1. Energy and the New Reality, Volume 1: Energy Efficiency and the Demand for Energy Services Chapter 3: Electricity from Fossil Fuels L. D. Danny Harvey harvey@geog.utoronto.ca harvey@geog.utoronto.ca This material is intended for use in lectures, presentations and as handouts to students, and is provided in Powerpoint format so as to allow customization for the individual needs of course instructors. Permission of the author and publisher is required for any other usage. Please see www.earthscan.co.uk for contact details. www.earthscan.co.uk Publisher: Earthscan, UK Homepage: www.earthscan.co.uk/?tabid=101807www.earthscan.co.uk/?tabid=101807

2. Recap: Primary to Secondary to End-Use Energy

3. Outline Electricity Basics Electricity from Fossil Fuels Cogeneration and Trigeneration Economics

4. Electricity Basics Electricity can be either direct current (DC) or alternating current (AC) In AC current, the voltage and current fluctuate up and down 60 times per second in North America and 50 times per second in the rest of the world The power (W) in a DC current is equal to current (amps) x voltage (volts): P=VI The power in an AC current is equal to the product of the root mean square (RMS) of the fluctuating current and voltage if the current and voltage are exactly in phase (exactly tracking each other): P=V rms x I rms The standard electricity distribution system consists of 3 wires with the current in each wire offset by 1/3 of a cycle from the others, as shown in the next figure

5. Figure 3.1 Three-phase AC Current

6. Figure 3.2 Two Pole Synchronous Generator Source: EWEA

7. Electricity demand continuously varies, and power utilities have to match this variation as closely as they can by varying their power production. The following distinctions are made: Baseload powerplants: these are plants that run steadily at full load, with output equal to the typical minimum electricity demand during the year. Plants (such as coal or nuclear) that cost a lot to build but are cheap to operate (having low fuel costs) are good choices Peaking powerplants: these are plants that can go from an off state to full power within an hour or so, and which can be scheduled based on anticipated variation in demand (natural gas turbines or diesel engines would be a common choice) Spinning reserve: these are plants that are on but running at part load – this permits them to rapidly (within a minute) vary their output, but at the cost of lower efficiency (and so requires greater fuel use in the case of fossil fuel power plants).

8. Electricity from Fossil Fuels Pulverized coal Integrated Gasification/Combined Cycle (IGCC) Natural gas turbines and combined cycle Diesel and natural gas reciprocating engines Fuel cells

9. Technical issues related to electricity from fossil fuels Full load efficiency Part-load efficiency Rates of increase of output Impact of temperature on output Auxiliary energy use

10. Figure 3.3 Generation of electricity from a conventional, pulverized-coal powerplant Source: Hoffert et al (2002, Science 298, 981-987)

11. The upper limit to the possible efficiency of a powerplant is given by the Carnot efficiency: η = (T in -T out )/T in So, the hotter the steam supplied to the steam turbine, the greater the efficiency. Hotter steam requires greater pressure, which requires stronger steel and thicker walls – so there is a practical limit to the achievable Carnot efficiency (and actual efficiencies are even lower)

12. Coal powerplant operating temperatures and efficiencies Typical: 590ºC, 35% efficiency Best today: > 600ºC, 42-44% efficiency Projected by 2020: 720ºC, 48-50% efficiency

13. Integrated Gasification Combined Cycle (IGCC) This is an alternative advanced coal powerplant concept Rather than burning pulverized solid coal, the coal is heated to 1000 º C or so at high pressure in (ideally) pure oxygen This turns the coal into a gas that is then used in a gas turbine, with heat in the turbine exhaust used to make steam that is then used in a steam turbine Efficiencies of ~ 50% are expected, but are much lower at present

14. Generation of electricity with natural gas Simple-cycle power generation Combined-cycle power generation Simple-cycle cogeneration Combined-cycle cogeneration

15. Simple-cycle turbine Has a compressor, combustor, and turbine proper Because hot gases rather than steam are produced, it is not restricted in temperature by the rapid increase in steam pressure with temperature Thus, the operating temperature is around 1200ºC

16. Figure 3.6a Simple-cycle gas turbine and electric generator Source: Williams (1989, Electricity: Efficient End-Use and New Generation Technologies and Their Planning Implications, Lund University Press)

17. Efficiency of generating electricity using natural gas One might expect a high efficiency from the gas turbine, due to the high input temperature (and the resulting looser Carnot limit) However, about half the output from the turbine has to be used to compress the air that is fed into it Thus, the overall efficiency is only about 35% in modern gas turbines

18. Figure 3.4 Turbine efficiency vs turbine size (power)

19. Figure 3.5 Efficiency and cost of a simple-cycle gas turbine with and without water injection

20. Due to the afore-mentioned high operating temperature of the gas turbine, the temperature of the exhaust gases is sufficiently hot that it can be used to either Make steam and generate more electricity in a steam turbine (this gives combined cycle power generation), or provide steam for some industrial process that can use the heat, or to supply steam for district heating (this gives simple cycle cogeneration)

21. Figure 3.6c Combined-cycle power generation using natural gas Source: Williams (1989, Electricity: Efficient End-Use and New Generation Technologies and Their Planning Implications, Lund University Press)

22. Figure 3.6b Simple-cycle cogeneration Source: Williams (1989, Electricity: Efficient End-Use and New Generation Technologies and Their Planning Implications, Lund University Press)

23. The energy can be cascaded even further, as follows: Gas turbine → steam turbine → useful heat as steam from the steam turbine (combined cycle cogeneration), or Gas turbine → steam turbine → steam → hot water (also combined cycle cogeneration), or Gas turbine → steam → hot water

24. Figure 3.6d Combined-cycle cogeneration Source: Williams (1989, Electricity: Efficient End-Use and New Generation Technologies and Their Planning Implications, Lund University Press)

25. Figure 3.7 Cogeneration system with production of steam and hot water Source: Malik (1997, M. Eng Thesis, U of Toronto)

26. State-of-the-art natural gas combined-cycle (NGCC) systems have electricity generation efficiencies of 55-60%, compared to a typical efficiency of 35% for single-cycle turbines However, NGCC systems are economical only in sizes of 25-30 MW or greater, so for smaller applications, only the less efficient simple-cycle systems are used Thus, a number of techniques are being developed to boost the electrical efficiency of simple gas turbines to 42-43%, with one technique maybe reaching 54-57%

27. In cogeneration applications, the overall efficiency (counting both electricity and useful heat) depends on how much of the waste heat can be put to use. However, overall efficiencies of 90% or better have been achieved

28. Reciprocating engines These have pistons that go back and forth (reciprocate) Normally they use diesel fuel – so these are the diesel generators normally used for backup or emergency purposes However, they can also be fuelled with natural gas, with efficiencies as high as 45%

29. Fuel cells These are electrochemical devices – they generate electricity through chemical reactions at two metal plates – an anode and a cathode Thus, they are not limited to the Carnot efficiency Operating temperatures range from 120ºC to 1000ºC, depending on the type of fuel cell All fuel cells require a hydrogen-rich gas as input, which can be made by processing natural gas or (in the case of high- temperature fuel cells) coal inside the fuel cells

30. Fuel cells (continued) Electricity generation efficiencies using natural gas of 40-50% are possible, and 90% overall efficiency can be obtained if there is a use for waste heat In the high-T fuel cells, the exhaust is hot enough that it can be used to make steam that can be used in a steam turbine to make more electricity An electrical efficiency of 70% should be possible in this way – about twice that of a typical coal-fired powerplant.

31. Figure 3.8 Cross section of a single fuel cell. Several such cells would be placed next to each other to form a fuel cell stack.

32. Figure 3.9 United Technologies Company 200-kW phosphoric acid fuel cell that uses natural gas as a fuel. 1=fuel processor, 2=fuel cell stack, 3=power conditioner, 4=electronics and controls Source: www.utcfuelcells.com

33. Figure 3.10 Solid Oxide Fuel Cell / Gas Turbine System

34. Figure 3.11a Electrical efficiency vs. load

35. Figure 3.11b Relative electrical efficiency vs. load

36. Summarizing the preceding slides and other information, Natural gas combined-cycle has the highest full- load efficiency (55-60%) and holds its efficiency well at part load Reciprocating engines have intermediate full- load efficiencies (40-45%) and load their efficiencies well at part load Gas turbines and micro-turbines have low full- load efficiencies (typically 25-35%, but ranging from 16% to 43%) and experience a substantial drop at part load Fuel cells using natural gas have intermediate full-load efficiency (40-45%) but this efficiency increases at part load

37. Capital Costs Today Pulverized coal powerplant with state-of- the-art pollution controls: $1200-1400/kW Natural gas combined cycle: $400-600/kW in mature markets, $600-900/kW in most developing countries Reciprocating engines: $600-1200/kW Fuel cells: $3000-5000/kW

38. Cogeneration

39. Cogeneration is the simultaneous production of electricity and useful heat – basically, take the waste heat from electricity generation and put it to some useful purpose. Two possible uses are to feed the heat into a district heating system, and to supply it to an industrial process

40. Figure 3.12 Proportion of electricity produced decentrally (overwhelmingly as cogeneration)

41. Technical issues Impact of withdrawing useful heat on the production of electricity Ratio of electricity to heat production Temperature at which heat is supplied Electrical, thermal and overall efficiencies Marginal efficiency of electricity generation

42. Four efficiencies for cogeneration: The electrical efficiency – the amount of electricity produced divided by the fuel use (later I’ll need to call this the direct electrical efficiency) The thermal efficiency – the amount of useful heat provided divided __by the fuel use The overall efficiency – the sum of the of two The effective or marginal efficiency of electricity generation – explained later

43. Impact of withdrawing heat In simple-cycle cogeneration, capturing some of the heat in the hot gas exhaust does not reduce the production of electricity, but the electrical production is already low In cogeneration with steam turbines, the withdrawal of steam from the turbine at a higher temperature than would otherwise be the case reduces the electricity production The higher the temperature at which we want to take heat, the more that electricity production is reduced

44. Figure 3.13 Example of the tradeoff between production of useful heat and loss of electricity production using steam turbine cogeneration Source: Bolland and Undrum (1999, Greenhouse Gas Control Technologies, 125-130, Elsevier Science, New York)

45. Thus, to maximize the electricity production, we want to be able to make use of heat at the lowest possible temperature. If the heat is to be provided to buildings, that means having well insulated buildings that can be kept warm with radiators that are not very hot

46. The alternative to cogeneration is the separate production of heat and electricity. The effective efficiency in generating electricity is the amount of electrical energy produced divided by the extra fuel used to produce electricity along with heat compared to the amount of fuel that would be used in producing heat alone. The extra amount of fuel required in turn depends on the efficiency with which we would have otherwise have produced heat with a boiler or furnace.

47. For example, suppose that we have a cogeneration system with an electrical efficiency of 25% and an overall efficiency of 80%. Then, the thermal efficiency is 80%-25%=55% - we get 55 units of useful heat from the 100 units of fuel. If the alternative for heating is a furnace at 80% efficiency, we would have required 68.75 units of fuel to produce the 55 units of heat. Thus, the extra fuel use in cogeneration is 100-68.75=31.25 units, and the effective electricity generation efficiency is 25/31.25=80%. I call this the marginal efficiency, because it is based on looking at things on the margin (this is a concept from economics).

48. With a little algebra, it can be shown that the marginal efficiency is given by n marginal = n el /(1-n th /n b ) where n el and n th are the electrical and thermal efficiencies of the cogeneration system, and n b is the efficiency of the boiler or furnace that would otherwise be used for heating

49. Figure 3.15 Marginal efficiency of electricity generation in cogeneration (η el = efficiency of the alternative, central powerplant for electricity generation)

50. Key points For a given thermal efficiency, the effective electrical efficiency is higher the higher the direct electrical efficiency However, very high effective electrical efficiencies can be achieved even with low direct electrical efficiencies if the thermal efficiency is high – that is, if we can make use of most of the waste heat To get a high thermal efficiency requires being able to make use of low-temperature heat (at 50-60ºC), as well as making use of higher temperature heat

51. Electricity:heat ratio Because the marginal electricity generation efficiency in cogeneration is generally much higher than the efficiency of a dedicated central powerplant, there is a substantial reduction in the amount of fuel used to generate electricity when cogeneration is used Thus, we would like to displace as much inefficient central electricity generation as possible when cogeneration is used to supply a given heating requirement This in turn requires that the electricity-to-heat production ratio in cogeneration be as large as possible (Remember – none of the gains that we’ve talked about occur if we can’t use the waste heat produced by cogeneration)

52. Figure 3.14 Electricity:heat output ratio in cogeneration

53. Figure 3.17 Dependence of overall savings through cogeneration on the electricity:heat ratio and on the central powerplant efficiency, assuming a 90% overall efficiency for cogeneration and 90% efficiency for the alternative heating system

54. Cost of Electricity

55. Issues related to the cost of electricity: Capital cost, interest rate, lifespan Fuel cost (impact of depends on efficiency) Fixed and variable operation & maintenance costs Baseload vs peaking costs Transmission line costs and transmission losses Amount of backup capacity

56. Figure 3.16 Capital cost of natural gas combined cycle cogeneration plants

57. Amortization of capital cost: CRF x C cap / (8760 x CF) units: $/kWh where CRF = i /(1-(1+i) -N ) is the cost recovery factor _i = interest rate _N = financing time period C cap = capital cost ($/kW) 8760 is the number of hours in a year CF= capacity factor (annual average output as a fraction of capacity)

58. Fuel contribution to the final cost: C fuel ($/GJ) x 0.0036 (GJ/kWh) / efficiency The cost of electricity from less efficient powerplants will be more sensitive to the cost of fuel than the cost of electricity from efficient powerplants, but more efficient powerplants will tend to have greater capital cost

59. Typical overnight capital costs and best efficiencies Pulverized coal: $1200-1400/kW,η= 0.45-0.48 IGCC: $1400-2600/kW today, η= 0.41-0.55 $1150-1400/kW hoped for, future NGCC: $400-600/kW, η = 0.55-0.60 Reciprocating engine: $600-1200/kW,η=0.40-0.46 Micro-turbine: $1800-2600/kW, η= 0.23-0.27 Fuel cells: $3000-5000/kW, η= 0.35-0.45 $1000-1500/kW hoped for, future NGCC/FC hybrid: $2000-3000/kW, η= 0.70-0.80

60. Figure 3.18 Cost of electricity from coal and natural gas

61. Figure 3.19 Cost of heat from boilers, electricity with or without cogeneration, and heat from cogeneration

62. Figure 3.20 Cost of electricity from central coal (at $2/GJ) and from natural gas (at $10/GJ)

63. Water requirements Most thermal powerplants use water to cool the condenser of a steam turbine and for other, minor, purposes There are two approaches: a once-through cooling system a recirculating system in a cooling tower Water use by power generation represents the largest or second largest use of water in most countries (with irrigation sometimes being a larger use)

64. In once-through systems, the water is returned to the source (but at a warmer temperature). Large volumes of water are needed – not available in arid regions In a recirculating systems, water that has removed heat from the condenser is sprayed through a cooling tower, where it is cooled by evaporation, then returns to the condenser This consumes water, but the amount that is withdrawn from the water source (lakes, rivers or groundwater) is smaller than in once-through systems

65. Typical water requirements Steam turbines (as in coal powerplants) Once through: 80-190 litres withdrawn per kWh of __generated electricity, ~ 1 litre/kWh consumed Recirculating: 1-3 litres/kWh withdrawn 1-2 litres/kWh consumed Natural gas combined cycle Once through: 30 litres/kWh withdrawn ~ 0.4 litres/kWh consumed Recirculating: 0.9 litres/kWh withdrawn 0.7 litres/kWh consumed

66. Bottom line: More efficient power plants, such as natural gas combined cycle powerplants, use less water per kWh of generated electricity than less efficient powerplants The water requirements can be a constraining factor in arid regions It is possible to use air rather than water to cool the condenser, but then the efficiency drops