Solar thermal energy
Solar thermal energy is a form of energy and a technology for harnessing solar energy to generate thermal energy for use in industry, and in the residential and commercial sectors. Solar thermal collectors are classified by the United States Energy Information Administration as low-, medium-, or high-temperature collectors. Low-temperature collectors are generally unglazed and used to heat swimming pools or to heat ventilation air. Medium-temperature collectors are also usually flat plates but are used for heating water or air for residential and commercial use.
High-temperature collectors concentrate sunlight using mirrors or lenses and are generally used for fulfilling heat requirements up to / 20 bar pressure in industries, and for electric power production. Two categories include Concentrated Solar Thermal for fulfilling heat requirements in industries, and concentrated solar power when the heat collected is used for electric power generation. CST and CSP are not replaceable in terms of application.
Unlike photovoltaic cells that convert sunlight directly into electricity, solar thermal systems convert it into heat. They use mirrors or lenses to concentrate sunlight onto a receiver, which in turn heats a water reservoir. The heated water can then be used in homes. The advantage of solar thermal is that the heated water can be stored until it is needed, eliminating the need for a separate energy storage system. Solar thermal power can also be converted to electricity by using the steam generated from the heated water to drive a turbine connected to a generator. However, because generating electricity this way is much more expensive than photovoltaic power plants, there are very few in use today.
History
demonstrated a solar collector with a cooling engine making ice cream at the 1878 Universal Exhibition in Paris. The first installation of solar thermal energy equipment occurred in the Sahara approximately in 1910 by Frank Shuman when a steam engine was run on steam produced by sunlight. Because liquid fuel engines were developed and found more convenient, the Sahara project was abandoned, only to be revisited several decades later. As of 2023, the world's largest thermal solar power plant is in the United Arab Emirates.Low-temperature heating and cooling
Systems for utilizing low-temperature solar thermal energy include means for heat collection; usually heat storage, either short-term or interseasonal; and distribution within a structure or a district heating network. In some cases a single feature can do more than one of these things. Some systems are passive, others are active.Heating is the most obvious application, but solar cooling can be achieved for a building or for district cooling by using a heat-driven absorption or adsorption chiller. There is a productive coincidence that the greater the driving heat from insolation, the greater the cooling output. In 1878, Auguste Mouchout pioneered solar cooling by making ice using a solar steam engine attached to a refrigeration device.
In the United States, heating, ventilation, and air conditioning systems account for over 25% of the energy used in commercial buildings and nearly half of the energy used in residential buildings. Solar heating, cooling, and ventilation technologies can be used to offset a portion of this energy. The most popular solar heating technology for heating buildings is the building integrated transpired solar air collection system which connects to the building's HVAC equipment. According to Solar Energy Industries Association over 500,000 m2 of these panels are in operation in North America as of 2015.
In Europe, since the mid-1990s about 125 large solar-thermal district heating
plants have been constructed, each with over 500 m2 of solar
collectors. The largest are about 10,000 m2, with capacities of 7
MW-thermal and solar heat costs around 4 Eurocents/kWh without subsidies.
40 of them have nominal capacities of 1 MW-thermal or more. The Solar District Heating program has participation from 14 European Nations and the European Commission, and is working toward technical and market development, and holds annual conferences.
Low-temperature collectors
Glazed solar collectors are designed primarily for space heating. They recirculate building air through a solar air panel where the air is heated and then directed back into the building. These solar space heating systems require at least two penetrations into the building and only perform when the air in the solar collector is warmer than the building room temperature. Most glazed collectors are used in the residential sector.Image:Transpired Air Collector.PNG|thumb|Unglazed, "transpired" air collector
Unglazed solar collectors are primarily used to pre-heat make-up ventilation air in commercial, industrial and institutional buildings with a high ventilation load. They turn building walls or sections of walls into low cost, high performance, unglazed solar collectors. Also called, "transpired solar panels" or "solar wall", they employ a painted perforated metal solar heat absorber that also serves as the exterior wall surface of the building. Heat transfer to the air takes place on the surface of the absorber, through the metal absorber and behind the absorber. The boundary layer of solar heated air is drawn into a nearby perforation before the heat can escape by convection to the outside air. The heated air is then drawn from behind the absorber plate into the building's ventilation system.
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.
Solar roof ponds for solar heating and cooling were 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.
Solar space heating with solar air heat collectors is more popular in the USA and Canada than heating with solar liquid collectors since most buildings already have a ventilation system for heating and cooling. The two main types of solar air panels are glazed and unglazed.
Of the of solar thermal collectors produced in the United States in 2007, were of the low-temperature variety. Low-temperature collectors are generally installed to heat swimming pools, although they can also be used for space heating. Collectors can use air or water as the medium to transfer the heat to their destination. The sun's free energy can also be used to heat water to fulfil domestic hot water demands, such as the hot water that comes out of taps. Solar thermal water heating systems can provide approximately 50% of a property's annual hot water demand which in turn can help homeowners make savings on their energy bills.
Heat storage for space heating
A collection of mature technologies called seasonal thermal energy storage is capable of storing heat for months at a time, so solar heat collected primarily in Summer can be used for all-year heating. Solar-supplied STES technology has been advanced primarily in Denmark, Germany, and Canada, and applications include individual buildings and district heating networks. Drake Landing Solar Community in Alberta, Canada has a small district system and in 2012 achieved a world record of providing 97% of the community's all-year space heating needs from the sun. STES thermal storage mediums include deep aquifers; native rock surrounding clusters of small-diameter, heat exchanger equipped boreholes; large, shallow, lined pits that are filled with gravel and top-insulated; and large, insulated and buried surface water tanks.Centralized district heating round the clock is also feasible with concentrated solar thermal storage plant.
Interseasonal storage. Solar heat can be effectively stored between opposing seasons in aquifers, underground geological strata, large specially constructed pits, and large tanks that are insulated and covered with earth.
Short-term storage. Thermal mass materials store solar energy during the day and release this energy during cooler periods. Common thermal mass materials include stone, concrete, 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.