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鈥檙e 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鈥攊f enough people can be persuaded to install them.
The unit in the Harrison鈥檚 garage is what鈥檚 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鈥檚 Combined Heat and Power
Association, CHP cuts fuel consumption by up to 35 per cent. That鈥檚 because the
waste heat from generating electricity鈥攚hich power stations normally
fritter away in cooling towers鈥攊s used to heat buildings and water
instead. And you don鈥檛 lose energy transmitting the electricity hundreds of
kilometres along overhead cables.
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Installing CHP units in houses seems like an obvious next step, but it hasn鈥檛
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鈥檙e
scaled down.
And there鈥檚 another problem: in a factory or apartment block, CHP鈥檚 main job
is generating electricity. The waste heat is put to good use, but it鈥檚 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鈥檝e 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
(快猫短视频, 18 November 2000, p 16).
Fuel cells burn gas to generate electricity, and they can be rigged up so that most of the
fuel鈥檚 energy goes into heat. They are also quiet and they have no moving parts, so
don鈥檛 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鈥檚 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鈥檚 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鈥攜ou don鈥檛 want to put your foot down and
then have to wait several seconds for your car to move.
But a short delay doesn鈥檛 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鈥檛 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 鈥渉ot end鈥�
the better. Advanced materials, particularly high temperature steels and
ceramics from the space programme, have recently allowed the hot end to rise to
1200 掳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 鈥渃ool end鈥� 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鈥�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鈥檚 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. 鈥淚t looks like a gas boiler
and it鈥檚 no noisier than a freezer,鈥� says Jeremy Harrison, who shares the house
with his wife and three children. He says the unit is doing its job
nicely鈥攊t handles the heating and hot water just as effectively as the old
boiler. That鈥檚 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鈥檛 disconnect from the mains altogether. There are
certain to be times when demand outstrips supply鈥擟HP could never meet the
spike in demand from, say, a kettle or electric shower. What鈥檚 more, when the
boiler is idle the house still needs a trickle of electricity鈥攁bout 150
watts鈥攖o 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鈥檚
plenty of home-made electricity around鈥攁nd plenty of demand for it. BG
Group says its unit will save an average household 拢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鈥攊t was designed for big generators, not individual
households鈥攁nd 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鈥檛 so simple. One advantage is the increased overall
efficiency鈥擲tirling 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鈥檙e 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鈥攁nd the report predicts
鈥渕arket saturation鈥� in as little as 10 years鈥攖hat 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鈥檚 61 per cent of Britain鈥檚 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鈥檚 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.
