Solar thermal

Solar thermal applications make up the most widely used category of solar energy technology. These technologies use heat from the sun for water and space heating, ventilation, industrial process heat, cooking, water distillation and disinfection, and many other applications.

Solar water heaters face the equator and are angled according to latitude to maximize solar gain.

Solar hot water systems use sunlight to heat water. Commercial solar water heaters began appearing in the United States in the 1890s. These systems saw increasing use until the 1920s but were gradually replaced by relatively cheap and more reliable conventional heating fuels. The economic advantage of conventional heating fuels has varied over time resulting in periodic interest in solar hot water; however, solar hot water technologies have yet to show the sustained momentum they had until the 1920s. Recent price spikes, erratic availability of conventional fuels, and other factors are renewing interest in solar heating technologies. Approximately 14 percent (15 EJ) of the total energy used in the United States is for water heating. In many climates, a solar heating system can provide 50 to 75 percent of domestic hot water use.

As of 2005, the total installed capacity of solar hot water systems is 88 GWth and growth is 14 percent per year. China is the world leader in the deployment of solar hot water systems with 80% of the market. Israel is the per capita leader in the use of solar hot water with 90 percent of homes using this technology. In the United States heating swimming pools is the most successful application of solar hot water.

Solar water heating technologies have high efficiencies relative to other solar technologies. Performance will depend upon the site of deployment, but flat-plate and evacuated-tube collectors can be expected to have efficiencies above 60 percent during normal operating conditions. In addition, solar water heating is particularly appropriate for low-temperature (25-70 °C) applications such as swimming pools, domestic hot water, and space heating. The most common types of solar water heaters are batch systems, flat plate collectors and evacuated tube collectors.

MIT's Solar House #1 built in 1939 utilized seasonal thermal storage for year round heating.

Heating, cooling and ventilation (HVAC) systems of buildings are closely interrelated. All seek to provide thermal comfort, acceptable indoor air quality, and reasonable installation, operation, and maintenance costs. Conventional HVAC systems account for roughly 40 percent of the energy used in the United States and European Union. Many solar heating, cooling, and ventilation technologies can be used to offset a portion of this energy.

Thermal mass materials store solar energy during the day and release this energy during cooler periods. Common thermal mass materials include stone, cement, and water. The proportion and placement of thermal mass should consider several factors such as climate, daylighting, and shading conditions. When properly incorporated, thermal mass can passively maintain comfortable temperatures while reducing energy consumption. More advanced thermal mass systems can be also be used for ventilation.

A solar chimney (or thermal chimney) is a passive solar ventilation system composed of a hollow thermal mass connecting the interior and exterior of a building. As the chimney warms, the air inside is heated causing an updraft that pulls air through the building. These systems have been in use since Roman times and remain common in the Middle east.

A Trombe wall is a passive solar heating and ventilation system consisting of an air channel sandwiched between a window and a sun-facing thermal mass. During the ventilation cycle, sunlight stores heat in the thermal mass and warms the air channel causing circulation through vents at the top and bottom of the wall. During the heating cycle the Trombe wall radiates stored heat.
Transpired air collectors are highly efficient and cost effective.

Solar roof ponds are a unique solar heating and cooling technology developed by Harold Hay in the 1960s. A basic system consists of a roof mounted water bladder with a movable insulating cover. This system can control heat exchange between interior and exterior environments by covering and uncovering the bladder between night and day. When heating is a concern the bladder is uncovered during the day allowing sunlight to warm the water bladder and store heat for evening use. When cooling is a concern the covered bladder draws heat from the building's interior during the day and is uncovered at night to radiate heat to the cooler atmosphere. The Skytherm house in Atascadero, California uses a prototype roof pond for heating and cooling.

A transpired air collector is a perforated sun-facing wall. The wall absorbs sunlight and pre-heats air as much as 22 °C as it is drawn into the ventilation system. These systems are highly efficient (up to 80 percent) and can pay for themselves within 3 to 12 years in offset heating costs.

Active solar cooling can be achieved via absorption refrigeration cycles, desiccant cycles, and solar mechanical processes. In 1878, Auguste Mouchout pioneered solar cooling by making ice using a solar steam engine attached to a refrigeration deviceThermal mass, smart windows and shading methods can also be used to provide cooling. The leaves of deciduous trees provide natural shade during the summer while the bare limbs allow light and warmth into a building during the winter. The water content of trees will also help moderate local temperatures.