Lets ask ourselves a few questions:
If there is a perfectly good source of mains power, why not use it?
Mains driven pumps typically will be rated at less than 50W, less than a lightbulb. The pump is only powered when there is solar gain. The power used is not negligible but it is small.
How long will the PV panel take to pay for itself?
As with all PV systems, a small solar panel to drive the pump will take a long time to offset its cost. I'd take a guess and say 30 years.
Is the level of control provided by a PV driven system as good as using a temperature controller?
Unless a proper differential temperature controller is used, there is no measurement of the hot water cylinder temperature. This means that theoretically, the pump could run when the cylinder is hotter than the solar panel. This would, of course, waste perfectly good heat.
Considering the above, we generally recommend a mains driven pump. We can and do supply PV driven pumps with PV solar panels to suit if customers want it.
Hope that helps.
2 comments:
As the developer of the Solartwin technology I must first declare a commercial interest. That said I do think that commercially focussed blogs such as this should not be used for myth making, particularly if they carry the Solar Trade Assocation Logo, since this is supposed to be about open and ethical selling.
At Solartwin we think that using mains power for pumps (and, incidentally, solar controllers, which are using electricity 24/7) is far from being a "perfectly good source" of energy.
Indeed these "parasitics" typically claw back about 3-8% of solar thermal energy savings a year, and about 20% of the carbon savings, according to independent DTI funded research on this matter. OK, I agree that mains pumps plus a wire to a power station are cheaper than a PV pumped system.
As for the author's "guess" that the PV panel will pay for itself in 30 years, I think this might be an exaggeration.
As for the red herrings about control, the level of control provided by a PV driven system is as good as mains solar because PV powered solar temperature controllers are fitted with all Solarrtwin's That are also available to all users of solar water heating systems at modest cost.
This is the second time that I have posted this comment and I hope that it will be posted soon.
Thanks for your comment Barry. We aren't trying to promote one way over another, I just tell it how I see it.
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