Mining the Sky – Today’s Energy Solutions
Written by Elizabeth Engler Modic – May 2012
SolarReserve’s first-generation solar power plant is a welcome sign of green progress. Once completion of the Crescent Dunes Solar Energy Project in 2013, it will generate roughly 480,000MW-h per year of clean, renewable electricity to power 75,000 homes during peak electricity periods. 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 is the company’s unique concentrating solar power (CSP) technology and innovative energy storage.
Tonopah Solar Energy, a subsidiary of SolarReserve, manages the project located on 1,600 acres northwest of Tonopah, NV, a historical silver mining town near the Nevada desert. The site selection was based on key ingredients for optimum solar energy – hours of direct sunlight, altitude (more intense sunlight), and DNI (Direct Normal Irradiance) strength. The power plant design includes a tall receiver tower and power block positioned centrally in a large circular field of mirrors known as heliostats. The 100ft receiver mounts on the 553ft concrete tower. Tower height is essential for plant efficiency, ensuring 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, as 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 110MW plant will utilize an ordinary steam turbine generator to produce electricity, but integrates a sophisticated hybrid cooling system using less than 600 acre-feet/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% thermal efficiency. Eventually, pumps into the generator for electricity production. This process is similar to a standard coal-fired power plant, but clean and free solar energy fuels this process.
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, made from an environmentally friendly mixture of sodium and potassium nitrate, is 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.
Challenges of Production
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 28ft x 24ft, with the pedestal made from large diameter piping that is 11ft to 16ft tall. Other metal components and trusses supporting the mirror structure range from 28ft to 30ft 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. Use in Tonopah will be the baseline 62.5m2 heliostat that translates to 17,608 heliostats in the collection field. The project has an ambitious plan to install 70 heliostats per day until completion.
Metrology is the Glue
For optimum performance, the solar plant design calls for an overall beam quality and accuracy of less than 1.5mRad for each facet of 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, North Kingstown, RI, 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. Due to its long-range measurement volume of 525ft when used with a standard corner cube, use of this laser tracking system is primarily for aerospace and other in-place measurement applications. Based on Leica’s Absolute interferometer (AIFM) technology, the portable CMM maintains precision measurement in all operating conditions, with multiple, built-in redundancies to ensure high accuracies.
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