Question about the practical benefits of heat pumps


by Low-Q
Tags: heat, practical, pumps
phyzguy
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#19
Oct17-13, 12:17 PM
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Quote Quote by ZapperZ View Post
Let me get this clear. You use heat pumps as a heat source in Norway??!!! And these are used to heat your houses during winter? REALLY??!!Zz.
This is not so unusual. I had a air-source heat pump for heating the house in the winter when I lived in suburban Seattle. It functioned as an air conditioner in the summer and as a heat source in the winter, not that you need air conditioning very often in Seattle. To be clear, it had back up electrical heating coils that kicked in if the temperature got so low that the heat pump couldn't produce enough heat, but this rarely happened. One other thing that limited the efficiency in my system was that if it was cold and humid (which is often the case in Seattle), then the outside coils would ice up, and the system would run backwards, pumping heat out of the house into the coils, until the ice had melted.
Crazymechanic
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#20
Oct17-13, 01:40 PM
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as Sophie already said and I could agree a heat pump for heating your house in winter while it's very cold outside may not be that effective as for lower temperature regions were winters are less cold , Now in Norway depending on the region you can go as low as -30, maybe more sometimes , so thats some serious temperature to deal with.

But as we know the efficiency and the potential savings depend on the temperature difference between the two sides of the pump so I would say in a cold winter climate you would have to bury your pipes deep under ground where the earth gets warmer , I don't know how deep that is probably very deep, and not practical but in theory it would work kinda good.
DrClaude
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#21
Oct17-13, 01:55 PM
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Here are some typical numbers for a geothermal installation in Sweden. If you make one borehole straight down, it will be 100-200 m deep. You can also bury pipes some 0.6-1.5 m under ground level and lay a looping pipe, covering some 400-600 m2.
mikeph
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#22
Oct17-13, 02:05 PM
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Quote Quote by ZapperZ View Post
I had someone gave me a calculation of how inefficient it is in the conversion of fuel into electricity at a typical turbine. I think the number is like 15% or thereabouts. Do we know what is the energy conversion efficiency for a hydroelectric turbine?
I nearly spat my tea out when I read 15%, with aero-engines it's hard to design something worse than 60%. I guess the electricity generation comes with its own losses, but I think it should be no lower than 50% in the new combined cycle generation of gas power plants.
Low-Q
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#23
Oct17-13, 02:45 PM
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Quote Quote by ZapperZ View Post
I guess that would make sense now, since if the idea is to reduce the use of hydrocarbons, then the use of electrical heat pumps, no matter how inefficient the "chain of events" are, will not use any hydrocarbons directly, when compared to the use of fuel for direct heating.

I had someone gave me a calculation of how inefficient it is in the conversion of fuel into electricity at a typical turbine. I think the number is like 15% or thereabouts. Do we know what is the energy conversion efficiency for a hydroelectric turbine?

Zz.
That is hard to say. The efficiency of a hydroplant is somewhat limited to the efficiency of the generators, which ends up around 80% - 90% output versus harnessed energy from the waterfall. Harnessed energy depends on how much water we allow to bypass through valves or between the turbine blades. Efficiency compared to the total kinetic energy of all the water in the river / waterfall might be rather low. But that isn't interesting as the water flow itself is done for free by nature.

Vidar
Low-Q
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#24
Oct17-13, 03:03 PM
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Quote Quote by Crazymechanic View Post
as Sophie already said and I could agree a heat pump for heating your house in winter while it's very cold outside may not be that effective as for lower temperature regions were winters are less cold , Now in Norway depending on the region you can go as low as -30, maybe more sometimes , so thats some serious temperature to deal with.

But as we know the efficiency and the potential savings depend on the temperature difference between the two sides of the pump so I would say in a cold winter climate you would have to bury your pipes deep under ground where the earth gets warmer , I don't know how deep that is probably very deep, and not practical but in theory it would work kinda good.
It is not very common for housholds to dig deep in the ground for these pipes. However, I see that some people spend some extra money in making a "ditch" around the house they're building, for laying pipes for their pump before the pipes are burried. It does not have to be very deep if the pipes covers a large area. Some people also drill holes several hundred meter into the rock. Very expensive though.

Vidar
CWatters
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#25
Oct18-13, 03:58 AM
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A correctly specified, designed and installed heat pump will definitely be cheaper to run than a standard electric heater. However if the specifier/installer does a bad job it's easy for a heat pump to perform badly.

Many early heat pumps had limited maximum power output. Worse still the maximum power output of an ASHP falls when the outside air temperature is cold - exactly the time when you need the most power. Most heat pumps are supplied with booster coils (COP=1) to provide the extra power needed when it's very cold. Normally the booster coil should only kick in when it's very cold but if the unit is undersize for the house then it kicks in too early and too frequently with massively increased bills.

The COP of the heat pump is also proportional to the temperature uplift. This means that they perform better when coupled to low temperature UFH or oversize radiators designed to operate at lower temperatures. Some installers have been replacing gas fired boilers with ASHP without changing the radiators.

This is also an issue for Domestic Hot Water which if stored in a tank needs to be >60C to prevent Legionnaires' disease. If the household needs a lot of DHW then that also reduces the average COP.

So to get the best out of a heat pump make sure the heat pump is correctly sized for the house. Big houses might need two! Make sure your house is well insulated so that it's rare for the booster to kick in. Couple it to a low temperature heating system. Minimise the amount of DHW needed.

I believe many of the bad/negative reviews are as a result of getting the design or installation wrong. Sometimes an installer should tell the owner they would be better to use mains gas.


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