8.1. CSP site selection and feasibility analysis

8.1. CSP site selection and feasibility analysis

Solar energy systems rely on the natural solar resource, which is unevenly distributed over the planet. Clearly, some locations may be considered very favorable for harvesting sunlight, while others - much less favorable for a number of reasons. Choice of the CSP development site has therefore a strategic importance for the long-term feasibility of the project. Meteorological availability of sunlight is the first, but not the only, limitation to be considered when planning a CSP site development. One should also look at the physical geography of a potential site, available land area, available infrastructure, energy market, political and social situation. All these factors, being location-specific, can have a critical influence on the success of the CSP development in the area. In essence, the project development approach to CSP is no different from other project developments, such as those involving conventional fossil fuel power plants, photovoltaic solar, wind, etc. However, CSP has its own specific features that require special knowledge when site selection and feasibility study are performed.

Any site targeted for CSP should be carefully examined with respect to the key criteria. The most important of them are listed below:

• Direct normal irradiance (DNI). This is the amount of solar radiation received by a unit area that is perpendicular (i.e., normal) to the incoming sun rays. DNI converts to energy, which, in turn, converts to monetary return from the project. It is generally believed that DNI should be at least 2000 kWh/m² per year to provide a viable energy yield [Lovegrove and Stein, 2012]; however, the threshold would depend on the local market, and should be assessed individually for each case. The accuracy of the DNI data is very important for estimating predicted energy output from the future plant. It is recommended to perform on-site DNI measurements for at least one year to characterize a complete seasonal cycle.
  

  Figure 8.1. World map of DNI.

  Credit: SolarGIS © 2014 GeoModel Solar. Refer to the SolarGIS website to get the DNI maps for specific locations.
• Available land area and topography. These conditions are different for different CSP technologies. For example, parabolic trough setup tolerates up to 2^o slope, while Fresnel reflector systems can tolerate up to 5^o slope. Solar tower systems can accommodate more variegated topography as long as the slopes properly support the arrangement of the heliostats. Typically, utility scale CSP plants requires substantial area of open land (~ 2-5 km²) free from obstructions. Typical land use by CSP plants was estimated by NREL at ~40,000 m²/MW [NREL, 2013].
• Soil and subsoil structures. The land should allow some leveling works to be performed. This requires the analysis of soil and rock conditions. Some soft soils may be unstable and undermine the alignment of optics; so, it may need to be replaced. Rocky formations underneath may bring extra costs for leveling works.
• Weather profile. Long-term data analysis of meteorological and satellite data helps to create predictions for the cloudiness in the area over the system lifetime. Also, strong tendencies for rain, water run-off, or flooding may have effect on site topography. Strong winds may have a direct impact on power generation since they create vibrations and deformations on concentrators and thus affect sunbeam focusing precision. Some sites benefit from wind-breaking barriers and other mitigation measures. Dry bulb and wet bulb temperatures are assessed to define the achievable cold-end temperatures of the power plant. This has effect on the steam turbine efficiency.
• Local water resources. Water is needed for CSP plants first of all as the cooling agent for steam turbine condenser. Both subsurface and surface water can be used. However, the locations with highest DNI are typically arid areas (e.g., deserts), where water is scarce. Associated need for water use authorization may become an additional obstacle for site approval. Other water use includes service, mirror cleaning. Water-steam cycle requires demineralized water, which would require an additional on-site water treatment plant.
• Grid connections. The power produced must be delivered to customers, so the connection to the high voltage electricity line is necessary. CSP plants of 20 MW and above capacity usually use the lines in the range of 60-400 kV. The distance to the grid should be minimal to avoid additional investments for power transmission. If auxiliary routes for power transmission need to be constructed, authorization and environmental concerns should be addressed up front.
• Proximity of roads. Roads are needed at the stage of plant construction for transport of materials and equipment to the site. Further, at the operation stage, access roads become permanent structures.

The simultaneous analysis of the multiple factors make the site assessment a complex iterative process. The main successive phases of this process are illustrated below (Lovegrove and Stein, 2012):

1. Market Analysis

2. Regional Stud and Site Identification

3. Feasibility Analysis

4. Project Qualification

5. Contract Closing

Each subsequent stage in the above scheme requires more specific information and additional expertise. This process of site characterization and selection is to some extent illustrated in the report by Stoddard et al. (2005) - the study that considered various alternatives for CSP plant development in New Mexico:

Water considerations

The water use strategy at the CSP facility is one of the keystones of the project, as it would influence many other factors and choices and may become a go/no-go tipping point for the project. This is because water shortage is being identified as a severe environmental problem in many regions, and, therefore, is subject to strict environmental regulations. So, let us take a closer look at the water requirements in CSP systems and associated technological options.

The biggest consumption point of water in CSP is cooling for the steam turbine condenser. Cooling systems can utilize both salt and fresh water, which can be taken from both surface and subsurface reservoirs. Agreement on water withdrawal should be reached prior to the project start, and that should be part of the project feasibility analysis. There are wet-cooling and dry-cooling systems. The wet cooling usually implies using a cooling tower, and that is the most efficient technology for cooling as long as water is affordable and continuously available for plant operation.

If you are not familiar with those, watch the following video (3:16) to see how they work:

Video: How Cooling Towers WorkV (3:16)

However, cooling towers use up to 85-90% of all process water. When water is in short supply, dry-cooling systems can be an alternative. Dry-cooling systems use ~10 times less water than wet cooling systems do; even truck-based supply may suffice. However, from an economic point of view, dry cooling systems are less beneficial. In dry cooling systems, air is used as heat transfer medium, and air has much lower heat transfer coefficient than water. Furthermore, the cooling effect of evaporation, which is the core mechanism of cooling in cooling towers, is not available. This results in lower efficiency of the water-steam cycle. Another drawback of the dry-cooling systems is additional power consumption by fans blowing air for cooling. For the above reasons, the wet-cooled projects have an economic advantage over dry-cooled projects. The decision involving the trade-off of water versus energy is to be made individually in each particular case based on available resources. One of the compromise options for water use is hybrid cooling tower, which combines dry cooling and wet cooling. In this technology, water is sprayed on the condenser allowing for evaporative action, but the water consumption is significantly lower compared to conventional wet cooling method.

According to water use estimation by Andrew Eilbert (Worldwatch Institute), on the average, CSP plants use only 120 Gal of water per megawatt-hour of energy. For comparison, this number is lined up against typical values for other types of power plants in Table 8.1. [Eilbert, 2010]. Visit the WorldWatch website for specific data on water use by different CSP plants in California.

Other water uses in CSP plants include:

• process (water-steam cycle)
• service (cooling rotating equipment)
• washing (mirror cleaning)

The water involved in water-steam cycle needs to be relatively pure and often needs to be de-mineralized. Requirements for water purity is specified by the turbine manufacturers. These requirements impose an additional limitation on the water sources. If raw water contains significant amounts of ions and other chemicals, a special water treatment plant may need to be added to the facility. Much of this water is recycled, and its total volume is not substantial.

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