Recently a MSc study (Besuijen, 2014) towards the design of large scale algae production systems in greenhouses has been carried out.
The study was done in close cooperation of Wageningen UR Greenhouse Horticulture, currently operating small scale algae production systems in greenhouses at their location in Blesiwijk , and Farm Technology group of Wageningen University , with in depth expertise on agricultural systems engineering. The advantage of algae production in a greenhouse is that optimal growing conditions can be controlled with resources that are already available or easy to access. To prevent competition for light with horticultural crops, the algae production system should be placed in a separate greenhouse compartment. Analyses in literature showed that on large-scale plants a reduction in the microalgae cost price can be achieved by reducing photobioreactor equipment cost, energy use in the harvest process and developments in algal productivity. A major drawback in scaling up tubular photobioreactors however, is the accumulation of excessive dissolved oxygen produced by photosynthesis. A two-phase type of tubular photobioreactor has been proposed in literature and practice as a solution to this drawback.
A simulation model for algae production in greenhouses, earlier developed by Slager (2011) , was adapted in this MSc study and was used to calculate the yearly algae production in a one-hectare greenhouse filled with photobioreactors in The Netherlands. Different configurations of one-phase and two-phase photobioreactor were simulated. An economic model was used for algae cost price calculations.
The study showed that the size of one-phase algae photobioreactor systems is limited due to oxygen accumulation. Due to the limitation in size they contain a relatively low production volume per photobioreactor. Many small systems have to be installed on a one ha greenhouse area. Therefore costs for pump, tank and sensor equipment are relatively high. The minimum cost price of algae produce in one-phase systems in greenhouse is calculated between 16 and 35 €•per kg dry matter algae. Two-phase systems can be larger and produce algae with a lower minimum cost price between 11 and 19 €•per kg dry matter algae. Depreciation of the photobioreactor and greenhouse equipment, labour, maintenance and electricity for illumination account for the highest percentage of the total cost.
In general the simulations showed that photo bioreactor configurations with vertically stacked double tube layers are preferred above one layer horizontal systems, due to more efficient use of the greenhouse ground surface and incident light. For the same reason it is economically interesting to find the optimum of horizontal and vertical distance of tubes. The number of tubes vertically is limited by the amount of incident light. Two-phase systems are more favourable than one-phase systems due since larger photo bioreactors can be installed per one ha greenhouse.
During a model calculation different assumptions are made, different model parameters have to be estimated. An uncertainty analysis has been carried out towards model results. The uncertainty in prediction of algae production has been estimated to be 30%, the uncertainty in cost price was 50%.
The developed model can be used to optimise algae photobioreactor configurations in greenhouses. The cost price of a system configuration can be estimated. Using this tool the economic feasibility of a system can be estimated at a given algae market price.