Monday, August 16, 2010

More reflections on Performance

16 August: The thermal comparison of a few days ago is slightly unfair, as I thought about it more, and discussed with Chris Wood.
    The final end product of most Solar Thermal panels is the Hot water that comes from your tap. The 1,100 kWh for the 4sqm of roof mounted flat panel takes the equation to the final product - it must take account of Stasis in the tank, and that people are not having baths all the time.
    The wall mounted Surya sunboxes may produce 3,000 kWh if we have a good year, and they have the advantage that they also contribute to space heating. However, for any of that heat to come out of a tap in the form of hot water, it has to be reprocessed by the heat pump. It is difficult to equate, but if the COP is 2.0 or 2.6, we should reduce it by a third, so that might still put the Sunbox production at about 2,200 useful, still twice that of the conventional solar thermal panel.
   Chris also points out that if you had a solar thermal panel and used heat from the storage tank for a heat pump that was heating the house, it can do better. If it never experienced stasis because it was sending heat to a tank from which heat was being removed by the heatpump, it would have a far better output with the much improved delta-T. However, this heat would then still have to be transformed by the heatpump to make it useful in the house. The moment you let the temperature rise in the tank to get hot water, you would have a poorer delta-T for the panel, and would be taking less to heat the house. And this whole process still needs a duplicate very large copper insulated tank and two loops. Overall... heating the ground is easy because there is always a beneficial delta-T as you cannot heat 8,000 tons of clay much more than a couple of degrees.

So I still think that the Suryas win.... hehe.... we will need a whole winter of readings to be sure about my projected figures.

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