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Contract No. DE-AC36-08GO28308

Advanced Supercritical Carbon 

Dioxide Power Cycle 

Configurations for Use in 

Concentrating Solar Power 

Systems

Preprint

Zhiwen Ma and Craig S. Turchi

To be presented at the Supercritical CO2 Power Cycle Symposium

Boulder, Colorado

May 24-25, 2011

Conference Paper

NREL/CP-5500-50787

March 2011
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1 

Advanced Supercritical Carbon Dioxide Power Cycle Configurations

for Use in Concentrating Solar Power Systems

Zhiwen Ma and Craig S. Turchi

National Renewable Energy Laboratory

1617 Cole Blvd., Golden, CO, USA

Email: zhiwen.ma@nrel.gov, craig.turchi@nrel.gov

Abstract

Concentrating Solar Power (CSP) utilizes solar thermal 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. The resurgent interest in CSP has been driven by renewable portfolio standards in southwestern 

states and renewable energy feed-in tariffs in Spain. CSP systems are 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 is 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 utilize oil or steam to transfer solar energy to the power block. These fluids have properties that 

limit plant performance; for example, the synthetic oil has an upper temperature limit of 400°C while direct steam 

generation requires complex controls and has limited storage capacity. Higher operating temperatures generally 

translate into higher thermal cycle efficiency and often allow for more efficient thermal storage. To obviate these 

limitations, alternative fluids are under investigation by research teams worldwide.

Supercritical carbon dioxide (S-CO2) operated in a closed-loop recompression Brayton cycle offers the potential of 

equivalent or higher cycle efficiency versus supercritical or superheated steam cycles at temperatures relevant for 

CSP applications. The S-CO2 pressure is higher than superheated steam but lower than supercritical steam at 

temperatures of interest. The high pressure required for S-CO2 makes application to trough fields difficult [4], and 

preliminary analysis suggests the fluid may be better suited for use in Power Towers. Even circulating high pressure 

S-CO2 through a large Power Tower would be a challenge due to the volume and pressure of fluid being moved [9]. 

However, a modular power tower design introduced in this paper can take advantage of S-CO2’s potential without 

prohibitive piping costs.

In the proposed design, a single-phase process using S-CO2 as both heat transfer fluid (HTF) and thermal power 

cycle fluid simplifies the power system configuration. The design is compatible with sensible-heat thermal energy 

storage, if desired. The simpler machinery and compact size of the S-CO2 process may also reduce the installation, 

maintenance and operation cost of the system. Brayton-cycle systems using S-CO2 have smaller weight and volume, 

lower thermal mass, and less complex power blocks versus Rankine cycles due to the higher density of the fluid and 

simpler cycle design. The lower thermal mass makes startup and load change faster for frequent start up/shut down 

operations and load adaption than a HTF/steam based system. The research will characterize and evaluate advanced 

S-CO2 Brayton cycle power generation with a modular power tower CSP system. 

Introduction

Supercritical CO2 operated in a closed-loop recompression Brayton cycle offers the potential of equivalent or higher 

cycle efficiency versus supercritical or superheated steam cycles at temperatures relevant for CSP applications [2, 

7]. A single-phase process using S-CO2 as both heat transfer fluid for solar collector and working fluid for power 

cycle will simplify the power plant and may reduce the installation, maintenance and operation costs of the system. 

The high pressure required for S-CO2 makes application to trough fields difficult due to the extensive piping in these 

plants [4], and the fluid may be better suited for use in Power Towers. In particular, modular towers can integrate an 

S-CO2 power block within each tower to minimize piping size and length while taking advantage of the small size 

and weight with S-CO2 turbine and compressor. 
2 

Modular S-CO2 Receiver/Generation Unit for Tower/Solar Field 

The CSP plant design described in this paper uses S-CO2 as both HTF and working fluid. The assumed capacity of 

the power block is approximately 5 to 10 MW. Each modular tower would house its own turbomachinery and 

multiple towers could be assembled in a single power park. Such a modular power park has been proposed by 

eSolar, although in their system a single power block is shared by multiple towers. The proposed configuration 

includes two-stage turbines to drive the CO2 compressor and generator respectively, as shown in Figure 1. Because 

of the compactness of the S-CO2 turbine/compressor, it is possible to reduce the size of the generation unit and 

integrate the generator unit in the tower as depicted in Figure 2. The benefits of integration include shorter piping 

and associated pressure loss, lesser thermal losses, and improved transient response. As a result, the system achieves 

high performance and significant cost benefits for CSP power generation. 

Figure 1. Dual-shaft, tower receiver S-CO2 Brayton Cycle solar thermal power system with thermal energy storage. 

There are two options for arranging the turbine/compressor/generator with the modular receiver generation set to 

operate with or without energy storage. The design options for the modular configuration are described below: 

1. Modular generation unit in the receiver without energy storage. 

This is a baseline design that uses a modular tower with a small heliostat field. Turbine/compressor size depends on 

power rating and design parameters, such as compression ratio, shaft speed, and/or selection of axial or radial flow 

for the compressor and turbine. In order to have the S-CO2 generation module integrated into the receiver, the design 

considers the power rating and generation configuration for power block. 

The power block design assumes a dual-shaft turbine layout to separate gas compression and power generation 

shafts. The gas compression and power generation can run at different shaft speeds – each at its own optimum 

condition – so that the power generation matches the grid power frequency at 3600 RPM without gearbox. The 

compressor and compressor turbine run at much higher speeds for better efficiency. The high shaft speed for the 

compressor and compressor turbine reduces their sizes and improves performance. The high shaft speed results in a 

small size and better sealed system, and is very suitable for small power unit, as indicated in Table 1. 

Figure 2 shows a face-to-face layout of compressors and the compressor turbine that improves thrust balance. A 

motor/brake system is inserted in the compressor and compressor turbine for startup and shutdown. Power turbine 

runs at constant speed in synchronization with the grid frequency. The face-to-face layout for power turbines also 

cancels the force exerted on the thrust bearing. 

Table 1 shows the turbine size, shaft speed, and CO2 mass flow rate for power rating of 0.3, 3 and 300 MW. At the 3 

MW level the turbine size is 15 cm (6 inch) with a speed of 50,000 RPM, and it is possible to locate the turbine in 

the receiver. The module power selected in the current design is between 5 MWe to 10MWe, so as to take advantage 

of S-CO2 Brayton turbine/compressor compactness. 
3 

The power rating will be designed for maximum system performance, size and weight, and commercial availability 

for a modular solar field. Future performance and cost models will study whether recompression benefits would 

outweigh the complexity and cost of additional compressor stage and heat exchanges. 

Table 1. Turbine and compressor size vs. power (Ref. [7]) 

Power Rate 

(MW) 

Turbine Wheel 

diameter (m) 

Desired Shaft 

Speed (RPM) 

CO2 Flow 

(kg/sec) 

0.3 0.04 125,000 3.5 

3 0.15 50,000 35 

300 1.5 3,600 3500 

Starting from the basic modular design, the energy storage option is considered by adding the thermal energy 

storage for the capability of continuous power generation. 

Figure 2. Schematic of a solar thermal tower receiver with embedded dual-shaft, S-CO2 Brayton Cycle 

power generation. 

2. Modular receiver with high-temperature, thermal energy storage integration 

Modular S-CO2 system plus thermal energy storage (TES) can reduce the impact of weather conditions on 

generation variability. Implementation of a large TES for longer storage hours may shift generation to accommodate 

peak hours or allow for continuous power generation. Unlike water/steam Rankine cycles, S-CO2 undergoes no 

phase change and can be matched to current molten salt TES technology. Using design option (1) as a starting point, 

this design adds TES tank in the field to store solar energy for use during peak demand or under no-solar heat 

conditions. The TES system would probably be ground-mounted and shared between towers. This design can 

provide short term storage for weather transition and load shift simply and economically. 

Several operating CSP plants use molten salt TES. The “two-tank” salt system maintains hot and cold salt in 

separate tanks. During discharge, the salt is pumped from hot tank to cold tank through heat exchangers that heat the 

HTF. The process is reversed during charging. The flexibility of the system allows for operating modes described as 

generation mode, charge mode, and discharge mode: 

• Generation mode: All HTF used for power generation turbine and compressor turbine. 

• Charge mode: Power generation turbine stops or operates at partial load, and part of the S-CO2 is sent to the 

storage system, while heat is stored in the thermal energy storage tank. 
4 

• Discharge mode: Compressor and compressor turbine are driven by the thermal energy from the storage 

tank instead of receiving heat from solar receiver. The power turbine is driven by the high-pressure, hightemperature S-CO2 from TES heat. This storage design provides short-term energy to drive the engine and 

bridge weather conditions such as passing clouds.

The shortcomings of a two-tank salt system include high system and material costs and a temperature cap (less than 

600 °C) for salt stability. Other TES technologies under development involve thermocline TES, use of phase-change 

material, or other low-cost, stable material for high performance and more economical operations. Figure 1 depicts a 

generic TES system. 

The advantage of a modular S-CO2 tower receiver/generator configuration is analogous to a modular dish-Stirling 

engine of system in terms of factory fabrication and easy deployment. Instead of the low power range (~10 kWe for 

a Stirling engine), the modular S-CO2 plant reaches multi-MWe levels that bring a scale advantage in terms of

maintenance and cost benefits. In addition, the s-CO2 process utilizes more reliable turbomachinery rather than 

reciprocating Stirling engines. The scale of 5-10 MWe is compatible with small tower systems under development 

by eSolar, which feature close-packed heliostat fields that have lower land usage than other CSP configurations. 

Conclusion

Supercritical CO2 operated in a closed-loop recompression Brayton cycle offers the potential of equivalent or higher 

cycle efficiency versus supercritical or superheated steam cycles at temperatures relevant for CSP applications. The 

S-CO2 pressure is higher than superheated steam but lower than supercritical steam at temperatures of interest. The 

high pressure required for S-CO2 makes application to trough fields difficult. A small tower design is recommended 

for simplicity in the power system. A single-phase process using S-CO2 as both HTF and thermal cycle fluids would 

simplify the power block machinery and is compatible with sensible-heat thermal energy storage. The uncertainties 

in the utilization of such a cycle are: the high pressure required and lack of experience with closed loop Brayton 

cycles. This is an area of active research for next-generation nuclear power plants.

Acknowledgment

This research is sponsored by the US Department of Energy under Agreement #15085.

References

[1] Vaclav Dostal, Pavel Hejzlar, Michael J. Driscoll, “High-Performance Supercritical Next-Generation Nuclear 

Reactors”, Nuclear Technology Vol. 154, pp. 265-282, June 2006.

[2] Vaclav Dostal, Pavel Hejzlar, Michael J. Driscoll, “The Supercritical Carbon Dioxide Power Cycle: Comparison 

to Other Advanced Power Cycles”, Nuclear Technology Vol. 154, pp. 283-282, June 2006.

[3] Argonne National Laboratory, "Performance Improvement Options for the Supercritical Carbon Dioxide Brayton 

Cycle," ANL-GenIV-103, 2007.

[4] Chapman, D.J., and D. Arias, “An Assessment of the Supercritical Carbon Dioxide Cycle for Use in a Solar 

Parabolic Trough Power Plant,” Proceedings o SCCO2 Power Cycle Symposium 2009, Troy, NY April 29-30, 2009.

[5] Wagner, M., “Simulation and Predictive Performance Modeling of Utility-Scale Central Receiver System Power 

Plants”, M.S. thesis, University of Wisconsin, Madison, 2008.

[6] Wright, S. A. et al., Operation and Analysis of a Supercritical CO2 Brayton Cycle, Sandia Report, No. 

SAND2010-0171, (2010).

[7] Turchi, C., Supercritical CO2 for Application in Concentrating Solar Power Systems, Proceedings of SCCO2 

Power Cycle Symposium 2009, Troy, NY April 29-30, 2009.

[8] Fuller, R. L., and Batton, W., Practical Considerations in Scaling Supercritical Carbon Dioxide Closed Brayton 

Cycle Power Systems, Proceedings of SCCO2 Power Cycle Symposium 2009, RPI, Troy, NY, April 29-30, 2009.

[9] Kelly, B., “Advanced Thermal Storage for Central Receivers with Supercritical Coolants,” Final Report under 

Grant DE-FG36-08GO18149, Abengoa Solar Inc, Lakewood, CO, June 15, 2010.
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