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Old 11-08-14, 11:23 PM   #1
F357
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Default Anybody ever build a natural gas heat pump?

So I've been thinking lately...

What if you take an automotive style A/C compressor, and drive it with a natural gas powered engine? Then, capture the waste heat from the engine.

From what I can tell an engine will require about 8,000 BTU in gas per hour per horsepower. If I can spin a 3 ton compressor with a 3hp engine(Is that possible?), it would take 24,000 BTU natural gas input.

The engine, if 33% efficient, would create 15,840 BTU in waste heat. If you are only 90% efficient at capturing that, you would get 14,250 BTU in waste heat, half from the exhaust and half from the engine block. Run the engine in a sealed box with a 10K BTU capable evaporator, and make another heat exchanger for the exhaust gasses.

Put the two together, and you have 50K BTU heat output for 24K BTU gas input. In my area gas cost $1.01 per therm. That would cost $0.24 per hour to run.

If I had an electric heat pump with an EER of 11 it would take 4,545 watts to make the same 50K BTU. That would cost $0.50 per hour at my electric rate of 11 cents/KWH. Hey, that's more than double!

In the summer, 24K BTU gas = 36K BTU cooling for $0.24 and all the hot water I need.
Electric 3 ton A/C would use 3,272 watts and cost $0.36.

Could any of that actually work or is this just the mumblings of a crazy person? I know natural gas powered heat pumps exist or have existed in the past, but I can't find any currently for sale. Anybody else ever try this? I am not taking into account maintenance costs on the engine(oil, air filter, spark plugs), but I still think it could be cheaper.

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Old 11-09-14, 07:38 AM   #2
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google search term: trigeneration





Cogeneration (CHP-combined heat + power) and trigeneration (CCHP-combined cooling, heat, power) plants have been well researched, and many are in operation today in industrial applications. They have been proven to work best with gas turbines and ammonia absorption chillers in medium to large to gargantuan scale systems. Cities and districts with utility-provided DHW are by far the largest and most experienced group of experts using combined systems. Not surprisingly, most of these users are in Europe and nations previously ruled by the British empire.

From a provider's point of view, it makes good sense:


powerwaterheat.com has a good website that explores the tip of the iceberg of this subject. They build and sell medium and small scale units that can work anywhere.

By and large, the cooling system portion of micro-chp systems has not been exploited. The majority of small systems are built as temporary systems for remote work camps or disaster relief operations. For this purpose, space cooling is not considered important.

ORNL and the DOE have been attacking this idea for decades.

http://web.ornl.gov/info/ornlreview/...2/text/gas.htm

Last edited by jeff5may; 11-09-14 at 11:31 AM..
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Old 11-09-14, 08:23 AM   #3
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Automotive air conditioning has a reputation for being very inefficient, a COP of between 1 and 2 is common. 7kw output is about 2 tons. A COP of 2 is 6.824 EER.

Here's a performance chart. It's not pretty.
http://www.sanden.com/objects/SD7H15_Performance.pdf

I think I'd rather run a cogeneration generator(important that it isn't oversized but can still handle the startup load) that is installed inside the house to run a mini-split heat pump. Not sure if they are okay with running off generator power or not so maybe you are stuck with a conventional compressor if you decide to go that route.

You could use automotive equipment, don't get me wrong. I'm thinking there's a better way to go about this.

I personally think your best bet is to run the heat pump off of grid electricity if it is available. I'd expect it to be cheaper, cleaner, and quieter than running an AC compressor off of anything other than electricity.

Either way, good luck.
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Old 11-09-14, 08:34 AM   #4
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Quote:
Originally Posted by MN Renovator View Post
Automotive air conditioning has a reputation for being very inefficient, a COP of between 1 and 2 is common. 7kw output is about 2 tons. A COP of 2 is 6.824 EER.
Interesting. Is there nothing else available? Car compressors are probably inefficient for a lot of reasons, I would bet mostly just to keep them cheap and light. But what about something more industrial? Maybe a compressor out of some kind of refrigeration unit?
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Old 11-09-14, 10:34 AM   #5
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Connect a big induction motor to the engine, such that the engine will turn it a little faster than synchronous speed. The motor will then become a grid tie generator. Then you can optimize the setup to just supply power and not worry too much about inrush or regulation.
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Old 11-09-14, 11:43 AM   #6
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consider this:

Converting heat to mechanical or electrical energy is very inefficient. If your goal is to provide heating and cooling, an ammonia or lithium bromide absorption system is the most efficient. If only heating is your goal, a condensing furnace or boiler combined with a heat recovery unit on the exhaust pipe is more efficient.
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Old 11-09-14, 12:26 PM   #7
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CHP is quite efficient if you have a use for all the heat.
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Old 11-09-14, 04:15 PM   #8
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Quote:
Originally Posted by jeff5may View Post
Converting heat to mechanical or electrical energy is very inefficient. If your goal is to provide heating and cooling, an ammonia or lithium bromide absorption system is the most efficient.
Except components for such a small system are nearly non existent. Compressor heat pump stuff is often available for less than scrap price. Also the ammonia needed is highly poisonous so that doesn't seem like something that should go into a homebrewed system.

Quote:
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a condensing furnace or boiler combined with a heat recovery unit on the exhaust pipe is more efficient.
How do you figure? Is that more than 200% efficient?

A cogenerator is not a money saving device at $1.01 per therm. A natural gas heat pump just might be, if I can find a compressor that is efficient enough, and if any of my numbers above are even remotely close. It is also very hard to collect 100% of the waste heat from a cogenerator, if you are not using a refrigeration compressor. (Small water cooled generators or engines with governors are very very expensive and hard to find) To make electricity at these gas prices would cost about 40% more than buying it.

If my math is correct it is cheaper to run a natural gas engine than it is to spin an electric motor of the same size, even if you throw away all the heat. But in a heat pump situation, that heat is very valuable.

I can tell you for a fact that York used to build and sell a gas heat pump like this, that was roughly 3-5 tons for household use in the 80s/90s. It is no longer in production, and I don't know why. Maybe it wasn't efficient enough. Or maybe there was just too much maintenance keeping the engine running. Or maybe the EPA killed it for some reason...

Last edited by F357; 11-09-14 at 05:25 PM..
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Old 11-09-14, 06:02 PM   #9
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You could use a small car engine running a somewhat lean mixture at low RPM.
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Old 11-09-14, 07:43 PM   #10
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Google Whispergen.

Interesting tech

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