kW fur Heizung und AC

kW fur Heizung und AC

Beitragvon Jan Treur » Do 6. Apr 2017, 07:10

To get a better approximation of the range for low and high temperatures it is useful to take into account the losses in kW power for heating or AC. I made a computational model with some parameters for this and I want to ask whether from your experiences the values for certain temperatures are too high, too low or OK. I assume preconditioning took place and this indicates only the average power in kW to maintain the interior temperature during the driving. Note that it sometimes fluctuates between lower and higher, so then an average should be considered.

This is the graph for the current settings (with interior set point 20 °C).

P.S. I modeled the case that a heat pump is used (until -10 °C), as you can see in the curve of the line. For resistance-based heating you have to draw the straight line from -10 °C to the point 0 at 20 °C; see the second picture.

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Re: kW fur Heizung und AC

Beitragvon DocDan » Do 6. Apr 2017, 14:29

Nice model. But I think the cloudiness will have to be considered. You have 0 kW with an outer temperature of 20 °C in the model. When the sun is shining, the inner temperature will be above 20 °C due to a small greenhouse effect.
Just my short thoughts, but I may be wrong.
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Re: kW fur Heizung und AC

Beitragvon umrath » Do 6. Apr 2017, 14:36

The data for the resistance based heating (model Trend in Germany) is definitely wrong.

When I test drove the car at -2 degree it used a lot more than the ~0,7 kW. It was rather in range of 2-3 kW.
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Re: kW fur Heizung und AC

Beitragvon Jan Treur » Do 6. Apr 2017, 16:49

Are you sure that this was for a very long time and not only in the first 5 or 10 minutes? I also know such numbers, but only in the first phase when no preheating was applied (but I have a heat pump).
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Re: kW fur Heizung und AC

Beitragvon Jan Treur » Do 6. Apr 2017, 16:52

DocDan hat geschrieben:
Nice model. But I think the cloudiness will have to be considered. You have 0 kW with an outer temperature of 20 °C in the model. When the sun is shining, the inner temperature will be above 20 °C due to a small greenhouse effect.
Just my short thoughts, but I may be wrong.


Yes, I was also thinking about that. Maybe I still have to add this effect. But still there can be a point where you don't need heating nor AC, but when it is sunny this point will be lower. Two days ago I had such a trip when it was sunny and 16 °C.
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Re: kW fur Heizung und AC

Beitragvon umrath » Do 6. Apr 2017, 17:52

Jan Treur hat geschrieben:
Are you sure that this was for a very long time and not only in the first 5 or 10 minutes? I also know such numbers, but only in the first phase when no preheating was applied (but I have a heat pump).


Yes, I am. It was during the test drive over 2x 550 km and I was scared when I saw that it's drawing that much energy all the time. It was even higher at the beginning (as high as 6 kW) if I'm not mistaken and it rarely dropped below 2 kW while driving.
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Re: kW fur Heizung und AC

Beitragvon Jan Treur » Do 6. Apr 2017, 17:56

OK, I have to adapt one or more parameters.
By the way, what was your set point for the interior? Also 20 °C?

Here I increased the heat loss rate of the car to 0.085:

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Zuletzt geändert von Jan Treur am Do 6. Apr 2017, 18:54, insgesamt 2-mal geändert.
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Re: kW fur Heizung und AC

Beitragvon umrath » Do 6. Apr 2017, 18:46

I'm not sure if the heat loss rate is responsible or if it's just bad efficiency.
I also don't understand why the conventional heating and the heat pump are equal in energy consumption beyond -10 degrees.
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Re: kW fur Heizung und AC

Beitragvon Jan Treur » Do 6. Apr 2017, 18:57

I have read that below -10 the efficiency of the heat pump is not better than the resistance heating; therefore the heat pump is not used below -10. The performance factor for a heat pump (COP or SPF) indeed decreases with temperature, as I know for domestic heat pumps.

I think the heat loss rate can be high, because the car has no 20 cm thick isolation as a house can have, and even no 10 cm.
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Re: kW fur Heizung und AC

Beitragvon umrath » Do 6. Apr 2017, 19:02

Hmm. I don't have any actual numbers for temperatures that low. But when driving the Premium (with heat pump) at -2 to -3 degree I was usually in the range of 0,6 kW while driving. A that sharp increase in energy consumption would astonish me because the heat pump had no problems to heat the car in no time. Why would it cease to work / be horribly inefficient just 7-8 degree lower?
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