Design and Performance of Evacuated Solar Collector Microchannel Plates

Abstract

Solar thermal collectors for buildings have traditionally used either a flat panel or evacuated tube design. A high vacuum (<1 Pa) can eliminate heat loss via gas conduction effects, thereby increasing efficiency in cold climates or in applications requiring elevated delivery temperatures e.g. for process heat. Combining the two established technologies, an evacuated flat panel design would be more architecturally elegant than tubular collectors and have a better fill factor, yet be thinner and more efficient than conventional flat panels. Beyond the efficiency gains from vacuum insulation, the collector design aims to limit temperature nonuniformity (with its increased radiative losses) by achieving homogeneous flow across micro-channels within the plate. A design methodology has been adopted that optimises the channel hydraulic diameter in terms of plate-to-fluid temperature difference and power required to pump the fluid. Two collectors (aluminium, stainless steel) have been designed and fabricated, with machined plates and laser-welded joints.

Investigating the effects of geometry in solar thermal absorber plates with micro-channels

“…There is an advantage in expending more power above this level to increase fluid velocity as it reduces the overall temperature rise (which also reduces thermal losses). The trends observed in this experimental study agrees with the theory presented by Moss and Shire [31]. In practice, optimum micro-channel geometry in plates will need to be sized based on fluid properties and operating conditions.…”

“…It indicates that the smaller the channel, the lower the power required for a given fluid velocity. This agrees with the predictions of Moss and Shire [31], however, their predictions further show that increasing the channel size slightly can improve performance at a pumping power requirement of less than 2% of collected solar energy. Therefore, larger channels may be used; this should be sized based on pumping power limit, required fluid temperature rise and fluid properties.…”

“…Dogruoz et al [30] stated that the wall heat transfer coefficient shows more dependence upon geometry than upon flow rate. Moss and Shire [31] presented a theoretical method for identifying the optimum channel size based on pumping power limit. This paper therefore investigates the effects of the channel geometry on the overall performance.…”

Detailed Modeling of Flat Plate Solar Collector with Vacuum Glazing

“…That has become possible by ultrahigh vacuum (1 33 × 10 −7 Pa) maintained by a getter pump powered by the sun. As for the latest studies, Moss and Shire [12] indicate an improvement from 25% for a conventional FPC to 60-65% for vacuum FPC when operating at 140°C above ambient temperatures. More recently, Shire et al [13] highlight that vacuum FPC collector could provide heat up to 200°C with efficiency greater than 50%.…”

Design and fabrication of a hydroformed absorber for an evacuated flat plate solar collector

“…A pair of 200 mm × 140 mm absorbers were built [27]. This was a labour-intensive exercise and it became clear that the machining and manual welding time would be impracticable for a larger (0.5 × 0.5 m) panel.…”