Microalgae are some of the world’s oldest organisms, phylogenetically. Microalgae can be found virtually everywhere, adapted to almost any habitat, from oceans and fresh-water lakes to deserts, arctic regions and the interior of rocks.
Because microalgae can, in principle, be easily and quickly propagated, these organisms have been the focus of applied research investigating the generation of valuable biomass. Decisive factors in the rising intersts for algae include its ability to accommodate large quantities of lipids under certain circumstances, and the possibility to propagate microalgae in water resources which are not usually exploitable (salt, waste and brackish water), in areas that have remained unsuitable for agriculture, in order to pull large quantities of CO2 out of the atmosphere. Microalgae would thus be able (at least in theory) to provide a sustainable source of biofuel without competing with agricultural production. On account of various technical advantages, such as energy fuel value, energy density and compatibility with existing technologies, many researchers consider biodiesel the preferred final source of energy to be produced out of algae.
At first glance, producing biodiesel from algae biomass is a simple technical process. The algae must be harvested from the culture medium and dried before the lipids contained within can be extracted and ultimately, converted into biodiesel. The essential feasibility of this procedure at laboratory scale has been demonstrated by many scientists. Already sixty-five years ago, scientists succeeded in extracting oils for technical applications from microalgae, diatoms in this case . In 1942, Harder and von Witsch obtained about two grams of oil per square metre of culturing area per day using a simple culturing system. Today researchers achieved results of an average of 5 to 10 grams oil per square meter per day, over productive months. While significantly greater yields were observed on certain days, the decisive factor for evaluating productivity is the average productivity over a period of several months.
Sunlight is the energy source behind microalgae growth. The availability of light limits the amount of biomass that can be produced. On average, annual solar radiation in Germany is approximately 1000 kWh. The maximum theoretically attainable efficiency of photosynthesis with respect to the energy of solar radiation is 11%. At a fuel value of about 6 KWh per kilogram for algae biomass, in Germany there is a maximum potential annual algae biomass accretion of approximately 100 grams per square meter per day (in the Sahara, this value would be approximately double). Considering an average oil content of 30%, theoretically a maximum of about 30 gram of extractable algae oil could be produced per square metre and day.
The yields produced are already up to 30% of the thermodynamic maximum. Even if, after 65 years of research and development, microalgal productivity could be increased above the currently achievable quantity, the increase would only be by a factor of 2 in optimistic scenarios.
The question of energy balance is an essential factor influencing sustainability of biomass production. How much energy is in the biomass and how much energy is required for production and refining? One kilogram of dried algae biomass has a fuel value of 27 MJ. The state of technology indicates employing a paddle mixer to continuously mix an algae culture, the mixer requiring about 2 MJ of energy to produce one kilogram of algae biomass. The fertilizers required are also obtained by expending energy; for of algae, the energy required for this ranges from 5 to 12 MJ. Pumps will be required for harvesting, replenishing and emptying basins and maintaining water levels. Assuming a water volume to be moved of about 10 to 60 cubic metres per kilogram of algae produced, and an energy requirement of 0.1 kW per cubic metre, up to 16 MJ of energy are required per kilogram of algae. Finally, consider the energy required for harvesting: even assuming that the algae could be sedimented spontaneously or by adding flocculants and thus be enriched by a factor of 10, it remains necessary to further concentrate the remaining volume before drying. Considering that a separator requires 1 kWh per cubic metre, there is a further increase of 3.6 MJ per kilogram of algae. A band filter press would require half as much energy. A further 7 to 8 MJ per kilogram are required for drying the algae.
With recourse to the usual current technology, at least 35 to 45 MJ of energy must be expended in order to produce a “value” of 27 MJ in dry algal biomass. This simple estimate does not take into account many steps in the total production process of actual biofuel such as biodiesel that also require energy. The energy costs for transport, supplying CO2 to the cultures, preparation and regeneration of culture media, cleaning procedures and finally, the extraction process, would have to be taken into account. It should also be taken into consideration that each step in the process is accompanied by a certain loss factor. The above estimate operates with the unrealistic assumption of zero loss during the procedures. The basic data used for this calculation are derived from practical experience at considerably smaller facilities. Higher energy costs are to be expected in larger facilities in the future, possessing multiple square kilometres, since liquids such as culture medium etc. will have to be transported over longer distances.
The negative energy balance is not the only problem in evaluating the sustainability of algae biodiesel. There is a range of other, unsolved problematic issues that will be briefly outlined.
As many studies have already shown, it is not possible to prevent undesired organisms in open ponds over an extended period (e.g. “wild” algae, bacteria, protozoa, water fleas and insects, such as mosquitoes). This could make the use of herbicides or pesticides necessary over large areas. Mixed cultures would become established, making it impossible to market the biomass as higher quality.
Since at least 3-10mm of liquid evaporate per day from /open ponds/, considerable quantities of fresh water will be required to prevent salt concentrations from rising.
It is currently unknown to what extent culture medium can be reused and what would be necessary to clean the waste water.
The land required for facilities of the size required may not be immediately usable for agriculture, however the land is valuable to the population in the area. Infrequently used areas of land are often valuable habitats for rare and protected plants and animals.
In conclusion, it can be determined that at first glance, microalgae appear to be an attractive source of biomass. A more detailed consideration of energy requirements, however, indicates that the practice is not sustainable because of the negative energy balance associated with the production process. Culture stability, media recycling and harvesting are still a significant challenge and require further research. Field demonstration projects are necessary to advance understanding the possible environmental risk of large scale microalgal monocultures.