SunsLab SnapShot Solar Two Demonstrates Clean Power for the Future The 10-megawatt Solar Two power tower pilot plant near Barstow, California, successfully completed operations in April 1999, having met essentially all of its objectives. It demon- strated the ability to collect and store solar energy efficiently and to generate electricity when needed by the utility and its customers. Based on the success of Solar Two, U.S. industry is actively planning the first commercial implementation of this technology. Solar Two represents a new generation of the initial high cost and risk of commer- solar energy technology, capable of producing cializing a new technology. The proposed clean, cost-effective, dispatchable electric Spanish project, ÒSolar Tres,Ó would use all power on a very large scale, without harmful the proven molten-salt technology of Solar pollutants or carbon emissions. Solar Two was Two, scaled up by a factor of three. Boeing, conceived and built on the site of its predeces- Bechtel, and their Spanish partners are currently sor, Solar One, by a consortium of U.S. investigating design options, preparing cost utilities and industry and the Department of estimates, and negotiating contract opportunities Energy (DOE). Over its three years of for a project start in 2000. operation, Solar Two achieved its overall goal While Solar Two was a demonstration of demonstrating advanced molten-salt power project, Solar Tres will be financed and built by tower technology developed over the past industry (with no DOE cost-share) as a money- decade at a scale sufficient to allow follow-on making venture. U.S. industry views this commercialization of the technology. Plant project as a springboard to near-term opportuni- operations successfully proved that solar energy ties for numerous full-scale (greater than 50 could be collected efficiently over a broad megawatts) commercial power tower plants in range of operating conditions and that the low- Spain, in other worldwide markets, and eventu- cost energy storage system operated reliably and ally in the U.S. Southwest. This dispatchable, efficiently. This unique storage capability utility-scale solar power could be a major allowed solar energy to be collected when the source of clean energy worldwide, offsetting as sun was shining and high-value, dispatchable much as 4 million metric tonnes of carbon electric power to be generated at night or when- equivalent (MMTCE) over the next 10 years. ever demanded by the utility, even when the sun was not shining. Still, much remains to be done. The risks of a large, capital-intensive plant demonstrating The next step... new technology remain high. With all the In addition to validating the design and lessons learned from Solar Two, DOEÕs technical characteristics of molten-salt receiver Concentrating Solar Power Program can play a and storage technology, Solar Two has also vital role in providing the solar experience and been successful in fostering commercial expertise of SunsLab (Sandia National interest in power towers. Two of the projectÕs Laboratories and the National Renewable key industry partners, the Boeing Company Energy Laboratory working together) to the and Bechtel Corporation, are currently pur- Solar Tres consortium. An active partnership suing commercial solar power tower plant with U.S. industry will be critically important opportunities in Spain. Solar energy premiums in assuring the success of this exciting renew- and other incentives under review in Spain able energy option. Among other things, Solar Concentrating create an attractive market opportunity, provid- Two demonstrated the success that such a Solar Power ing the economic incentives needed to mitigate DOE/industry partnership can bring. Program In the beginning... The major challenges facing researchers in making the The Solar Two story really began when the United concept work were first to develop very large mirrors that States became interested in alternatives to imported oil in could accurately focus sunlight on the receiver as the sun the late 1970s as the global political situation threatened moves across the sky. The receiver had to be able to han- our energy supply. At that time, the DOE asked its national dle the high solar fluxes (up to 1000 times the intensity of laboratories to explore sources of renewable energy, normal sunlight) and temperatures up to 565¼C. The heat including innovative ideas for harvesting the power of the then had to be stored to generate steam when power was sun, the wind, and the heat of the earth. In the area of needed. Testing at the NSTTF proved the concept was solar energy, the power tower concept was developed. possible. For Solar One, water was chosen as the heat-transport Solar One is Born medium. The idea was to pump water up to the Engineers at Sandia National Laboratories in receiver to make steam, which was then brought Albuquerque, New Mexico, and Livermore, California, down to the base of the tower to generate studied this power tower concept and determined that the electricity or to thermally charge an enormous system held promise to generate electricity on a scale holding tank containing oil, rock, and sand. large enough to power entire cities. They proposed a test facility to investigate the concept and qualify components By 1982, Solar One was ready to go into oper- and systems for larger-scale evaluation at a pilot plant. As ationÑwith its field of 1,818 heliostats encircling a result, the National Solar Thermal Test Facility (NSTTF) and facing a tower rising 90 meters into the air and was built at Sandia in Albuquerque in 1976, followed soon rated to produce 10 megawatts of power. The plant after by the 10-megawatt Solar One pilot plant near operated successfully until 1988, as planned, proving Barstow, California. that power towers can produce electricity efficiently and reliably, operating with 96% availability during its final year. It generated more than 38,000 megawatt-hours during its lifetime and consistently ran at its 10-megawatt rating. Solar One did have deficiencies, however, including inefficiencies in its storage system and difficulties operating under intermittent cloud cover. Solar One Begets Solar Two Like the legendary phoenix, the idea for Solar Two (tower and receiver at right) arose from Solar One. This time, it was a matter of making improvements to a known and proven conceptÑ and retrofitting the original plant to incorporate the Storage is the key to molten-salt power towers. These two improvements. Although the water/steam receiver tanks provide three full hours of storage at Solar Two. concept worked, researchers knew that certain salts when melted could store the heat much more efficiently and cost-effectively. Thus, use of a large storage system would not only increase the value of The National Solar Thermal Test Facility produced power, but would actually decrease the At the NSTTF, built at Sandia National Laboratories overall cost as well. Much of the equipment at in Albuquerque in 1976, the 63-meter tall tower has Solar One, including the large heliostat field, tower, 222 computer-controlled heliostats that can direct the and turbine, was reused, but a new receiver, storage sun into any of four test bays to produce a total system, control system, and connecting piping were thermal capacity of 5 megawatts. The facility has been required. The DOE and a consortium of utility and used for six major industry partners (see back page) shared the costs power tower of the Solar Two effort. development The engineering, manufacturing, and construction programs, including of Solar Two lasted from 1992 into 1995, with development and initial startup and testing beginning in 1996. Solar qualification testing Two operated from April 1996 to April 1999, of the molten-salt when operations were completed, after having technology used at proven the promise of the concept. Solar Two. Solar Two Performance... Molten-Salt Power Tower The Bottom Line Technology Solar TwoÕs overall goal was to demonstrate advanced In a molten-salt power tower, sun-tracking heliostats can molten-salt power tower technology. To this end, its concentrate solar energy up to 1000 times onto a central, design objectives, developed early in the project, were to: tower-mounted receiver. Molten nitrate salt, which is a ¥ Simulate the design, construction, and operation of the clear liquid with properties like water at temperatures above its 240¡C melting point, is pumped from a large storage first commercial plants. tank to the receiver, where it is heated in tubes to temper- ¥ Validate the technical characteristics of a molten-salt atures of 565¡C. The salt is then returned to a second large plant. storage tank, where it remains until needed by the utility for ¥ Improve the accuracy of economic projections for com- power generation. At that time, the salt is pumped through mercial projects by increasing the database of system per- a steam generator to produce the steam to power a conven- tional, high-efficiency steam turbine to produce electricity. formance, and capital, operating, and maintenance The salt at 285¡C then returns to the first storage tank to be costs. used in the cycle again. ¥ Collect, evaluate, and distribute information to foster interest in commercialization of power towers. ¥ Stimulate interest in forming a commercial consortium. All of these objectives were successfully met during the project. Over its three-year operating lifetime, daily operation of Solar Two became relatively routine, with various performance records broken on a fairly regular basis. Key attributes of solar power tower plants, including efficient collection and storage of the sunÕs energy and dispatch of electricity as needed by the utility grid, were confirmed. Specific technical accomplishments included demonstra- The Dispatchability Advantage tion of: The major advantage of Solar Two is that the plant can produce power when the local utility needs it mostÑ ¥ Dispatchability: Using its unique and extremely during peak demand periods in the afternoon and early efficient thermal storage system, Solar Two delivered evening, for example.