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Improved Concentrating Solar Power Systems

National Renewable Energy Laboratory

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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:

 

Integration of Gas-Solid Fluidization and Circulating Fluidized Bed in Power Tower Concentrating Solar Power Plant with Particle Receiver and Solid Thermal Energy Storage

(ROI 11-92, US 9,347,690)    

This invention introduces a novel CSP system by using gas-solid two-phase flow to replace liquid HTF, and uses low-cost solid particles as thermal energy storage media. The system resolves the current low temperature freezing issue of salt, and high temperature stability issue of both salt and oil, and achieves high performance with both high temperature operation and high heat transfer rate by the stable solid particles. System cost is significantly lower than salt and high grade steel structure container system. The system performance and temperature range can be a platform to serve multiple power generation methods including sub/super critical steam cycle, S-C02 Brayton cycle, air-Brayton and combined cycle. 

 

Integration of Chemical Looping Process Based on Fluidized-Bed System for Concentrating Solar Power Energy Conversion and Storage

(ROI 12-40, US 61/807,982)

This process 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 reduction reaction from solar heat will be stored in a silo in chemical energy form, supplied to a fluidized-bed heat exchanger and release heat through an oxidization process to power generation as needed by the grid. Different types of metal oxides are selected that have higher reduction kinetics and oxidization conversion rates. High quality energy can be stored and retrieved, thereby supporting higher efficiency power cycles than the current salt-based CSP system. The approach will transform solar energy for continuous baseload power generation. Chemical energy storage can serve as long-term storage for days, months, or seasons. The storage technology overcomes day-to-day variation of renewable generation, and can shift generation from low power demand season to high demand season to meet load requirements and fundamentally resolve renewable resource variation.

 

Fluidized-Bed Heat Exchange Designs for Different Power Cycle in Power Tower Concentrating Solar Power Plant with Particle Receiver and Solid Thermal Energy Storage

(ROI 12-74)

These heater designs 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 (PFB-GTCC) systems. 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.

 

Enclosed Particle Receiver Design for a Fluidized Bed in Power Tower Concentrating Solar Power Plant

(ROI 13-05)

This process describes development approach and design consideration for the near-blackbody, enclosed particle receiver that is a critical component in the CSP thermal system design using gas/solid two-phase flow as the heat-transfer fluid. The gas/solid particle system uses FB technology for heat exchange and packed particles for TES when it is implemented in a CSP plant. This receiver design operates at >800°C with reduced thermal losses, and the materials used in the design can tolerate >I,000°C, providing excellent potential for application to solar tower systems to support all types of high efficiency power cycles.

The receiver described in this disclosure, when integrated with FB-CSP thermal system design, overcomes many issues facing the current molten-salt-based CSP plant design; specifically, salt freezing at low temperature and salt stability for high temperature operation. The design employs a working mechanism similar to a blackbody furnace while minimizing thermal losses by convection and radiation, resulting in high thermal efficiency at high particle temperature. The receiver design advances CSP solar energy collection with significant benefits in both plant cost and performance.

 

Planar-Cavity and Cooled-Flare Particle Receiver Design

(ROI 14-72)

This design introduces a variety of particle receivers that use the near-blackbody (NBB) principle adapted to available materials and easy manufacturing methods in order to address potential material-related restrictions in the use of the concept for different potential applications and implementation. Specifically, this addresses the need for a high temperature, specular-reflective material for flux spreading. To avoid the shortcoming of the receiver design depending on stable, high temperature reflective materials, the receiver uses a planar-cavity configuration for flux distribution to match the solar heat collection with the particle flow.  

 

Benefits

 

·        Simplified systems

·        Reduced installation, maintenance and operation costs

·        Increased efficiency

 

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
Technology Status
Technology IDDevelopment StageAvailabilityPublishedLast Updated
NREL ROI 11-92, 12-40, 12-74, 13-05, 14-72ProposedAvailable02/17/201602/17/2016

Contact NREL About This Technology

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