this area is expensive. Some additional centers for such research in academia with industrial alliances could be beneficial. It will be necessary to collect multifunctional teams from different engineering disciplines for such studies.

In the midterm to long term, organic-polymer-based solar cells hold promise for mass production at low cost. They have an appeal for being cast as thin films at very high speeds using known polymer film casting techniques. Currently, the efficiency of such a system is quite low (in the neighborhood of 3 to 4 percent or lower), and stability in sunlight is poor. However, owing to the tremendous development in conducting polymers and other electronics-related applications, it is anticipated that research in such an area has a high potential for success.

Similarly, the search for a stable dye material and better electrolyte material in dye-sensitized cells (Grätzel cells) has a potential to lead to lower-cost solar cells. There is a need to increase the stable efficiency of such cells; a stable efficiency of about 10 percent could be quite useful.

In the long run, the success of directly splitting water molecules by using photons is quite attractive. Research in this area could be very fruitful.

Department of Energy Programs for Solar Energy to Hydrogen

The current DOE target for photoelectrochemical hydrogen production in 2015 is $5/kg H2 at the plant gate. Even if this target is met, solar hydrogen is unlikely to be competitive. Therefore, beyond 2015 a much more aggressive cost target for hydrogen production by photoelectrochemical methods is needed.

Since photoelectrochemical hydrogen production is in an embryonic stage, a parallel effort to reduce the cost of electricity production from PV modules must be made. A substantial reduction in PV module cost (lower than $1/Wp), coupled with a similar reduction in electrolyzer costs (much below $125/kW at a reasonably high efficiency of about 70 percent based on lower heating value), could provide hydrogen at reasonable cost. In the long run, considering the environmental issues associated with fossil fuels and considering the limitless supply of solar energy, this has a potential to be quite attractive. This option will be especially attractive if advances in battery technology are unable to substantially increase the electricity storage density (based on mass of battery) and greatly reduce the cost of batteries. Therefore, it is recommended that thin-film technologies and other emerging PV technologies that hold the promise for cost reduction be aggressively pursued. As stated earlier, it means that more efficient and robust methods for thin-film coating must be developed. Organic-polymer-based solar cells should also be funded. There is tremendous development underway in conducting polymers for light-emitting diodes and other display technologies. The potential of these materials for solar cell PVs must be actively explored.

Summary

All of the current methods and the projected technologies of producing hydrogen from solar energy are much more expensive (greater than a factor of 3) when compared with hydrogen production from coal or natural gas plants. This is due partly to the lower annual utilization factor of about 20 percent (as compared with say, wind, at 30 to 40 percent). This high cost puts enormous pressure on the need to reduce the cost of a solar energy recovery device. While an expected future installed module cost of about $1/Wp is very attractive for electricity generation and deserves a strong research effort in its own right, this cost fails to provide hydrogen at a competitive value. The raw material cost for crystalline silicon-wafer-based technologies is a large fraction of the $1/Wp value and is therefore less likely to provide hydrogen economically. On the other hand, thin-film technologies do not use much raw material in thin films themselves but require tremendous progress in the deposition technology. There is a need for a robust deposition method that would have a potential to reduce cost much below $1/Wp. Emerging polymer-based technologies have a potential to provide low-cost devices to harness solar energy. It is apparent that there is no one method of harnessing solar energy that is clearly preferable. However, it appears possible that new concepts may emerge that would be competitive. The benefits of such developments would be very substantial.

In the future, as the cost of the fuel cell approaches $50 per kilowatt, the cost of an electrolytic cell to electrolyze water is also expected to approach a low number (about $125/kW). With such low-cost electrolyzer units, the electricity cost of about $0.02 to $0.03/kWh is expected to result in a competitive hydrogen cost. It is also estimated that for a photoelectrochemical method to compete, its cost must approach $0.04 to $0.05/kWh. The order-of-magnitude reductions in cost for both hydrogen processes are similar.



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