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Old 03-10-13, 10:05 PM   #1
Vapor Compression Goon
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Default Liquid to Liquid Heat Pump Water Heater

When I run my hand across a warm drain pipe I don’t think of dirty water going down the drain, I think “Jackpot!”. That, my curious friend, is some primo grade heat available just going to waste. I see hot water as a very precious thing. Water has an unusually large capacity to hold heat and being a fluid, it’s not terribly hard to recover it; certainly easier than getting heat from the air.
I'm fascinated by the heat pump water heaters, but I know they have extreme limitations when installed in cold basements and such. I'm curious about recovering that warm greywater going to the sewer. Not just a copper preheater wrapped around the the drain pipe, but a system that pumps that quality energy out of the effluent and into a water tank for consumption.

Here's some ramblings from my blog to explain further:

"These machines are neat and deserve attention, but also deserve improvement in my opinion. As I explained before, their performance is determined by the environment in which they are installed. Sometimes, they are well suited to that environment providing cooling/dehumidification and hot water. Other times, not so much. I would not label this as ‘appropriate technology’. Now, if installed to recover humid bathroom heat during a shower, that certainly makes sense. At the very time hot water is being used, it is readily available floating about the room. Of course, much of the hot water is still going down the drain, partially recoverable by a heat exchanger. I want that heat.

I loathe heating and cooling air. Not only does it require a lot of surface area to exchange heat because the heat capacity of air is relatively low, but it takes a lot of energy to blow it around too.

I propose, and would someday like to build a heat pump system which recovers and reuses as much of the heat in a domestic hot water system within reason. This would require, rather than a atmospheric coil absorbing heat from the air, a more passive coil bathed in the warm effluent of a home or apartment building. Much like the simple copper heat recovery heat exchanger, this setup would provide the most heat to the system when it is being consumed. The warmer the effluent temperature, the higher the evaporator pressure, the smaller the compression ratio and thus a higher coefficient of performance. I would think significantly higher than a conventional heat pump water heater. The warmer the water reaching the evaporator, the higher the performance. Hot water supply lines are often insulated to provide the warmest water at the spigot; with a recovery system like this it is advisable to insulate the drains as well. To keep the temperature difference small between the warm effluent water and the evaporator so the compressor does not have to do much extra work, the water would probably have to travel through an insulated tank similar to a water heater tank where the warmest water rises to the top and the coldest water sinks to the bottom; just like a water heater. A submerged coil in this warm region could absorb heat and pump it into fresh water in the domestic system while influent is deposited somewhere in the middle or the top and effluent runs through a vented overflow sourced at the cold water in the bottom. Performance of a system such as this could be further improved by the appropriate application of Phase Change Materials with the system. Both tanks, the recovery tank and the domestic hot water tank, could be fitted with a latent heat storage mass or masses capable of absorbing excess heat energy when it is available, then release it when needed; there by increasing the capacity of the heating system, lowering power consumption and/or decreasing the overall size.

It sounds big, complicated and impractical. Probably the first one would be, but I think such a system could be made that would be low cost, long lasting and very successful. It could be implemented in existing structures with some ingenuity or configured into new homes as part of an overall energy management philosophy. Solar water heating in climates where this is an option should be explored first, but in other areas much more consideration must be made for the energy we have on hand."

Has anyone else seen an attempt at something like this? Any ideas/thoughts?

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