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Author Topic: ASHP controls  (Read 2916 times)
acresswell
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« on: October 19, 2011, 05:23:41 AM »

I don't own a heat pump and don't plan to own a heat pump (so I apologise if I don't know enough about them), but I know that they are many people's preferred option. If my neighbours are going to use them then at least I want them to use as little electricity as possible.

I'm guessing that the standard controls for a heat pump are basically a thermostat in the thermal store that demands heat.  The heat pump then runs to provide this until the thermostat is satisfied. 

Knowing that external air temp makes quite a difference to the CoP of an ASHP, wouldn't it make sense to make the decision about when to run the ASHP by taking account of the likely external air temp?

e.g. in a standard situation, the target temp of the thermal store might be reached by 3pm and the ASHP would switch off, even though there was a lovely 15-20 degrees of external air temp.  By midnight, the temp of the thermal store has dropped slightly and this triggers the ASHP to run, but the external air temp is now 5 degrees C.

We can realistically predict that air temps will be highest in the day and lowest at night, so I'm suggesting that the controller should allow slight over-heating (advance loading?) of the thermal store in the afternoon/ early evening so that it will have to run less when the external air temp drops.

Equally, in the early morning, the demand for heat would be postponed slightly (letting the thermal store temp drop a couple of degrees further than usual) until the sun came up and air temps started to rise.

There's probably some fundamental flaw with this, or maybe the manufacturers are already doing it.
What do you think?
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EccentricAnomaly
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« Reply #1 on: October 19, 2011, 10:32:00 AM »

Agree completely. The controller should be looking at the weather forecast on the web and working out the likely demand then working out the trade-off between different electricity tariffs during the day and night vs the forecast outside air temperatures and humidities to minimize costs. If you have your own PV it should also be looking at the forecast sun to work out how to make good use of the local generation. Ditto wind, of course.
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A.L.
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« Reply #2 on: October 19, 2011, 10:36:41 AM »

hello,

depends on wether you want to minimise electricity, and therefore CO2, used or minimise cost

assume a COP of 3 during the day and 2 during the night and a daytime electricity cost of 15p/kWh and nightime cost of 6p/kWh

daytime bias will minimise electricity use, nightime bias will minimise cost

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or maybe the manufacturers are already doing it.

in my experience manufacturers/installers fit system and leave sharpish!
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acresswell
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« Reply #3 on: October 19, 2011, 05:29:37 PM »

As I don't own one, my main thinking was to make most efficent use of electricity from the grid in order to reduce carbob emissions...  so I suppose it ought to take account of the local grid's capacity and any generation from renewables. 
 
If you/your neighbour had a load of solar PV and was feeding back into the local grid in the middle of the day, then it'd be a sensible time to run the ASHP.
Whereas if you had wind/hydro that was feeding back into the grid at night (when other loads are switched off) then I suppose it'd be better to run the ASHP then, even though the CoP would be less good.
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guydewdney
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« Reply #4 on: November 02, 2011, 11:16:52 PM »

my (slightly - no - very - geeky) brother has done something along these lines. His system looks at the temp drop of the hosue, and the outside, and the time / sun. It then works out if its worth firing up the heating - eg at 7am, its just a bit chilly, but its going to get sunny soon - wait a bit.
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SimonHobson
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« Reply #5 on: November 08, 2011, 07:56:42 PM »

assume a COP of 3 during the day and 2 during the night ...
That's generous, they drop to unity COP in cold weather. In fact, I believe many units have an electric heater and simply stop trying to pump heat once the outside temperature drops too low.
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