3 ms·

Note that I'm not proposing a home generator, but using the gas combustion engine to directly power the heat pump compressor. That way all possible power is ex

Note that I'm not proposing a home generator, but using the gas combustion engine to directly power the heat pump compressor. That way all possible power is extracted, both mechanical energy and waste heat

solardev 3y ago
Basically it's the input vs output efficiency difference of your home generator vs the big ass grid one. For each unit of input fuel (like natural gas), the grid generator can get more electricity out of it than a small home gen can. It's true that you don't capture the waste heat of the home generator if you use grid power. But that's a lot of work to install and maintain too. You'd have to have a heat exchanger with the generator running in some sort of shell. You can't run the gen indoors because of exhaust gases. You also can't have it running too hot with no ventilation. So you'd have to enclose and insulate the generator outdoors, capture its heat output, heat exchange it with the home, tie it to the home thermostat, and still have a way to shunt extra heat (water heater, external exhaust, whatever) so the gen doesn't overheat. You usually can't just throttle down the generator either because they have some optimal RPM at which they run, and your heat pump depends on its output. It's just a lot of overhead for what would probably be a small improvement in efficiency (maybe not even that, if the decreased efficiency in home generation is bigger than the captured heat output). Now, at a municipal level or bigger, having to install such systems in homes, along with the natural gas infrastructure, is a much tougher thing than just running wires to each house. Many cold places outside major cities don't have reliable nature gas delivery to the home. It's a marginal improvement to a hard to fill niche, vs the simplicity of running a mini split off grid power. That can be installed in a few hours. I think a regular electric heat pump plus supplemental wood burning or kerosene stove in the home would give you simpler and more consistent results, personally, at the cost of having to breathe their exhaust fumes.
simonblack 3y ago
Note that I'm not proposing a home generator, but using the gas combustion engine to directly power the heat pump compressor. Isn't that ass-backwards? You can use the gas directly to run a heat pump . You're probably too young to remember kerosene fridges.
- m463 3y ago
  Are you talking about absorption refrigerators? I know RVs can have these based on ammonia that can refrigerate your food using propane or electricity. I thought they were not efficient at all, and used a different principle compared to a heat pump.
  - simonblack 3y ago
    Are you talking about absorption refrigerators? That's another name for them. I thought they were not efficient at all, and used a different principle compared to a heat pump. The principle is different but very similar: a continuous cycle of pressurisation, followed by expansion where that part of the system takes in heat from the surroundings and making them cooler. Kerosene has an efficiency approx 90%, according to one site I read. Using gas lowers that efficiency to about 70%. (I don't know why the difference in efficiencies, I would have thought it would be the same, no matter the heat source.)
themoonisachees 3y ago
Since nobody seems to be entertaining your idea: Sure. Maybe. The specifics aren't really worked out because nobody has bothered to work them out. Maybe it's better, maybe not. If you manage to find a way to extract 99% of the waste heat of the engine without choking it, you're probably still not getting 99.9% efficiency out of it, because ICEs do not always have stocheometric air/fuel mixes, and generally have a lot of inefficiencies in converting all of the energy present in the fuel, especially a small 2-stroke engine as seen in home generators. On top of that, you would need to devise a way to run a heat pump compressor off of an ICE, which is probably possible but you'd have to make that. So if you're a company looking to make a product to disrupt the electric heat pump industry, you would have to solve all of these problems, and at the end of the day you'd be barely more efficient than burning fuel in an electric plant and transmitting the power that way. Since current costs to the consumer are ballooning, you may edge out electricity in costs, but I highly doubt you're gaining that much efficiency because there isn't much to be had (modern natural gas electric plants are unfathomably efficient)
midasuni 3y ago
The best gas heat pumps I’ve seen have about 1.8 ratio. Better than a gas boiler, similar to a mid range electric one driven by a gas power plant, but way less than one driven by a grid with a decent amount of renewable (in terms of heat produced per unit of co2) On the assumption that grids are reducing their co2 load then an electric heat pump will continue to get more efficient while a gas one won’t.
iancmceachern 3y ago
I heard of "old guys" doing this for rural cabins in Colorado. They would take old 4 cylinder VW pancake motors, convert 2 of the cylinders to compress air, run the remaining 2 on gas, and then use the waste heat to heat the cabin. Then you have compressed air, heat, and electricity (you use the motor to spin a generator)

solardev 3y ago

Basically it's the input vs output efficiency difference of your home generator vs the big ass grid one. For each unit of input fuel (like natural gas), the grid generator can get more electricity out of it than a small home gen can. It's true that you don't capture the waste heat of the home generator if you use grid power. But that's a lot of work to install and maintain too. You'd have to have a heat exchanger with the generator running in some sort of shell. You can't run the gen indoors because of exhaust gases. You also can't have it running too hot with no ventilation. So you'd have to enclose and insulate the generator outdoors, capture its heat output, heat exchange it with the home, tie it to the home thermostat, and still have a way to shunt extra heat (water heater, external exhaust, whatever) so the gen doesn't overheat. You usually can't just throttle down the generator either because they have some optimal RPM at which they run, and your heat pump depends on its output. It's just a lot of overhead for what would probably be a small improvement in efficiency (maybe not even that, if the decreased efficiency in home generation is bigger than the captured heat output). Now, at a municipal level or bigger, having to install such systems in homes, along with the natural gas infrastructure, is a much tougher thing than just running wires to each house. Many cold places outside major cities don't have reliable nature gas delivery to the home. It's a marginal improvement to a hard to fill niche, vs the simplicity of running a mini split off grid power. That can be installed in a few hours. I think a regular electric heat pump plus supplemental wood burning or kerosene stove in the home would give you simpler and more consistent results, personally, at the cost of having to breathe their exhaust fumes.

simonblack 3y ago

Note that I'm not proposing a home generator, but using the gas combustion engine to directly power the heat pump compressor. Isn't that ass-backwards? You can use the gas directly to run a heat pump . You're probably too young to remember kerosene fridges.

m463 3y ago
Are you talking about absorption refrigerators? I know RVs can have these based on ammonia that can refrigerate your food using propane or electricity. I thought they were not efficient at all, and used a different principle compared to a heat pump.
- simonblack 3y ago
  Are you talking about absorption refrigerators? That's another name for them. I thought they were not efficient at all, and used a different principle compared to a heat pump. The principle is different but very similar: a continuous cycle of pressurisation, followed by expansion where that part of the system takes in heat from the surroundings and making them cooler. Kerosene has an efficiency approx 90%, according to one site I read. Using gas lowers that efficiency to about 70%. (I don't know why the difference in efficiencies, I would have thought it would be the same, no matter the heat source.)

themoonisachees 3y ago

Since nobody seems to be entertaining your idea: Sure. Maybe. The specifics aren't really worked out because nobody has bothered to work them out. Maybe it's better, maybe not. If you manage to find a way to extract 99% of the waste heat of the engine without choking it, you're probably still not getting 99.9% efficiency out of it, because ICEs do not always have stocheometric air/fuel mixes, and generally have a lot of inefficiencies in converting all of the energy present in the fuel, especially a small 2-stroke engine as seen in home generators. On top of that, you would need to devise a way to run a heat pump compressor off of an ICE, which is probably possible but you'd have to make that. So if you're a company looking to make a product to disrupt the electric heat pump industry, you would have to solve all of these problems, and at the end of the day you'd be barely more efficient than burning fuel in an electric plant and transmitting the power that way. Since current costs to the consumer are ballooning, you may edge out electricity in costs, but I highly doubt you're gaining that much efficiency because there isn't much to be had (modern natural gas electric plants are unfathomably efficient)

midasuni 3y ago

The best gas heat pumps I’ve seen have about 1.8 ratio. Better than a gas boiler, similar to a mid range electric one driven by a gas power plant, but way less than one driven by a grid with a decent amount of renewable (in terms of heat produced per unit of co2) On the assumption that grids are reducing their co2 load then an electric heat pump will continue to get more efficient while a gas one won’t.

iancmceachern 3y ago

I heard of "old guys" doing this for rural cabins in Colorado. They would take old 4 cylinder VW pancake motors, convert 2 of the cylinders to compress air, run the remaining 2 on gas, and then use the waste heat to heat the cabin. Then you have compressed air, heat, and electricity (you use the motor to spin a generator)