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Chapter 3


Concentrating solar power electricity generation

CSP is a proven technology at the utility scale. The longevity of com- ponents has been established over two decades, O&M aspects are understood, and there is enough operational experience to have enabled O&M cost-reduction studies not only to recommend, but also to test, those improvements. In addition, field experience has been fed back to industry and research institutes and has led to improved components and more advanced processes. Importantly, there is now substantial experience that allows researchers and developers to better under- stand the limits of performance, the likely potential for cost reduction, or both. Studies (Sargent and Lundy LLC Consulting Group, 2003) have concluded that cost reductions will come from technology improvement, economies of scale and mass production. Other innovations related to power cycles and collectors are discussed below.

CSP is a technology driven largely by thermodynamics. Thus, the thermal energy conversion cycle plays a critical role in determining overall per- formance and cost. In general, thermodynamic cycles with higher temperatures will perform more efficiently. Of course, the solar collec- tors that provide the higher-temperature thermal energy to the process must be able to perform efficiently at these higher temperatures, and today, considerable R&D attention is on increasing the operating tem- perature of CSP systems. Although CSP works with turbine cycles used by the fossil-fuel industry, there are opportunities to refine turbines such that they can better accommodate the duties associated with thermal cycling invoked by solar inputs.

Considerable development is taking place to optimize the linkage between solar collectors and higher-temperature thermodynamic cycles. The most commonly used power block to date is the steam tur- bine (Rankine cycle). The steam turbine is most efficient and most cost effective in large capacities. Present trough plants using oil as the heat transfer fluid limit steam turbine temperatures to 370°C and turbine cycle efficiencies to around 37%, leading to design-point solar-to-electric efficiencies of the order of 18% and annual average efficiency of 14%. To increase efficiency, alternatives to the use of oil as the heat transfer fluid—such as producing steam directly in the receiver or using molten salts—are being developed for troughs.

These fluids and others are already preferred for central receivers. Central receivers and dishes are capable of reaching the upper tem- perature limits of these fluids (around 600°C for present molten salts) for advanced steam turbine cycles, whether subcritical or supercritical, and they can also provide the temperatures needed for higher-efficiency cycles such as gas turbines (Brayton cycle) and Stirling engines. Such high-temperature cycles have the capacity to boost design-point solar- to-electricity efficiency to 35% and annual average efficiency to 25%. The penalty for dry cooling is also reduced, and at higher temperatures thermal storage is more efficient.

The collector is the single largest area for potential cost reduction in CSP plants. For CSP collectors, the objective is to lower their cost while

Direct Solar Energy

achieving the higher optical efficiency necessary for powering higher- temperature cycles. Trough technology will benefit from continuing advances in solar-selective surfaces, and central receivers and dishes will benefit from improved receiver/absorber design that allows collec- tion of very high solar fluxes. Linear Fresnel is attractive in part because the inverted-cavity design can reduce some of the issues associated with the heat collection elements of troughs, although with reduced annual optical performance.

Improved overall efficiency yields a corresponding decrease in the area of mirrors needed in the field, and thus, lower collector cost and lower O&M cost. Investment cost reduction is expected to come primarily from the benefits of mass production of key components that are specific to the solar industry, and from economies of scale as the fixed price associ- ated with manufacturing tooling and installation is spread over larger and larger capacities. In addition, the benefits of ‘learning by doing’ can- not be overestimated. A more detailed assessment of future technology improvements that would benefit CSP can be found in ECOSTAR (2005), a European project report edited by the German Aerospace Center.


Solar fuel production

The ability to store solar energy in the form of a fuel may be desirable not only for the transportation industry, but also for high-efficiency electric- ity generation using today’s combined cycles, improved combined cycles using advances in gas turbines, and fuel cells. In addition, solar fuels offer a form of storage for solar electricity generation.

Future solar fuel processes will benefit from the continuing development of high-temperature solar collectors, but also from other fields of science such as electrochemistry and biochemistry. Many researchers consider hydrogen to offer the most attraction for the future, although intermedi- ate and transitional approaches are also being developed. Hydrogen is considered in this section, with other solar fuels having been covered in previous sections.

Future technology innovation for solar electrolysis is the photoelectro- chemical (PEC) cell, which converts solar irradiance into chemical energy such as H2. A PEC cell is fabricated using an electrode that absorbs the solar light, two catalytic films, and a membrane separating H2 and oxygen (O2). Semiconductor material can be used as a solar light-absorbing anode in PEC cells (Bolton, 1996; Park and Holt, 2010).

Promising thermochemical processes for future ‘clean’ hydrogen mass production encompass the hybrid-sulphur cycle and metal oxide-based cycles.The hybrid-sulphur cycle is a two-step water-splitting process using an electrochemical, instead of thermochemical, reaction for one of the two steps. In this process, sulphur dioxide depolarizes the anode of the electrolyzer, which results in a significant decrease in the reversible cell potential—and, therefore, the electric power requirement for the elec- trochemical reaction step. A number of solar reactors applicable to solar thermochemical metal oxide-based cycles have been developed, including


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