“Leave the algae alone”. Low-tech Magazine. 4 Apr. 2008 – transparent aquariums (called closed bio-reactors) solve all the problems of open ponds. These bioreactors can be placed inclined or suspended from the roof of a greenhouse so that they can catch more sun on a given surface. And since they are closed, no other organisms can enter. However, this method introduces a host of other issues. Bioreactors have a higher efficiency, but they also use considerably more energy.
First of all, you have to build an array of structures: the glass or polycarbonate containers themselves, the metal frames, the greenhouses. The production of all this equipment might consume less energy (and money) per square meter than the production of solar panels, but you need much more of it because algae are less efficient than solar plants. Moreover, in closed bioreactors, CO2 has to be added artificially. This is done by bubbling air through the water by means of gas pumps, a process that needs energy. Furthermore, the containers have to be emptied and cleaned regularly, they have to be sterilized, the water has to be kept at a certain temperature, and minerals have to be added continuously (because also here, just as with cellulosic ethanol, “waste” materials are being removed). All these processes demand extra energy.