Thermal Advantages for Solar Heating Systems with a Glass Cover with Antireflection Surfaces

摘要

Experimental and theoretical investigations elucidate how a glass cover with antireflection surfaces can improve the efficiency of a solar collector and the thermal performance of solar heating systems. The transmittances for two glass covers for a flat-plate solar collector were measured for different incidence angles. The two glasses are identical, except for the fact that one of them is equipped with antireflection surfaces by the company SunArc A/S. The transmittance was increased by 5-9%-points due to the antireflection surfaces. The increase depends on the incidence angle. The efficiency at incidence angles of 0° and the incidence angle modifier were measured for a flat-plate solar collector with the two cover plates. The collector efficiency was increased by 4-6%-points due to the antireflection surfaces, depending on the incidence angle. The thermal advantage with using a glass cover with antireflection surfaces was determined for different solar heating systems. Three systems were investigated: Solar domestic hot water systems, solar heating systems for combined space heating demand and domestic hot water supply, and large solar heating plants. The yearly thermal performance of the systems was calculated by detailed simulation models with collectors with a normal glass cover and with a glass cover with antireflection surfaces. The calculations, which were based on the measurements mentioned above, were carried out for different solar fractions and temperature levels of the solar heating systems. These parameters influence greatly the thermal performance associated with the antireflection surfaces. For increasing solar fraction the thermal advantage decreases, whereas for increasing temperature level the thermal advantage increases. For instance, for small solar domestic hot water systems the extra yearly thermal performance by using antireflection glass as the cover plate for the collectors is 10% and 4%, respectively, for yearly solar fractions of 25% and 60%, respectively. For solar heating plants the extra thermal performance with collectors using antireflection glass is 12% and 20%, respectively, for a mean solar collector fluid temperature of 60°C and 100°C, respectively.