Solar thermal collectors absorb the heat energy inherent in sunlight and use that energy to heat buildings. These systems are readily scalable for residential and commercial applications. Utility-scale installations employ mirrors and other means to concentrate solar thermal energy to create steam that drives powerful turbine-electric generators.
Thermodynamic solar panels use flat plate collectors to absorb solar heat energy and the thermal energy available from the ambient air and transfer that heat for beneficial reuse.
Solar Water Heaters
Solar water heaters are a cost-effective and environmentally friendly means to produce hot water for heating and domestic use in residential and commercial buildings, often reducing the need to operate fossil-fuel-fired or electrical heaters by 60% or more.
At their core, solar water heaters consist of a solar collector and a storage tank. The solar collector is commonly a flat-plate or tube-style configuration that concentrates the thermal solar energy to heat water directly (or indirectly using a heat-transfer fluid such as a glycol-water mixture). The heated water is stored in a well-insulated storage tank for use as demand requires.
Solar water heaters can be either passive or active systems.
Passive systems rely on gravity and the inherent convection circulation arising when heating water to supply a building's heating and domestic water requirements. Such systems are reliable and durable and require minimal maintenance.
Two options for passive systems are popular with users.
- Integral-collector storage systems incorporate one or several storage tanks enclosed within an insulated box with the transparent side of the box facing the sun. Such systems are best suited for warmer climates where sustained freezing is not a concern. As much of the heat energy is lost overnight, they may be incapable of meeting high early morning hot water demands.
- Thermosiphon systems are particularly well-suited for new home construction projects. These systems rely upon the convection motion that occurs when water is heated – warm water from the solar collector rises to the storage tank mounted above the collector, and cooler water in the tank descends to the bottom of the collector, completing the circulation route. For cold climate operation, indirect thermosiphon systems employ a mixture of glycol (antifreeze) and water as the heated medium, and the water lines are adequately insulated. For freeze-protection and aesthetic reasons, the storage tank is often located out-of-sight in the attic.
Active solar water heaters require electric motors and associated controllers to circulate heat-transfer liquids through the solar collector.
- Direct active circulation systems heat potable water pumped through the collector. They operate best where the water is not too hard or acidic and where warmer climates reduce the risk of prolonged freezing conditions. During freezing conditions, such systems may recirculate warm tank water through the system to avoid icing up.
- Indirect circulation systems are ideal for colder climates, where pumps force a water-glycol antifreeze mix through the solar collector, transferring the collected thermal energy to potable water in a heat exchanger. A food-grade glycol is commonly used. A drain-back feature ensures the antifreeze mix in the collector drains by gravity to its storage tank when the pumps shut off, preventing freezing and possible damage to exposed lines.
Solar Space Heating
Solar space heating uses the sun to heat the interiors of homes and buildings directly or indirectly. They can involve passive and active heating processes separately or combined to produce the desired effect.
Passive solar space heating incorporates large south-facing windows (in northern latitudes) and heat-absorbing products and materials to collect the thermal energy from the sun and deliver that heat to interior spaces.
- Direct gain systems use sunlight to heat interior floors and surfaces that release their stored heat during the day and night.
- Indirect gain systems employ walls to collect solar heat energy to heat interior spaces.
- Isolated gain systems employ the solar heat collected from a remote area, such as a sunroom attached to a home, relying on convection to carry the warm air to other rooms.
Active solar space heating systems use electric pumps and fans to distribute solar thermal energy to the interior spaces of a home or building. Fossil-fuel or electric heating systems are standard backups when solar thermal energy is insufficient.
Solar Space Cooling
Solar energy can cool homes and buildings in several ingenious ways. Solar-generated electricity can power conventional air conditioning equipment. Hybrid air conditioners employ solar thermal collectors to supplement the heat produced by the compression of low-pressure refrigerant to a hot vapor within the refrigeration cycle, reducing compressor run time and saving energy and wear on the mechanical system.
Other cooling options include:
- Solar open-loop air conditioning using desiccants – this method applies solid desiccants such as silica gel or zeolite or liquid alternatives like lithium bromide and lithium chloride to draw the moisture from air passing over them. Removing water vapor in this manner cools the air by evaporation. Solar thermal energy cyclically dehumidifies and regenerates the desiccants to sustain the cooling effect. A PV-powered motor can operate a low-speed circulation fan and a slowly rotating disk containing the desiccant.
- During cold weather, an energy recovery ventilation (ERV) system is a form of air-to-air heat exchanger using the warm interior exhaust air of a home to raise the temperature of incoming fresh and cold ventilation air (a process known as preconditioning). The same ERV conditions a home's interior during the summertime by using the cooler indoor exhaust air to lower the temperature of hot incoming ventilation air. A low-energy motorized fan powered by solar-generated electricity can supply the ventilation requirements. An ERV system can substantially reduce the cost of heating and cooling a home.
- Achieving passive solar cooling requires construction designs that slow the heat transfer rate into a building in the summer while efficiently eliminating unwanted heat. Incorporating heat-reflective colors and materials and perhaps a green (living) roof, selectively sizing and locating windows, and installing suitable insulation and air gaps within the wall and roof assemblies can drastically reduce the thermal energy load entering interior spaces. Passive cooling makes the most sense for new construction projects – retrofitting existing homes and buildings can be challenging and expensive.
Solar Pool Heaters
Solar pool covers act as simple water heaters by absorbing the sun's heat energy. They also act as insulation to retain the water's warmth.
Sophisticated solar pool heaters apply non-glazed or glazed solar collector panels and one or more pumps (these could be the existing pool circulation pump and control system) to circulate heated water from the collectors to the pool and back. Non-glazed collectors are commonly heavy-duty black UV-resistant plastic or rubber panels. More expensive glazed collectors are constructed of copper tubing on an aluminum backing plate with tempered glass. They are efficient devices often used to heat pools located in colder climates.