Researchers at Sandia National Laboratories are inventing and refining more efficient and less expensive ways to turn sunlight into electricity through concentrating solar power (CSP) while also improving energy storage options. Although there are CSP plants in operation already – in California, Nevada, Arizona and throughout the world – current research could make this technology more competitive with other sources of energy and a viable large-scale source of renewable energy for the grid.
As more renewable energy sources are added to local electrical grids, there is an increasing need for renewable energy with storage to balance supply throughout the day and night. Well-known renewable energy options such as photovoltaic (PV) solar panels and wind turbines are naturally intermittent, and available storage options for those technologies are expensive and limited. During peak daylight hours, sunny locations like California and Hawaii are already seeing over-generation of PV and wind electricity on some days, leading to curtailment and even negative pricing. Meanwhile, even with higher penetrations of PV and wind energy, these sources can’t keep up with the spike in demand for electricity during evening hours and require significant ramp-ups of other resources. This trend of midday over-generation followed by a steep ramp-up at night when solar PV and wind are waning, known as the “duck curve,” is expected to continue as more renewable energy sources are included in the nation’s power supply each year.
Large-scale energy storage is becoming a critical need, and the future of renewable energy technologies will depend on the development of inexpensive and efficient energy storage that can be dispatched when needed. Unfortunately, many current energy storage solutions, including large-scale battery storage, compressed air and pumped hydro, are too expensive for large-scale use or are limited because of unique geographical and resource requirements.
A different type of solar energy
Conventional power plants use coal, natural gas or radioactive decay to heat and produce high-temperature, high-pressure steam or air, which spins a turbine generator to produce electricity. In a CSP power plant, concentrated heat from the sun is used as the heat source instead of fossil fuels. Researchers at Sandia Labs are investigating CSP’s potential as a reliable and efficient 24-hour renewable energy source to complement other renewables on the grid by pairing it with innovative energy storage options and refining the methods used for absorbing the concentrated sunlight.
Researchers at Sandia collect concentrated sunlight by using a “power tower” or central receiver system. In this setup, a large number of mirrors are placed around a central tower to reflect and concentrate sunlight on a thermal receiver at the top of the tower. The receiver collects the heat from the concentrated sunlight to be used immediately in the power cycle or stored for later use when the sun is not shining.
The current state-of-the-art method for capturing heat in a central receiver system is molten nitrate salts. The molten salts flow through a panel of tubes at the top of the receiver where the concentrated sunlight beam is focused. The salts heat up to 565°C and then are stored in large insulated tanks. The stored salt can efficiently and inexpensively retain its heat for days to weeks, but it is typically used in the next 10 to 15 hours for on-demand electricity production. To convert stored molten salt into electricity, the molten salt is pumped from insulated storage tanks to a heat exchanger, where the heat from the salt turns water to steam for use in a conventional steam-Rankine power cycle. The ability to use CSP-generated electricity immediately or on demand could help balance supply to the grid during peak and nighttime hours, especially as researchers work to make the technology more efficient and less expensive.
Making CSP competitive
The U.S. Department of Energy (DOE) launched the SunShot Initiative in 2011 with the goal of making solar electricity cost-competitive with other power sources by the end of the decade. As part of this program, the DOE set cost and performance targets for both PV and CSP systems. For CSP, the goal is to develop a heat storage and transfer system that reduces the cost of solar electricity to $0.06/kWh and achieves a thermal-to-electric cycle efficiency greater than 50%. According to a publication from the National Renewable Energy Laboratory, today’s most advanced operational CSP systems have lowered the cost of CSP electricity by nearly 50% and reached cycle efficiencies of approximately 40%. However, these systems have not kept pace with the falling costs of PV systems.
In February, the DOE hosted a workshop for CSP stakeholders that defined three potential technology pathways to research for the next generation of CSP central receiver designs: molten salt, particles and gas. These materials are being studied for their potential to integrate CSP systems with a supercritical carbon dioxide (sCO2) Brayton cycle as the new power cycle that can achieve 50% thermal-to-electric efficiency. To make the SunShot goals feasible with the new power cycle, the collected and stored solar energy must be delivered to the sCO2 heat transfer system at a temperature above 700°C. This means that the current use of molten nitrate salts needs to be replaced or modified because these salts decompose when they reach 600°C.
Searching for the next generation of CSP technology
Sandia Labs designed, built and tested the world’s first high-temperature falling particle receiver system for CSP as part of a DOE SunShot-funded project, performed in collaboration with Georgia Tech, Bucknell University, King Saud University and the German Aerospace Center. Instead of molten nitrate salts, the system uses sand-like ceramic particles to capture and store heat from concentrated sunlight. These particles can reach higher temperatures than salt and do not have the same potential for corrosion, decomposition or freezing.
The Sandia Labs falling particle receiver system includes a particle elevator, top and bottom hoppers, and a cavity receiver that can accommodate either a free-falling curtain of particles or a staggered array of porous chevron-shaped mesh structures that slow the particle flow. The particles fall through the beam of concentrated sunlight at the top of the receiver and are heated to a desired temperature. The particles can then either be stored for later use or run through a heat exchanger to generate electricity. Aside from the particle lift, the entire system is based on the gravity-driven flow of the particles through each component, reducing parasitic power consumption.
Recent on-sun tests of Sandia Labs’ 1 MW falling particle receiver have demonstrated particles being heated to over 800°C and thermal receiver efficiencies up to 80%. The particle receiver design is scalable for power generation up to 100 MW, which is enough to power 60,000 to 70,000 homes. The design also enables new high-temperature solar-driven applications, including thermochemical storage, solar fuels, industrial process heating, water treatment, and integration with the sCO2 power cycle. A particle-to-sCO2 heat exchanger and an sCO2 flow system are currently being designed for integration with the Sandia particle receiver system.
Further development, modeling and testing are under way to study the opportunities and challenges associated with a solarized sCO2 system and other particle-, salt- and gas-based CSP systems that have the potential to reach SunShot goals. The next steps for these technologies are integrated system tests and scaled-up pilot demonstrations to confirm the ability of each technology to meet the SunShot technical targets and market requirements such as ramp rates, reliability and availability. With this work under way, researchers are closing in on the goal of refining a CSP system with energy storage that can provide the grid with a reliable 24-hour source of renewable energy at competitive prices.
Dr. Clifford Ho is a distinguished member of the technical staff at Sandia National Laboratories, where he currently conducts research to improve the performance of CSP technologies. Kristen Meub is a corporate communications specialist at the lab, where she writes about energy research. All images in this article are courtesy of Sandia National Laboratories.
