SolarReserve’s first-generation solar power plant is a welcome sign of green progress. Once the Crescent Dunes Solar Energy Project is completed in 2013, it will generate roughly 480,000 megawatt hours per year of clean, renewable electricity to power 75,000 homes during peak electricity periods.
The company’s unique, concentrating solar-power technology and innovative energy storage was developed by the same Rocketdyne engineers and scientists who designed Apollo rockets, Space Shuttle engines, and the solar power system for the International Space Station.
Tonopah Solar Energy, a subsidiary of SolarReserve, manages the project, which is located on 1,600 acres northwest of Tonopah, a historical silver mining town in Nevada. The site selection was based on key elements for optimum solar energy—hours of direct sunlight, altitude (for more intense sunlight), and the strength of direct normal irradiance (the amount of solar radiation from the direction of the sun).
The power plant design includes a tall receiver tower and power block positioned centrally in a large circular field of large flat mirrors known as heliostats. The 100-ft receiver is mounted upon the 553-ft concrete tower. Tower height is essential for plant efficiency and ensures the heliostat array can concentrate sunlight onto the receiver.
Comprised of thousands of tracking mirrors in a two square-mile area, the solar collection field operates whenever there is ample solar resource to collect energy. SolarReserve’s propriety motion-control software lies on a central computer that sends angle location data to the heliostat field, and the mirrors move simultaneously as the sun travels across the sky. The energy is stored and delivered to the grid anytime, even after sundown. The 110-megawatt plant will utilize an ordinary steam turbine generator to produce electricity, but integrates a sophisticated hybrid cooling system that uses less than 600 acre-feet per year of water, a scarce commodity in the Southwest.
The power tower receiver glows as it soaks up the sun during the day, but remains dormant at night. Its high heat flux hardware is a unique blend of liquid rocket engine heat-transfer technology and molten salt handling expertise. Inside the receiver, concentrated sunlight heats molten salt to more than 1,000°F. The liquid then flows into an insulated storage tank maintaining 98-percent thermal efficiency. It is eventually pumped to the generator to produce electricity. This process is similar to a standard coal-fired power plant, but it is fueled by clean and free solar energy.
The solar power plant utilizes components primarily manufactured in the United States, as opposed to competing technologies using mostly imported parts and assemblies. Bar a few one-of-a-kind components, the plant employs readily available materials (mirrors) and established technologies (steam generators and turbines). The inexpensive molten salt is made from an environmentally friendly mixture of sodium and potassium nitrate, the same ingredients used in garden fertilizer. This configuration of materials and equipment will enable SolarReserve to provide electricity at or below prices from traditional sources such as coal or natural gas.
Project manager Gary Raczka is responsible for the design, manufacture, and installation of the solar collector system in the field, as well as the software that drives the mirrors to focus on the receiver. Heliostat design is a large team effort, and his staff includes mechanical engineers, software developers, and a growing engineering workforce to support not only the heliostat production, but also the entire solar plant.
For the build aspect of the project, the team works closely with contract manufacturers well versed in handling large mechanical components. To get a feel for the size of these heliostat assemblies, the mirror surface is approximately 28 ft × 24 ft. The pedestal is made from large-diameter piping that is 11 ft to 16 ft tall. Other metal components and trusses supporting the mirror structure range from 28 ft to 30 ft long.
The company cut its teeth on small test facilities, and then went on to create three different designs and sizes of heliostats supported by various structural configurations. Each SolarReserve plant is roughly the same in terms of tower size, general field layout, and overall square meters of glass in the field. In Tonopah, the baseline 62.5 square-meter heliostat will be used, which translates to 17,608 heliostats in the collection field. The project has an ambitious plan to install 70 heliostats per day until completion.
For optimum performance, the solar plant design calls for an overall beam quality and accuracy of less than 1.5 milliradian (mRad) for each facet and the entire heliostat field. This high-precision specification dictated the early need for dimensional control and verification. Raczka found specialized expertise at Hexagon Metrology Services in North Kingstown, Rhode Island, to verify designs and build confidence into all aspects of manufacture and assembly. Rina Molari, a seasoned metrologist from Hexagon, was excited to be a part of this groundbreaking endeavor.
Because the project entailed working with sizeable parts in formidable outdoor settings, Molari employed a portable Leica AT901 laser tracker for its ability to handle numerous quality assurance tasks. This laser tracking system is primarily used for aerospace and other in-place measurement applications due to its long-range measurement volume of 525 ft when used with a standard corner cube. Based on Leica’s Absolute Interferometer technology, the portable coordinate measuring machine (CMM) maintains precision measurement in all operating conditions, with multiple, built-in redundancies to ensure high accuracies.
High beam quality is imperative when strong focusing of a beam is required, so targets for tolerances and beam quality were built into the initial solar plant design. Two aspects of beam quality are pointing accuracy and slope error. Pointing accuracy is the focus of the beam’s center, and its proximity to a point is determined by the SolarReserve control system. Slope error is determined by comparing a glass surface to the original design intent. During the early stages of the product development, the laser tracker was used to create the desired mirror flatness by using the real-time feedback capability of the tracker and its software. Deviations were found in the glass, and the company refined its manufacturing processes to produce the desired end product.
During the production phase, Molari verifies that the mirrors are tilted at the correct angle, and rechecks the flatness of each 4 ft × 4 ft facet. “Assembly has to be conducted on site due to the size of the components,” explains Molari. “The flattening of the mirrors has to be accurate to ±0.002 in. These tolerances make it impossible to use tools or jigs to build this light structure rigid enough to hold the mirrors. The mechanics of this project are a marvel to me. From the beginning, we have assisted in verifying integrity of their design-to-assembly process.
“The Leica AT901 is my go-to metrology tool for this type of challenging work,” says Molari. “I know I can measure and achieve a solution for any dimensional control issue such as inspecting the cant of each mirror, analyzing those data, then comparing the measurement with a neighboring facet to ensure we induce the correct angle. The work environment was taxing in these remote sites. The laser tracker was on the ground, and I had to use a scissor lift to access the 10-ft tall assembly. Wind, sand, and very high temperatures were often present during these inspection jobs. The robust construction and thermal stability of the tracker renders it very reliable in these circumstances.”
The laser tracker integrates a unique PowerLock feature that proved to be indispensable for this project. This active vision technology automatically locks onto any moving reflective target, without user intervention, as soon as it is within view of the sensor. If a line of sight is obstructed by a co-worker or a part of an assembly, the Absolute Interferometer instantly reestablishes a broken laser beam and starts measuring the moving target.
“You can focus on measuring a part without worrying about a broken laser beam,” says Molari. “When working outdoors, PowerLock is even more helpful as it is extremely difficult to see a laser spot in the sunshine or in the shade. There were many obstructions during assembly as you are working through trusses and mirror structures. When there was no direct line of site to a point we wanted to measure, I used the wireless Leica T-Probe to get at those hard-to-reach areas. You must have an agile tool set in the field.”
The Crescent Dunes Solar project has the potential to kick the alternative energy door wide open. At the construction site, there is a sense of excitement as heliostat arrays are erected one by one. Overall, the project’s benefits are palpable from reducing reliance on fossil fuels and lowering greenhouse gas emissions, to a stable, flexible electricity to improve grid reliability. Other bonuses for the local economy are new green jobs, and additional tax revenue. On a recent visit to Tonopah, town representatives awarded SolarReserve’s staff with commemorative “Mining the Sky” coffee cups to show their support for the area’s largest asset.
Raczka also visualizes the possibilities. “Solar has been around a long time, but what differentiates our product is its improved efficiency with very high quality optics and thermal storage,” says Raczka. “Some of our utility customers want power 24/7, as opposed to another that only wants help through the peak loads. Depending on their power requirements, we can design storage tanks and size the generators to serve our customer’s needs. Secondary, but very important, is our very aggressive internal cost targets have spawned innovation to reduce the cost of heliostats.
“As production ramps up on heliostats and other components, the price comes down,” adds Raczka. “In the future, we will be able to produce electricity per kilowatt less expensively. If we modernize our nation’s energy grid to leverage new energy technologies, a solar power plant on 100 square miles in the desert could possibly power most of the United States with a proper distribution system.”