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Glossary · Solar

Solar thermal collector

A device that absorbs sunlight and transfers the heat to a fluid, mainly for domestic hot water, space heating, district heating and industrial process heat.

Solar thermal collectors convert sunlight directly into heat, with optical efficiencies far higher than PV converts it into electricity. The main non-concentrating types:

  • Flat-plate collectors: a selectively coated absorber in an insulated, glazed box; best for 30–80 °C.
  • Evacuated-tube collectors: absorbers in vacuum glass tubes that cut convective loss; better in cold climates and up to about 120 °C.
  • Unglazed collectors: plastic mats for swimming pools.
η = η0 − a1 · (Tm − Ta) / G − a2 · (Tm − Ta)² / G

Under ISO 9806, flat plates typically have an optical efficiency η0 of 0.75–0.82 and a1 of 3–4 W/m²K; evacuated tubes have lower η0 (0.6–0.75) but a1 around 1–1.5 W/m²K. Efficiency falls as the gap between collector and ambient temperature grows.

Global installed capacity is roughly 550 GWth, about three-quarters of it in China, mostly thermosiphon water heaters (IEA SHC, Solar Heat Worldwide). A residential hot-water system typically covers 50–70% of annual demand in central Europe and 70–90% in the Mediterranean, with thermal storage in a hot-water tank. Large collector fields feed district heating in Denmark, with seasonal pit storage. In buildings, solar thermal increasingly competes with PV driving a heat pump. Concentrating collectors used for power are covered under CSP.

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Related terms

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  • Concentrated solar power (CSP)

    Power plants that use mirrors to concentrate direct sunlight onto a receiver, heating a fluid that drives a steam turbine, usually with built-in thermal storage.

  • Thermal energy storage

    Storing energy as heat or cold, in sensible, latent (phase-change) or thermochemical form, for later use as heat or for power generation.

  • District heating

    A network of insulated pipes distributing hot water or steam from central heat sources to many buildings for space heating and hot water.

  • Heat pump

    A device that uses a refrigeration cycle to move heat from a cooler source (air, ground, water) to a warmer sink, delivering 2–5 units of heat per unit of electricity.

  • Irradiation (insolation)

    Solar energy received per unit area over a period, usually expressed in kWh/m² per day or per year; the time integral of irradiance.