The culture velocity within the solar receiver is very important, as the cells must be evenly distributed throughout the tubing to avoid extended periods within the dark zones located at the centre of the tubing. The maximum velocity obtained within the system is dependent on the size of micro-eddies in comparison to the algae cell dimension. Acien Fernandez et al. [1] found that the maximum velocity in an exter­nal loop reactor (ELR) for Phaeodactylum tricornutum strain was 1 m/s. This veloc­ity was obtained by a specific power input of 170 W/m3. However, the actual velocity used in the ELR was 0.5 m/s due to issues associated with the mechanical properties of the solar receiver. The velocity must be high enough to ensure turbulence, thus preventing bio-sedimentation. However, the liquid velocity cannot be applied at gratuitous speeds, as this could potentially cause damage to the algal cells. Generally the liquid velocity must comply with two constraints: a turbulent Reynolds number and a micro eddy length that is significantly larger than the cell dimensions [1].

1.3.3 pH

The pH of the cultivation system increases as the algal cells photosynthesise. The consumption of carbon dioxide and the production of dissolved oxygen from pho­tosynthesis can significantly alter the pH thereby impeding growth. The cultivation system requires a relatively neutral environment, usually maintained at pH ~8 [34]. To prevent variations in culture pH, appropriate control systems are incorporated to monitor the pH. Another technique to control variations in pH is to employ carbon dioxide injection points along the tube run. This prevents excessive culture pH and any carbon limitation that may occur [23]. However, this is not economically viable when considering large algal plants.