Did you know ? From the field to the wheel: Photovoltaic is 40 times more efficient than the best biofuel (source: shutterstock/alphaspirit) By Michael Carus Managing Director nova-Institute Hürth, Germany Solar radiation in Germany in gigajoules per hectare per year 36,000 (+/- 10 to 12 % depending on region) Photosynthesis About 2% of 20,000 GJ per hectare and cultivation period: 400 GJ per hectare per year Mechanical and chemical processes > Biofuels 50 to 135 GJ per hectare per year (bioethanol, biodiesel, BTL) Degree of efficiency of distribution and combustion engine (fuel > wheel) About 35% 18 – 47 GJ per hectare per year (bioethanol, biodiesel, BTL) Photovoltaic cell > national grid Total degree of efficiency about 10%: 3,600 GJ per, hectare per year Inverter (DC > AC) Efficiency 90% Network losses: 6% Remainder for the car battery: 3,050 GJ per hectare per year From battery to vehicle wheel Total efficiency about 60% 1,800 GJ per hectare per year (solar electric car) The yield per hectare per year varies between a factor of 40 (BTL) and 100 (biodiesel) What will be the future of mobility? Which solution is both land-efficient and sustainable? On the one hand we have all different kinds of biofuels, like biodiesel, bioethanol and BTL (biomass to liquid), and on the other hand there is e-mobility sourced by renewable energy sources. Today we would like to compare the land efficiency, or the average energy yield per hectare for different biofuels, with that of a solar driven electric car - from the agricultural field to the car wheel. As a region we have chosen Germany just as an example. For most other regions the relationship of the results will not be so different - if there is more sun, the yield of the crops (as long they have enough water) and of the solar panels will increase almost in the same order. In regions with very long growing periods, or even two growing seasons per year, and sufficient water supply, the yield will be relative higher. In Germany the average solar radiation per hectare per year is about 10,000,000 kWh or 36,000 Gigajoules (GJ). This energy is used by the leaves of the crops as well as by the photovoltaic cell to transform and store energy. 1) Biofuels The leaves of crops use the solar radiation by photosynthesis. The theoretical maximum conversion efficiency of solar energy to biomass is 4.6% for C3 crops and 6% for C4 crops (maize, sugar cane, miscanthus), the best yearround efficiencies realized are no more than 3% (Langeveld 2010). So a realistic value of the photosynthesis in plant cells is about 2%, this is not very efficient. Because crops normally are only 100 – 150 days in the fields (spring and summer) the full yearly solar radiation cannot be taken into 58 bioplastics MAGAZINE [01/12] Vol. 7
Did you know ? account – we have to reduce the 36,000 GJ to around 20,000 GJ per hectare and growing period. That means that 400 GJ per hectare per year (2% of 20,000 GJ) are transferred to bioenergy in biomolecules. Further mechanical and chemical processing to biofuels will reduce the efficiencies and the yields significantly. In the range covering biodiesel from rapeseed/canola, bioethanol from wheat, and sugar beet to BTL (biomass to liquid) the energy yields are between 50 and 135 GJ per hectare per year. That means that between 0.3 and 0.7% of the solar energy is converted to biofuel. Finally the internal combustion engine has an efficiency of about 35% (biofuel to wheel). 65% of the energy is lost as heat. This brings us a final yield of between 18 and 47 GJ per hectare per year or a total efficiency of between 0.1% and 0.2% related to the solar radiation of 20,000 GJ per hectare over the growing period. This does not look like the solution for the future! (biodiesel) more efficient compared to the system of energy crops plus a biofuel driven car! That is one reason why the nova-Institute thinks that biofuels are an intermediate technology that should be substituted by solar (and wind) energy in the next 20 – 30 years. To switch from biomass to solar will set free huge amounts of land for other applications, such as bioplastics: we should rather use biomass for bio-based chemistry and materials which cannot be produced by sun and wind. Sources: Langeveld, J.W.A. 2010: Biomass availability. In: Langeveld et al. (editors): The Biobased Economy. Earthscan, London 2010. Remark: Where is the energy lost in the crop? Light-use efficiency of the average leaf of a crop is similar to that of the best photovoltaic (PV) solar cells transducing solar energy to charge separation (approx. 37%). In photosynthesis most of the energy is lost, being dissipated as heat during synthesis of biomass. (Langeveld 2010) 2) Solar electricity Photovoltaic panels have a realistic efficiency of 10% as a yearly average today, and they work during the full year. The latest commercial systems have already efficiencies up to 15% and it is expected this will increase to 20 – 40% in the future. Today from the 36,000 GJ average solar radiation solar panels can earn 3,600 GJ of electricity (DC) and an inverter transforms this to AC electricity, suitable to feed into the national grid. Modern electrical inverters have efficiencies of ca. 90%. There are also losses in the grid, typically in Germany about 6%. Thus, of the original solar radiation about 3,050 GJ reached the battery of the car. The system battery (ca. 65%) and electric motor (ca. 95%) have a total efficiency of ca. 60%. That means that finally 1,800 GJ are transmitted to the car wheel – or as a percentage of the solar radiation: 5%. This is much better than with biofuels. Conclusion: Crops and solar panels are using the same source of energy to transform, via biofuels or electricity, into mobility, i.e. solar radiation. The photovoltaic panel and electric car system is 40 times (BTL) to 100 times iBIB 2012 International Business Directory for Innovative Bio-based Plastics and Composites Pictures: nova-Institut, Sainsbury’s, Proganic For the 2 nd time worldwide: An entire overview of all suppliers of bio-based plastics and composites! In spring 2012 iBIB 2012 the second international directory of major suppliers of biobased plastics and composites will be published. Becoming an iBIB 2012 participant will enable you to reach about 50,000 potential industrial clients from all over the world. The print version will be distributed by the publishers and partners at trade fairs, exhibitions and conferences worldwide The PDF-version will be distributed widely by email and websides Online-database with detailed index to reach your supplier in a target oriented way iBIB 2012 : 250 pages – 100 companies, associations, R&D – 20 countries Book your page(s) now at: www.bio-based.eu/iBIB Deadline: 17 th February 2012 In cooperation with www.bio-based.eu/iBIB Book now: www.bio-based.eu/iBIB Due to strong demand the new deadline for registration is: February 17 th Publisher nova-Institute GmbH | Chemiepark Knapsack | Industriestrasse 300 | D-50354 Hürth Dominik Vogt | Phone: +49 (0)2233 4814 – 49 | dominik.vogt@nova-institut.de bioplastics MAGAZINE [01/12] Vol. 7 59
