Combined heat and power (CHP) cogeneration
Last updated: March 6, 2009.
The world's fast running out of oil, coal, and natural gas, but that's probably a good thing on balance because these dirty old "fossil fuels"
are accelerating the problem of global warming
and threatening major change to the climate. Unfortunately, we can't just stop using fossil
fuels overnight: something like 80-90 percent of world energy still
comes from them. Until renewable energy, such as
solar and wind power,
comes fully on stream, what can we do instead? One solution is
to swap some of our power plants over to a different system called
combined heat and power (CHP), also known as
cogeneration. CHP plants make better use of the fuel we put into them, saving
something like 15-40 percent of the energy in total. They're good for
our pockets and good for the planet. So let's take a closer look!
Photo: A modern CHP power generator. Photo by Dennis Jones courtesy of
US Department of Energy/NREL.
How does CHP work?
A conventional power plant makes electricity
by a fairly inefficient process. A fossil fuel such as oil, coal, or natural gas is burned in a giant
furnace to release heat energy. The heat is used to boil water and make steam, the steam drives a turbine, the turbine drives a generator, and the generator makes electricity. (You can find out more in our main article on power plants.)
The trouble with this is that energy is wasted in every step of the process—sometimes quite
spectacularly. For example, the water that's boiled into steam to drive the
steam turbines has to be cooled back down using giant cooling towers in the open
air, wasting huge amounts of energy. Now a fuel-driven power plant has to work by heating
and cooling—that's what the laws of physics say—but surely
we don't have to waste quite so much energy in the process?
Photo: A typical coal-fired power plant. Photo by Warren Gretz courtesy of US DOE/NREL (US Department of Energy/National Renewable Energy Laboratory).
Instead of letting heat escape uselessly up cooling towers, why not simply pipe it as hot water to homes and offices instead? That's essentially the idea behind CHP: to capture the heat that would
normally be wasted in electricity generation and supply it to local
buildings as well. Where a conventional power plant makes electricity
and wastes the heat it makes as a byproduct, a CHP power plant makes
both electricity and hot water and supplies both to consumers.
Cogeneration (the alternative name for CHP) simply means that the
electricity and heat are made at the same time.
Types of CHP
The actual efficiency of a CHP plant depends on how well it supplies the heat it
produces. Since the heat is generally carried as hot water, the
efficiency is greatest when the power plant is closest to the
buildings it's serving. In other words, CHP works best as a
decentralized form of energy supply with more and smaller power
plants built very close to local communities. Cutting the distance
between power plants and consumers also makes the electricity supply
more efficient: since the electrical power has to travel down shorter
lengths of wire, less energy is lost due to resistance. Taking
decentralization to its logical conclusion, it can even work out
efficient for offices, schools, hotels, and homes to have their own
mini or micro CHP power plant producing their electricity and hot
water where it is consumed and sending any unwanted electricity to
the power grid for other people to use.
Photo: A micro-CHP turbine produced by Capstone Turbine Corporation. It's powered by LPG (in this case, propane) supplied from the white tank at the back. Photo by Jim Yost courtesy of US Department of Energy/NREL.
In theory, you could make a CHP plant simply by sending the waste hot water from a
conventional plant to local buildings. In practice, CHP plants make
energy in completely different ways using entirely different heat
engines (the machines that burn fuels to release heat). Smaller CHP
plants often use what are essentially internal combustion engines
(similar to gasoline engines in cars and diesel engines in trucks) to
drive electricity generators, with heat exchangers recovering waste
heat in hot water. Larger plants use very efficient gas and
steam turbine engines. In future, CHP plants are likely to use fuel cells
burning hydrogen gas.
None of this means CHP is a new or untested idea. The world's first proper power plant
(built at Pearl Street in New York City by Thomas Edison in 1882) was
essentially a CHP design: it supplied both heat and power to nearby
buildings in Manhattan. CHP was a brilliant idea we somehow lost in
the decades that followed, largely in the rush to create huge power
plants that burned inexpensive coal. It's an idea we urgently need to
rediscover in these environmentally challenging times.
Photo: A CHP power unit that runs on woodchips. The chips are loaded in at one end,
converted to a gas inside the machine, and then the gas is burned to fuel the heat and power engine.
The photo on the right shows the internals of the unit, including the electricity generating unit
(colored orange) made by Generac. Both photos by Jim Yost courtesy of US Department of Energy/NREL.
Pros and cons of CHP cogeneration
The efficiency advantages of CHP speak for themselves, but there are environmental
benefits too. Every tonne of fossil fuel we avoid burning stops
carbon dioxide from entering the atmosphere and reduces, just a
little bit, the problem of global warming. Burning fewer fossil fuels
also reduces air pollution and related problems such as
water pollution and acid rain.
Replacing huge power plants with more CHP plants that are much smaller makes us less dependent on the
centralized energy network and, in theory, major system failures and
outages (blackouts). Just like conventional power plants, CHP plants
can run off virtually any fuel, from oil, gas, and oil to methane gas
produced in landfill sites or power made by burning trash in
municipal incinerators.
CHP has few obvious disadvantages.
One problem is that the technology is currently
more expensive and complex, so building CHP plants typically requires
greater initial investment. Energy savings eventually pay back the
investment, but more money still has to be spent upfront to begin
with. Maintenance costs can also be greater for CHP. Another problem
is that smaller-scale CHP plants produce electricity more expensively
than larger-scale ones. Much more seriously, fossil-fuelled CHP plants
reinforce our dependency on the very fuels we should be trying to
eliminate (though it is possible to run them on greener fuels
such as biomass). And some critics argue that
CHP is less efficient than alternative technologies such as heat pumps,
which could be a far better way to tackle climate change.
Generally, there seems to be a growing consensus that cogeneration is the way forward for large-scale
electricity and heating supplies, and we're likely to see thousands
more CHP plants appearing all over the world in the coming decades.
Further reading
There are some excellent sources of further reading on the Web. We recommend you try
these two sites to start with:
