Transition Paths Toward Carbonless Energy

Lawrence Livermore National Laboratory (LLNL) has developed a network optimization model (Figure 6.1) to examine these three stages of integrating renewables into utilities (reliability, intermittent intensive electric systems, and carbonless transportation). By constructing and analyzing model scenarios of future electricity and transportation systems attempts we quantify the

Electric demand

Cars and trucks demand

Electric demand

Cars and trucks demand

Natural gas

Nuclear

Wind

Photovoltaic

Figure 6.1: Schematic of a coupled utility electric generation and transportation system using nuclear, fossil, and renewable primary energy sources, with electricity and hydrogen as energy carriers.

Natural gas

Nuclear

Wind

Photovoltaic

Figure 6.1: Schematic of a coupled utility electric generation and transportation system using nuclear, fossil, and renewable primary energy sources, with electricity and hydrogen as energy carriers.

characteristics of transition paths to carbonless utilities and transportation. The model calculates the economically optimum energy system to meet scenarios of electricity and/or transportation demands, arriving at the desired system structure in terms of energy supply sources, conversion technologies, and storage capacities. It simultaneously determines optimal operation of the system components, using patterns of electricity demand (Iannucci et al.,

1998), transportation fuel demand, energy available from solar and wind production, and capacities of long and short term hydrogen storage technologies. A more detailed model description and table of key assumptions used to generate these scenarios is given in Appendix 6.A.

Although there are many transition paths to a carbonless future, we wish to try to identify paths that are economically and strategically advantageous. Using the model discussed above, we have evaluated a broad range of possibilities, using various levels of nuclear, natural gas and renewable generation in combination with a transportation sector using natural gas and/or hydrogen in various amounts. In order to explore a representative example for which data was readily available, we chose to design scenarios based on US Energy Information Agency projections of electricity and transportation demands of the United States in 2020 (EIA, 2000). Under this scenario, within two decades, the United States will demand roughly 5 trillion kWh of electricity (about one third of world demand) and 4.6 trillion kWh of transportation fuel.

A subset of model results are shown as points in Figure 6.2. Each point corresponds to the projected cost of an electricity and transportation fuel system which can achieve the given level of overall carbon emissions from both sectors. These scenarios trace a transition which is efficient in terms of reducing carbon emissions for minimum cost, the efficient frontier. From an economic perspective an optimum emission reduction path should fall on this frontier since any other approach will cost more, have higher emissions, or both.

Figure 6.3 shows hourly variations in electricity demand and generation for a ten day period representative of selected cases along the efficient frontier. These cases illustrate the changes in generation patterns and structure along the efficient frontier as natural gas is displaced by renewables and hydrogen transportation is ultimately phased in.

6.2.1 High efficiency use of low carbon fuels

Our starting point is a scenario that efficiently uses natural gas and nuclear electric generation while transportation is fueled by natural gas (lower right point in Figure 6.2. In assessing different carbon reduction strategies, there is generally consensus that improved efficiency and fuel switching to natural gas are "no regrets" measures. To take this into maximum account, we have therefore chosen to measure scenarios using solar and wind electricity against a "no regrets" technically advanced carbon-conscious scenario fueled by natural gas.

Our 2020 US reference scenario relies essentially on very efficient use of natural gas. Hydroelectric and nuclear power (hereafter combined for

700 650 600 550

450 400 350 300

The transportation served by H2 is indicated as: None: normal symbol, no letter 100% cars: letter "c" 100% cars and trucks: letter "t 100% cars, trucks, aircraft: letter "a"

100 200 300 400 500

Carbon emissions (mmTonnes/yr)

-A—350 GW gas —X—250 GW gas -e—100 GW gas —•—0 GW gas -----Slope of $500/tonne C emissions reduction

Figure 6.2 The efficient frontier for reducing US 2020 overall carbon emissions. Moving from right to left, natural gas generation capacity is reduced and replaced by renewable generation. Carbon emissions can be reduced to around 250 mmt (million metric tonnes) by displacing natural gas electric generation. Further carbon reductions are achieved by replacing natural gas fueled transportation with hydrogen-fueled transportation. Each case meets 5 trillion kWh of electricity demand and 4.6 trillion kWh of fuel demand and includes 100 GW of nuclear capacity in addition to the gas-fired generation capacities indicated.

simplicity and referred to as nuclear) provide nearly 20% of electricity and 10% of the generating capacity of a 1000 GW US electricity system. Natural gas fuels the remaining 80% of US electricity generation at 57% efficiency, in addition to the all the automobiles, trucks, and aircraft in the US transportation sector. The natural gas infrastructure is assumed to meet all these demands as efficiently as theoretically possible. Fuel cycle greenhouse gas emissions are neglected and natural gas leakage is assumed to be reducible to negligible levels. 250 million light-duty vehicles in the US are assumed to achieve an average fleet economy of 80 mpg through the use of lighter and more aerodynamic automobiles. Driving is assumed to rise only slightly to 14000 miles/yr. Natural gas at refueling stations is priced at $6.93-8.31/GJ

(equivalent to $0.83-1.00/gallon of gasoline). Natural gas for utility electric generation is priced somewhat lower at $5.54/GJ.

Even with all the progress assumed in this scenario, US electricity and transportation fuel use is projected to produce 642 million metric tonnes of carbon emissions in 2020 and cost roughly $350 billion/yr. Rapid and full implementation of the "no regret" strategy, throughout the entire infrastructure, manages relatively moderate emission reductions relative to current levels (1000 mmtC/yr). Rising energy use driven by population and especially economic growth partially offset a near doubling of electric generation efficiency, a tripling of automobile fuel economy, and a near halving of the carbon intensity of fuels. Global emission targets consistent with stabilizing greenhouse gases at an equivalent doubling of carbon dioxide would limit worldwide gas fuel emissions to about 4000 mmtC/yr (Fetter, 1999). An eventual US target based on per capita share and adjusted to reflect transportation and utilities emissions alone would be about 100 mmtC/yr. Obviously, reducing carbon emissions to this level from the advanced fossil scenario (642 mmtC/yr) will ultimately require a massive shift from fossil energy to carbonless sources.

6.2.2 Displacing natural gas generation with renewables

To efficiently offset the greatest amount of carbon emissions, direct displacement of gas generation with wind and solar electricity is likely to be the best first step. The gas fired electric generation sector is projected to account for substantially more emissions (394 mmtC/yr) than a natural gas transportation system (248 mmtC/yr) which also has more complex fuel infrastructure issues than electric utilities. Employing carbonless sources in the utility sector also circumvents the energy penalties of converting wind or solar electricity to transportation fuel.

This forms the basis of a "utilities first" approach, in which fossil electric generation is successively displaced by renewables and ultimately eliminated. Further emission reductions are accomplished by displacing natural gas transportation fuel with renewable hydrogen, first in automobiles, then freight trucks, and ultimately in aircraft. The corresponding sequence of scenarios is shown in Figure 6.2. The cost breakdowns (Figure 6.4) and energy flows (Figure 6.5) indicate the dramatic shifts in energy supply, storage, emissions, and marginal cost as carbon emissions are reduced, and ultimately eliminated, using the "utilities first" strategy.

Power Generation (TW)

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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