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Chapter 3

Dish systems include an ideal optical reflector and therefore are suitable for applications requiring high temperatures. Dish reflectors are paraboloid and concentrate the solar irradiation onto a receiver mounted at the focal point, with the receiver moving with the dish. Dishes have been used to power Stirling engines at 900°C, and also for steam genera- tion. There is now significant operational experience with dish/Stirling engine systems, and commercial rollout is planned. In 2010, the capac- ity of each Stirling engine is small—on the order of 10 to 25 kWelectric. The largest solar dishes have a 485-m2 aperture and are in research facilities or demonstration plants.

In thermal storage, the heat from the solar field is stored prior to reaching the turbine. Thermal storage takes the form of sensible or latent heat storage (Gil et al., 2010; Medrano et al., 2010). The solar field needs to be oversized so that enough heat can be supplied to both operate the turbine during the day and, in parallel, charge the thermal storage. The term ‘solar multiple’ refers to the total solar field area installed divided by the solar field area needed to operate the tur- bine at design point without storage. Thermal storage for CSP systems needs to be at a temperature higher than that needed for the work- ing fluid of the turbine. As such, system temperatures are generally between 400°C and 600°C, with the lower end for troughs and the higher end for towers. Allowable temperatures are also dictated by the limits of the media available. Examples of storage media include molten salt (presently comprising separate hot and cold tanks), steam accumulators (for short-term storage only), solid ceramic particles, high-temperature phase-change materials, graphite, and high-tem- perature concrete. The heat can then be drawn from the storage to generate steam for a turbine, as and when needed. Another type of storage associated with high-temperature CSP is thermochemical stor- age, where solar energy is stored chemically. This is discussed more fully in Sections 3.3.5 and 3.7.5.

Thermal energy storage integrated into a system is an important attri- bute of CSP. Until recently, this has been primarily for operational purposes, providing 30 minutes to 1 hour of full-load storage. This eases the impact of thermal transients such as clouds on the plant, assists start-up and shut-down, and provides benefits to the grid. Trough plants are now designed for 6 to 7.5 hours of storage, which is enough to allow operation well into the evening when peak demand can occur and tariffs are high. Trough plants in Spain are now operat- ing with molten-salt storage. In the USA, Abengoa Solar’s 280-MW Solana trough project, planned to be operational by 2013, intends to integrate six hours of thermal storage. Towers, with their higher temperatures, can charge and store molten salt more efficiently. Gemasolar, a 17-MWe solar tower project under construction in Spain, is designed for 15 hours of storage, giving a 75% annual capacity fac- tor (Arce et al., 2011).

Thermal storage is a means of providing dispatchability. Hybridization with non-renewable fuels is another way in which CSP can be designed to be dispatchable. Although the back-up fuel itself may

Direct Solar Energy

not be renewable (unless it is biomass-derived), it provides significant operational benefits for the turbine and improves solar yield.

CSP applications range from small distributed systems of tens of kW to large centralized power stations of hundreds of MW.

Stirling and Brayton cycle generation in CSP can be installed in a wide range from small distributed systems to clusters forming medium- to large-capacity power stations. The dish/Stirling technology has been under development for many years, with advances in dish struc- tures, high-temperature receivers, use of hydrogen as the circulating working fluid, as well as some experiments with liquid metals and improvements in Stirling engines—all bringing the technology closer to commercial deployment. Although the individual unit size may only be of the order of tens of kWe, power stations having a large capacity of up to 800 MWe have been proposed by aggregating many modules. Because each dish represents a stand-alone electricity generator, from the perspective of distributed generation there is great flexibility in the capacity and rate at which units are installed. However, the dish technology is less likely to integrate thermal storage.

An alternative to the Stirling engine is the Brayton cycle, as used by gas turbines. The attraction of these engines for CSP is that they are already in significant production, being used for distributed generation fired with landfill gas or natural gas. In the solarized version, the air is instead heated by concentrated solar irradiance from a tower or dish reflector. It is also possible to integrate with a biogas or natural gas combustor to back up the solar. Several developments are currently underway based on solar tower and micro-turbine combinations.

Centralized CSP benefits from the economies of scale offered by large- scale plants. Based on conventional steam and gas turbine cycles, much of the technological know-how of large power station design and practice is already in place. However, although larger capacity has significant cost benefits, it has also tended to be an inhibitor until recently because of the much larger investment commitment required from investors. In addition, larger power stations require strong infra- structural support, and new or augmented transmission capacity may be needed.

The earliest commercial CSP plants were the 354 MW of Solar Electric Generating Stations in California—deployed between 1985 and 1991—that continue to operate commercially today. As a result of the positive experiences and lessons learned from these early plants, the trough systems tend to be the technology most often applied today as the CSP industry grows. In Spain, regulations to date have mandated that the largest capacity unit that can be installed is 50 MWe to help stimulate industry competition. In the USA, this limitation does not exist, and proposals are in place for much larger plants—280 MWe in the case of troughs and 400-MWe plants (made up of four modules) based on towers. There are presently two operational solar towers of 10 and 20 MWe, and all tower developers plan to increase capacity in


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