Archive for May, 2008

How much energy can algae produce?

May 28, 2008

Some say algae can grow faster than any other crop growing on land. The numbers range up to 150 (300) tonnes algal biomass/ha.year, which is several times higher than the best known arable crop. Can algae really deliver this enormous amount of biomass? At the first congress on algae in The Netherlands, Eugène Roebroeck from Lgem made some interesting calculations.

There is only a limited amount of energy reaching the earth’s surface which plants can use to grow.

First, let’s look at the sun and the energy it provides. The sun provides a limited amount of energy per square meter. At the tropics the sunshine is very intense, in contrary to the poles where there is little solar energy. Clouds are also an important factor; regions with more clouds recieve less solar energy at ground level. The World Metereological Organisation has combined these two factors and calculated the annual solar energy available at any given location.

The annual average of solar energy for the Netherlands (and for Belgium) is 110 W/m². This is the maximum energy you can use. But since the spectrum of sunlight has a range from 250 to 2700 nm, containing next to visible light also ultraviolet radiation and infrared radiation, not all of this energy can be used. The figure underneath shows the distribution of energy over the wavelengths. The red area between the two vertical lines is approxymately the energy algae can use: only 43% of the total energy of the sun.

Figure: the total solar spectrum. The red area is the energy at ground level. The red area between the two vertical lines is the part algae can use.

When we know that algae can only use 43% of 110W/m², we see that algae can only use 48 W/m².

Then, we have the efficiency of the photosynthetic apparatus of plants. Research shows that we need about 8-10 photons per captured CO2 molecule. Researchs on trees show that trees can take up only 5-6% of the available photons. This means we have in practice 3 W/m² which the algae can effectively use. Extrapolated to biomass this means we have a potential of 40 tonnes biomass/ha.year.

Since algal growth can be improved to reach higher effeciencies than the data shown above, the maximum value for photosynthetic efficiency can be 10%, which means a maximum potential of 5 W/m² or 68 tonnes biomass/ha.year.

In literature, this theoretical 10% efficiency is applied to areas (for example, the Sahara area) with much higher solar power (350 W/m² instead of 110W/m²), which results in the theoretical production rates of ~150 tonnes biomass/ha.year. This is the maximum amount of energy algae can capture per surface area.

But what has been proven? The NREL reported production rates of 50 tonnes algal biomass/ha.year, so this value can be achieved.

Is this enough to compete with other energy crops? Literature (Van Sark et al., 2006) suggests 8-12 tonnes DRY weight/ha.year for current bioenergy crops. At the congress some people argued that sugar beets can reach productivities of 25 tonnes dry mass/ha.year.