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Renewable Energy: solar thermal: mixed flat plate and evacuated tube collectors
(trying to bump this by reposting here, rather than Heating and Cooling) Hi,
so, here's how I understand things: 1) evacuated tube collectors (ETC) maintain efficiency at higher inlet temperatures; 2) flat plate collectors (FPC) are more efficient at low inlet temperatures; 3) ETC are more expensive than FPC by both per joule and per sq_m.
what's to stop us trying to optimise using the benefits of both in a mixed system?
I'm thinking: 6-8sq_m of FPC in series with ~2sq_m of ETC.
Is there any technical constraint that would prevent this being viable? (apparently the MCS certificate can't handle it but that's bureaucracy, eh?)
As the system is in series the FPC may become a radiator when the ETC starts working at a higher temp, unless your store is large and lowers the flow temp into the FPC sufficiently. I am not sure if it is efficiency that varies really, more to do with losses (which can be calculated as efficiencies I suppose depending on what you measure). The question is why would you want to do this if you have 10m^2 to play with.
If the fluid flow rate is fairly high (as it usually is) then the temperature differentials across the panel and tank heat exchanger inputs and outputs tend to be small - likely less than 10 °C. The panels operate at the tank temperature plus a little bit.
To operate with the large differentials needed for significant differences between the flat plate and ET temperatures would require a low flow rate. I suppose that's quite possible and would save a bit of pumping energy but would imagine it's a bit trickier to control. E.g., does the viscosity of anti-freeze change much with temperature?
As I understand it, yes, it's all about the simultaneous losses rising to equal (or even exceed) solar input, and so: 1) heat pipe collectors maintain efficiency (somewhat) at lower outside air/radiant scenery/sky temperatures; 2) flat plate collectors (FPC) are less efficient at lower ditto because more succeptible to simultaneous re-radiation loss to cold scenery/sky; 3) evacuated tube (non heat pipe) collectors (ETC) are just as succeptible as FPCs to simultaneous re-radiation loss, but a bit less succeptible to simultaneous conduction loss to cold outside air.
1)'s efficiency is relatively unaffected by thermofluid temp 2) and 3's) efficiency rises sharply as thermofluid temp is lowered - but how low can that go and still be useful?
I have a hunch that 2) and 3) would be more efficient, in mid winter, that 1), provided thermofluid temp were lowered to say 22C. Our theoretical calcs show efficiency of 60% in mid winter, with suitably optimised (but simple) FPC, at that thermofluid temp. At 60%, with big (but simple) collector, insolation is adequate to space-heat a near-PH building by solar alone, right thro Dec/Jan (depending on horizon obstructions).
Not actually looked closely at it, thought I would let you precis it for me. Does seem to get thicker as it gets colder, though not sure if that is a problem, would depend on concentration and pump type/performance I guess. GSHP us a cold transfer fluid I think.
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