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Micro Combined Heat-Power Unit
Efficient extraction of space heat and electricity, from natural gas.
By Se TH MASIA
Seth Masia is managing
editor of SOLAR TODA Y.
exhaust out to
building exterior
air cleaner
electric
power out
In November, Honda introduced its micro combined heat-power (MCHP) appliance in
the United States ( hondapowerequipment.com). Since 2003, the company has sold
more than 60,000 of these systems in Japan and elsewhere. A competing machine,
made by WhisperGen in New Zealand ( whispergen.com), has just arrived in North
America after successful introductions in Britain and the Netherlands. The German
Sener Tec Dachs CHP is marketed broadly in Europe ( senertec.com).
In a CHP, a water-cooled piston engine drives an electric generator. About 20 percent
of the energy in the fuel comes out as electric power, and 80 percent comes out as heat.
That’s typical for an internal combustion engine. In a CHP, most of the heat is recovered
and used for either space or domestic water heating. Where a standard natural gas
furnace converts 50 percent of the fuel energy to useful household heat, the CHP converts 85 to 95 percent of its fuel energy to useful household power, split between heat
and AC current.
The Honda MCHP uses a tiny 165cc single-cylinder, water-cooled four-stroke
engine, burning natural gas and driving a 1.2-kilowatt generator. Engine
heat — the equivalent of 3. 25 kilowatts (k W) — is recovered by circu-
lating water through the cylinder jacket and through an elaborate
heat exchanger in the oversized muffler/catalytic converter.
That coolant, at up to 80˚C (176˚F), goes to another heat
exchanger to warm air or heat domestic water. The size
of a dishwasher, the MCHP runs when space heat is de-
manded. It hums along at about 44 decibels, roughly
the equivalent of a hushed library. U.S. Freewatt
( freewatt.com) packages the MCHP with a gas
furnace or with a backup generator/battery bank
combination designed to keep a home warm
and functional through an extended blackout.
An off-grid 12- or 24-volt version can go into
boats and recreational vehicles.
intake duct
generator
cylinder
head
The WhisperGen uses natural gas to run a
four-cylinder Stirling engine, quiet enough
for installation under a kitchen counter.
It produces 1 k W of electric power and 7
to 12 k W of hot water output. Sener Tec’s
Dachs (Badger) CHP, designed to run a
small business or apartment building, runs
a 580cc four-stroke engine to drive a 5. 5 k W
generator. It puts out 12. 5 k W in heat. Versions
are available to run on natural gas, liquid
petroleum gas, fuel oil or biodiesel. ST
oversize
crankcase
with oil sump
natural gas in
hot water out
cold water in
ILLUSTRATIOn By KURT STRUve
oil filter
muffler with catalytic converter and
water-cooled heat exchanger
