gen produced. For example, in the committee’s analysis of costs discussed in Chapter 5 (summarized in Table G-9), for the future optimistic case the cost of hydrogen is calculated to be $6.18/kg (Dist PV Ele-F). For this case, the cost of the installed PV panels, including all of the general facilities, is estimated to be $1.011/Wp, and is used in conjunction with an electrolyzer that is assumed to take advantage of all of the advancements made in the fuel cell. The PV part is responsible for $4.64/kg, and the electrolyzer is $1.54/kg. Compared with this, the cost of hydrogen from a future central coal plant at the dispensing station is estimated to be $1.63/ kg with carbon tax (CS Coal-F). This cost implies that for a PV-electrolyzer to compete in the future with a coal plant, either the cost of PV modules must be reduced by an order of magnitude or the electrolyzer cost must drop substantially from $125/kW. A factor contributing to this is the low utilization of the electrolyzer capital. It has been proposed to use electricity from the grid to run the electrolyzer when solar electricity is unavailable. This use increases the time on-stream for the electrolyzer; however, in the long term, for solar to play a dominant role in the hydrogen economy, it cannot rely on power from the grid to supplement equipment utilization. Therefore, while electricity at $0.098/kWh from a PV module can be quite attractive for distributive applications where electricity is directly used, its use in conjunction with electrolysis to produce hydrogen is certainly not competitive with the projected cost of hydrogen from coal.

Direct Production

Research is being done to create photoelectrochemical cells for the direct production of hydrogen (Grätzel, 2001).31 In this method, light is converted to electrical and chemical energy. The technical challenge stems from the fact that energy from two photons is needed to split one water molecule. A solid inorganic oxide electrode is used to absorb photons and provide oxygen and electrons. The electrons flow through an external circuit to a metal electrode, and hydrogen is liberated at this electrode. The candidate inorganic oxides are SrTiO3, KTaO3, TiO2, SnO2, and Fe2O3. When successful, such a method holds promise of directly providing low-cost hydrogen from solar energy.

Regarding production costs, it seems that a photoelectrochemical device in which all of the functions of photon absorption and water splitting are combined in the same equipment may have better potential for hydrogen production at reasonable costs. However, it is instructive to do a quick “back of the envelope” analysis for the acceptable cost by such a system. It is assumed that cost per peak watt for a photoelectrochemical device is the same as that for the possible future PV modules (see Table E-48 of Appendix E.) It is further assumed that this energy is recovered as hydrogen rather than as electricity. Therefore, a recovery of 39.4 kWh translates into a kilogram of hydrogen. This implies that 4729 kWe worth of solar plant in the Dist PV-F spreadsheet will produce about 576 kg/day of hydrogen (assuming an annual capacity factor of 20 percent). At the total cost of $0.813 million per year, this gives $3.87/kg of hydrogen! This cost is still too high when compared with that of hydrogen from coal or natural gas plants. It implies that photoelectrochemical devices should recover hydrogen at an energy equivalent of $0.4 to $0.5/Wp. This cost challenge is similar to that for electricity production from the solar cells.

Advantages and Disadvantages of Solar Energy

Solar energy holds the promise of being inexhaustible. If harnessed, it can meet all of the energy needed in the foreseeable future. It is clean and environmentally friendly. It converts solar energy into hydrogen without the emission of any greenhouse gas. Because of its distributed nature of power production, it contributes to the national security.

There are certain challenges associated with the use of solar energy. The intermittent nature of sunshine, on both a daily and a seasonal basis, presents a number of challenges. A backup system, or a storage system for electricity/hydrogen, is needed for the periods when sunshine is not available and power demand exists. Furthermore, this intermittent availability means that four to six times more solar modules have to be installed than the peak watt rating would dictate. This intermittency also implies that a significant decrease in the module cost is required. Another challenge is to ensure that no toxic materials are discharged during the fabrication of solar cells and over the complete life cycle of the cell. Such questions have been raised in the context of cadmium-containing solar cells, and public perception in such cases will play a key role.

Challenges and Research and Development Needs

Large-scale use of solar energy for hydrogen economy will require research and development efforts on multiple fronts. In the short term, there is a need to reduce the cost of thin-film solar cells. This reduction will require the development of silicon deposition techniques that are robust and provide high throughput rates. New deposition techniques at moderate pressures with microcrystalline silicon structures for higher efficiencies are needed. Inline detection and control and the development of better roll-to-roll coating processes can lead to reductions in the manufacturing costs. Increased automation will also contribute to the decreased cost. Issues related to a large decrease in efficiency from small laboratory samples to the module level should be addressed. In the short run, thin-film deposition methods can potentially gain from a fresh look at the overall process from the laboratory scale to the manufacturing scale. The research in

31  

Nathan Lewis, California Institute of Technology, “Hydrogen Production from Solar Energy,” presentation to the committee, April 25, 2003.



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