Thursday, April 28, 2011

Time around the circuit

27 April '11: Watching the summer time circulation, I can see the temperature going down, and the temperature of the returning liquid. So that should give another indication of how much heat is deposited in the ground. 

 I just calculated the volume of liquid in the loop from the Heat pump into the ground (not counting the loop that goes through the panels) It's a pity we can't do a sort of Pooh Sticks method to find out how long the glycol takes to get round.
Upper: When I had to fill the upper system it required 20 litres of neat glycol and 40 litres of water, that is 60 litres in total. (I had a 20 litre container for pouring, so know this precisely)
Lower: For the lower loop, we have near-enough 200 metres combined length and a 40mm pipe all the way round, whose internal diameter is 32mm, which has a cross sectional area of 0.0008 m2, which gives an internal pipe volume of 160 litres.
 Total: This makes 220 litres total for both parts of the loop.

As the slow pump speed is 300 litres/hour (5 litres a minute), it must take about 44 mins for one entire circulation to take place at the slower pump speed. (When the GHSP is running, it goes at 18 litres/minute, more than 3 times that speed).
The temp diff between going down and returning is about 2.5 - 3.5 degs C.

With specific heat of the liquid of 4.0 kJ/kgK, that suggests: 4 x 300 x 3.5 *1000 / 3600 

That is equivalent to 1.17 kilowatt - i.e. in one hour, just over 1 kilowatt hour of heat deposited. As the data from the energy meter confirms a figure close to that, then I am glad to find agreement. 1.17 kW is almost exactly the average capture we got throughout last Summer.
I have long noticed that during the Equinox and Winter when the GSHP is running and forcing down the loop temperature, the delta-T improves and the rate moves up to 1.5 or even 2.0.

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