Abstract
A flat-plate solar thermal collector’s efficiency can be much improved if the enclosure in which the solar absorber is housed can be evacuated. This would result in a high performance, architecturally versatile solar thermal collector capable of supplying clean energy efficiently for use in applications including residential hot water and space heating. This paper focuses on the design of evacuated enclosures for flat-plate solar collectors, in which the solar absorber is completely surrounded by a thin layer (4–10mm) of thermally insulating vacuum, resulting in a thin solar thermal collector (depth<20mm). The expectations, requirements and applications of these solar collectors are discussed along with a description of the enclosure concept under consideration. Potential seal materials are identified and their limitations examined. Finite element modelling results are presented of a study investigating how the glass cover of such enclosures are mechanically stressed when subject to atmospheric pressure loading and differential thermal expansion of dissimilar components. Finite element model predictions are validated against preliminary experimental strain measurements for existing experimental enclosures. It is demonstrated that with a suitably low temperature sealing process vacuum the designed enclosure can successfully withstand imposed stresses.
Highlights
Solar thermal collectors conventionally come in two forms; non-evacuated, glazed, flat plate (FP) collectors and evacuated tube (ET) collectors
FP collectors have a larger solar absorber area to gross area ratio when compared with ET collectors but their thermal performance is Abbreviations: FP, flat plate; ET, evacuated tube; VFP, vacuum flat plate. ⇑ Corresponding author
Poorer, especially at elevated temperatures. This is due to FP collectors typically employing a solar absorbing plate that fills a large proportion of the collector area, whilst ET collectors employ absorbing tubes which are individually enclosed in larger evacuated glass tubes
Summary
Evacuated enclosures for flat plate solar thermal collectors have design similarities to vacuum glazing, but are subject to different design constraints and operational conditions as there are more mechanical design options. The process of sealing a vacuum glazing and ensuring the support pillar array is evenly distributing mechanical stresses over the glass surface, theoretically possible, is known to be significantly more complex when using tempered glass. This complexity most likely extends to vacuum enclosures for flat plate solar collectors with front and rear glass covers. FEM models are validated against preliminary experimental measurements of enclosure strain gained using digital image correlation (DIC)
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