The concept of an evacuated flat plate (EFP) collector was proposed over 40 years ago but, despite its professed advantages, very few manufacturers have developed commercial versions. This situation suggests both technical difficulties in manufacturing a competitively-priced sealed for life panel and a lack of awareness of the benefits of such panels. This paper demonstrates an evacuated flat plate simulation that closely models experimental efficiency measurements. Having established the validity of the model, it compares published data for a commercial EFP collector with predictions for an optimal design to investigate whether any further efficiency improvement might be possible. The optimised design is then evaluated against alternative solar energy devices by modelling a number of possible applications. These comparisons should inform choices about solar options for delivering heat: EFP collectors are well-suited to some of these applications. An evacuated flat plate solar thermal collector with a 0.5x0.5 m absorber was tested under a solar simulator. The test conditions spanned the range 200<G<1000 W/m², 0 ≤ TM ≤ 52°C. Evacuating the enclosure reduced the measured heat loss coefficient by 3.7 W/m2K: this was a close match to predictions and corresponds to an increase in aperture efficiency from 0.3 to 0.6 at TM/G = 0.06 m²K/W. Further model predictions incorporating commercialsolar coating propertiessuggest that evacuated flat plate collectors should have a 50% greater efficiency than conventional flat platesfor an absorber mean surface temperature 100°C above ambient. As a measure of installed performance, the mean annual heat output was predicted by a transient analysis scheme using efficiency curves, absorber heat capacity and historical weather data for Coventry, UK. An optimised evacuated flat plate collector could supply 104% more heat to an 85°C district heating main than a conventional flat plate. The availability parameter increased by 120% due to the reduction in critical radiation level. In Winter if the necessary absorber temperature is above 58°C an optimised EFP collector requires the least panel area to meet a heating demand. Conversely if temperatures below 58°C can be utilised a smaller panel area is possible using a heat pump powered by a PVT panel. A further option would be to replace each 1m² of PVT panel with 0.3m² of PV panel in a country receiving more solar radiation. Evacuated flat plate collectors are a possible alternative to concentrating collectors for Organic Rankine Cycle power generation. The annual output for all the modelled collectors was found to be a quadratic function of delivery temperature: this enabled a novel optimisation of ORC source temperature. Predictions for concentrating and non-concentrating ORC plant are compared with a PV/thermal alternative. The ORC output is significantly less than a PV panel would achieve; applications needing both heat and power are better served by PVT panels. This is an original and novel result.
Moss, R.W.Henshall, PaulArya, F.Shire, G.S.F.Hyde, T.Eames, P.C.
Faculty of Technology, Design and Environment
Year of publication: 2018Date of RADAR deposit: 2018-01-18