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Old 11-08-14, 11:23 PM   #1
Helper EcoRenovator
Join Date: Sep 2014
Location: Oregon
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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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