Abstract
Reducing mixing in both flat panel and tubular photobioreactors can result in a positive net energy balance with state-of-the-art technology and Dutch weather conditions. In the tubular photobioreactor, the net energy balance becomes positive at velocities < 0.3 ms-1, at which point the biomass production cost is 3.2 €/kg dry weight. In flat panel reactors, this point is at an air supply rate < 0.25 vol vol-1 min-1, at which the biomass production cost is 2.39 €/kg dry weight. To achieve these values in flat panel reactors, cheap low pressure blowers must be used, which limits the panel height to a maximum of 0.5 m, and in tubular reactors the tubes must be hydraulically smooth. For tubular reactors, it is important to prevent the formation of wall growth in order to keep the tubes hydraulically smooth. This paper shows how current production costs and energy requirement could be decreased.
Highlights
Electrical power is used for mechanical mixing in both open and closed photobioreactors and is necessary to keep the algae suspended, to provide sufficient mass transfer which denotes the exchange of oxygen and carbon dioxide, to obtain a certain level of light integration.Light integration means shifting the algae between dark and light zones in the light path whereby the resulting productivity increases towards that of a homogenously illuminated culture, illuminated by the time-averaged light intensity
The mixing costs for flat panel reactors consist of the value of the depreciation and cost of the energy consumption of the blowers or compressors delivering the compressed air for sparging, the pressure of which has a dramatic effect on these factors
The conclusion from table 2 is that if the back pressure from the flat panel reactors is kept below 100 mbar, either radial or side channel blowers can be used with the same result on the economics of production
Summary
Full light integration implies high frequency of flashing. For the microalga Chlamydomonas reinhardtii, for example, full light integration implies light flashes of only millisecond duration (Vejrazka et al 2011), which in practical photobioreactors requires a high and energetically costly level of turbulence. Optimizing a photobioreactor in terms of cost or energy is a complicated process requiring functional relationships between, on the one hand, oxygen, carbon dioxide and irradiation and, on the other, productivity. It should be kept in mind that these processes are overlaid by daily and seasonal cycles of light and temperature. Until these functional relationships have been developed, optimization remains a trial-and-error process.
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