Could the energy cost of moving water sink the burgeoning algae industry?
C&EN recently checked in with a number of leading algae-growing firms to learn more about their current plans for profiting from the prolific green slime. Though eventually many hope to make money in the large market for biofuels, most firms say that other products like chemicals and high-protein fish food will go first.
Building large-scale algae-growing systems is still too expensive to make fuels profitable. The key to bringing down costs is in the engineering of the infrastructure. A recent study by researchers at the University of Texas at Austin looked at the energy costs of moving water into and around algae-growing systems (Environ. Sci. Technol., 2011, 45 (13), pp 5861–5868).Researchers Cynthia Murphy and David Allen presented a startling conclusion:
Energy output in the form of algal biodiesel and the total energy content of algal biomass are compared to energy inputs required for water management. The analysis indicates that, for current technologies, energy required for water management alone is approximately seven times greater than energy output in the form of biodiesel and more than double that contained within the entire algal biomass.
Seven times greater? Ouch. Like any model, this one began with a host of assumptions. Importantly, the model did not assume that algae farmers would be using fresh water, but did assume algae would be grown in open ponds (except for the inoculation vessels). In fact, the main problem is not the water itself, but the need to move it around from place to place.
First water from various sources (saline, fresh, reclaimed from the facility) needs to be obtained and pumped into the inoculation area and the algae pond. More water would be added to compensate for algae removed, evaporation and other “leaks” from the system. Evaporation would concentrate salts in the pond, and may require compensating amounts of fresh water for “blow down.” Cleaning after each growing season would require removing the water and replacing it.
In addition, energy would be required to remove water from the harvested algae, and then to return that reclaimed water to the system. The researchers also included in the model the embodied energy of the plastics used to contain the algae in ponds (and the lifespan of the plastics).
There is no way, of course, to compare the assumptions in the model to any particular firm’s proprietary growing system. But I did pose the question of water energy to the companies I spoke with that use open ponds.
Cellana’s CEO Martin Sabarsky said, “Water is a big issue. It’s an issue for biofuels generally. You have to deal with it on the backend too. We’ve developed and are continuing to optimize cost effective technology to handle water issues at the back end including dewatering. “
And Sapphire President Cynthia Warner commented, “It is true that to optimize the process and get costs down, you have to minimize water movement, maximize efficiency. Using sophisticated equipment is key.”
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