Thursday, January 10, 2013

PV-Thermal evaluation by Solar Blogger

9 Jan 2013: There's an interesting article on the Solar Blogger website by Stuart Elmes. It's about PVT (Photo-Voltaic-Thermal) panels, and he has a more technical PDF linked to it. The link was sent to me by David B. of Newform.

PVT panels are, in essence, a PV panel with a mesh of liquid cooling tubes behind the PV surface. For higher PV capture, the panel is uncovered, but for a higher thermal capture, the panel has a glass cover over it and insulation behind.

   I am a great believer in PVT, but the only properly productive application for it that I can see is when it is associated with low-grade thermal storage and heat pump.
   It could also good for those who have a limited area of roof, and haven't enough space. If you have enough roof surface to play with, the best place for PV might not be the best place for thermal. Ideally, a house needs 28-30 square metres of PV to attain the 4 kW that is recommended for the Feed-in-Tariff in the UK - a smooth, unobstructed and unshaded area facing south is the best. PV arrays normally have an air space under them and they work best on sunny days with a cooling northerly breeze that can get under the panels.
   For solar thermal, the size can vary from 1sqm to 4sqm. Above that, it gets a bit silly unless you are addicted to hot showers all summer. So Solar thermal can often work well in smaller locations on hipped roofs, or large dormers, providing you can get a south aspect. Another criterion for best place might be closer to the optimum positions of a tank and the plumbing.
   The idea of PVT is that you get a larger area for thermal, and the PV element is cooled by the liquid passing under it.

   The paradox is this: for you to get useful heat to a water tank, you want the panel to run very hot, but for the PV panel to run well you want it to run very cool. Stuart's article explains the paradox well.

   So for a solar thermal water tank you want the circuit to run at 50-70ºC, and when the tank reached 60ºC, the circulation would stop. If you ran PV at these temperatures it would be unworkable. If you want PV to be optimised, the purpose of the liquid has to be to cool the PV, so it should be circulating at 15-20ºC. A normally air cooled PV panel reaches 40-50ºC surface temperatures on a hot summer's day, but the PV cells run better is the wind is blowing.
   If the water tank reaches optimum temperature and turns off the pump, the sun will make the PV even hotter, and as there would be insulation behind the PVT panel, there's nothing to stop the liquid temperature getting dangerously high. Stuart's article refers to the risk of temperatures as high as 220ºC. (I am informed by Newform that this might be a mistake, as Powervolt panels permitted to stagnate seriously haven't gone higher than 100ºC, although the glass topped Powertherms might get hotter.)

Therefore, for thermal storage for a heat pump, the panels running at 15-20ºC would be warming an infinitely large tank size, the earth, whose base temperature would be 10-12ºC, so there will always be a benefit. There is no risk of any of the system reaching stasis and turning off because earth has a vast thermal capacity in relation to the size of the panel array or the size of the building above it.
    Having a larger surface area than normal solar thermal panels (28sqm instead of just 2 or 4sqm) the solar thermal gain for the store - large volume of low grade thermal energy is going to be very good! One thing that my experiment has shown is that the ground does not get 'hotter', but the volume of earth that is warmed up to a moderately warm temperature gets larger.
    One thing about any of these systems, is that they must keep running. If the system circulates reliably everything is the right temperature. If a future owner of this house just turned the solar circulation off in summer, there would be problems if temperatures rise and there isn't sufficient capacity for expansion of the liquid.

In this house, we were within a gnat's whisker of having PVT, and I had already emailed Newform with preliminary arrangements for delivery and invoicing. 
Four reasons we changed direction: 
  • My wife rebelled against the cost of the PVT, when we have no real need for the extra electricity, 
  • I had problems with the battery I intended to use for the PV power, and 
  • I was offered a large amount of polycarbonate, aluminium and two large radiators, an offer so good, I had to accept it. I also had a good collection of OSB and timber, enough to make an entire sunbox without buying any materials other than plumbing connections.
  • I considered my research priorities, and the general thrust of these are to prove the efficacy of a black collector in a polycarbonate sunbox. With only 4 sqm of roof to choose from, there is only space for ONE more device - had to be a Sunbox.
If I was starting this house again, with the knowledge I have now, I have no hesitation in saying that I would use PVT panels in association with the borehole and heat pump - instead of the existing array. But these were not available or known to me in 2009.

Refs: http://www.solarblogger.net/2012_10_01_archive.html
https://dl.dropbox.com/u/72783679/Assessment_of_the_Effectiveness_of_Combined_PV-Thermal_Solar_Panels.pdf

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