Performance of evacuated flat plate solar thermal collectors

Roger Moss,Stan Shire,Paul Henshall,Farid Arya,Philip Eames,Trevor Hyde

doi:10.1016/j.tsep.2018.09.003

Abstract

Heat losses from a flat panel solar collector can be significantly reduced by lowering the internal pressure to <0.5 Pa. Compared with conventional collectors, the resulting increase in efficiency is predicted to give a significant gain in annual heat output in the context of a temperate climate with low median irradiance.Two experimental collectors were built and tested to investigate theoretical aspects of evacuated flat plates and develop appropriate technological solutions. One had a metal tray to the rear, the other used two sheets of glass. An array of pillars supports the glass against atmospheric pressure. The experimental procedure was designed to minimise measurement uncertainty. Testing under a solar simulator, with and without a vacuum, showed a sudden drop in heat loss as the pressure was reduced below 0.5 Pa. When evacuated the heat loss coefficient fell from 7.43 to 3.65 W/m2 K and the efficiency at a nominal test condition of ΔT = 60 °C, G = 1000 W/m2 increased from 36% to 56%. Heat losses from absorber to glass were within 9% of the predicted level. This demonstrates that the heat loss mechanism is well understood.

Highlights

IntroductionThe research programme summarised in this paper was a collaboration between the University of Warwick, Loughborough University and Ulster University
The research programme summarised in this paper was a collaboration between the University of Warwick, Loughborough University and Ulster University. It aimed to deepen the theoretical understanding of stresses in vacuum enclosures, develop practical techniques for their fabrication, demonstrate the performance benefits and disseminate the resulting knowledge
The production of low temperature heat accounts for 47% of fossil fuel consumption and 25% of carbon dioxide emissions in the UK

Summary

Introduction

The research programme summarised in this paper was a collaboration between the University of Warwick, Loughborough University and Ulster University. It aimed to deepen the theoretical understanding of stresses in vacuum enclosures, develop practical techniques for their fabrication, demonstrate the performance benefits and disseminate the resulting knowledge. Two evacuated flat panel collectors were built and tested under a solar simulator. The production of low temperature heat accounts for 47% of fossil fuel consumption and 25% of carbon dioxide emissions in the UK. Solar thermal collectors can play a part in satisfying this demand. Solar collector efficiency falls as the fluid to ambient temperature difference increases and/or as illumination levels decrease: the highest possible efficiency is desirable if the collector is intended for use in cold conditions, on overcast days or with high delivery temperatures

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