Solar thermal panels are devices that convert solar radiation into heat and transfer it to a heat transfer fluid (typically water or an anti-freeze fluid) to be stored or to reach the point of use. Solar thermal systems are usually implemented with a water storage tank, to reduce the production-demand fluctuations, ensure high operational safety and good performance, and guarantee at least 25 to 30 years of lifetime. The most common application is solar heating to produce thermal energy and domestic hot water (DHW) which represents more than 90% of the applications on the global market . In this case, solar thermal panels are included in a solar system, where collectors are usually coupled with a water storage tank. Solar thermal systems can also be employed in large scale heating systems including district heating networks [2,3]. Solar thermal collectors can be mainly divided in two types: tracking and stationary solar collectors .Tracking solar collectors are able to track the sun and concentrate the direct solar radiation through lenses and mirrors to the receiver. These collectors are suitable for medium-to-high temperature applications and include parabolic trough collectors, linear Fresnel reflectors, parabolic dishes, and central receiver systems . On the other hand, stationary solar collectors are installed in a fixed position and are more suitable for low-temperature thermal energy. The most typical solar thermal stationary modules are Flat-Plate Collectors (FPC) and Evacuated Tube Collectors (ETC).
FPC is the main solar collector technology installed in Europe with more that 2 million m2 installed per year . FPCs are suitable for low-medium temperature applications including domestic hot water, heating, preheating and combined systems. FPC consists of tubes carrying a heat transfer fluid placed in an insulated, weather-proof box with a dark absorber material and thermal insulation material. FPC can be installed as a thermosiphon system, in which the circulation of the heat transfer fluid is induced by the density difference caused by the increase in temperature, or as an active system with a pump . The simplicity of construction makes FPCs a relatively low-cost solution, which can be installed as single modules on roofs, or manufactured in a larger format for roof or facade integration or for ground-mounted systems. For the production of FPC, different materials can be used including copper, aluminium, staineless steel combined with a glazing to achieve a high efficiency.
In ETC systems, the absorbing plate and heat pipe are located in vacuum-sealed glass tubes to improve solar radiation absorption and reduce heat transfer losses, thus achieving a greater performance that allows heat production even in winter with low-light conditions and cold ambient temperatures. Both FPC and ETC are insulated to prevent heat losses, however ETC has a greater performance ratio(up to 80% ) due to its design with vacuum insulated glass tubes. Moreover, ETCs are suitable to hot, mild, cloudy or cold climates while FPCs have some limitations .There are two types of ETC systems. One type allows the heat transfer fluid to flow in and out of each tube absorbing directly the solar radiation, while in the other type, a copper heat pipe is used inside the tubes as efficient thermal conductor, which contains a small amount of non-toxic fluid as heat transfer medium that undergoes an evaporating–condensing cycle releasing heat from the tip of the heat pipe to the heat transfer fluid.
Solar thermal panels ares a mature technology available on the market (TRL 9) suitable for domestic applications and able to operate in different ranges of temperature. Solar district heating is also mature with more than 340 systems installed around the world.