Ask HN: Why not natural gas powered heat pumps?
It has been said on here previously that it is more efficient to have a gas powered generator that sends electricity to a house, to run a heat pump, then it is to heat the house directly by burning natural gas. My question is why not both? Use natural gas to the house to run a combustion engine, that in turn runs the compressor on the heat pump. And use the waste heat off the engine to also add to the home eating. Wouldn't that be effectively a very efficient way to heat a home (compared to direct natural gas, or electric heat pumps that run from non-renewable electric source)?
34 comments
[ 3.0 ms ] story [ 79.0 ms ] threadBut a renewable energy powered heat pump is better.
It’s more about utilities taking on the difficult work of making electricity greener (I recently learned California burns a substantial amount of natural gas for electricity production! But at least they are able to continually pick up the solar percentage in the daytime) and homeowners taking on the work of making their own consumption cleaner.
Also it’s a natural security measure to try to reduce reliance on fuels that we may need to import one day.
The shutoff at 25F heat pump thing is people installing a device not suited for their climate. I live in IN and 90% of the people around me bought a 25F crap heat pump “for the environment”, and are actually doing more harm and eating $600 electric bills. People don’t do enough research before buying heat pumps.
(Unsure how corrosive the gases are, but shouldn’t be given the amount of thin metal and/or ABS (PVC?) vent pipe used)
https://www.epa.gov/chp/what-chp
That being said, I’d much rather leave electricity generation to a utility, maintaining generators is expensive and you’re screwed if your primary generator goes down without utility backup power or a secondary backup generator.
If you take 1 unit of gas and burn it you get ate 1 unit of heat
If you take 1 unit of gas and make electicity in a power plant you get 0.5 units of electricity and can make that into 1.5 units of heat
If you take 1 unit of gas and make electicity in a small home generator you make 0.1 j it’s of electicity and make 0.3 units of heat
The ratios are likely to be off on the specifics but that’s how a heat pump powered by grid electicity from grid gas is better than gas directly and gas directly is better than a local generator.
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.
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.
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.)
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)
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.
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)
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.
Firstly, consider the capital costs of an engine + natural gas pipeline + heat pump (excluding electrical motor) versus either a natural gas pipeline + burner _or_ a heat pump + electric motor. A high guess on the lifetime of a suitable natural gas engine under consumer quality maintenance (variable and generally late) would be 10,000 runtime hours. On heating duty cycle that's about 5 years. You can reduce runtime hours with more starts and stops, but that reduces the engine life -- probably more than it saves.
So the extra capital costs of the engine must amortize over five years out to less than the lost efficiency. Importing electricity has economies of scale in capital costs. Direct burning of natural gas has low capital costs.
Secondly, there's the efficiency question. A moderate-sized heating system has a peak power of around 100,000 BTU/hr or about 29 KW. That's about 37 horse power. Engines of that size are piston engines with around 20% efficiency. Therefore out of every 5 units of fuel-heat you get 1 unit of mechanical energy you have 4 units of waste heat. That mechanical energy put through a heat pump will net you around 3 units of heat for a total of 7 units of heat for every 5 units of fuel.
That's better than just burning it, but 140% thermal efficiency is only ~45 points better than a natural gas furnace and comes with much higher maintenance costs (oil changes, filters, etc.).
Importing electricity has economies of scale in production, even if generated using natural gas, because the 'engines' are of more efficient types with 40-60% thermal efficiency. Further, an electric motor is low maintenance and long lasting. So for every 5 units of fuel-heat in a power plant you can get between 6 and 9 units of heat through an electrical heat pump. Not too different overall, but with much lower per-house capital and maintenance costs.
So compared to a natural gas furnace it's not a win because of the capital costs and maintenance costs versus the much cheaper equipment; a ~$3000 engine every five years is $600 per year, which buys a lot of natural gas. Compared to an electric heat pump it's not a win because the efficiency of engines that small aren't great.
https://www.robur.com/en-us/applications/solutions/solutions...
https://www.robur.com/en-us/products/gahp-a
Since large centralized power generators are much more efficient, yet still put out prodigious waste heat, why not use that waste heat to heat homes?
You get the benefits of an electric heat pump while recovering a large portion of the plant's waste heat.
District heating plus heat pumps is probably the most efficient you can make a combustion plant.
Once we get off of fossil fuels, we can recover waste heat from industrial processes, or convert the district heating system to be purely electric or concentrated solar or something.
Pushing all the "spare" thermal energy from nuclear plants into the sea or lakes has been a tremendous waste.
Apparently while others talk about it, it is actively being worked on in China. https://www.nucnet.org/news/city-of-haiyang-first-in-country...
https://en.wikipedia.org/wiki/Cogeneration?wprov=sfla1
I have seen reports (do not have reports to link to sadly) on massive losses in the pipe infrastructure.
While there is loss in the electric grid too, at least it wont contribute to more emissions, as well as probably being overall less costly than the lost gas.
Because the vast majority of the waste heat created by the relatively inefficient generator would be used for heating the home, the electricity produced is nearly free. If you run the heat pump compressor directly from the engine, even more so.
The main caveat is that the generator (which would need to be a well insulated, liquid cooled type) would be expensive and would require rebuilding every 10000 hours or so.
The efficiency of a small generator is about 1/2 of that of a power plant, but between harvesting 90percent of all waste heat and having no distribution losses, the fuel consumption cost would typically end up lower than the electricity cost of an all electric system.
This co-generation design is actually pretty common here fuel oil is the main heating fuel, but it should work even better on gas.