Microalgae as Second Generation Energy Plants

The use of microalgae for the production of biofuels has many ecological and economical advantages. First of all, microalgae show a much higher efficiency in converting solar energy to biomass. From the biomass of corn grown on one hectare, about 2,000 m3 biogas (methane) can be obtained; however, biomass of microalgae grown on the equivalent area produces about 200,000 m3 (Solarbiofuels 2008). In microalgal biomass, the percentage of compounds suitable for the production of biofuels (e.g., starch, oil) is much higher than in crop plants, because there is no need to divert energy to the synthesis of fibre material, vascular and absorption tissues, etc. Microalgal cultures can be grown on a relatively small area that may not be appropriate for agriculture and - at least in the case of 'indoor systems' -they do not need irrigation and produce a high constant yield irrespective of outside environmental conditions such as temperature and draught. For producing a given amount of biomass, indoor cultures need about 1,000 times less water than crops. Preliminary data show that - on the same area - (Chisti, 2007) microalgal cultures produce about 15x more oil for biodiesel production than rapeseed does. To cover 25% of the US demand for transportation fuels by corn (Table 1), an area of about 4.6x the area that is currently used for US agriculture is needed. When oil from oil palms is used, about 12% of the agricultural area is required; however, when microalgal cultures are used only 2-5% of that area would be sufficient.

3.1. CULTURE SYSTEMS FOR MICROALGAE

For a large-scale culture of microalgae, two systems are used, the so-called 'indoor' and 'outdoor' systems (Figs. 1 and 2). Outdoor systems are sometimes also called 'open raceway ponds' and have a long tradition that can be traced back in history

Table 1. Crop efficiencies for the production of biodiesel. (Modified after Chisti, 2007.)

Crop

Yield of biodiesel (L x ha-1 x a-1)

Land area required (percentage of area covered currently (ha x 106) by crops in the USA)a

Corn

157

843

463

Soybean

451

294

162

Rapeseed

1,206

110

60

Jatropha

1,892

70

38

Oil Palm

5,991

22

12

Microalgae"

58,700

9

5

Microalgae0

136,900

4

2

a To cover about 25% of all transportation fuels needed in the USA per year. b Oil content in algal biomass (by weight): 30%. c Oil content in algal biomass (by weight): 70%.

a To cover about 25% of all transportation fuels needed in the USA per year. b Oil content in algal biomass (by weight): 30%. c Oil content in algal biomass (by weight): 70%.

Microalgae Raceway Pond
Figure 1. Outdoor system for the cultivation of microalgae: 'open raceway pond' Seambiotic (Israel) by permission.
Microalgae Raceway Pond

for centuries and were used already by the Mayas. Microalgae, usually cyanobacte-ria as, e.g., Spirulina spp., were grown in small lakes, ponds and ditches, harvested and spread nearby for drying and subsequently used as animal food. Modern systems consist of pools of different shapes, in which microalgae are grown in a shallow layer (20-40 cm) that is permanently agitated to guarantee optimal growth conditions. Most outdoor systems are run today in countries providing optimal natural conditions concerning temperature and sunshine such as Hawaii, Australia and Japan. The largest outdoor facilities spread on an area of about 440,000 m2 (Spolaore et al., 2006) and produce about 8-12 g dryweight of algal biomass per m2 and day (Ackermann, 2007).

Indoor systems are closed bioreactors, in which algae are grown under defined conditions of temperature, light, nutrient supply, etc. (Pulz, 2001). The largest commercially used indoor system has a volume of about 600 m3 that is contained in glass tubes 500 km in length (Ackermann, 2007). Production of algal biomass amounts to 32 g m-2 and day (sunlight only).

3.2. COSTS

A comparison of pros and cons of both systems shows that open systems are more cost-efficient than closed systems when optimal and constant climatic conditions are given, e.g., optimum temperature and sufficient sunshine. However, it is also obvious that open systems are generally more prone to changes in environmental conditions, as to the input of spores, germs and particulate matter from the atmosphere and to extreme weather conditions such as thunderstorms, hurricanes, etc. Continuous production of algal biomass is easier to achieve by closed systems that, however, require higher financial investments. Various tests have shown that in closed systems, the maximum cell density that can be obtained under optimum conditions is about 30 times higher than that in open systems. Taking into account the economic advantage of a constant and predictable production of high-quality indoor systems presumably is more cost-efficient than in open systems. Estimates (Chisti, 2007) calculate the costs for the production of 1 kg of algal biomass to US$2.85 in closed systems, whereas in open systems it is about US$3.89. When algae are used for the production of biodiesel (30% of algal biomass is process-able), costs of 1 L of biodiesel obtained from open systems would amount to US$1.81, from closed systems to US$1.40. The same author calculates for 1 L biodiesel made from palm oil US$0.66 and made from petroleum about US$0.49. However, it should be mentioned that calculations are a matter of discussion depending not only on the oil content of algal biomass but also on general operating costs of the facility that in part may be effected by environmental conditions such as ambient temperature and duration of sunshine. Other authors calculate for 1 L biodiesel made from microalgae costs ranging from about US$5.35 (Dimitov, 2007) to US$0.16 (Gunzburg, 1993). At any rate, most calculations show that costs of biofuels made from microalgae are still higher than costs of petrofuels. However, in recent times, the market price of petrofuel is steadily pointing upwards and the break-even point may be reached sooner than anticipated.

3.3. HIGH-VALUE PRODUCTS

It is not astonishing at all that up to now any economic success of indoor and outdoor systems was achieved by the production of high-value products such as pharmaceuticals, cosmetics, products for healthcare, natural colours, unsaturated fatty acids, essential amino acids, etc. Those high-value substances allow a realistic competition of microalgal cultures with classical production methods such as isolation of linolic acids from herbs, etc. They allow microalgae cultures to bring in their specific advantages such as production under reliable sterile conditions, no risk of contamination with human viruses, prions, etc. Thus, it is conceivable to use microalgal cultures also for genetic engineering techniques to obtain, for example, specially designed antibodies, recombinant proteins, etc. Appropriate techniques are available for microalgae, e.g., in Chlamydomonas sp. (green algae) genetic manipulation has been successful (Patel-Predd, 2007). An interesting advantage of microalgae over crop plants concerning genetic manipulation might be that there should be no public concern about bringing 'manipulated' organisms into the ecosystem, because algal systems are not in contact with the environment.

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