Abstract
Solar thermal collectors for buildings use a heat transfer fluid passing through heat exchange channels in the absorber. Flat plate absorbers may pass the fluid through a tube bonded to a thermally conducting plate or achieve lower thermal resistance and pressure drop by using a flooded panel or microchannel design. The pressure drop should be low to minimise power input to the circulating pump.A method is presented for choosing the optimum channel hydraulic diameter subject to geometric similarity and pumping power constraints; this is an important preliminary design choice for any solar collector designer. The choice of pumping power is also illustrated in terms of relative energy source costs.Both microchannel and serpentine tube systems have an optimum passage diameter, albeit for different reasons. Double-pass and flooded panel designs are considered as special microchannel cases. To maintain efficiency, the pumping power per unit area must rise as the passage length increases. Beyond the optimum pumping power the rise in operating cost outweighs the increase in collector efficiency.
Highlights
Solar thermal collectors generally extract heat to a fluid that passes through a tube bonded to the absorber plate, passages embedded inside the plate or a flooded panel.For a given absorber area, the designer must select the tube diameter and length and choose between a single pipe or a microchannel arrangement with multiple passages
This paper describes a methodology for choosing the optimum channel size for a given solar collector plate area in terms of the allowable pumping power and fluid properties
The optimum pumping power can be determined by a simulation based on flow resistance around the installed system together with relative energy costs and collector efficiency characteristics
Summary
Flat plate solar collectors usually rely upon the flow of a waterbased coolant to extract heat from the absorber; the alternative using heat pipes (Deng et al, 2013; Xu et al, 2015) is not considered here. In some installations using stratified hot water tanks it has been found to be beneficial (Duffie & Beckman) to use a very low flow rate that results in a high coolant temperature rise across the panel, even though the panel efficiency can be reduced as a result. They distinguish between these ‘‘low flow” cases (0:002 À 0:006 kg=m2 s) and the more usual ‘‘mixed out” tank assumption with heat being passed to a constant temperature heat sink and typical mass flow rates % 0:015 kg=m2 s. The following analysis in terms of Tpm assumes a mixed out system where there is no advantage to be obtained from a high fluid temperature rise
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
