Alternative Refrigeration: Solar Thermoelectric

An Off-Grid Solution to Vaccine Storage

Bass Connections in Energy 2018April 30, 2018

Ananya ChaureyChad Curd

Katelyn McCrackenNarendran Narasimhan

Sam Pickerill

Contents

Abstract 2

Research & Ideation 4Common Refrigeration Technologies 4Developing Refrigeration Technologies 8Comparing Refrigeration Technologies 12

Technical Design 15Prototype Overview and Goals 15Refrigerator Compartment 15Thermoelectric Module (TEM) Assembly 18Temperature Controller 20PV Array and Battery Sizing 21Small-Scale Prototype 22Prototype Construction 23

Testing 25Experimentation 25Testing Conclusions 31

Environmental Analysis 33

Social Impact 35

Market Analysis 37

Future Plans 38

Conclusion 39

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Abstract

Nigeria experiences the world's worst vaccination rate, with a coverage rate of approximately 42%1. Although there have been concentrated efforts to improve vaccination coverage by the government, the country has only witnessed an improvement of about 3% per year, far below the goals set by the UN. A lack of cold chain technology, poor education and training, and a burdened healthcare system are some of the main issues faced by organizations. This project hopes to provide an alternative off-grid solution for vaccine storage, namely solar thermoelectric refrigeration. The advantages of such a system, such as the lack of moving parts, transportability, and absence of a refrigerant, could allow for reliable and continuous maintenance of cold chain temperatures of 2oC - 8oC in varying environmental conditions. Testing on our small-scale prototype, 15% of the full-scale volume, demonstrates that cold chain temperatures are attainable. While, the full-scale model only maintained a steady state temperature of 10.8oC, the validity of this technology modality has still been confirmed. We have provided a platform through which further design modifications can be made. This is an important step in incorporating this type of technology in remote regions of Nigeria and working together with entities such as Africa Power Storage can prove to be instrumental in improving vaccine distribution. This would alleviate the massive burden on Nigeria’s current healthcare system and provide a more robust cold chain technology. Additionally, the refrigerator proposed could potentially save 159 kg of CO2 per year, and decrease the infant mortality, resulting in a healthier and more productive workforce.

Introduction

Despite recent advancements in vaccination techniques, nearly 20% of the world’s children remain unvaccinated to some of the world’s most dangerous diseases.1 This is due in large part to difficulties within the supply chain, and the inability to maintain critical cold chain temperatures of 2oC - 8oC when transporting vaccines to remote and energy poor regions. Our project aims to minimize this barrier by providing a solar cooling option which is cheap, portable, and low maintenance enough to be readily adopted and utilized in these areas.

We have selected Northern Nigeria as our initial interest area for the project due to the region’s demonstrated need and solar potential. Nigeria has the lowest vaccination rate in the world at 42%, leaving 2-3 million children unvaccinated in the country.2 This has partially resulted in the country’s 9% child mortality rate, the third worst in the world.3 The lowest coverage rates occur in the northern half of the country, which bordering the southern edge of the Sahara desert also

1 “Vaccination, rattling the supply chain.” Bulletin of the World Health Organization. 2011.2 Adebowale, Nike “Measles: Nigeria has highest number of unimmunized children in world” World Health Organization and UNICEF. Sep. 20173 “Levels and Trends in Childhood Mortality 2017.” World Health Organization. 2017.

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features a high solar irradiance of 5300 W·h/m² (Figure 1). While outside factors including religion and education barriers hinder vaccination rates as well, the combination of limited electricity access (57%) and high average temperatures (26.1°C) also makes maintaining cold chain in the region a major challenge.4

Figure 1: Map of solar potential across the states of Nigeria, with warmer colors indicating higher potential. Pinpointed is Kano, the most populous state in the country, which also has electricity access and vaccination rates

below the national average.5

Regionally, many of Nigeria’s neighbors experience similar cold chain difficulties and associated low vaccination rates. Nearly 50% of the 19 million children who are unvaccinated worldwide live in Africa, and the continent as a whole demonstrates several overlapping regions of low electricity access and high solar potential. The successful demonstration of the effectiveness of solar cooling in Northern Nigeria offers the potential to expand this technology throughout the region and address the vaccination problem on a more global scale.

Previous attempts to introduce solar cold chain technologies within the region have failed for a variety of reasons, including an inability to provide the maintenance required to consistently keep refrigeration systems operational. Because of this issue, our project utilizes a thermoelectric module, a solid-state refrigeration method that minimizes the need for new parts and outside technical expertise.

Research & Ideation

4 Antai, Diddy. "Faith and child survival: the role of religion in childhood immunization in Nigeria." Journal of biosocial science 41.1. 2009.5 “State of Electricity Access Report” World Bank Energy Sector Management Assistance Program. June 2017.

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Our ideation process began by researching different refrigeration methods to determine which one would best serve off-grid locations. We researched both commonly-used methods and methods that are currently being developed and refined. Each method was evaluated primarily on four different criteria: ability to maintain cold chain temperatures, design simplicity, innovative potential, and environmental impact. These evaluations led us to our final choice of refrigeration technology: thermoelectric refrigeration.

Common Refrigeration Technologies

Evaporative CoolerThe majority of modern-day cooling devices use a liquid-to-vapor phase change to provide cooling. The simplest example of this concept can be seen in an evaporative cooler, which is also known as a swamp cooler. The layout for an evaporative cooler can be seen below in Figure 2.

Figure 2: A graphic depicting evaporative cooling.6

Most evaporative coolers have an inlet for outside air, an evaporation membrane, and an outlet for cooled air. To begin the cooling process, outside air is pumped through the inlet by a fan and is forced through the membrane. The membrane is supplied with water by a reservoir stored in the cooler. As the air moves across the water in the membrane, heat from the air is used to evaporate the water. As a result, the temperature of the air decreases and the humidity of the air increases. A fan is then used to pump the low-temperature air into the space to be cooled. Evaporative coolers lose water through evaporation, so the water reservoirs must be refilled regularly.

Vapor Compression

6 McGloin, Joe. “Almost Free, Portable, Indoor, Home-Made Evaporative (Swamp) Cooler.” Instructables, 28 Sept. 2017.

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Vapor compression refrigeration is a much more common method of cooling, but operates via similar principles to the evaporative cooler. Vapor compression refrigeration utilizes the vapor compression cycle, a process by which a refrigerant is evaporated and condensed in a continuous cycle. It is commonly found in most refrigerators and air conditioning units, along with other applications like electronics cooling.7 This cycle can be seen below in Figure 3 below.

Figure 3: A diagram detailing the vapor compression cycle.8

Cooling occurs when the refrigerant enters the evaporator. At this point, the refrigerant is at a temperature below its boiling point and a low pressure. Refrigerants used in this process almost always have boiling points below the freezing point of water. For example, one common refrigerant, R-134a, has a boiling point of -14.9°F at 1 atm of pressure.9 The evaporator is then exposed to the chilled air, such as the interior of a refrigerator or the vents of an air-conditioned house. The refrigerant uses heat from the air to evaporate, which cools the air further. This air can then be circulated through the space it is meant to cool.

Once the refrigerant passes through the evaporator, it moves to the compressor. The compressor raises the pressure of the refrigerant when it flows through and provides the mechanical force to allow the flow of refrigerant through the entire cycle. When the refrigerant reaches the compressor, it is in gaseous form, but still at a low pressure and low temperature. After the refrigerant flows through the compressor, it has a high pressure and a high temperature. It then flows to the condenser, which is a heat exchanger that usually interacts with ambient air. This ambient air could be the outside of the refrigerator or the area outside an air-conditioned house. The ambient air absorbs heat from the high-pressure refrigerant, which causes the refrigerant to undergo a phase change from a gas to a liquid. After the refrigerant passes through the

7 Burnett, James. “Advances in Vapor Compression Electronics Cooling.” Electronics Cooling, vol. 20, 11 May 2014.8 Salah, Ahmad. “What Is An Absorption Chiller?” Blogspot.9 National Refrigerants, Inc. “R-134a.” R-134a, 2010.

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compressor, it is a high-pressure liquid. This liquid then proceeds through an expansion valve. The expansion causes a sudden drop in pressure, which also causes a sudden drop in the temperature of the refrigerant. The temperature drop is enough to make the refrigerants temperature lower than the chilled air. This allows the refrigerant to once again pass through the evaporator and restart the entire cycle.

Vapor AbsorptionWhile it is not nearly as common as vapor compression refrigeration, vapor absorption refrigeration is another cooling method found in refrigerators, especially RV (recreational vehicle) refrigerators. Vapor absorption refrigerators use a vapor absorption cycle, which is very similar to the vapor compression cycle. Both cycles utilize a condenser, expansion valve, and evaporator. However, in the vapor absorption cycle, the electrical compressor is swapped out for an absorption system that relies more on heat energy. This can be seen when comparing Figure 3 with Figure 4, where the compressor from the compression cycle is replaced with an absorber and generator in the absorption cycle.

Figure 4: Diagram of the vapor absorption cycle.10

Much like the compression cycle, refrigerant exits the evaporator as a low-pressure gas in the absorption cycle. The differences start when the gas enters the absorber. The absorber has a basin of liquid solvent that the gaseous refrigerant dissolves into. This liquid solution is then pumped to the generator and brought to a higher pressure. A pump is also used in the vapor compression cycle to compress the gaseous refrigerant, but the pump in the vapor absorption cycle uses significantly less power. This occurs because bringing a liquid to a high pressure requires significantly less work than bringing a gas to a high pressure. Once the high-pressure liquid solution passes through the pump, it flows to the generator. Some outside heat source heats the liquid in the generator. Commonly used heat sources include propane, electrical, and solar heaters. This heat evaporates the refrigerant out of the solution, which results in the separation of the high-pressure, high-temperature gaseous refrigerant and the liquid solvent. The liquid solvent

10 Salah, Ahmad. “What Is An Absorption Chiller?”

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returns to the absorber, while the refrigerant flows to the condenser. The refrigerant then continues through the expansion valve and the evaporator, where it can repeat the cycle.

The absorption cycle can also be altered to operate with no pump, thus creating a refrigeration cycle with no moving parts. This altered process, called a single-pressure absorption cycle, involves replacing the pump with a system driven by buoyancy forces. Much like the absorption cycle detailed above, in the single-pressure system, a solution of gaseous refrigerant and a solvent leaves the absorber. However, instead of entering a pump, this solution flows directly to the generator. The solution heats up, and the gaseous refrigerant comes out of solution. The buoyancy force of the gaseous bubbles drives the liquid solvent up a tube, until it reaches the separator. Once it reaches the separator, gravity forces the solvent back to the absorber while the gaseous refrigerant continues to the condenser. The separator can be seen below in Figure 5.

Figure 5: The separation of refrigerant and solvent in a single-pressure absorption cycle.11

Developing Refrigeration Technologies

Solar Thermo-MechanicalAlthough vapor compression cycles have dominated the air cooling systems market for decades, there has been renewed effort into finding alternative energy generation technologies since the energy crisis of the 1970’s. One such technology is solar thermo-mechanical cooling, given

11 Rodriguez-Muñoz, J L, and J M Belman-Flores. “Review of Diffusion-Absorption Refrigeration.” Renewable and Sustainable Energy Reviews, vol. 30, Feb. 2014, pp. 145–153.

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certain advantages it holds over conventional vapor compression cycles12. These systems convert the heat gained from a solar collector into usable mechanical work, which can be used to directly compress the working fluid in a vapor compression cycle or to power an organic Rankine cycle. The primary advantages of this system are the ability to maintain low temperatures, robustness in off grid locations, and the ability to convert the mechanical work into electricity when cooling is not required.

A solar thermo-mechanical cooling method that directly compresses the working fluid in a vapor compression cycle is known as an ejector cooling system. A simple ejector cycle is shown below in Figure 6. These systems are very similar to vapor compression cycles, with the only difference being that the mechanical compressor is replaced by an ejector, thus making this system a thermally driven one.

Figure 6: Ejector Cooling System 12

Heat energy gained in the collector runs through a heat storage tank, and then through a generator. The loop is completed with a pump that recirculates the fluid. Heat is then absorbed by the working fluid in the power loop, resulting in a high temperature and pressure vapor at point 1. The vapor flows through the ejector, as shown in Figure 7, where the entertained flow mixes with the primary flow at the nozzle exit. This is because the vapor at the nozzle exit now has a lower pressure than the entertained flow. This mixture is then ejected out of the ejector at a higher pressure and lower velocity than at the nozzle exit.13 At point 2, the pressure is slightly higher than the condenser pressure, which results in the vapor rejecting heat to the environment and becoming a liquid. Some of this liquid is pumped back to the generator, and some of it is expanded through a throttling valve at point 4. This liquid-vapor mixture passes through the evaporator, absorbing heat from the environment, and causes a cooling effect. The refrigeration

12 Zeyghami, Mehdi, et al. “A Review of Solar Thermo-Mechanical Refrigeration and Cooling Methods.” Renewable and Sustainable Energy Reviews, vol. 51, Nov. 2015, pp. 1428–1445., doi:10.1016/j.rser.2015.07.011.13 Zeyghami, Mehdi, et al. “A Review of Solar Thermo-Mechanical Refrigeration and Cooling Methods.”

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loop is completed when the emerging flow from the evaporator is fed into the ejector as he entertained flow. Although these systems have a relatively low efficiency, the easy construction and maintenance of the ejector cycle is a proven advantage. Operational conditions, ejector design, and working fluid can all affect the performance of the ejector cycle.

Figure 7: Ejector schematic.13

The other form of solar thermo-mechanical refrigeration, the solar Rankine cycle, is also receiving more attention. This is due to the improvement in organic Rankine cycle technology and the increased availability of environmentally friendly refrigerants. Solar Rankine cycles work by using the heat energy gained to indirectly drive a conventional vapor compression cycle. Figure 8 below shows a simple expander-compressor-coupled solar Rankine cycle. The generator gains heat energy via the heat collector. On its way to the expander, the refrigerant passes through the generator and becomes a high-pressure, high-temperature vapor. As the vapor passes through the expander, it does mechanical work which is used by the compressor in a conventional vapor compression cycle. The fluid emerging from the expander enters the condenser where heat is rejected to the environment. This fluid then enters the generator as a liquid, and the cycle is complete.

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Figure 8: Solar Rankine cycle.14

Currently solar thermo-mechanical systems cannot compete with traditional solar cooling systems in terms of efficiency. However, with more research and development, the advantages of these robust systems can be exploited. A wide temperature range, and easy maintenance are some advantages that could prove useful in varying environments and off grid locations. Another potential advantage is its ability to provide electricity when disconnected from a refrigeration unit.

Ranque-Hilsch Vortex TubeAnother developing refrigeration technology is the Ranque-Hilsch vortex tube. A mechanical device with no moving parts, it can separate high pressure compressed gas into lower pressure hot and cold streams. With a sufficient mass flow rate of inflowing gas, the hot stream can reach temperatures of 200oC, while the cold stream can go down to -50oC. The basic principle behind the vortex tube is that a stream of gas has both high energy (hot) particles, as well as low energy (cold) particles, which can be separated into two distinct streams. Compressed gas enters the tube tangentially through the inlet and starts spinning in a vortex. This inlet, along with the rest of the system, can be seen below in Figure 9. The higher energy particles have higher momentum and thus get pushed to the outer fringes of the tube. At the hot outlet, a control valve allows the hotter stream swirling on the outer fringes to escape, while the cooler gas is reflected back into the tube. The cold stream swirling in the inner parts of the tube then escapes through the cold outlet. The control valve can be used to adjust the fraction of gas coming out of the cold side, otherwise known as the cold fraction. A higher cold fraction allows for a higher volume, but higher temperature of cold gas, while a lower cold fraction allows for a lower temperature, but lower volume.

While the vortex tube itself has no moving parts and requires no maintenance, it suffers from a low relative efficiency and requires a steady stream of high pressure compressed air. For our needs, the amount of compressed air required could only be achieved by high power compressors, which also require high maintenance.

14 Zeyghami, Mehdi, et al. “A Review of Solar Thermo-Mechanical Refrigeration and Cooling Methods.”

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Figure 9: Vortex Tube principle15

Thermoelectric CoolingThe final alternate method of refrigeration we considered was thermoelectric cooling. Thermoelectric cooling utilizes devices called thermoelectric modules that are based on the Peltier Effect. The Peltier Effect, discover in 1834 by Jean Charles Athanase Peltier, a French physicist, is the conversion of electrical energy to a temperature gradient. Thermoelectric modules consist of an array of several N-type and P-type semiconductor junctions connected electrically in series and thermally in parallel via ceramic plates. When DC power is applied to the module in which the current flows from the N-type element to the P-type element, one side of the module is cooled, and the other side is heated. This occurs because of electron transport between the N-type and P-type elements. Electrons begin at a low energy level in the P-type element, absorb heat energy at the cold junction, continue on at a high energy level to the N-type element, and eventually reach the hot junction where the heat gained by the electrons is dissipated while they continue back to the P-type element at a low energy level. Thermoelectric module performance is governed by parameters such as electrical resistivity of materials, electrical conductivities of the semiconductor elements, the total thermal conductivity between the two junctions, and the Seebeck coefficient, a material property describing the magnitude of an induced thermoelectric voltage in response to a temperature difference across the material.16 This process is depicted below in Figure 10.

Figure 10: Schematic of a thermoelectric module depicting the process behind the Peltier Effect.17

15 Pneumatics and Sensors Ireland. Sourced from http://www.psireland.ie/partners/vortec16 Enescu, Diana, and Elena Otilia Virjoghe. "A review on thermoelectric cooling parameters and performance." Renewable and Sustainable Energy Reviews, vol. 38, no. 2014, Oct. 2014, pp. 903-16. Accessed 27 Apr. 2018.17 Enescu, Diana, and Elena Otilia Virjoghe. "A review on thermoelectric cooling parameters and performance."

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Thermoelectric cooling is widely used today in a variety of applications such as electronic cooling (e.g., PC-processors), portable food and beverage coolers, temperature control car seats, and air conditioners. Though some research is being done in applying this method to vaccine storage and cold chain maintenance, a successful design is yet to be implemented in the realm of vaccine refrigerators. Advantages of thermoelectric cooling include: high reliability, no mechanical moving parts, small size, low weight, and quiet operation. Additionally, thermoelectric modules require DC power. Depending on the application, this can be an advantage or disadvantage. For this particular application of a solar powered vaccine refrigerator, a DC power requirement is an advantage as it can easily be integrated with solar panel and battery arrangements. The disadvantage to thermoelectric cooling is its low COP. Typical values range from 0.2 to 0.5.18 This low energy efficiency is not particularly attractive. However, the advantages of no moving parts, small size, low weight, and lack of working fluid have the potential to outweigh low COP values in this application.

Comparing Refrigeration Technologies

The first concern in selecting a refrigeration method is considering whether that method is capable of maintaining cold-chain temperatures for a long period of time. Thus, evaporative cooling was quickly disregarded as a cooling method. While evaporative coolers are a simple and efficient method of cooling buildings, they are rarely used in refrigeration applications. The minimum temperature of cooling that they can provide has a lower thermodynamic limit, which is a function of the relative humidity and air temperature of its ambient surroundings. For example, an average August day in Kanos, Nigeria, is 20°C with 68% humidity.19 In these conditions, the lowest temperature that can be produced via evaporative cooling is 15.6°C.20 This is significantly higher than the 8°C needed to maintain cold chain.

The other refrigeration methods discussed above are all capable of providing the necessary cooling to provide cold-chain temperatures. Thus, some secondary considerations were implemented to decide which of the remaining methods would be best. Three primary considerations were used: design simplicity, innovative potential, and environmental impact.

To be an effective vaccine refrigerator in off-grid locations, the refrigerator must be simple and adaptable. In this case, adaptability means that it must be capable of travelling frequently and must be easily fixed in case of a part failure. This need for adaptability was highlighted by a member of the non-profit Africa Power Storage, who said that one of his solar refrigerators was “being used as a filing cabinet” after the compressor broke. Simplicity is enhanced by having fewer parts and less complex componentry. The thermo-mechanical and solar absorption cycles

18 Zhao, Dongliang, and Gang Tan. "A review of thermoelectric cooling: Materials, modeling and applications." Applied Thermal Engineering, vol. 66, no. 1-2, May, pp. 15-24. Accessed 27 Apr. 2018.19 “Relative Humidity in Kano, Nigeria.” ClimaTemps20 “Evaporative Coolers.” Big Ladder Software, Lawrence Berkeley National Laboratory, 2015.

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are both advantageous in regard to simplicity because they require no electrical energy. This means they do not require any complex electrical componentry or a battery. However, the vortex vapor tube and thermoelectric modules both require very few mechanical components, which is also advantageous. Componentry alone does not reveal a clear front-runner in terms of simplicity, but thermoelectric refrigeration distinguishes itself in terms of ease of repair. Thermoelectric refrigeration requires very few parts, and each of these parts can be easily sourced, ordered, and repaired from large vendors. This is not true of the other refrigeration methods, which require uncommon componentry and/or the use of a refrigerant. Refrigerants are particularly important with regards to reparability, because refrigerators that utilize refrigerants must be drained and refilled during repairs. Thus, the most simplistic refrigeration method was deemed to be thermoelectric refrigeration.

Solar-powered vaccine refrigerators are already on the market, which makes the innovative potential of the refrigeration method an important factor. Creating a product that stands out amongst its peers is advantageous for business success. Vapor compression and absorption refrigeration are already common in the refrigeration market, so they would likely not stand out in the solar refrigeration market. On the other hand, thermo-mechanical, vortex vapor tube, and thermoelectric refrigeration are all refrigeration methods not yet explored for vaccine refrigeration. Thus, these methods provide the highest potential for successfully entering the solar vaccine refrigeration market.

The environmental footprint of the refrigeration method was also an important consideration. It is advantageous to create a refrigerator that minimizes environmental impact, both to stay ahead of environmental regulations and to pair the refrigerator’s social benefits with environmental benefits. Efficiency is an effective measure of environmental impact, so it was weighed heavily when choosing a refrigeration method. One commonly used metric for judging refrigerator efficiency is the coefficient of performance (COP). The COP is determined by dividing the amount of heat removed by the amount of work required. Vapor compression has by far the highest efficiency of all the refrigeration methods, with a COP of about 3.5.21 Vapor absorption is the closest, with a COP of about 1.2.22 The remaining refrigeration methods all have COPs below 0.5, so vapor compression refrigeration is clearly the most efficient method.23 However, some of the efficiency gains of compression and absorption are offset by their use of environmentally harmful refrigerants. On the other hand, vortex vapor tubes circulate air and thermoelectric refrigerators have no working fluid. This makes both of these options environmentally friendly in their own right.

21 Roy, Ranendra, and Bijan Kumar Mandal. Thermodynamic Analysis of Modified Vapour Compression System Using R-134a. International Conference on Recent Advancement in Air Conditioning and Refrigeration, Nov. 2016.22 United States DOE. Absorption Chillers for CHP Systems. Absorption Chillers for CHP Systems, 2017.23 Zehygami, Mehdi, et al. “A Review of Solar Thermo-Mechanical Refrigeration and Cooling Methods.” Renewable and Sustainable Energy Reviews, no. 51, 2015, pp. 1428–1445. Researchgate.

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After considering simplicity, innovative potential, and environmental impact, a Pugh Matrix was created with a few other important factors in order to quantify the decision-making process. This Pugh Matrix can be seen below in Table 1.

WeightSolar

Absorption Cycle

Vortex Vapor Tube Compression

Solar Thermo-

mechanical

Solar Thermo-electric

COP 1.0 5.0 2.0 2.0 3.0

Scale 0.7 5.0 3.0 4.5 3.0

Feasibility 1.0 5.0 5.0 2.0 5.0

Maintenance 0.3 3.0 4.0 4.0 5.0

Manufacturability 0.5 4.5 5.0 4.0 3.0

Environmental Impact

1.0 2.5 4.0 2.5 5.0

Price 0.7 5.0 2.0 4.5 3.0

Innovation 0.5 3.0 5.0 5.0 4.0

Total 24.15 20.7 18.5 22.2

Table 1: Decision matrix demonstrating highlighting advantages and disadvantages of the different concepts considered.

The thermoelectric module is an advantageous option in terms of simplicity and innovative potential. It also scored highly in the Pugh Matrix in Table 1. It is lightweight, small, requires few moving parts, and is compatible with DC solar and battery power. With this in mind, it was decided that a thermoelectric module would be used to cool the refrigerator.

Different renewable energy technologies were also discussed - solar, hydro, wind, and kinetic - which would be coupled together with a battery to power the refrigerator and charge a battery when there is an excess load. After consideration of the environmental restrictions, solar powered refrigeration was the best option. Not only is there an abundance of solar irradiance in northern Nigeria, but also concentrated effort to incorporate solar power with different applications.

Technical Design

Prototype Overview and Goals

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The purpose of the prototype for this project was to provide a full-scale model to demonstrate proof of concept. In addition to providing proof of concept, prototyping allows for viewers to better understand the proposed design.

For our prototype to be an effective solution to the difficulties in maintaining cold chain, it had to be capable of maintaining temperatures within the refrigerator compartment that comply with cold chain temperature requirements. Cold chain temperatures serve to ensure vaccine quality and for most refrigerated vaccines the acceptable range of temperatures that comply with the cold chain is 2°C to 8°C. In order to maintain cold chain temperatures, a certain amount of heat must be removed by our chosen refrigeration method, the thermoelectric module. Thus, the primary driver behind the design of our system revolved around the heat transfer to the refrigerator compartment from the surroundings, which is equal to the amount of heat that must be removed from the refrigerator compartment. The design of the refrigerator compartment dictated this value, while this value drove consequent design decisions.

In brief, our prototype system consists of a refrigerator compartment, a thermoelectric module (TEM) assembly, a temperature controller, a battery, and solar photovoltaic (PV) array. The design and/or selection of these components is discussed in subsequent subsections.

Refrigerator Compartment

The refrigerator compartment was designed with several different parameters in mind: geometry, heat transfer, and weight. It has to be an acceptable geometry for vaccine storage. The heat transfer from the surrounding air to the compartment had to be minimized while also maintaining a practical design. Weight also had to be minimized to allow for transportability of the compartment.

Because vaccine vials are stored, transported, and distributed in rectangular packages, we chose to design a prismatic refrigerator compartment.24 Utilizing a geometry similar to that of the smaller packages of vaccines to be transported and stored in our compartment allows for efficient use of storage space. The inner volume of the compartment was chosen to be approximately 0.85 ft.3 (25L), allowing for storage of about 400 0.5mL vaccine doses in single-dose vials, or 2,000 0.5mL doses in ten-dose vials.25 A prismatic design has the added benefit of being easily manufactured.

24 Norman, Bryan A., Jayant Rajgopal, Jung Lim, Katrin Gorham, and Leila Haidari. "Modular vaccine packaging increases packing efficiency." Vaccine, vol. 33, no. 27, 17 June 2015, pp. 3135-41. Accessed 27 Apr. 2018.25 Drain, Paul K., Carib M. Nelson, and John S. Lloyd. "Single-dose versus multi-dose vaccine vials for immunization programmes in developing countries." Bulletin of the World Health Organization, vol. 81, 2003, pp. 726-31. Accessed 27 Apr. 2018.

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Secondary to the compartment geometry were the heat transfer and heat removal requirements. These values were governed by insulation choice and thickness. While we researched a multitude of insulation options from novel super-insulators such as aerogels and glass microspheres to more traditional choices such as rigid polyurethane foam and rigid polyisocyanurate foam, we selected rigid polyisocyanurate foam for its high availability and low thermal conductivity value of 0.023 W/m·K. This value is one of the lowest for common insulation materials.26 In addition, rigid foam insulation is also easy with which to manufacture. It can be cut to size as needed, stacked in multiple layers for added insulation, and easily installed into prismatic compartments. Polyisocyanurate is even beneficial in terms of its own manufacturing, as its manufacturing requires no blowing agents that contribute to ozone depletion.27

In order to determine the heat transfer to the refrigerator compartment through the insulation, a thermal study was performed on the insulation in SolidWorks. The interior of the compartment was set at 2°C and the exterior of the temperature was set at 40°C. These values were chosen to represent the extreme case. An inner temperature of 2°C represents the absolute minimum cold chain temperature. An exterior temperature of 40°C accounts not only for the fact that average annual maximum temperature in Kano, Nigeria is 33°C, but also for the fact that temperatures may rise up to 38°C.28 This process was iterated for different insulation thicknesses until a value that was reasonable considering the capabilities of our chosen refrigeration method was found. For an insulation thickness of 3in, the total heat transfer across the insulation from the environment to the interior was found to be 8W. The simulation results for this thickness is shown below in Figure 11 as a heat flux plot.

26 "Insulation materials and their thermal properties." greenspec. Accessed 27 Apr. 2018.27 "Rigid Foam Insulation." Green Building Advisor, 9 Aug. 2012. Accessed 27 Apr. 2018.28 World Weather and Climate Information. Accessed 27 Apr. 2018.

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Figure 11: Results of SolidWorks thermal study showing heat flux across 3 inches of rigid polyisocyanurate foam insulation of refrigerator compartment.

In order to weatherproof and protect the insulation, exterior and interior shells were also designed. The exterior shell is made of 0.25 in. thick UV-Stabilized High-Density Polyethylene Plastic (HDPE) and the interior shell is made of 0.06 in. thick polypropylene plastic. The material of the inner shell differs from that of the exterior shell for three reasons: it is not necessary that the interior be UV-resistant, polypropylene is cheaper, and polypropylene is available in thinner options.

A layer of HDPE with a groove sits on top of the insulation between the exterior and interior shells to cover the insulation as well as to provide a housing for a hollow surface-mount seal. The seal is compressed by a layer of HDPE on the lid when closed. This provides an airtight seal to minimize leaks.

Finally, the lid and base are constructed separately and joined together by a hinge-and-latch mechanism. The latch has the option to add a padlock for added security when storing vaccines. A simple pull-handle is included on the lid for ease of opening. The TEM assembly fits into a square hole cut through the outer shell, insulation, and inner shell in the back face of the compartment. An exploded view of the design is shown below in Figure 12.

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Figure 12: Exploded view of refrigerator compartment design, including TEM assembly

Thermoelectric Module (TEM) Assembly

The selection of the specific TEM was driven by the heat removal requirements of the designed refrigerator compartment. As mentioned above, the heat transfer to the refrigerator compartment from the environment occurs at a rate of 8W for the given conditions. This means that the TEM must be capable of removing heat at a rate of at least 8W. In addition, the power requirements of the TEM to remove heat at this rate had to be compatible with a PV array of a reasonable size. The specifics of sizing the PV array and battery subsystem are discussed later.

In order to select a specific TEM, we examined product data sheets to obtain the information we needed to select a module that complied with our requirements. The data sheets provide performance curves for each module at different operating conditions. These performance curves give information about heat pumped, current drawn, and input voltage as functions of module temperature difference at a given hot side temperature. After examining several different modules from several different manufacturers, we opted to use a two-stage thermoelectric

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module from CUI Inc. (Model Number: CP854705-2). At a hot-side temperature of 50°C and a temperature difference of approximately 40°C, this unit is capable of pumping approximately 15W of heat at 7.5V and drawing 3.4A. Using the relationship between current and voltage:

P = IVthe power required to operate the module can be calculated. This power requirement is 25.5W.

TEMs operate best when the temperature difference between the hot side and the cold side is minimized. This is accomplished by increasing the heat transfer rate at both the hot side and cold side. Heat transfer rate is given by:

Q' = UA(Ts – T ∞)

It is directly proportional to U, the overall heat transfer coefficient, A, the surface area of the object in question, and ¿¿) the temperature difference between the surface of the object and the temperature of the surrounding fluid. As we could not directly control the temperature of either the surface of the object or the temperature of the surrounding fluid, we were left with increasing the overall heat transfer coefficient and/or increasing surface area of the object. By implementing active heat sinks - finned heat sinks with fans to force airflow over the fins - we accomplished both. The fins increase surface area and the forced flow of air by the fans increases the convective heat transfer component of the overall heat transfer coefficient. The TEM assembly can be seen below in Figure 13.

Figure 13: TEM Assembly showing TEM, finned heat sinks, and fans.

After initial tests, it was found that the direction of the fans has a significant impact on system performance. The assembly performs best when both the exterior and interior fan are directed towards the respective finned heat sink. This can be explained by the fluid dynamics principle of continuity. Applying this principle to mass, it states that the mass flow rate entering a system must be equal to the mass flow rate exiting the system. In other words, mass must be conserved. Assuming constant density, this relationship can be expressed as:

ṁin = ṁout

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Ainvin = Aoutvout

where ṁis the mass flow rate, A is the cross-sectional area through which the mass is flowing, and v is the velocity of the flow. The velocity of the air before the fan is smaller than the velocity of the air after the fan. This means that the area through which the air flows is smaller after the fan than before. The concentration of high-speed flow after the fan onto the finned heat sink improves convective heat transfer, thus improving TEM performance by removing excess heat from the hot side of the module.

Temperature Controller

The goal of the temperature controller for the refrigerator is to ensure that the temperature will remain within the 2oC - 8oC range. Furthermore, it was important from a technical and social standpoint that we design a controller that is both simplistic - in case parts need to be fixed, or code needs to be changed - and easy to build. A variety of different controller modalities were brainstormed and researched, and certain technologies stood out. Specifically - PWM, PID control, voltage regulators, potentiometers, thermostatic control, and a thermoelectric controller. Essentially, for all the considered controllers a feedback loop consisting of a temperature sensor will be used to control the power of the Thermoelectric module and thus maintain the temperature within the refrigerator at a predetermined value.

After doing research into these different models, it was found that the Thermostatic type controller would be the most apt for this specific application. PWM, although effective in varying voltages, would have required an electronic hardware setup that would prove to be too complex, and would defeat the goal of creating a simplistic system. As the refrigerator is driven by a 12V power supply, the inbuilt PWM functionality in the arduino would be restrictive, as the full 0-12V range of power would not be attainable. It was also found that in certain cases PWM signals, over long periods of time, tend to deteriorate the thermoelectric modules. Likewise, PID control would allow for accurate and sensitive control of the voltage, based on the temperature feedback. Proportional and derivative gain may be implemented to allow for gradual power changes to meet temperature requirements. Similar to PWM, PID would have required hardware that was too complicated, and unattainable using the arduino. Voltage regulators and potentiometers were both simple models that could have been implemented, but both modalities proved to be extremely lossy when working at low power ranges. Purchasing a TEC controller was also investigated, which would have eliminated the need to design and build a custom controller. However, these controllers were expensive, and also comprised of unnecessary functionalities that would draw too much power.

Thermostatic control was the only control system that was both simple and easy to design. A temperature probe is connected to an arduino that actively monitors the temperature within the refrigerator compartment. This information is processed and depending on whether the temperature is above or below a predetermined value, a HIGH or LOW signal is sent to a relay,

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which controls the power input to the TEM module. From the initial tests conducted, it is evident that the temperature is not susceptible to rapid changes, and only gradually reaches the temperature that the refrigerator has been designed for. This means that a thermostatic control is suitable for this application, and rapid switching of the thermoelectric module is not expected. Our specific controller turns off at 2oC, but only turns back on at 6oC. This will ensure maximum power efficiency, as the module would not be switching on and off at the same temperature. Figure 14 below depicts a simplified version of the control system that has been implemented.

Figure 14: Simplified Controller Schematic.

Having adopted this control system, the next step would be to incorporate other pertinent functionalities. The particle mesh is a microcontroller, that provides live temperature and GPS data, allowing for dynamic monitoring of cold chain status. As the refrigerator is designed for mobility, the particle mesh would enable users to identify where the cold chain might be breaking and react accordingly. Fixing an LCD screen with the option of changing the critical temperature, would also enable for more customization. This might increase power efficiency based on environmental conditions, i.e. irradiance, as the refrigerator could potentially be turned off for longer periods of time in cooler conditions.

PV Array and Battery Sizing

Since we wanted our vaccine refrigerator to work in off-grid locations, we decided to design it to run using solar photovoltaic (PV) panels and a battery. Further, we chose to run the system at 12V DC using a lead-acid battery since it is durable, cheap and easily available in Nigeria. Designing the system at 12V also allows it to be run using car or truck batteries in emergencies.

The first step in sizing the PV panel and the battery was to estimate the power (Watts, W) and energy (Watt-hours, Wh) requirements of our refrigerator. As mentioned previously our TEM consumes 25 W of power to provide the required amount (15 W) of cooling. Thus, in an hour, the TEM would consume about 25 Wh of electric energy. Similarly, other components’ energy requirements are listed in Table 2 below.

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Component Quantity Power draw (W) Hourly Energy consumption (Wh)

TEM 1 25 25

Fans 2 3 per fan 6

Arduino controller 1 1 1

Total - 32 32Table 2: Theoretical power and energy consumption of refrigerator

Accounting for air leaks, we decided to over-design our PV and battery system, and assumed an hourly energy consumption of approximately 45Wh. For a full day backup of 24 hours, this translated to 1,080 Wh of energy. Lead-acid battery life depends on the depth of discharge and most manufacturers recommend a depth of discharge no more than 60% (in other words, a minimum state of charge of 40%). However, due to size and weight of such a large battery, we chose to lay more emphasis on battery weight rather than lifetime. Ultimately, we decided to go with a 1,500 Wh battery, which results in a maximum depth of discharge of 70%.

Coming to the size of the solar PV panel, we first looked at northern Nigeria’s solar insolation. We found data for a town called Maiduguri, located in the north-east of the country. Maiduguri’s annual average solar insolation is 5.91 kWh/m2/day, which translates to a solar panel output of 1.07 kWh/m2/day (assuming 18% panel efficiency). This is almost equal to our system’s daily energy needs, and thus, our ideal solar PV panel should be 1m2 in size. Looking at the latest products, a 1m2 panel has a peak power rating of 250-300 W DC.

Small-Scale Prototype

Before constructing the full-scale prototype, a small-scale version was designed and constructed with the goal of validating the performance of the TEM assembly. The primary design requirement was that the small-scale prototype have similar heat removal requirements as the full-scale prototype. This resulted in an interior storage space volume of 0.125 ft3 (15% the volume of the full-scale prototype) with 0.5 in of polyisocyanurate insulation for a total heat transfer rate of 11W. The higher heat transfer rate of the small-scale prototype is acceptable, as it models leaks and inefficiencies that might be encountered in the full-scale design. The results of the thermal study performed in SolidWorks using the same parameters as the full-scale prototype is shown below in Figure 15 as a heat flux plot.

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Figure 15: Heat flux plot of SolidWorks thermal study on small-scale prototype

An inner and outer shell were 3D-printed out of ABS plastic to mimic the HDPE exterior shell and polypropylene interior shell of the full-scale model. No seal, latch, hinge, or handle were included. The small-scale prototype assembly is shown below in Figure 16.

Figure 16: SolidWorks rendering of small-scale prototype design

The testing results for the small-scale prototype are discussed later in the Testing section.

Prototype Construction

There were three separate components that had to be constructed for our full-scale prototype: the refrigerator compartment, the TEM assembly, and the temperature controller.

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In order to construct the refrigerator compartment, we began by assembling the UV-Resistant HDPE 0.25 in. sheets. The distributor we chose cut the larger 4.5 ft. x 9 ft. sheet to size so we simply assembled the exterior shell base and lid by sanding the edges of the smaller sheets and adhering them together with TAP Poly-Weld Adhesive. This adhesive was selected because of its ability to adhere HDPE, a plastic that is usually very difficult to adhere with traditional glues. Next, we cut the rigid polyisocyanurate foam insulation down to size and placed it in the base and lid, respectively. We then epoxied the polypropylene plastic sheets (also cut to size by the distributor) to the interior surface of the insulation in both the base and the lid. The top surface of the base insulation is covered by 0.5 in. thick UV-Resistant HDPE, into which we milled a 0.3 in. deep groove. This groove is for the seal, which we cut to length, sealed together with silicone sealant, and epoxied to the HDPE. We drilled holes into the lid for the latch, handle, and hinges. Holes were also drilled into the base for the latch and the two hinges. Once the holes had been drilled and tapped, the respective components were assembled onto the compartment.

Before assembly began, we initially milled 3.6 in. square holes into one sheet each of the HDPE and polypropylene, as well as a 3.15 in. square hole through the insulation layer into which the TEM assembly would be inserted. The smaller size of the insulation layer hole allowed for the snug placement of the TEM assembly into the refrigerator compartment, while the larger holes in the shells allowed space for airflow around the finned heat sinks. However, after some testing, it was discovered that the performance of the TEM assembly was hindered by the fact that the finned heat sinks were almost completely surrounded by insulation because of the thickness of the insulation layer. As a result, the design of the cutout for the TEM assembly had to be updated. On the interior of the compartment, the hole of both the polypropylene plastic and 2 in. of depth of the insulation was expanded to a 5.6 in. square. This allowed for the simultaneous shifting of the hot-side fins completely outside the refrigerator compartment volume and complete exposal of the cold-side heat fins inside the compartment. The thermal study, discussed above, was repeated to ensure that these design updates would not significantly alter the heat transfer requirements of the system. This can be seen below in Figure 17.

Figure 17: SolidWorks thermal study of full-scale prototype after design updates.

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As indicated by the green and orange shade coloring the face of the compartment to which the design updates were done, the heat flux through this face increased relative to the original model. However, the total heat transfer rate rose by only 0.4W to 8.4W. This value still falls within the heat pumping capabilities of the selected TEM and thus the design updates were deemed acceptable.

Construction of the TEM assembly was quite simple. We began by assembling the active heat sinks, which involved bolting one fan to the finned side of each. The TEM was then connected to the active heat sinks, one on each side, via thermal paste.

The thermostatic controller for this system was setup by first programming the arduino to read in temperature through the DS18B20 Temperature Probe and vary the output signal to a specific digital pin. The TEM was connected through the relay in a normally closed fashion, and the arduino output pin was connected to the relay. An LCD display will also be connected to the arduino to display the current temperature and the state of the TEM (ON/OFF). The electronics will be mounted inside of the refrigerator.

Once these three components were constructed, the TEM assembly was inserted into the refrigerator compartment and the TEM assembly, temperature controller, battery, and PV array were electrically connected.

Testing

Experimentation

When testing, we had several goals in mind. The primary goal was to prove that the refrigerator was capable of reaching cold-chain temperatures. This was tested by running the system beginning at ambient temperatures and allowing the compartment to cool off as much as possible. Additional goals included demonstrating the ability of the refrigerator to maintain cold-chain temperatures with a load and exploring the performance of the compartment when not cooled by the TEM. To add a load to the system, cans of chilled soda were placed in the compartment after the system had reached steady-state to simulate the placement of vaccines in the compartment. To explore the performance of the compartment with no cooling, the system was run until steady-state was achieved. Power to the system was then shut off and the compartment allowed to return to ambient. All temperatures were measured with a thermocouple suspended from the lid of the compartment. Current through and voltage across the TEM were also measured during testing for purposes of comparison to manufacturer-supplied performance curves.

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The small-scale prototype was tested first in the aforementioned methods before the full-scale model was constructed and tested. The results of these tests are shown below in Figures 18, 19, and 20, respectively.

Figure 18: Plot of temperature vs. time and power vs. time for cooling of small-scale prototype from ambient to

cold chain temperatures.

Figure 19: Plot of interior temperature of compartment vs. time and soda can temperature vs. time showing steady-state of small-scale prototype with one 12 oz. can of soda as a load.

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Figure 20: Plot of interior temperature of small-scale prototype as a function of time when TEM is disconnected with a 12 oz. can of soda as in interior load

As can be seen in Figure 20, the small-scale prototype achieved cold chain temperatures when operated at an ambient temperature of 23°C. After 13.5 minutes, cold chain temperatures were met (7.6°C) and after 37.5 minutes, the interior compartment surpassed cold chain temperatures reaching 1.8°C. Figure 18 also presents the power requirements of the TEM during testing. Interestingly, the power requirements remained relatively constant throughout the duration of the test, despite a changing temperature difference across the TEM. The small-scale prototype also functioned well with an interior load. Once the interior temperature of the refrigerator compartment reached steady state (~ 6°C in this case), one 12oz can of chilled soda was placed inside to simulate the placement of vaccines in the compartment. As can be seen in Figure 19, both the interior temperature of the compartment and the temperature of the soda remained stable. Finally, after collecting sufficient data on the device with the interior load, the TEM assembly was shut off and the compartment allowed to warm. The behavior of the temperature of the compartment during warming can be seen in Figure 20.

After the successful testing of the small-scale prototype, the full-scale prototype was constructed and tested. Similar tests were run on the full-scale prototype for comparison purposes. In Figures 21 and 22, below, the test results from cooling the compartment from ambient temperatures and from adding a load are shown.

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Figure 21: Plot of temperature vs. time and power vs. time for cooling of full-scale prototype from ambient to cold chain temperatures.

Figure 22: Plot of interior temperature of compartment vs. time and temperature of cans vs. time for full-scale prototype. Demonstrates the process of cooling to steady-state and the effects of adding six 12 oz. soda cans.

As can be seen in Figure 21, the full-scale model achieved approximately a 10°C temperature difference between ambient and interior temperatures with a trend similar to that of the small-scale prototype. Also similar to the small-scale prototype, the power requirements of the TEM during the cooling process remained relatively constant.

Another test was done to see how the refrigerator would react to the addition of a load. The results can be seen above in Figure 22. The refrigerator was cooled to approximately steady-state and then six 12 oz. cans of chilled soda were added to the compartment to again simulate the addition of vaccines. Six were added to keep similitude between the test of the small-scale and the full-scale. One can is approximately 10% of the interior storage volume of the small-scale

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prototype and six cans are approximately 10% of the interior storage volume of the full-scale prototype. As can be seen in Figure B, the temperature of both the compartment and the cans fell immediately after the addition of the cans but slowly rose for the duration of the test (1.8 hours). It is hypothesized that both temperatures would continue to rise until reaching the steady state temperature of ~10.8°C achieved before the addition of the cans, similar to the results of the small-scale prototype test.

Since the addition of cooled liquids clearly created a decrease in temperature, a new test took a step beyond cooled liquids to determine the cooling performance of the refrigerator with ice. In this test, the refrigerator was started at an ambient room temperature of 21°C. 1.5 kg. of ice was immediately added to the refrigerator and the temperature of the system was recorded over time. The results of this test can be seen below in Figure 23.

Figure 23: Plot of refrigerator temperature with 1.5 kg. of ice inside.

With 1.5 kg. of ice in the refrigerator, the refrigerator went from 21°C to cold chain temperatures in 26 minutes. The rate of cooling dropped from there, but at 74 minutes, the refrigerator still reached a minimum temperature of 5.6°C. The vertical line at 74 minutes shows the moment at which the refrigerator was turned off. With the refrigerator off, the refrigerator warmed only 3.9°C over the next 60 minutes and seemed to have reached a steady temperature at 9.5°C. At 132 minutes, the fridge was briefly opened, which increased the temperature to 9.9°C. Interestingly, over the next 12 minutes, the fridge cooled back down to 9.7°C. The final vertical line at 144 minutes indicates when the fridge was once again turned on. The temperature decreased 2.7°C in 8 minutes to return to the cold-chain temperature of 7°C.

The steady warming temperature of 9.5°C that the refrigerator reached with 1.5 kg. of ice was a notable observation, so another test was done to further investigate this phenomenon. For two trials, the refrigerator was brought below 11°C and then cooling power was disconnected. For the

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first trial, the inner temperature was measured with the fridge completely empty. During the second trial, 1 kg. of ice was inserted into the fridge and the temperature was once again measured. The results of this test can be seen below in Figure 24.

Figure 24: Plot of the fridge temperature with the TEM off. The fridge contains 1 kg of ice for one trial and is empty for the other.

Both the empty refrigerator and the refrigerator containing ice underwent just over 1°C of warming over the first two minutes. From there, the empty fridge continued to warm at a considerably quicker rate than the refrigerator containing ice. Over the course of an hour, the temperature of the empty refrigerator increased from 11.3°C to 17.5°C. While the rate of warming slowed, the empty fridge was still experiencing noticeable increases in temperature when the trial ended. The warming likely would have continued at continuously slower rate until the fridge temperature was equal to ambient temperature. The trial in which the refrigerator containing 1 kg. of ice was warmed had notable differences to the empty refrigerator. While the two refrigerators behaved similarly over the first two minutes, the temperature of the ice-cooled refrigerator almost entirely stopped increasing after 15 minutes. Once the refrigerator reached 11.3°C, the temperature did not deviate more than .1°C for the next 35 minutes. After 50 minutes, the refrigerator door was briefly opened and then closed again. The temperature increased to 13.1°C, but then returned back to 11.3°C once more. The temperature most likely would have started increasing once all of the ice melted, but the consistency of the temperature with ice inside is a promising sign for possible refrigerator improvements.

Finally, one additional test was completed to examine the effects of leaks on the performance of the full-scale prototype. Due to a slight manufacturing error, the hinges were placed such that a small gap exists at the rear of the compartment between the seal and the lid. During testing, the hinges were removed, and the lid fully met the seal. However, for one test, the hinges were reattached to reestablish the gap. This allowed for the testing of the effect of leaks on the

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performance of the compartment. The performance of the refrigerator with and without leaks is shown below in Figure 25.

Figure 25: Plot of temperature vs. time for full-scale prototype with and without leaks.

In the test with leaks, the compartment initially cooled much more quickly, but stabilized at the same temperature as the test without leaks. This indicates the small gap between the seal and the lid did not have a significant impact on the performance of our compartment and further indicates that other possible small leaks resulting from manufacturing errors likely did not have a significant impact.

There are a few other valuable tests that our team did not perform, but still should be carried out down the road if more work is done on this project. One such test would be to measure the performance of the refrigerator in climates similar to that of our target markets. Temperatures in Nigeria can reach up to 40°C, so it is vital that our refrigerator maintains cold chain temperatures even in that extreme heat. Furthermore, tests should be done on how long it takes the cooling assembly to completely discharge the battery. This would be valuable information because it would give a better sense of whether the battery that is currently utilized in the prototype has sufficient storage capacity.

Testing Conclusions

While the full-scale prototype successfully demonstrated our design, it was unsuccessful in achieving proof of concept. However, the small-scale prototype was successful in proving the feasibility of thermoelectric refrigeration for vaccines. The success of the small-scale prototype and lack thereof of the full-scale prototype indicate that some redesign and optimization of the full-scale prototype is necessary moving forward.

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There are a few different approaches that could be taken to update the full-scale product for success. First and foremost, the refrigerator compartment could be altered to reduce the necessary cooling power. The volume could easily be reduced to a size somewhere between the current small-scale and full-scale prototypes while still having enough room to store vaccines. As mentioned before, the current full-scale prototype can hold an estimated 400 single-dose vials of vaccine. If the inner refrigerator compartment was reduced to a 9’’x 9’’x 9’’ cube, the refrigerator could still hold approximately 200 single-dose vials. The insulating material could also be altered to improve fridge performance. Though the polyisocyanurate seemed to be sufficient for achieving cold chain temperatures, the difficulties associated with installing the TEM assembly and allowing for sufficient airflow through the heat sinks caused a loss of robustness of the original design. Aerogels are a particularly attractive option as some have conductive heat transfer coefficients 36% (k = 0.0145 W/m·K) less than rigid polyisocyanurate foam insulation.

Another approach to improving the performance of the full-scale product is changing aspects of the TEM assembly. For one, more TEM assemblies could be added to the system. While the performance curves of the TEM indicate that one module is sufficient, in application this is not the case. TEMs are complex devices and further understanding of their performance would be beneficial. As well as adding additional TEMs, better heat sinks could be used. There are several options for improving the performance of the heat sinks in the TEM assembly: more powerful fans, larger finned arrays, or alternate heat sink designs (such as a liquid-cooled heat sink).

One final method of improving the current design could entail shifting away from a battery and toward a direct-drive system. Instead of using excess solar power to charge a battery, a direct-drive system uses excess solar power to freeze water. Lead-acid batteries are able to distribute approximately 80% of the energy they receive, so using any excess power directly in the cooling process would eliminate the energy losses caused by the battery.29 The battery is also by far the heaviest and most expensive component of the current design, so eliminating the battery would save significant cost and weight. The advantages of ice cooling were shown in the testing of the full-scale prototype. With 1.5 kg. of ice, the refrigerator was able to reach 5.6°C, which is over 5°C lower than the minimum temperature reached without ice. Also, when the cooling element is turned off, the melting process allows the refrigerator to maintain a steady-state temperature for extended periods of time. For example, a near-constant temperature of 9.5°C was reached for over an hour with 1.5 kg. of ice. The challenges of this improvement would be largely electrical in nature. A method for supplying the TEM with a consistent power would be necessary, along with a more robust controller to direct cooling power either directly to the refrigeration compartment or to the ice reservoir.

29 “Lead Acid – Lithium Ion Battery Test Centre.” Lithium Ion Battery Test Centre, ITP Renewables.

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These design updates could be implemented individually or in combination. In summary, despite the fact that the full-scale prototype was not completely successful, it was an important first step in exploring the possibility of thermoelectric refrigeration for vaccine storage and provided a basis for potential future designs.

Environmental Analysis

A common method to analyze the environmental impacts of a product is to conduct a life cycle assessment on its components. The four major components (by mass) in our refrigerator were PV panel, lead-acid battery, high-density polyethylene (HDPE), and aluminum heat sinks. We chose to evaluate these components on the following metrics:

● Life-cycle CO2 emissions● Toxic chemical emissions● Indoor air quality● Overall waste generation

Life-Cycle CO2 EmissionsThe biggest environmental advantage of our refrigerator is its potential to run completely off solar energy. If run using Nigeria’s electricity grid, the refrigerator would consume about 400 kWh of electricity and emit 168 kg of CO2 a year (Nigeria’s electric grid emits 420gCO2/kWh). To put this into perspective, this means that approximately 35 of our refrigerators will remove as much CO2 emissions as taking a car off the road in the US. However, it is important to take into account any CO2 emitted during the manufacturing process. The annual CO2 emissions calculations are summarized below.

● Lead-acid battery:Annual CO2 emissions = 0.5kWh energy input/1 kWh battery*420gCO2/kWh3 year lifetime

● PV Panel:Annual CO2 emissions = 24gCO2/kWh generated* annual kWh generated

● HDPE:Annual CO2 emissions = 1.45kgCO2/kg HDPE10 year lifetime

● Aluminum:Annual CO2 emissions = 0.467kgCO2/kg aluminum10 year lifetime

● Raw materials processing for conventional refrigerator:Annual CO2 emissions = 1385 kWh energy input/cubic meter refrigerator size*0.025

cubic meter size*420gCO2/kWh10 year lifetime● Manufacturing:

Annual CO2 emissions = 520 kWh energy input/cubic meter refrigerator size*0.025 cubic meter size*420gCO2/kWh10 year lifetime

TEM Refrigerator Conventional Refrigerator

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Component/Process

Mass/size used

Annual CO2 emitted (kg)

Component/Process

Energy input (kWh/cu.m)

Annual CO2 emitted (kg)

Battery 1.5 kWh 0.10530 Raw materials processing

138531 1.45

PV Panel 1.0 sq.m 9.373232 Manufacturing 52033 0.55

HDPE 7.5 kg 1.087534 Electricity consumption

NA 168

Aluminum 0.3 kg 0.001435

Electricity consumption

400 kWh 0

Total 10.5671 Total 170

Table 3: Life-cycle CO2 emissions comparison

Thus, the total annual CO2 emissions reduction from our refrigerator is 159.4 kgCO2 equivalent.

Toxic Chemicals EmissionsA study on the life-cycle emissions of lead-acid batteries in China found that for each kWh of battery capacity, 32 grams of lead leaked into the environment36. It also found that 85% of the emissions were during the recycling phase, while 10% were during battery repurposing. Further, these emissions were observed after China conducted a major overhaul of its environmental regulations and enforcement in the lead-acid battery recycling industry. Thus, we expect Nigeria to have much higher lead emissions since its environmental regulations are much more lax than China’s. Assuming a battery lifetime of 3 years, we expect a minimum of 10 grams of lead emissions per year.

Indoor Air QualityDue to the lack of a compression refrigeration cycle, our refrigerant features no harmful refrigerants such as R-124A and hydrochlorofluorocarbons (HCFC). This nullifies the potential

30 Shwartz M. Stanford scientists calculate the carbon footprint of grid-scale battery technologies. Source: https://news.stanford.edu/news/2013/march/store-electric-grid-030513.html 31 Boustani et al. Appliance Remanufacturing and energy savings. MIT 2010. Source: http://web.mit.edu/ebm/www/Publications/MITEI-1-a-2010.pdf 32 Carbon footprint of solar panels under microscope. Source: http://www.ecolise.eu/carbon-footprint-of-solar-panels-under-microscope/ 33 Boustani et al. Appliance Remanufacturing and energy savings. MIT 2010. Source: http://web.mit.edu/ebm/www/Publications/MITEI-1-a-2010.pdf 34 NREL U.S. Life Cycle Inventory Database. Source: https://www.nrel.gov/lci/ 35 NREL U.S. Life Cycle Inventory Database. Source: https://www.nrel.gov/lci/ 36 Liu, W. et a. Life cycle assessment of lead-acid batteries used in electric bicycles in China. Source: https://www.sciencedirect.com/science/article/pii/S0959652615009063

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of any refrigerant leaks that could cause indoor air quality issues. Further, most refrigerants used commercially belong to the HCFC or HFC families, which are known to have extremely high global warming potentials.

Overall Waste GenerationWe expect our refrigerator to have a much lower waste footprint since it uses fewer components than a conventional refrigerator. Further, most of the components used in our refrigerator are recyclable with well-developed recycling centers all around the world. We expect significant recycling available for HDPE, aluminum, and lead-acid battery. In theory, the solar panel should be recyclable. However, since it is a relatively new technology commercially, recycling centers are not well-developed in most parts of the world.

Social Impact

The primary benefit of our product is social in nature. It is an undeniable good to expand the access of modern medicine techniques to all people, and we hope that this project can be a small but significant step in minimizing child mortality and maximizing general health for a region that faces many challenges. Improving access to vaccinations directly addresses the United Nations’ Sustainable Development Goal 3 on good health and well-being. Despite considerable improvements in access to medicine over the past 30 years, we are far from an acceptable global level. Every day, 16,000 children die globally from preventable diseases, totaling nearly 6 million per year.37 This shows that in order to reach the UN’s stated goal of bringing that number to 0 by 2030, much work needs to be done. For the past 12 years Nigeria’s infant mortality rate has decreased steadily by between 3% and 4% per year, but continuing at this rate will still fail to reach the UN’s 2030 goal of an under-5 mortality rate less than 12 deaths per 1,000 individuals.38

This trend is shown below in Figure 26.

37 “The State of the World’s Children 2016.” UNICEF. 2016.38 “Mortality rate, under-5.” The World Bank. 2016.

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Figure 26: If Nigeria’s infant mortality rate continues to decrease at the present rate it will still fall short of the UN goal for 2030.

Improving cold chain capabilities in the region delivers an important societal and ethical benefit, but these benefits can be difficult to fully quantify in an economic sense. However, there are several measurable factors which can be used to help quantify the benefit provided by increased vaccination coverage. These factors include lost economic productivity due to illness or death, lifetime treatment costs, and strains put on the country’s public health system. Using these variables, one can estimate the minimum economic benefit provided by an improved vaccination infrastructure. The important, but more difficult to quantify ethical incentive of improving individuals’ quality of life sits atop this minimum economic value when considering the true value of cold chain improvement projects.

For this estimation we focused on the state of Kano in Northern Nigeria. With a population of 9.3 million, Kano is the largest state in Nigeria, while also lying within a geographical area with relatively high solar potential. Kano also lies significantly below the national averages for electricity access and vaccination coverage.39

Since prevalence, mortality rates, treatment costs and other factors vary from disease to disease, we decided to focus on Hepatitis B, a disease that affects 24.2 million Nigerians despite being

39 “State of Electricity Access Report” World Bank Energy Sector Management Assistance Program. June 2017.

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preventable with vaccination at birth40. Knowing the unvaccinated population in Kano, the prevalence, treatment cost, and mortality rate for Hep B in the region, we utilized the WHO formula to determine the economic value of each year-life saved in Kano via vaccination. Factoring in the number of additional refrigeration units needed to reach full vaccination levels in Kano provides an estimated economic-social value added of $3,771 per unit per year. While this number is an estimate and does not encompass the entirety of social value added by increased vaccination coverage, it is able to show that these coolers have a very short payoff period on investment in the large scale of societal goods.

Market Analysis

Thermoelectric solar coolers currently exist on the market in two categories, separated by designed use. The first category is recreational coolers, targeted towards an audience focused on keeping food and drinks cold during camping, tailgates, or other outdoor events. The Wagan Solar eCube is a typical example of products within this category, which retail from $950-$1250. The second grouping of thermoelectric solar coolers on the market are those designed specifically for medicinal purposes, and this is the category which relates best to our product. One company currently operating within this market space is SunDanzer, who offers two medicinal refrigerator products, priced at $2,495 and $3,600. Haier, Dometic, Vestfrost and Sure Chill all also offer solar direct drive coolers that are prequalified by the WHO to carry vaccines, although only Sundanzer and Sure Chill are rated for hot climates.

The total price for our final full-scale prototype is $601.10. A price breakdown for all the materials used can be found in Table A of the Appendix. This price does not include the price of solar panels, as our product is intended to integrate the design with existing PV arrays. However, for the purposes of cost estimation, we will assume that a solar array must be sold alongside the refrigerator. Solar panels that meet the requirements of our system range can be found for $150, thus raising the price of the system to approximately $750. Given that the total cost to produce our prototype was only $750, our aim is to provide a medical quality cooler at a price point lower than these current competitors. With an estimated $106,500 budget for fixed costs, the price for our product would be approximately $2,149 for 100 units sold. At greater than 100 units sold, this price could decrease further. A more detailed breakdown of cost can be found in Tables B and C of the Appendix. This low price not only gives our product a unique space in the market, but also allows greater economic access for the impoverished regions that we are targeting.

In order to gain legitimacy in the market, the refrigerator must be able to clear the guidelines set by the World Health Organization for vaccine refrigerators. The WHO sets guidelines based on the type of refrigerator, such as solar absorption or combined power compression cycle.

40 Ikobah, Joanah, et al. "The prevalence of hepatitis B virus infection in Nigerian children prior to vaccine introduction into the National Programme on Immunization schedule." Pan African Medical Journal 23.1 2016.

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Unfortunately, given the lack of existing thermoelectric coolers, the WHO does not have any standards currently in place for solar thermoelectric coolers. That said, one standard that is standard across most solar powered refrigerators is a requirement that they maintain cold chain temperatures for up to five consecutive days with little to no solar irradiance. Thus, before coming to market, it would be necessary to run tests to ensure that cold chain could be maintained without power for five straight days.

Once the refrigerator has cleared the necessary regulatory hurdles, it is important to link up with as many potential customers as possible. Thankfully, we got a head-start by reaching out to potential partners ahead of time. We have since partnered with Africa Power Storage, a UK-based company that currently provides solar inverters and power storage to homes in Nigeria, Uganda, Zimbabwe and elsewhere within the region. The founders of APS have a background working in vaccine cold chain storage in Nigeria and can help us connect the product to local clinics in areas of need. APS has an ongoing project focusing on cold chain storage and has expressed enthusiasm in providing support in this industry.

For the future of this product we have discussed with Zui Dighe, member of another Duke design team, the potential for integrating cloud-based temperature tracking into our design. Utilizing a Particle Mesh chip, our refrigerators would be able to report on GPS and temperature data to monitor cold chain and identify any areas where proper temperatures are not maintained. Implementing this technology would provide important data which would be valuable both for this team and for local agencies.

Future Plans

Solar thermoelectric vaccine refrigeration has potential to make an entrance into the market as a legitimate competitor. Our team has proven that thermoelectric cooling can be used to bring a refrigerator down to cold chain temperatures, a thermoelectric module can be run on solar power, a refrigerator can be built from common materials, and a simple Arduino controller can be used to regulate refrigerator temperatures. If these four pieces are refined and brought together while maintaining the unit price of $2,149, this technology could have a legitimate future. That said, there are some technical and business steps that need to be taken before that future can be realized.

As it stands, the current full-scale prototype never reached vaccine cold chain temperatures as designed. However, as was mentioned in the testing conclusions section, there are multiple avenues to improving the refrigerator’s technical performance. Some of the simpler avenues include scaling down the refrigerator’s volume and using a more effective heat sink configuration. Some of the more technically challenging avenues include adding more thermoelectric modules to increase cooling power, researching and installing more effective

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insulation, and removing the battery to create a direct-drive system. All of these options should be considered moving forward, although it is possible that not all of them need to be implemented to achieve cold chain temperatures. If the end goal is to become WHO certified, then these improvements must be made until the refrigerator can maintain cold chain temperatures for five days in rainy conditions.41

As important as creating a working chain refrigerator is, it is not impactful without a robust business plan. It will be necessary to come alongside more non-profit organizations, like Africa Power Storage, to create a demand for the refrigerators. There may also be demand from national and state governments, so outreach should also be done with public officials in target markets. The success of the business model could be further improved by shifting material sourcing and manufacturing to the markets that the refrigerator will be implemented in. From a business perspective, this will most likely result in substantially lower shipping and labor costs. This would also create a greater economic multiplier effect, as people in these markets could enjoy both increased vaccination rates and an increase in monetary revenue.

Conclusion

Nigeria has the world's lowest vaccination rate, currently at a staggering 42%. This is primarily due to a dearth of cold chain technology, lack of education, and an inefficient healthcare system. The goal of this project was to address the cold chain technology problem. Current off-grid vaccination coolers are unreliable and complicated, which in some extreme cases leads to their use as bookshelves instead of refrigerators. The primary problem is not availability of vaccines, but with the successful distribution and maintenance of these vaccines. Too many vaccines are spoiled on a daily basis because they are stored at the wrong temperature. We propose an alternative off-grid solution to this problem, namely solar thermoelectric refrigeration. There exists a huge opportunity in the field of thermoelectric refrigeration, to provide a reliable and easy-to-use platform to maintain cold chain temperatures at 2o C - 8o C. Other technologies exist in the market, e.g. solar absorption, vortex vapor tubes, and solar thermo-mechanical systems, but the thermoelectric modules provide certain unique advantages. They are small, have no moving parts, require no refrigerants, and can easily be powered by D.C power sources which allows for coupling with solar panels.

A small-scale prototype, 15% the volume of the full-scale model, was constructed to prove the validity of the thermoelectric approach. Rigorous testing was conducted to evaluate not only the plausibility of cold chain temperatures, but also the performance with internal loads (i.e. a soda can). These tests proved to be successful, and a temperature of 1.8o C was achieved. This demonstrated the feasibility of thermoelectric refrigeration as a potential option for vaccine storage. The full-scale prototype on the other hand was only able to manage a temperature

41 World Health Organization. “Solar Power System for Compression-Cycle Vaccine Refrigerator or Combined Refrigerator and Water-Pack Freezer.”, WHO, 2010.

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differential of roughly 10o C, and a steady state temperature of 10.8o C (the full-scale prototype did manage to maintain the internal temperature with six soda cans). It should be noted that these tests were conducted in an ambient temperature of roughly 21o C, and do not replicate the conditions in Nigeria. However, with certain design modifications cold chain temperatures should be attainable, including: a smaller compartment volume, more effective insulation (e.g. Aerogels), stronger module heat dissipation with larger heat sinks, stronger fans, and potentially more thermoelectric modules.

Although addressing cold chain technology issues will help alleviate vaccine storage, tackling vaccination in Nigeria is more than just an engineering problem. It is important to couple technical advancement with education, and reinforcement through the healthcare system. It is vital we partner with organizations like Africa Power Storage, to ensure the effective distribution, and usage of our solar refrigerators. Higher vaccination rates will lead to a more productive workforce and lessen the burden on Nigeria’s already suffering healthcare system. As explained earlier, $3,771 per unit per year is recovered by improving coverage of Hep B in the Kano region of Nigeria. Environmental benefits of the thermoelectric module are the absence of any harmful refrigerant and mitigation of carbon emissions. The refrigerator coupled with a panel 1m2 in size has the potential of abating 159 kg of CO2 per year. The only downside of our system is the lead acid battery, that will emit roughly 10g of toxic lead every year.

Overall, this project has provided the framework for future teams to develop our design, and potentially create a reliable, and effective solution for vaccine storage in Nigeria. There are undeniable social goods associated with this project, and with the advantages of thermoelectric modules, successful distribution and maintenance of vaccines at cold chain temperatures could be achieved, allowing us to move one step forward towards the UN’s goal for vaccination rates.

Appendix

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Table A: Bill of Materials for Final Prototype.

Table B: A cost breakdown of the different fixed costs and variable costs for refrigerator manufacturing.

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Table C: A time breakdown for each step of the manufacturing process.

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