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Storing Heat in Liquid Systems - Liquid systems store solar heat in tanks of water or in the masonry mass of a radiant slab system. In tank type storage systems, heat from the working fluid transfers to a distribution fluid in a heat exchanger exterior to or within the tank. There are different types and configurations of heat exchangers available, which are not discussed in detail here.
Most storage tanks require one to two gallons (3.8 to 7.6 Liters) of water for each square foot (0.093 square meter) of collector area. Tanks are pressurized or unpressurized — which type is used depends on the overall system design. Before choosing a storage tank, you should consider several factors, including cost, size, and durability, where to place it (in the basement or outside), and how to install it. You may need to construct a tank on-site if a tank of the necessary size will not fit through existing doorways. Tanks also have limits for temperature and pressure, and must meet local building, plumbing, and mechanical codes. You should also note how much insulation is necessary to prevent excessive heat loss, and what kind of protective coating or sealing is necessary to avoid corrosion or leaks.
Specialty or custom tanks may be necessary in systems with very large storage requirements. They are usually stainless steel, fiberglass, or high temperature plastic. Concrete and wood (hot tub) tanks are also options. Each type of tank has its advantages and disadvantages. All types require careful consideration for their location, due to their size and weight. It may be more practical to use several smaller tanks rather than one large one. The simplest storage system option is to use standard domestic water heaters. They are designed to meet building codes for pressure vessel requirements, are lined to inhibit corrosion, and it is easy to attach pipes and fittings.
Distributing Heat for Liquid Systems - There are different ways to distribute the solar heat: with a radiant floor, with hot water baseboards or radiators, or with a central forced-air system. In a radiant floor system a solar-heated liquid circulates through pipes embedded in a thin concrete slab floor, which then radiates heat to the room. Radiant floor heating is ideal for liquid solar systems because it performs well at relatively low temperatures. A carefully designed system may not need a separate heat storage tank, though most systems do for temperature control. A conventional boiler or even a standard domestic water heater can supply backup heat. The slab is typically covered with tile. Radiant slab systems take longer to heat the home from a "cold start" than other types of heat distribution systems. Once they are operating, however, they provide a consistent level of heat. Carpeting and rugs will reduce the system's effectiveness.
Hot-water baseboards and radiators require water between 160° and 180° F (71° and 82° C) to effectively heat a room. Generally, flat-plate liquid collectors heat the transfer and distribution fluids to between 90° and 120° F (32 ° and 49° C). Therefore using baseboards or radiators with a solar heating system requires that either the surface area of the baseboard or radiators be larger, that the distribution liquid be heated further (with the backup system), and/or that a medium temperature solar collector, such as an evacuated tube collector, be used.
It is possible to incorporate a liquid system into a forced-air heating system, and there are different options for doing so. The basic design is to place a liquid-to-air heat exchanger in the main room-air return duct prior to it entering a conventional forced air heating system. Air returning from the living space is heated as it passes over the solar heated liquid in the heat exchanger. Additional heat is supplied as necessary by an additional heat exchanger or electric heating element in the conventional heater. The solar heat exchanger must be large enough to transfer sufficient heat to the air, especially when the heat exchanger liquid is at 90°F (32°C).
Controls
Controls for solar heating systems are usually more complex than those of a conventional heating system, because they have to analyze more signals and control more devices (including the conventional, backup heating system). Solar controls use sensors, switches, and/or motors to operate the system. The system uses other controls to prevent freezing or extremely high temperatures in the collectors.
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