Maria L. Ghirardi and Wade Amos
BioCycle May 2004, Vol. 45, No. 5, p 59
While commercially-viable methods require more research, clearly there is a path to get from here to there to generate a clean, renewable hydrogen from algae.
A few years ago, researchers at the University of California, Berkeley, and the National Renewable Energy Laboratory (NREL) discovered a physiological way to manipulate algal cultures to photoproduce hydrogen (H2) without the need to continuously remove oxygen (O2). The procedure was based on the selective effect of sulfate deprivation on photosynthetic O2 evolution without corresponding effects on other cellular functions. This demonstration renewed interest in algal H2 production as a possible future means to produce H2 gas on a commercial basis.
In order to estimate the economic potential of the algal H2-producing system and to identify key areas for research emphasis, NREL performed a cost analysis of the system. This study used H2-production data obtained from a laboratory-scale photobioreactor, assumed periods of 12 h:12 h light:dark (as opposed to the continuous illumination conditions used in the laboratory), and scaled up the system to supply enough H2 to fuel 100 cars per day (300 kg H2/d). As expected, the estimated H2 selling price of the batch system was over $720/kg H2. Compared to the current cost of H2 at $1.20/kg, the batch algal H2 system is clearly too expensive for commercial consideration. However, the analysis identified the following factors as being the major cost-drivers of the system:
a) Low H2 yield per g alga of the system;
b) Long recovery time and the cost of cycling the cultures from sulfur replete to sulfur deprived conditions; and
c) High cost of the reactor material.
The first factor listed above is a consequence of the large number of pigment molecules associated with the light-harvesting systems (antennae) in algae that provide excitation energy to the photosynthetic H2-production apparatus. These large antennae arrays are very useful if the cultures are grown at low light intensity, which allows the algae to capture and utilize a large fraction of the incident light energy to drive H2 production. However, under high intensity solar irradiation (i.e., full sunlight), the large antennae will not be able to utilize all of the absorbed light for H2 production and we convert excess light into heat and/or fluorescence. Professor Anastasios Melis, currently supported by the USDOE at UC Berkeley, is addressing this issue, by engineering algae with smaller light-harvesting antennae. Mutants with a 40 percent decrease in antennae were found to more efficiently utilize high-intensity light. This will allow researchers to pack the cells at much higher cell density in a photobioreactor with concomitant improvements in light conversion efficiency. The ultimate goal of this research is to produce mutants with ten times less pigments than the wild-type strain, which would result in a parallel increase in H2 productivity per unit of photobioreactor surface area.
In addition, the low H2 yield observed with sulfur deprived cultures is also the result of the absence of CO2 fixation by H2-producing cultures, which in turn causes the nondissipation of a proton gradient that builds up across the photosynthetic membranes. This gradient down-regulates the light induced transport of electrons from H2O to the hydrogenase enzyme and gives rise to the low rates observed for H2 production. In order to by-pass the down-regulation problem, Dr. James Lee at Oak Ridge National Laboratory is designing an artificial proton channel that, when expressed at the same time that H2 is being produced, will restore the electron transport rates to their maximum level.. This improvement is also expected to significantly increase the rates of H2 photoproduction with the sulfur deprivation algal system.
Our research group at NREL is addressing the sulfur cycling issue (factor b). The multiple cycle batch system described above can in fact be converted to a continuous chemostat like, dual reactor system that does not require expensive culture manipulation and photoproduces H2 at rates comparable to the average rates obtained with the batch system. At this point, we have produced H2 through the system for over 3 months without a shutdown.
Finally, there is the need to develop a H2-impermeable bioreactor material with a cost of about $0.25/m2, comparable to that of polyethylene (factor c).
By taking into consideration the four expected improvements described above, we determined a best case scenario for algal H2 photoproduction. In this scenario, the algal cell concentration in the reactor was increased by a factor of 10 and the specific hydrogen yield by another factor of 10. Continuous H2 photoproduction eliminated the need for a recovery period and for sample manipulation, and it was assumed that a cheap reactor material would become available. These calculations yielded a H2-selling price of $1.26/kg, well within the DOE target value and the current price of H2.
It is clear that commercially viable algal H2-photoproduction will require considerably more research effort. However, it is also clear that there is a path to get from here to there, and that algae have the potential to play a significant role in the future generation of clean, renewable hydrogen, as shown in Figure 1. Moreover, developments in other research areas, such as engineering organisms with an oxygen tolerant Fe-hydrogenase that are able to function in an aerobic atmosphere, will contribute even further to improving the economics of the system.
Maria L. Ghirardi and Wade Amos are with the National Renewable Energy Laboratory in Golden, Colorado.