Solar Systems

4. Solar Systems

4.1 Active and Passive Systems

An active solar system uses collectors to absorb the sun’s heat and needs mechanical components to transfer the heat to a storage system and to circulate it to supply buildings with hot water and space heating. The mechanical parts can be pumps, fans, or other controls.

A passive solar system for heating or cooling doesn’t require mechanical devices because the structure itself serves as a collector and storage medium. It relies on design features such as proper building and room orientation towards the sun, large south-facing windows, and insulating shutters and overhangs for summer shading to maximize solar gain in winter and minimize it in summer. Passive solar is best suited for new construction and space heating and cooling.

A solar greenhouse is one of the best passive heating systems for a house. Having numerous south-facing windows helps to heat a house too. Using passive solar heating combined with a solar electric system, and backed up by an active system, is a healthful alternative to using nonrenewable energy sources that create pollution.

4.2 Solar Water Heat

A passive solar water heater in one of its simplest forms consists of a tank painted black, mounted on a reflective surface and sealed into an airtight box that has a glazed front that lets the sun’s rays in to be absorbed into the black tank (black is the most heat-absorbent of all colors).

A recycled hot water tank can be painted black and used as a collector, resulting in an extremely lowcost solar water heater. The tank is tripped of its outer covering and surrounded by flexible plastic sheeting. The tank is then mounted on 3/4″ plywood covered with shiny metal sheets that reflect as much sun onto the tank as possible.

A typical flat-plate solar collector for heating water is made up of the following parts: the glazing is usually something like double strength window glass. The water tubes used to be made of copper; now usually aluminium or steel are used for economic reasons. The flat plate may be any metal (copper, aluminium, steel) that has good thermal conductivity and is reasonable in cost. The metal plate must be coated with a solar radiation-absorbing paint or plating. Flat black paint, properly applied to prevent peeling and cracking, will do a good job for ordinary domestic solar water heaters. The insulation may be any low-conductivity material available (usually something like glass wool) that can withstand temperatures up to 200°F. The casing holds the solar collector together and, together with the glazing, makes it water- and dust-proof. A simple wooden box, adequately painted and fitted with a hard-board base, will do. When water flows through the collector, it is heated, starting the solar cycle to work in your house.

One of the most widely-used passive designs for water heating is the thermosyphon hot water heater, which combines a flat-plate solar collector end an insulated water storage tank mounted high enough above the collector so that the cold water will go downward (heat rises, cold settles), where it will be heated by the collector and rise into the storage tank. This slow but continuous circulation continues as long as sun shines on the collector. In a good sunny location with no shadows, a 4′ x 8′ collector will give 40 to 50 gallons of hot water a day.

4.3 Solar Heating Systems

A simple and inexpensive air heater can be made with a cover glass (plastic film may also be used), a corrugated plate of sheet steel or aluminium painted black, a space through which the air can flow, a layer of insulation and a Masonite or plywood backing to keep the assembly waterproof. The air can be made to flow by a fan or blower, or, if the system is properly designed, it will rise due to convection (the “chimney effect”) because the heated air is lighter than the cold air outside.

Air heaters are less expensive than water heaters used to heat air (not the same as the solar hot water heaters just discussed), because there is no need to worry about freezing, and any leakage which occurs will not cause the kind of damage water can create. The pumps used may be larger, more expensive and more power-consuming than those used with some solar water heating systems, though. Also, the ducts used to carry the air are larger and more costly than the pipes used with water systems. Each type has its advantages and disadvantages.

The simplest of all solar air heaters uses a heavy south-facing concrete wall painted a dark color and covered with a sheet of glass. An air space runs between the concrete and glass, and the chimney effect causes the heated air to rise. Openings at the top and bottom of the wall let cold air enter the air space and warm air to reenter the room. The air then circulates around the room. Small electric baseboard heaters can be used for heat during long periods of bad weather.

Solar space heating may be accomplished in many ways, but one must first estimate how much heat the structure will need during adverse winter conditions and at night. The solar heater must be able to provide not only heat during the sunny days but also have additional capacity for heat storage.

The best, method presently available for storing heat ,or cold in large amounts is large water tanks filled, or almost filled, with water. We can store about three times as much warmth as cold, since we cannot use such a large temperature range with cold without running into the complication of freezing the water. The mechanics of storing heat in water are simple and water is available almost everywhere.

Heat can also be stored by means of rock beds. These can’t freeze or leak, but their capacity is limited. However, they can be safely used under a building since not much can happen to them once they’re put in place.

In considering all these options for solar systems, we must remember that the space of a lesson does not permit in-depth construction details—there are hundreds of books on solar technology of all sorts, and one must refer to other sources in order to learn the specifics. There have been many, many experiments made with various building materials, designs, and theories, and there are always several methods available for arriving at the same effect, whether this be heating, cooling, or whatever. An individual must determine what best meets his needs as to what’s best for his climate, living structure and finances.

4.4 Solar Cooling Systems

There is no way to use the heat of the sun directly to produce cooling, however, we can use the heat to produce hot water or steam, and with that we can refrigerate, using the process known as absorption refrigeration. This was first discovered in 1824, then, about 100 years later, this principle was used for household refrigerators.

Cooling can be achieved with the aid of a humidifier and by controlling the heat radiation of the thermal mass. The thermal mass itself can be used for cooling during the summer by opening the windows and exposing it to the cool evening void. The stored heat is then radiated back to the depths of space. One way to cool a building which is tight and well-insulated is to close it up during the day. This is done with massive adobe houses. Insulated shutters, thermal curtains, or window quilts can help to keep the heat out and the coolness in.

An example of solar cooling is the “Sytherm Systems” developed by Harold Hay. These systems have large water containers on the roof that are cooled at night and keep the building cool during the day. During the winter days, they collect warmth and radiate it into the dwellings at night.

Shade roofs are roofs with extremely large overhangs and will cool a building; they are especially good in the tropics. Placement of windows to allow breezes through a structure is also helpful in cooling a building. Perhaps the very best way to keep a building cool is to build it underground in the layer of the earth that is always naturally cool in the summer.

4.5 Solar Electricity

About a century ago, a Frenchman, Becquerel, found that sunlight could produce minute amounts of electricity when it entered a very special kind of “wet cell” battery. Later, other workers found that sunlight could change the resistance of certain metals and that very small amounts of electricity would be generated when sunlight illuminated discs of selenium or certain types of copper oxide. These devices were useful as light meters but didn’t produce enough power to do anything more than move a pointer on a meter or activate a very sensitive relay. In 1954, a new treatment for ultra-pure silicon was discovered which gave it the property of generating electricity from sunlight with a conversion efficiency of 6%. This was 10 times better than any previous efficiency for the direct conversion of sunlight into electricity, and the invention was immediately applied to a small transistorized radio transmitter and receiver.

In 1957, the space program found a unique application for the silicon solar battery. NASA put silicon cells on its first permanent satellite, and they worked so well that all but one of the satellites orbited since that time have been powered by increasingly complex arrays of silicon solar cells. Communication satellites use tens of thousands of these cells. Each cell alone contributes a small amount of power, but silicon cell technology has advanced so rapidly that tens of thousands of individual cells can be connected together, rapidly and reliably. Today the communications satellite has become the standard means of intercontinental communication for voice, television, and even computer language.

The cost is still a bit high for installation of great panels of solar cells on every rooftop, but great strides are being made, and the cost has already been reduced from $1,000 per watt to $20 per watt with more reductions on the way. Ways of producing less expensive silicon cells are being intensely studied. Some specialists say the cost will come down to $2 per watt within another decade.

Solar cells come in a wide variety of sizes. There are larger units for supplying large amounts of power, and small photovoltaic devices to supply operating power for devices such as electronic watches, calculators, and flashlights. These small solar devices are called microgenerators, and are actually made up of several extremely small solar cells connected in series.

A solar electric system has no moving parts and usually requires little, if any, maintenance. The two main considerations in the design of any solar electric power system are, first how much sunlight is available at the proposed site and how it varies with the seasons of the year (this tells us the size of the solar electric generator needed to supply any given amount of power), and the second consideration is the characteristics of the load including the average current requirement and the duty cycle (this tells us how much storage battery capacity we will need to keep the system operating when sunlight isn’t available).

Solar cells can be used in radio and television, in agriculture (for irrigation, pumping water, charging storage batteries at remote locations, etc.), at construction sites where electricity isn’t yet available, in remote areas, for work or recreation, and so on.

Solar arrays should face due south, but “trackers” have been developed, whereby the solar panels are mounted so that they can move, so as to remain pointed in the correct direction at all times for maximum sunlight. A small sensor on the array provides electrical signals that tell the control system which way to turn the array to get the most sun.

4.6 Solar Water Distillers

A solar water distiller consists of a water-tight compartment painted black to absorb the solar radiation which enters through the glass roof of the still. Water which is brackish or impure flows through the box in a four- to six-inch deep channel, where the intense solar heat in the box forces the water to evaporate and to condense on the inner side of the roof where it is drained off to a holding tank. The end result is pure drinking water.

The ocean rescue still, developed in 1940 by Dr. Maria Telkes, can be used to make drinking water from ocean water.

Dr. M. Kobayashi of Tokyo developed a solar still that could extract water from virtually any kind of soil, and tested it at the top of Mt. Fuji where the soil is volcanic ash and in the arid deserts of Pakistan, and he has never failed to produce water that is pure and potable.

4.7 Solar Food Dryers

Solar energy has always been used to dry crops of fruits and vegetables. Essentially this was done by exposing the food to the sun’s rays and hoping it wouldn’t rain. A more “sophisticated” technology has evolved to use the sun’s thermal power and minimize contamination from dust and airborne debris, insects and their larvae, and animal or human interference. The drying area must be covered with a transparent material. A drying “hot box” is constructed and insulated (glass wool is preferred since it can survive any temperature and does not support insect, life). Ventilation holes at the top and bottom allow air to enter and carry away the moisture. An access door makes loading and unloading easier. The interior of the cabinet should be painted black and the exteriors of the side and rear panels painted with aluminium paint. Drying trays can be made with galvanized wire mesh. Where electricity is available, a small fan may be used to draw air through the dryer, but it is not necessary.

The dryer should be glazed, preferably with two layers of glass, fitted in with adequate room for thermal expansion. Ventilation is essential so that moisture can escape and can be provided by screened air holes in the bottom, sides, and back of the cabinet. Such a dryer can keep produce dry during rain storms, so the glazed top should be watertight. The ventilation is also needed to prevent overheating, since a hot box of this type can readily attain temperatures above 200° F.

4.8 Solar Cookers

The first solar cooker was probably the one built in Bombay in 1880, and several other ingenious ovens have originated in India, as well as in other areas of the world. We won’t go into detail since the Hygienic way of life doesn’t advocate cooking food, but the student must at least be aware that the technology is available—even if we don’t cook, we all know people who do. Solar cooking is cleaner than gas cooking, which sends its toxic combustion products into the room.