The world's 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: about 83 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 at how they 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—much of which literally disappears into thin air! 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 fossil-fuel power plant in Didcot, England, before older parts of it were demolished. This one now burns natural gas; a previous plant on the same site burned coal and oil. The cooling towers on the right waste energy, but they're nevertheless an essential feature of plants like this.
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.
Artwork: How much more efficient is combined heat and power? A conventional plant is at best about 60 percent efficient (for every 100 tons of coal or other fuel it burns, 40 tons is completely wasted); a CHP plant can be 90 percent efficient or even more. That gives a huge saving in fuel and operating costs and a major environmental benefit.
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 apartment buildings 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.
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 steam 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.
How does micro CHP work?
Here (greatly simplified) are the basic components of a typical micro CHP unit. In practice, there are multiple heat exchangers, noise silencers, and other components I've deliberately omitted for the sake of clarity.
Artwork: How a typical micro CHP unit works. Small microturbines like this are about 0.76m long, 2m high, and 2m deep, and weigh around 750–1000kg.
Fuel (coal, natural gas, oil, or biomass) is added at one end.
The engine (roughly the same size as a four-cylinder car engine) burns the fuel by ordinary combustion.
Something like 30–1000kW of electricity is produced, which can be used for conventional power or as an emergency
Exhaust gases from the engine flow through one or more heat exchangers,
which remove most of their waste heat.
A catalytic converter (similar to the one in a car) removes some of the pollution from the gases.
The (relatively clean) exhaust emerges through a tailpipe or chimney.
Cold water flowing into the heat exchanger picks up heat from the exhaust gas and exits at a much higher
temperature. If it's hot enough, it can be piped directly into radiators or fed into a conventional
central-heating boiler for further heating. A unit like this will produce about 40kW–150kW of thermal energy (heat).
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.
Pros and cons of CHP cogeneration
“The true net gains from combined heat and power are often much smaller than the hype would lead you to believe.”
Professor Sir David MacKay
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
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-fueled 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).
Some critics argue that CHP is overhyped and less efficient than alternative technologies such as
ground-source heat pumps, which, deployed at scale, could be a far better way to tackle climate change. The late Cambridge physicist
David MacKay, for example, pointed out
a theoretical flaw with the technology. Heat isn't wasted in conventional power plants in quite such an arbitrary way as we often assume: the "lost" heat is actually a fundamental part of making electricity as efficiently as possible in a conventional power plant—and optimizing the process to recover that heat can reduce the efficiency with which the electric power is produced. That's a problem, because electricity is a much more useful form of energy than heat (we can
do far more things with it). As MacKay puts it: "The true net gains from combined heat and power are often much smaller than the hype would lead you to believe." Property developers see the argument very differently, however: the cost of electricity from your own, small CHP unit can be half that of buying in from a utility—and there's the added advantage of having, in effect, a backup source of power (a major attraction for places such as hospitals or computer data centers, where power outages must be avoided at all costs).
Chart: Does CHP risk locking us into fossil fuel power? Chart drawn by Explainthatstuff.com using data
downloaded in April 2016 and July 2020 from the DOE CHP Installation Database, maintained by ICF International Inc. and funded by the U.S. Department of Energy.
The chart above confirms that about three quarters of current CHP systems in the United States are powered by natural gas, coal, or oil (blue, red, and yellow slices), with renewables (notably biomass) and various types of waste powering about a quarter. If anything, the domination of CHP by fossil fuels (natural gas in particular) is growing: the outer ring shows 2020 figures, while the inner ring shows the data for 2016, so there's been a noticeable shift to gas. Please note that this chart based on the total number of installations using each fuel type, not the total power generated by each fuel type, which would be a slightly different chart showing (I believe) even greater domination by natural gas and coal.
How will CHP develop in future?
Criticisms notwithstanding, 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. Take the United States as an example. According to a March 2016
report by the US Department of Energy (DOE), "The United States has the potential for more than 240 gigawatts (GW) of efficient CHP in industrial facilities and commercial buildings," at over 291,000 sites, which is equivalent to about 100 large (2–3GW) coal or nuclear power plants. That's about three times more CHP capacity than today (the comparable US figure, for 2020, is 81.6GW), but a massive 60 times more CHP installations (compared to 4579 CHP sites in July 2021).
While existing CHP installations tend to be big units, typically based in places like chemical, refining, or manufacturing plants, the DOE sees massive potential for expansion into a much wider range of large buildings, from
retail stores and universities to office complexes, hotels, and hospitals.
A Competitor Emerges for Solar Panels by Kate Galbraith. The New York Times. April 4, 2012. Explores the argument that micro CHP could be an attractive alternative to solar panels on smaller buildings.
Energy by Chris Woodford. New York/London, England: Dorling Kindersley (DK), 2007. One of my own books, this is a guide to where energy comes from, how we can harness it, and how we use it throughout the world. For ages 8–12.
Power and Energy by Chris Woodford. New York: Facts on File, 2004. Another of my books, this one is an illustrated history of human attempts to harness energy. Most suitable for ages 10–15, though of interest to older readers too.
↑BP Statistical Review of World Energy 2021, "Primary energy: Consumption by fuel," p.11, gives a figure of 83.1 percent of the world's primary energy supplied by oil, gas, and coal (462.77 out of 556.63 exajoules).
↑ Using the latest US data downloaded in July 2021 from
the DOE CHP Installation
Database, maintained by ICF International Inc. and funded by the U.S. Department of Energy.
↑ Indeed, some appear to suggest the heat was the more important part.
In Chapter 2: "Ground-based gas turbine combustion: metrics, constraints, and system interactions" of Gas Turbine Emissions [by Vigor Yang and Timothy C. Lieuwen (eds), Cambridge 2013, p.33], Vincent McDonell and Manfred Klein describe Edison's Pearl Street plant as "a cogeneration facility to produce industrial steam, with by-product electricity for local street lighting."
↑ These figures are based on the compact Capstone C65 and taken from its product datasheet, provided by Capstone Power Solutions Ltd.
↑ 30–1000kW are figures for the smallest and largest microturbines in Capstone's current range (the C65 and the C1000S). Smaller and larger outputs are possible.
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