Technology for production of biofuels has been a popular research area for many biochemists in the past ten years because finding alternatives to the use of fossil fuels can have important effects on the future of human life. Although the technology is not able to produce low cost, efficient biofuels yet, Borak et al. (2013) promote the possibilities and efficiency of biofuel production through crops. Their “PETRO approach” is used to evaluate the new crops, not only on the capture of solar energy but also the capture of carbon in atmosphere. The reverse of the carbon combustion cycle naturally occurs within plants, which use photosynthesis to convert carbon dioxide from the air to usable fuels. The energy source products from various crops are similar, but the efficiency of conversion of sunlight into energy varies. The authors summed up the total loss of energy during each process and organized data on the final energy content for each crop. They showed that using the PETRO approach for evaluating potential crops as biofuels can lead to more detail-based discussion in the scientific community. —Chieh-Hsin Chen
Borak, B., Ort, DR., Barbaum, JJ., 2013. Energy and carbon accounting to compare bioenergy crops. Current opinion in biotechnology 24, 369-375.
Because of the depletion of fossil fuels and the atmospheric increase of atmospheric carbon dioxide from combusting the fossil fuels, scientists are eager to find alternative fuels. The two main barriers to production of alternative fuels are the costs and shortage of potential stocks; thus the production of liquid fuels from crops has become one of the top environmental goals for future research. On a fundamental level, the concept of biofuel is replacing the process of mining with the process of agriculture; the process shift is significant because biomass has significantly higher carbon oxidation state than fossil fuels. The approach of biofuel essentially reversed the combustion of carbon-based fuels capturing the byproduct CO2 converting and storing as usable energy via carbon fixation utilizing plants and other terrestrial plant matter.
To further enhance the ability of the biofuel production, a detailed evaluation of efficiency of the crops and productivity of conversion of energy will be useful; however a systematic methodology for evaluation is currently lacking in the field. Different research groups studying in different geographical areas use a variety of non-comparative assumptions and approaches to calculation yields. The data consistency became one of the difficulties for further biofuel research. The authors introduced the Plants Engineered To Replace Oil (PETRO) approach that included the input of raw materials (sunlight, carbon dioxide and water), trace process of conversion by plants, and the output of liquid fuels. Photosynthesis reverses the combustion of fuels and stores the carbon energy along with solar energy in the plant; the production of biofuel extracts and concentrates carbon energy from plants, usually as ethanol, converting it into usable fuel.
Although plants have evolved effective photosynthetic pathway to capture light, they are not as efficient as we would like; the result of evolution does not aim for maximizing the benefits of wither for producing food or fuels. That C3 plants only utilize 4.6% of the solar energy and C4 plants utilize 6.0% suggests substantial room for improvement but even with these low capture levels of photons much of the captured energy is subsequently lost.
The authors looked into the difference in loss of carbon energy between C3and C4 plants in four levels: captured, harvested, purified, and processed. C3 plants lose half of their usable carbon energy in photorespiration and more in respiration. C4 plants lose less in respiration, but C4 plants lose more than half seasonally. Overall the final usable carbon energy in C3 plants is about 0.69% and 3.0% in C4 plants. But there are also some losses of carbon energy in the processing steps.
Although energy loss in the processing steps is small compared to the loss from photorespiration or seasonality, it is the first step that can be improved through conventional engineering. With careful genetic selection and engineering of crops, we will be ale to control the seasonality and other growth process of crops. The authors introduce data that show the differences in final fuel product of four common crops used in biofuels: maize, soybean, sugarcane, and switchgrass. The final energy content of the four different crops is very similar, but they have significant differences in overall fuel yield. Sugarcane contains the most overall fuel yield and soybean the least.