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(d) Carbonless energy system serving electric demands and fueling hydrogen cars. Since the wind capacity had reached its limit in the previous case, all additional energy to make energy for the fuel cell and the cars must come from PV.

400 Annual Fuel and Electricity Cost

6OO 5OO 4OO 3OO 2OO

Carbon Emissions (mmTC/yr)

Figure 6.4 Cost breakdown for electricity and transportation systems using the "utilities first" approach to reducing carbon emissions along the efficient frontier in

Figure 6.2.

6OO 5OO 4OO 3OO 2OO

Carbon Emissions (mmTC/yr)

400 Annual Fuel and Electricity Cost

Figure 6.4 Cost breakdown for electricity and transportation systems using the "utilities first" approach to reducing carbon emissions along the efficient frontier in

Figure 6.2.

400 300

Carbon Emissions

Figure 6.5 Energy flows for electricity and transportation systems using the "utilities first" approach to reducing carbon emissions along the efficient frontier in Figure 6.2. 5000 TWh of electricity and 4600 TWh of transportation fuel are delivered for end-uses. As carbon emissions are reduced, natural gas is displaced from electric generation, then from transportation. Losses are shown for hydrogen production, storage, and reconversion to electricity in fuel cells.

400 300

Carbon Emissions

16000 14000 12000 10000 8000

Delivered 6000 Energy (TWh/yr)

4000 2000 0

-2000 -4000

Figure 6.5 Energy flows for electricity and transportation systems using the "utilities first" approach to reducing carbon emissions along the efficient frontier in Figure 6.2. 5000 TWh of electricity and 4600 TWh of transportation fuel are delivered for end-uses. As carbon emissions are reduced, natural gas is displaced from electric generation, then from transportation. Losses are shown for hydrogen production, storage, and reconversion to electricity in fuel cells.

6.2.3 Integrating intermittent renewables

Initially, displacing natural gas generation with low cost wind energy is projected to reduce emissions and system costs (Figure 6.2, 500 GW scenario). Wind capacity can be fully absorbed by the electric grid, except during occasional nighttime hours of very low demand, and very high wind availability. Solar electricity will just begin to be cost effective, even though it is twice the cost of wind per kWh, augmenting generation during periods of higher (daylight) electric demand. Energy storage at this level of renewable penetration is minimal, accounting for roughly 2% of generation (similar to pumped hydro today). This storage essentially serves the same function as pumped hydro, reducing the cost of generating capacity. More than 95% of renewable generation will serve the grid directly, cost-effectively reducing total system emissions to two-thirds of the US 2020 reference case. Intermittent electricity, predominately wind, will account for 50% of the utility mix.

Later scenarios (Figure 6.2, 350 GW scenario) indicate integration issues increase the cost of emission reductions. Displacing more natural gas requires wind capacity levels near daytime peaks, as wind electricity costs much less than solar. At night wind capacity is high relative to demand, and must be stored, displacing natural gas only indirectly (and less efficiently) through fuel cells. As the mismatch between electric demand and wind generation grows, high cost solar electricity will cost-effectively contribute, flattening net electric demand patterns. This enables excess wind to be more cost-effectively used in electrolyzers, producing hydrogen. This hydrogen will be needed by fuel cells to meet peak demands in excess of the gas-fired capacity, accounting for roughly 5% of delivered electricity. Emissions are reduced to 404mmtC/yr at a marginal cost of $150/tonneC.

Reducing gas capacity further (Figure 6.2, 250 GW scenario) limits the ability of fossil generation to compensate for seasonal mismatches between demand and renewable (principally wind) supply. This will substantially increase solar generation, which matches seasonal demand patterns well. In winter, when solar is only partially available, and short term hydrogen storage is not large enough to cover periods between windy days, energy intensive liquid hydrogen will be used extensively. Fuel cell generation will grow to 10% of delivered electricity, being required every windless night and cloudy day. Carbon emissions will fall to 362 mmtC/yr at a marginal cost of approximately $300/tonneC.

Reducing emissions to 305 mmtC/yr (Figure 6.2,100 GW scenario) will raise the wind capacity to the assumed maximum of 1.0 TW (Schipper and Meyers, 1992; Fetter, 1999). Additional displacement of natural gas will require increasingly costly solar capacity. Reduced gas generating capacity will also lead to greater fuel cell use (18% of delivered electricity). On days when wind and solar generation coincide, hydrogen will be sent to long term hydrogen storage since compressor capacity is exceeded. Liquid hydrogen flow will grow relative to capacity. Relying increasingly on high cost solar generation is the chief factor contributing to a marginal cost of $400/tonneC reducing emissions to 305 mmtC/yr.

Removing natural gas generation entirely from the utility sector (achieving 248 mmtC/yr), will increase solar generation to levels above peak daytime demands, leading to routine storage of excess solar electricity as compressed hydrogen to power late afternoon fuel cell generation. Fuel cell generation will peak at 20% of total electric demand. A totally carbonless utility system will require 7 trillion kWh of carbonless generation to reliably deliver 5 trillion kWh of end-use electricity, eliminating utility emissions at a marginal cost of $400/tonneC (Figure 6.2, 0 GW scenario). Further carbon reductions must then come from the transportation sector, essentially fueled by additional solar power.

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Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

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.

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