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 storage or the point of use. Solar thermal systems are usually implemented with a water storage tank, to reduce the production-demand fluctuations, and have high operational safety, a great performance ratio, and can account for at least 25 to 30 years of lifetime.
Solar thermal collectors can be mainly divided into two types: tracking solar collectors and stationary . The first is able to track the sun concentrating it through lens and mirrors to the receiver using direct solar radiation. These collectors are suitable for medium to high-temperature applications and include parabolic trough collectors, linear Fresnel reflectors, parabolic dishes, and central receivers . Stationary solar collectors are, on the other hand, in a fixed position and are more suitable for low-temperature thermal energy (<100ºC). The most typical solar thermal stationary modules are Flat-Plate Collectors (FPC) and Evacuated Tube Collectors (ETC). This factsheet in particular focuses on ETC.
Components of evacuated tubes solar collectors
Source: Supankanok, R., Sriwong, S., Ponpo, P., Wu, W., Chandra-Ambhorn, W., & Anantpinijwatna, A. (2021). Modification of a Solar Thermal Collector to Promote Heat Transfer inside an Evacuated Tube Solar Thermal Absorber. Applied Sciences, 11(9), 4100.
In the ETC, the absorber plate and heat pipe are located in vacuum-sealed glass tubes to improve solar radiation absorption and reduce heat transfer losses, achieving a greater performance ratio 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, ETC has a greater performance ratio to its design with vacuum-insulated glass tubes. Moreover, ETCs are capable to work in hot, mild, cloudy, or cold climates while FPC has some limitations .
There are two types of ETC, the first one allows the heat transfer fluid to flow in and out of each tube absorbing directly the solar radiation, while the second feature a copper heat pipe inside the tubes as an efficient thermal conductor, which contains a small amount of non-toxic fluid as heat transfer medium that undergoes an evaporating–condensing cycle releasing flowing and releasing heat at the tip of the heat pipe to the heat transfer fluid.
Evacuated tubes solar collectors in rooftop installations
ETC, are usually combined with auxiliary systems (e.g. biomass boiler, heat pumps, etc.) to supply the needs when no solar radiation is available. ETC can be used for domestic applications for space heating and domestic hot water (DHW) supply (60-90ºC). For process heating in industries, ETC can reach higher temperatures (150ºC/180ºC) combined with high-temperature heat pumps. ETC combined with adsorption/absorption can be used for cooling supply. The main drawbacks are its mechanical fragility and mostly economic due to a greater initial investment cost .
ETC is a mature technology available in the market (TRL 9) suitable for domestic applications and able to operate in different range of temperatures. The most common application of ETC is solar heating to produce thermal energy domestic hot water (DHW) which represents more than 90% of the global market share of applications . In this case, ETC is included in a solar system, where collectors are usually coupled with a water storage tank (and a back-up system).
ETC can be also used in domestic applications to produce thermal energy for space heating or space cooling coupling them with an absorption chiller or liquid desiccant-based air conditioning systems. Solar thermal can also be employed in large-scale heating systems including district heating networks [5,6].
Another application where ETC can be integrated is pool heating or industrial applications to produce steam or heat for food processing or industrial product drying.