The potential of using algae for harvesting energy is huge. Yet in Spain, its potential has long been overlooked. Only now is interest starting to grow. Just like for agriculture, successful large scale algae cultivation requires selection of the most suitable species for the intended purpose, an understanding of the optimal environmental conditions for growth, and consideration of nutritional requirements. For arable farming these factors have been developed and optimised over the course of many centuries. However, algae aquaculture is in its infancy and knowledge of best practice is comparatively sparse. Due to the immediate need for fossil fuel alternatives, intensive studies are therefore required to quickly understand the best means for commercialising energy and fuel production from algae.
We also live in an era where climate change, food shortages and droughts are becoming of increasing concern. There is therefore a need to understand the sustainability impacts of algae cultivation at scale to prevent any unfavourable outcomes occurring from its commercial exploitation.
Diesel from algae, ethanol from algae and methane from algae have yet a cost price for the processing of these energy products from algae very high and exceeds the selling price. The raw material costs are the most important factor in the production price. But even if algae would be freely available the downstream processing cost still exceeds the selling price. If algae are to be used for energy production, an important decrease in the cost of production and processing is required.
The idea that algae can be used as a renewable source of energy is not an entirely new concept. Production of methane gas from algae was proposed as early as the 1950s. Following the 1970s oil crisis, the US Department of Energy’s Office of Fuels Development initiated the ‘Aquatic Species Program’ with a focus of developing renewable fuels from high oil yielding algae. The project ran for almost 20 years from 1978 and cost over $25 million. It was terminated during the 1990s once cheap oil became more readily available. In the years following, interest in algal-based technologies subsided. However, the algae industry has been rejuvenated in recent years due to a growing need to find low carbon, renewable forms of energy. Alongside EnAlgae, there are now dozens of projects and more than 100 companies worldwide researching, and investing in, algal biotechnologies.
Microalgae essentially act as single-celled bioreactors, using sunlight and CO2 to produce a range of valuable products. Many species have high oil contents, which can be extracted and converted to biodiesel. Microalgae can also be used to produce other biofuels such as butanol, hydrogen, methane, ethanol, vegetable oil and aviation fuels. Moreover, these organisms can be utilised to manufacture high value products such as Omega-3 oils which can sell for up to $160 per kilogram. There even lies the potential for microalgae to produce anti-cancer drugs and medicines for malaria.
Where are they grown?
Microalgae have been cultivated on an industrial scale for decades, predominantly for manufacturing human and animal nutrition products, and have been most typically grown in open ponds or raceways. A desire to produce more specific products and use strictly regulated cultivation conditions has meant that use of closed photobioreactor systems have become increasingly common.
How do they compare to other renewable resources?
Microalgae have a photosynthetic biomolecular toolkit that is around five times more efficient than that found in plants and can generate up to 50 times more oil per acre than traditional crops used to produce vegetable oil, according to the San Diego Centre for Algae Biotechnology. They can also help to deliver significant reductions in greenhouse gas (GHG) emissions by replacing products extracted from fossil oil. Yet, importantly, algae have no requirement for agricultural land and so have very little interference with food supply chains.
Microalgae can already be used to generate a suite of valuable, bio-based products and this list can only be expected to grow. With the cost of genomic sequencing having reduced substantially over recent years and genetic engineering techniques becoming ever more advanced, the attractiveness of using microalgae in commercial applications has never been greater.
Downstream processing to energy carriers
Diesel from algae
The downstream process of algae to diesel involves the recovery of intracellular lipid, and the subsequent conversion to the fatty acid methyl ester (FAME) via a transesterification reaction to make biodiesel.
Harvested algae must be dried and milled prior to supercritical carbon dioxide (sCO2) extraction to separate lipid from the cells. Under conditions of high pressure and elevated temperature, CO2 acts as solvent to remove neutral lipids, such as triacylglycerides. A pressure drop is used to collect various fractions from the biomass. In this example, only the lipid fraction is considered. Following sCO2 extraction, the remaining protein-rich biomass can be sold for other purposes. The lipid is collected and reined prior to transesterification using methanol and a catalyst. Further purification is required to reduce the contaminants present in the FAME mixture to acceptable levels for biodiesel. Glycerol, which is a by-product of the transesterification reaction, can be purified and sold.
Ethanol from algae
First step is to disrupt the algal cell walls by ball milling so that the cell’s content is released. The algae paste can be milled directly after harvesting (wet milling) or can be milled after a drying step (dry milling). The fact that wet milling makes drying unnecessary, means that drying energy is saved, and the risk of damaging the cell’s content – protein– is avoided. However, the energy requirements for wet milling are quite high as well, and the milling equipment is more expensive than a simple dry mill of comparable throughput. In case of dry milling, the resulting product is rehydrated. The next step is enzymatic hydrolysis of the released polymeric carbohydrates. Of the resulting monomeric carbohydrates, only glucose is readily fermented into ethanol by common yeast. Through distillation the ethanol can be upgraded from a concentration of 2.7% to 94% ethanol. Protein is a (high-value) by-product of the process.
Methane from algae
Digestion is an organic process where organic matter is broken down by microorganisms to methane (CH4) and CO2 under anaerobe conditions. Co-fermentation is fermentation of various biomass flows in a fermentation installation (co-digester) at the same time, whereby biogas is produced. In practice co-fermentation is often seen as manure being fermented with other organic material such as maize, barley, potatoes et cetera. These materials are added to increase the profitability of the fermentation process. In this case the co-digester will be fed with algae paste only. The biodiesel and ethanol models are based on 10 tonnes algae dry matter, whereby the amount of biomass can be scaled up. Because the minimum algae volume to produce methane in a digester is about 1,500 tonnes algae, the comparison of the cost price for the energy carriers will be based on this large algae biomass volume.