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Basic concepts of solar design Conversion of solar energy Solar collectors Uses of solar energy Solar heating systems Solar cooling systems Design of solar buildings

INTRODUCTION Solar energy is converted for use in buildings in three ways: biochemicaa/, electrical and thermal. The devices used for collecting solar energy include flat Hate collectors, concentrating or focusing collectors and photov~ltaic collectors. Solar energy is used for space heating and cooling, water heating, cooking, drying, distillation, evansion and steam engines and for electricity generation. There are active and passive solar heating and solar cooling systems. Passive heating systems include direct gain systems, thermal storage walls, sun spaces, thermosiphon systems, and mixed systems. Natural cooling systems utilize radiant night cooling, specialized radiators, convective and evaporative cooling, roof ponds, earth cooling and comfort venti/ation. The design of solar buildings involves data collection, bioclimatic analysis, solar analysis, thermal analysis, choice of appropriate system and integration,

BASIC CONCEPTS OF SOLAR DESIGN

The idea of free and endless energy in today's world of high fuel and energy costs may seem unrealistic. Yet the sun, the main source of energy on earth produces

energy free of charge and practically endlessly. This energy can be harnessed to provide all or most of building energy requirements at little or no cost. The main obstacles were knowledge, cost and technology and as long as fuel costs remained low people were ,satisfied to leave things as they were. Rapid depletion of mineral resources and the oil crisis however brought rapid change. The technology is now much more reliable, prices have fallen considerably and there is better awareness of solar issues.

In Nigeria the solar question remains not only unanswered but largely unasked. This is as a result of the nature of the technology involved and high costs. Luckily, the climate of Nigeria does not require winter heating and little auxiliary energy is needed to maintain thermal comfort. This is however subject to appropriate design of houses to take maximum advantage of environmental conditions to enhance thermal comfort. The architect thus needs a good understanding of how solar energy works. The abundance of solar energy in Nigeria, the rising cost of energy and gradual depletion of petroleum resources are indications of future interest in solar energy.

CONVERSION OF SOLAR ENERGY

There are three basic ways in which solar energy may be converted for use in buildings - biochemical, electrical and thermal.

Biochemical conversion of solar energy involves photo-biochemical processes. Photosynthesis involves the use of solar energy in the conversion of carbon dioxide and water into carbohydrate and oxygen. The carbohydrate is consumed by humans and other animals for energy. Solar energy can be used in production of biogas. Sunlight is used to grow algae which are then fermented anaerobically to produce methane for the operation of internal combustion engines and for cooking.

Electrical conversion of solar energy can be done directly using thermoelectric or photoelectric processes. Thermoelectric conversion is the direct conversion of solar

energy into electrical energy by means of thermocouples. A potential difference for the generation of electrical energy is achieved by cooling one dnd heating the other junction of the thermocouple. A thermopile is made up of thermocouples joined in seyies. Photoelectric or photovoltaic conversion is the production of electricity using the light content of solar radiation. Photovoltaic cells made from silicon are commonly used? See figure 13.1.

Thermal conversion. When radiant energy falls on a surface, some of it is absorbed. The nearer the surface is to a matt black surface, the more the radiation absorbed. Some of this energy is transmitted to other parts of the body by conduction and some r-mitted by conductive and radiant processes. The solar energy is thereby converted to heat energy. The heat produced can be trapped by covering the absorber plate with a sheet of glass thus creating a greenhouse effect.

SOLAR COLLECTORS +ve P-N

There are several devices used for collecting solar energy. They may be broadly junctirn : ' divided into three groups - flat plate collectors, concentrating or focusing collectors

& '6 i-

and photovoltaic collectors. -Le 4yer

The flat plate collector is made up of an absorber plate of black metal backed with insulation and a glass or plastic cover. The cover is separated by an airspace from the absorber which may have tubing built into it for circulating fluid. See figure 13.2.

Concentrating or focusing collectors are of many types. They use various devices for concentrating solar radiation on the surface of an absorber. The heat gain is thus from an area much larger than that of the absorber. The result is much Fig,,re 13. , higher equilibrium temperatures. Concentration is achieved through the use of multiple plane mirrors, cylindrical reflectors, spherical reflectors and cylindrical or A photovolfdic cell circular Fresnel lenses. See figure 13.3.

Figure 13.2 h flat- pbte cdlectm

Photovoltaic collectors convert radiant energy directly into electrical energy using photovoltaic cells.

USES OF SOLAR ENERGY

Solar energy can be used in houses in many ways ranging from heating and cooling to cooking and electricity production.

Space heating utilizes solar energy for heating of buildings through using the building as a collector or the use of special building elements or collectors.

Space cooling can be achieved through mechanically driven compression type refrigeration or by using absorption type refrigeration. The energy to run these plants is obtained from the sun.

Water heating for domestic use or in swimming pools is done using the thermo- siphon principle. The water is heated in the collector, rises and is replaced by cooler water;

Cooking. Solar energy can be used for cooking using either the direct (focusing) cooker or the box (oven) type solar cooker. In the focusing cooker the pot containing the foodstuff is placed at the focus of a parabolic mirror. The solar oven is an insulated chamber with a window on one side to admit radiation.

Drying of various agricultural crops or products is achieved by exposing them to the sun in a covered tray of some sort or by blowing hot air through or over them.

Distillation of non-potable water for drinking wingsthe box type distiller.

Expansion engines and steam engines based on solar energy may be used for water pumping.

Electricity can be produced fkom solar energy in two ways. The first involves direct conversion by photovoltaic or thermoelectric processes. Alternatively, solar energy may be used to produce mechanical work which will then be used to drive electric generators.

SOLAR HEATING SYSTEMS

In solar design of buildings solar heating systems are used to supply some of the energy needed both for space and domestic water heating. There are two types of systems - passive and active.

Passive solar heating systems use only natural means for heating of buildings. This can be done in several ways. In the direct gain system energy is collected through windows and other glazing and stored within the building structure. Diffusion may be used to spread the incoming radiation evenly through the use of diffusing glass or blinds. Thermal storage walls are in the form of vented Trombe walls, unvented Trombe walls, water walls or phase change walls. The wall is used for heat storage and is located closely behind the solar collection glazing. A sun space is usually a strongly solar driven direct gain space sharing a common wall with the main building. There should be the possibility of convectively connecting or isolating the two spaces. The thermal storage roof system uses heat storage material located on the building roof. For example, a roof pond ig a thermal storage roof using water. Thermosiphon systems transport heat and mass by natural convection of a fluid such as air or water giving rise to air thermosiphon or water thermosiphon. Two or more of these systems may be combined to give a mixed passive system. See figure 13.4.

(a) two-rnrrror concenkator

- .

(b) paraboloid ref Iccf-or

paraboloid

Active solar heating systems are systems in which the flow of energy is forced by mechanical means such as a pump or fan. These systems are usually more compli- Figure 13.3 cated and more expensive than passive systems. There are also problems with maintenance and availability of spare parts. For these reasons hybrid systems with 150cueing and ~oncentr6 f ;q cd\wfor5

a minimal number of active components are becoming more popular.

non- diff~ljing diredgain d;ff using direct* in m ass Trom be w.ll wder Trom be W l l

s u n 5 p a e T hermodphon thermal storage - rmf pond

Figure 13.4

Pa55i v e wlar heating' 5 j t e m s

SOLAR COOLING SYSTEMS

Solar cooling systems are used for cooling and ventilation of buildings. The first concern in solar coo- is however how to avoid cooling loads and not how to cool down the building. If excessive heating can be minimized, then the problem of providing sufficie~t cooling will be half--soltred. Cooling loads are due to sunshine through windows or on the outside of walls or roofs, hot air entering the building or heat conducted from hot outside air to the inside. Cooling loads can usually be avoided through good design involving the judicious use of shading devices, vegetation, colours and insulation. There are two types of solar cooling systems - the passive cooling systems known as natural cooling and active cooling systems.

Natural cooling systems are passive solar cooling systems that depend solely on natural means for the cooling of buildmgs. Radiant night cooling directly cools the roof mass from longwavenet heat loss to the night sky. The benefits of this type of cooling increases if the roof is covered with insulating materials during the day to prevent heat gain. Radiant cooling with specialized radiators makes use of metallic plate iadiators for longwave-radiation.- The heat storage mass consisting of the walls and the roof may be cooled at night by convective cooling using the cool outdoor air. See figure 13.5. In the day time the structural mass can then be used as a heat sink with interior ventilation deliberately kept low to avoid heat gain. Evaporative cooling of buildings may be either direct or indirect. It employs the latent heat of evaporation of water for cooling. The wind force is used to produce natural air flow through moist elements installed in windows and openings. -The air is humidified while the dry bulb temperature falls thus causing evaporative cooling. Roof ponds also lower the temperature of the building structure through evapora- tion. Earth cooling uses the earth as a heat sink to lower the temperature of buildings while encouraging heat loss from the earth's surface. Heat loss isachieved

buildings act a5 du&

by covering the ground with a layer of gravel and irrigating it. Comfort ventilation Figure 3.5 uses ambient air for cooling. See figure 13.6. Passive mlar venhlation 4y4fem5

night

Roof pand

Dessi ca nf cool; ng Indud ventiIa+ior,

Figure 13.6

fa45ive 5&r cooling 5ydm5

Active cooling systems These are systems that use some mechanical means for cooling. They can be either open cycle or closed cycle.

DESIGN OF SOLAR BUILDINGS

The design of buildings incorporating effective passive solar systems and energy conservation principles is a complex one and much depends upon the architect and his understanding of solar energy. It is important to understand that each house should be a solar house - the rule rather than the exception. In this wise, all houses, even those made from conventional - building materials and using conventional construction should take maximum advantage of environmental copditions to enhance thermal comfort. Many houses may be kept comfortable throughout the year by applying these principles. There are however climates where this is not achievable - cooling or heating devices have to be installed. This is when the question arises - renewable solar energy or conventional energy? This however assumes that our major concern is thermal comfort. The question may arise much earlier if the use of solar powered cooking, artificial lighting and domestic water heating is being considered.

The following steps are usually involved in solar design:

DATA COLLECTION This involves the collection of climatic data for the specific site including outdoor air temperatures, humidity or vapour pressure, wind speed and direction, global radiation on a horizontal plane, hours of sunshine, cloudiness and precipitation.

BIOCLIMATIC ANALYSIS This involves the determination of the thermal stress and comfort conditions using any of the thermal indices. The aim is to determine the nature of thermal stress and how to relieve it.

SOLAR GEOMETRY The bioclimatic analysis would have shown us when in thexyear there is thermal stress and how it should be relieved - by cooling or by heating. In either case the position and movement of the sun at this period must be known. This will allow the design of sun-shading devices to block the sun and prevent overheating on one hand or the provision of means of harnessing solar energy for space heating on the other. Solar geometry is often determined using graphic methods (sun- path diagrams), instrumental methods or computer programs.

THERMAL ANALYSIS This is an analysis of the heat balance of the building. The first analysis is done on the basis of the climatic and site data to produce sketch design guides. The performance of the building is checked continuously as amendments are made. At the end of the design process the whole building will be analyzed to confirm its performance. There are several methods in use for annual heating load calculations. The traditional method uses the degree day and the total building heat loss per day. A more popular method is the Solar Load Ratio (SLR) method and of course there are computer programs.

The Choice of a passive solar, active solar or conventional system of heating or cooling is based on many factors such as client demand, cost effectiveness, fashion, tradition, availability and maintenance. If a solar system is chosen then several questions involving the following have to be answered: type of device, efficiency, air or water system, position and size of collector; type and size of storage, auxiliary heat supply and manual or automatic control.

The integration of the chosen system into the building will cause a change in the existing thermal balance. New calculations need to be made, thus bringing us back to the thermal analysis stage. The results of this analysis may necessitate an adjustment of the solar system parameters. This process is repeated until an optimal solution is found.

TESTS AND EXERCISES

13.1 Describe three types of solar collectors. 13.2 Describe three types of passive heating systems. 13.3 Write briefly on solar cooling and how it may be applied in Nigeria. 13.4 Discuss the following terms:

a. Trombe wall b. roof pond cooling c. active solar system d. sun-space e. thermal storage

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