Some important figures for Algae

Posted by Rachel

There seems to be a lot of conflicting information about some of the figures related to algae, especially when it comes to determining the yield of oil by the plant. The textbook that I showed the group on Wednesday (Biological Energy Resources; M Slesser, C Lewis; 1980) gives a short-term high yield of 24g/m^2 which seems distinctly average when compared with yields from other crops. However, this is by now an old-ish book so looking at more recent statistics on the internet I have managed to come up with the following stats (hopefully I have covered most of what we discussed on Wednesday).  First of all, it must be remembered that different species of algae produce different amounts of oil. Some species (such as Diatoms) can produce 50% oil, whereas some species will produce a little less.

Crop lifecycle: Micro-algae are the fastest growing photosynthesizing organisms. They can complete an entire growing cycle every few days.

Yield:

• At an assumed recovery rate of 30% of the weight of algae, 45.6 tonnes of oil/hectare/year can be produced from Diatom algae.
• Under optimum growing conditions micro-algae will produce up to 4 lbs./sq. ft./year or 15,000 gallons of oil/acre/year.  

Energy density: The only reliable data I could find for energy density is regarding algae biodiesel (produced from a coal-fired powerstation). There were also some other useful figures for this scenario; see below:

• Algae yield: 100 tonnes/ha.
• 2.2 tonnes carbon dioxide needed / 1 tonnes algae
• Water needed 4m³/m². Most of the water is lost due to evaporation, some is consumed by the algae, and some is lost in the harvesting of the algae.
• A 500 MW coal fired power station produces 3.67x10⁶ tonnes of carbon dioxide
• 3.5 barrels of biodiesel per tonne algae produced
• 6 MJ methane per tonne algae generated
• Energy density of LNG is 15.2 kWh/kg and density is 448 kg/m³

Please see http://www.oilgae.com/algae/oil/biod/ref/ref.html for more information. There are also a lot of links about algae produced oil.

A note about species of algae to consider:
Diatoms appear to be one of the more promising species to consider in the production of energy resources although research into them is still ongoing. The link above has some more information on them towards the bottom of the page if anyone is interested.