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:- 85% overall efficiency.
- 1 kW electric output.
- 3 kW heat output.
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:
- This choice requires the utility to handle the difference between base-load and peak requirements. It would be trivial to trade off heating duties between a bigger cogenerator and the furnace section to approximate the daily demand curve and smooth the load at the utility. Instead, the cost of winter peaking is put where it's the most expensive (and least efficient) to meet it.
- The design removes most capability of generation for backup power.
- The design foregoes the capability of generation for other loads, such as charging electric cars. In other words, it's not designed with the future in mind.
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. ¶ 4/30/2005 12:08:00 AM
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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.
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.
# posted by 
: 5/01/2005 7:25 PM
: 5/01/2005 7:25 PM
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.
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"
"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"
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.
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.
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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