Energy Implications of the Scenarios

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Carbonaceous fossil fuels (coal, oil and natural gas) represent approximately 80% of the primary global energy supply today (Nakicenovic et al., 1998). The carbon free component of the global energy supply is composed of traditional renewables 10% firewood (presumed sustainably burned), 5% hydropower and 4% nuclear fission. Collectively, the "high tech" renewables (solar electric, solar thermal, wind, biomass, and geothermal) produce less than 1%. In this section we examine what the IIASA scenarios, the IPCC BAU scenario and the WRE scenarios (which lead to specific future equilibrium levels of atmospheric CO2) imply about the need for carbon free global power supply over the 21st century.

First of all, let us examine some results of the IIASA scenarios. Table 2.1 is a compilation of the total world energy consumption associated with the six scenarios. Values for total primary energy for 1990 and 2050 are read from Table 5.1 of Nakicenovic et al. (1998). Values for the year 2100 are those read from graphs in that reference. Since Nakicenovic et al. give values in Gtoe (gigatons oil equivalent) a conversion factor 1Gtoe X 1.33 = 1TWt is used.

According to the six IIASA scenarios as little as 8 TWt or as much as 13.6 TWt of energy must be supplied by renewables plus nuclear power by 50 years hence. In 100 years the range is from 20 to 42 TWt, that is, from nearly two to about four times global energy consumption today.

Most global energy projections include a reference scenario supposed to represent a "business as usual" or "no policy" projection (Nakicenovic et al., 1998; Raskin and Margolis, 1995). In the IIASA scenarios it is probably best represented by case A1. To characterize "business as usual" energy demand and carbon emissions over the 21st century, the IPCC developed the IS92a (BAU) scenario, incorporating widely accepted population projections and the current consensus on economic development of the World Bank and the United Nations (Leggett et al., 1992). Note that the BAU scenario was not meant to represent particularly high or low rates of energy use, but is a nominal reference case in the absence of intervening policy actions.

To understand the implications of the BAU scenario and the WRE scenarios

Table 2.1 Primary energy consumption from six IIASA scenarios. Renewables plus nuclear means energy from renewables such as solar, wind, hydroelectricity, biomass and nuclear sources and is shown in boldface. Units are TWt (terawatts thermal). Note that the required energy production from renewables and nuclear by 2050 is in all cases comparable to total world energy production in 1990.

Table 2.1 Primary energy consumption from six IIASA scenarios. Renewables plus nuclear means energy from renewables such as solar, wind, hydroelectricity, biomass and nuclear sources and is shown in boldface. Units are TWt (terawatts thermal). Note that the required energy production from renewables and nuclear by 2050 is in all cases comparable to total world energy production in 1990.

Scenario

Energy (TWt)

1990

2050

2100

A1

Total

12

33.3

60

Renewable plus nuclear

2.7

11.3

30

A2

Total

33.3

60

Renewables plus nuclear

9

30

A3

Total

33.3

60

Renewables plus nuclear

13.

42

B

Total

26.6

46.6

Renewables plus nuclear

9.6

26

C1

Total

18.6

28

Renewables plus nuclear

8

20

C2

Total

18.6

28

Renewables plus nuclear

9.1

23

for future energy needs Hoffert et al. (1998) examined the individual terms in the Kaya identity (Kaya, 1989). In general, the rate at which carbon is emitted (as CO2) by energy production is given by the identity

expressing the population (N), per capita gross domestic product (GDPIN), primary energy intensity (E'IGDP), E' being the rate of energy use, and carbon intensity (CIE). Here we express the primary energy consumption from all fuel sources in watts and the gross domestic product in (1990 US)$ per year so their ratio, the energy intensity, has units of watt years per 1990 US$. Carbon intensity, the weighted average of the carbon to energy emission factors of all energy sources, has the units of kgCIwatt year. For example, from the Kaya identity, fossil fuel emissions in 1990 were MC = 5.3 x 109 persons times $4100 per person per year times 0.49 watt year per $US (1990) times 0.56 kgC per watt year, or about 6.0 GtCIyr. This corresponds to approximately 1.1 metric tons of carbon per person emitted to the atmosphere per year as CO2 (in 1990). At this time per capita emissions were significantly higher in the developed than in the developing nations (Hammond et al., 1994), but the projected growth of emissions is far greater in developing nations. The rate of population growth is also projected to be much larger in developing nations (Alcamo et al., 1995)

To illustrate the relative contributions of the factors in the Kaya identity, Hoffert et al. (1998) evaluated each of them globally over the 210 year period from 1890 to 2100, from historical data before 1990 (Nakicenovic, 1996), and from documents defining IPCC scenarios after 1990 (see Hoffert et al., 1998 for details).

One way to reduce the rate of increase of carbon emissions is to increase the economic productivity of energy, which we define as the inverse of the primary energy intensity, E'/GDP. Data from individual nations indicate that the E'/GDP generally increases during economic development as countries establish heavy infrastructure, and declines only after some lag as economic productivity rises and the economy shifts structurally to less energy intensive activities (for example, to services). Energy intensities of China and India are two to five times the global mean, but they are decreasing. To focus on energy supply issues Hoffert et al. (1998) provisionally accepted the IS1992a projections of 1% per year improvements. Note that the IIASA projections range from 0.8% to 1.4%. Achieving this will depend crucially on technology and structural changes adopted by individual nations. We will see later how larger or smaller values change our estimates of the nature of future energy supplies.

Another opportunity for emission reductions of CO2 is the continuation of the "decarbonization" of the past 100 years that is reflected in the decreasing carbon intensity of the global energy mix (Nakicenovic, 1996). The carbon intensity is the mass of carbon produced from a unit of energy for any given carbon fuel, in kgC/watt year. For example, for direct combustion of natural gas the factor is 0.46 kgC/Watt year (Nakicenovic, 1996), while that for oil is 0.60 and for coal, 0.77. Emission factors decrease as the carbon to hydrogen ratio of the fuel decreases. For wood the factor is 0.89 kgC/watt year. To compute the emission factor for the fuel mix,

C/E=( ^ C X Ei)/^ E, i i where E; is the ith source of energy characterized by an emission factor C.

In 1990 the value of C/E was 0.56, slightly below whzat it would be if global energy came completely from oil. At that time global energy sources were 2.54 TWt from natural gas, 3.90 TWt from oil and 3.14 from coal, with 1.3 TWt from nuclear and renewables, which are assumed to emit no CO2.

Both of these factors are decreasing, and they are assumed (by the IPCC and other scenarios) to continue their decline throughout the 21st century. The other two factors in the Kaya identity will continue to increase, however.

Table 2.2 Global energy production in TWt, world GDP in 1012 $US (1990) and world population in billions as projected by the IPCC IS92a (BAU)

scenario

Table 2.2 Global energy production in TWt, world GDP in 1012 $US (1990) and world population in billions as projected by the IPCC IS92a (BAU)

scenario

Year

Gas

Oil

Coal

Nuclear

Renew.

Total

GDP

N

1990

2.54

3.90

3.14

0.54

0.76

10.87

21.8

5.3

2000

3.11

4.44

3.71

0.73

0.95

12.92

20.0

6.24

2025

4.44

5.23

6.97

1.78

4.03

22.44

59.3

8.40

2050

4.09

4.56

10.84

2.60

7.51

29.61

97.8

9.95

2075

2.71

3.76

16.34

4.07

10.95

37.83

161.3

10.83

2100

1.33

2.95

21.84

5.55

14.39

46.06

265.9

11.30

To project into the future according to the IPCC 1992a (BAU) scenario we use E from Pepper et al. (1992), N from Leggett et al. (1992, IPCC) and GDP from the IPCC projections. The population in 1990 was 5.3 billion, and projections are for 8.4 billion in 2025 and 11.3 billion in 2100. The IPCC assumes increases in economic growth of 2.9% per year from 1990 until 2025 and 2.3% from 1990 until 2100. The energy mix and its evolution along with world GDP are shown in Table 2.2.

From these numbers one can calculate each of the terms in the Kaya identity. We note that in the BAU scenario 10.11 TWt of renewable plus nuclear (i.e., non-fossil) energy is required globally by 2050 and 19.94 TWt by 2100, even though carbon emissions reach approximately 20 GtC/year, and the CO2 content of the atmosphere nearly triples. These results are within the range of values in the IIASA scenarios.

We now ask the question "How much renewable energy will be required to follow the WRE scenarios that maintain atmospheric CO2 levels below specified targets?" First assume that the technology mix implied by the BAU scenario, that is, the value of C/E calculated from Table 2.2, is maintained. In that case the differences between the primary power available for the BAU and the primary power available for the CO2 stabilization scenarios represent shortfalls that would presumably have to be made up entirely by carbon free energy sources to achieve the global economic targets of the BAU scenarios. To find the global primary power available for CO2 stabilization scenarios the carbon emission rates are computed by running the carbon cycle model of Jain et al. (1995) in an inverse mode in which the atmospheric concentration paths are constrained to follow those in the WRE stabilization scenarios. Under the assumptions of the BAU scenario economic targets it is now possible to calculate the primary power consumption (from the fossil fuel mix) that results in these emission rates from the Kaya identity.

Figure 2.1 The evolution of factors governing the rate of global fossil-fuel carbon emissions in the Kaya identity. Historical curves and future projections computed for the IPCC IS92a (BAU) scenario. Reprinted with permission from Nature (29 October, 1998), © 1998 Macmillan Magazines Limited.

The top curve in Figure 2.1 shows the evolution of primary power required to meet the economic goals of the BAU scenario. Also shown are the contributions from the fossil fuels: coal, oil, and natural gas. The dashed lines shown on that figure represent the primary power from fossil fuel that is allowed following the WRE scenarios for stabilization of CO2 at various levels. Carbon emissions from the various scenarios are shown in Figure 2.2.

Figure 2.2 Fossil-fuel emissions and primary power in the 21st century for the IPCC IS92a (BAU) and the WRE CO2 stabilization scenarios. Reprinted with permission from Nature (29 October, 1998), © 1998 Macmillan Magazines Limited.

The differences between the energy requirements of the BAU scenario and the WRE scenarios must presumably be made up from nuclear and renewable sources. These are shown in Figure 2.2C. Even in the BAU scenario, approximately 10 TWt must be produced from renewables and nuclear energy by 2050, and about 20 TWt by 2100, more or less in agreement with the IIASA scenarios. To maintain CO2 levels at twice preindustrial levels 30 TWt must be produced from non-fossil sources by 2100.

The numbers are rather daunting. Let us begin to examine their implications by noting the probability that the population will be at least as large as that used in most scenarios and with the hopeful assumption that we will be able to increase the per capita wealth of developing nations at some nominal rate, reasonably close to that assumed by the IIASA or the IPCC scenarios. What other factors in the Kaya identity can be manipulated? The candidates are the primary energy intensity (E'/GDP) and the carbon intensity (C/E). Reducing the carbon intensity implies a switch to carbon free energy.

As we have seen, in the opinions of the many experts involved in the IIASA and IPCC studies, improvements (decreases) in the primary energy intensity will be between 0.8 and 1.4% per year. Among those who believe that it can be improved much more rapidly is Art Rosenfeld (personal communication, 2000). Rosenfeld points out that historically the US primary energy intensity has improved from 2.26 watt years per $US (1990) in 1850 to 0.49 in 1995, or a sustained decrease of about 1% per year. However, there were periods when the decrease reached as high as 5% per year. Between 1855 and 1880 the improvement was nearly 50% as we switched from wood to coal; during the 1920s there was another rapid decrease as we switched from coal to natural gas and oil. There has also been a recent decrease greater than 1% per year (about 2.2% per year since 1975), mostly due to energy price increases during the OPEC years (about 1974 until 1985) and the recent rapid growth in the US economy.

There is some evidence that the primary energy intensity is improving rapidly in China (Rosenfeld, personal communication), having declined by 5% per year for the past 25 years.

Figure 2.3 shows how improvements in the primary energy intensity affect our conclusions about the need for more non-fossil energy sources in the future. If improvements can be sustained at 2% per year the need for carbon free power remains modest even until 2100. However, periods of very rapid improvement in the past have been driven by changes in technology (changing from wood to coal and from coal to oil and gas) or by the scarcity of a resource (during the OPEC years). A sustained decrease of more than 1% per year in the future will be very difficult to manage without substantial policy intervention in the form

Figure 2.3 Twenty-first century trade-offs between carbon free power required and energy intensity for CO2 stabilization at twice the preindustrial atmospheric concentration. Reprinted with permission from Nature (29 October, 1998), © 1998 Macmillan

Magazines Limited.

Figure 2.3 Twenty-first century trade-offs between carbon free power required and energy intensity for CO2 stabilization at twice the preindustrial atmospheric concentration. Reprinted with permission from Nature (29 October, 1998), © 1998 Macmillan

Magazines Limited.

of carbon taxes or other incentives. Whether this can be implemented globally, as it would have to be, remains seriously in question. It will require a supranational authority having the power to somehow impose its will on sovereign nations. Whether this is possible or even desirable is problematic (Pendergraft, 1999).

The factors in the Kaya identity are not likely to be independent of each other. The rate of population growth, for example, depends on the GDP per capita and on education (particularly of women). Per capita wealth is not likely to increase very rapidly without abundant, inexpensive energy. The most abundant and inexpensive source of energy in China is coal, and China is rapidly developing an energy infrastructure based on coal. It is difficult to see how they can reduce CIE very far below that of coal, 0.77 kgC/watt year. Once this infrastructure is established, they will not want to replace it soon. If an international treaty is proposed to tax emissions of CO2, they will in all probability not cooperate. It is likely that "mandatory reductions" of CO2 emissions as proposed in Kyoto would in any case only be supported by developed countries, and developing countries will account for most of the emissions within the next several decades.

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Getting Started With Solar

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