1.3.1 Conversion of Solar Energy to Biomass and Electricity

Photosynthesis is the driving mechanism behind microalgae biomass production but only requires a small fraction of the incident solar energy, primarily in the blue and red portions of the solar spectrum. In conventional cultivation of microalgae, the remainder of the incident solar energy simply heats the algae ponds, causing the water in them to evaporate and increase salinity which is a significant problem in biomass production. With microalgae cultivation often occurring in hot, semi-arid locations, this incidental heating is essentially a waste of the solar energy. Instead, it would be advantageous to be able to capture this unused portion of the solar spectrum and convert it to electricity for use at the cultivation site (Moheimani and Parlevliet 2013).

Figure 1.3 illustrates how the solar spectrum can be divided between the growth of microalgae and the production of electricity by a photovoltaic device (solar cell). Irradiance falling on the Earth’s surface is well defined in the standard ASTM G-173-03 (ASTM 2008). This is the AM1.5 solar spectrum as shown in Fig. 1.3. Of this spectrum, only a fraction is used by photosynthesis by a microalgae culture. Some 48.7 % of the incident solar energy is considered to be photosynthetically active radiation (PAR) in the region between 400 and 700 nm (Zhu et al. 2008). However, it is clear from the absorption spectra of Nannochloropsis that some parts of the spectrum are absorbed more strongly than others. As such, the growth and performance of photosynthetic organisms are strongly linked to the quality and quantity of available light (Lindstrom 1984; Smith 1983) with only some parts of the spectrum being used in photosynthesis. In comparison, highly efficient crys­talline silicon solar cells can absorb light strongly across the solar spectrum as shown by the spectral response of a PERL cell (Zhao et al. 1996) shown in Fig. 1.3. This suggests that although these consumers of solar energy (microalgae and solar cells) would appear to compete for the same resource, if the irradiance could be split between the two, the full utilisation of the solar spectrum would be possible. The shaded regions in Fig. 1.3 illustrate the portions of the solar spectrum that can be delivered to electrical generation and to microalgae cultivation without reducing the productivity of the microalgae. This would allow the production of biomass and electricity from the one facility.

The concept of the coproduction of electricity and agricultural production has been previously used in photovoltaic greenhouses. These are a building integrated photovoltaic system whereby solar modules are integrated into the structure of the building (Panda et al. 2011). Photovoltaic greenhouses use photovoltaic modules in the parts of the greenhouse whereby any reduction in overall PAR would not alter the growth of the plants, while the use of semi-transparent or opaque elements on the greenhouse can reduce the PAR and result in decreased productivity (Perez — Alonso et al. 2012). This would be due to a reduction in the irradiance the plants required for photosynthesis. To overcome this issue, we propose the use of a

semi-transparent solar module that is specifically designed to transmit the irradiance required by the microalgae and convert the remainder to electricity via a photo­voltaic system. This solar module or filter can be located above the microalgae ponds (Moheimani and Parlevliet 2013).

There are a number of advantages to this system. By reducing the total irradiance incident upon the microalgae pond, the temperature of the culture would be reduced which would result in lower evaporation and a more stable salt content in the pond. As the microalgae are still receiving the portion of the spectrum required for photosynthesis, there would be no reduction in productivity. The electricity gen­eration by the photovoltaic aspects of the system can be used on site to power motors and electronic systems to reduce the running costs of a facility. Alterna­tively, the electricity can be used to power additional lighting to increase the period of illumination on the microalgae or to increase the irradiance in specific parts of the solar spectrum. Using additional lighting powered by the otherwise wasted portions of the solar spectrum can increase the productivity of the microalgae. The style of system we have proposed (Moheimani and Parlevliet 2013) can improve the via­bility of microalgae growth for industrial purposes.