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. The absorber is a key component of a flat plate collector: in the context of an evacuated panel, absorber design poses a number of technical challenges.A flooded panel absorber has been designed for use in evacuated flat plate solar collectors. The aim was to obtain higher efficiency, in a low out-gassing material, than would be possible using a conventional serpentine tube design.Initial plans for a micro-channel plate were modified when optimisation analysis showed that a flooded panel could achieve as good performance with easier fabrication. The absorber plate is made from hydroformed stainless steel sheets welded together and features an array of through-holes for the glass-supporting pillars with the square panel subdivided into two rectangles connected in series for ease of fabrication and better flow distribution. The coolant flow was modelled in Star-CCM+. FEM simulations based on tensile test data informed the choice of sheet thickness and weld radius around the holes to withstand the 1 bar pressure differential.Hydroforming is an effective method for producing sheet metal components, e.g. plates for heat exchangers or solar absorbers. As a thermal engineering experimental technique, the tooling is significantly cheaper than press tools since the mould does not need a matching die. In a research context, the ability to form plates in-house and explore profile and tooling options at low cost is very useful and might find application in other fields such as experimental heat exchangers.A hydroforming facility was built using 85 mm thick steel sheet and a 25 MPa hydraulic pump. This proved highly effective at forming 0.7 mm stainless steel sheet. A total of eight absorbers were fabricated and successfully leak tested using helium. Two variants were made: one kind for use in enclosures with a metallic rear tray, the other for enclosures with glass on both sides. The collector efficiency factor is estimated to be 3% higher than for commercial tube-on-plate designs.
Highlights
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
At the nominal UL = 1 W/m2 K condition the efficiency factor is at least 3% higher than for the aluminium tube on plate result and 7% higher than expected in practice with a tube spot-welded to a stainless steel plate
A flooded panel absorber made from hydroformed plates was adopted because it offered higher efficiency than a serpentine tube arrangement and was easier to manufacture than a micro-channel architecture
Summary
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 competitivelypriced sealed for life panel and a lack of awareness of the benefits of such panels. Researchers at Warwick, Loughborough and Ulster universities have developed and tested a number of evacuated flat plate collectors as part of an investigation into technical issues that might hinder manufacture and innovations that might improve efficiency. The dissemination of these results aims to increase awareness of the efficiency benefits to end users. The need for pillars or ribs to support the glass without any thermal conduction path from the plate adds complexity
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