Mining the Sky - Production Challenges

The 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 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. x 24 ft. The pedestal is made from large diameter piping that is 11 to 16 ft. tall. Other metal components and trusses supporting the mirror structure range from 28 – 30 ft. long.

The company cut its teeth on small test facilities, 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 62.5sq-m 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.

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 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 (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. 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 (AIFM) technology, the portable 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 determined by the SolarReserve control system. Slope error is determined by comparing a glass surface to the original design intent.  In 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 their manufacturing processes to produce the desired end product.