Some questions about electric vehicles

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You've shown that electric cars are more energy-efficient than fossil cars. But are they better if our objective is to reduce CO2 emissions, and the electricity is still generated by fossil power-stations?

This is quite an easy calculation to do. Assume the electric vehicle's energy cost is 20 kWh(e) per 100 km. (I think 15 kWh(e) per 100 km is perfectly possible, but let's play sceptical in this calculation.) If grid electricity has a carbon footprint of 500 g per kWh(e) then the effective emissions of this vehicle are 100gCO2 per km, which is as good as the best fossil cars (figure 20.9). So I conclude that switching to electric cars is already a good idea, even before we green our electricity supply.

Electric cars, like fossil cars, have costs of both manufacture and use. Electric cars may cost less to use, but if the batteries don't last very long, shouldn't you pay more attention to the manufacturing cost?

Yes, that's a good point. My transport diagram shows only the use cost. If electric cars require new batteries every few years, my numbers may be underestimates. The batteries in a Prius are expected to last just 10 years, and a new set would cost £3500. Will anyone want to own a 10-year old Prius and pay that cost? It could be predicted that most Priuses will be junked at age 10 years. This is certainly a concern for all electric vehicles that have batteries. I guess I'm optimistic that, as we switch to electric vehicles, battery technology is going to improve.

I live in a hot place. How could I drive an electric car? I demand power-hungry air-conditioning!

There's an elegant fix for this demand: fit 4 m2 of photovoltaic panels in the upward-facing surfaces of the electric car. If the air-conditioning is needed, the sun must surely be shining. 20%-efficient panels will generate up to 800 W, which is enough to power a car's air-conditioning. The panels might even make a useful contribution to charging the car when it's parked, too. Solar-powered vehicle cooling was included in a Mazda in 1993; the solar cells were embedded in the glass sunroof.

I live in a cold place. How could I drive an electric car? I demand power-hungry heating!

The motor of an electric vehicle, when it's running, will on average use something like 10 kW, with an efficiency of 90-95%. Some of the lost power, the other 5-10%, will be dissipated as heat in the motor. Perhaps electric cars that are going to be used in cold places can be carefully designed so that this motor-generated heat, which might amount to 250 or 500 W, can be piped from the motor into the car. That much power would provide some significant windscreen demisting or body-warming.

Are lithium-ion batteries safe in an accident?

Some lithium-ion batteries are unsafe when short-circuited or overheated, but the battery industry is now producing safer batteries such as lithium phosphate. There's a fun safety video at

Is there enough lithium to make all the batteries for a huge fleet of electric cars?

World lithium reserves are estimated to be 9.5 million tons in ore deposits (p175). A lithium-ion battery is 3% lithium. If we assume each vehicle has a 200 kg battery, then we need 6 kg of lithium per vehicle. So the estimated reserves in ore deposits are enough to make the batteries for 1.6billion vehicles. That's more than the number of cars in the world today (roughly 1 billion) - but not much more, so the amount of lithium may be a concern, especially when we take into account the competing ambitions of the nuclear fusion posse (Chapter 24) to guzzle lithium in their reactors. There's many thousands times more lithium in sea water, so perhaps the oceans will provide a useful backup. However, lithium specialist R. Keith Evans says "concerns regarding lithium availability for hybrid or electric vehicle batteries or other foreseeable applications are unfounded." And anyway, other lithium-free battery technologies such as zinc-air rechargeables are being developed []. I think the electric car is a goer!

The future of flying?

The superjumbo A380 is said by Airbus to be "a highly fuel-efficient aircraft." In fact, it burns just 12% less fuel per passenger than a 747.

Boeing has announced similar breakthroughs: their new 747-8 Intercontinental, trumpeted for its planet-saving properties, is (according to Boeing's advertisements) only 15% more fuel-efficient than a 747-400.

This slender rate of progress (contrasted with cars, where changes in technology deliver two-fold or even ten-fold improvements in efficiency) is explained in Technical Chapter C. Planes are up against a fundamental limit imposed by the laws of physics. Any plane, whatever its size, has to expend an energy of about 0.4 kWh per ton-km on keeping up and keeping

Figure 20.29. Airbus A380.

moving. Planes have already been fantastically optimized, and there is no prospect of significant improvements in plane efficiency.

For a time, I thought that the way to solve the long-distance-transport problem was to revert to the way it was done before planes: ocean liners. Then I looked at the numbers. The sad truth is that ocean liners use more energy per passenger-km than jumbo jets. The QE2 uses four times as much energy per passenger-km as a jumbo. OK, it's a luxury vessel; can we do better with slower tourist-class liners? From 1952 to 1968, the economical way to cross the Atlantic was in two Dutch-built liners known as "The Economy Twins," the Maasdam and the Rijnsdam. These travelled at 16.5 knots (30.5 km/h), so the crossing from Britain to New York took eight days. Their energy consumption, if they carried a full load of 893 passengers, was 103 kWh per 100p-km. At a typical 85% occupancy, the energy consumption was 121 kWh per 100 pkm - more than twice that of the jumbo jet. To be fair to the boats, they are not only providing transportation: they also provide the passengers and crew with hot air, hot water, light, and entertainment for several days; but the energy saved back home from being cooped up on the boat is dwarfed by the boat's energy consumption, which, in the case of the QE2, is about 3000 kWh per day per passenger.

So, sadly, I don't think boats are going to beat planes in energy consumption. If eventually we want a way of travelling large distances without fossil fuels, perhaps nuclear-powered ships are an interesting option (figures 20.31 & 20.32).

What about freight?

International shipping is a surprisingly efficient user of fossil fuels; so getting road transport off fossil fuels is a higher priority than getting ships off fossil fuels. But fossil fuels are a finite resource, and eventually ships must be powered by something else. Biofuels may work out. Another option will be nuclear power. The first nuclear-powered ship for carrying cargo and passengers was the NS Savannah, launched in 1962 as part of President Dwight D. Eisenhower's Atoms for Peace initiative (figure 20.31). Powered by one 74-MW nuclear reactor driving a 15-MW motor, the Savannah had a service speed of 21 knots (39 km/h) and could carry 60 passengers and 14 0001 of cargo. That's a cargo transport cost of 0.14 kWh per ton-km. She could travel 500 000 km without refuelling. There are already many nuclear-powered ships, both military and civilian. Russia has ten nuclear-powered ice-breakers, for example, of which seven are still active. Figure 20.32 shows the nuclear ice-breaker Yamal, which has two 171-MW reactors, and motors that can deliver 55 MW.

Figure 20.30. TSS Rijndam.
Figure 20.31. NS Savannah, the first commercial nuclear-powered cargo vessel, passing under the Golden Gate Bridge in 1962.
Figure 20.32. The nuclear ice-breaker Yamal, carrying 100 tourists to the North Pole in 2001. Photo by Wofratz.

"Hang on! You haven't mentioned magnetic levitation"

The German company, Transrapid, which made the maglev train for Shanghai, China (figure 20.33), says: "The Transrapid Superspeed Maglev System is unrivaled when it comes to noise emission, energy consumption, and land use. The innovative non-contact transportation system provides mobility without the environment falling by the wayside."

Magnetic levitation is one of many technologies that gets hyped up when people are discussing energy issues. In energy-consumption terms, the comparison with other fast trains is actually not as flattering as the hype suggests. The Transrapid site compares the Transrapid with the In-terCityExpress (ICE), a high-speed electric train.

Fast trains compared at 200 km/h (125mph)

Transrapid 2.2 kWh per 100 seat-km ICE 2.9 kWh per 100 seat-km

The main reasons why maglev is slightly better than the ICE are: the magnetic propulsion motor has high efficiency; the train itself has low mass, because most of the propulsion system is in the track, rather than the train; and more passengers are inside the train because space is not needed for motors. Oh, and perhaps because the data are from the maglev company's website, so are bound to make the maglev look better!

Incidentally, people who have seen the Transrapid train in Shanghai tell me that at full speed it is "about as quiet as a jet aircraft."

Notes and further reading page no.

119 A widely quoted statistic says "Only 1% of fuel energy in a car goes into moving the driver." In fact the percentage in this myth varies in size as it commutes around the urban community. Some people say "5% of the energy goes into moving the driver." Others say "A mere three tenths of1 percent of fuel energy goes into moving the driver." [4qgg8q] My take, by the way, is that none of these statistics is correct or helpful.

- The bicycle's performance is about the same as the eco-car's. Cycling on a single-person bike costs about 1.6 kWh per 100 km, assuming a speed of 20 km/h. For details and references, see Chapter A, p262.

- The 8-carriage stopping train from Cambridge to London (figure 20.4) weighs 275 tonnes, and can carry 584 passengers seated. Its maximum speed is 100 mph (161 km/h), and the power output is 1.5 MW. If all the seats are occupied, this train at top speed consumes at most 1.6 kWh per 100 passenger-km.

Figure 20.33. A maglev train at Pudong International Airport, Shanghai.

"driving without wheels; flying without wings."

Photo by Alex Needham.

Photo by Alex Needham.

Figure 20.34. Nine out of ten vehicles in London are G-Wizes. (And 95% of statistics are made up.)

120 London Underground. A Victoria-line train consists of four 30.5-ton and four 20.5-ton cars (the former carrying the motors). Laden, an average train weighs 228 tons. The maximum speed is 45 mile/h. The average speed is 31 mph. A train with most seats occupied carries about 350 passengers; crush-loaded, the train takes about 620. The energy consumption at peak times is about 4.4 kWh per 100 passenger-km (Catling, 1966).

121 High-speed train.

A diesel-powered intercity 125 train (on the right in figure 20.5) weighs 410 tons. When travelling at 125 mph, the power delivered "at the rail" is 2.6 MW. The number of passengers in a full train is about 500. The average fuel consumption is about 0.84 litres of diesel per 100 seat-km [5o5x5m], which is a transport cost of about 9 kWh per 100 seat-km. The Class 91 electric train (on the left in figure 20.5) travels at 140 mph (225km/h) and uses 4.5 MW. According to Roger Kemp, this train's average energy consumption is 3 kWh per 100 seat-km [5o5x5m]. The government document [5fbeg9] says that east-coast mainline and west-coast mainline trains both consume about 15 kWh per km (whole train). The number of seats in each train is 526 or 470 respectively. So that's 2.9-3.2 kWh per 100 seat-km.

- the total energy cost of all London's underground trains, was 15kWh per 100p-km. ... The energy cost of all London buses was 32kWh per 100p-km. Source: [679rpc]. Source for train speeds and bus speeds: Ridley and Catling (1982).

- Croydon Tramlink.,,

123 ... provision of excellent cycle facilities ... The UK street design guide [] encourages designing streets to make 20 miles per hour the natural speed. See also Franklin (2007).

124 A fair and simple method for handling congestion-charging. I learnt a brilliant way to automate congestion-charging from Stephen Salter. A simple daily congestion charge, as levied in London, sends only a crude signal to drivers; once a car-owner has decided to pay the day's charge and drive into a congestion zone, he has no incentive to drive little in the zone. Nor is he rewarded with any rebate if he carefully chooses routes in the zone that are not congested.

Instead of having a centralized authority that decides in advance when and where the congestion-charge zones are, with expensive and intrusive monitoring and recording of vehicle movements into and within all those zones, Salter has a simpler, decentralized, anonymous method of charging drivers for driving in heavy, slow traffic, wherever and whenever it actually exists. The system would operate nationwide. Here's how it works. We want a device that answers the question "how congested is the traffic I am driving in?" A good measure of congestion is "how many other active vehicles are close to mine?" In fast-moving traffic, the spacing between vehicles is larger than slow-moving traffic. Traffic that's trundling in tedious queues is the

Car (100km): 80kWh

Train: 3kWh

Figure 20.35. 100 km in a single-person car, compared with 100 km on a fully-occupied electric high-speed train.

Figure 20.36. Trams work nicely in Istanbul and Prague too.

most densely packed. The number of nearby vehicles that are active can be sensed anonymously by fitting in every vehicle a radio transmitter/receiver (like a very cheap mobile phone) that transmits little radio-bleeps at a steady rate whenever the engine is running, and that counts the number of bleeps it hears from other vehicles. The congestion charge would be proportional to the number of bleeps received; this charge could be paid at refuelling stations whenever the vehicle is refuelled. The radio transmitter/receiver would replace the current UK road tax disc.

126 hydraulics and flywheels salvage at least 70% of the braking energy Compressed air is used for regenerative braking in trucks; say "hydraulic launch assist" captures 70% of the kinetic energy. [5cp27j]

The flywheel system of flybridsystems. com also captures 70% of the kinetic energy.

Electric regenerative braking salvages 50%. Source: E4tech (2007).

- Electric batteries capable of delivering 60kW would weigh about 200kg. Good lithium-ion batteries have a specific power of 300 W/kg (Horie et al., 1997; Mindl, 2003).

- the average new car in the UK emits 168g CO2 per km. This is the figure for the year 2006 (King, 2008). The average emissions of a new passenger vehicle in the USA were 255 g per km (King, 2008).

- The Toyota Priushas a more-efficient engine. The Prius's petrol engine uses the Atkinson cycle, in contrast to the conventional Otto cycle. By cunningly mixing electric power and petrol power as the driver's demands change, the Prius gets by with a smaller engine than is normal in a car of its weight, and converts petrol to work more efficiently than a conventional petrol engine.

- Hybrid technologies give fuel savings of 20% or 30%. For example, from Hitachi's research report describing hybrid trains (Kaneko et al., 2004): high-efficiency power generation and regenerative braking are "expected to give fuel savings of approximately 20% compared with conventional diesel-powered trains."

127 Only 8.3% of commuters travel over 30 km to their workplace. Source: Ed-dington (2006). The dependence of the range of an electric car on the size of its battery is discussed in Chapter A (p261).

- Lots of electric vehicles. They are all listed below, in no particular order. Performance figures are mainly from the manufacturers. As we saw on p127, real-life performance doesn't always match manufacturers' claims.

Th!nk Electric cars from Norway. The five-door Th!nk Ox has a range of 200 km. Its batteries weigh 350 kg, and the car weighs 1500 kg in total. Its energy consumption is approximately 20 kWh per 100 km. www. think. no

Electric Smart Car "The electric version is powered by a 40 bhp motor, can go up to 70 miles, and has a top speed of 70 mph. Recharging is done through a standard electrical power point and costs about £1.20, producing the equivalent of 60 g/km of carbon dioxide emissions at the power station. [cf. the equivalent petrol-powered Smart: 116 g/km.] A full recharge takes"/>
Figure 20.37. Th!nk Ox. Photo from

about eight hours, but the battery can be topped up from 80%-drained to 80%-charged in about three-and-a-half hours." []

Berlingo Electrique 500E, an urban delivery van (figure 20.20), has 27 nicad batteries and a 28 kW motor. It can transport a payload of 500 kg. Top speed: 100 km/h; range: 100 km. 25kWh per 100 km. (Estimate kindly supplied by a Berlingo owner.) [4wm2w4]

i MiEV This electric car is projected to have a range of 160 km with a 16 kWh battery pack. That's 10kWh per 100 km - better than the G-Wiz - and whereas it's hard to fit two adult Europeans in a G-Wiz, the Mitsubishi prototype has four doors and four full-size seats (figure 20.38). [658ode]

EV1 The two-seater General Motors EV1 had a range of 120 to 240 km per charge, with nickel-metal hydride batteries holding 26.4 kWh. That's an energy consumption of between 11 and 22 kWh per 100 km.

Lightning (figure 20.39) - has four 120 kW brushless motors, one on each wheel, regenerative braking, and fast-charging Nanosafe lithium titanate batteries. A capacity of 36 kWh gives a range of 200 miles (320 km). That's 11 kWh per 100 km.

Aptera This fantastic slippery fish is a two-seater vehicle, said to have an energy cost of 6 kWh per 100 km. It has a drag coefficient of 0.11 (figure 20.40). Electric and hybrid models are being developed.

Loremo Like the Aptera, the Loremo (figure 20.41) has a small frontal area and small drag coefficient (0.2) and it's going to be available in both fossil-fuel and electric versions. It has two adult seats and two rear-facing kiddie seats. The Loremo EV will have lithium ion batteries and is predicted to have an energy cost of 6 kWh per 100 km, a top speed of 170 km/h, and a range of 153 km. It weighs 600 kg.

eBox The eBox has a lithium-ion battery with a capacity of 35 kWh and a weight of 280 kg; and a range of 140-180 miles. Its motor has a peak power of 120 kW and can produce a sustained power of 50 kW. Energy consumption: 12 kWh per 100 km.

Ze-0 A five-seat, five-door car. Maximum speed: 50mph. Range: 50 miles. Weight, including batteries: 1350 kg. Lead acid batteries with capacity of 18 kWh. Motor: 15 kW. 22.4 kWh per 100 km.

e500 An Italian Fiat-like car, with two doors and 4 seats. Maximum speed: 60 mph. Range in city driving: 75 miles. Battery: lithium-ion polymer.

MyCar The MyCar is an Italian-designed two-seater. Maximum speed: 40 mph. Maximum range: 60 miles. Lead-acid battery.

Mega City A two-seater car with a maximum continuous power of 4 kW and maximum speed of 40 mph: 11.5 kWh per 100 km. Weight unladen (including batteries) - 725 kg. The lead batteries have a capacity of 10 kWh.

Xebra Is claimed to have a 40 km range from a 4.75 kWh charge. 12 kWh per 100 km. Maximum speed 65 km/h. Lead-acid batteries.

Figure 20.38. The i MiEV from Mitsubishi Motors Corporation. It has a 47 kW motor, weighs 1080 kg, and has a top speed of 130 km/h."/>
Figure 20.39. Lightning: 11 kWh per 100 km. Photo from
Figure 20.40. The Aptera. 6kWhper 100 km. Photo from www. aptera. com.

Figure 20.41. The Loremo. 6kWhper 100 km. Photo from

TREV The Two-Seater Renewable Energy Vehicle (TREV) is a prototype developed by the University of South Australia (figure 20.42). This three-wheeler has a range of 150 km, a top speed of 120km/h, a mass of 300 kg, and lithium-ion polymer batteries weighing 45 kg. During a real 3000 km trip, the energy consumption was 6.2 kWh per 100 km.

Venturi Fetish Has a 28 kWh battery, weighing 248 kg. The car weighs 1000 kg. Range 160-250 km. That's 11-17 kWh per 100 km. www.venturifet

Toyota RAV4 EV This vehicle - an all-electric mini-SUV - was sold by Toyota between 1997 and 2003 (figure 20.43). The RAV4 EV has 24 12-volt 95Ah NiMH batteries capable of storing 27.4 kWh of energy; and a range of 130 to 190 km. So that's an energy consumption of 14-21 kWh per 100 km. The RAV4 EV was popular with Jersey Police force.

Phoenix SUT - a five-seat "sport utility truck" made in California - has a range of "up to 130 miles" from a 35 kWh lithium-ion battery pack. (That's 17 kWh per 100 km.) The batteries can be recharged from a special outlet in 10 minutes.

Modec delivery vehicle Modec carries two tons a distance of 100 miles. Kerb weight 3000 kg. www

Smith Ampere Smaller delivery van, 24 kWh lithium ion batteries. Range "over 100 miles."

Electric minibus From www. smithelectricvehicles. com:

40 kWh lithium ion battery pack. 90 kW motor with regenerative brakes. Range "up to 100 miles." 15 seats. Vehicle kerb weight 3026 kg. Payload 1224 kg. That's a vehicle-performance of at best 25 kWh per 100 km. If the vehicle is fully occupied, it could deliver transportation at an impressive cost of 2 kWh per 100p-km.

Electric coach The Thunder Sky bus has a range of 180 miles and a recharge time of three hours.

Electric scooters The Vectrix is a substantial scooter (figure 20.44). Its battery (nickel metal hydride) has a capacity of 3.7 kWh. It can be driven for up to 68 miles at 25 miles/h (40 km/h), on a two-hour charge from a standard electrical socket. That's 110 km for 3 kWh, or 2.75 kWh per 100 km. It has a maximum speed of 62mph (100 km/h). It weighs 210 kg and has a peak power of 20 kW. www. vectrix. com

The "Oxygen Cargo" is a smaller scooter. It weighs 121 kg, has a 38 mile range, and takes 2-3 hours to charge. Peak power: 3.5 kW; maximum speed 28 mph. It has two lithium-ion batteries and regenerative brakes. The range can be extended by adding extra batteries, which store about 1.2 kWh and weigh 15 kg each. Energy consumption: 4 kWh per 100 km.

Figure 20.42. The TREV. 6 kWh per 100 km. Photo from www.uni"/>
Figure 20.43. Toyota RAV4 EV. Photo by Kenneth Adelman,"/>
Figure 20.44. Vectrix: 2.75 kWh per 100 km. Photo from

129 the energy-density of compressed-air energy-stores is only about 11-28 Wh per kg. The theoretical limit, assuming perfect isothermal compression: if 1 m3 of ambient air is slowly compressed into a 5-litre container at 200 bar, the potential energy stored is 0.16 kWh in 1.2 kg of air. In practice, a 5-litre container appropriate for this sort of pressure weighs about 7.5 kg if made from steel or 2 kg using kevlar or carbon fibre, and the overall energy density achieved would be about 11-28 Wh per kg. The theoretical energy density is the same, whatever the volume of the container.

130 Arnold Schwarzenegger .. .filling up a hydrogen-powered Hummer. Nature 438, 24 November 2005. I'm not saying that hydrogen will never be useful for transportation; but I would hope that such a distinguished journal as Nature would address the hydrogen bandwagon with some critical thought, not only euphoria.

Hydrogen and fuel cells are not the way to go. The decision by the Bush administration and the State of California to follow the hydrogen highway is the single worst decision of the past few years.

James Woolsey, Chairman of the Advisory Board of the US Clean Fuels Foundation, 27th November 2007.

In September 2008, The Economist wrote "Almost nobody disputes that ... eventually most cars will be powered by batteries alone."

On the other hand, to hear more from advocates of hydrogen-based transport, see the Rocky Mountain Institute's pages about the "HyperCar"

- In the Clean Urban Transport for Europe project the overall energy required to power the hydrogen buses was between 80% and 200% greater than that of the baseline diesel bus. Source: CUTE (2006); Binder et al. (2006).

- Fuelling the hydrogen-powered car made by BMW requires three times more energy than an average car. Half of the boot of the BMW "Hydrogen 7" car is taken up by its 170-litre hydrogen tank, which holds 8 kg of hydrogen, giving a range of 200km on hydrogen []. The calorific value of hydrogen is 39 kWh per kg, and the best-practice energy cost of making hydrogen is 63 kWh per kg (made up of 52 kWh of natural gas and 11 kWh of electricity) (CUTE, 2006). So filling up the 8 kg tank has an energy cost of at least 508 kWh; and if that tank indeed delivers 200 km, then the energy cost is 254 kWh per 100 km.

The Hydrogen 7 and its hydrogen-fuel-cell cousins are, in many ways, simply flashy distractions.

David Talbot, MIT Technology Review Honda's fuel-cell car, the FCX Clarity, weighs 1625 kg, stores 4.1 kg of hydrogen at a pressure of 345 bar, and is said to have a range of 280 miles, consuming 57 miles of road per kg of hydrogen (91 km per kg) in a standard mix of driving conditions [czjjo], [5a3ryx]. Using the cost for creating hydrogen mentioned above, assuming natural gas is used as the main energy source, this car has a transport cost of 69 kWh per 100 km.

Honda might be able to kid journalists into thinking that hydrogen cars are "zero emission" but unfortunately they can't fool the climate.

Merrick Godhaven

132 A lithium-ion battery is 3% lithium. Source: Fisher et al. (2006).

- Lithium specialistR. Keith Evans says "concerns regardinglithium availability... are unfounded." - Evans (2008).

133 Two Dutch-built liners known as "The Economy Twins." www.ssmaritime. com/rijndam-maasdam.htm. QE2: www .qe2. org. uk.

134 Transrapid magnetic levitation train.

21 Smarter heating

In the last chapter, we learned that electrification could shrink transport's energy consumption to one fifth of its current levels; and that public transport and cycling can be about 40 times more energy-efficient than car-driving. How about heating? What sort of energy-savings can technology or lifestyle-change offer?

The power used to heat a building is given by multiplying together three quantities:

power used average temperature difference x leakiness of building efficiency of heating system

Let me explain this formula (which is discussed in detail in Chapter E) with an example. My house is a three-bedroom semi-detached house built about 1940 (figure 21.1). The average temperature difference between the inside and outside of the house depends on the setting of the thermostat and on the weather. If the thermostat is permanently at 20 ° C, the average temperature difference might be 9 ° C. The leakiness of the building describes how quickly heat gets out through walls, windows, and cracks, in response to a temperature difference. The leakiness is sometimes called the heat-loss coefficient of the building. It is measured in kWh per day per degree of temperature difference. In Chapter E, I calculate that the leakiness of my house in 2006 was 7.7kWh/d/°C. The product average temperature difference x leakiness of building is the rate at which heat flows out of the house by conduction and ventilation. For example, if the average temperature difference is 9 ° C then the heat loss is

Finally, to calculate the power required, we divide this heat loss by the efficiency of the heating system. In my house, the condensing gas boiler has an efficiency of 90%, so we find:

9 °C x 7.7kWh/d/°C ^ TA71 ,, power used =-—-= 77kWh/d.

That's bigger than the space-heating requirement we estimated in Chapter 7. It's bigger for two reasons: first, this formula assumes that all the heat is supplied by the boiler, whereas in fact some heat is supplied by incidental heat gains from occupants, gadgets, and the sun; second, in Chapter 7 we assumed that a person kept just two rooms at 20 °C all the time; keeping an entire house at this temperature all the time would require more.

OK, how can we reduce the power used by heating? Well, obviously, there are three lines of attack.

Figure 21.1. My house.

1. Reduce the average temperature difference. This can be achieved by turning thermostats down (or, if you have friends in high places, by changing the weather).

2. Reduce the leakiness of the building. This can be done by improving the building's insulation - think triple glazing, draught-proofing, and fluffy blankets in the loft - or, more radically, by demolishing the building and replacing it with a better insulated building; or perhaps by living in a building of smaller size per person. (Leakiness tends to be bigger, the larger a building's floor area, because the areas of external wall, window, and roof tend to be bigger too.)

3. Increase the efficiency of the heating system. You might think that 90% sounds hard to beat, but actually we can do much better.

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