Friday, April 30, 2010

Summer performance - some things different from expected

30 April : Midday Observation with sun shining :-
  • Air temp in boxes 32ºC, 
  • Glycol going down to ground, 23ºC, 
  • ....and returning back up at 12ºC. 
             That's charging!

Summer Performance - the unexpected
I have also realised that my intentions for night air charging are possible, but unnecessary. When heating is going on (late March/early April) the boxes worked at night because air temp was better than glycol temp - that was helpful. Now that it is summer (we hope it stays that way) I have noticed three things:
  • Morning Startup: The sunboxes don't start up by themselves on cloudy days, until the heat-pump lowers the glycol sufficiently : good! this is saving pump power and the heat we would get down would not be worth the pump power. On cool days, the boxes come on when GSHP demands, support the GSHP in Realtime while it does a water heating cycle, then they turn off shortly after the GSHP goes back to sleep. This is good! On sunny mornings, the cold sensor is at Room temperature, say 22ºC, so they will start up by themselves if the Sunbox temp rises to 27ºC. (Startup is dependent on the temperature difference - I am finding that 5 degrees C is pretty well perfect balanced - ensures that pump is not on for excessive hours, but when it is, the heat being claimed is worth getting.).
  • Day gains-Night gains: The amount of power we get on sunny days is sooooo much that it's not necessary to chase after small gains based on night time air temperatures (but I am willing to change my mind later in summer, depending on the datalogger finding.) Night time action only occurs if the GSHP demands realtime heat. There's no point in interseasonal night charging.
  • Temperature reversal : The earth below the house is getting 8-16 kWhr per day. This disperses well, so there is no danger of chilling. At the end of a day, the temperature below is quite acceptable, sometimes higher than the morning temp. e.g. before we did this, the evening temperature might be 5.8º but in the morning it had recovered somewhat. The reverse is happening now. At the end of the day when we put heat down during the day, but draw little up, the temp is high in the evening perhaps 12.8º, but in the morning, this heat has moved outwards and the stable temperature (stable meaning that for 12 hours, no heat in and no heat out), it is about 11.8º. 
Forget Overheating risk
If this is the case, there is no need to have louvres top and bottom to 'flush air through' - this was thought essential for summer night time gains. The daytime performance is better if the boxes are very airtight. The boxes never overheat - I was worried about this, and the top louvre was intended mainly to meet the overheating risk. However, the hotter the boxes get, the quicker the pump pulls heat from the boxes and buries it, keeping the sunbox air temperature below 40ºC (about the same as Baghdad in summer, or your blood temperature). The only overheating risk is if the pump is not working, but in this case there is an expansion bottle in the loft to cope with swelling of the liquid mass. There are cooling louvres top and bottom, in case I stop the pump for maintenance, or try running the GSHP for a week with sunboxes turned off, for testing purposes.

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