Renewables, Solar

Central Solar Power for the Southwest

Issue 7 and Volume 112.

By Cara Libby, EPRI

As utilities in the United States seek to develop renewable energy projects, one promising option is central station solar power (CSSP). CSSP includes solar thermal technologies, such as central receiver and parabolic trough, as well several photovoltaic (PV) technologies. Solar thermal is of particular interest due to the emerging ability to integrate thermal storage into the plant design, thus improving operating flexibility and enabling the plant to operate during more hours of the day.


Testing a parabolic trough in New Mexico. Photo courtesy Sandia National Laboratory.
Click here to enlarge image

Over the next five years, close to six gigawatts of concentrating solar power capacity are planned worldwide, including several new large-scale plants in the U.S. Southwest. This trend is expected to continue as energy companies broaden their generation mix and prepare to meet state renewable portfolio standards.

The Electric Power Research Institute (EPRI) recently completed a feasibility study for the development of a 50 to 500 megawatt (MW) CSSP plant in New Mexico by mid-2011. The study was backed by six utilities, including Public Service Company of New Mexico, El Paso Electric, San Diego Gas & Electric, Southern California Edison, Tri-State Generation & Transmission Association and Xcel Energy.

The study included a comprehensive evaluation of potential plant sites, solar technologies, plant designs, economics and financial incentives, and regulatory and environmental issues. A key requirement was that the plant be installed and operational by mid-2011. As a result, only sites with access to firm transmission were considered for further study and technologies were evaluated based on demonstrated commercial readiness. The levelized cost of electricity was used as the basis for quantitatively comparing different technology and design options.

Based on a technical and environmental analysis of potential plant sites, two locations met the plant siting requirements and were selected for further consideration. Within the study’s framework, parabolic trough technology was identified as the most mature and economical technology for this application.

The results of this feasibility study provide key information for understanding the parameters involved for future deployments of solar technology in the American Southwest and other parts of the world.

Site Assessment

The study initially identified six siting regions encompassing most of central, western and southwestern New Mexico. An initial project team review of water availability and transmission resources within these regions eliminated four areas from further consideration and left two areas to be studied in more depth.

Specific sites within the two preferred siting regions were determined using U.S. Geological Survey 7.5-minute quadrangle maps. Siting specialists examined the maps and located eight potential greenfield sites. All of these sites were field-observed to verify features and collect property information. Four sites were eliminated based on results of the field reconnaissance activities’ this left four candidate sites to be studied in more detail.

A scoring system was developed specifically for this siting study to evaluate and rank the candidate sites. The criteria included direct normal irradiance/capacity factor, access to the transmission grid, natural gas located in the vicinity, potential for water availability, constructability considerations, area available for expansion, and other factors.

Ultimately two sites were selected for further consideration. One site is in central New Mexico inside the Albuquerque-Santa Fe load center. The other is in southwestern New Mexico where the solar resource is best.

A request for information (RFI) process was used to collect information on the current status of the solar industry. The following solar thermal technologies were evaluated:

Central receiver. These plants use a field array of large mirrors, called “heliostats,” that track the sun and focus its light onto a central receiver mounted on top of a tower. Commercial operation is currently being demonstrated at the 10 MW PS-10 plant in Spain.

Compact linear Fresnel reflector. This technology consists of rows of solar collectors that reflect solar radiation onto a tower-mounted linear receiver, consisting of a series of steel tubes surrounded by a reflective surface. CLFR is still early in development and has never generated standalone electricity.

Dish/engine. This system uses a parabolic reflector to focus sunlight onto a receiver located at the focal point of the dish. The sunlight heats a working fluid, which transfers the heat to a small engine used to generate electricity. Dish/engine technology has been demonstrated at small scale, but there are currently no operating commercial plants.

Parabolic trough. These plants use a field of linear parabolic collectors to redirect and concentrate sunlight onto a tube receiver. Solar parabolic trough is a commercially proven technology, with plant sizes up to 80 MW successfully operating in the United States.

Four PV technologies were also evaluated: fixed, flat-plate with crystalline silicon; fixed, flat-plate with thin-film; one-axis tracking, flat-plate with crystalline silicon; and two-axis tracking, concentrating PV.

PV technologies rely on materials that produce electric currents when exposed to light, utilizing semiconductors such as silicon. Flat-plate crystalline silicon is the most mature PV technology available today. Thin film is a less mature development that is capturing a growing share of PV markets. Although its performance trails crystalline silicon, it is easier to manufacture and has lower capital costs. Concentrating PV (CPV) systems use concentrating optics or lenses that gather sunlight and concentrate its intensity onto small PV cells, minimizing the amount of PV material needed. CPV systems can provide higher conversion efficiencies than conventional flat-plate systems. They have, however, been slow to gain a commercial foothold until just recently in central station installations. PV panels can be fixed mounted or mounted on single- or dual-axis drives. Although over 10 GW of PV is deployed worldwide, the largest PV plants are just over 20 MW.

Based on three criteria—commercial status, current levels of deployment and ability to ramp up manufacturing for a 2011 project—three technologies were selected for further evaluation: central receiver, parabolic trough and PV. The cost and performance of the selected technologies were then analyzed in more detail to compare the levelized cost of electricity (LCOE) for several technology and design options.

The solar thermal electric plant design cases that were assessed included plant capacity, thermal energy storage capacity, natural gas hybridization, and cooling designs. Sensitivities around natural gas cost and annual radiation were also investigated. Due to the simplistic and modular nature of PV, only a 50 MW plant size was evaluated.

The LCOE was calculated to compare technology and plant design options using a common set of assumptions and ultimately determine which technologies and plant designs would be most economical for a CSSP plant in New Mexico during the time frame being considered.

The study showed that securing financial incentives will be essential for any solar plant. This study identified the types of incentives that will have the greatest impact on the LCOE. Stacking all the existing financial incentives resulted in LCOE reductions of 30 percent or more.

Technology Selection

Parabolic trough technology was selected for this project based on the combination of large-scale, commercial operating experience, low cost of energy relative to other solar technology options and operating flexibility through thermal storage or hybridization.

Thermal storage and natural gas hybridization both had the effect of lowering the LCOE compared to the base case design. Adding thermal storage to the trough plant design was shown to improve the plant’s capacity factor and slightly lower the cost of energy. Similarly, hybridized solar thermal plant designs supplemented with natural gas firing also produce lower-cost electricity than the base case. Although wet cooling lowers the LCOE by about 2 percent on average, dry cooling is preferred due to water availability issues in New Mexico.

Central receiver technology was not selected due to the limited experience with large-scale, molten salt central receiver plants, the scale-up risk and the high cost to finance the first large plants.

The project team concluded that system effects, such as grid stability, and integration costs need to be better understood before a 50 MW or larger PV project can be prudently developed. The LCOE for each of the four PV technologies was determined to be significantly higher than parabolic trough and central receiver technologies.

An assessment was conducted to determine the major environmental and regulatory issues associated with project development at each of the two candidate sites. A 15-month regulatory plan was developed to guide the permit application process. Overall, the assessment found that most project impacts will be acceptably minor or approved with mitigation measures.

Parabolic trough was recommended for this specific project based on the technical and economic feasibility of developing a 50 to 500 MW plant in the mid-2011 timeframe. Two viable siting areas were investigated and will be considered for development. The results of this project will be beneficial to any energy company or project developer considering a central solar plant project. The cost data for a range of technology options and design configurations can be used in planning and siting solar plants in other locations.

Author: Cara Libby is a Project Manager in EPRI’s Generation Sector. She works in the Office of Innovation with a focus on emerging renewable energy technologies. Before joining EPRI in 2006, Ms. Libby worked as a Project Leader in the Energy Systems Laboratory at GE Global Research Center. Ms. Libby is a mechanical engineer with a B.S. degree from Johns Hopkins University and an M.S. degree from Stanford University. A free public summary report is available on EPRI’s website at www.epri.com/seig.