A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

SOLAR ENERGY

DOI: 10.1615/AtoZ.s.solar_energy

A solar energy thermal conversion system should seek to provide the optimal combination of efficient performance, low initial and running costs, robustness and durability. Such a system consists of components for energy collection, distribution and storage. These may be discrete items, or so inextricably linked as to be synonymous. The system components are sized in relation to the solar energy resource and to the nature and pattern of energy utilization.

Solar Selective Surfaces

High efficiency thermal solar energy collection requires a large absorption of shortwave solar radiation, low emission of emitted longwave thermal radiation and suppression of convective heat losses. Ninety five percent of the solar radiation spectrum lies in the wavelength range 0.3 to 2 μm; ninety nine percent of thermal radiation at 325 K lies in the range 3.0 to 30 μm.

Solar selective surfaces have a high solar spectrum absorptance and a low emittance in the thermal spectrum.

Consider a heat balance between absorber surface and glass cover of the solar collector shown schematically in Figure 1. The energy absorbed by the absorber per unit area is given by

(1)

where I is the incoming solar radiation, α is the solar absorptance, and τ is the transmittance of the glass cover. The heat lost by thermal radiation qrad per unit area is given by

(2)

(ε or εg ≈ 1), where ε and εg are emittance of the absorber and of the glass cover, respectively. A low ε or a low εg is favorable. A low εg requires an infrared coating on the glass cover. Though low emittance is clearly beneficial, at lower temperatures natural convection across the cavity is the dominant mode of heat loss, this introduces the possible inclusion of convection suppression devices (known as transparent insulation materials) or evacuation of the cavity.

Components and heat transfer mechanisms of a liquid-heating flat-plate collector.

Figure 1. Components and heat transfer mechanisms of a liquid-heating flat-plate collector.

To obtain a selective surface with a high α and a low ε the material needs to have a low reflectance ρ in the solar, and a high reflectance in the thermal (infrared) spectrum. Materials that have this property are semiconductors like silicon and germanium. Photons having energies greater than that of their typical band gap will be absorbed: the band gap energy of silicon is 1.11 eV (equivalent wavelength 1.2 μm); the band gap energy of germanium is 0.67 eV (equivalent wavelength 1.9 μm). However, these materials have an appreciable solar reflectance (of ≈ 0.3), and so require an appropriate coating. Metals such as copper, nickel and aluminum exhibit high infrared reflectances (e.g., ≥ 0.95 for clean and polished surfaces); however, they also have low solar absorptances. To overcome this, a thin (0.4 ≈ 1.5 μm) layer of a material with high solar absorptance and good infrared transmittance is applied to the metal.

Black nickel is a nickel-zinc-sulfide complex that has the required properties. An absorptance of 0.96 can be obtained; the polished nickel substrate can give a low emittance, typically 0.08. Copper oxide on copper shows absorptances and emittances of, typically, 0.9 and 0.15, respectively. "Black chrome" selective absorbing surfaces comprise a thin surface layer of chrome in an amorphous chromium oxide matrix deposited on a polished metal surface.

Nonmetallic materials which have a high infrared reflectance, compared with that of metallic surfaces, and a low solar reflectance, are termed "heat mirrors". When a thin layer of such a material is combined with a solar-absorbing substrate, the resulting tandem exhibits good selective properties.

Convective Heat Transfer in Solar Energy Collectors

In determining the heat loss by buoyancy-driven convection between the collector plate and the glass cover of a flat-plate collector, when mounted at an angle between 0° and 75° from the horizontal, the following correlation of Nusselt Number, Nu, in terms of Grashof Number, Gr, and Prandtl Number, Pr, is employed:

(3)

where β is the inclination of the system to the horizontal.

The superscript + denotes a positive value for the quantity in brackets if this quantity is greater than zero and a value of zero otherwise. The characteristic length in determining Gr is taken as the thickness of the air cavity.

Heat losses from the absorber of a line-axis concentrating collector to the surrounding environment ensue by radiation, conduction and convection. Under steady-state conditions, the interactions of these three heat-transfer modes lead to a particular temperature distribution being established, which is characteristic of the geometry and the applied temperature difference between the absorber and the ambient environment. The internal convection correlation for Nusselt number is:

(4)

where H is the height of the cavity, W is its half-width and b is the inclination of the system.

Solar Energy Collectors

The principal components of a water-heating flat-plate collector are shown in Figure 1. It consists of an absorbing surface, which absorbs insolation and transmits it in the form of heat to a working fluid, commonly air or water. In an evacuated-tube collector, each absorber fin is enclosed in a separate cylindrical glass envelope. The latter generally has only one protrusion of pipework, against which the glass is sealed, through which heat is removed. Evacuation of the envelope prevents convective heat loss from the absorber plate to the glass. The transfer of heat from the collector is usually accomplished by employing a Heat Pipe absorber.

Solar energy collectors are devices employed to gain useful heat energy from the incident solar radiation. They can be of the concentrating or the flat-plate type and can be stationary or track solar azimuthal position in either two or three areas. The range of solar collectors is illustrated in Figure 2.

Solar energy collector types.

Figure 2. Solar energy collector types.

The useful heat gained by a collector can be expressed as

(5)

and the following heat balance, the Hottel-Whillier-Bliss equation expresses the thermal performance of a collector under steady state:

(6)

Collector efficiency is defined as the ratio of useful heat gain over any time period to the incident solar radiation over the same period, i.e.,

(7)

thus

(8)

and therefore

(9)

The general steady-state test procedures for flat-plate collectors is to determine Qu and measure I, Ti and Ta, by operating the collector under nearly steady-state conditions in test facilities (i.e., either indoor or outdoor). Instantaneous efficiencies are plotted against (Ti − Ta)/I and the intercept (FR(τα)e) and the slope (−FRUL) determined. These parameters are not constant, UL depends on temperature and wind speed, and FR being a weak function of UL. However the long-term performance of many solar-heating collectors can be characterzed by a thus determined intercept and slope. Illustrative examples of characteristic areas for air and water heating collectors are given in Figure 3.

Typical Hottel-Whittlier-Bliss performance characterizations for generic air-heating solar energy collectors.

Figure 3. Typical Hottel-Whittlier-Bliss performance characterizations for generic air-heating solar energy collectors.

The absorber material in a flat-plate collector, in addition to having a high absorptance of the incident radiation should also have a low emissivity, good thermal conductivity, and be stable thermally under temperatures encountered during operation and stagnation. It should also be durable, have low weight per unit area and, most importantly, be cheap.

A good cover material should have a high transmittance in the visible range of the spectrum and a low transmittance to infrared radiation in order to effectively trap in reradiated heat from the absorber. Other qualities of a good cover material include low heat absorptivity, stability at the operating temperatures (should withstand high temperatures under stagnation conditions), resistance to breakage, durability under adverse weather conditions, and low cost. The variation of the radiative transmittance of a "transparent" material is determined by its chemical composition, molecular structure and fabrication. Though most plastic films have transmittances to visible light greater than 0.85, they exhibit wide variations in transmittance to infrared from 0.01 (for polymethyl methacrylate) to 0.77 (for polyethylene).

Glass has been very widely used as a cover material due to its high transmittance to visible light, very low transmittance to infrared radiation and its stability to high temperature. Its high cost, low shatter resistance and relatively large weight per unit (which increases the cost of supporting structures) have encouraged the consideration of alternative cover materials. Plastics have been used, their major limitations being their stability at collector operating temperatures and their durability under weather conditions, particularly degradation under ultraviolet radiation. However, most plastics have been chemically treated to overcome at least some of these shortcomings. Some plastic covers show high transmittance to visible light and equally low transmittance to infrared. Plastics weigh about 10% of the same area of glass. The overriding factor in the choice of materials for the design of cheap and simple solar energy collectors is cost, particularly those that heat air, thus certain desired material properties may be compromised during design and construction.

A transpired collector is an unglazed perforated absorber plate through whose holes solar heated air is drawn by the action of a fan. The efficiency of a transpired collector is given by:

(10)

where αc is the collector absorptance, hr is a linearized radiative heat loss coefficient, εhx is the absorber heat exchange effectiveness, hc is the convective heat coefficient, ρ is air density, cρ is specific heat at constant pressure and Vo is suction velocity.

A line-focus compound parabolic concentrating collector as shown in Figure 4 is characterized by an acceptance half-angle θa, which determines the maximum attainable concentration ratio, which is given by

(11)

This maximum concentration ratio can be achieved only by a full-height compound parabolic concentrator, i.e. no truncation is applied at the top of the reflectors, and if the absorber is of optically correct area, i.e. the area of the absorber is 1/Cmax of the aperture area. In a real application, with a tubular absorber, the concentration ratio is expressed as

(12)

The value given by the above equation E is lower than that given by I/sin θa because of truncation of the concentrator top, undertaken normally to reduce the capital cost, and oversizing of the absorber's diameter, to allow for optical scatter introduced by imperfections arising during manufacture and operation.

Components and geometry of a compound parabolic concentrating solar energy collector with a tubular absorber.

Figure 4. Components and geometry of a compound parabolic concentrating solar energy collector with a tubular absorber.

For a parabolic trough concentrating collector, which tracks the Sun continuously, so any ray entering the concentrator parallel to its axis will, either after reflection or directly, intercept the tubular absorber. The concentration ratio for a parabolic trough concentrating collector is also given by W/πD.

Unlike flat-plate collectors, only a fraction of the diffuse insolation is exploitable by concentrating collectors. This can be shown by considering the radiation exchange between the absorber and the aperture in a concentrating collector. If ER−A and EA−R represent the exchange factors for the radiation exchange between absorber and aperture and aperture and absorber, respectively, then the following equation applies:

(13)

For a compound parabolic concentrating collector the exchange factor ER−A is unity, as any ray emitted from the absorber will either directly, or after one or more reflections, reach the aperture. Thus

(14)

If an isotropic distribution is assumed for the diffuse radiation then the exchange factor EA−R also represents the exploitable part of the diffuse insolation of a compound parabolic concentrating collector, gD, CPC:

(15)

For a parabolic trough concentrating collector, ER−A < 1 as the absorber can "view" itself on the reflector. Thus, the exploitable part of the diffuse insolation of a parabolic trough concentrating collector, βD, PTC is less than 1/C. The diffuse insolation absorbed by the absorber can be given by

(16)

where βD is a correction coefficient accounting for the part of diffuse insolation which reaches the absorber directly, i.e. is not attenuated by reflection losses and ρ accounts for end effects. The total insolation absorbed by the absorber, Iu,

(17)

where γ is the intercept factor accounting for the optical losses occurring in a real parabolic trough concentrating due to optical errors and Ieff represents the effective insolation at the concentrator's aperture, given by

(18)

To evaluate the thermal performance of a parabolic trough concentrating collector Ieft is employed.

The optical efficiency ηopt of a parabolic trough concentrating collector is defined as the ratio between the insolation Iu absorbed by the absorber and the total hemispherical insolation on the plane of the collector, Itot, i.e.,

(19)

Thus

(20)

A compound parabolic concentrating collector exploits a greater part of the available diffuse insolation compared with a parabolic trough collector, although this advantage diminishes as the concentration ratio increases. A compound parabolic concentrating collector also maintains a superior acceptance angle.

Solar Ponds

Solar ponds are unitary solar energy collectors and heat stores. In a nonconvecting solar pond, part of the incident insolation absorbed is which is stored as heat in lower regions of the pond.

A salt-gradient, nonconvecting solar pond consists of three zones as shown in Figure 5.

  • The upper-convecting zone (UCZ), of almost constant low salinity at close to ambient temperature. The UCZ, typically 0.3 m thick, is the result of evaporation; wind-induced mixing and surface flushing. Wave-suppressing surface meshes and placing windbreaks near the pond keep UCZ as thin as possible.

  • The nonconvecting zone (NCZ), in which both salinity and temperature increase with depth. The vertical salt gradient in the NCZ inhibits convection and thus provides the thermal insulation.

  • The lower-convecting zone (LCZ), of almost constant, relatively high salinity (typically 20% by weight) at a high temperature. Heat is stored in the LCZ, which is sized either to supply energy continuously throughout the year for power generation or provide interseasonal heat storage for space heating. As the depth increases, the thermal capacity increases and annual variations of temperature decrease. However, large depths increase the initial capital outlay and require longer start-up times.

Schematic vertical crosssection through a salt-stratified nonconvecting solar pond.

Figure 5. Schematic vertical crosssection through a salt-stratified nonconvecting solar pond.

Salt gradient lakes, which exhibit an increase in temperature with depth have occurred naturally in Transylvania, in California and Washington State, USA, in the Arctic, Venezuela, western Uganda, and a lake on the east coast of the Sinai Peninsula.

The application of solar ponds for electric-power production usually employs an organic vapor Rankine cycle engine to convert solar-pond heat to mechanical work, and then into electricity. However, to obtain a low cost per unit generated power, solar ponds of several square kilometers are required.

The site for a solar pond should be near a cheap source of salt, an adequate source of water, in- cur low land costs, and have an all-year solar exposure. The underlying earth structure should be free of stresses and fissures. If not, then increases in temperature may cause differential thermal expansions, which could result in earth movements. The pond must not pollute aquifers nor lose heat via underground water streams passing through an aquifer. Any continuous drain of heat will lower the pond's storage capability and effectiveness. Stormy regions should be avoided in order to limit wind surface mixing effects.

Species of freshwater and saltwater algae grow under the conditions of temperature and salt concentration that exist in a stratified solar pond. Algae and cynobacteria growth will inhibit solar transmittance and, for the latter, possible be toxic too. Different algae and organic bacteria species are introduced by rain water and airborne dust. To prevent algae formation, copper sulfate, has been added at a concentration of about 1.5 mg l−1.

A solar pond will cease to function without maintenance of the vertical salt gradient stratification. The stability of the salt gradient is maintained by:

  1. controlling the overall salinity difference between the two converting layers,

  2. inhibiting internal convection currents if they tend to form in the NCZ,

  3. limiting the growth of the UCZ.

Salt slowly diffuses upwards at an annual average rate of about 20 kg m−2 as a result of its concentration gradient. This rate varies and is dependent upon the ambient environment conditions, type of salt and temperature gradient. Surface washing by fresh water and injecting brines of adequate density at the bottom of the pond usually maintain an almost stationary gradient. UCZ growth caused by surface flushing is diminished if the velocity of the surface washing water is small. Surface temperature fluctuations will result in heat being transferred upwards through the UCZ by convection, especially at night, and downward, more slowly, by conduction. The thickness of the UCZ also varies with the intensify of the incident insolation.

The higher the temperature of the UCZ, and the lower the humidity above the pond's surface, the greater will be the evaporation rate caused by insolation and wind action. Excessive evaporation results in a downward growth of the UCZ. Evaporation can be compensated by surface water washing, as well as reduce the temperature of the pond's surface especially during periods of high insolation. Windbreaks will reduce evaporation rates. Though evaporation can be the dominant mechanism in surface-layer mixing under light-to-moderate winds, it is minor under strong winds.

Solar Water Heaters

Solar energy water heaters are categorized as either active or passive. An active system requires a pump to drive the heated fluid through the system, whereas a passive system requires no external power. Distributed systems comprise a solar collector, hot water store and connecting pipework; they may be either active or passive. In the former, a pump actuated by temperature sensors via a control circuit is required to convey the fluid from the collector to the store. In a thermosyphon solar water heater, fluid flow is due to buoyancy forces occurring in a closed circuit comprising a (usually flat-plate) collector hot water store and the connecting pipework. These forces are produced by the difference in densities of the water in the collector (which is heated by the sun), and that of the cooler water in the store.

In a two-phase thermosyphon system the natural circulation circuit contains a fluid with a low boiling point at the pressure prevailing in the system. The liquid absorbs heat when passing through the collector and boils. The gas rises to a heat exchanger, where it gives up its latent heat to the storage medium and returns to the collector in liquid state to recommence the cycle.

An integral passive solar water heater comprises one or more tanks, painted black or applied with a selective absorber surface, within a well-insulated box, possibly with reflectors and covered with single, double or even triple glazing of glass or plastic or a transparent insulation material. The first integral passive solar water heaters, just bare tanks of water left out to warm in the sun, were used in some rural areas of southwest USA in the late 1800s. The first commercially manufactured solar water heater was a derivative of such systems, patented in Maryland, USA, in 1891.

To predict analytically the performance of solar energy water heaters, three alternative broad approaches can be identified; simplified models, correlation of performance characteristics from either the rigorous simulation or monitoring of generic systems and rigorous detailed simulation models.

The first two approaches are used to estimate a system's long-term performance and to determine the system size that achieves the optimum solar fraction of total hot water derived. Because of the simplifications inherent in the first approach, such models are limited by the range of operating conditions and system configurations over which the simplifying assumptions are valid. Models in this category often require experimentally determined information, which is only obtainable once the system has been constructed. The second approach cannot be applied reliably to these systems whose correlation of dimensions and climatic conditions has not been determined. For a thermosyphon system, the rate and the direction of the flow are dependent on prevailing weather conditions and on the geometry of the pipe-work. An exact mathematical model of the behavior of the latter system requires the simultaneous solution of the coupled energy and momentum equations.

Solar Drying

The objective in Drying an agricultural product is to reduce its moisture content sufficiently to prevent deterioration within a safe storage period. Drying is a dual process of heat transfer to the product from the heating source, and mass transfer of moisture from the interior of the product to its surface; then from the surface to the surrounding air.

In solar drying, solar energy is used as either the sole or a supplemental source of heat; air flow can be generated by either forced or natural convection. The heating procedure could involve the passage of preheated air through the product, by directly exposing the product to solar radiation or a combination of both. The major requirement is the transfer of heat to the moist product by convection and conduction from surrounding air mass at temperatures above that of the product, or by radiation mainly from the sun and to a little extent from surrounding hot surfaces, or conduction from heated surfaces in contact with the product. Water starts to vaporize from the surface of the moist product when the absorbed energy has increased its temperature sufficiently from the water vapor pressure of the crop moisture to exceed the vapor pressure of the surrounding air. The rate of moisture replenishment to the surface by diffusion from the interior depends on the nature of the product and its moisture content. If diffusion rate is slow, it becomes the limiting factor in the drying process, but if it is sufficiently rapid, the controlling factor may be the rate of evaporation at the surface. The latter is the case at the commencement of the drying process. In direct radiation drying, part of the solar radiation may penetrate the material and be absorbed within the product itself, generating heat in the interior of the product as well as its surface, thus hastening thermal transfer. The solar absorptance of the product is an important factor in direct solar drying, most agricultural materials have relatively high absorptances of between 0.67 and 0.09. Heat transfer and evaporation rates must be controlled closely for an optimum combination of drying rate and acceptable final product quality. Solar energy dryers vary mainly as to the mode of utilization of the solar heat and the arrangement of their major features, a classification with illustrative cross-sectional drawings is given in Figure 6.

Generic solar dryers.

Figure 6. Generic solar dryers.

Solar Refrigeration

The solar operation of conventional electrical refrigerators, working on a compression cycle requires the conversion of solar energy into electricity. The dc electricity produced from photocells usually can be used only to operate conventional refrigerators after being converted to ac electricity. An alternative solar operated refrigerator in which the refrigerator itself is also of "conventional" design, involves solar thermal conversion using high temperature solar energy collectors. The high temperature thermal energy produced may be transformed subsequently into mechanical energy via a heat engine, which then drives a refrigerator compressor. High temperature concentrating collectors most suited to this application need daily tracking of the sun.

Design and production of medium temperature solar energy thermal collectors is, however, a relatively simple technology. At most, these would require seasonal tilt-angle adjustments only. Intermittent vapor-absorption refrigeration plants work on a 24 h cycle comprising heating and refrigeration processes matched to the diurnal operation of the sun: undergoing heating process during the day and producing "cold" at night.

Porous solids, termed adsorbents, can physically and reversibly adsorb large volumes of vapor, termed the adsorbate. The concentration of adsorbate vapors in a solid adsorbent is a function of the temperature of the "working pair" (i.e., mixture of adsorbent and adsorbate) and the vapor pressure of the adsorbate. The dependence of adsorbate concentration on temperature, under constant pressure conditions, makes it possible to adsorb or desorb the adsorbate by varying the temperature of the mixture. This forms the basis of the application of this phenomenon in the solar-powered intermittent vapor sorption refrigeration cycle.

Water-ammonia has been the most widely used sorption refrigeration pair. The efficiency of such systems is limited by the condensing temperature. For example, cooling towers or desiccant beds have to be used to produce cold water to condense ammonia at lower pressure. Among the other disadvantages inherent in using water and ammonia as the working pair are: heavy gauge pipe and vessel walls are required to withstand the high pressure, the corrosiveness of ammonia, and the problem of rectification (i.e., removing water vapor from ammonia during generation).

One system employs solid absorption using calcium chloride as the absorbent and ammonia as the refrigerant. A reversible chemical reaction takes place when the refrigerant is absorbed by the solid absorbent. This results in physical changes in the mixture. When ammonia is absorbed into calcium chloride, swelling of the mass up to 400% takes place. To overcome this a small quantity of another salt was added to calcium chloride and then ammonia was mixed to prepare a paste, which was subsequently heated in a controlled manner to produce a new granulated absorbent. The heat of adsorption and desorption for the working pair is high; almost twice the latent heat of evaporation of ammonia, a large combined solar collector/absorber area is required.

An adsorbent-refrigerant working pair for a solar refrigerator requires the following characteristics:

  1. a refrigerant with a large latent heat of vaporization,

  2. a working-pair with high thermodynamic efficiency,

  3. a small heat of desorption under the envisaged operating pressure and temperature conditions and,

  4. a low thermal capacity.

In addition, the operating conditions of a solar-powered refrigerator (i.e., generator and condenser temperature) vary with its geographical location.

Solar refrigeration is employed primarily to cool vaccine stores. The need for such systems is greatest in peripheral health centers in rural communities of developing countries. In the absence of grid electricity, the vaccine cold chain can be extended to these areas through the use of autonomous solar-energy operated vaccine stores.

Passive Solar Design of Buildings

The environmental function of a building is to mediate between the external climate (with its seasonal and diurnal variation of temperature, illuminance and wind speed) and the more stable conditions, which are normally required for human comfort.

Climate conscious urban planning, passive solar heating, natural cooling and daylighting of buildings facilitate low energy consumption, comfortable internal conditions and a more benign effect on the wider environment.

Building design is subject to a diverse set of physical constraints (e.g., site, internal arrangement) and functional requirements (e.g., structure, use, circulation areas), successful passive solar design is reconciled harmoniously with these. In temperate climates passive solar design seeks usually to provide heating in cool weather while avoiding overheating in warm weather. Heating involves the distribution, storage and conservation of collected solar energy, overheating prevention involves shading and ventilation. Passive solar design reduces the auxiliary energy load (for heating, ventilating, cooling and/or lighting) and thus the running costs of the building and provides a pleasant, redundant with attractive, attractive internal environment, and, possibly, additional usable space. A purely passive solar building uses no additional energy to collect solar heat. Buildings can incorporate passive features, which have active elements, such as fans or moveable insulation; these are termed "hybrid". However, many building features, which are commonly regarded as "passive" systems transport energy to the point of use via small fans or motors.

Direct gain is a term used to refer to the system of a room with a southerly facing window. Using direct gain presents particular challenges which building designers must overcome if it is to be used effectively, these include: thermal discomfort due to radiant temperature asymmetry, glare problems, damage from u.v. light to fabrics and finishes and summertime overheating. Direct gain incurs little or no extra cost with simple, and virtually self-functioning, operation.

When solar gain is made to a space with no provision for auxiliary heating and then is distributed to the heated space, that space or element is termed an indirect gain feature. The heat distribution is usually by air convection or conduction through an adjoining wall or a combination of the two. Such unheated, occupiable, indirect gain features include conservatories, sunspaces and atria. Nonoccupiable indirect gain features include Trombe walls, mass walls and water walls. Natural circulation of air between the conservatory and the heated building occurs when windows or doors open into the conservatory. In some cases purpose-built ducts are provided to ensure a low resistance to air flow. Buoyancy-driven flow is induced by the temperature difference between the warm conservatory and the cooler adjacent room. In summer such action needs to be prevented in order to avoid overheating. If natural-circulation heat gain is to be encouraged, then "flap valves" have to be provided to prevent nocturnal and winter reverse flow when the temperature in the conservatory is less than in the heated building. There is no heat gain from the circulation of air between the conservatory and the building, until the temperature of the air in the conservatory is greater than the heated building temperature. In another indirect gain feature, the Trombe-Michel wall, the thermal coupling to the space to be heated is virtually the same as with conservatories, but the relative magnitude of the conductive and convective heat transfers are different, the conductive being larger in the case of a Trombe-Michel wall. The latter delays the delivery of solar heat. They are thus ideally suited to providing heating in the early part of a cold night after a hot sunny day, conditions encountered frequently in arid and mountainous regions. Trombe-Michel walls, which are simply solid glazed walls unlike conservatories provide no additional usable floor area.

Mass walls may be formed by the application of transparent insulation materials to the facade.

Isolated gain passive solar features are those that may be decoupled thermally from the building. This is accomplished via a thermally insulated separating wall, as in thermosyphoning air panels, or by location above the building, as in roof space collectors. A more controllable heat gain combined with—if well designed—an avoidance of summer overheating, is the primary advantage of isolated gain.

Isolated indirect elements do not provide heat storage unless it is insulated during periods of nonsolar-collection, or is remote with a controlled convective link, otherwise a net heat loss will ensue. Isolated gain collectors such as the thermosyphoning air panel (TAP) overcome some of the disadvantages of indirect gain collectors by dispensing with heat storage and relying totally on convective heat gain. Heat input is almost immediate while heat loss during nongain periods are low when the collector is isolated from the heated space. This design is ideally suited to the task of providing daytime heat in cool or cold climates. A TAP operates in the same manner as the natural-convection mode of a Trombe wall. However, the absorber is often made of metal, usually aluminum or steel, and the unit is insulated to prevent heat loss to, or from, the building. The problem most commonly associated with passive solar energy systems is control of the heat output. This is not a problem for a TAP as all that is required is for an inlet or exit vent to be closed and thermosyphoning ceases.

The Barra-Costantini system is a natural-convection dual-pass solar air heating system. It has the attributes of a Trombe-Michel wall though the heat storage is remote and may be decoupled from the collection of solar energy. A low thermal capacity dual-pass absorber solar collector is decoupled from the south wall of the building; the collector also acts as additional exterior insulation. Buoyancy-driven heated air is distributed throughout the building via channels within the ceilings. Thermal energy storage is provided within these ceilings.

A roof-space solar energy collector, is essentially a pitched roof which is partially of fully glazed on its southerly aspect. Solar-heated air from the roof-space collector is conveyed by an automatically controlled fan via a duct either directly into the living space or as a preheated supply to a warm-air space-heating system. When the air from the roof-space solar energy collector is a lower temperature than the set level of the room thermostat, the air stream emerging from the roof-space collector is a preheated supply to the gas-fired auxiliary system. A roof-space collector involves the passive collection and active distribution of solar heat and is thus generically a hybrid solar energy system. Ventilation is employed to prevent overheating in high summer. The advantage of a roof-space collector is that it has a low initial capital cost as its physical construction does not differ greatly from that of a conventional pitched roof.

Since passive systems are designed to maximize solar gains, there is a high risk of overheating, not only in the summer but also towards the end of the heating season when most systems should be operating at their maximum performance. The thermal discomfort of unwanted solar gains can be avoided by preventing the initial solar gain by using shading devices or by rejecting the solar gains by ventilating and/or absorbing the solar gain in thermal mass.

With fixed shading devices, the seasonal geometry of solar radiation permits some control of unwanted solar radiation. However, care must be given to the orientation, inclination and the geometry of fixed overhangs and fins. An important advantage of fixed shading devices is that they are self operating.

For a building where the solar input forms a significant proportion of the heating, a responsive control of solar gain is needed. This cannot be provided by fixing shading devices. In temperate climates, buildings have daily variations in heat demand within the same season. Indeed, a building may go from energy surplus to deficit within a few hours. Consider the latitude 52°N: though a fixed horizontal south-facing overhang 1 m wide will completely shade a window about 2 m high in midsummer, unfortunately it will also shade about 10% in midwinter and about 50% in the spring, a time when the performance of a passive system should be at its best.

Furthermore, in many climates, the annual variation in mean daily solar position and mean daily ambient temperature are not in phase. Though the solar motions are the same in September as in May, the corresponding ambient temperatures are not the same. For example, in England a fixed shading device that provides the shading desired in a warm September, unfortunately will also shade during the cooler May when solar gain is useful. Though movable shading devices do not suffer from this lack of response, they may present mechanical difficulties.

Shading devices also influence the view through glazing: an overhang, an opaque blind, a Venetian blind and solar control film may all reduce the solar gain of an aperture by the same amount, but they will alter the view through that aperture very differently.

The most efficient shading is provided by external devices (e.g. awnings), as the solar energy is rejected before it enters the collector. However, external shading devices are usually expensive since they have to be weatherproof. Weatherproofing has control implications; an awning must be withdrawn if the wind is strong even if it is sunny. The control linkage may also be difficult to install and maintain.

Indoor shading devices reflect the solar radiation which has passed through the glazing into the collecting element or zone, back out through the glazing. They are not as efficient as external shades because some of the radiation is reflected and scattered by the glazing back into the collector, and some of the radiation is absorbed onto the surface of the shading device. An important function of all types of shading devices is that they protect the occupants from direct radiation. Direct radiation elevates the effective temperature several degrees above air temperature, thus lowering the threshold temperature at which thermal discomfort is reached.

Ventilation must be considered as a second line of defence against overheating since heat can only be removed when the temperature of the building is already above ambient. To keep the temperature elevation above ambient small by ventilation alone, very large ventilation rates would be required. This may be inconvenient or in some cases impossible to attain. However, ventilation is an important complement to shading—particularly when it converts away gains made by absorption onto internal shading surfaces. Ideally in such circumstances these gains will be removed well away from the occupants. Ventilation is important in spaces with large areas of horizontal glazing such as atria or covered courtyards. If substantial openings at the top and inlets at ground level are provided, the ventilation can be induced by the "stack effect" even on days of zero windspeed. This vertical flow will prevent the build up of hot air in the upper zone of the atrium.

Solar Stills

Basin stills using single effect distillations have been used supplying large quantities of water for isolated communities or for supplying small amounts of water for functions such as battery charging. The conventional basin-type solar still consists of an insulated shallow basin lined or painted with a waterproof black material holding shallow depth (5 to 20 cm) of saline or brackish water to be distilled and covered with a single or double sloped glass sheet, sealed tightly to reduce vapor leakage, A condensate channel along the lower edge of the glass pane collects the distillate. The still can be fed with saline water either continuously or intermittently, but the supply is generally kept as twice the amount of fresh water produced by the still, depending on the initial salinity of the saline water.

Solar radiation transmitted through the transparent (cover) is absorbed in the water and basin and therefore water temperature becomes high compared to the cover. The water loses heat by evaporation, convection, and radiation to the cover and by conduction through the base and edges of the still. The evaporation of water from the basin increased the moisture content in the enclosure and condensation ensures on the underside of the cover, which collected the condensate channels.

Solar Cookers

Solar cookers are broadly of three types:

  1. Direct or focussing type.

  2. Indirect or box type.

  3. Advanced or separate collector and cooking chamber type.

The difference between each of them is as follows:

  1. Direct or focusing a solar energy concentrator focus of solar radiation on a focal area at which the cooking pot/pan is located. In these cookers the convection heat loss from cooking vessel is large and the cooker utilizes only the direct solar radiation.

  2. Slot box: in these cookers an insulated hot box (square, rectangular, cylindrical) painted black from inside with double glazing is used. To enhance the solar radiation plane sheet reflectors (single or multiple) are used. Here the adjustment of cooker toward the sun is not so frequently required as in case of direct type solar cooker. This is a slow cooker and takes a long time for cooking and many of the dishes cannot be prepared with this cooker.

  3. Indirect: In these cookers, the problem of cooking outdoors is avoided to some extent. The heat or the solar heat is directly transferred to the cooking vessel in the kitchen. The cookers use either a flat-plate or focussing collector which collect the solar heat and transfers this to the cooking vessel.

REFERENCES

Duffie, J. A. and Beckman, W. H. (1991) Solar Engineering of Thermal Processes, J. Wiley and Sons, New York.

Rabl, A. (1985) Active Solar Collectors and Their Applications, Oxford University.

Reddy, T. A. (1987) The Design and Sizing of Active Solar Thermal Systems, Clarendon Press, Oxford. DOI: 10.1016/0306-2619(88)90062-1

Norton, B. (1992) Solar Energy Thermal Technology, Springer, Heidelgurg.

Clarke, J. A, (1986) Energy Simulation in Building Design, Adam Hilger, London. DOI: 10.1016/0378-7788(86)90026-5

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