Editing Solar thermal collector (section)

==General principles of operation==

A solar thermal collector functions as a heat exchanger that converts solar radiation into thermal energy.<ref name="Duffiep236">{{cite book |last1=Duffie |first1=John A. |last2=Beckman |first2=William A. |title=Solar Engineering of Thermal Processes |date=2013 |publisher=Wiley |page=236 |isbn=978-1-118-41280-0 }}</ref> It differs from a conventional heat exchanger in several aspects. The solar energy flux (irradiance) incident on the Earth's surface has a variable and relatively low surface density, usually not exceeding 1100 W/m<sup>2</sup> without concentration systems. Moreover, the wavelength of incident solar radiation falls between 0.3 and 3 μm, which is significantly shorter than the wavelength of radiation emitted by most radiative surfaces.<ref name="Duffiep236" />

The collector absorbs the incoming solar radiation, converting it into thermal energy. This thermal energy is then transferred to a heat transfer fluid circulating within the collector.<ref name="Kalogiroup240">{{cite book |last=Kalogirou |first=Soteris A. |title=Solar Energy Engineering: Processes and Systems |date=2004 |publisher=Academic Press |page=240 |isbn=978-0-12-397270-5 }}</ref> The heat transfer fluid can be air, water, oil, or a mixture including glycol (an antifreeze fluid), especially in forced circulation systems.<ref name="Kalogiroup240" /> Concentration systems may utilize phase change materials such as molten salts.<ref name="Steinhagen">{{cite book |last1=Müller-Steinhagen |first1=Hans |last2=Trieb |first2=Frank |title=Concentrating Solar Power and Desalination Plants |date=2004 |publisher=Elsevier |isbn=978-0-08-044495-6 }}</ref> The thermal energy of the heat transfer fluid can then be used directly or stored for later use.<ref name="Kalogiroup270">{{cite book |last=Kalogirou |first=Soteris A. |title=Solar Energy Engineering: Processes and Systems |date=2004 |publisher=Academic Press |page=270 |isbn=978-0-12-397270-5 }}</ref> The transfer of thermal energy occurs through convection, which can be either natural or forced depending on the specific system.<ref name="hoanggiangsolar">{{cite web |title=Sửa chữa máy nước nóng năng lượng mặt trời |url=https://hoanggiangsolar.com/sua-may-nuoc-nong-nang-luong-mat-troi/ |website=hoanggiangsolar.com |access-date=27 June 2023}}</ref><ref name="Duffiep478">{{cite book |last1=Duffie |first1=John A. |last2=Beckman |first2=William A. |title=Solar Engineering of Thermal Processes |date=2013 |publisher=Wiley |page=478 |isbn=978-1-118-41280-0 }}</ref>