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Old 03-10-13, 10:05 PM   #1
marx290
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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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Old 03-16-13, 08:03 PM   #2
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It has been done before and should be done again so I await your results. If I can find any specs from Greg Allen's waste heat pump, done in the 80s, I will post it but you may be able to google it if available. It is a good starting point.
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Old 03-16-13, 08:29 PM   #3
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It's one of a few projects I would really like to take on, but the effluent drainage in my home (rented) is all cast iron and there is no place to divert it into a heat exchanger tank. To recover heat through the iron pipe would probably require excessively low evaporator temperatures. The only effluent I have access to is the discharge from my washing machine into a mop sink, but we wash with cold water . I might be able to pull off a small under sink water heater in my kitchen that recovers heat from the drain water under the sink while I do the dishes.
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Old 03-17-13, 12:12 PM   #4
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Quote:
Originally Posted by marx290 View Post
I might be able to pull off a small under sink water heater in my kitchen that recovers heat from the drain water under the sink while I do the dishes.
Very interesting post.

Your head is moving in the right direction... now we just have to get your hands involved.

R290 is a very interesting, economical and efficient refrigerant. However, regarding an apartment-dweller engaging in R290 experimentation, I think it is a bad idea.

R134a can be bought from auto supply stores and R134a vapor compression machines are beginning to filter into second hand stores at encouraging prices.

Small water coolers with very small R134a compressors are showing up. They are nicely sized for a project such as you have in mind. I suspect that their compressors might not be so efficient, but the size is right. However, their cap tubes are integrated into the lines which return refrigerant from the evaporator. It would be difficult to scavenge those parts for your use... not impossible, but difficult. If you can pick up one of those little units for less than $15 it would be worth investigating... try the Goodwill bins. They sell all manner of useful stuff by the pound.

An easier bet would be to look for R134 de-humidifiers, as their cap tubes are easily separated for your use. You would want the smallest you could find, which would be about 20 pt. per day. These units would probably be a bit too big, but worth a try. Again, try the Goodwill bins.

Getting your hands involved will definitely improve your thinking.

As a sport shoe maker should have said, "Just make it."

Best,

-AC
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Old 03-17-13, 04:23 PM   #5
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I do have some concerns about dangers associated with R-290. (I use filtered BBQ Propane like many hobbyists, but calling it R-290 is close enough I suppose.) The work I've done with it before was in a pretty open air garage where, with some common sense and small charge quantities, I wasn't too at risk of an ignitable mixture. Now, in the cupboard under the sink could be quite a different matter and a few ounces of propane accidentally released, although unlikely, could possibly explode I suppose. Even with a low pressure kill switch and combustable gas alarm, it's probably not worth the risk.

I have a Type I EPA certification for refrigerants (never used it), so there are a couple of refrigerants I could get a hold of however, I have to say I'm not a fan of the proprietary halogenated refrigerants because of their expense, polluting nature of synthesis and global warming potential. Of course, the hydrocarbons might replace the need for most of them if it weren't for the flammability issue. It seems the HC refrigerants are doing just that in certain industries in some other countries, but that is in well tested equipment. For the experimental hobbyist, those flammability dangers are perhaps a greater danger. I will heed your warning and appreciate your suggestion. In cases where I don't feel I can use propane safely, I will consider using R-134a. I read a post somewhere of yours AC_Hacker, describing a refrigerant leak resulting in the denser gas sinking down a set of steps into a basement or something. Have you or anyone you know ever had an unsafe experience using propane as a refrigerant? Ignition even?

In regards to refrigerant control; I played with capillary tubes for a while, but quickly found them to be very limiting. I had a good deal of success using a brass flow control to control refrigerant control, just as the "Temperature Man" used to manually adjust a flow valve in ice plants, I could play with the orifice size and adjust it for a given set of conditions. It is certainly not as reliable as a capillary tube (I would imagine flash gas probably deteriorates the innards of the valve), and a manual valve obviously doesn't self regulate like a TXV or EXV, but it's a hell of a lot of fun and serves to teach the hobbyist what an automatic valve cannot. More of a needle valve with greater precision in control is something I'm looking for.
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Old 03-17-13, 05:45 PM   #6
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Quote:
Originally Posted by marx290 View Post
I read a post somewhere of yours AC_Hacker, describing a refrigerant leak resulting in the denser gas sinking down a set of steps into a basement or something. Have you or anyone you know ever had an unsafe experience using propane as a refrigerant? Ignition even?
I live in a house by myself so I'm not endangering my family... only myself. I have found myself literally wrapped in dense propane vapor, and if there had been a spark, I'd not be here to tell you to be careful.

When I have worked with propane, I have done it outside when possible, and when that was not possible I had power vents going and made sure my quantities were very, very small.

But the apartment thing would be unethical. Your neighbors would have nothing to gain from your experimenting and possibly everything to lose. Don't do it.

If you're in your very own digs, then the risk is to you alone.

Brad_C had some excellent advice regarding using a full face shield at all times when working with the stuff.

To answer your question, no I haven't had any explosions or ignitions. But then again, I did ride motorcycles for twenty years with no crashes... not because I was lucky, but because I was very careful.

The first time I did work with organic refrigerant, I realized that I had an electric heater with a thermostatic switch that could arc, in the very same space that I was working... very bad, super bad.

Quote:
Originally Posted by marx290 View Post
In regards to refrigerant control; I played with capillary tubes for a while, but quickly found them to be very limiting. I had a good deal of success using a brass flow control to control refrigerant control, just as the "Temperature Man" used to manually adjust a flow valve in ice plants, I could play with the orifice size and adjust it for a given set of conditions. It is certainly not as reliable as a capillary tube (I would imagine flash gas probably deteriorates the innards of the valve), and a manual valve obviously doesn't self regulate like a TXV or EXV, but it's a hell of a lot of fun and serves to teach the hobbyist what an automatic valve cannot. More of a needle valve with greater precision in control is something I'm looking for.
If you read through some of Brad_C's posts, you'll see that he scavenged an electronically controlled valve and then rolled his own controller. Pretty cool stuff, I think.

Best,

-AC

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