Solar electricity has to compete with biofuels in urban traffic. However, biofuels have very low energy productivity per hectare and high requirements for fertilizers, pesticides and water.
The share of traffic in the world’s secondary energy consumption is about 27% (21% of primary energy), it grows by an average of 1% per year, and its fuel is almost exclusively oil. Could biofuels play an important role in combating both transport greenhouse gas emissions and oil dependence?
Currently marketed first-generation biofuels represent only about 3 percent of the world’s road transport fuels, and therefore cannot be considered a transition technology. This limited penetration of first-generation biofuels is partly due to potential competition with food crops and market prices (sugar and ethanol prices on international markets).
Concerns that these technologies will not meet expectations have led to the development of second generation (cellulosic ethanol, biomass to liquid, pyrolysis oil, dimethyl ether) and third generation biofuel technologies (algae biodiesel, third generation biofuels). processes) that still need to be expanded to become cost competitive.
With the exception of sugarcane ethanol, traditional biofuels have serious disadvantages related to raw materials. The current costs of rapeseed biodiesel and ethanol from grain or beets are much higher than the costs of gasoline and diesel, and substantial subsidies are needed to make them competitive. These high costs are due to the low net energy yield of most centuries (100–200 GJ/ha per year in the long term), the high quality agricultural land required, and intensive management requirements. For comparison, the typical radiation output is 50,000 GJ/ha per year and the typical electrical output of solar panels is 10,000 GJ/ha per year. Biofuels’ very low energy productivity per hectare, along with their high fertilizer, pesticide and water requirements, severely limit pit-to-wheel reductions in fossil energy use, which in turn limit environmental benefits. Perennials (220-550 GJ/ha per year), grasses (220-260 GJ/ha per year) and sugar cane (500-650 GJ/ha per year) have somewhat higher net energy (but still pale in comparison with solar).
Brazil has been the world’s leading promoter of biofuel sales for over 30 years Proalcohol program. Mandates for large-scale blending of biofuels with vehicle fuels have appeared in several other countries in recent years based on national greenhouse gas (GHG) reduction targets, but also apparently due to the desire of agricultural companies to receive production subsidies.
In the long term, however, it can be argued that biofuels will play a transitional role in electric vehicles, reducing carbon dioxide emissions and producing solar electricity. In terms of energy conversion efficiency, biofuels used for transportation can be largely ignored as a transition technology. The following figure shows a direct comparison with sugarcane ethanol combined with flex-fuel cars and solar-to-electricity (PV) conversion + electric cars.
A 1 hectare (100 mx 100 m) sugarcane plantation produces 7,000-8,000 liters of ethanol per year, which enables a medium-sized flex-fuel car to drive about 54,000 km/year in the best case. If the same area is covered with a dense set of low-cost commercially available solar modules with a conversion efficiency of 20% and a moderate 1,500 kWh/kWp per year, the electricity produced in the same 1 ha in one year enables the electric version of the same medium-sized car to drive more than 18 million km/year (about 350 times more!). The reason for this huge difference is that solar energy conversion is 50-100 times more efficient than typical photosynthesis efficiency, and also that electric motors are about three times more efficient than combustion engines.
Global sales of battery electric cars (excluding hybrids) are about 10% of light vehicle sales and grow by about 50% per year. This suggests the likelihood that half of vehicle sales will be battery-electric by 2026, and that the majority of vehicles on the road will be battery-electric by 2040, when the current fossil fuel fleet becomes obsolete and retired. Vehicles running on biofuels and hydrogen have a negligible market share.
In the low, mid-latitudes of the sun zone, where 80% of the world’s population lives, rooftop electricity can cover most of the electricity needs of homes each year. Rooftop PV can also cover the additional need for electricity represented by the new fleet of electric vehicles. You can drive an electric car about 8,000 km per year with a kilowatt-power roof electric system.
Biofuels suffer from irreparably poor conversion of solar energy into useful energy and compete with food production for land, water, pesticides and fertilizers. Basic physics and economics destroy the notion that bioenergy can play a significant role in global energy systems.
Authors: Prof. Andrew Blakers (ANU) and Prof. Ricardo Rüther (UFSC)
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ISES, the International Solar Energy Society, is a UN-accredited membership organization founded in 1954. It works towards a world with 100% renewable energy for all, used efficiently and wisely.