The Ergosphere
Saturday, April 30, 2005

Throw it back

Honda has announced a cooperative venture with Climate Energy LLC of Massachusettsto produce a "micro-cogenerator" (photo link) based on a Honda engine (hat tips:  Green Car Congress via Peak Oil Optimist).  The specifics are:

I make that about 21% electric efficiency and 64% heating efficiency [1].

Poster Gene DeJoannis at the GCC discussion notes that 3 kW of heat is only about 10 kBTU/hr, which is not enough to supply the peak winter heating requirements of a small row house, let alone a single-family home.  It follows from this that the cogenerator is not capable of functioning as a stand-alone heating system; it would require extra heat.  The concept is that the engine runs continuously, and heat requirements beyond the CHP system are provided by a conventional combustion boiler.  This is included in the auxiliary furnace.  (These graphics answer some questions regarding the odd-looking cogenerator efficiency numbers; it appears that the cogen exhaust does not allow recovery of the latent heat of the water vapor and is also a separate unit from the air handler; as a consequence, heat losses are higher from the cogenerator side than the backup furnace.)  It also appears that the full heat demand of a house can be met by the pair, and that the $8000 cost of the two units covers the entire heating plant.  Earlier, I had objected that a $8000 capital expense is very hard to pay off with a $600/year revenue stream; it appears that the incremental cost of the cogenerator over a conventional furnace is considerably smaller, and the payoff quicker.

The stated purpose of this cogen is to meet the average electrical demand of the typical house, without generating surplus power to feed the grid.  I believe that this is a mistake:

IMHO, a properly-designed system would be able to handle contingencies.  If the generator was capable of 3 kW and 30,000 BTU/hr, it could supply a 1 kW average electric load by operating at a 1/3 duty cycle.  It would also be able to crank up to full output and 30,000 BTU/hr to handle cold snaps, and help to feed the heat pump of the house down the block.  When the homeowner came home with a Prius+ or the like, the system could be programmed (perhaps via a Bluetooth or WiFi connection) to react to the car plugging in and boost generation to charge it on power from natural gas instead of oil.

But this isn't going to happen, because it's just too small.  The designers thought too small.

This one looks like it's under the limit.  It needs to grow some; throw it back.

[1]  The low efficiency is a disappointment too.  Cummins claims BSFC as low as 0.32 lbm/hp-hr for some of their diesels; assuming #2 diesel at 19,110 BTU/lbm this works out to over 41% thermal efficiency (and that's the higher heating value to boot).  It ought to be possible to achieve much better than 21% efficiency from a gas-fired reciprocating engine; perhaps this requires the freedom to begin with a clean sheet of paper. 

Would probably make more sense with a fairly large vehicle with a big engine, preferably a diesel for efficiency and longevity. 1KW is just silly...won't even handle the lighting load in many houses.

Give 'em credit for trying, though.
Sounds very similar to the business model followed by domestic CHP in the UK: size the unit so that the generated electricity can always all be used. This offsets the cost of the most expensive electricity on the grid, and minimises the capital cost by making a small unit.
Philip:  I'm not sure what you mean by "offsets the cost of the most expensive electricity on the grid".  A small generator running continuously has the operating characteristics of a base-load plant, which generates the cheapest electricity (fired by the cheapest fuel); if you wanted to offset the most expensive electricity you'd configure the cogenerator system so that it is suited for peak-shaving, cutting the fraction of demand which is met by the most expensive (least used, highest-cost fuel) generators.
Firstly, in the UK most people have access to piped natural gas which is reasonably priced, likewise a reliable electricity supply at about five times the cost. Our climate varies somewhat with location but the temperature does not reach extreme values, it does however fluctuate in an unpredictable and sometimes rapid manner. Gas prices are currently increasing owing to diminishing supplies.

1kW is a very useful amount of electricity. The best fluorescent lighting gear will provide 100lumens/watt - assuming luminaires are designed and not merely styled. Further savings can be made by using dimmable gear and controlling it with photo-cells and movement detectors. This will leave a considerable amount for other purposes.

3kW would provide ample hot water with some surplus for space heating. This could be supplemented by a modest solar panel. Additional heat input could be provided by using gas for cooking including boiling water. A gas fire could provide an instant source of heat.

The house could be very well insulated and have a carefully controlled ventilation system. Warm, moist, air could be extracted from bathroom and kitchen and passed through a heat-pump powered by the mechanical or electrical output of the CHP unit. Likewise the waste water and the exhaust from the gas fire and the CHP unit itself. The extracted heat can then be used to heat incoming air.

Thus the CHP unit is producing 3kW of heat plus 1kW of electrical power all of which is dissipated inside the house, add to this any other sources of heat within the house including people. A large proportion of this heat is recovered by the ventilation system and returned to the house.

It is important not to use an overated CHP unit, as for a large part of the time, during the night and on summer days, the unit will just be ticking over. Electrical peak requirements can easily be provided from the grid. Likewise sudden heating demand can be met with an auxiliary gas heater.

Of course for those of you who experience long cold winters or require cooling in the summer, the requirements and economics of the system will be quite different.

I hope you can understand my terminology, I certainly find lbm/hp-hr quite incomprehensible, but I suppose you did put men on the moon using such units.
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Alan, thanks for dropping by.

"1kW is a very useful amount of electricity."

Oh, don't get me wrong; I never meant otherwise (my place is well-lit on less than 100 watts).  What I'm claiming is that 3 kW on a 0-1.0 duty cycle is much more useful than 1 kW on a 1.0 duty cycle, especially if you are burning your highest-quality fuel to make it.

"3kW would provide ample hot water with some surplus for space heating."

But 9 kW of heat and a well-insulated storage tank would let you make the electricity when it was most valuable, rather than when you needed heat; you could heat 300 liters of water during the afternoon electric demand peak and then shut down, satisfying overnight heat needs from the tank and electric requirements from cheap base-load plants or wind power.  It would let you exploit the full cogeneration potential of the fuel during cold snaps as well as during mild and moderate weather; you could get the most out of the heating fuel for old structures which are not (yet) superinsulated.

Storing heat has its own benefits, such as synergy with wind power.  With the addition of an electric heating element in the water tank you could shut down the cogenerator when the wind was high and heat both air and water without burning fuel; if you can't store hot water for a day or so, you cannot take best advantage of spot conditions like that.  As gas gets more expensive it is going to be important to be able to use other sources when possible.

"It is important not to use an overated CHP unit, as for a large part of the time, during the night and on summer days, the unit will just be ticking over."

During the summer?  It shouldn't be used at all when you don't need heat; you would be wasting fuel in a 21% efficient engine when the same fuel could have been used in a 55% combined-cycle plant.

As for "just ticking over", that is the ideal situation.  Think of a Listeroid equivalent running at 400 RPM for minimum demand.  By my calculations, those units are a bit over 30% efficient.  At the 650 RPM full speed they are capable of about 4.5 kW mechanical, and by extension, about 10 kW thermal.  That will warm the air, heat the water, light the lights and run the heat pump of the guy next door all on the same fuel.  If you can only make your own electricity and have to burn more fuel for heat, the guy next door needs electricity generated somewhere else (with the heat thrown away) and total fuel consumption goes up.

"I certainly find lbm/hp-hr quite incomprehensible, but I suppose you did put men on the moon using such units."

Indeed we did... and don't forget, we beat the Russians.

If you look elsewhere you will see that I am bi-lingual; I speak both Imperial and MKS.  I will probably be leaning toward MKS in the future, but I reserve the right to exile that Gallic stuff to subtitles when the mood strikes me.

        You use a lot of arsenic, if arson is your calling
        Francium and Gallium are frankly pretty galling.

        -- Doctor Jane, "Battle With The Elements"
Thank you Engineer-Poet for the feedback on my comment. I have been browsing around The Ergosphere and have been finding it very interesting. Keep up the good work.

You claim to be bi-lingual, a polyglot would be a better description. SI MKS cgs Imperial and any mixture (cal/kg?). I was glad to be finished with slugs and poundals when I left school (40 years ago).

I would like to comment on philip sargent's remark about the UK business model. We can choose from any number of gas and electricity 'suppliers' (t'was not always so but I will spare you the politics). We get both of ours from Powergen who have been buying Whispergen CHP units. Now Powergen buys electricity and gas from other people and it is distributed by someone else's cables and pipes and the meters are read by other people again. Now it suits Powergen to buy base-load electricity at rock-bottom price and have its own gas customers generate the higher value extra for the winter day-time load. This still leaves the short-term peaks but they were there anyway. A more cynical advantage is that the consumer is locked-in to Powergen as the supplier of gas and electricity, as the customer for surplus electricity, and as the maintainer of the CHP unit (and probably the rest of the heating system) - very neat.
I have a ~1800 sq. foot house, 2 floors + finished basement, part of a duplex. Forced air natural gas heat and natural gas water heater. Looking at my bills, I average about .8 KWe usage during the winter, and a little over 5 KWt in gas. But that 5 KWt is the actual energy content before my 20+ year-old furnace gets a hold of it; 50% efficiency would be a generous estimate there. So in reality, I'm averaging about .8 KWe and 2.5 KWt in actual usage.

So unless I misunderstand the specs, 1.2 KWe and 3.2 KWt in usable output sounds like more than enough for my house, particularly if I replaced my inefficient windows/skylights while I was at it. I'd surely use the capacity of the backup furnace and grid electricity at times, but not very often; surely not often enough that it would justify the capital costs of something three times more powerful.

I admit my house (small, but relatively efficient due to adjoining structure and being partially underground) is not typical, but it goes to show that this heating unit size does have some value. And there's also the purely practical point - anything much bigger than this system would not fit in the space between the door and the wall.

I'm not looking to spend a lot of time creating a custom system here - I'm just a guy who needs an upgrade and would like to make the responsible choice if it's practical. Assuming I'm not waiting for the next generation, do you really think this thing is inferior to, say, a 93% efficient furnace with no electrical generation capacity at all? Because in the real world, that's the choice.
What's ideal and what's practical at the moment are often two rather different things.  Two years ago I was very interested in hybrids, but they were in such tight supply I wound up buying a diesel.  I'm getting about 40 MPG instead of 46-50, but I'm not getting 22-26 anymore either.
Point taken. All told, a regular old 93% furnace is not a whole lot less efficient than this thing.

Now that I think about it, the real problem with this thing (in the context of a small house like mine, anyway) is that the thermal output can only be used to heat air. They should release a line of products that are compatible with one another. If the same working fluid can be used to heat the water and run the dryer, it becomes usable year round, and you're saving a lot more money/energy.
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