Ecological Impacts Dependent on Size of Facility Footprint
The land use efficiencies for proposed projects are directly related to the size of the facility footprint. The total amount of land disturbed has implications for dust emissions related to aeolian wind processes, carbon sequestration by biological soil crust, and regional albedo.
Aeolian Wind Processes: Dust Emission
Solar development has the potential to release large quantities of dust, especially if facilities are constructed in locations where dust is currently sequestered, or if multiple facilities are constructed in relative proximity to one another. Dust release could have indirect impacts to both the facility site as well as off-site areas downwind. Dust plays a role in nutrient cycling; since much of the plant-essential nutrients are stored in the top few millimeters of soil, excess dust release may significantly deplete nutrients on site.1 Additionally, dust storms may result in a “sandblast” effect downwind of a solar development-induced dust release, causing increased wind erosion and disturbance in adjacent areas, and triggering larger dust emissions in the dust storm’s path.2 Large dust depositions could also bury landscapes in a layer of dust, halting photosynthetic activity and reducing fertility if plants and soil crusts are covered.3
Aeolian Wind Processes: Human Health
Any process or activity, whether it is natural wind movement or vehicles driving on unpaved roads, can resuspend dust particles and contribute to PM10 pollution.4 Therefore the construction of solar facilities is likely to contribute to PM10 pollution, but the potential relative contribution of facilities to the overall PM10 concentrations in the California desert region remains unknown. How dust emissions are managed on site, such as the application of water to suppress construction dust, will also influence the contribution of a facility to overall PM10 pollution concentrations. Addressing potential increases in PM10 pollution is crucial to the health and well-being of residents in desert communities.
Biological Soil Crusts: Carbon Sequestration
Solar development could affect the carbon sequestration potential of the California desert; the extent of this impact may be dependent on total size of disturbed area (in other words, the facility footprint), in addition to the carbon sequestration potential at the specific facility location. In the 2004 article “Carbon Sequestration in Dryland Ecosystems,” author Lal states that the worlds arid land ecosystems have a large potential to sequester carbon, and that degradation of these lands often results in emission of carbon dioxide, CO2, into the atmosphere.5 In a two-year study of the Mojave Desert published in 2008, Wohlfahrt et al. found the ecosystem to be a significant net sink for CO2. The authors attribute a significant portion of the desert soil’s carbon sequestration capabilities to the expansion and growth of biological soil crust organisms.6 However, assertions about desert soils as carbon sinks have been met with skepticism by some scientists. In a 2008 article by Stone in Science, Jayne Belnap, an ecologist for the USGS and world renowned authority on soil crusts, says: “There is no way that all the CO2 absorption observed in these studies is due to biological crusts, as there are not enough of them active long enough to account for such a large sink.”7
Despite the controversy, the potential for desert soils to act as long-term carbon sinks has important implications for solar development in the California desert. The grading of land necessary for facility construction could eliminate the ability of the soil to sequester carbon and might result in the release of large amounts of carbon into the atmosphere. Whether developing large swaths of desert for solar energy production or leaving the desert soil intact has greater potential for reducing carbon emissions warrants further study.
Biological Soil Crusts: Carbon Uptake and Avoided Carbon Emissions
Because of the potential for biological soil crusts to fix carbon across the California desert landscape, we examine potential carbon uptake by desert soils compared with the range of potential avoided carbon emissions that would result from the construction of solar facilities in the California desert. Our calculations indicate that potential carbon taken up by biological soil crusts is far less than the amount of carbon offset by the production of solar energy. This calculation does not take into account the amount of carbon emitted by the solar panel manufacturing process, which can be a significant portion of the life-cycle carbon emitted by this product, and is only intended to provide a rough comparison of whether desert soils or solar energy production are more effective at reducing overall carbon emission. Our results must be understood within the context of our assumptions and parameters, which are as follows:
- Rates of carbon uptake determined by the 2008 Wohlfahrt et al. paper are average for the California desert and that rates can be applied to the entire California desert.
- If these utility-scale solar facilities were not built, the carbon that would have been produced by conventional energy sources can be estimated by using CO2 emitted per MWh of electricity produced, using a statewide average coefficient of 0.138 metric tons CO2 per MWh for California’s grid.8
- Average nameplate capacity of proposed solar facilities in California = 427 MW
- Average number of acres for proposed solar facilities in California = 3,797 acres
- Highest average operating efficiency for proposed solar technologies = 39.9 percent (dish/engines)
- Lowest average operating efficiency for proposed solar technologies = 11 percent (Thin Film PV)
Wohlfahrt et al. found that their study sites in the Mojave Desert took up 102 and 110 g C m−2 during 2005 and 2006, respectively. We used the average of the two years to calculate the annual rate of carbon uptake per acre of undisturbed desert at 428,967 g C/acre/yr.a Operating efficiencies for Thin Film PV at 11 percent efficiency and dish/engine at 39.9 percent efficiency were used to calculate a lower and upper range of avoided emissions: 4,081,211 to 14,803,667 g C/acre/yr.b, c A comparison of carbon uptake to avoided emissions is shown in Figure 1.
The estimated range of avoided carbon emissions from use of solar energy is 4 to 14.8 million grams carbon per acre per year, depending on the operating efficiency of the solar technology, while the amount of annual carbon uptake is much smaller at approximately 429,000 grams carbon per acre per year. Under these assumptions and parameters, solar facilities are much more effective at reducing overall atmospheric carbon than desert soils.
a. Calculating the amount of carbon uptake by desert soils: (106 g C / m2) * (1 m2 / 0.000247105 acre) = 428,967.443 g C/acre/yr
b. Calculation for Thin Film PV: (427 MW) * (0.138 t CO2/MWh) * (0.11) * (8,766 hr/yr) = 56,819.98 mt CO2/yr
(56,819.98 mt CO2/yr) * (106 g / mt) * (12g C / 44g CO2) = 1.55 * 1010 g C/yr
(1.55 * 1010 g C/yr) ÷ (3,797 acres) = 4,081,211 g C/acre/yr
c. Calculation of Dish/Engine: (427 MW) * (0.138 t CO2/MWh) * (0.399) * (8,766 hr/yr) = 206,101.58 mt CO2/yr
(206,101.58 mt CO2/yr) * (106 g / mt) * (12g C / 44g CO2) = 5.62 * 1010 g C/yr
(5.62 * 1010 g C/yr) ÷ (3,797 acres) = 14,803,666.6 g C/acre/yr
Biological Soil Crusts: Albedo
Another unanticipated consequence of solar development might be its effect on regional climate through alteration of soil albedo. Albedo is a measure of how reflective a surface is to the sun’s radiation.9 A surface that is more reflective to this radiation (e.g. a light-colored surface) would have a higher albedo than a surface that absorbs more of the sun’s radiation (e.g. a dark-colored surface). In a 2003 paper by Belnap and Eldridge, the authors state that the trampling of dark-colored biological soil crusts exposes lighter soils and can increase albedo; they also caution that large-scale changes in surface color may lead to changes in regional climate patterns.10 There is concern that eliminating the soil crusts, which are dark in color, over several thousand acres of desert and replacing those dark surfaces with reflective panels and mirrors, could also affect regional climate. The effect that modifying several thousand acres of surface albedo will have on the regional climate of the California desert warrants further study.