DEPARTMENT OF ARCHITECTUREFEDERAL UNIVERSITY OF TECHNOLOGY,

AKURE.COURSE:

APPLIED CLIMATOLOGY (ARC 810)TOPIC:

WRITE UP ON: ACTIVE AND PASSIVE SOLAR HEATING SYSTEMS IN NIGERIA.

WRITTEN BY:Olajubu o.oMATRIC NO:ARC/03/1939

IN PARTIAL FULFILMENT FOR THE REQUIREMENT OF THE AWARD

OF M.TECH IN ARCHITECTURE.

LECTURER:

PROF (ARC.) O.O GUNSOTE

AUG,2011

ABSTRACT

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Nigeria is situated in the equatorial region and receives abundant solar radiation.The first area that is most appropriate for the large-scale application of solar energy is domestic water heating. sector This paper examines the active and passive solar heating systems in Nigeria, the current state of development, and the dissemination of the systems and their future prospects in Nigeria. The place of government policies and programs in the general development of solar energy systems are also highlighted. Passive solar energy means that mechanical means are not employed to utilize solar energy. The basic benefit of active systems is that controls (usually electrical) can be used to maximise their effectiveness. A "purely passive" solar-heated house would have no mechanical furnace unit, relying instead on energy captured from sunshine, only supplemented by "incidental" heat energy given off by lights, computers, and other task-specific appliances (such as those for cooking, entertainment, etc.), showering, people and pets. There are three approaches to passive systems – direct gain, indirect gain, and isolated gain. The goal of all passive solar heating systems is to capture the sun’s heat within the building’s elements and release that heat during periods when the sun is not shining. At the same time that the building’s elements (or materials) is absorbing heat for later use, solar heat is available for keeping the space comfortable (not overheated). Active solar energy systems require a solar collector (a device used to store energy) and controls linked to pumps or fans that draw heat from storage as necessary.

1.0 INTRODUCTION

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Solar energy is a radiant heat source that causes natural processes upon which all life depends. Some of the natural processes can be managed through building design in a manner that helps heat and cool the building. The basic natural processes that are used in passive solar energy are the thermal energy flows associated with radiation, conduction, and natural convection. When sunlight strikes a building, the building materials can reflect, transmit, or absorb the solar radiation. Additionally, the heat produced by the sun causes air movement that can be predictable in designed spaces. These basic responses to solar heat lead to design elements, material choices and placements that can provide heating and cooling effects in a home. The conventional sources of energy are finite in nature and pose severe threats to man’s environment. The industrial and economic development which have been made possible through the harnessing of conventional energy technologies have brought about significant environmental degradation and climate change with severe impact on human and aquatic life. The world oil crisis in the 1970’s and the climatic shifts noticed by the World Meteorological Organization (WMO) in the 1980’s have stimulated a global action to mitigate the impact of conventional sources of energy on the environment. The activities of the United Nation’s Intergovernmental Panel on Climate Change (IPCC) highlight these facts. Energy from the sun is not only infinite (in a practical sense) but also abundant; enough to take care of mankind’s energy requirements. The sun radiates a hundred billion megawatts. Out of this, the earth receives about two hundred billion megawatts. It was estimated that by the year 2000, the energy requirement for the total population of the earth would be 15 million megawatts. Studies relevant to the availability of solar energy resources in Nigeria indicate the viability of solar energy for domestic and industrial uses. The annual average solar radiation is between 3.7KWm-2 day-1 along the coastal areas and 7.0KWm-2day-1 in the semi-arid zone of the country. The average figure approximates to 5.4KWm-2day-1. It is estimated that Nigeria receives 5.08 x 10 12 KWh of energy per day from the sun and if solar appliances with 5% efficiency are used to cover 1% of the country’s surface area, 2.541 x 10 6MWh of electricity will be produced. This tremendous amount of electrical energy is equivalent to 4.556 million barrels of oil per day. The Federal Government of Nigeria responded to the global concern on energy diversification by setting up four energy research centres in the 1980s. Two of these centres, the National Centre for Energy Research and Development, (NCERD) University of Nigeria, Nsukka and the Sokoto Energy Research Centre have the mandate to research, develop, disseminate and conduct training on solar and other renewable energy technologies. This mandate is being carried out in several fields under solar/alternative energy namely; photovoltaics, solar thermal, geothermal, and biomass technology. Several solar water heating systems, which fall within the solar thermal field, have been designed and fabricated at NCERD.

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Considering the large population of the country, the ever-increasing tariffs on electricity, and the need for hot water for household, institutional, and industrial uses particularly during the harmattan period, there is the need to disseminate solar water heating systems in the rural and urban areas.

2.0 PASSIVE SOLAR HEATING SYSTEMS IN NIGERIA

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Passive solar heating systems are systems that uses only natural means for heating of buildings.Two primary elements of passive solar heating are required:

South facing glass Thermal mass to absorb, store, and distribute heat

There are three approaches to passive systems – direct gain, indirect gain, and isolated gain. The goal of all passive solar heating systems is to capture the sun’s heat within the building’s elements and release that heat during periods when the sun is not shining. At the same time that the building’s elements (or materials) is absorbing heat for later use, solar heat is available for keeping the space comfortable (not overheated).

Direct Gain

In this system, the actual living space is a solar collector, heat absorber and distribution system. South facing glass admits solar energy into the house where it strikes directly and indirectly thermal mass materials in the house such as masonry floors and walls. The direct gain system will utilize 60 – 75% of the sun’s energy striking the windows.

Figure 1Thermal mass in the interior absorbs the sunlight and radiates the heat at night.

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In a direct gain system, the thermal mass floors and walls are functional parts of the house. It is also possible to use water containers inside the house to store heat. However, it is more difficult to integrate water storage containers in the design of the house.

The thermal mass will temper the intensity of the heat during the day by absorbing the heat. At night, the thermal mass radiates heat into the living space.

Direct gain system rules of thumb (Austin):

A heat load analysis of the house should be conducted. Do not exceed 6 inches of thickness in thermal mass materials. Do not cover thermal mass floors with wall to wall carpeting; keep as bare as

functionally and aesthetically possible. Use a medium dark color for masonry floors; use light colors for other lightweight walls;

thermal mass walls can be any color. For every square foot of south glass, use 150 pounds of masonry or 4 gallons of water

for thermal mass. Fill the cavities of any concrete block used as thermal storage with concrete or other

high mass substance. Use thermal mass at less thickness throughout the living space rather than a

concentrated area of thicker mass. The surface area of mass exposed to direct sunlight should be 9 times the area of the

glazing. Sun tempering is the use of direct gain without added thermal mass. For most homes,

multiply the house square footage by 0.08 to determine the amount of south facing glass for sun tempering.

Indirect Gain

In an indirect gain system, thermal mass is located between the sun and the living space. The thermal mass absorbs the sunlight that strikes it and transfers it to the living space by conduction. The indirect gain system will utilize 30 – 45% of the sun’s energy striking the glass adjoining the thermal mass.

There are two types of indirect gain systems:

Thermal storage wall systems (Trombe Walls) Roof pond systems

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Figure 2Thermal Mass Wall or Trombe Wall Day and Night Operation

Isolated Gain

An isolated gain system has its integral parts separate from the main living area of a house. Examples are a sunroom and a convective loop through an air collector to a storage system in the house. The ability to isolate the system from the primary living areas is the point of distinction for this type of system.

The isolated gain system will utilize 15 – 30% of the sunlight striking the glazing toward Sunrooms (or solar greenhouses) employ a combination of direct gain and indirect gain system features. Sunlight entering the sunroom is retained in the thermal mass and air of the room. Sunlight is brought into the house by means of conduction through a shared mass wall in the rear of the sunroom, or by vents that permit the air between the sunroom and living space to be exchanged by convection.

The use of a south facing air collector to naturally convect air into a storage area is a variation on the active solar system air collector. These are passive collectors. Convective air collectors are located lower than the storage area so that the heated air generated in the collector naturally rises into the storage area and is replaced by return air from the lower cooler section of the storage area. Heat can be released from the storage area either by opening vents that access the storage by mechanical means (fans), or by conduction if the storage is built into the house.

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Figure 3Day and Night Operation of a Sunroom Isolated Gain System

The sunroom has some advantages as an isolated gain approach in that it can provide additional usable space to the house and plants can be grown in it quite effectively.The convective air collector by comparison becomes more complex in trying to achieve additional functions from the system. This is a drawback in this area where space heating is less of a concern than in colder regions where the system would be used longer. It is best to use a system that provides more than one function if the system is not an integral part of the building. The sunroom approach will be emphasized in this information since it can provide multiple functions.

Efficiency and economics of passive solar heating

Technically, PSH is highly efficient. Direct-gain systems can utilize (i.e. convert into "useful" heat) 65-70% of the energy of solar radiation that strikes the aperture or collector. To put this in perspective relative to another energy conversion process, the photosynthetic efficiency theoretical limit is around 11%. There are six primary passive solar energy configurations

direct solar gain indirect solar gain isolated solar gain heat storage

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insulation and glazing passive cooling

Direct solar gain

Direct gain attempts to control the amount of direct solar radiation reaching the living space. This direct solar gain is a critical part of passive solar house designation as it imparts to a direct gain. The cost effectiveness of these configurations are currently being investigated in great detail and are demonstrating promising results.

Indirect solar gain

Indirect gain attempts to control solar radiation reaching an area adjacent but not part of the living space. Heat enters the building through windows and is captured and stored in thermal mass (e.g. water tank, masonry wall) and slowly transmitted indirectly to the building through conduction and convection. Efficiency can suffer from slow response (thermal lag) and heat losses at night. Other issues include the cost of

Isolated solar gain

Isolated gain involves utilizing solar energy to passively move heat from or to the living space using a fluid, such as water or air by natural convection or forced convection. Heat gain can occur through a sunspace, solarium or solar closet. These areas may also be employed usefully as a greenhouse or drying cabinet. An equator-side sun room may have its exterior windows higher than the windows between the sun room and the interior living space, to allow the low winter sun to penetrate to the cold side of adjacent rooms. Glass placement and overhangs prevent solar gain during the summer. Earth cooling tubes or other passive cooling techniques can keep a solarium cool in the summer. Measures should be taken to reduce heat loss at night e.g. window coverings or movable window insulation. Examples:

Thermosiphon Barra system Double envelope house Thermal buffer zone[20]

Solar space heating system

Heat storage

The sun doesn't shine all the time. Heat storage, or thermal mass keeps the building warm when the sun can't heat it. In diurnal solar houses, the storage is designed for one or a few days. The usual method is a custom-constructed thermal mass. These include a Trombe wall, a

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ventilated concrete floor, a cistern, water wall or roof pond. In subarctic areas, or areas that have long terms without solar gain (e.g. weeks of freezing fog), purpose-built thermal mass is very expensive. Don Stephens pioneered an experimental technique to use the ground as thermal mass large enough for annualized heat storage. His designs run an isolated thermosiphon 3m under a house, and insulate the ground with a 6m waterproof skirt.

Insulation

Thermal insulation or superinsulation (type, placement and amount) reduces unwanted leakage of heat. Some passive buildings are actually constructed of insulation.

Special glazing systems and window coverings

The effectiveness of direct solar gain systems is significantly enhanced by insulative (e.g. double glazing), spectrally selective glazing (low-e), or movable window insulation (window quilts, bifold interior insulation shutters, shades, etc.). Generally, Equator-facing windows should not employ glazing coatings that inhibit solar gain.There is extensive use of super-insulated windows in the German Passive House standard. Selection of different spectrally selective window coating depends on the ratio of heating versus cooling degree days for the design location.

Equator-facing glass

The requirement for vertical equator-facing glass is different from the other three sides of a building. Reflective window coatings and multiple panes of glass can reduce useful solar gain. However, direct-gain systems are more dependent on double or triple glazing to reduce heat loss. Indirect-gain and isolated-gain configurations may still be able to function effectively with only single-pane glazing. Nevertheless, the optimal cost-effective solution is both location and system dependent. Skylights admit harsh direct overhead sunlight and glare either horizontally (a flat roof) or pitched at the same angle as the roof slope. In some cases, horizontal skylights are used with reflectors to increase the intensity of solar radiation (and harsh glare), depending on the roof angle of incidence.

Operable shading and insulation devices

A design with too much equator-facing glass can result in excessive winter, spring, or fall day heating, uncomfortably bright living spaces at certain times of the year, and excessive heat transfer on winter nights and summer days.Although the sun is at the same altitude 6-weeks before and after the solstice, the heating and cooling requirements before and after the solstice are significantly different. Heat storage on the Earth's surface causes "thermal lag." Variable

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cloud cover influences solar gain potential. This means that latitude-specific fixed window overhangs, while important, are not a complete seasonal solar gain control solution. Control mechanisms (such as manual-or-motorized interior insulated drapes, shutters, exterior roll-down shade screens, or retractable awnings) can compensate for differences caused by thermal lag or cloud cover, and help control daily / hourly solar gain requirement variations. Home automation systems that monitor temperature, sunlight, time of day, and room occupancy can precisely control motorized window-shading-and-insulation devices.

Exterior colors reflecting - absorbing

Materials and colors can be chosen to reflect or absorb solar thermal energy. Using information on a Color for electromagnetic radiation to determine its thermal radiation properties of reflection or absorption can assist the choices. Landscaping and gardens- Energy-efficient landscaping materials for careful passive solar choices include hardscape building material and "softscape" plants. The use of landscape design principles for selection of trees, hedges, and trellis-pergola features with vines; all can be used to create summer shading. For winter solar gain it is desirable to use deciduous plants that drop their leaves in the autumn gives year round passive solar benefits. Non-deciduous evergreen shrubs and trees can be windbreaks, at variable heights and distances, to create protection and shelter from winter wind chill. Xeriscaping with 'mature size appropriate' native species of-and drought tolerant plants, drip irrigation, mulching, and organic gardening practices reduce or eliminate the need for energy-and-water-intensive irrigation, gas powered garden equipment, and reduces the landfill waste footprint. Solar powered landscape lighting and fountain pumps, and covered swimming pools and plunge pools with solar water heaters can reduce the impact of such amenities.

Sustainable gardening Sustainable landscaping Sustainable landscape architecture

Other passive solar principles

Passive solar lighting

Passive solar lighting techniques enhance taking advantage of natural illumination for interiors, and so reduce reliance on artificial lighting systems. This can be achieved by careful building design, orientation, and placement of window sections to collect light. Other creative solutions involve the use of reflecting surfaces to admit daylight into the interior of a building.

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Window sections should be adequately sized, and to avoid over-illumination can be shielded with a Brise soleil, awnings, well placed trees, glass coatings, and other passive and active devices.

Another major issue for many window systems is that they can be potentially vulnerable sites of excessive thermal gain or heat loss. Whilst high mounted clerestory window and traditional skylights can introduce daylight in poorly oriented sections of a building, unwanted heat transfer may be hard to control.Thus, energy that is saved by reducing artificial lighting is often more than offset by the energy required for operating HVAC systems to maintain thermal comfort.

Various methods can be employed to address this including but not limited to window coverings, insulated glazing and novel materials such as aerogel semi-transparent insulation, optical fiber embedded in walls or roof, or hybrid solar lighting at Oak Ridge National Laboratory.

Interior reflecting

Reflecting elements, from active and passive daylighting collectors, such as light shelves, lighter wall and floor colors, mirrored wall sections, interior walls with upper glass panels, and clear or translucent glassed hinged doors and sliding glass doors take the captured light and passively reflect it further inside. The light can be from passive windows or skylights and solar light tubes or from active daylighting sources. In traditional Japanese architecture the Shōji sliding panel doors, with translucent Washi screens, are an original precedent. International style, Modernist and Mid-century modern architecture were earlier innovators of this passive penetration and reflection in industrial, commercial, and residential applications.

Passive solar water heating

There are many ways to use solar thermal energy to heat water for domestic use. Different active-and-passive solar hot water technologies have different location-specific economic cost benefit analysis implications. Fundamental passive solar hot water heating involves no pumps or anything electrical. It is very cost effective in climates that do not have lengthy sub-freezing, or very-cloudy, weather conditions. Other active solar water heating technologies, etc. may be more appropriate for some locations. It is possible to have active solar hot water which is also capable of being "off grid" and qualifies as sustainable. This is done by the use of a photovoltaic cell which uses energy from the sun to power the pumps

Comparison to the Passive House standard in Europe

There is growing momentum in Europe for the approach espoused by the Passive House Institute in Germany. Rather than relying solely on traditional passive solar design techniques, this approach seeks to make use of all passive sources of heat, minimises energy usage, and

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emphasises the need for high levels of insulation reinforced by meticulous attention to detail in order to address thermal bridging and cold air infiltration. Most of the buildings built to the Passive House standard also incorporate an active heat recovery ventilation unit with or without a small (typically 1 kW) incorporated heating component.The energy design of Passive House buildings is developed using a spreadsheet-based modeling tool called the Passive House Planning Package (PHPP) which is updated periodically. The current version is PHPP2007, where 2007 is the year of issue. A building may be certified as a 'Passive House' when it can be shown that it meets certain criteria, the most important being that the annual specific heat demand for the house should not exceed 15kWh/m2a.

Design tools

Traditionally a heliodon was used to simulate the altitude and azimuth of the sun shining on a model building at any time of any day of the year. In modern times, computer programs can model this phenomenon and integrate local climate data (including site impacts such as overshadowing and physical obstructions) to predict the solar gain potential for a particular building design over the course of a year. GPS-based smart phone applications can now do this inexpensively on a hand held device. These tools provide the passive solar designer the ability to evaluate local conditions, design elements and orientation prior to construction. Energy performance optimization normally requires an iterative-refinement design-and-evaluate process. There is no such thing as a "one-size-fits-all" universal passive solar building design that would work well in all locations.

Levels of application

Pragmatic

Many detached suburban houses can achieve reductions in heating expense without obvious changes to their appearance, comfort or usability. This is done using good siting and window positioning, small amounts of thermal mass, with good-but-conventional insulation, weatherization, and an occasional supplementary heat source, such as a central radiator connected to a (solar) water heater. Sunrays may fall on a wall during the daytime and raise the temperature of its thermal mass. This will then radiate heat into the building in the evening. This can be a problem in the summer, especially on western walls in areas with high degree day cooling requirements. External shading, or a radiant barrier plus air gap, may be used to reduce undesirable summer solar gain.

Annualised

An extension of the "passive solar" approach to seasonal solar capture and storage of heat and cooling. These designs attempt to capture warm-season solar heat, and

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convey it to a seasonal thermal store for use months later during the cold season ("annualised passive solar.") Increased storage is achieved by employing large amounts of thermal mass or earth coupling. Anecdotal reports suggest they can be effective but no formal study has been conducted to demonstrate their superiority. The approach also can move cooling into the warm season.

Examples:

Passive Annual Heat Storage (PAHS) - by John Hait Annualized Geothermal Solar (AGS) heating - by Don Stephen Earthed-roof

Minimum machinery

A "purely passive" solar-heated house would have no mechanical furnace unit, relying instead on energy captured from sunshine, only supplemented by "incidental" heat energy given off by lights, computers, and other task-specific appliances (such as those for cooking, entertainment, etc.), showering, people and pets. The use of natural convection air currents (rather than mechanical devices such as fans) to circulate air is related, though not strictly solar design.

Passive solar building design sometimes uses limited electrical and mechanical controls to operate dampers, insulating shutters, shades, awnings, or reflectors. Some systems enlist small fans or solar-heated chimneys to improve convective air-flow. A reasonable way to analyse these systems is by measuring their coefficient of performance. A heat pump might use 1 J for every 4 J it delivers giving a COP of 4. A system that only uses a 30 W fan to more-evenly distribute 10 kW of solar heat through an entire house would have a COP of 300.

Zero Energy Building

Passive solar building design is often a foundational element of a cost-effective zero energy building. Although a ZEB uses multiple passive solar building design concepts, a ZEB is usually not purely passive, having active mechanical renewable energy generation systems such as: wind turbine, photovoltaics, micro hydro, geothermal, and other emerging alternative energy sources.

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3.0 ACTIVE SOLAR HEATING SYSTEMS IN NIGERIA

Solar trackers may be driven by active or passive solar technology

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Active solar heating systems are means in which the flow of energy is forced by mechanical means such as a pump or fan.

Active solar technologies are employed to convert solar energy into another more useful form of energy. This would normally be a conversion to heat or electrical energy. Inside a building this energy would be used for heating, cooling, or off-setting other energy use or costs. Active solar uses electrical or mechanical equipment for this conversion. Solar energy collection and utilization systems that do not use external energy, such as a solar chimney, are classified as passive solar technologies. Passive solar relies on the inherent thermo-dynamic properties of the system or materials to operate. They do not need external energy sources.

Solar hot water systems, except those based on the thermosiphon, use pumps or fans to circulate fluid (often a mixture of water and glycol to prevent freezing during winter periods) or air, through solar collectors, and are therefore classified under active solar technology.

Solar trackers, used to orient solar arrays may be driven by either passive or active technology, and can have a significant gain in energy yield over the course of a year when compared to a fixed array. Again passive solar tracking would rely on the inherent thermo-dynamic properties of the materials used in the system rather than an external power source to generate its tracking movement. Active Solar Tracking would utilise sensors and motors track the path of the sun across the sky. This action can be caused by geographical and time data being programmed into the controls. However, some systems actually track the brightest point in the sky using light sensors, and manufacturers claim this can add a significant extra yield over and above geographical tracking.

The basic benefit of active systems is that controls (usually electrical) can be used to maximise their effectiveness. For example a passive solar thermal array which does not rely on pumps and sensors will only start circulating when a certain amount of internal energy has built up in the system. Using sensors and pumps, a relatively small amount of energy (i.e. that used to power a pump and controller) can harvest a far larger amount of available thermal energy by switching on as soon as a useful temperature differential becomes present. Controls also allow a greater variety of choices for utilising the energy that becomes available. For example a solar thermal array could heat a swimming pool on a relatively cool morning where heating a domestic hot water cylinder was impractical due to the different stored water temperatures. Later in the day as the temperature rises the controls could be used to switch the solar heated water over to the cylinder instead.

The downside to Active Solar heating systems is that the external power sources can fail (probably rendering them useless), and the controls need maintenance. Most solar collectors are fixed in their array position mounting, but can have a higher performance if they track the path of the sun through the sky (however it is unusual for thermal collectors to be mounted in this way).

SOLAR WATER HEATERS

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The solar collector is the main component of a solar hot-water system. It transforms radiant energy from the sun in the spectral range 0.3-3μm, into usable heat. Two distinct processes are dealt with by the collector:

(a) The absorption of radiant energy, which requires the highest possible transmission coefficient, τ for the transparent cover and the highest possible absorption coefficient, α for the absorber plate. The effective parameter will be the product (τα).

(b) The loss of energy in the infra-red spectrum due to radiation losses between the absorber plate and the transparent cover; natural convection losses between the absorber plate and the transparent cover, and conduction losses through the back and edges insulation.

Types of Solar Water Heating Systems

Water heating is one of the simplest applications of solar heat and one of the least expensive. A great many types of water heating systems have been conceived of by investors and solar engineers; the different types varying in design configuration and system make up. Generally, solar water heaters can be active or passive, stand-alone or hybrid systems. Active Solar Water Heating System According to Daniel ,an active solar system is one having an assembly of collectors, storage device, and transfer fluid which converts solar energy into thermal energy and in which energy in addition to solar input is used to accomplish the transfer of thermal energy. In active solar water heating systems, electric pumps, valves, and controllers are used to circulate water or other heat-transfer fluids through the collectors. They are usually more expensive than passive systems but are also more efficient. Active systems are usually easier to retrofit than passive systems because their storage tanks do not need to be installed above or close to the collectors. Passive Solar Water Heating System This system transfers heat by means of natural circulation by convection (i.e. they do not require pumps to function). They naturally modulate the circulation flow-rate in phase with the radiation level. Here, the warmer, less dense water rises to the top of system, displacing the colder water to the lowest point. Passive solar water heaters also known as thermosyphon systems are generally more reliable, easier to maintain, and possibly longer lasting than active systems. They can be built with inherent freeze

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resistance so they can be used in areas that are subject to extended periods of freezing temperatures. Basic Components of a Solar Water Heating System The basic elements of most common solar water heaters are the flat-plate collector, heat transfer fluid, and the storage tank. Other components such as heat exchanger, pumps, pipe network, valves, auxiliary energy source, and control systems can be included depending on the intended design, operation, and application of the system.

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REFERENCES

Balcom, J.D. (1977) ; Designing passive solar building to reduce temperature swings

"Glazing - Overview". Archived from the original on December 15, 2007. Retrieved 2008-01-14.

Springer, John L. (December 1954). "The 'Big Piece' Way to Build". Popular Science 165 (6): 157.

"Introductory Passive Solar Energy Technology Overview". U.S. DOE - ORNL Passive Solar Workshop. Retrieved 2007-12-23.

"Passive Solar Design". New Mexico Solar Association.

Chiras, D. The Solar House: Passive Heating and Cooling. Chelsea Green Publishing Company; 2002.

"Zero Energy Buildings". Fsec.ucf.edu. Retrieved 2010-03-16.

"Two Small Delta Ts Are Better Than One Large Delta T". Zero Energy Design. Retrieved 2007-12-23.

Annualized Geo-Solar Heating, Don Stephens- Accessed 2009-02-05

Shurcliff, William A.. 1980 Thermal Shutters & Shades - Over 100 Schemes for Reducing

Heat Loss through Windows. "U.S. Department of Energy - Energy Efficiency and Renewable Energy - Sunspace Orientation and Glazing Angles". Retrieved 2011-03-28.

Solar Heat Gain Through Glass". Irc.nrc-cnrc.gc.ca. 2010-03-08. Retrieved 2010-03-16.

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