Friday, April 16, 2010

Excellent results with a working GSHP!

16 April : I am getting some interesting results now that the GSHP is working correctly. It comes on, does its job efficiently, and now spends long periods in sleep because the house is well enough insulated not to need more heat for a while. So I can take accurate readings of the sunbox performance. I can also rely on the reading from the down loop pipe as I made my special copper connector, and took care to calibrate the sensor precisely last night.

For example today... with the GSHP slumbering, the sunboxes are the only heating device... this morning after the GSHP had been off since the night before, the deep ground was 12.0. After a morning heating, the ground had been cooled around the pipes to around 8 degrees (during this warming cycle, the sunboxes contributed about 1/3 of the heat). During the lunch hour, the house was warm, the GSHP sleeping and the sunboxes were only recharging the earth.

Time Sunbox  Looptemp   From Sunbox to loop
1245 23.0     9.8> 9.8  12.2
1300 36.0    10.9>10.9  14.1
1315 27.8    11.3>11.3  14.3
1330 24.6    12.1>12.1  14.1
Heating restarted 1340
1345 26.6     8.5>11.8   9.7
[in this table, 
['Loop temp' is glycol coming up into GSHP and going out again]
['From Sunbox' temp is 50-50 mixture of glycol from the GSHP and from Sunbox]
['Sunbox' temp is air temp in the sunbox, just above the black plastic slabs. The weather is alternating sun and cloud]
[when the GSHP is sleeping the In and Out temperatures level out because GSHP is allowing glycol to pass directly through, and at steady state this is indicating the deep earth temperature immediately around the deep pipes.]

This clearly shows some immediate 'realtime' benefit. The ground temperature is rapidly recovering from the morning's heating cycle. In merely 45 mins, the deep-earth temp around the pipes increased from 9.8º to 12.1º. The liquid going down was consistently warm, and coming up cooler, having given its heat to the deepground, and is sent up to retrieve more. The moment that heating resumed, the sunboxes contributed directly to that, as can be seen from the significantly reduced temperature from Sunbox, still giving about 1.2 degs benefit.

As an observation, when a heating cycle is running, the algorithm in the GSHP is usually requiring about 3/0 - 3.6 degsC difference between out and in. On a nice day like today, the Sunbox/GHP combined return temperature is consistently contributing about 1/3 of that,  e.g. 1.2 degs in the example above.

Here is a similar sampling from later in the same day, again during a period of GSHP sleep, external air temp in the shade is 14.6º. The loop temp shows the returning temperature of glycol from deep in the ground loop. The house internal air temp is stabilised by good solar gain through the west facing house windows, so the GSHP is remaining dormant.

Time Sunbox  Looptemp   From Sunbox to loop
1515 35.7    13.3>13.3  16.0
1530 34.7    13.5>13.5  16.5
1545 32.4    13.6>13.6  16.4
1600 29.8    13.7>13.7  16.3
1630 33.7    13.7>13.7  15.9

1700 32.7    13.7>13.7  15.5
1800 24.5    13.6>13.6  14.1

Heating restarted 1845
1900 15.6     8.9>11.1   9.2

As I take these samples, it gives me a clearer idea of where we need to put sensors for datalogging. These are so far the most valuable real-time readings we could have to prove the concept, and of course the regular meter readings (daily, weekly, monthly) are also vital for longterm analysis.

The last 5 readings of that group are especially interesting. What it suggests is that the ground recovers heat to a certain extent, but when the delta-T between the borehole pipe and the surrounding soil is high enough, e.g. at a glycol temperature of 13.7º, the rate at which heat moves out through the soil away from the pipes begins to equal the rate at which it is being put down there - the soil does not just get hotter, but the temperature distribution gets smoother.

We don't expect the soil to get hotter, what we hope for is for the body of warm soil to be larger, with warmth reaching out further, but not so far that it will not come back. So, two solid cylinders of soil 5 metres radius all at 13.7º without any cold 'rings' by the end of the summer would be a good result, and anything more than this would be an excellent result.

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