2005-03-29: See below
(note: this topic was handled briefly in the
"Where to go from here?"
There is one universal truth about heat engines: none can
convert 100% of its input energy into work (in the physics sense).
Some fraction of the input energy is always going to be converted to
low-temperature heat which must be exhausted to the "heat sink", which
is typically the environment (see
). This exhaust heat is often called "waste heat".
One feature of waste heat is that it isn't always necessary to waste
it. If there is a need for heat at or below the exhaust
temperature of the engine, the engine's heat exhaust may be usable as a
heat source instead of burning fuel directly. This system of
generating power is called cogeneration.
Cogeneration effectively uses fuel energy two or more times:
the first time as high-grade (high-temperature, low entropy) heat as
input to the engine, and the waste heat is used a second time as
lower-grade (lower temperature, higher entropy) heat for other
purposes. These other purposes include industrial process heat,
distillation of water or alcohol, input to absorption chillers, or space
If both heat and electricity are required at the same time,
cogeneration can achieve much higher overall efficiences than the
separate generation of heat and electricity. For instance, a
cogenerator which takes 100 kWh of fuel to produce 30 kWh of electricity
and 70 kWh of heat can eliminate 70 kWh of fuel that would otherwise be
burned in a furnace to provide heat.
Very high effective efficiency
If the efficiency of cogeneration is rated in terms of additional
fuel burned versus additional energy produced, it can be extremely
high. A modern home furnace may burn natural gas and capture 95%
of the energy as space heat. A gas-fired cogenerator with an
electric efficiency of 30% and an overall efficiency of 90% will lose
10% of the total energy of the gas, but for each unit of electricity it
uses only 1.33 additional units of gas; its effective efficiency is a
whopping 75%. If the heat losses can be reduced to the same 5% as
the furnace, this figure increases to 86%! This contrasts very
favorably with centralized powerplants which may burn gas with perhaps
50% efficiency, or coal at efficiencies as low as 33%. If fuel is
being burned to produce low-temperature heat, cogeneration is a good way
of getting electricity with very little extra fuel and fuel cost.
Domestic cogeneration (also called combined
heat and power
) is one form of distributed electric
generation. Like standard furnaces and water heaters, domestic
cogenerators burn fuel and produce low-grade heat; unlike furnaces and
water heaters, they turn part of their fuel into electricity.
Some attractive technologies have impediments to their
adoption. Gas turbines are inefficient in very small sizes and
fuel cells remain too expensive for common use. A building which
needs ~390,000 BTU/hr of heat and can use or sell 60 kW of electricity
can use a Capstone ICHP system, but this is about ten times too big for
most homes. For this and other reasons small cogenerators use
internal combustion engines, either Otto (spark ignition) or Diesel
Broad applicability, no availability
Just about every home which needs winter heat and is not
all-electric is a possible site for cogeneration. Heating fuel is
delivered to most homes either by pipeline or by truck.
Home-heating oil is almost identical to number 2 diesel fuel, and
engines can be run quite well on either propane or natural gas.
Homes appear to be natural sites for cogeneration.
Yet for all the availability of fuel, small cogenerators are very
scarce. Perhaps this is due to a lack of true markets in
electricity, where homeowners with an excess could sell as easily as
they buy. Regardless of the reason, devices which look like they
ought to be common household appliances are not for sale.
In the absence of a market in small cogenerators, it is difficult to
determine what they would cost in volume production. Superficially
similar systems have per-kW prices all over the map. Examples:
- A gasoline-powered Generac portable generator rated at 7550 W
continuous retails for $1150 at Home Depot, or $152/kW (2005 dollars).
German FCEV study compared the cost of fuel cells vs. conventional
powertrains. The study estimated a cost of $2450 ($39/kW) for
gasoline up to $6170 ($89/kW) for a diesel parallel hybrid, with CNG and
conventional diesel in between. This study used 1997 dollars; 2005
figures would be about 20% higher.
California study found that a gas-fired, 500 kW reciprocating
cogenerator would cost $1075/kW installed. The page is dated 2002;
the figure would be about 8% higher in 2005 dollars.
These systems are not directly comparable. Here is a table of
costs and properties:
|| Emission controls
||No cooling jacket or exhaust heat recovery.
Engine speed too high for long life.
||Presumably includes radiator and exhaust.
||Presumably includes radiator and exhaust.
||Presumably includes radiator and exhaust.
||Presumably includes full heat recovery.
The comparisons are interesting. The 550 kW cogenerator's
price is off the chart; clearly it is a low-volume product with few
economies of scale, and is not comparable to the other examples.
The one-lung engine in the Generac and vehicle powertrains are
high-volume products. Curiously, the cost of the vehicle
powertrains all run roughly 3-5 times the retail price of the Generac,
despite their outputs being nearly ten times as great; their cost
appears to scale with cylinders rather than power. The complexity
of a 4-cylinder engine is roughly 4 times as much as a 1-cylinder
engine. Complexity and expensive components would appear to
account for the extra cost of CNG over gasoline (costly tanks,
high-pressure plumbing), and diesel over CNG (high-precision injectors
and injector pumps). It seems fairly safe to posit that cost is
roughly proportional to cylinder (parts) count.
What do we need?
What are the properties required of a domestic cogenerator?
- Long life: Should run 10 years with only normal maintenance.
- Serviceable: Should be easily refurbished or replaced as
maintenance or obsolescence requires.
- Quiet: Should be suitable for installation in a basement
utility room or crawlspace.
- Low pollution: Should restrict pollution to marginal
increments over conventional heating plants.
- Standard form factor: Should fit in the footprint of a
standard heating plant and be otherwise suitable for retrofit
There are some difficulties unique to cogenerators which must be
- Pollution regulations: Federal law imposes more stringent
pollution limits on generators connected to the grid than those which
are not. CNG-fired piston generators require catalytic NOx
reduction with ammonia injection to meet these requirements.
Anhydrous ammonia is clearly not an acceptable material for use in
domestic heating plants, for reasons of both safety and law
enforcement. NOx controls are primarily required to limit
production of photochemical smog on hot days. It may be reasonable
to give waivers to heating plants which only operate in chill or cold
- Grid regulation: The utility must be able to limit
backfeeding so that grid protection measures remain effective. If
the available technology for demand-side management is any indication,
this should not present large difficulties.
What's the 2010-model, million-selling home cogenerator likely to look
- It'll come in one or more boxes, totalling about the size of a
- It will have a one- or two-cylinder, long-stroke engine operating
at a speed well under 2000 RPM, perhaps as low as 1000 RPM. This
engine will operate on something like the Atkinson cycle, for improved
efficiency and lower exhaust noise.
- Maintenance will ordinarily be limited to annual changes of the
engine air filter and an oil reservoir/filter cartridge.
- The engine module will have a heavy, sound-absorbing
housing. This may be a light shell filled with sand or water
on-site. There may also be active noise cancellation for the
intake and exhaust.
- Gas- and propane-fired generators will use catalytic converters
for emissions control. Diesel generators (retrofitted for oil
furnaces or where propane delivery is not available) will use catalysts
to reduce gaseous emissions and filters to trap particulates.
- The treated exhaust will feed into a condensing heat
exchanger. The cooled exhaust will be cool enough to exit through
PVC pipes, like today's condensing furnaces and high-efficiency water
- Engine thermal efficiency will be on the order of 35%.
Alternator and electronic losses will reduce this somewhat. Good
design will recover this energy as space heat, e.g. by using the house
air to cool the electronic heat sinks.
- Units will probably have some stand-alone capability, so that
they can provide heat and essential electricity during winter grid
outages. This would require a small starting battery, some
additional sophistication in the starting and power conditioning
electronics, and a resistor bank (dump load) capable of handling excess
This cogenerator will have perhaps two cylinders. Its cooling
system will be roughly comparable to a vehicle's radiator. The
alternator will have fewer parts, less bulk and lower operating loads
than an automotive transmission. It will not need a separate
starter. It will have electronic controls on the order of a
vehicle. The exhaust system will be more sophisticated, including
not merely muffling, catalytic aftertreatment and possible filtration
but heat exchange and handling of condensate. The fuel supply is
either piped in or assumed to be in place, so there are no additional
expenses there. Its cooling fan will be the house air circulation
It appears that the entire cogenerator system will have roughly the
same complexity as half a 4-cylinder car engine, and ought to be able to
sell for roughly half the price: $1500 for natural gas or propane,
$2500 for oil-fired diesel. This purchases a system capable of
producing 5 to 10 kW continuous, plus 30,000 to 60,000 BTU/hr of heat
(roughly 6000 BTU/hr/kW).
What does it get the owner?
Apparently, one hell of a lot. The average gas-heated home in the
USA uses 50 million BTU of gas per heating season; at current retail
prices of $.60/therm, it costs about $300 a year. If an owner
replaces an older, 80%-efficient furnace with a cogenerator with 60%
thermal efficiency and 35% electric efficiency, gas consumption rises by
1/3 to 66.7 million BTU and annual fuel cost rises by about $100.
However, the cogenerator also produces 23.4 million BTU (6840
kilowatt-hours) of electricity; at a retail price of 8 cents/kWh, this
electricity is worth about $550.
The typical household uses about 1 kilowatt average; over a 180-day
heating season, one would expect the house to use only about 4300 kWh of
electricity. The cogenerator would produce considerably more
electricity than the household could consume, and it seems unlikely that
the utility's other customers (who might have their own cogenerators)
could be expected to consume it. What else could the cogenerator's
output help to displace?
Why not gasoline? If the household has an electric or plug-in
hybrid vehicle, the cogenerator could feed it during the heating
season. If the electricity was fed to a vehicle using 250 Wh/mile,
the excess 2520 kWh could drive the car for 10,080 miles. If the
car would otherwise have gotten 35 miles per gallon of gasoline, that is
a savings of 288 gallons worth nearly $590 at today's pump price.
The final question: Where do you get the increased 1/3 in
natural gas consumption? Everything comes at a price, and in areas
where electricity is generated by burning coal the likelihood is that
some of it would come from petroleum, as fuel oil instead of
gasoline. If about 1/4 of the cogenerators switched to fuel oil,
the average household would burn 16.7 million BTU of oil and 50 million
BTU of gas. At 19,110 BTU/lb this would be 847 pounds of fuel, or
114 gallons. This displacement would not occur in areas where
electricity is generated with natural gas.
One unresolved issue is how much fuel is displaced at the electric
powerplant. If the house draws power from a coal-fired powerplant
with a heat-rate of 10200 BTU/kWh, the saved 4320 kWh would displace 44
million BTU of coal, or about 2 tons. The savings in CO2
emissions would be 7.3 tons. A gas-fired powerplant with a heat
rate of 6800 BTU/kWh would reduce its emissions by a mere 1.69 tons.
Last, I'm going to assume that heating oil costs, and is taxed,
the same as motor fuel so that there is no tax subsidy of electric
vehicles via oil-burning cogenerators. Last, for simplicity's sake,
I'm going to assume that the chemical formula of both gasoline and fuel
oil is Cn
- The cogenerator can reduce electricity expenditures on the order
of $350 per year.
- In combination with an electric or plug-in hybrid automobile,
motor fuel costs could be cut by as much as $576/year, offset by $228 in
- It appears that winter carbon emissions of the average household
could be reduced by more than 2/3.
- In areas where the marginal electric plant is a gas-fired
combined cycle plant with a heat rate of 6800 BTU/kWh, cost is cut by
over $800/year and the net carbon emission is cut by almost half.
- Raw rate of return on the cogenerator, assuming the vehicle is already
owned, is 33% to 50% per year.
This analysis does not include other improvements which could easily be levered
in. For instance, any building with central air already has most of the
hardware required to heat with a heat pump; using a heat pump, external
electricity can displace fossil fuel. Wind power is widely
available in the winter across much of the nation and could displace fuel for
lighting, heating and transportation at the same time. The potential
savings in gas consumption is immense, and cogenerators could easily
compensate for variations in wind intensity on a minute-by-minute basis.
Other issues and savings
The widespread use of cogeneration would improve many other things as well.
- Load on the electric transmission and distribution system would be slashed
during the winter, along with its losses.
- Vulnerability of the electric distribution system to disruption by e.g.
ice storms would be greatly reduced.
- Manageability of the electric grid would be greatly improved, as utilities could
request electricity to be converted to heat in dump loads or fed to the grid with a response time of seconds.
- Households would have a supply of electricity during winter grid outages as
long as heating fuel held out. In combination with a plug-in hybrid car,
the house would effectively have a complete uninterruptible power supply.
- In areas where electricity is gas-fired, net gas consumption would be reduced
and natural gas prices would fall. This would reduce total costs in a way
not captured by the above accounting.
Some people have been asking if we can afford to adopt measures like cogeneration.
Unless I'm terribly wrong, it looks like we can't afford not to.
(As always, corrections cheerfully accepted. I'll credit them to the originator.)
: Tom DC/VA thinks that
my estimate for cogenerator cost is low, by a factor of
. I can't see
how a mass-produced piston cogenerator in the 5-10 kW range could cost more than a 60 kW
automotive powerplant; the engine itself would have about half the parts count, and the
cooling system is roughly the same complexity but can be made more rugged (cheaper materials,
lower warranty costs) because it does not have vehicular weight constraints. The only
things that might do that are emissions regulations much more stringent than vehicles (already
noted), or difficulty in muffling.
Randall Parker writes
wind's lack of schedulability is a problem. I agree, but I don't think that this is a
reason to write it off; its value should be measured by what it can save. More on this