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A thought on efficiency

A thought on efficiency

A thought on efficiency

(OP)
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

RE: A thought on efficiency

Please share you economic calculations so that we have something to comment on.  Have you generated a pro forma that details all of the variables including indirect & direct costs for the added liability? When you are done, let us know what the IRR should be for this project?  Is this the best investment for your money?     

RE: A thought on efficiency

Err, this is just CHP, right?

- Steve

RE: A thought on efficiency

Well, the first time I saw it was in the early 70s on Tomorrow's World. Fiat  engine (1500cc?) running on gaseous gas as opposed to the other stuff. Supplies electrickery for a hotel via an AC generator, and hot water.

Second time I saw it was when I built it, large diesel engine supplied process power and heat for an agricultural installation. Warm up time was accelerated by using immersion heaters in the cooling system.




 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: A thought on efficiency

Cogeneration has been around a very long time, economics is the driver as to whether it is a reasonable approach.  The economics must be viewed on a case by case basis based on the environment.  There is an economy of scale when it comes to first-cost, maintenance, design, liabilities, fuel costs, etc.  If the economics show it is profitable, you do it, otherwise you don't unless you are a government agency or a hobbyist.

RE: A thought on efficiency

Well, you're not going to beat grid power for efficiency as that runs off a dedicated generator and fuel of choice. You're not going to beat a burner for heating efficiency, as there are no moving parts or expensive replacement parts. So, unless you need the portability and grid independence, there is no need for such a system.

Thus, they exist (as others have noted), but they are not in general use because they are more expensive and most people don't need the portability and grid independence.

RE: A thought on efficiency

not going to beat grid power for efficiency

um, cogeneration?
 

RE: A thought on efficiency

ivymike:
Not sure what you're saying here exactly. I'm talking about individual cogeneration units versus basic grid power. For efficiency in terms of kwh/$$, individual cogeneration cannot compete. If you're talking about industrial cogeneration units, I didn't read that as what the OP was about, and I believe that is in fact being done. I probably agree with whatever your were trying to say.

RE: A thought on efficiency

I thought that most new/upgraded base line power plants in the USA were cogeneration and efficient.  At least they are in my area (no coal is used here, only natural gas fired turbines with HRSGs).  I also thought that in areas where coal is used, even these plants use cogeneration with the recovered low grade heat being used for commercial/industrial applications when it is economical.  In contrast, peaking generation sets in my area are not cogeneration.  I can think of two power plants that have pushed their efficiency even further with trigeneration where they use the low grade heat for heating/tempering of water for commercial/industrial usage.  When you consider the economy of scale and the fact that the large electric utility companies can buy natural gas cheaper than I can, residential cogeneration is not even worth considering in these areas.

For some isolated areas where cheep energy is not available, residential cogeneration might make sense.  
 

RE: A thought on efficiency

Honda is already testing the waters in the NE US:

http://www.hondapowerequipment.com/products/homeenergy/freewatt.aspx

A brochure I have shows 290W -> 100W plant/outlet loss from the grid vs. 118W -> 100W CHP/outlet loss from their system.  

Fuel price volatility may be another variable that makes innovation tough for residential application.  Last year oil was 2X gas in cost- don't know how coal stacks up, but who can predict where those will fall over the payback period for an unconventional residential system?
 

RE: A thought on efficiency

Jsteve - oh, "efficiency" in kwh/$.  I thought you were talking about efficiency (of energy conversion), as in the OP.

Zapster - if by "most" you mean "less than 8% of production" then you're probably right.  Perhaps we should all define our terms more carefully.

 

RE: A thought on efficiency

ivymike,  as I stated "thought new/upgraded base"  not the entire base load.  Are you stating that only 8% of new natural gas power plants are cogen?  Or that only 8% new/upgraded base sets are cogen?   Or are you talking about 8% of the total generating capacity of the united states both base and peaking sets have cogen capabilities.  Please define what you are saying because I have no idea what your 8% represents.  

RE: A thought on efficiency

Reading misc. articles on the internet, one might get the idea that cogeneration is commonly used at relatively small facilities (not utilities) to reduce the money they spend on electricity.  

To quote from several internet sources:

http://en.wikipedia.org/wiki/Cogeneration
By 1978, Congress recognized that efficiency at central power plants had stagnated and sought to encourage improved efficiency with the Public Utility Regulatory Policies Act (PURPA), which encouraged utilities to buy power from other energy producers.  Cogeneration plants proliferated, soon producing about 8 percent of all energy in the U.S.[13] However, the bill left implementation and enforcement up to individual states, resulting in little or nothing being done in many parts of the country.

http://www.cogenworks.com/gtf_didyouknow.html
Although cogeneration has been in use for nearly a century, in the mid-1980s relatively low natural gas prices made it a widely attractive alternative for new power generation. In fact, gas-fired cogeneration is largely responsible for the decline in conventional power plant construction that occurred in North America during the 1980s. Cogeneration accounted for a large proportion of all new power plant capacity built in North America during much of the period in the late 1980s and early 1990s.

"The average efficiency of fossil-fueled power plants in the U.S. is 33% and has remained virtually unchanged for 40 years. This means that two-thirds of the energy in the fuel is lost–vented as heat–at most power plants in the United States. CHP systems achieve effective electrical efficiencies of 50% to 70%. This improvement in efficiency is an excellent pollution prevention strategy that reduces emissions of air pollutants and carbon dioxide, the leading greenhouse gas associated with climate change."

http://www.epa.gov/chp/markets/index.html
http://www.epa.gov/chp/markets/municipalities_fs.html
Combined heat and power (CHP) offers many benefits over separate heat and power for a wide variety of applications and users, such as: Industrial manufacturers
Institutions Commercial buildings Municipalities residential structures

Some sectors are well-suited to expand their use of CHP because: (1) CHP technology is a strong technical fit for these facilities' needs; and (2) CHP systems can potentially generate significant bottom-line cost savings for these industries and organizations.

Is My Facility a Good Candidate for CHP?
If you answer "yes" to 3 or more of these of these questions, your facility may be good candidate for CHP.
Do you pay more than $0.06/kilowatt hour (kWh) on average for electricity (including generation, transmission, and distribution)? Are you concerned about the impact of current or future energy costs? Is your facility located in a deregulated electricity market? Are you concerned about power reliability? Does your facility operate more than 5,000 hours per year? Do you have thermal loads throughout the year (including steam, hot water, chilled water, process heat, etc.)?

 

RE: A thought on efficiency

... and by the way, when you say "natural gas fired turbines with HRSGs,"  I would've assumed that those are used for bottoming cycles in combined cycle power plants (to generate more electricity), not for "cogeneration" where the heat itself is a desired product.  Do you know which is the case?

Living in the relatively isolated (apparently) Houston area, I was unable to get "cheep" energy, and paid $0.18/kWh all summer.  Some of my coworkers were paying $0.24/kWh.  How "cheep" is it where you are?
 

RE: A thought on efficiency

Simply put, the gas heater is only on when the thermostat tells it to put out heat.
The gas burning power plant would have to run longer to come up to temperature to put out the same heat as the furnace. If you have use for continuous heat and rotary drive power off the engine, then yes it is a great idea.
And unless the cost of electrical power is high enough, it usually ends up costing you to generate the power you sell back to the power company. Unless the fuel you use is free.  

RE: A thought on efficiency

"The average efficiency of fossil-fueled power plants in the U.S. is 33% . . ."  A surprising figure to me, but very much in line with Honda's figures.  Now what if their system were used to heat a home and charge a plug-in electric's batteries overnight?

RE: A thought on efficiency

ivymike:
I was focusing on why, given better thermal efficiency, cogeneration was not used at the residential level, which is what I read as the main purpose of the OP. And I inadvertently threw the overused term "efficiency" back in there with a different meaning. Great posts, btw.

RE: A thought on efficiency

(OP)
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

RE: A thought on efficiency

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.

 

RE: A thought on efficiency

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.

RE: A thought on efficiency

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.  

RE: A thought on efficiency

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

RE: A thought on efficiency

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: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers for professional engineers
 

RE: A thought on efficiency

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.

RE: A thought on efficiency

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: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers for professional engineers
 

RE: A thought on efficiency

(OP)
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    

RE: A thought on efficiency

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...
 

RE: A thought on efficiency

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.

RE: A thought on efficiency

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.
 

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