We are now in a position to understand why this new high-efficiency solar electric technology provokes a fresh look at the challenge of generating hydrogen from water using sunlight. In addition to generating solar electricity at low cost, CPV systems have the potential to produce hydrogen through an electrolysis process. The generation of electrolytic hydrogen from solar energy is critically important to the world's long-term energy needs for several reasons. The feedstock (water) and supplied energy (solar) are inherently carbon free so that on a life cycle basis the total carbon emissions will be significantly less than from fossil-based options for generating hydrogen. And there is the potential to generate hydrogen near its markets, thus minimizing transportation costs. In the past, the principal criticism of photovoltaics for generating hydrogen has been the high cost of PV electricity and the inefficiencies of the conversion processes, particularly the PV process.
As we have seen, CPV systems have the potential for generating lower-cost electricity, primarily due to developing high-efficiency multijunction III-V solar cells with efficiencies above 40%. But it is the heat boost from CPV systems that can dramatically improve and enhance the electrolysis efficiency of water in a high-temperature solid-oxide electrolyzer. This heat boost—40% was measured in the 1990s by the company Solar Systems in Australia above 1100oC13,14—has been substantiated in recent theoretical analyses.15 This new pathway provides significant engineering and economic benefits for generating electrolytic hydrogen from solar energy, thereby creating opportunities for PV to contribute to future transportation markets directly with low-cost hydrogen or by producing liquid hydrogen-carrier fuels such as methanol.16
Solar-to-hydrogen conversion efficiencies of 40%, including optical losses, are attainable in the near-term (within the next few years) using high-efficiency III-V multijunction solar cells, whereas efficiencies of 50% and higher are realistic targets within 5 to 10 years. These efficiencies are dramatically higher, by roughly a factor of 3 or 4, than those of any of the other methods previously considered for generating electrolytic hydrogen from solar electricity.16 These results, based on the long-term potential for CPV systems to be mass produced at costs of less than $ 1/W, lead to hydrogen production costs comparable with the energy costs of gasoline— recognizing that 1 kg of hydrogen has the energy equivalent of one U.S. gallon of gasoline.17,18
temperature electrolysis cell
Fig. 6. Schematic of system shows sunlight reflected and focused on the receiver, with reflected infrared directed to a fiber-optics light pipe for transport to a high-temperature solid-oxide electrolysis cell. Solar electricity is sent to the same electrolysis cell, which is able to use both heat and electricity to split water.
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