Energy wasted in the average home.
SOME people make their own bread; some make their own clothes. But the
Harrison family from Chester in north-west England has taken self-sufficiency
to new heights. They're making their own electricity.
The Harrisons are one of a handful of families in Britain and the Netherlands
testing miniature power stations this winter. A boiler-sized unit in the
garage burns natural gas to supply the family with hot water and central
heating, just as their old boiler did. But the new unit also generates
electricity, which keeps the bills down. Good news for the Harrisons. And
good news for the environment too—if enough people can be persuaded to
install them.
The unit in the Harrison's garage is what's known as a combined heat
and power (CHP) plant. CHP has been commonplace in factories, schools and
other large buildings for decades. According to Britain's Combined Heat
and Power Association, CHP cuts fuel consumption by up to 35 per cent.
That's because the waste heat from generating electricity—which power stations
normally fritter away in cooling towers—is used to heat buildings and water
instead. And you don't lose energy transmitting the electricity hundreds
of kilometres along overhead cables.
Installing CHP units in houses seems like an obvious next step, but
it hasn't been an option until now. Large CHP units generally run on either
a gas-burning internal combustion engine or a gas turbine, but these are
totally unsuitable for houses. Internal combustion engines are noisy and
juddery and need frequent maintenance. Turbines, meanwhile, rapidly dwindle
in efficiency as they're scaled down.
And there's another problem: in a factory or apartment block, CHP's
main job is generating electricity. The waste heat is put to good use,
but it's a by-product. Domestic CHP has to do it the other way round. You
want hot water and heating on demand, not just when you've switched on
a light or the television.
Together, these problems mean that there is no off-the-peg technology
suitable for household systems. Many people assumed that fuel cells would
fit the bill (New Scientist, 18 November 2000, p 16). Fuel cells
burn gas to generate electricity, and they can be rigged up so that most
of the fuel's energy goes into heat. They are also quiet and they have
no moving parts, so don't need regular maintenance. Last month Plug Power
of Latham, New York, and German heating firm Vaillant installed the first
domestic CHP fuel cell in an apartment block in Gelsenkirchen, Germany.
But there are no units for single homes just yet.
This winter, however, personal CHP has become a reality. A few homes
in Britain and the Netherlands are making their own electricity, and not
a fuel cell in sight. Instead, the prime mover is a venerable technology
called the Stirling engine.
The Stirling engine was devised in 1816 by Scottish inventor Robert
Stirling. It has a sealed tank of gas with a heat source at one end and
a heat sink at the other, plus a piston arrangement inside. The heat flow
from one end of the tank to the other is regulated to produce fluctuations
in pressure, which drive the piston (see Diagram).
Stirling engines can be extremely efficient, but they have one big
problem: they respond very slowly. That's because they are external combustion
engines, so the heat has to be conducted from its source into the chamber
before it can take effect. In a car's internal combustion engine, on the
other hand, the fuel explodes inside the cylinders and does its work almost
instantly. This precludes Stirling engines from most uses—you don't want
to put your foot down and then have to wait several seconds for your car
to move.
But a short delay doesn't matter in CHP. And the engines have advantages
that make them ideal for the cupboard under the stairs. They are quiet
and efficient, can be scaled down, and need no more maintenance than your
average boiler.
So why haven't they been put in personal power stations before? The
reason, says David Moriarty, managing director of New Zealand-based Stirling
engine company WhisperTech, is that the right materials have only just
become available.
To work well, a Stirling engine needs to get hot. Its efficiency is
a function of the temperature difference across it, so the hotter the "hot
end" the better. Advanced materials, particularly high temperature steels
and ceramics from the space programme, have recently allowed the hot end
to rise to 1200 °C, making the engines efficient enough to offer real
energy savings.
In a personal CHP unit, burning gas directly heats one end of the Stirling
engine. The "cool end" draws waste thermal energy away and uses it to heat
your water.
WhisperTech is one of two companies that have been putting CHP plants
in homes this winter—20 in Britain and about the same number in the Netherlands.
Its unit replaces boilers that sit on the floor and produces about 6 kilowatts
of heat plus 1 kilowatt of electricity. The competing unit, from British-based
BG Group, is wall-mounted and produces up to 15 kilowatts of heat and 1.1
kilowatt of electricity. It's currently installed in 12 houses in Britain.
Both units cost more than a conventional boiler, but the energy savings
should recoup the difference in three to four years.
The one in the Harrison house is a WhisperTech. "It looks like a gas
boiler and it's no noisier than a freezer," says Jeremy Harrison, who shares
the house with his wife and three children. He says the unit is doing its
job nicely—it handles the heating and hot water just as effectively as
the old boiler. That's a big plus: no one wants to change their lifestyle
to accommodate the whims of a central heating system.
Of course, as the unit only produces electricity when the heating or
hot water is on, the house can't disconnect from the mains altogether.
There are certain to be times when demand outstrips supply—CHP could never
meet the spike in demand from, say, a kettle or electric shower. What's
more, when the boiler is idle the house still needs a trickle of electricity—about
150 watts—to keep the fridge and freezer ticking over and the TV on standby.
And in summer the unit will generate scarcely any electricity. But during
the cold, dark months, the heating and hot water are in regular use so
there's plenty of home-made electricity around—and plenty of demand for
it. BG Group says its unit will save an average household £200 a
year in electricity bills.
Staying linked up to the mains is useful for another reason. Just as
there are times when your demand outstrips supply, the opposite is sometimes
true. Then you can export your excess electricity to the grid, which is
what the Harrison family do with their overspill. At the moment the process
is costly and bureaucratic—it was designed for big generators, not individual
households—and the Harrisons actually give their excess electricity away.
But the government is reviewing the system and intends to make it easier
for little producers to sell to the grid.
So much for saving money. How about the planet? Totting up the environmental
benefits isn't so simple. One advantage is the increased overall efficiency—Stirling
CHP units are up to 90 per cent efficient, compared with about 60 per cent
for the average old British boiler. But a modern condensing boiler is just
as efficient, as it recirculates its flue gases to extract as much heat
as possible.
However, a condensing boiler only gives you heat, so the CHP has a
big plus: you're cutting down on electricity generated by wasteful power
stations. Their conversion efficiency is less than 40 per cent, and they
fritter away a further 5 per cent as they distribute the electricity around
the country.
According to a report published last year by energy consultancy EA
Technology, an average household with a CHP unit will reduce its energy
use by about 20 per cent. Once they are widespread—and the report predicts
"market saturation" in as little as 10 years—that could make a big dent
in carbon dioxide emissions. If every one of the 13 million British homes
suited to a CHP unit installed one, the country would reduce its carbon
emissions by 16 million tonnes a year. That's 61 per cent of Britain's
commitment under the Kyoto global warming treaty.
Other countries with cold winters where gas boilers are the norm could
make similar savings, as long as the electricity market is liberalised
enough to allow people to sell their surpluses back to the grid. The Netherlands
and Germany already fit the bill and the whole EU must liberalise its electricity
market in the next eight years.
Not surprisingly, governments are eager to promote this technology.
The British government, for example, is considering offering grants to
anyone willing to install a CHP unit. But that's no guarantee of success.
Condensing boilers have been around for 10 years and also qualify for energy
efficiency grants in Britain, yet only about 8 per cent of households have
one.
But if domestic CHP does take off, it promises to keep you cosy, save
you money and fight climate change. Not bad for a bit of home-made electricity.
How the Stirling engine converts heat energy into electricity.