Active Solar Space Heating

Solar Air Heating Systems

Solar air heating systems use air as the working fluid for absorbing and transferring solar energy.Solar air collectors (devices to heat air using solar energy) can directly heat individual rooms or can potentially pre-heat the air passing into a heat recovery ventilator or through the air coil of an air-source heat pump.

Air collectors produce heat earlier and later in the day than liquid systems, so they may produce more usable energy over a heating season than a liquid system of the same size. Also, unlike liquid systems, air systems do not freeze, and minor leaks in the collector or distribution ducts will not cause significant problems, although they will degrade performance. However, air is a less efficient heat transfer medium than liquid, so solar air collectors operate at lower efficiencies than solar liquid collectors.

Although some early systems passed solar-heated air through a bed of rocks as energy storage, this approach is not recommended because of the inefficiencies involved, the potential problems with condensation and mold in the rock bed, and the effects of that moisture and mold on indoor air quality.

Solar air collectors are often integrated into walls or roofs to hide their appearance. For instance, a tile roof could have air flow paths built into it to make use of the heat absorbed by the tiles. Air entering a collector at 70°F (21.1°C) is typically warmed an additional 70°–90°F (21.1°–32.2°C.). The air flow rate through standard collectors should be 1–3 cubic feet (0.03–0.76 cubic meters) per minute for each square foot (0.09 square meters) of collector. The velocity should be 5–10 feet (1.5–3.1 meters ) per second.

Most solar air heating systems are room air heaters, but relatively new devices called transpired air collectors have limited applications in homes.

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What are my Options?

What are my Options?

Room Air Heaters. Air collectors can be installed on a roof or an exterior (south facing) wall for heating one or more rooms. Although factory-built collectors for on-site installation are available, do-it-yourselfers may choose to build and install their own air collector.  A simple window air heater collector can be made for a few hundred dollars.

The collector has an airtight and insulated metal frame and a black metal plate for absorbing heat with glazing in front of it.  Solar radiation heats the plate that, in turn, heats the air in the collector.  An electrically powered fan or blower pulls air from the room through the collector, and blows it back into the room.  Roof-mounted collectors require ducts to carry air between the room and the collector.  Wall-mounted collectors are placed directly on a south-facing wall, and holes are cut through the wall for the collector air inlet and outlets.

Simple "window box collectors" fit in an existing window opening. They can be active (using a fan) or passive.  In passive types, air enters the bottom of the collector, rises as it is heated, and enters the room.  A baffle or damper keeps the room air from flowing back into the panel (reverse thermosiphoning) when the sun is not shining. These systems only provide a small amount of heat, since the collector area is relatively small.

Transpired Air Collectors. Transpired air collectors use a simple technology to capture the sun's heat to warm buildings.  The collectors consist of dark, perforated metal plates installed over a building's south-facing wall.  An air space is created between the old wall and the new facade.  The dark outer facade absorbs solar energy and rapidly heats up on sunny days—even when the outside air is cold.

A fan or blower draws ventilation air into the building through tiny holes in the collectors, up through the air space between the collectors and the south wall. The solar energy absorbed by the collectors warms the air flowing through them by as much as 40°F. Unlike other space heating technologies, transpired air collectors require no expensive glazing.

Transpired air collectors are most suitable for large buildings with high ventilation loads, a fact which makes them generally unsuitable for today's tightly sealed homes.  However, small transpired air collectors could be used to pre-heat the air passing into a heat recovery ventilator or could warm the air coil on an air source heat pump, improving its efficiency and comfort level on cold days.  No information is currently available on the cost effectiveness of using a transpired air collector in this way.

Liquid-Based Active Solar Space Heating. Solar liquid collectors are most appropriate for central heating. They are the same as those used in solar domestic water heating systems. Flat-plate collectors are the most common, but evacuated tube and concentrating collectors are also available. In the collector, a heat transfer or "working" fluid such as water, antifreeze (usually non-toxic propylene glycol), or other type of liquid absorbs the solar heat. At the appropriate time, a controller operates a circulating pump to move the fluid through the collector.

The liquid flows rapidly through the collectors, so its temperature only increases 10°–20°F (5.6°–11°C) as it moves through the collector. Heating a smaller volume of liquid to a higher temperature increases heat loss from the collector and decreases the efficiency of the system. The liquid flows to either a storage tank or a heat exchanger for immediate use.  Other system components include piping, pumps, valves, an expansion tank, a heat exchanger, a storage tank, and controls.

The flow rate through the collector should be between 0.02 and 0.03 gallons per minute per square foot of collector when water is the heat transfer fluid (0.82 to 1.22 liters per minute per square meter of collector).  Other flow rates apply for different heat transfer fluids. The total flow rate, used to size the collector pump, is the product of the above flow rate times the total collector area.

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.

Most storage tanks require 1–2 gallons (3.8–7.6 Liters) of water for each square foot (0.093 square meter) of collector area. Tanks are pressurized or unpressurized, and the type used depends on the overall system design. Before choosing a storage tank, you should consider several factors, including cost, size, 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 designed so 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 well designed system may not need a separate heat storage tank, though some 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.   They are also clean, silent and extremely comfortable.  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 solar-heated liquid be heated more with the backup system, 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, or heating coil, in the main room-air return duct prior to the furnace. 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 the furnace. The coil must be large enough to transfer sufficient heat to the air at the lowest operating temperature of the collector.