A thought on efficiency

[hydrae]

A thought on energy efficiency:

There are quite a few houses that are heated with natural gas, right now if the furnace is brand new it is 90% efficient, older 70%.

Is there an engine out there that is very quiet, extremely reliable such that it could run a 50% duty cycle for year between services and a decade between overhauls?

This engine could be installed inside a heat exchanger that would extract the heat from both the cooling system and exhaust to heat the house.

Now you are taking fuel and turning it into useful work and heat. You are still getting between 70% and 90% of the useful heat into the house plus maybe turning that electric meter backwards.

OK tear this apart, why is nobody doing this?

Hydrae

 

[hydrae]

Thanks all

The Honda and Marathon solutions are what I was interested in.

Too bad they are not available yet where I live (Northwest USA).

Hydrae

 

[ivymike]

I don't run my heater much here in Houston. Last winter, when gas prices were higher, my highest bill was $43 for 39CCF of gas. At 900BTU/cf, that'd be 3.51E6 BTU. In the same month I used 624kWh of electrity.

imagine that I had a genset at home with the following parameters:
air cooled
engine thermal efficiency 20%
generator efficiency 80%
engine power 3.75kW
genset power output 3kW

If I run that genset for an hour, I'll generate 3kWh electricity. I'll burn about 64kBTU of fuel (for a cost of $0.78). In the wintertime (last winter) that electricity would have cost me $0.13/kWh to $0.17/kWh ($0.39 to $0.51) depending on the month. Waste heat from the genset would be about 15.75kWh or 54kBTU. If I could capture 60% of the waste heat (seems low), that'd be about 9.45kWh or 32.4kBTU into the house. My house is equipped with a pair of 66kBTU/hr 80% efficient furnaces, so I'd be looking at the equivalent heat input (to house) of running both on a 30% duty cycle. The same heat provided by my heaters would cost $0.50 for gas and (x) for electric. For that particular hour, I'd come out somewhere between $0.09 and $0.23 ahead (plus whatever (x) comes out to) if I could use the electricity or run the meter backward. If I could only get a fraction of the purchase price of electricity back, then I wouldn't likely come out ahead... then again, I wouldn't necessarily expect that combined heat/power would make much sense in Houston, TX. In Illinois or Michigan we might get a very different answer.

A 30% duty cycle is a bit much in the daytime around here, but perhaps more reasonable in the evening/night during the winter. As luck would have it, I also use more electricity in the evening than during the day.

 

[romke]

the idea of cogeneration of electricity and heat is not new. on a small scale for home use FIAT had their TOTEM system ("total energy module") back in the seventies. at this moment in some european countries experiments are carried out with a heater combined with a stirling engine fuelled by natural gas. the problem in all cases is that you do not need all the heat you produce and all the electricity at the same time. in winter this usually poses no problem and excess electricity can be transferred to the national grid, but when you need all the electricity using all the heat cogenerated can be a problem. the newest approach with the stirling engines in a way solves the problem in such a way that primarily heat is generated for heating and hot water in a heating system and electricity only when there is temporarily excess heat that then through the stirling engine can be converted to electricity.

 

[dicer]

There are IC engines that get close to 50% effciency. And even better so, would a cogeneration system.
But to do that you have to use most all the heat from the system. If you sized it right then it should accomplish what you want. But like I said unlike the furnace, the cogen will be running all the time. So you need a use for the excess heat/motive power. I haven't kept up with the cost of electrical power. But the way it works is, we pay something like .07 / KWH and if we manufacture electricity to sell back we only get the grid price of say, .01 /kwh, and that will not cover the costs to generate it.
Now if you have close neighbors that need some cut rate power you can sell to them and make it worth it.

 

[globi5]

http://www.sunmachine.com/english/produkte_zukuenftigeprodukte.htm

 

[rmw]

I live a little northeast of ivymike-about 10 miles. What we need here is a similar system that would drive a generator that would run our A/C compressors 2/3 of the year (and as heat pumps in the winter) while using the waste heat from the exhaust and jacket water to drive an absorption type A/C system (or heat) in parallel. Since our A/C derived electric bills are much greater than our heating costs, this is the system that would appeal to me.

But remember that even at all that with that theorhetically efficient machine, you still lose a lot of the exhaust heat because you can't heat it below just the north side of 300 degF due to condensation and acid corrosion problems in the exhaust system, or alternatively build it out of some prohibitively expensive materials.

All of this is theorhetical and nice banter for engineers but the reality is that it would take the lifetimes of most of my children and their children to amortize the costs of such equipment on an individual housing unit basis hence the truth of Zapster's first post notwithstanding his confustion with the terms cogeneration and combined cycle.

rmw

 

[patprimmer]

I would think if you ran the exhaust to a high point then had it on a continuous downhill slope through the heat exchangers, the acidic condensation would then drain by gravity.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[mbt22]

I think the point was the corrosion, rather than the condensate itself. Flue gas heat recovery systems have a nasty tendency to rot if you cool below the dewpoint.

I'm speculating now, but perhaps ultra low sulphur fuels might mitigate the problem. Just carbonic acid rather than sulphuric, but I don't know enough about corrosion to decide whether this makes enough difference to the metallurgy.

 

[patprimmer]

Carbonic vs sulphuric makes a huge difference.

The lower the temp the less the corrosion once you have already got below the condensation point.



Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[hydrae]

There are heat exchangers mass produced now, in use, on the 90plus gas furnances that are condensing. They have been out for 20 years now, the flue is a pvc pipe. There is a PVC condensate drain line routed to the sewer. So extracting that heat for the house out of the exhaust gases should not be a big concern. My thought is just replace that burner with an engine, have the engine cycle with the demand for heat. The use of the device would be primarily for heat, power is just the byproduct.
Hydrae

 

[ivymike]

I was going to get around to mentioning it - my last house (in IL) had a >90 efficiency furnace, and used PVC for the entire visible length of both the combustion air inlet and exhaust tube. The PVC connected to the combustion side of the heat exchanger somewhere inside the furnace enclosure. Always thought it was neat that the exhaust was so cool...

 

[Tmoose]

Ours has a little sump with a pump triggered by a float when the liquid condensate level rises. When running the furnace off a generator after last weeks ice storm I remember to plug the pump AND the furnace in to 110.

 

[ivymike]

my one used gravity drain for the condensate, with a check valve. The latter became important when I installed a radon mitigation system, because the check valve would not allow water to exit the furnace if 2" vacuum was applied... so I had to put an air gap in between. Didn't figure that out until the blower for the combustion side was making fish tank sounds.