Land plants may have an advantage over aquatic plants as they are able to photosynthesise using leaves that can make use of the rapid diffusion of gases in air which is about 10 times faster than that in water [ 12 - 14 ]. Thus, cyanobacteria and algae in water may need to be well stirred to support rapid photosynthesis and growth [ 12 ]. Fortunately for oceanic photosynthesisers, surface waters are often vigorously mixed and where nutrients are available primary production can proceed rapidly.
Those oceanic primary producers represent only about 0. Large ocean area provides a significant potential for biomass production, though nutrients are often limiting and harvesting oceanic biomass is difficult and challenging. Because of relative ease of harvesting, and the longer life time of land plants, nearly all current bioenergy harvesting is from terrestrial plants. Thus, about half of the incident solar energy is unavailable to higher-plant photosynthesis, which is accounted for in the coefficient 0.
The amount of energy in a unit mass of plant material also varies, being about MJ kg -1 for typical biomass, but as much as MJ kg -1 for oilseeds [ 17 , 18 ]. The observed minimum quantum requirement of mol photons per mol CO 2 assimliated in C3 photosynthesis represents an absolute limit on biofuel production from sunlight, in spite of claims for biomass production usually by algal systems that would correspond to significantly smaller quantum requirements [ 20 ]. That range corresponds to C3 photosynthesis in the absence of photorespiration, which in the current atmosphere increases minimum quantum requirement to about 14 mol mol But due to light saturation, and other factors below , biomass production, especially over an annual cycle, cannot approach limits set solely by minimum quantum requirements [ 17 , 20 ].
The potential maximum efficiency of converting solar energy to biomass energy is estimated at about 4. C4 plants can be more efficient than C3 plants because they are able to suppress photorespiration through a combination of biochemical and anatomical innovations that arose relatively recently in plant evolution.
A country's vision for developing renewable and sustainable energy resources is typically propelled by three important drivers – security, cost, and. In , in association with the American Society of Plant Biologists, the 1st Pan- American Congress on Plants and BioEnergy was convened.
These innovations presumably were a response to declining global atmospheric [CO 2 ] during the past million years. Actual maximum conversion efficiency is generally lower than the calculated potential efficiency at around 3. The actual photosynthetic efficiency of mature C3 forest stands was also calculated to be between 2. Of course, plants are self-regenerating and self-maintaining whereas photovoltaic cells are not. Light saturation under bright conditions and associated photoinhibition in Photosystem II;. Plants are living organisms that spend about half of each day in the dark, when they need to use previously generated carbohydrate stores to keep themselves metabolically active and growing.
Minimum energy losses associated with biomass production. This analysis indicates that a theoretical maximal photosynthetic energy conversion efficiency is 4.
These results are still generally applicable. The current bioenergy enterprises are focussing on C4 crops such as sugarcane, maize, sweet sorghum, switchgrass and miscanthus presumably due to the higher energy conversion efficiency. Due to the apparently low actual energy conversion efficiency in whole-plant photosynthesis i.
Engineering C3 crops to use C4 photosynthesis. There is an ongoing ambitious research program, led by the International Rice Research Institute IRRI , to convert the normally C3 rice to a C4 system by transforming rice to express Kranz anatomy and the C4 metabolic enzymes [ 28 ]. This would improve the efficiency of rubisco as a catalyst of CO 2 assimilation [ 18 , 24 ]. Another complication for engineering an improved rubisco is that it is composed of eight large chloroplast-encoded subunits and eight small nuclear-encoded subunits, and assembling modified subunits in chloroplasts remains a challenge [ 30 ].
Minimising, or truncating, the chlorophyll antenna size of chloroplast photosystems. This would potentially improve solar conversion efficiency by up to 3-fold in high light, which normally saturates photosynthesis [ 31 ]. Improving the recovery rate from the photoprotected state. This would potentially increase carbon uptake by crop canopies in the field [ 32 ]. The xanthophyll photoprotection system protects plants from damage from absorption of excess light the reduction of photosynthesis by dissipation of photons by NPQ.
High-yielding rice are reported to recover more quickly from photoinhibition than traditional varieties [ 33 ]. C3 crops include wheat, rice, cotton, barley, soybean, bean, chickpea, algae, palm and peanut. C3 photosynthesis of CO 2 forming fructose 6-P can be summarised by:. Because 8 mol photons PAR contain on average 1. That efficiency occurs only in low light, however; under a bright sun, C3 photosynthesis becomes light-saturated.
In addition, the process of photorespiration, which is relatively rapid in C3 plants, especially at higher temperature, is a significant constraint on CO 2 assimilation in C3 plants. As much as one third of the C assimilated in C3 photosynthesis can be almost immediately lost to photorespiration with present atmospheric CO 2 concentration higher CO 2 concentration not only stimulates photosynthesis, but inhibits photorespiration.
Moreover, that efficiency does not account for plant respiration, and some respiration is essential for growth and maintenance processes. C4 plants include maize, sugarcane, sorghum, millet, miscanthus and switchgrass. The C4 system involves the specialised metabolism and Kranz leaf anatomy to concentrate CO 2 in the bundle sheath cells. Normal C3 photosynthesis takes place in the bundle sheath cells in C4 plants, but because the CO 2 concentration there is quite high, photorespiration is greatly suppressed. The C4 cycle, which concentrates the CO 2 in bundle sheath cells, requires two ATP to assimilate a CO 2 in the mesophyll, release it in the bundle sheath and regenerate the CO 2 acceptor in the mesophyll.
Monitoring soil quality to assess the sustainability of harvesting corn stover. Economic growth and development worldwide depend increasingly on secure supplies of reliable, affordable, clean energy, and climate stability and environmental security require carbon neutral alternatives. Science Insider. Shutterstock Wathanachai Janwithayayot. Soil quality impacts of residue removal for bioethanol production. But field studies in the Midwest suggest that planting native switchgrass, with a few other plant species thrown in for good measure, could actually help restore the grassland ecosystems that once covered the middle of the continent.
Some CO 2 leakage from the bundle sheath is inevitable, and this requires that the C4 cycle operates more quickly than the C3 cycle in C4 plants. Hence, C4 photosynthesis may require at least 2. In spite of the extra energy cost of the C4 cycle, C4 photosynthesis responds better to bright sunlight and to higher temperatures than C3 photosynthesis because of suppressed photorespiration.
At cooler temperatures e. In addition, many C4 plants are sensitive to low temperature. That efficiency is relatively insensitive to temperature, at least over the normal range experienced by typical C4 crops during daylight hours. Commonly cultivated CAM crassulacean acid metabolism plants include agave Agave spp.
CAM plants are well adapted to arid and semi-arid habitats. They open their stomata at night and take up CO 2 in the dark to form malic acid, which is then metabolised to release CO 2 for photosynthesis during the following day, but with their stomata closed [ 35 , 36 ]. By closing the stomata during the day, less water is lost, resulting in high water use efficiencies with a trade-off of lower growth rates. CAM plants have been suggested to have potential for food, fibre and biofuel production in dry marginal lands [ 38 , 39 ].
The energy contained in food consumed per person is only about 10 MJ day -1 equivalent to 2 kcal per day, 10 Btu or W [ 40 ]. Globally, one third of food, around 1. Global population is projected to increase to billion within years [ 43 ]. In developing countries, food consumption per person is rising with increased consumption of animal protein with the livestock revolution [ 44 ]. Average annual meat consumption is projected to rise from 32 kg person -1 in to 52 kg person -1 by [ 45 ].
In ancient civilisations, most of the energy used for farming was provided by animals and the nutrients were derived from animal manure. During and after the Green Revolution, dependence on non-renewable fossil fuels resulted in a conversion of fossil energy into food energy, but in an inefficient way.
The dependency of agriculture on fossil fuels has resulted in commodity food prices being closely linked with global energy prices [ 48 ]. Hence, food prices tend to fluctuate and trend upwards in parallel with energy prices. It is instructive to compare maize production in Mexico using human labour with a hoe and sickle returning The U. Energy use in grain and legume production [ 49 ]. In developing countries, populations tend to have a cereal-based diet and are effectively at a lower trophic level in the food chain, while populations in developed countries tend to consume more meat and operate at a higher trophic level.
Production of livestock, on average, may require 4 kg of wheat for the production of 1 kg of meat [ 40 , 50 ].
Therefore, in developed countries where kg of cereal and kg of meat are consumed per year, the total need for food and feed is kg of cereal per person per year [ 40 ]. Overfishing of the ocean predators e. To increase the energy efficiency of our primary food production system, we should focus on primary production in agriculture e. In addition to providing food and feed, plants are an important source of fuel. Indeed, biofuels are not a new concept.
In B. More recently, the German inventor Rudolph Diesel demonstrated his engine that ran on peanut oil at the World Fair in Paris. That NPP, however, includes vast amounts of biomass that cannot be physically or economically harvested including national parks. Replacing fossil fuels with renewable energy sources derived from sunlight, such as solar, hydro or biomass is very challenging as these energy sources have a lower energy density than fossil fuels and are generally more expensive [ 54 ].
Since the beginning of civilisation, humans have depended on biomass for cooking and heating, and many developing countries in Asia and Africa are still dependent on traditional sources of biomass. The automative industry currently uses relatively energy inefficient internal combustion engines to burn liquid fuels e. Electric car motors have a 7. Hydrogen fuel cells may replace electric motors in the future but this is still in the developmental phase.
In the meantime, liquid biofuels are the transition renewable alternative to fossil fuels. Globally, liquid biofuels can generally be classified into three production sources; maize ethanol from the United States, sugarcane ethanol from Brazil and rapeseed biodiesel from the European Union. For the rest of this chapter, we use the term biofuels to refer to liquid biofuels. First generation biofuels refer to the traditional or conventional supply chains based on food crops, whereas second generation biofuels require more complex and expensive processes and are generally operating in pilot plants and not yet widely available on the market.
The first generation of biofuels is produced from starches, sugars and oils of agricultural food crops, including maize, sugarcane, rapeseed and soybean. Carbohydrates are fermented to bioethanol, which is mixed with gasoline as a transporation fuel. Biodiesel, a supplement or replacement to traditional diesel, is also produced from animal fats tallow.
Due to food and energy security concerns, many countries are promoting bioenergy crops that can be grown on land not suited for food production, so that the two systems are complementary rather than competitive [ 58 , 59 ]. Second generation biofuels refer to the range of feedstocks e.
There is a fine line between a first and second generation biofuel. For example, sugarcane is a first generation biofuel feedstock sucrose but co-generation for electricity using sugarcane residue bagasse as a fuel is also possible, and sugarcane residues may serve as future feedstocks in second generation ligno-cellulosic bioethanol production [ 61 ].
Ligno-cellulosic bioethanol is based on the conversion of lignocellulosic compounds, made up of chains of about 10 glucose and other small organic molecules, into sugars with sophisticated methods of acid or enzymatic hydrolysis. Those sugars can then be converted to fuel using tradiational methods. This means that non-food products such as cereal and wood residues can be converted to ethanol instead of remaining as a waste by-product. There are a few examples of commercial ligno-cellulosic plants.
For example, Swiss company Clariant opened a ligno-cellulosic plant in Germany in that can produce up to 1 Mg of cellulosic ethanol from 4 Mg of wheat straw [ 62 ]. Where lignin cannot be converted to small sugars easily through biochemical processes, it can be burnt for co-generation of bio-electricity. Another potential bioethanol feedstock is agave Agave spp. Agaves are well-suited for biofuel production as they can be grown in sandy soil with little or no irrigation and are less likely to be weedy.
Production of ethaonol and bioenergy from sisal juice from the sisal leaves and stems is under pilot testing at the Institute for Production Innovation at the Uninversity of Dar es Salaam and Aalborg University [ 65 ]. The first field experiment of blue agave Agave tequilana as a biofuel crop was planted in in the Burdekin River Irrigation Area of Queensland, Australia [ 35 ]. Blue agave can acheive strong growth rates by potentially switching from CAM to C3 photosynthesis if there is sufficient water supply [ 66 ].
Approximately 0. In theory, this crop area 0. In the meantime, new and novel feedstock conversion technologies are being developed such as fast pyrolysis and supercritical water treatment that can now convert nearly any biomass feedstock, such as wood residues, agricultural residues e. Hydrogen H 2 is designated as a third generation biofuel, when it is produced from biomass by algae or enzymes [ 1 ]. H 2 is a fuel whose combustion produces only water, although future technological breakthroughs are needed before H 2 can be produced economically.
The advanced biofuels value chain . There is also a move to source oilseed from non-food dedicated energy crops grown on marginal land. These crops might include jatropha Jatropha curcas , pongamia Millettia pinnata , Indian mustard Brassica juncea , and microalgae. The recent failure of jatropha as an energy crop in India and other developing countries due to a lack of bioenergy policy highlights the need for investment in research and policy development before starting on large-scale investments [ 67 ].
Pongamia is a tropical tree legume Fabaceae family and is a native of India and northern Australia. It has been used as a biofuel crop in India for some time, and is well-suited to marginal land as it is regarded as both a saline- and drought-tolerant species.
In India, the de-oiled cake of pongamia i. Opportunities exist for a sustainable pongamia agroforestry program to supply biodiesel in northern Australia, although substantial infrastructure investment in processing plants would be needed [ 59 ]. Indian mustard is another potential annual oilseed crop being developed in India and Australia. It is drought-tolerant annual rainfall mm and many varieties can express greater osmotic adjustment than canola [ 70 ].
Indian mustard is now part of a four year rotation at the Watson Centre. Microalgae can be cultivated in open raceway ponds or closed photobioreactors, harvested, extracted and then converted into a suitable biofuel such as biodiesel. Raceway ponds are shallow no more than 30 cm deep raceways and contents are cycled continuously around the pond circuit using a paddlewheel. Most commercial algal producers are currently using open raceway ponds as these require lower capital costs to set up but may result in increased evaporation and risks of contamination [ 72 ].
Photobioreactors are closed systems which offer better control over contamination and evaporation but have higher capital and operating costs than open raceway ponds [ 72 ]. Surface fouling due to competitors e. Despite the development of microalgae as a feedstock for biodiesel production, there are problems scaling up from laboratories to commercial production [ 74 , 75 ]. Key limitations to algal production in raceways or photobioreactors are 1 the need for stirring, 2 provision of nutrients for optimal growth and 3 very large surface areas required to capture significant amounts of sunlight [ 12 ].
Other problems are pathogen attack, ageing of algal cultures, and lack of system optimisation [ 76 ]. Life cycle analysis LCA is a tool to take into account the inputs and outputs of a food or biofuel crop production system, including the growing of the crop and its subsequent processing; the technique is also used to assess the energy efficiency and impact of food and biofuel crops on greenhouse gases [ 79 ].
Ecologists can relate an LCA to a foodweb or ecosystem model that traces the fluxes of energy through the system. Net energy value NEV is an efficiency term calculated as the difference between the usable energy produced from a crop and the amount of energy required for the production of that crop [ 79 ]. Three annual crop management systems, conventional several tillage operations for weed control, seedbed preparation, seeding , conservation reduced, minimum and no-till systems , and organic intensive tillage for seeding, weed control were compared in Canada and Spain [ 80 - 82 ].
However, fertiliser and pesticide rates were often increased in response to increased soil water, resulting in a similar total energy use by conventional compared with conservation systems [ 80 - 82 ]. In contrast, there was a reduction in energy input in organic systems due to the use of organic fertilisers instead of synthetic pesticides and fertilisers [ 47 ].
In terms of energy output:input ratio, organic farming in Spain was 2. Inclusion of a leguminous forage crop e. Legume-rhizobial associations are effective solar-energy-driven systems fixing atmospheric N 2 into ammonia with minimal CO 2 emissions compared to industrial nitrogen-fertiliser production. Legumes fix nitrogen and thus reduce synthetic N fertiliser use in farming systems; they also enhance the productivity of subsequent crops through breaks in the disease cycle [ 84 ]. Pulses contribute about 21 million Mg of fixed-N per year globally, accounting for one third of the toal biological N 2 fixation in agroecosystems [ 85 ].
The energy efficiency of biofuels can also be termed the fossil energy ratio FER expressed as the ratio of the amount of fuel energy produced to the amount of fossil fuel energy required for that production [ 79 ]. Life cycle assessments for biofuels have also shown that Brazilian sugarcane, agave, and switchgrass ethanol could achieve positive energy balances and substantial greenhouse gas offsets, while maize in the United States and China offers modest or no offsets [ 64 ].
The bioenergy created in sugarcane and agave ethanol, and in palm oil, is at least four times the amount required to produce it, while maize in the United States and China release almost as much energy when they are burnt as the energy that is consumed in growing and processing them Figures 5 and 6 [ 54 , 86 ].
Sugar crops usually produce more ethanol per ha with a better energy balance than starch crops because sugar crops produce higher sugar amounts per ha than starch crops; and sugar sucrose can be directly fermented, whereas starch polymers have to be hydrolysed before being fermented by yeast [ 1 ]. In general the energy gain and conversion of solar energy into biomass in the sub-tropics is substantially greater than any achievable in temperate zones [ 87 ], possibly due to the longer growing season and higher levels of solar energy over an annual cycle.
For example, the FER of sugarcane in Brazil was 8. There is already evidence of a land-grab with countries e. Many countries may never be able to establish a position of energy or food independence or anywhere near approaching it [ 88 ]. For example, Sweden is importing Brazilian bioethanol as its main source of renewable transportation energy, due to the climatic constraints of growing biomass for liquid fuels within Sweden [ 88 ].
FER of microalgal-based Chlorella vulgaris biodiesel produced in raceways is 0. This current negative energy balance is unacceptable unless the production chain can be fully optimised with heating and electricity inputs decarbonised [ 89 ]. Estimated ranges of fossil energy ratio FER of selected fuel types [54, 86].
Note: The ratios for cellulosic biofuels are theoretical. In some jurisdictions, forest biomass is increasingly consisting of elements essential to functioning forest ecosystems, including standing trees, naturally disturbed forests and remains of traditional logging operations that were previously left in the forest. Environmental groups also cite recent scientific research which has found that it can take many decades for the carbon released by burning biomass to be recaptured by regrowing trees, and even longer in low productivity areas; furthermore, logging operations may disturb forest soils and cause them to release stored carbon.
In light of the pressing need to reduce greenhouse gas emissions in the short term in order to mitigate the effects of climate change, a number of environmental groups are opposing the large-scale use of forest biomass in energy production. The New Scientist described a scenario in a September article which illustrated why the journal believed bioenergy can be bad: Suppose you cut down a year oak tree in your garden and use the logs to heat your house instead of coal.
Wood emits more carbon dioxide than coal per unit of heat gained and the roots left in the soil emit more carbon dioxide as they rot. If you plant another tree it will soak up that carbon dioxide in about 50 years. But if you had left the original tree in place it would have soaked up the carbon dioxide from the coal and more.
It could take centuries before cutting down the tree would give any benefit. But the world needed to cut carbon dioxide over the next few decades if the global warming was to be kept below 3 degrees C. Recently, a new company called Mango materials used bacterial fermentation to produce an intracellular biopolymer, polyhydroxyalkanoate from methane. The great advantage of biopolymers is that it is biodegradable which makes it environment friendly. Also, because methane would be converted into biopolymer that would reduce methane emissions.
Chief Executive Officer Molly Morse said that the unused methane would be enough to produce more than three billion pounds of biopolymer. From Wikipedia, the free encyclopedia. For other uses, see Bioenergy disambiguation. This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources.
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