Wednesday, February 9, 2011

Interseasonal News from Sweden

8 Feb '11: I heard from Tommy in Sweden, the one I mentioned earlier who has being doing some experimental interseasonal charging on his house.  He runs the heat from his solar panels directly to the heat pump (well, the liquid is circulated to the panels). When there is no Sun, the GSHP uses the ground loop as normal, and when the Sun is shining and the GSHP asleep in summer, he can charge the earth, for future use.

That's interesting, and I would be able to fiddle with the plumbing here to do this (I have sketched a layout establishing that I would would need at least three three-port solenoid valves) but we would be hesitant about doing this as this IVT heat pump would not accept hot liquid directly from a solar thermal panel - it assumes that if the return from the loop is in the 20s, something has gone wrong and shuts down.
   So for the moment, my very simple 'Trickle-or-Whoosh' system using one 3-port valve is working well, and for the system in East Leake, we are using 'Trickle-while-we-work' layout with a 2 port solenoid valve, both cheap and effective.
  In the Swedish environment, being a lot further north, Tommy's system would have a lot to offer, if you can get the control mechanisms right.

He writes:  Our system is somewhat different from yours, as we are using the sun to heat the house and the hot water directly as much as possible. In order to do so we have a control system especially designed for us.
  When the sun hasn´t been able to heat up the solar panels enough for us to use the sun directly (when there is a need), we use the solar energy to preheat the brine before it enters the heat pump.
  During winter this is the only possible way for the control system to use the solar energy, as there are so few solar hours. When the heat pump is not running the bore hole will be recharged as in your system. For example my 14m2 of solar panels where recharging my 70m bore hole for about 3 hours during the fantastic winter day yesterday.
  As we are using the solar energy directly we cannot use “pool solar panels”, instead we use Solar Keymarked solar panels from Watt ( ). “Pool solar panels” are effective during the recharging cycle, but for direct heat they are not useful.

See below, some interesting follow up comments from Dr Chris Wood one of my colleagues at the University.


  1. David,

    You could still incorporate solar thermal panels - all you would need is a mixer valve/circuit to ensure the return to the heat pump does not exceed 20degC. In other words it would do this at the point after the heat pump and before the panels. It could monitor the outlet tempertaure and when on approaching a set point it would slowly adjust to send more directly to the borehole.... or some kind of arrangment in the fashion... i dont think it would be too difficult.


  2. just thinking about it for a second.... i dont think that direct connection to the panels would cause a problem with the fluid being over 20 deg C. Imagine this.. a solar thermal system of 4sqm... now at optimum assume 800W per sqm (but mmost of the time much less particulaurly in winter when the heat pump will be used predominately). So we have 3.2kw heat input. The heat pump is 5.6kW heat ouput, thus at a COP of 4 (yes 4 is higher than usual but remember we are assuming that we are now at an elevated source tempertaure compared to the norm from the ground), thus the heat extraction from the 'environment' is 4.2kW. Hence the heat pump can take out more than the solar can give (whilst both are operating). Therefore the highest temperature that could come back would be ground temperature circa 11degC. Your difficulty and problems would be when you have circulated hot fluid through the ground loop i.e a day in June - all day the ground has been absorbing heat and maybe the fluid tempertaure is now up to 40degC and so it the immediate ground around the borehole. The heat pump comes on a mid night to heat the domestic hot water and is suddenly hit by a slug of fluid well in excess of 20degC.. yes the heat pump will record a fault and shut down as the evaporating pressure will be sky high.


  3. Dear Chris, this is one thing that my friend in East Leake was worried about. For example if the pump was not pumping for a while due to a timeclock going off, and you suddenly get it working again, the liquid in the panels or radiator could have built up a mighty temperature and would hit the heat pump. In my layout of the system, they have to go through the 'earth battery' first, and no amount of built up heat in a small panel can stay that hot if pumped 100 metres through a few thousand tonnes of cold clay. He was also worried about the plastic piping and joints in the ground loop if suddenly 80 degree liquid is flowing at the start of the loop. But it would flow for such a short time - the volume of liquid in 4 sqm of panel being small compared to the whole loop, I cant see it causing permanent damage.

  4. Last summer, the GSHP was only needed for hot water, but when it came on, it was getting incoming liquid of sometimes 16 or 17 because the Sunboxes had had some hours earlier of putting liquid down at over 20. We noticed that it got on with its business quickly. I also added more insulation to the tank to reduce demand. The ratio of DHW to Heating used to be 27-73 over first three years, and since July 2010 it is improved to 20-80 percent.

  5. (I should mention that the statistics on the GSHP were reset to Zero in July 2010, which is how I know the new percentage balance of DHS to SpaceHeating.)

  6. You certainly would not be up to 80degC by circulating through the loop, but you could get higher than 20degC quite easily in the scenario I have given.

  7. By 80ยบ, I meant that if the system was not circulating (because of a power shortage), but the sun was shining in summer, the black panels would just get hotter and hotter. When power was restored and liquid began to move, there would be a very brief passage of very hot liquid.


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