That is a very good question.

First of all you should focus on the cost of the raw materials: water, nutrients (including CO2) and light - every single one of these items has an associated energy requirement and carbon footprint which should be accounted for.

Depending on the system, the energy consumption in microalgae cultivation should account for (1) pumping/paddle wheel, (2) nutrient supply, (3) de-oxygenation, (4) light and (5) cooling/heating. Since most of the work published refers to small scale or even pilot-scale, there is not much data on this and usually limited by its scale. If no data can be found I would advise you to work with fixed mass-energy balances, lets say for producing 1 kg algae.

1. If you know the flow and total head, you are able to calculate the theoretical power consumption from a pump. Chiaramonti et al. (2013) have reviewed the energy balance in raceway ponds - Applied Energy vol 102.

2. If you consider the nutrients to be added batch wise there is no energy in that - but note that producing these has a cost and an associated energy. However, the supply of CO2 will consume energy - again, this depends on the flow and time of supply. Pires et al (2012) have reported some estimated costs in Renewable and Sustainable Energy Reviews Vol 15 Issue 5.

3. Most algae culturing systems do not degas mechanically since there is a positive pressure that allows it to be removed from the system - I think!

4. The supply of light will be geographically dependent. If outdoors, on a early basis you could assume 12 hours daylight and 12 hours artificial light. I have not seen any costs associated with light supply. H. Jeong et al (2013) have compared "normal" light with different LEDs in Renewable Energy vol 54.

5. Cooling/heating requirements will be different at different latitudes. A simple heat transfer calculation should help towards a realistic estimate depending the starting mass-energy balance.

A quick search in the literature will provide you insight and a good starting point. There are quite a few papers on life cycle analysis of microalgae and these usually report on estimates for the production of algae depending on the production system adopted.

Dear Mikko Kouhia

Unfortunately, I am not able to answer your question. However, I forward the contact info below:

Jorge Alberto Vieira Costa ([email protected])

Thank you for your email and good luck

Best regards

Fabiana

The work of Chisti on raceways gives an estimated energy budgets for raceways for oil production systems with AD. Yes pumping is an important factor, but the sources of nutrients and embedded energy associated with them also is also a big factor. Nitrogen requirements are substantial for growth and the provision of fertilizer based nutrients can account for over double that of pumping energy. It essential to use waste nitrogen sources to reduce net energy input and recycle any N that is not taken out as product. Most probably through a coupled AD process as is commonly suggested.

As Fabiana and Robert mentioned this is a complex topic. I would suggest the following paper, which calculates the maximum theoretical algal oil/biomass production using very sound set of transparent assumptions.

Weyer, Kristina M., Daniel R. Bush, Al Darzins, and Bryan D. Willson. “Theoretical Maximum Algal Oil Production.” BioEnergy Research 3, no. 2 (October 8, 2009): 204–213. doi:10.1007/s12155-009-9046-x.

In addition, I will also suggest that you look into some papers that deal with the LCA of Algae. There are a lot of them (including sander and Murthy, 2010 We published) that are based on lab/pilot scale data. I would suggest that you look into a recent paper with commercial data:

Passell, Howard, Harnoor Dhaliwal, Marissa Reno, Ben Wu, Ami Ben Amotz, Etai Ivry, Marcus Gay, Tom Czartoski, Lise Laurin, and Nathan Ayer. “Algae Biodiesel Life Cycle Assessment Using Current Commercial Data.” Journal of Environmental Management 129 (November 15, 2013): 103–111. doi:10.1016/j.jenvman.2013.06.055.

Hope this helps.

Regards,

Murthy

Thank you all for your insights and suggestions. System boundary in my case will be set at mill level and nutrient/CO2/heat supply will be from secondary or "waste" sources. Those LCAs are a good point: even though I recognized that there are many different assessments made, I failed to comprehend that they naturally have to include assumptions for energy usage.

Heat transfer calculations for the reactors themselves are indeed a good way to get rough estimates of the heating/cooling requirement. I'll definitely do those calculations in the course of this project.

In this paper there is nice inforrmation about that..

Bioresource Technology. Volume 102, Issue 1, January 2011, Pages 207–214

This manuscrip can you help

Comparative energy life-cycle analyses of microalgal biomass production in open ponds and photobioreactors.

Orlando Jorquera, Asher Kiperstok, Emerson A Sales, Marcelo Embiruçu, Maria L Ghiradi

Is undoubtedly one of the key questions in microalgae culture systems...

But unfortunatly there are more that one aswers to your question, because algae energy consumption depends mainly of cultivation system, harvesting strategy and extraction methods, the combination of this factors will affect greatly you energgy consumption. An easy example is the harvesting strategy, which as you know can be carried out by one step or two, and the selection of one or other will change greatly the consumption.

So you have to analyze you specific case of study, the data in paper are interesting, but rarely extrapolable to a real case