Sandia National Laboratories Molten Salt Test Loop

Engineering firm: Bridgers & Paxton Consulting Engineers Inc.
2013 MEP Giants rank: 56
Project: Sandia National Laboratories Molten Salt Test Loop
Address: Albuquerque, N.M., United States
Building type: Research/lab/high-tech
Project type: New construction
Engineering services: Automation & Controls, Code Compliance, Electrical/Power, HVAC, Lighting
Project timeline: April 2010 to July 2012
Engineering services budget: N/A
MEP budget: $8.43 million

Challenges

The Molten Salt Test Loop (MSTL) project is a first-of-a-kind concentrated solar power (CSP) power plant test facility that allows manufacturers and university research facilities around the world to come to Sandia National Laboratories to perform accelerated lifecycle tests of power plant size components in flowing molten nitrate salt. There are currently no facilities in the world capable of performing accelerated lifecycle testing of power plant size components in a 585 C (1085 F) high temperature molten nitrate salt environment.

The objective of the MSTL project was to advance solar energy application technology in the power generation arena beyond any current existing low temperature technology. The completed test platform allows a variety of actual power plant size components to be tested to support the ultimate goal of large scale solar electrical generation. The MSTL at Sandia National Laboratories’ National Solar Thermal Test Facility (NSTTF) was designed by Bridgers & Paxton Consulting Engineers, Inc., to provide three parallel test platforms for the performance and evaluation of concentrating solar collectors and power plant size components in flowing molten nitrate salt simulating solar power plant conditions.

A key characteristic of CSP is its built-in thermal inertia, which provides stability in power plant output during changes in solar radiation, such as when clouds pass over. This technology consists of pumping molten salt through various types of solar collectors such as parabolic troughs, linear Fresnel, or elevated tower receivers. These technologies involve converting sunlight into thermal energy for use in a heat-driven turbine engine for generating electrical power. In a functioning test loop, mirrors are used to track the sun and focus concentrated solar energy onto solar collectors, which contain circulating molten salt. The molten salt is an excellent energy storage media, which can then be collected in large storage vessels (>500k gal tanks) for later use to generate electrical power. This energy can be stored well into the night and generate power when needed. The potential CSP generating capacity in the United States is 7,500 gigawatts (GW), which is several times greater than the entire U.S. electrical grid capacity. During an assumed lifetime of 30 years, a CSP system would produce about 20 times the energy required for its manufacture while significantly reducing greenhouse gas emissions and reliance on fossil fuel power generation.

Solutions

This one-of-a-kind structure required components that were not manufactured in the United States, due the unique materials necessary to flow molten salt at high temperatures. This required a great deal of research and communication with various companies around the world to identify companies that could meet the owner’s requirements. Molten salt is very corrosive at high temperatures and will permeate through valve packing material. B&P’s design approach took into consideration Sandia’s requirement to design this testing facility for a 20-year operational life; in doing so, B&P took great care in specifying materials that would handle extreme temperature cycling, pressure cycling, and extreme corrosive environments. The design also included numerous design approaches that could handle three major types of failures in order to prevent salt freeze up within the system. These approaches included: positive drain back to salt furnace during particular power outage events, redundant heat trace capability, and a unique approach to piping insulation which accounts for piping expansion and contraction while maintaining a high level of insulation efficiency.

An additional example of an innovative technique is that B&P’s design mechanically controls the temperature of the extended valve bonnet to maintain it at 275 C while salt flows through the valve at 585 C (1085 F). This unique design approach will substantially extend the life of the valve bonnet packing material and reduce downtime for maintenance. Challenges included 16 months for the pump manufacturer to design and build the 9-stage vertical turbine pump, which ultimately weighed 14,500 lbs. and was 26 ft in height. Another challenge was the complex piping design, which accounted for thermal expansion and thermal cycling of the piping test loops and required a unique design approach. B&P’s approach to the analysis was to neutralize movement and mechanical loads at the three test loop/customer interface connection points and subsequently protect the resident equipment. Included in the objective was maintaining a robust hard pipe system, avoiding the use of any flexible expansion compensation devices. To meet the objective, the pipe needed to be fully restrained at the interface points by restricting movement in all 6 degrees of freedom (3 translational and 3 rotational axes); however, restricting movement comes with limitations.

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