Method for Optimizing the Field Coils of Internally Illuminated Photobioreactors

We present a method for optimizing the design of held coils for a new type of photobioreactor (PBR). In those PBRs, photosynthetic active microorganisms or cells are grown. A novel concept for the illumination of PBRs was necessary, as the external illumination of those reactors leads to a limited penetration depth of light. The solution we propose is an internal illumination via wireless light emitters (WLEs). This increases the manageable culture volume of photosynthetic active microorganisms or cells. The illumination system is based on floating light emitters, which are powered wirelessly by near held resonant inductive coupling. The floating light emitters are able to illuminate a PBR more homogeneously than external illumination systems. We designed held coils to produce an intermediate frequency electromagnetic held inside the reactor. We chose a magnetic flux density B = 0.95 mT and a frequency f = 178 kHz. To reach optimal efhciency, the magnetic held has to be as homogeneous as possible. We demonstrate how this optimization can be performed with the method of differential evolution. Using the optimized held coil design, the minimum magnetic held was increased by almost 10% keeping the number of ampere-turns constant. A comparison of the light prohles over the cross section of both reactor types show clearly that the internal illumination via WLE results in a more uniform light distribution and a higher average light intensity in the reactor. An impact of the magnetic held on the green alga Chlamydomonas reinhardtii could be excluded by comparing the growth rate, the achieved cell dry weight and the maximum photosynthetic yield with and without applied magnetic held. Furthermore, a negative effect of the potentially occurring mechanical stress due to the suspended WLE can be excluded. A culture which is mechanically stressed by 125 WLE per liter shows the same maximal growth rate as a control culture. Finally, the comparison of an externally and a via WLE internally illuminated PBR shows the same growth rates in the exponential growth phase. However, the growth rate in the linear phase is more than twice the rate of the externally illuminated control culture, resulting in a more than 80% higher achieved biomass.