Climate Legislation Necessitates High Area-Efficiency Solar

Energy policy and environmental policy are inextricably linked. Over recent years a growing scientific and political consensus has been forming worldwide that the environmental threat posed by global warming is so great that we need to act urgently to curtail green house gas (GHG) emissions. The United Nations, the United States, several other countries, and the State of California have all endeavored to introduce policies to curb GHG emissions. Over the coming years we expect policies to be formulated and legislation enacted to limit GHG emissions.

The principal source of GHGs today is the burning of fossil fuels, notably coal and natural gas, used for electricity generation, and oil, diesel, and gasoline used for transportation. Thus, to make any serious reduction in GHGs emissions legislation will have to discourage use of fossil fuels for energy generation, and instead encourage renewable technologies.

To illustrate what kind of legislation to expect, in 2006 under Governor Arnold Schwarzenegger, the State of California took action to lower its greenhouse gas emissions, in order to mitigate the risk of climate change. The Global Warming Solutions Act of 2006 (Assembly Bill 32) requires that by 2020 the GHG emissions in California will be down to their 1990 levels. This represents an 11% reduction in current levels and a 30% reduction from the “business as usual” scenario if historical emissions trends had continued unabated to 2020 [REF]. In the same year, Senate Bill 1368 (SB 1368) imposed a requirement on all retail providers of electricity that they must limit their GHG emissions to 1,100 lbs (0.5 metric ton) of CO2 per MWh of electricity generated. Higher emissions are only allowed if the excess CO2 can be permanently captured and stored. SB 1368 created a de facto moratorium on new coal power plants in California because coal-based power generation cannot meet the emissions target without carbon capture and sequestration—technologies that are not yet mature and, even if feasible, are sure to lower the efficiency of coal power plants and increase electricity costs. As ever more aggressive global climate change treaties are enacted even stricter measures to reduce GHG emissions are likely thereby extending the legislative impact to natural gas power plants too.

GHG emissions reduction legislation will have a profound impact on the renewable energy industry in two ways. It will increase demand for renewable power generation, and this in turn will obligate us to devote more land to renewable power generation. In Fig. 1 we illustrate how a given level of CO2 reductions will translate into a required increase in land area. The blue curve in Fig. 1 shows the per capita CO2 emissions of a typical American as a function of time assuming that legislation will be enacted to reduce CO2 emissions by 5% per year. This is the rate of decrease believed to provide a 74%-91% probability of keeping global warming to less than 2 degrees Celsius. Taking account of the differing amounts of CO2 generated by bituminous versus sub-bituminous coal, the two most common types of coal currently used in the United States, we calculate that one metric ton of coal releases approximately 1.975 metric tons of CO2 assuming no carbon capture or sequestration is used. Hence, by assuming 5% reduction on CO2 emissions per year, we can estimate how much coal must not be burned, and therefore how much energy must be generated from alternative (renewable) sources. The result is shown as the red curve in Fig. 1. As the average amount of coal-fired power retired goes up, emissions go down (as is desired). The vertical rulers on the right hand side of Fig. 1 then translate the amount of coal-fired power retired into equivalent sizes of squares of land needed, and the effective percentage of the continental United States land area given up, if the coal-fired power retired were to be replaced with alternative (renewable) fuel sources.

What is not generally appreciated is the staggering amount of area required to replace the coal-fired power from renewable energy sources. Using a traditional option, e.g., SiPV solar on typical trackers, some 2.7% of the land area of the continental U.S. would be needed. By contrast using XE-CPV technology only about 0.27% of the continental U.S. land area would be needed. Note the factor of 10% is not arbitrary, see the TRIANGLE symbols on the Power and Scale Comparisons figure here for current examples of roughly 10% land-efficient systems. Thus, by using XE-CPV we can foresee a path to achieving the desired reductions in CO2 emissions using the least amount of U.S. land given over to renewable power generation. At the scales in question this represents an enormous saving in land area.

The analysis here is not meant to be exact, or even predictive of what will come to pass as a GHG reduction plan. It is, however, representative of the order of magnitude of the land resource problem that will arise as we close coal and natural gas power plants and switch to renewable power generation. Policies need to be constructed to force utility-scale renewable power systems to be maximally area-efficient to give us the best shot at realizing the reductions in GHG emissions required to mitigate global warming and making a viable transition to a renewable energy generation infrastructure.

 

Figure 1. The blue curve shows per capita CO2 emissions for a US citizen assuming a 5% reduction per year. The red curve shows the corresponding amount of coal-fired power that would need to be retired to meet these reductions. As the amount of coal-fired power retired increases, the CO2 emissions decrease (as is desired). The rulers to the right of the figure translate the retired coal-fired power into an equivalent land area, or percentage of the continental U.S., if the retired power were to be replaced using current approaches to solar energy. Assuming SiPV on trackers with a fill fraction of 10% roughly 2.7% of the land area of the continental U.S. would need to be given over to renewable generation to bring emissions to zero. The rightmost ruler shows that the retired power can be achieved using far less land area (0.27% of the continental U.S.) if XE-CPV is used instead of traditional solar power. Note, the 10% packing efficiency we assumed is not an arbitrary choice. It reflects what we actually see in the largest solar installations currently under development—see TRIANGLE symbols, which correspond to about 10% land efficiency, on the Power and Scale Comparison figure here.