Solar Power Shines

March 2013


More solar energy reaches the earth in one hour than the combined worldwide consumption of energy by human activities in one year. Even though the sun supplies far more energy than we can possibly use, the challenge is making solar energy cost-competitive with fossil fuels and other alternative energy sources.

Today's concentrating solar power (CSP) systems use mirrors (sometimes 100,000 or more) and sophisticated tracking systems to reflect and concentrate sunlight, which is then converted to heat to generate electricity. CSP technologies are typically deployed in four system configurations:

Concentrating solar power systems use mirrors and sophisticated tracking systems to reflect and concentrate sunlight, which is then converted to heat to generate electricity.
  • Parabolic trough. This type of linear concentrator is the most mature CSP technology, with over 500 megawatts operating worldwide. Rows of reflectors concentrate sunlight onto tubes that contain synthetic oil.
  • Linear Fresnel. Also a linear concentrator technology, this reflector system consists of slightly curved mirrors mounted on trackers on the ground, which reflect sunlight onto a receiver tube fixed in space above the mirrors.
  • Power tower systems. Sun-tracking mirrors known as heliostats focus sunlight onto a receiver at the top of a tall tower. This solar energy heats a heat-transfer fluid that is used to generate steam.
  • Dish engine. Consisting of a solar concentrator and power conversion unit, this system uses a parabolic dish of mirrors to direct and concentrate sunlight onto a central engine that produces electricity; hydrogen gas or helium is the working fluid.

Both parabolic troughs and linear Fresnels have only a single curvature of mirrors (focused along a line or tube) and therefore are oriented north-south and track on a single axis from east to west over the course of a day. Power towers and dish engines track in two directions and have higher efficiency by using dual curved mirrors.

Technology Improvements Abound

The Department of Energy's (DOE) SunShot Initiative was established in 2011 to make solar energy cost-competitive with other forms of energy by the end of the decade. A key concern is the lack of high-operating-temperature heat transfer fluids, which DOE has identified as one of the biggest barriers to widespread adoption of solar thermal power generation.

The SunShot Initiative recently announced two new projects for developing new heat transfer fluids. The University of Arizona ($5 million over five years) is leading a multi-university team to develop molten salt-based fluids as possible alternatives to traditional heat transfer fluids. The University of California-Los Angeles ($5.5 million over five years) will collaborate with Yale University and UC-Berkeley to investigate liquid metals as potential heat transfer fluids.

"The temperature limit of synthetic oil is 400°C and for the current available molten salt is 550°C," said Peiwen Li, director of the University of Arizona's Energy and Fuel Cell Laboratory and principal investigator for the Arizona project. "New DOE projects are setting a target of 800°C."

Li plans to make a heat transfer fluid from multiple salts that can work in a temperature range from 250°C to 800°C. "This means the fluid will not freeze at temperatures above 250°C or degrade below 800°C," he said.

Professor Sungtaek Ju of UCLA's Department of Mechanical Engineering, the lead on UCLA's project, will use a novel material synthesis system to rapidly screen metal alloys with the desired thermophysical properties. The search space is being defined through thermochemical modeling efforts and the application of rapid screening tools.

"Our goal is to develop new types of low-melting point alloys and associated structural materials with several constraints, including high-temperature stability to allow the power cycle to run at temperatures beyond 650°C, thereby achieving high cycle efficiency and low levelized cost of electricity," said Ju.

Moving Forward

"The primary challenge that is driving all development work on this technology is how to reduce the installation and operating costs to the point where the generated electricity is cost-competitive with other conventional forms of electricity generation," indicated Scott R. Hunter, senior research scientist at Oak Ridge National Laboratory.

Because solar energy is an inconsistent source of power, the best way to make it more competitive is to increase its storage capability—this would increase a CSP plant's flexibility in meeting utility power demands. It would also make CSP more competitive with photovoltaic technologies, which have become less expensive in recent years.

"Solar plants, when integrated with thermal energy storage, extend electricity production into later parts of the day and after sundown, when it is valued most by utilities and other power producers," said Kristin Hunter, communications director for BrightSource Energy, which is currently building the world's largest solar thermal energy plant in California. "This capability reduces the cost of renewable power by increasing a plant's capacity factor."

"Virtually every sub-discipline in mechanical engineering plays a role in CSP—heat transfer, fluid mechanics, high-temperature materials and coatings, thermodynamic power cycles, lean manufacturing, assembly and automation, metrology, sensing and control, and computational modeling," added Ranga Pitchumani, professor of mechanical engineering at Virginia Tech and director of the Concentrating Solar Power Program for the SunShot Initiative. "These system components must also be engineered to endure the harsh temperature, corrosive, and erosive environments in which CSP systems operate."

Mark Crawford is an independent writer.

Register here for a free webinar on Concentrated Solar Power technology.

Virtually every sub-discipline in mechanical engineering plays a role in concentrating solar power.

Dr. Ranga Pitchumani, Virginia Tech and Concentrating Solar Power Program, SunShot Initiative


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