Sunday, November 15, 2009

Heat Pump Water Heaters

I added a couple new entries in the Heat Pump Water Heater Section

The first is a Tom Gocze video that describes installing a Geyser Heat Pump Water Heater -- a good video -- informative and funny in places.  Tom is a long time friend of Build-It-Solar and has contributed a lot of helpful information.  He does some radio and TV work in Maine, so if you are in place where you can get these, they are very good.  Tom knows renewable  energy well, and has a good, practical approach to energy matters.


Heat pumps in general remove heat from one place and "pump" (transfer) it to another place.  For the heat pump water heater, the heat is removed from the household air and transferred to the water in the water heater tank.  The efficiency of the whole process is about 200% (that is they have a Coefficient of Performance (COP) of about 2).  This compares to a regular hot water heater which has a heating efficiency of nearly 100%.  In addition, during the summer they have the added benefit that they provide some free air conditioning -- that is, they cool and dehumidify the household air some. 

The cost of the units are around $700 to $1400.  Some of them (but not all) qualify for the 30% federal tax rebate, and some states may also offer some incentives.

On the down side the cool air they provide, which is a plus in the summer becomes a negative in the winter.  In the winter the cooled air has to be heated back up by your space heating system, and this is a cost.  It seems to me that in effect it will reduce the efficincy back to about what a regular electric water heat is for the winter.  
This paper on a Building America demonstration home shows a way to hook up the heat pump water heater in such a way that in the summer, the cooling air is used to cool the kitchen area, but in the winter the ducts are switched around so that the heat pump air is taken from the crawl space and does not cool the house air (or at least not as much).  This seems like a worthwhile improvement, but does complicate the installation.

Does anyone have any ideas on a winter hookup that would not steal warm air from the house?  Seems like there ought to be a way to do this?

From a carbon emissions point of view, I get these numbers -- the
- Regular electric water heater:  1.5 lbs CO2 per KWH heat out
- Heat pump water heater: 0.75 lbs CO2 per KWH heat out (COP of 2)
- Regular NG gas (EF0.65):  0.75 lbs CO2 per KWH heat out
- Tankless NG gas water heater (EF0.85): 0.6 lbs CO2 per KWH heat out

So, they are substantially better than a straight electric water heater, about the same as a conventional gas water heater, and not quite as good as a tankless gas heater.
These numbers are based on the 1.5 lbs of CO2 per 1 KWH of electric power (the US grid average), and 0.5 lbs of CO2 per 1 KWH of heat output from NG.


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