Energy Efficient Technologies in Solar Water Pumping

Dr. A.G.Thosar K.A.Joshi Head, Department of Electrical Engineering Senior Undergraduate aprevankar@gmail.com kamlesh_joshi@live.com

Government College of Engineering, Aurangabad, Maharashtra

Abstract

As a promising renewable alternative to

conventional water pumping system for large, medium and small applications, solar water pumping systems has a potential to contribute significantly. The traditional solar water pumping systems like bushed DC motor (small applications) and Induction Motors (medium and large applications) are getting alternative of the energy efficient motors like Permanent Magnet Brush-less DC Motors (PMBLDC). This replacement with energy efficient one is proving to be conducive for the growing need of energy conservation. This work deals with the basics of solar power water pumping and a brief comparison of conventional and energy efficient technologies. Index Terms- MPPT, PMBLDC, Solar Water Pumping. 1. Introduction

The sun is the natural source of energy for an

independent water supply. Solar pumps operate anywhere that the sun shines, and the longer it shines, the more water they pump. Photovoltaic modules, the power source for solar pumping, have no moving parts, require no maintenance and last for decades. A properly designed solar pumping system will be efficient, simple and reliable. The output of the solar power system varies throughout the day and with changes in weather conditions. A solar water pumping system is essentially an electrically driven pumping system. Electricity, in this instance, is produced by the sunlight energizing photovoltaic (solar) modules. The nature of variable electricity in the form of direct current (DC) is quite different from conventional, steady alternating (AC) current from the utility grid or a generator. To use solar energy economically, the pumping system must utilize the long solar day, drawing a minimum of power. Because solar energy varies from one location to another, and over the course of a day, system design is important. Adequate water storage ensures that water is available whenever needed, and balances daily variations in water supply and demand. Thus a small pump only running when the sun shines, plus water storage, can provide the average requirement for water supply.

The consumption of fossil fuels also has an environmental impact, in particular the release of carbon dioxide (CO2) into the atmosphere. CO2 emissions can be greatly reduced through the application of renewable energy technologies, which are already cost competitive with fossil fuels in many situations. Good examples include large-scale grid-connected wind turbines, solar water heating, and off-grid stand-alone PV systems. The use of renewable energy for water pumping systems is, therefore, a very attractive proposition.

Windmills are a long-established method of using renewable energy; however they are quickly phasing out from the scene despite success of large-scale grid-tied wind turbines. The key to PV’s success is the low labor and maintenance costs relative to the other options. The long-term economics make PV pumps superior to most other remote watering options, except where gravity feed is available. One study completed by the Bureau of Land Management (USA) compared solar water pumping systems to generator systems. For one 3.8 gpm system with a 275 foot design head, the PV system cost only 64% as much over 20 years as the generator system did over only 10 years. This remote solar site also used only 14% as many labor hours. Inexpensive diesel or gas generators have low initial costs but require consistent maintenance and have a design life of approximately 1500 hours. Small to medium sized solar pumping systems often cost less initially than a durable slow speed engine driven generator [1].

Figure 1: Typical solar water pumping for irrigation system

A brief comparison of different stand-alone type water pumping system is delineated below:

Table 1 : Comparison of various water pumping system

System Type

Advantages Disadvantages

PV Powered System

1.Low maintenance 2.Unattended operation 3.Reliable long life 4.No fuel and no fumes 5.Easy to install 6.Low recurrent costs 7.System is modular and closely matched to need

1.Relatively high initial cost 2.Low output in cloudy weather

Diesel (or Gas) Powered System

1.Moderate capital costs 2.Easy to install 3.Can be portable 4.Extensive experience available

1.Needs maintenance and replacement 2.Site visits necessary 3.Noise, fume, dirt problems 4.Fuel often expensive and supply intermittent

Windmill 1.No fuel and no fumes 2.Potentially long-lasting 3.Works well in windy sites

1.High maintenance 2.Seasonal disadvantages 3.Difficult find parts thus costly repair 4.Installation is labor intensive and needs special tools

2. Basics of solar power water pumping

The photovoltaic effect produces a flow of electrons. Electrons are excited by particles of light and find the attached electrical circuit the easiest path to travel from one side of the solar cell to the other. Envision a piece of metal such as the side panel of a car. As it sits in the sun, the metal warms. This warming is caused by the exciting of electrons, bouncing back and forth, creating friction, and therefore, heat. The solar cell merely takes a percentage of these electrons and directs them to flow in a path. This flow of electrons is, by definition, electricity. Photovoltaics or solar electric cells convert sunlight directly into electricity. This electricity is collected by the wiring in the module, then supplied to the DC pump controller and motor, which, in turn, pumps water whenever the

sun shines. At night, or in heavy cloud conditions, electrical production and pumping ceases.

Figure 2: Basics of solar water pumping

Because solar energy varies from one location to another, and over the course of a day, system design is important. Adequate water storage ensures that water is available whenever needed, and balances daily variations in water supply and demand. Thus a small pump only running when the sun shines, plus water storage, can provide the average requirement for water supply. For the best electrical and mechanical performance, all components of the solar pumping system must be carefully matched. Correct sizing of the pump, motor and controlling devices, will allow the system to operate at the highest efficiency to ensure economical water pumping[2].

3. PV pumping system elements

Solar water pumping systems consist of three basic

components:

Figure 3: PV pumping system components

• Power source (photovoltaic solar modules)

Solar or photovoltaic (PV) cells are made of

semiconducting materials that can convert sunlight directly into electricity. When sunlight strikes the cells, it dislodges and liberates electrons within the material which then move to produce a direct electrical current (DC). This is done without any moving parts. PV cells are combined to make modules that are encased in glass

or clear plastic. Modules can be aggregated together to make an array that is sized to the specific application. Most commercial PV cells are made from silicon, and come in three general types: monocrystalline, multicrystalline, and amorphous.

Figure 4: Mechanical solar tracking structure

Single crystal or monocrystalline cells are made using silicon wafers cut from a single, cylindrical crystal of silicon. This type of PV cell is the most efficient, with approximately 15% efficiency (defined as the fraction of the sun’s energy that is converted to electrical power), but is also one of the most expensive to produce. They are identifiable as having individual cells shaped like circles or rectangles.

Multicrystalline or polycrystalline silicon cells are made by casting molten silicon into ingots, which crystallize into a solid block of intergrown crystals. The size of the crystals is determined mostly by the rate at which the ingot is cooled, with larger grains made by slower cooling. Cells are then cut from the ingot. Multicrystalline cells are less expensive to produce than monocrystalline ones, due to the simpler manufacturing process and lower purity requirements for the starting material. However, they are slightly less efficient, with average efficiencies of around 12%.

Figure 5: Solar assembly

Amorphous silicon PV cells are made from a thin layer of noncrystalline silicon placed on a rigid or flexible substrate. They are relatively easy to manufacture and are less expensive than monocrystalline and polycrystalline PV, but are less efficient with efficiencies of around 6%. Their low cost makes them the best choice where high efficiency and space are not important.

• Motor/pump (or motor/compressor) assembly Off–the–shelf, mass produced motors and pumps

can be used for solar water pumping. Special pumps and motor have also been developed for solar systems.

Figure 6: Typical variation of pump flow rate with time of day (average day based on four years solar data)

The power from a solar system and the volume of

water pumped varies with the amount of solar radiation. This means that the system must be designed to work efficiently over a range of voltage and current levels. • Electrical Motors

Solar water pumps that are currently available use

the following types of motors: AC synchronous motors AC asynchronous induction motors DC series motors DC permanent magnet motors DC permanent magnet brushless motors

• Pumps

1. Centrifugal pumps are designed for a fixed head

and their water output increases with rotational speed. Centrifugal pumps are not self–priming and are seldom used for suction lifts greater than 4–5m. Solar powered floating pumps are often of this type. At low heads, centrifugal pumps are usually more efficient than positive displacement pumps.

2. Positive displacement pumps have a water output which is directly proportional to speed. Helical shaped rotor pumps have very few moving parts, operate at low speeds and are able to handle dirty water. The flow is non pulsating and ideal for long distance pumping. The efficiency of the pump increases with head and consequently at higher heads, positive displacement pumps can be more efficient than centrifugal pumps.

3. Air lift pumps A specialized application of this principle is used to keep the water in reservoirs in good condition. Compressed air bubbling from the bottom of the reservoir eliminates the thermal

stratification that can contribute to the contamination of the water supply.

Table 2: Brief comparison of centrifugal and positive

displacement pump Centrifugal Pump Positive Displacement

Pump High-speed impellers Volumetric movement

Large flow rates Lower flow rates Loss of flow with higher

heads Flow rate less affected by

head Low irradiance reduces ability to achieve head

Low irradiance has little effect on head

Potential grit abrasion Unaffected by grit • Power controllers for matching the

changing electrical output of the array to suit the motor/pump

There are several types of power controllers available:

• Impedance matching devices (such as power maximizes) • DC to AC inverters (used with AC pumps only) • Switches and protective controllers. Impedance matching devices such as power

maximizes, sometimes called maximum power point trackers (MPPT’s), control the output of the array so it will operate close to its maximum efficiency (power) over a range of sunlight levels. • Maximum Power Point Trackers

The maximum power point (MPP) corresponds to

the biggest rectangle that can fit beneath the I –V curve. Clearly, significant efficiency gains could be realized if the operating points for pumping loads could somehow be kept near the knee of the PV I –V curves throughout the ever-changing daily conditions. Devices to do just that, called maximum power trackers (MPPTs), are available and are a standard part of many PV systems. There are some very clever, quite simple circuits that are at the heart of not only MPPTs but also linear current boosters (LCBs) as well as a number of other important power devices. The key is to be able to convert dc voltages from one level to another.

Figure 7: Basic MPPT system

A boost converter is a commonly used circuit to step up the voltage from a dc source, while a buck converter is often used to step down voltage. The circuit underneath is a combination of these two circuits and is called a buck-boost converter. A buck-boost converter is capable of rising or lowering a dc voltage from its source to whatever dc voltage is needed by the load. The source in this case is shown as being a PV module and the load is shown as a dc motor, but the basic concept is used for a wide variety of electric power applications. The transistor switch flips on and off at a rapid rate (on the order of 20 kHz) under control of some sensing and logic circuitry that isn’t shown. Also not shown is a capacitor across the PVs that helps smooth the voltage supplied by the PVs [3].

Figure 8: A Buck-Boost converter for MPPT system

4. Water storage – efficient and effective

Storing water in a good sized cistern or stock tank has many advantages. It is less expensive and more efficient than storing energy in batteries, giving your system a flywheel effect over cloudy days and letting the pump work at a slower continuous pace over the day. As a rule of thumb, the tank should be able to store 3 or 5 days worth of water. Generally speaking, animals, plants and humans use less water on cloudy days. Conversely, the sunniest days are when we consume the most water and when the solar modules are providing the pump with the most power.

Figure 9: Water storage system

While batteries may seem like a good idea, they have a number of disadvantages in pumping systems. They reduce the efficiency of the overall system. The solar modules operating voltage is dictated by the battery bank and is reduced substantially from levels which are achieved by operating the pump directly. Batteries also require additional maintenance and under

and over-charge protection circuitry which adds to the cost and complexity of a given system. For these reasons, only about five percent of solar pumping systems employ a battery bank [4]. 5. Economics of solar water pumping

The economy and reliability of solar electric power make it an excellent choice for remote water pumping. A solar pump minimizes future costs and uncertainties. The fuel is free. Moving parts are reduced to as few as one. A few spare parts can assure you many years of reliable water supply at near-zero operating costs[4].

Table 3: Economic comparison of two solar water pumps Motor pump

Output @ 5kWhm/m3/ day insolation

Head (m)

Solar Array (Wp)

System Price US$

Submerge borehole

40 20 1200 7000-8000

positive displace-ment

6 100 1200 7500-9000

6. Various solar power pumping

technologies

Solar water pumping technologies exists from a long period of time. Most conventional solar water pumping includes a brushed DC motor for small and medium applications while for large applications of lift head of more than 30 meters, a conventional 3-phase induction motor are widely used. With the advent of new technologies like Permanent Magnet Brushless DC Motor (PMBLDC), Shaft- less PMBLDC motors are proving to very energy efficient and cost effective as well. Hence it becomes a mandatory effort to study the electrical characteristics of these technologies. • Conventional 3-φ Induction motor

An AC submersible pump with a specially designed inverter is used in the SPV Deep Well Water Pumping System. The inverter converts the DC Voltage supplied by the SPV array into a three phase AC voltage with variable frequency. The frequency and thus the speed of submersible motor vary with intensity of the solar irradiation. The inverter unit incorporates the main switch and has LEDs for indicating the operating conditions. The submersible pump is installed below the water level in a bore well or opens well. Delivery is obtained through a riser pipe of sufficient length. The pump is energized through a drop cable, which has a tough sheath. The pump has corrosion proof parts and water lubricated bearing and requires minimal maintenance. These systems can pump water from a depth up to 125 meters, which can be stored, if needed, in tanks of suitable capacity[5].

The electrical parameters of Conventional 3-φ Induction motor are stated below:

Table 4: Electrical parameters for Conventional 3-φ

Induction motor Electrical Parameters Value

Rated Power 1 hp Voltage 230Volts

Full Load Current 3.9A Power Factor 0.71

Per phase Resistance 2.71Ω Per phase Inductance 12mH

Efficiency 71%

• Permanent Magnet Brush-less DC motor (PMBLDC)

There is a growing trend among the pump

manufacturers to use them with brushless DC motors (BLDC) for higher efficiency and low maintenance. However, the cost and complexity of these systems will be significantly higher. Later pumps are driven by various types of rotors. AC induction motors are cheaper and widely available worldwide. The system, however, needs an inverter to convert DC output power from PV to AC power, which is usually expensive, and it is also less efficient than BLDC motor-pump systems [6]. Table 5: Electrical parameters for Permanent Magnet Brush-

less DC motor (PMBLDC) Electrical Parameters Value

Rated Power 1 hp Voltage 230Volts

Full Load Current 3A Power Factor 0

Per phase Resistance 1Ω Per phase Inductance 5mH

Efficiency 90% 7. A brief comparison of two technologies

with there electrical and hydraulic specifications

The solar water pumping differ with respect to the

type of motor (AC motor / BDC motor) and the type of pump (centrifugal pump / displacement pump). The pumping system costs make up for a small part of the total PVP costs but its influence on the overall efficiency is considerable. For this reason intensive investigations of these system concepts are still an important issue.

Following table delineates the comparison of both the electrical and hydraulic parameters of two types of technologies:

Table 6: Characteristics of two technologies

Parameters Conventional 3-φ Induction

motor

Permanent Magnet Brush-less DC motor

PV Generator Generator area: Pnominal

15.36m2

1.91kWp

3.41m2

0.43kWp Power Conditioning Pdc,max Pac,max

PWM 3- φ inverter

4.8kW 3.5kVA

MPPT

0.8kW

Motor Unit Pnominal Pump Head Flow Rate

Submersible 1100W

Centrifugal 18m (Nominal)

7m3/h (Nominal)

Submersible 600W

Eccentric screw 25-90m (range)

2.5 m3/h (Nominal)

Hydraulic System Geodetic Head

15m

30m

To compare the two systems the performance ratio

(PR) serves as a measure of system performance. It is defined as the net energy to overcome the geodetic head divided by the energy expected from the PV generator, when performing at STC efficiency. (Standard Test Conditions: 1000 W/m² , 25 °C. The performance ratio of Conventional 3-φ Induction motor is 21 % that of PMBLDC is 48 %.

A brief comparison of component efficiency of both the conventional 3-φ Induction motor and Permanent Magnet Brush-less DC motor is depicted below: Table 7: Comparison of component efficiency Component Conventional 3-

φ Induction motor

Permanent Magnet Brush-less DC motor

PV Generator

Voltage Tracked 79%

MPPT 80%

Threshold 93% 99% Inverter 92% - Motor 67% 90% Pump Multi-stage

Centrifugal 49%

Eccentric Screw 69%

Piping System 97% 97% The analysis of losses is as follows:

The losses when operating at conditions different from STC are 13 % for temperature and irradiation effects. 7% are due to spectral, reflection and mismatch losses. In centrifugal pumps a minimum rotational speed of the impeller is required to generate head. This

implies that 7 % of the DC power cannot be transformed into hydraulic power (threshold). An eccentric screw pump needs a high starting torque which may be overcome by the motor with the help of modern electronic power conditioning units at a very low electric input power. Therefore the threshold in PMBLDC results in losses of only 1 %. The inverter losses in Conventional 3-φ Induction motor are in the range of 8 % of the inverter input. The power conditioning of PMBLDC is included in the motor.

The main motor losses (core and copper) act in the asynchronous motor in both rotor and stator while they occur in BLDC motors in the stator only. Conventional 3-φ Induction motor losses amount to 33 %, in PMBLDC they are only 10 %.

Hydrodynamic losses are the main losses in the centrifugal pump and they increase with the second power of velocity. The main losses of an eccentric screw pump are due to friction between rotor and stator. These losses are almost linear to the rotational speed of the rotor. Therefore the pump of PMBLDC has an annual efficiency of 69 % that of Conventional 3-φ Induction motor reaches 49 % when pumping [5, 6].

8. Conclusion

Solar water pumps can provide simple and low labor watering options for farms that require water in remote areas. The paper has presented basics of solar water pumping which includes a study of working of solar water pumping to the MPPT system and water storage technology. Optimal operation of the PV generator was insured by MPPT. To reduce the cost of a system, water conservation must be practiced. PV modules are expensive, and reducing water use in any manner will save on the installed cost.

A brief comparison of conventional 3-φ Induction motor and PMBLDC is also taken into consideration. The comparison reveals that PVP systems consisting of BDC motor and eccentric screw pump are superior to conventional asynchronous motor and multistage centrifugal pump systems.

9. Acknowledgment

Authors are gratefully acknowledged to the Department of Electrical Engineering (Government College of Engineering, Aurangabad) for the invaluable support. Authors acknowledge the kind permission advices and contributions of Dr. W.Z. Gandhare (Principal, Government College of Engineering, Aurangabad).

Authors acknowledge the immense support given by Pranav Kulkarni and Bhushan Pathak (Senior Undergraduate, Department of Electrical Engineering, Government College of Engineering, Aurangabad).

10. References [1]H.J. Helikson, D.Z. Haman and C.D. Baird, “Pumping Water for Irrigation Using Solar Energy”, Florida Cooperative Extensive Services, University of Florida, November 1991, pp. 1-4.

[2] C.W. Sinton, R. Butler and R.Winnett,” Guide to Solar Powered Water Pumping Systems in New-York State”, New York State Energy Research and Development Authority, Albany, New York,2003, pp. 1-16. [3]Masters, G.M., Renewable and Efficient Renewable Power Systems, Wiley Interscience, USA, 2004.

[4] Kyocera Solar Inc., “Solar Water Pumping Applications Guide”, Scottsdale, USA, September 2001, pp. 4-35. [5] A. Moussi, A. Torki, “An Improved Efficiency Permanent Magnet Brushless DC Motor PV Pumping System”, LARHYSS Journal, N0.01, May 2002, pp. 140-156. [6]H. Bloos, M. Genthner, D. Heinemann, A. Janssen and R. Moraes, “Photovoltaic Pumping System- A Comparison of Two Concepts”, Euro 96, Carl von Ossietzky Universität Oldenburg, 1996, pp. 583-588.