Solar heating and cooling
The use of solar energy to produce heating or cooling fortechnological purposes. Beneficial uses include distillation of sea water toproduce salt or potable water; heating of swimming pools; space heating;heating of water for domestic, commercial, and industrial purposes; cooling byabsorption or compression refrigeration; and cooking. See also Solar energy.
Distillation
Production of potable water from sea water by solar distillation isaccomplished in several parts of the world by use of glass-roofed solar stills(see illustration). Production of salt from the sea has been accomplished forhundreds of years by trapping ocean water in shallow ponds at high tide andsimply allowing the water to evaporate under the influence of the Sun.See also Distillation.
Roof-type solar still.
Swimming pool heating
Swimming pool heating is a moderate-temperature application which, undersuitable weather conditions, can be accomplished with a simple unglazed anduninsulated collector. For applications where a significant temperaturedifference exists between the fluid within the collector passages and theambient air, both glazing and insulation are essential.
Space heating
Space heating can be carried out by active systems which use separatecollection, distribution, and storage subsystems, or by passive designs whichuse components of a building to admit, store, and distribute the heat resultingfrom absorbing the incoming solar radiation within the building itself.
Passive systems can be classified as direct-gain when they admit solarradiant energy directly into the structure through large south-facing windows,or as indirect-gain when a wall or a roof absorbs the solar radiation, storesthe resulting heat, and then transfers it into the building. Passive systemsare generally effective where the number of hours of sunshine during the wintermonths is relatively high, where moderate indoor temperature fluctuations canbe tolerated, and where the need for summer cooling and dehumidification ismoderate or nonexistent.
Active systems may use either water or air to transport heat fromroof-mounted south-facing collectors to storage in rock beds or water tanks.The stored heat may be withdrawn and used directly when air is the transferfluid. When the heat is collected and stored as hot water, fan-coil units aregenerally used to transfer the heat to air which is then circulated through thewarmed space. Standby energy sources are included in designs for activesystems, since some method of providing warmth must be included for use whenthe Sun's radiant energy is inadequate for long periods of time. The standbyheater may be something as simple as a wood-burning stove or fireplace, or ascomplex as an electrically powered heat pump. See also Heat pump.
Servicewater heating
Solar water heating for domestic, commercial, orindustrial purposes is an old and successful application of solar-thermaltechnology. The most widely used water heater, and one that is suitable for usein relatively warm climates where freezing is a minor problem, is thethermosiphon type. A flat-plate collector is generally used with a storagetank which is mounted above the collector. A source of water is connected nearthe bottom of the tank, and the hot water outlet is connected to its top. Adowncomer pipe leads from the bottom of the tank to the inlet of the collector,and an insulated return line runs from the top of the collector to the upperpart of the storage tank which is also insulated.
The system is filledwith water, and when the Sun shines on the collector, the water in the tubes isheated. It then becomes less dense than the water in the downcomer, and theheated water rises by thermosiphon action into the storage tank. It is replacedby cool water from the bottom of the tank, and this action continues as long asthe Sun shines on the collector with adequate intensity.
Forapplications where the elevated storage tank is undesirable or where very largequantities of hot water are needed, the tank is placed at ground level. A smallpump circulates the water in response to a signal from a controller whichsenses the temperatures of the collector and the water near the bottom of thetank. Heat exchangers may also be used with water at operating pressure withinthe tubes of the exchanger and the collector water outside to eliminate thenecessity of using high-pressure collectors.
Cooling
Cooling can be provided byboth active and passive systems.
The two feasible types of activecooling systems are Rankine cycle and absorption. The Rankine cycle system usessolar collectors to produce a vapor (steam or one of the fluorocarbonsgenerally known as Freon) to drive an engine or turbine. A condenser must beused to condense the spent vapor so it can be pumped back through thevaporizer. The engine or turbine drives a conventional refrigeration compressorwhich produces cooling in the usual manner. See also Rankine cycle; Refrigeration.
Passive cooling systems make use ofthree natural processes: convection cooling with night air; radiative coolingby heat rejection to the sky on clear nights; and evaporative cooling fromwater surfaces exposed to the atmosphere. The effectiveness of each of theseprocesses depends upon local climatic conditions. See also Solar cell.
