This article discusses various aspects of concentrating solar power (CSP) systems. CSP system ensures that more solar energy reaches the earth in one hour than the combined worldwide consumption of energy by human activities in one year. The article also focuses on various challenges posed by the CSP systems as alternative energy sources. Some CSP systems focus sunlight onto a line, where tubes contain a working fluid, such as synthetic oil, which is heated and pumped to heat exchangers to produce high-pressure steam. These systems are oriented north–south and track on a single axis from east to west over the course of a day. Technological improvements have been made in nearly all the sub-components of CSP systems over the past few years. Research efforts include developing novel materials and heat-transfer fluids, designing receivers that can achieve high temperatures, and building higher efficiency heat collectors. The study shows that nearly every part of the CSP system presents rich opportunities for mechanical engineers to contribute their expertise. In particular, the challenging SunShot Initiative goals call for innovations and ingenious system designs to drive costs down, while improving efficiencies.
Archimedes, the Greek engineer from the third century B.C., defended his home of Syracuse from a besieging Roman fleet by focusing sunlight reflected off highly polished shields onto the ships, setting them afire. As a historical matter, it’s probably a myth, but it neatly illustrates the power of solar energy. When concentrated to a high degree, sunlight generates intense heat.
—Capturing that heat and putting it to work has captivated minds of engineers since Archimedes’s time, if not before. But it has taken advances in materials science and control technologies to bring this ancient dream to the cusp of practical power production.
Photovoltaic cells, which turn light into electricity, are the most common means of capturing solar energy. Concentrating solar power systems are more complex: mirrors—sometimes 100,000 or more—controlled by sophisticated tracking systems reflect and concentrate sunlight, which is then converted to heat to generate electricity via steam turbines or heat engines. CSP systems range in size from large, utility-scale operations to smaller units that power individual buildings or plants.
The potential for CSP is enormous. More solar energy reaches the earth in one hour than the combined worldwide consumption of energy by human activities in one year. A recent study by the International Energy Agency’s SolarPACES group, in conjunction with the European Solar Thermal Electricity Association and Greenpeace International, suggested that concentrating solar power systems could provide up to 25 percent of the world’s electricity needs by 2050.
In spite of the promise of CSP, the challenge is making solar thermal energy cost-competitive with fossil fuels and other alternative energy sources. This means increasing the efficiency of CSP technologies, as well as making them more affordable.
“The primary challenge that is driving all the 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,” said Scott R. Hunter, senior research scientist at the Oak Ridge National Laboratory in Tennessee. “This is driving the design of lower-cost mirror materials, thermal storage, scaling to larger mirrors and facilities, and higher operating temperatures.”
Solar Thermal in Focus
The flux of energy streaming in as sunlight is roughly 1,000 W per square meter. Getting that flux concentrated enough to be able to do work with it is the challenge for all CSP technologies.
Some CSP systems focus sunlight onto a line, where tubes contain a working fluid, such as synthetic oil, which is heated and pumped to heat exchangers to produce high-pressure steam. Such systems are oriented north-south and track on a single axis from east to west over the course of a day. The most mature form of this linear concentrator technology uses single-piece parabolic reflectors; facilities using this configuration have more than 500 MW nameplate capacity. Linear Fresnel reflector systems consist of slightly curved mirrors mounted on trackers on the ground, which reflect sunlight onto a receiver tube fixed in space above the mirrors.
To get higher concentrations—and thus higher temperatures—other CSP systems focus sunlight to a single point. In so-called power tower systems, numerous 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, which then powers a conventional turbine generator. BrightSource Energy of Oakland, Calif., is currently constructing the $2.2 billion Ivanpah project in California, a power tower that will be the world’s largest solar thermal energy plant and power about 150,000 homes.
A smaller version of the two-dimensional array called a dish engine uses a mirrored parabolic dish that tracks the sun across the sky. A power conversion unit at the focal point produces electricity. Dish engines are currently the most efficient CSP technology, with a record greater than 31 percent. The power block of a dish engine system is a Stirling engine, which uses hydrogen gas or helium as the working fluid.
Power towers and dish engines track in two directions and have higher efficiency by using dual curved mirrors (focusing at a point or an area on the receiver). Consequently the tracking is more complex to keep the sun’s image focused on the receiver.
Currently about 500 MW of electricity is generated by CSP in the U.S. Five new plants are under construction with financing from the U.S. Department of Energy loan guarantee program, for start-up in 2013 or early 2014, which will collectively add another 1.3 GW. Some of these plants are among the largest CSP plants in the world.
The Department of Energy’s SunShot Initiative was established in 2011 to make solar energy cost-competitive with other forms of energy by the end of the decade. This support has been a key driver in advancing CSP research.
Many of the Technologies have been shown to Work, but the Issue is more on how ot Reduce the Cost of Large-Scale Manufacturing and Deployment.
“The SunShot Initiative has set an aggressive target for CSP technologies to achieve cost parity with other forms of energy on the grid by the year 2020,” said Ranga Pitchumani, a professor of mechanical engineering at Virginia Tech and director of the Concentrating Solar Power Program for the SunShot Initiative. “This calls for at least a 75 percent reduction in costs in order to achieve a levelized cost of electricity (LCOE) of 6 cents/kWh electric or less without subsidy.”
Technological improvements have been made in nearly all the sub-components of CSP systems over the past few years. Research efforts include developing novel materials and heat-transfer fluids, designing receivers that can achieve high temperatures, and building higher-efficiency heat collectors.
“Some of the key technology issues are molten-salt storage, advanced concentrator technologies, and advanced thermal receiver approaches,” said Thomas R. Mancini of TRMancini Solar Consulting in Albuquerque, N.M. “Many of the technologies have been shown to work, but the issue is more on how to reduce the cost of large-scale manufacturing and deployment.”
Hot to Handle
One area of emphasis is heat-transfer fluids. The efficiency of the heat engines at the heart of a solar thermal plant is directly tied to its operating temperature, but the heat-transfer fluids in current use can’t stand up to the highest heat levels.
“A heat-transfer fluid that can operate at very high temperature as a liquid, with no decomposition, will result in higher thermal efficiencies for CSP,” said associate professor Peiwen Li, director of the University of Arizona’s Energy and Fuel Cell Laboratory. “The temperature limit of synthetic oil is 400 °C and for the current available molten salt is 550 °C. New DOE projects are setting a target of 800 °C.”
It’s a Dirty Job But . . .
It’s pretty simple: dirty mirrors and solar panels don’t concentrate as much solar energy.
A team headed by Scott Hunter of Oak Ridge National Laboratory’s Measurement Science and Systems Engineering Division recently received more than $2 million from the Department of Energy’s SunShot Initiative to develop low-cost, self-cleaning reflector coatings for concentrating solar power collectors. The goal is to develop a transparent superhydrophobic coating that can be applied to the surface of solar collector mirrors. The coating will keep collector mirrors clear of debris by preventing dust from sticking to the mirror surface, maximizing the amount of reflected sunlight from the collector mirrors and decreasing cleaning costs.
Hunter plans to engineer the coatings so they can be easily applied to the front surface of CSP collector or heliostat mirrors. If all goes well after two years of field testing, the coatings will be ready for application in operating facilities.
“By using transparent superhydrophobic coatings on collector mirrors, we’ll be able to create high-performance and low-maintenance concentrating solar power electricity generation,” he said.
The reflector coatings are expected to provide as much as a 90 percent reduction in mirror cleaning and maintenance costs, and provide about a 20 percent improvement in the average amount of reflected solar energy.
Li is a principal investigator for a five-year, $5 million project funded by the SunShot Initiative to develop molten salt-based fluids as possible alternatives to traditional heat-transfer fluids. Li’s objective is 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 and will not degrade below 800 °C,” Li said. “Other requirements for the heat-transfer fluid include low cost and favorable properties like low vapor pressure, low viscosity, and high thermal conductivity.”
Another five-year, $5.5 million SunShot project will team researchers from the University of California-Los Angeles, University of California-Berkeley, and Yale University to investigate the use of metal alloys as a heat-transfer fluid in CSP systems operating at temperatures in excess of 800 °C. Professor Sungtaek Ju of UCLA’s Department of Mechanical Engineering is the lead on that project, which 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. A combination of modeling and experimental tools, including high temperature corrosion flow loops, will be used to verify that the metal alloys identified can meet all the needs of a CSP plant.
“Our main 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,” Ju said. “We also want minimal corrosion and creep, high heat-transfer performance, low viscosity, and high heat capacity.”
New high-operating-temperature fluids, the DOE believes, can be used to realize “thermodynamic power conversion cycles capable of 50 percent or greater thermal-to-electric conversion efficiencies, thereby reaching a levelized cost of electricity target of 6 cents per kWh,” Ju said. “Of course, the cycle efficiency is a function of the heat source temperature and we want to push it as high as possible.”
New Heat-Transfer Fluids can be used to Reach Conversion Efficiencies of 50 Percent or Greater.
The SunShot Initiative is funding other research that might be able to deliver new technologies to meet various technical and cost targets within the next three to five years, Pitchumani said. These include highly efficient reflector materials integrated with low-cost structures for collectors, lean solar field manufacturing and assembly approaches, self-aligning and tracking heliostats, self-cleaning mirrors, solar selective coatings for enhanced absorption with lower radiative loss, and corrosion-resistant materials and coatings.
“High-temperature, higher-efficiency power cycles, such as the supercritical CO2 cycle at the 1 MW and 10 MW scales, and the solar-integrated Brayton cycle, will trend toward higher (greater than 50 percent) efficiency operation with dry cooling,” Pitchumani said. “Some of these have broader relevance beyond the solar industry to the nuclear and fossil industries as well.”
The SunShot Initiative is also investing in several thermal energy storage technologies. A CSP plant can be more flexible in meeting utility power demands if it has reliable thermal storage, and storage makes it more competitive with photovoltaics, which have dropped in price in recent years.
In one project, General Atomics is developing a method for storing the thermal energy produced by a CSP system in chemical bonds, which promises significantly higher energy storage density than sensible and latent energy storage methods. Another approach being developed by Terrafore, in collaboration with the Southwest Research Institute, focuses on a packed bed of encapsulated phase-change materials.
Solar thermal is also an excellent fit for a variety of industrial applications, such as thermal enhanced oil recovery, where solar energy is used to produce steam to “steam flood” oil wells that are low pressure and viscous, allowing for easier resource extraction.
“Solar thermal enhanced oil recovery is projected to be a $16 billion market by 2020,” said Kristin Hunter, communications director for BrightSource. “Estimates indicate that more than 50 percent of global oil reserves require enhanced oil recovery. Thermal EOR represents a major market opportunity in the U.S. and in oil producing countries around the world.”
Nearly every part of the CSP system presents rich opportunities for mechanical engineers to contribute their expertise. In particular, the challenging SunShot Initiative goals call for innovations and ingenious system designs to drive costs down, while improving efficiencies.
“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,” Pitchumani said. “In addition, there is tremendous opportunity to develop reliable and cost-effective solutions for system components such as actuators and drives, pumps, valves, fittings, and pressure vessels. These solutions must also be engineered to endure the harsh temperature, corrosive, and erosive environments in which CSP systems operate.”