Friday, August 14, 2015

Enquiry about the Heat Pump and Degree Days

14 August 2015: I had an email from Shane Slater who has been reading my blog, and he asked some good questions and I said to him that I will probably post them on here.

Here is abridged version of his email:
While looking for Heat Pump diurnal consumption data, I’ve come across your site Charging the Earth. Its excellent.
    Perhaps uniquely, you show graphs of heat pump consumption V degree days. This is a methodology I hope to use in a future model of European housing space heating demand (based on degree days) and consequent Heat Pump demand.
     I am interested in that, while the correlation you show is good, it seems on some of your graphs that in severe cold weather, the heat pump consumption is significantly higher, while in mid-season/summer the opposite is true – the HP load is close to zero even though there is a non-zero Degree day.
     Do you have any thoughts as to why this might be the case? You have said that you turn it off at night but I would have thought that would reduce the HP demand relative to the degree days. 
     I was wondering whether wind effects could play a role i.e. cold but windy weather results in a higher heat loss rate and therefore higher HP consumption? And in summer, perhaps solar gain means the Heat Pump can be switched off?
     I’d be grateful for your help – and in any event congratulations on your website.

My reply:
Hi, thank you for reading the blog, and I apologise that I have not been keeping up-to-date much because most of the work was done five years ago.

1. It seemed logical in 2010 that one should correlate heat pump consumption to degree days so I made that graph. Every winter and summer is slightly different, and the degree days is a perfect way to record this variation. 

2. Degree days go into negative when there are certain days which are so hot that in some countries you would have air-conditioning. In a British load-bearing-wall house there is no need for air-conditioning because there is good insulation and thermal capacity, and openable windows.

3. There is always some consumption in mid summer because we need hot water. The heating function of the heat pump is mostly turned off from May to mid september because the house is warm enough without heating in those months. (Sometimes turned on for a day or two in very cold spells.)

4. In very cold weather, the heat pump consumption goes up, because it goes into 'additional heat mode' if you cannot get enough heat from the ground at a satisfactory rate. This is direct electric heating mode, with no COP. This has not occurred since we had the solar charging. 

5. In cold weather, the cooling of the ground (to feed the HP) is such that it creates a favourable delta T with the solar sunbox panels. Thus, we get some ground recharging even on the cold days if there is a smidgeon of sunshine, or on the more average winter day, if the air temperature is above freezing. The sunbox panels face south, and are vertically positioned, so have some advantage in winter sun angles.

6. We set the clock on the HP to go off during the night (between 930pm and 730am) in both summer and winter. This is not just a desire to improve our metering figures, it is because we have a well insulated house, and most people sleep better if the air temperature drops a few degs C. Nobody can sleep well if they are sweaty and hot, and can hear water pumping around the house at 3 in the morning. My wife often insists on an open window even in winter, so what would be the point of heating? I compromise with her by having the window shut but the bedroom door open so that the roof ventilates to the whole volume of the house. 

7. My wife is disabled, so we have to have the house a degree or two more than we ever had it when she was not disabled. Even at this higher temperature, our consumption figures are satisfactory.

8. Having a HP, we have underfloor heating, so no radiators, and in recent times, I have had to buy a local electric radiant heater for my wife to have in winter. This has increased our house consumption, and must very slightly reduce the HP consumption, but its hard to quantify that. 

9. Wind chill is significant if you have draughty windows, but we do not (house is only 8 years old). Wall insulation is good, and so is the loft insulation. We also wear adaptive clothing, i.e. teeshirts in summer, and woolies in winter. 

1 comment:

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