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.

google search term: trigeneration

http://reginnovations.org/wp-content...strategies.jpg

http://www.trigeneration.com/trigeneration_diagram.gif

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:
http://www.lotuslive.org/energy/files/cogeneration.png

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

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.

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?

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.

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.

CHP is quite efficient if you have a use for all the heat.

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.

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...

You could use a small car engine running a somewhat lean mixture at low RPM.

Google Whispergen.

Interesting tech

Randen

Combined Heat and Power
 

This Combined Heat and Power idea has been discussed in some degree of detail HERE.

It could save some time.

I'm not sure why The fascination with car A/C compressors for hacking?

Really well designed, efficient, working piston compressors are widely available.

Maybe it's because folks just happen to have a car compressor laying around and want to do something with it.

Use The Google
 

By the way, if you "Use The Google" regarding the question originally asked in this thread, you will find a really significant amount of information.

Give it a try...

-AC

The First and Second Law of Thermodynamics say that when converting one form of energy to another, there will always be losses. Converting chemical energy (burning any kind of fuel) to mechanical energy has a lot of lost energy (put your hand next to any running engine ... that is heat energy you are feeling).

We have used these lossey devices ("engines") for hundreds of years because the fuel was cheap and plentiful. Relative speaking, these fuels still are.

Electric to mechanical energy (a motor) is actually pretty efficient compared to an "engine".

I'm not sure that I see your point here...

-AC

Natural gas is chemical energy. Converting it into mechanical energy will never be overly efficient, especially on a small scale.

Power companies buy their "chemical energy" (or nuclear energy) at wholesale and the economies of scale minimize the losses in converting it to electrical energy. Even though there are losses in the electric transmission line and some in the electric motor inside the compressor, it is not likely that a natural gas "powered" heat pump (compressor) will EVER be cost effective !


When fuel cells (which convert NG to electricity directly) become cheap, the above statements probably will go out the window !

BTW, the Japanese have developed small fuel cells because in certain parts of that country electricity is hard to come by (especially when all the nuclear plants were shut down). NG is expensive in Japan, but it those areas it was available.

We used a number of KWHs of electricity running machines and a few years ago we were burning furnace oil to heat.

Hey, why don't we get a diesel genset and with the oil we were burning and sending up the chimney. Why don't we power the machines at the same time. It actually made economic sense.

The diesel would power the generator providing us with electricity and heat the shop as well with its waste heat. It can make a lot of sense.

Two rationalities came to mind:

1/ The cost of diesel fuel was rising rapidly.

2/ We didn't care for another machine to maintain.


Randen

Yes, I know a lot about CHP. That is not what I am talking about. I have no plans to generate electricity.

I was just asking what is available. Suggestions?


Yeah, I'm pretty familiar with google. The question originally asked was "anybody ever build a natural gas heat pump?", and thus, google cannot answer that question because I was asking the members of the forum. But beyond that, if you do a search for natural gas engine driven heat pumps you find almost nothing. Most results are about industrial sized gas burning ammonia absorption systems. I'm not asking about commercial systems, I'm wondering if anybody else has ever tried to do this themselves.

You seem to be missing the point. Yes, I know exactly how much waste heat an engine produces, that's why I did the math for you in my first post. The engine in this situation is nothing more than a gas burner, that heat is not wasted, I am harvesting it.

Do you have any information to back that up? All the math I provided in my first post shows an approximate 50% savings. Is my math wrong?

That is not true. Commercial power generators throw away their heat. If you can use that heat for something else to displace a fuel you would otherwise be buying for heat, then you can save significant money. It is also much more efficient energy wise, if you care about that sort of thing.

I totally agree with you there. On the other hand, they've actually been lowering the price of natural gas! Maintenance is one of the biggest possible issues, but this is also something of a hobby.

York Triathlon
 

Triathlon (heat pump) - Wikipedia, the free encyclopedia

York made one like this in the late 90's, never caught on.

My question is this: do you have an abundant supply of free natural gas?

If so, it is economical at any efficiency. At some low temperature, you would not move enough BTU's with the heat pump to keep up with the thermal losses of the home. You could then rev up your engine and harvest the additional heat generated.

If not, most individuals in the field switch from air-source heat pumps to straight gas furnaces at a balance point somewhere around 20 degF due to the shortcomings of the heat pump system. It is just not wise to run the heat pump constantly at low COP when you can cycle a gas boiler or furnace to provide the same heat.

Above this balance point (most of the time in the USA), a heat pump makes sense, regardless of how you power it.

From Wikipedia


I actually worked alongside some guys trying to develop a Wankel engine to run the A/C compressor in the late 1980s. The engine would run on natural gas. It was aimed at small businesses (convenience stores, fast food restaurants, etc).

The system worked fine, but there were durability issues. When run continuously, the oil and spark plugs had to be changed every 2-4 weeks ! Also wear on the seals (Wankel equivalent to piston rings) was high.

The motivation was a predicted shortfall of electric power in the SE (check the population growth in GA from 1980 through 2010).

Setting out to burn NG is likely to encourage fracking, but I love the idea otherwise. I'm appalled that we can't buy NG heat pumps based on free pistons, without all the extra parts. Electronic valve control is making it easier to opmimize, too. Having the waste heat to temper the intake should make things easy.

It just makes me crazy, absolutely crazy, when old guys use bold print...

Feast your elderly eyes on this.

Sincerely,

-AC

It's interesting. If you look at that report, they are using running temps well in excess of our needs in the resi environment. The COP is around 1.6 and while there are savings to be had, I think a resi gas HP switching between ASHP and propane or NG (when needed) could have a COP of 5 and eliminate the ground loops. Also note they don't even look at it running at ambient temps anywhere below freezing

I find a lot of engineering articles still use water temps and boilers running at what we would have seen 20-30 years ago. 80% efficient copper fin boilers, as though they have never heard of low temp radiation or condensing equipment before. So much for real energy reduction.:mad:

Disappointed it's only for hot water. Would have been nice if they also had chilled water output. The real key is sizing it down to a residential scale.

Must have been problems with that particular company's engine. In Mazda rotaries, seal life is well over a thousand hours at high power, as long as 2stroke oil is used for apex seal lube instead of crankcase oil. No issues with crankcase oil, either. They do need extremely cold plugs when run at high power, though. Worst thing should have been rpm issues. It's difficult to get decent power at less than 4-5K rpm.

Charlie

I think that it mostly has to do with where you place the metering device.

With heat pump application, you get the added benefit of "wasted heat" from the engine & exhaust.

-AC

With 96% efficient natural gas furnaces I don't see there being much room for improvement for the complexity of running an engine to drive a compressor for the heat pump cycle. Gas furnaces are reliable because its a very simple concept, fire and hot air. It seems you are considering this more for cooling and reducing your electric usage or in lieu of electricity. It might make more sense to use a NG powered generator to run your hvac. And try to capture the exhaust heat into hot water. The york units had briggs and straton motors. I don't think these motors were considered efficient. From a technician's stand point I wouldn't want to repair a york triathlon, sounds very specialized and proprietary.

The problem with a 96% efficient gas furnace is in the method they are using to get the efficiency number, I fathom.

The heat loss threw the rear vent would need to be factored in to get a idea of its efficiency. I am not a pro on the subject I feel about 40% ? of the heat is being lost out the back vent. They do breath I am thinking a heat recovery unit off the vent would prove to improve the efficiency number to more of its true potential.

Generally the name plate ratings show input btu and output btu to derive the efficiency. I am curious about under what conditions and runtimes they do their tests. The usual testimony for 96% furnaces are the lukewarm to touch pvc flues. Cant imagine too much heat escaping through that lukewarm pipe.

Not much heat could be escaping from that type of a unit , interesting it must have a built in heat exchanger.

My Direct Vent Gas fireplace is 85 or 88% efficient , the Vent system is crude to say the least its Made by Heat n Glo

Usually direct vent means it exhaust vents into the living space, as in there's no flue. They sometimes say its 100% efficient because there's no loss out the flue. Of course you are also getting 100% of the carbon monoxide.
All furnaces with blowers have a heat exchanger to keep the carbon monoxide out. Even the run of the mill 78-80% efficient ones built in the last 25 years.

Your thinking of a No Vent fireplace

With My Fireplace No air leaves my house or enters it from the Fireplace , its sealed.


here is a Direct Vent Fireplace

https://www.youtube.com/watch?v=QobD_22S_vc

I'm re-visiting my options for replacing a gas furnace, and see that Yanmar is offering both CHP units, and pure gas-powered heat pumps with exhaust heat recovery. https://www.yanmarenergysystems.com/vrf/ Has anyone here tried an off-the-shelf system?

If you find some prices, I'd love to know them. I've read about natural gas heat pumps being available in Asian markets for years, but haven't seen any available here in the USA.

Thanks,

Charlie

Before I ask for prices, I want to have a size in mind, and my current furnace has no indication of BTU output by the numbers. Is there a decent formula for estimating that from the bills on the cold months? On the very worst days, it runs half the time.

No brand/model number? If you can find that, you can research the model to determine btu.

The most discouraging thing for me was that it looked like the smallest model was 8 tons. That might be about right for max output for my whole house, but we currently have 2 units and we close off over half the square footage unless we have a house full of company. I'd wonder if it could ramp down to less than a quarter of capacity efficiently when we don't need much in our mild climate down south (MS, USA).
Charlie

Model number? How many burners? Is it a natural gas furnace? Is the exhaust vented using metal or plastic pipe?