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SOLAR THERMAL WATER HEATING

Solar Thermal can be divided into two basic categories: Domestic Hot Water & Pool Heating.

Domestic Hot Water Systems

The principles involved in heating household water with the sun are not complex or new. However, state-of the art solar-heating equipment must be designed, installed and used properly to effect dollar and energy savings for the consumer.

The chief component of any solar water-heating system is the collector. Solar collectors absorb the radiant energy of the sun and change it into heat energy. The flat-plate collector is the predominant type used in solar domestic hot water (DHW) systems.

A typical flat-plate collector consists of a rectangular surface (absorber) with a series of fluid tubes running lengthwise along it. Liquid (in Florida, usually water) from the storage tank flows through the tubes. The absorber is usually composed of a good conductor material, such as copper or aluminum, and is coated black to absorb as much sunlight as possible. As the plate is heated, by exposure to sunlight, it transfers the heat to the liquid in the tubes.

The bottom and sides of the absorber are insulated to minimize heat loss. In addition, a translucent glass or plastic cover placed above it allows sunlight to strike the absorber while reducing the amount of heat that can escape.

In most systems, the collector is mounted on the roof of the dwelling, oriented to the south or within 45 degrees east or west of due south, and tilted between 20 and 40 degrees to horizontal. Systems may be ground mounted with the same orientation and tilt as roof-mounted systems.

When collectors are mounted on sloping roofs, they should be mounted parallel to the roof because of structural and aesthetic considerations. If the natural roof slope does not correspond to the optimum collector tilt, the collector still should be mounted parallel to the roof. The collector area may be increased if necessary.

The storage tank is the second major component in the DHW system. A conventional electric tank modified for solar service may be used, although specially designed, sized and super-insulated solar tanks are available.

In a pumped system, cold water is drawn from the bottom of the tank, circulated through copper piping to the rooftop collector, heated in the collectors, then returned to the storage tank. In a thermosiphon system, pumps and controls are not necessary. Here, the tank is elevated slightly above the collector. Cold water flows downward, entering the bottom of the collector; as the water is heated, it rises naturally and is returned to the tank.

The storage tank should be large enough to provide one day's hot water demand - i.e., 20 gallons per day per adult and 15 gallons per day per child. Standard tank sizes available are 40, 52, 66, 82, 100, and 120 gallons. When converting from electric water heating to solar, an additional tank may be added to provide the needed capacity, or the conventional tank may be replaced with a solar-designed tank. Solar tanks are constructed with more insulation and piping connections than the conventional tanks, although the conventional tank may be retrofitted with an external layer of insulation and extra ports.

Backup heating can assist during cloudy weather and periods of excessive hot-water consumption. In converted electric water heaters, this is accomplished by leaving the upper element connected (the lower element must be disconnected) and setting the thermostat at 120 degrees F. If your home is equipped with a dishwasher (without an automatic water heater feature), a setting of at least 130 degrees F is recommended. Tanks designed specially for solar are equipped with a single electric element in the upper portion of the tank. In both electric and solar tanks it is desirable to leave the electric current to the element turned off as much as possible to maximize solar contribution. A special switch may be installed in a convenient place for this purpose.

As a rule of thumb for Florida, the storage tank size (in gallons) should be between 1.5 and 2.5 times the size of the collector area. For example, a family of four using an average of 80 gallons on hot water a day would need an 82-gallon tank with between 32 and 54 square feet of collector area. However, individual calculations specific to the area and collector model are preferable. Several sizing procedures are available to the consumer and the installer. (Request GP-10 from the FSEC Public Information office.) Generally, a solar system will provide a range of 60 to 80 percent of a household's hot water needs, taking into consideration cloudy weather and periods of excessive hot-water consumption.

Systems that use pumps and controls to circulate the water usually have these two components mounted near or on the storage tank. The pump is a small (1/100 to 1/12 hp) circulating type made of bronze, brass or stainless steel. The controller (which regulates when and how long, and in some cases, how fast the pump operates) is usually a solid-state electronic device. It senses when he collector is able to heat the water in the tank and turns the pump on. When the water in the tank approaches the collector temperature (within 3-5 degrees), the controller turns the pump off.

Other devices are available to control the flow of water between the tank and collector. Timers are used to operate the pump in some systems, but you must exercise vigilance during unfavorable weather. A snap-switch control is also available. This device activates the pump when the collector heats to a specified temperature. The hot water in the collector is then returned to the tank, and cooler water is fed to the collector. A more recent addition to the array of available controllers is a photovoltaic (PV)-powered device. Sunlight striking the PV panel is converted to electricity, which powers the pump.

The controller also may be designed to provide freeze protection by turning the pump on to run warm water through the collector when its temperature falls near freezing. This type of freeze control results in a loss of hot water and will not work in the case of a power failure, which often accompanies freezing conditions. Other methods of freeze protection include manual and automatic draining of the collector, or use of a closed-looped rather than an open-looped system. Here, anti-freeze rather than water is used as the heat transfer fluid. Freeze protection is an essential feature to any solar system, even in usually warm climates.

Pool Heating Systems

Solar pool heating is an economically attractive solar technology. Swimming pools are abundant and most of them must be heated during the winter to maintain comfortable swimming conditions. Desired pool temperatures vary from a minimum acceptable level of 72 degrees F to a maximum of 110 degrees F, the latter for therapeutic use. For the average user, temperatures ranging from 78 degrees F to 80 degrees F for spring and fall and 76 degrees F for winter are considered comfortable.

The small differential between the average temperature of an unheated pool and the desired temperature for swimming allows the use of a very simple, efficient collector. These systems require no separate storage tank since the pool itself serves as the storage. In most cases, the pool's filtration pump is used to force the pool water through the solar panels or plastic pipes. However, in some retrofit applications an additional pump may be required to handle the needs of the new solar system.

The cool water inlet of a typical solar pool system is inserted into the circulation line between the filter and the pool. When adequate sunshine is available, the filtered pool water is circulated through the collector tubes, where it is heated by solar radiation and returned to the pool. All the water in the pool should be circulated through the filter about once every 8 to 12 hours.

Manufacturers have designed a variety of collectors for swimming pool heating. Most are constructed of black plastic material and consist of tubes running in a parallel fashion. Unlike the flat-plate collector used in domestic hot water systems, the pool collector is not covered by glass or plastic.

The larger the solar collector area, the greater the expected temperature increase. The rule-of -thumb for sizing a swimming pool collector system is that the collector area should be at least 50% of the pool-surface area, which will increase the temperature approximately 5 degrees F. Collector area double the pool surface area will increase the pool temperature roughly 14 degrees F. For an individual estimate, consult the FSEC pool-sizing guide (GP-??).

For smaller solar pool systems, the collectors should be mounted at an angle approximately equal to the local latitude. This will maximize solar collection during the spring and fall. For larger systems, where all year pool heating is desired, collectors should be mounted at a tilt angle equal to latitude plus 15 degrees to maximize winter collection. In most cases, however, pool collectors can be mounted most conveniently on a roof so that the slope of the roof determines collector tilt.

Ideally, collectors should face south. However, an orientation of 45 degrees east or west of due south will not significantly reduce performance, as long as the collectors are not shaded. Passive techniques can be incorporated in new pool construction to minimize the expense of supplementary pool heating. Their use can significantly reduce the amount of collector area needed. Three of the most effective passive methods are: siting the pool in the sun, reducing the wind velocity at the water surface, and using pool covers.

Since more than 75% of the solar energy striking the pool surface is absorbed by the water, your pool should be located to receive the maximum available sunlight. Note that screen enclosures reduce the amount of energy that strikes the pool surface. In this case, more heat energy will be required to maintain comfortable temperatures.

Windbreaks should be used wherever possible to reduce heat loss. Solid fences, tall hedges, buildings, trees and hills will protect the pool from wind. Take maximum advantage of these obstructions.

Pool covers can reduce heat loss due to evaporation and are effective in lengthening the swimming season. They also help keep the pool clean, thereby lowering the cost of chemicals and filter maintenance. Depending on materials and the number of hours of use, temperature increases of 5 degrees F to 10 degrees F may be expected from a pool cover. A 5 degree F temperature increase is reasonable when the cover is used 12 hours per day, or 10 degrees F when the cover is used 20 hours per day.

Lightly colored covers permit the entrance of solar energy. Black opaque covers are least effective in terms of collecting or conserving heat because light will not penetrate their surfaces. Covers are commercially available and can be expected to last three to five years.

Written by: Florida Solar Energy Center


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