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Improved Concentrating Solar Power Systems

National Renewable Energy Laboratory

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Table: Particle System Design Specifications
Table: Particle System Design Specifications

Technology Marketing Summary

Concentrating Solar Power (CSP) systems utilize solar energy to drive a thermal power cycle for the generation of electricity. CSP technologies include parabolic trough, linear Fresnel, central receiver or “power tower”, and dish/engine systems. Considerable interest in CSP has been driven by renewable energy portfolio standards applicable to energy providers in the southwestern United States and renewable energy feed-in tariffs in Spain. CSP systems are typically deployed as large, centralized power plants to take advantage of economies of scale. A key advantage of certain CSP systems, in particular parabolic troughs and power towers, is the ability to incorporate thermal energy storage. Thermal energy storage (TES) is often less expensive and more efficient than electric storage and allows CSP plants to increase capacity factor and dispatch power as needed – for example, to cover evening or other demand peaks.

Current CSP plants typically utilize oil, molten salt or steam to transfer solar energy from a solar energy collection field, tower or other apparatus to the power generation block. These fluids are generally referred to as a “heat transfer fluid” and are typically flowed through a heat exchanger to heat water to steam or to heat an alternative “working fluid” which is then used to drive a turbine and generate electrical power. Commonly utilized heat transfer fluids have properties that in certain instances limit plant performance; for example, synthetic oil heat transfer fluid has an upper temperature limit of 390°C, molten salt has an upper temperature limit of about 565°C while direct steam generation requires complex controls and allows for limited thermal storage capacity.

Current state-of-the-art two-tank molten salt storage costs are relatively high, and impose temperature limitations upon a practical system. For example, a typical two-tank molten salt storage system will freeze at temperatures under 200°C and become unstable above 600°C. Proposed single-tank thermocline TES systems have the potential to displace 75% of the expensive molten salt with low cost rocks or pebbles. Even so, the cost of a thermocline TES system will still be high due to the cost of the remaining 25% salt or other required elements such as a stainless steel tank. In addition, molten salt may still limit the highest operating temperature of the overall CSP system for the power generation and thereby limit system efficiency. In addition, TES salt transportation and conditioning can take several months, which negatively impacts capital investment.

In contrast to a molten-salt CSP system, highly stable solid particles can be stable at temperatures beyond 1000 °C, thus able to drive high efficiency thermal power cycles for high solar-to-electricity conversion efficiency. The particle-thermal system can support one of the promising high-efficiency s-CO2 power cycles. Sand-like solid particles are ordinary materials that are vastly available and can be processed inexpensively to be used in the particle-CSP system. Our patented designs are able to achieve high thermal performance, and lead to a low-cost, high-performance CSP system with the large-scale energy storage to serve for dispatchable power generation.

Description

Engineers at the National Renewable Energy Laboratory (NREL) have developed a number of improved CSP methods and systems to achieve high thermal performance.

One novel CSP system that, alongside resolving the issue of salt and oil’s high temperature stability and salt’s low temperature freezing, uses low-cost solid particles as thermal energy storage media. This system has a temperature range that can serve as a platform for multiple power generation and leverages the technology of circulating fluidized bed boilers to drive hot gas-solid two-phase flow through a boiler or heat exchanger to heat working fluid and to replace high transfer fluid (HTF). Another improved CSP method uses chemical reactions suitable for solar application in a closed-loop system that consists of a high-temperature, high-performance receiver to perform a reduction reaction and a fluidized-bed heat changer for oxidization reaction. The product of this reduction reaction will be stored in a silo in chemical energy form, supplied to a fluidized-bed heat exchanger, and will release heat through an oxidization process to power generation as needed by the grid. In addition, NREL engineers developed CSP system designs that cover potential power cycle methods of current steam power generation, future s-CO2 power systems, and proven high-efficiency pressurized fluidized-bed gas turbine combined cycle systems (GTCC). The particle-CSP system can be a self-dependent thermal subsystem and integrated with any power tower solar field to fundamentally transform solar energy collection, conversion, and storage.

Furthermore, NREL engineers have designed a near-blackbody enclosed particle receiver that is a critical component in the CSP thermal system and that operates at >800°C with reduced thermal losses and with materials that can tolerate >1,000°C. This design overcomes the issue of salt-freezing at low temperatures and salt stability at high temperatures through employing a working mechanism similar to a blackbody furnace that minimizes thermal losses through convection and radiation. NREL engineers have also developed a novel CSP planar-cavity and cooled-flare particle receiver design that addresses the need for a high temperature, specular-reflective material for flux spreading. This novel design uses a planar-cavity configuration for flux distribution to match the solar heat collection with the particle flow and introduces a variety of particle receivers that use the near-blackbody (NBB) principle adapted to available materials and easy manufacturing methods.

Benefits
  • Simplified systems
  • Low system cost by using inexpensive, stable solid particles
  • Reduced installation, maintenance and operation costs
  • Increased efficiency with the high-temperature ability to drive advanced power cycles
Applications and Industries
  • Power generation
  • Concentrated Solar Power (CSP)
  • Energy storage

 

Patents and Patent Applications
ID Number
Title and Abstract
Primary Lab
Date
Patent 9,347,690
Patent
9,347,690
Methods and systems for concentrated solar power
Embodiments described herein relate to a method of producing energy from concentrated solar flux. The method includes dropping granular solid particles through a solar flux receiver configured to transfer energy from concentrated solar flux incident on the solar flux receiver to the granular solid particles as heat. The method also includes fluidizing the granular solid particles from the solar flux receiver to produce a gas-solid fluid. The gas-solid fluid is passed through a heat exchanger to transfer heat from the solid particles in the gas-solid fluid to a working fluid. The granular solid particles are extracted from the gas-solid fluid such that the granular solid particles can be dropped through the solar flux receiver again.
National Renewable Energy Laboratory 05/24/2016
Issued
Application 20140298822
Application
20140298822
Chemical Looping Fluidized-Bed Concentrating Solar Power System and Method
A concentrated solar power (CSP) plant comprises a receiver configured to contain a chemical substance for a chemical reaction and an array of heliostats. Each heliostat is configured to direct sunlight toward the receiver. The receiver is configured to transfer thermal energy from the sunlight to the chemical substance in a reduction reaction. The CSP plant further comprises a first storage container configured to store solid state particles produced by the reduction reaction and a heat exchanger configured to combine the solid state particles and gas through an oxidation reaction. The heat exchanger is configured to transfer heat produced in the oxidation reaction to a working fluid to heat the working fluid. The CSP plant further comprises a power turbine coupled to the heat exchanger, such that the heated working fluid turns the power turbine, and a generator coupled to and driven by the power turbine to generate electricity.
National Renewable Energy Laboratory 04/02/2014
Filed
Technology Status
Technology IDDevelopment StageAvailabilityPublishedLast Updated
NREL ROI 11-92, 12-40, 14-72ProposedAvailable02/17/201602/17/2016

Contact NREL About This Technology

To: Erin Beaumont<erin.beaumont@nrel.gov>