Abstract
The concept of an evacuated flat plate collector was proposed over 40 years ago but, despite its professed advantages, very few manufacturers have developed commercial versions. This paper demonstrates the reduction in heat loss coefficient and increase in efficiency resulting from evacuating a flat plate collector: it is hoped that these results will stimulate interest in the concept. Evacuated tubes are now mass-produced in large numbers; evacuated flat plate collectors could in principle replace these tubes if the technical difficulties in creating extended metal-glass seals can be overcome. The experimental experiences described here should indicate targets for future research.Two different designs of evacuated flat plate solar thermal collector, each with a 0.5 × 0.5 m flooded panel black chrome plated absorber, were tested under a solar simulator. The cover glasses were supported by an array of 6 mm diameter pillars. Inlet and outlet temperatures were monitored via PT100 RTDs and glass temperatures were measured using thermocouples. Inlet temperature was controlled by a fluid circulator connected to a header tank with a Coriolis mass flow meter to measure fluid flow rate. Testing was conducted indoors with and without the use of a fan to cool the top cover glass. 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/m2 K: this was a close match to predictions and corresponds to an increase in aperture efficiency from 0.3 to 0.6 at TM/G=0.06m2K/W. The poor efficiency under non-evacuated conditions was due to the black chrome absorber coating being less selective than commercial panel coatings.The solder seals were developed from experience with vacuum glazing but the increased gap led to reliability issues. A vacuum pump maintained the enclosures under a high vacuum (<0.1 Pa) during testing. The enclosure based on a thin rear metal tray proved to be more effectively sealed than the more rigid enclosure with glass on both sides: the latter developed leaks as the front to rear temperature difference increased. The biggest challenge in the manufacture of evacuated flat plate collectors is to ensure a long-term hermetic seal such that no pumping is required.
Highlights
The flat covers on Evacuated flat plate (EFP) collectors are more attractive than bundles of evacuated tubes and, combined with the high efficiency, make them suitable for integration into roofs or building facias
Used flat plate collector materials such as flexible sealants are unsuitable for high vacuum conditions and the mechanical and thermal properties of alternative, vacuum-compatible materials introduce a number of design challenges
There was no illumination so the net heat flux into the collector was negative
Summary
Evacuated flat plate (EFP) solar thermal collectors are anticipated to combine the high fill factor, ease of cleaning and visual aesthetics of flat plate collectors with the low heat loss coefficient of evacuated tubes. An array of ribs or pillars is required to support the glass cover against atmospheric pressure loading Such collectors can operate efficiently in low illumination conditions and achieve “medium” to “high” delivery temperatures for industrial applications, a field that has recently attracted interest. Cg d h k ṁ p r w tg ηA η0,τα specific heat capacity of glass (J/kg K) absorber-glass gap (m) heat transfer coefficient (W/m2 K) metal conductivity (W/m K) fluid mass flow rate enclosure internal pressure (Pa) radius (m) for radial conduction glass mass/unit area (kg/m2) glass time constant (s) efficiency based on absorber area transmission-absorbance product collectors can absorb diffuse light and operate without tracking the Sun. The flat covers on EFP collectors are more attractive than bundles of evacuated tubes and, combined with the high efficiency, make them suitable for integration into roofs or building facias. Two different designs of experimental EFP collector were built, each using a flooded panel absorber but with different enclosures
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