The power output under constrained lighting situations from both a crystalline and amorphous silicon solar module has been calculated using the model outlined earlier. This is independent of the species of algae used as the power produced by the photovoltaics relies only on the portion of the spectrum they would receive. The threshold determines the portions of the spectrum provided to the photovoltaics as shown in Fig. 15.5. The power produced is not zero when the entire PAR is provided to the microalgae as there is an extensive part of the spectrum beyond PAR which photovoltaic devices can convert to electricity. It is clear that crystalline silicon solar cells are much more efficient in the regions outside of PAR than amorphous silicon is. This is due to the extended spectral response of crystalline silicon into the infrared part of the spectrum and the higher efficiencies of the crystalline solar cells (Fig. 15.3). The energy generated by these crystalline silicon solar cells can be used to power additional lighting to add more irradiance to the microalgae. If a LED system with external quantum efficiencies in the order of 55 % (Krames et al. 1999b) is used, a substantial amount of additional illumination can be provided to the microalgae.

The model we have created for this scenario is highly dependent on the absorption spectra of the microalgae. The power generated by the PV cells can be directed to a series of LEDs which will provide additional illumination to the microalgae. As can be seen in Fig. 15.6, the augmented power absorbed by the

microalgae is distinctly higher from some species when compared to others. Notably, this is chaetoceros and tetraselmis. The reason for this is apparent in Fig. 15.2. These two species of microalgae have a much greater absorbance than the other species in the measured absorbance data. This does not necessarily mean they are the most productive and only that they absorb the greatest portion of irradiance. The second trend which appears in the graphs in Fig. 15.6 is that a system aug­mented with electricity generated by crystalline silicon solar cells will generate more power and have a larger portion able to be provided back to the microalgae via LEDs. This highlights the importance of a highly efficient collecting device. For the remainder of this work, we will focus on results from the crystalline silicon parts of the model which are a best-case scenario.

A more useful visualization of this data is to examine the change in the amount of power absorbed by the microalgae as the threshold is changed. Figure 15.7 shows the change in power absorbed by various microalgae species when compared to the minimum situation. That is, the situation where all irradiance between 400 and 700 nm is transmitted through to the microalgae. Additional illumination is still provided by LEDs using the IR and UV parts of the spectrum. When compared to this baseline, it can be seen that even if all the irradiance in PAR is transmitted to the algae, there is a boost of 20 % in the total amount of power absorbed by the microalgae. This is from the additional irradiance provided by the LEDs which are powered by infrared radiation (>700 nm) captured by the crystalline silicon photovoltaics. As the threshold is increased, the total amount of power provided to the microalgae decreases and beyond 50 %, there would be no net benefit for this system. At first glance, this would seem to indicate there is not a great deal of advantage in filtering the light as described in this model and combining electricity and biofuel production. However, it needs to be recognized that much of the power being absorbed by the microalgae may not be assisting in photosynthesis. There is

still significant absorption in other portions of the spectrum. If, for example, the green portion of the spectrum is not required for photosynthesis, it can be entirely removed from the spectrum provided to the microalgae and instead converted to blue or red light which will be absorbed efficiently by the microalgae.

A better comparison would be to the 50 % threshold. This equates to a full-width half-maximum band-pass around the main absorption peaks of the microalgae as seen in Fig. 15.2. This situation is shown in Fig. 15.8. From this figure, it can be seen that there are gains in the amount of irradiance absorbed by the microalgae by

in excess of 100 % in some situations and for some species. This is a significant increase in irradiance absorbed by the microalgae and should lead to more pro­ductive growth.

A combination of these two energy production methods (solar energy and chemical energy) can efficiently use the whole solar spectrum. We recognize that an area covered by PV panels of the same scale as a microalgae farm would produce more electrical energy than the algae can store as chemical energy. However, the advantage of our proposed method is the production of chemical energy for transportation or other high value crops and can increase the productivity of mic­roalgae systems.

This suggests that a combination of the two energy production systems would allow for a full utilization of the solar spectrum allowing both biofuel and electricity production from the one facility. This makes efficient use of available land, or it can enhance biofuel production by management of the spectrum and the addition of targeted illumination. Therefore, we propose a co-production system that uses an active filter or photovoltaic system above a microalgae pond to capture and effi­ciently convert the whole solar spectrum into usable energy or products. While the mechanism for splitting the spectrum is not fully determined as yet, there are several candidate options, including a specifically tailored semitransparent thin-film PV, luminescent solar concentrators, or other advanced energy harvesting flat glass panel that match the spectrum not used by the microalgae. One excellent candidate technology system that can transmit arbitrary visible light wavebands, capture the infrared part of the spectrum, concentrate it on the edge of a glass panel, and convert it to electricity has been recently developed and patented (Rosenberg et al.

2013) .