Design, development, and experimental characterisation of a novel building integrated solar thermal vacuum flat plate collector

Solar thermal flat plate and evacuated tube collector systems have been developed and used as an efficient low carbon heat source for water heating in domestic applications up to the temperatures in the range of 60 to 80° C for many years. At higher temperatures beyond 150° C, the standard collectors used have low efficiency, typically less than 20% at 1000 W/m² solar irradiance, due to the increased rate of heat loss from the collector restricting the range of potential applications. This thesis reports the design, development, and experimental characterisation of a novel solar thermal collector capable of operating efficiently in the temperature range of 150 to 200°C.

A novel design for a vacuum flat plate solar thermal collector was developed composed of a compound parabolic concentrator (CPC) with a tubular absorber, metal enclosure, and glass aperture cover. An initial prototype vacuum enclosure was developed in which a set of thin parallel vertical supports were used to support the glass aperture cover when evacuated to a pressure of 0.1Pa. FE modelling was used to predict the deformation and stresses in the glass and support structure to understand the structural design safety limits when evacuated to a pressure of 0.1Pa. Experimental validation was achieved by comparison to measurements obtained using digital image correlation (DIC) on the initial developed prototype.

Radiative heat transfer from a body to its surrounding is proportional to the fourth power of the absolute temperature and is governed by the Stefan-Boltzmann law. At higher temperatures, greater radiative heat losses occur from component surfaces in the infra-red wavelengths. In the developed vacuum solar collector assembly, significant radiative heat losses from the absorber were predicted to occur due to the high operating temperatures, 150 to 200° C. In this research, to reduce the radiative heat losses from the absorber surface, the surface properties of the absorber were modified by developing and applying a solar selective absorption coating using binary metal salts dissolved in an aqueous solution along with chelating and esterifying agents. A dip coating technique was used for the absorber coating process with more than 90% absorption in the visible wavelengths with less than 10% emission in the infra-red wavelengths achieved.

A prototype vacuum flat solar thermal collector incorporating an array of parallel CPC troughs with an aperture size of 500mm x 500mm was manufactured, assembled, and characterised in the lab with the heat loss coefficient (U-value) of the collector for different fluid inlet temperatures from 60° C to 160° C determined. The heat losses from the collector to ambient increased non-linearly with the increase in the fluid inlet temperature. Based on the experimental measurements, the U value of the collector was calculated to be 3 W/m2.K when operating at a temperature of 160° C. The measured U-value of the collector was compared to values determined using an analytical thermal network analogy and a CFD simulation including a Monte Carlo radiation model. From the U values calculated at different fluid inlet temperatures, the efficiency of the solar collector was calculated to be 46% when operating at 160° C assuming 1000 W/m² solar beam irradiance. The efficiency values have been extrapolated considering the collector operating temperature up to 200° C and the calculated value of efficiency at this temperature was 36%.

Comparing the efficiency of the novel collector with the evacuated tube collector (Enersol HP 70-24) manufactured by EnerTech GMBH™, the increase in the efficiency was around 80% while operating in the range of 150 to 200° C. And the efficiency has been doubled compared to the traditional flat plate collector with double glazing (SF500-15DG) manufactured by Savosolar™. However, the evacuated flat plate collector manufactured by TVP solar™ has shown similar improvement in the efficiency at high temperature operating range. The efficiency of the novel collector is approximately 10% higher than the TVP solar™ collector in the desired operating range of the temperatures.

This research shows that the flat evacuated solar thermal collector with a CPC developed in this research could have potential applications in the decarbonisation of industrial processes requiring heat in the range of 150 to 200° C. It could also be used for charging portable thermal storage units operating in this temperature range. Future work is required to further improve the optical efficiency of the collector with enhanced vacuum retention techniques to increase the life span of the collector.