Wednesday, April 28, 2010

Reverse cycle cooling enquiry from P. King

28 April: I had an email from a GDL friend, Paul King (I think he is in the USA):
Question: "Hi David - very interested to see what you were doing with sunboxes! Could an additional string to the bow be using an additional air source heat pump to charge the reservoir during summer possibly using off peak tariff  electricity (cooling the house when too hot, or the surroundings when temperatures are mild), or using excess hot water from solar hot water system during day."

Answer: Hi Paul, Thanks for having a look through the blog!
The essence of our idea is to use the [free] solar heat to charge the earth - and the pump is mostly driven by power from the PhotoVoltaic roof. So for us, 'off peak' is in the middle of the day - when the roof is able to drive many of the appliances in the house and not import any electricity from the grid. We certainly wouldn't want to expend the running cost of an air source pump (air conditioner) to do what can be done for free.
    If we were in a hot climate, we would have considered a reverse cycle heatpump, but ours is for heating only. The thermal stability of the house is such that we would never really need summer cooling - and if we did, the first thing I would do would knock up some aluminium sections to make Shading devices for the south facing windows - a passive method, not an active one.
   I originally intended an off-the-shelf solar thermal panel, but this produces heat at a much higher temperature for water heating - and we can manage with something cruder as we are quite pleased to get heat in the 15-25 range (Celsius). 'Sunspace' is also a method of capturing heat, so think of our boxes as miniaturized sunspaces.
    Actually, my enthusiasm for plumbing leads me to think about the idea (if cooling was needed) of bypassing the existing loops to a heat exchanger, so that the ground loop (at a temp. averaging 12 deg C) could be exchanging heat with the underfloor heating loop, making it a cooling loop.
   We are not having solar thermally assisted hot water, as this would confuse the readings that we are taking of the improvements in performance of the heatpump as a result of the earth charging. The hot water is virtually free as during summer, the PV roof produces such a surplus that no imported power is needed during hours of bright sky.
      Hope you (and any other readers) can post a comment on the earth charging blog. thanks...

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