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Domestic Energy use in the UK - Power Conversion, Transport and Use

An A-level R&A project by Graham Lamburn (Spring 2000)

This project mainly looks at the efficiencies of using gas or electricity for heating our homes and comes up with a dramatic and surprising conclusion. In the UK, at present, using electricity to heat homes, instead of directly burning gas, wastes 280 TWh of energy - enough to boil a body of water 9 times the volume of Lake Windermere! (2,304,000,000 cubic metres!).

The overall efficiency of an electric heating system is on average only about 36%, compared with typical modern gas central heating efficiencies of about 80% or higher. Thus gas should be only used as a direct source of heat, rather than via conversion to electricity, and the Government should NOT allow more gas-powered electricity generating stations to be built unless these are small local ones that are 'combined heat and power' and that supply the 'waste' heat directly to other local uses.

Retrospective Summary:

I was initially made aware of this issue by press coverage from the new Yorkshire Super-Grid power line (see http://www.revolt.co.uk). I then had a look through my physics textbooks to see what they offered on this subject. They confirmed that it should be an interesting and worthwhile topic for me to research. My father gave me two old Open University texts [Refs. 7 and 8], that he felt might be helpful.

Reference 2 had an interesting energy flow diagram that came from the Department of Energy, but was printed in 1984 and was rather out of date. I next obtained the latest version of the flow diagram, contained in Reference 4, which I obtained from the HMSO. This provided me with the bulk of the data that I used, and also gave further references to follow up, including the National Grid Company Seven Year Statement.

I have concluded that, in general, it is very wasteful to use electricity to heat our homes. I find it surprising that we do not have an obvious national energy policy to give guidance on these matters, although on a commercial website [Ref. 11] I have found that a government-funded "Energy Saving Trust" will refund £200 of the cost of approved condensing gas boilers.

Background to this project:

In February 2000 there was some media coverage [Ref. 1] about a long new 400 kV power line that the National Grid Company are starting to build through Yorkshire in order to bring electricity from Scotland and the North East, down to the South East. One claim that the objectors made was that the electricity should be generated near to where it was needed as large amounts of energy were wasted as heat carrying the power along high power electricity lines.

On starting to research into this subject, I discovered one statistic, [page 17, Ref.4] which states that 29% of energy in the UK is used for domestic purposes, mainly heating. I thought that it would be interesting to find out the difference in efficiency of using gas or electricity for this purpose.

If there are significantly different efficiencies between these two types of system, then this should be taken into account in national energy policy. I was also interested to find out more about the U.K. electricity system.

Data Sources:

My investigation started with my two physics textbooks [Ref. 2, pp.405 - 470, and Ref. 3, pp185 - 190]. These provided me with the background to these issues and referred to other useful sources of data. I needed to discover what energy sources and fuels were used for background heating purposes and then estimate the power losses in both electricity generation and in the long distance transport of electricity along wires. This could be compared with the losses involved in piping gas to the houses and using that as the energy source to produce heat directly.

One particularly interesting diagram in Ref.1 was an 'Energy Flow Chart 1983' that had been extracted from a UK Government Department of Energy publication. I decided that it would be useful to obtain a more up-to-date version and compare them. It is now published in an annual publication from the Department of Trade and Industry [Ref. 4] and I obtained the latest edition, which is for 1999. I found this to have many tables of energy use relevant to my project.

This referred to another publication that seemed likely to be useful. This is an annual report from the National Grid Company that sets out UK electricity generation and transmission. I obtained a 1996 copy [Ref. 5] and also looked at the 1999 version that is on the NGC web site [Ref. 6]. This helped me understand the reason that the new Yorkshire powerline [Ref. 1] was being built. I discovered that too much electricity is generated in Scotland and the North East of England, whereas the South and South West areas of England do not generate enough for their needs. This can be clearly seen in Appendix 1 (from NGC SYS), which shows typical major power flows around England, and Appendix 2 (NGC SYS Fig. 8.1), which shows the need for large amounts of extra electricity in Southern England. Appendix 3 (from NGC SYS fig A.1) shows the geographic location of major electricity power stations and National Grid power lines within England.

Two Open University publications [Ref. 7 & 8] also proved useful, especially Ref.8 which briefly addressed the actual issue that I was investigating.

Other web sites that I looked at were Friends of the Earth, who run an energy efficiency campaign, and OFGEM (the Office of the Gas and Electricity Markets), the newly combined electricity and gas UK Energy Regulators.

The Scale of the Problem:

Table 1 - Energy consumption by final user from Ref. 4 p. 17

Year Domestic Industry Transport Other Uses
1960 29% 42% 17% 12%
1998 29% 22% 34% 14%

The total energy used in the U.K. in 1998 is given as 234,931,000 tonnes of oil equivalent. This is the equivalent of 9,836,561 TJ or 2,732,247,500 MWh. One kWh is generally known as one "unit" of electricity. A conversion table is given in Appendix 4.

In carrying out this research, I have been surprised at the level of losses in the generation and transmission of electricity through U.K. power-lines.

Losses occur in the conversion of raw materials into electricity, and "transport supply losses" due to resistive losses in the cables and equipment. I will discuss the overall efficiency of the "electricity cycle", from conversion and production, to usage in homes.

In 1998, the major electricity producers supplied a maximum load of 56.3 GW (Gigawatts) of electricity. Nuclear and conventional fossil fuel power stations ran at a thermal efficiency of just over 36%. The more efficient combined cycle gas turbine stations managed almost 47% efficiency, and produce about 22% of the total electricity. About 2% is produced by "renewables" (i.e. mainly hydro-electric, with negligible amounts supplied by wind and solar energy). [These figures are taken from tables 5.5 and 5.7 in Ref. 4]. This gives an overall electricity generation efficiency of approximately:

          ((76 x 0.36) + (22 x 0.47)) x 100 / 98 which equals 38.5 % overall efficiency

The 2% renewable sources are being ignored because it is very difficult to assess their thermal efficiencies and they are responsible for a much smaller amount of energy production.

Graph 1 data on fuel used in generation is taken from Table 5.2, Ref.4, p162, and shows the fuels used to generate U.K. electricity over the last 5 years, in Tonnes of Oil Equivalent x 106.

Graph & Table 1 - Energy sources for UK electricity generation

Energy Sources to generate electricity

  1994 1995 1996 1997 1998
Coal 37.10 36.11 33.40 28.85 30.00
Oil 4.05 3.62 3.49 1.89 1.41
Gas 9.86 12.54 16.40 20.92 22.16
Nuclear 21.20 21.25 22.18 22.99 23.29
Hydro 0.44 0.45 0.29 0.35 0.40
Other Fuels 1.06 1.15 1.23 1.43 1.15
Net Imports 1.45 1.40 1.44 1.42 1.07
Total 75.16 76.52 78.43 77.85 79.48

Electricity generation and distribution losses vs time

Graph 2: Overall total UK electricity generation and associated distribution losses

Graph 2 shows over 26 TWh energy loss in transmission and distribution during 1998 due to unwanted heating effects in cables and substation equipment, out of a total of 350 TWh generated. This represents a loss of 7.4%.

As the overall thermal efficiency of electricity generation is only 38.5% (see above) this results in total losses of:

350 x 0.615 + 26 = 241 TWh

This represents 69% of the original energy, which has been lost as wasted heat to the atmosphere by the time the electricity has reached the end user. Of course, all the electricity usefully utilised also eventually ends up as heat.

This means that although almost all the electricity being used in the house heater is converted to heat, this represents only 31% of the original energy used to create the electricity. Although electricity is a very easy to use source of energy, it can clearly be seen that with the concern over global warming we should now be using it only where necessary.

Gas production, distribution and Use

Although "town gas" used to be made from coal, virtually all fuel gas used in the U.K. now comes from the North Sea Natural Gas fields. Ref. 4 (p. 143) shows a total of 955,342 GWh of gas input into the national transmission system in 1998, of which 20,225 GWh (2.1%) is lost due to distribution losses, metering differences and transfers.

After visiting a number of websites for gas boiler manufacturers [Refs. 11 - 13] I discovered that ordinary modern gas boilers are typically 70 - 80% efficient, and condensing gas boilers are 87 - 96% efficient. These extract most of the heat from the waste flue gases, reducing this source of energy loss and markedly increasing the performance of the boiler.Taken with the 2.1% national gas distribution loss, this means that thermal efficiencies of using gas directly for heating purposes can exceed 90%.

Space Heating

110 TWh of electricity [table 5.3 Ref. 4] was used in 1998 for domestic purpose. About 90% of energy (99 TWh) used for domestic purposes is used for heating [Ref. 8, p.50, & Ref. 3, p.188]. Allowing for the 7.4% distribution loss means that 107 TWh of electricity has to be generated. As shown above, average electricity generation efficiency is only 38.5%, and therefore 278 TWh of primary fuel energy would have to have been used to produce 99 TWh of useful heat, wasting about 280 TWh.

The overall efficiency of an electric heating system is on average therefore only about 35.6%, compared with typical gas central heating efficiencies of about 80%.

The 280 TWh of wasted energy is an enormous value, and would be able to boil a body of water 9 times the volume of Lake Windermere:

Surface area of Lake Windermere: 2.56 x 107 m2
Average depth of Lake Windermere: 10m
Volume of water in Lake Windermere: 2.56 x 107 x 10 = 2.56 x 108 m3
Volume of new body of water (Lake Windermere x 9): 2.56 x 108 x 9 = 2.304 x 109 m3
Density of Water: 1000kgm-3
Mass of new body of water: 2.304 x 109 x 1000 = 2.304 x 1012 kg
Specific Heat Capacity of Water: 4200Jkg-1K-1
Amount of Energy in 1 Wh: 3600J
Amount of Energy theoretically supplied to water: 3.6 x 1015 x 280 = 1.008 x 1018 J
Therefore temperature rise of water: 1.008 x 1018 / (4200 x 2.304 x 1012)
(assuming no change of state to steam) = 104.2K (104.2 °C)

In comparison, if the 99 TWh of heat energy was produced by gas at 90% efficiency, only about 110 TWh of gas would be used, and only 11 TWh would be lost, compared with the 280 TWh lost if electricity was used. The 11 TWh would heat the same body of water by only 4.1K (4.1 °C).

I was unable to find figures for the number of buildings heated by different methods, however Ref.2 [Table 9.3, p.245] does give average expenditure on fuel per consuming household, with and without both gas and electric central heating (CH) during 1997/98. "All" is described as including only those households who mainly consumed either gas or electricity for heating purposes. From this data I should be able to approximately calculate the % of homes with different types of heating.

Electricity:
With electric CH £9.48 / wk    Without electric CH £6.27 / wk    All households £6.58 / wk
Let the percentage of houses with electric CH = a, and without electric CH = b,

Therefore a + b = 100 and 9.48a + 6.27b = 100 x 6.58
6.27a + 6.27b = 627
3.21a = 31
a = 9.7% (to 1DP)
b = 90.3% (to 1DP)

Gas:
With gas CH £7.62 / wk    Without gas CH £4.28 / wk    All households £7.16 / wk
Let the percentage of houses with gas CH = c, and without gas CH = d,

Therefore c + d = 100 and 7.62c + 4.28d = 100 x 7.16
4.28c + 4.28d = 428
3.34c = 288
c = 86.2% (to 1DP)
d = 13.8% (to 1DP)

Other:
This leaves about 4% of homes with other main forms of heating.
Doing the similar calculations for the 1980, 85, 90 and 95 data in the same Table gives:

Table 2 - Percentage of house with different main forms of heating

Year Gas C.H. Elect. C.H. Other
1980 51 % 18 % 31 %
1985 64 % 9 % 27 %
1990 75 % 10 % 15 %
1995 84 % 10 % 6 %
1997-98 86 % 10 % 4 %

The steady increase in homes using gas central heating therefore appears to be a trend that at is favourable as far as energy wastage is concerned, although it is probably more due to prices than energy efficiency considerations.

National Grid Transmission Losses

On reading their Seven Year Statement [Ref. 5], it was clear that much energy is wasted by electricity generated in the north of England and Scotland being carried down to Southern England to be used. Although some of this is still generated in coal-fired power stations, in recent years new power stations in the Northeast have used North Sea gas and oil. Appendix 1 shows National Grid power lines and the location of electricity generating stations. Appendix 2 shows typical electric power flows around England. Appendix 3 shows the opportunities for new electricity power stations, which occur mainly in the south and south-west of England.

This supports the idea that it would be much less wasteful to take more gas to these areas than more electricity. There is a good national gas grid pipeline system already in place.

Discussion

Upon looking at the two flow diagrams of U.K. energy [Ref.2 p.432 and Ref.4] the first factor which became clear was that they were both giving levels and values involving different units (Thousand million therms and millions of tonnes of oil equivalent respectively). As I result, my first step in the analysis was to convert the therms to equivalent tonnes of oil (for conversion table, see Appendix 4).

Table 3

Type of energy source Domestic Energy Use (Tonnes of Oil Equivalent x 106) in 1983 Domestic Energy Use (Tonnes of Oil Equivalent x 106) in 1998 Percentage change from 1983 to 1998
Gas 22.4 30.6 + 37
Coal 7.3 2.3 - 68
Petroleum Products 2.3 3.5 + 52
Electricity 7.1 9.4 + 32

Over the last 15 years the usage of each of the different types of energy source has increased by a relatively large amount, with the exception of coal. This is likely to be because of the methods of heating houses on winter evenings having changed from remaining open coal fires to gas fires and central heating systems. Petroleum products includes domestic transportation (personal vehicles) and has therefore risen by a proportionally larger amount mainly due to the increasing proportion of vehicles per person.

I was slightly disappointed about the fact that the Friend's of the Earth website [Ref. 9] was oriented strictly around saving money and did not refer to the efficiency differences between gas and electricity, especially in relation to heating systems.

The OFGEM site [Ref. 10] was also similarly price-orientated, concerning itself with the prices for the consumers, and also contained no references regarding overall thermal efficiencies of the different energy sources. I would have thought that an "Energy Regulator" should be concerned with the larger issues.

It is now clear to me that heating for all purposes should ideally be done using a primary fuel like gas (or possibly oil), and electricity used only where it is impractical to pipe a gas supply.

One recent development (Ref.4, Chapter 6) are "Combined Heat and Power" electricity generating stations which are built near to houses (as in Scandinavia) or industries that need heat. The systems are designed so that much of the heat that is usually wasted in the generation process is piped into nearby buildings to be used for useful purposes.

References

1. Press and T.V. covering REVOLT (Rural England Versus Overhead Transmission Lines) newsletter February 2000.
2. "A-Level" physics textbook - "Revised Nuffield advanced science - Physics" (Student's Guide 1) ISBN 0 582 35415 3, Section G: "Energy Sources"
3. "A-Level" physics textbook - Advanced Physics - Volume 1 by Tom Duncan ISBN 0 7195 5199 4, Chapter 10: "Energy and its uses"
4. Digest of United Kingdom Energy Statistics ISBN 011 5154639 (Department of Trade and Industry publication of the Government Statistical service)
5. National Grid 1996 Seven Year Statement (SYS)
6. NGC Website: http://www.ngc.co.uk
7. "The man-made world", The Open University Press, 1973
         ISBN 0 335 02506 4, Case Study 1, "The Electricity Supply Industry"
8. Energy conversion, Power and Society. The Open University Press, 1975
         ISBN 0 335 02520 X, pp 3-11 and p. 50.
9. "Be a super energy saver", Friends of the Earth website: http://www.foe.org.uk
10. OFGEM (U.K. Energy Regulator)
13. Potterton plc. Website http://www.potterton.co.uk

Appendix 4

To:

(Multiply by)

From:

Thousand tonnes of oil equivalent Terajoules (TJ) Gigawatt hours (GWh) Million Therms
Thousand tonnes of oil equivalent 1 41.87 11.63 0.3968
Terajoules (TJ) 0.02388 1 0.2778 0.009478
Gigawatt Hours (GWh) 0.08598 3.6 1 0.03412
Million Therms 2.52 105.5 29.31 1