a cooperative venture with
Climate Energy LLC
of Massachusettsto produce a "micro-cogenerator"
) based on a Honda engine (hat tips:
Green Car Congress
Peak Oil Optimist
- 85% overall efficiency.
- 1 kW electric output.
- 3 kW heat output.
I make that about 21% electric efficiency and 64% heating efficiency
Poster Gene DeJoannis at
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
, and heat requirements beyond the CHP system are
provided by a conventional combustion boiler. This is included in
. (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.
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
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
This one looks like it's under the limit. It needs to grow some;
throw it back.
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.