It is often assumed that the high land use intensities of solar and wind make electrification the transportation fuel of choice versus next generation biofuels in a net zero world. Not so fast.
High level let us calculate the land requirement per Watt (W) of mobility, that is actual energy used to move transportation vehicles around, with noted assumptions along the way. At the very least this pathway opens the door for a discussion on the subject.
The National Renewable Energy Laboratory (NREL) presents the land requirement for modern wind power plants based on name plate capacity to be around 34.5 hectares (ha) per Megawatt (MW) of nameplate capacity (Denholm et al, 2009). Solar had a higher energy intensity at 5.5acres/MW (Ong et al, 2013). Converting units that is approximately 45W/m2 for solar and 2.9W/m2 for wind.
To generate about a million MW of power to meet US demand that would be approx. 24000km2 of solar or 385000km2 of wind. Nebraska is 200000km2 so you only need to convert a tenth of the State to solar panels and you can electrify the entire US grid right? Unfortunately, no as this does not account for converting the US transportation fleet to electric cars. But more importantly the numbers presented are if every solar panel and wind turbine is always generating the installed capacity which is not the case.
Capacity Utilization is the actual power produced divided by the nameplate capacity, usually expressed in percent. Between 2015-2021 the average capacity factor in Germany for solar is 10.5% and 22.2% for wind as calculated by Emil Fridman, Project Lead at Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in 2021. Multiply the solar generation per m2 number by 0.105 and the wind power generation per m2 by 0.222. You now obtain 4.7W/m2 for solar and 0.64W/m2 for wind. To make a million MW you now need 237760m2 for solar and around 1746000m2 for wind. All of Nebraska has to be covered in solar panels.
Lest you think these requirements are ultra conservative let us examine existing wind and solar plants built in optimal locations.
The Canadian Wind Energy Association (CanWEA) reported that wind farm capacity had reached 13,413 MW of nameplate capacity by the end of 2019. Statistics Canada reports in CANSIM Table 25-10-0015-01that wind farms produced a total output of 32.8 Terawatthours (TWh) during 2019—or 5.1% of total electricity output.
Bhadla Solar Park in India is currently the world’s largest solar power plant, with 2,245 Megawatt of peak capacity. It covers 4,500 hectares of land. Assuming average output of 12% of peak capacity over the course of the year, this comes to a power density of roughly 6 W/m2 (Antweiler, 2020).
Note as more and more wind and solar plants are built that they will move further and further away from optimal locations offsetting technological improvements. And these calculations do not include the energy cost of producing the solar panels which must be replaced at regular intervals. Replacing a Nebraska worth of solar panels every twenty to forty years is no doubt energy intensive.
But what about next generation energy crops. Next generation energy crops are cellulosic versus current crops such as canola and soybeans in which vegetable oils and/or sugars must be grown. Generally, these grass, protein and woody based crops grow faster than foodstuffs. Energy crops such as switchgrass and poplar trees can be grown in most locations at 1-4 tonnes per acre per year on a dry basis. Some such as Giant King Grass can be grown in optimal locations at up to 44tonnes/acre-year. A transportation conversion yield of 20% to gasoline and diesel can be achieved with modern processes such as Hydrothermal liquefaction.
For Giant King Grass that is 44tonnes/acre-year x 0.20 fuel conversion = 9.68 tonnes/acre-year of fuel. Dividing by 365 that is 0.026 tonnes of fuel per acre-day. Gasoline and diesel have an energy density of around 45MJ/kg such that 1 tonne per day (1000kg/day) is equivalent to 1875MJ/hr to 520833W. Therefore 0.026tonnes of fuel per acre-day multiplied by 520833W results in 13812W/acre which divided by 4046W/m2 results in 3.4W/m2. The land intensity of giant King Grass is in the same order of magnitude as solar panels at 4.7W/m2 and superior to wind at 0.6W/m2. This is without any steel required. For switchgrass and poplar trees this drops to 0.4W/m3 to 0.7W/m2. Note that this is still in line with wind but less than solar. However, keep in mind no rare Earth minerals are required for energy crops in comparison to solar panels.
However, we have not accounted for energy efficiencies to get true “mobility Watts” given losses to energy transmission and the engines themselves. For EVs 5% transmission energy losses, 5% inversion AC/DC, 5% battery charge efficiency, 5% inversion Dc/AC and 10% engine efficiency for losses of around 30%. Internal combustion engine losses are around 65%. Accounting for these losses the net mobility land intensity for solar is around 3W/m2, wind is 0.4W/m2 and biofuels are between 0.3W/m2 and 2.2W/m2.
But what about the energy to build and construct solar panels and wind turbines? This is difficult to incorporate into the land intensity however if we assume the total energy requirement to produce a PV panel is 1,060 kWh/m2 (Bakers et al, 2000) we can estimate. The power generated by solar panels is approximately 4.7W/m2 that to generate 1060kWh will take twenty-five years (25). Assuming a life time of thirty-years this leaves 16% of the power released to the grid. Wind does better according to a publication by Kubiszewski, Cleveland and Endres (2010) estimating the plant will produce 95% excess power (assuming a 20-year life). This would leave solar at 0.48W/m2, wind at 0.4W/m2 and biofuels at 0.3-2.2W/m2 with next generation biofuels having the best mobility W per square meter.
Assuming an average energy crop and not accounting for the power input into wind turbines proposed land intensities in mobility Watts per square meter are presented in figure 1.
The results seem to indicate that next generation energy crops are competitive, if not better, than that solar and wind on a land use basis. The risks seem inherently higher for wind and solar given life cycles of 10-40 years versus the perpetual life of replanted energy crops. Next generation fuel plants can be self-recuperating generating their own power and recycling nutrients and fertilizers. They can also sell excess power to the grid or use that power to sequester carbon dioxide in saline aquifers, depleted oil and gas formations or basalt.
One must also acknowledge that not all land is created equally. Switchgrass can be grown on ranch land with co-use. Tumbleweed can be grown in the open desert. Macroalgae can be grown offshore. Some of the biomass for next generation fuels can also be provided by already generated agricultural, wood and municipal waste. Solar and wind installations generally require new materials and must be placed in optimal locations.
In summary these quick calculations indicate that the land intensities of next generation biofuels are comparable, or even better than, solar and wind power. Renewable gasoline and diesel also allow for conversion of the existing transportation fleet with all its affordability and convenience. It may be best to have solar and wind focus on decarbonizing the electrical grid while utilizing next generation renewable gasoline and diesel for transportation energy requirements.
What is it they say about assumptions?
Note: Comments for this article are active, feel free to discuss below.
References:
Antweiler, W. (2020). How much land is needed for wind and solar farms?. Werner’s Blog. Retrieved from: https://wernerantweiler.ca/blog.php?item=2020-04-30
Bakers A. Weber, K. (2000). The Energy Intensity of Photovoltaic Systems. Engineering Department, Australian National University. Retrieved from: http://www.ecotopia.com/apollo2/pvepbtoz.htm
Denholm, P, Hand, M., Jackson, M & Ong, S. (2009 August). Land-Use Requirements of Modern Wind Power Plants in the United States, National Renewable Energy Laboratory. Retrieved from: https://www.nrel.gov/docs/fy09osti/45834.pdf
Kubiszewski, I. Cleveland C. Endres, P. (2010 January). Meta-analysis of net energy return for wind power systems. Renewable Energy Volume 35, Issue 1. Retrieved from: https://www.sciencedirect.com/science/article/abs/pii/S096014810900055X
Paul Denholm, Maureen Hand, Maddalena Jackson, and Sean Ong: Land-Use Requirements of Modern Wind Power Plants in the United States, National Renewable Energy Laboratory, Technical Report NREL/TP-6A2-45834 August 2009. Retrieved from Land-Use Requirements of Modern Wind Power Plants in the United States (nrel.gov)
Ong, S. Campbell, C., Denholm P. Margolis, R & Heath G. (2013 June). Land-Use Requirements for Solar Power Plants in the United States. National Renewable Energy Laboratory. Retrieved from: https://www.nrel.gov/docs/fy13osti/56290.pdf
This is definitely a surprise going through the calculation.
When you use the straight name plate pre-capacity-factor land intensity you think solar is the clear winner but incorporate the capacity factor and the cost to build and maintain the panels and solar doesn’t look so rosy versus wind and next generation biofuels.
Note that the highest biofuel land intensities beating solar though require celloustic biomass to drop in fuels, something not necessarily out there today. I think the closest you come is ExxonMobil and Global Clean Energy Holdings with their camelina crops, but that is still oil based (though non foodstuff).
Will continue to watch this closely!