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Thrust 2: Multiscale Concentrated Solar Power

SERIIUS' Multiscale Concentrated Solar Power (CSP) research thrust focuses on three coupled activities. They are designed to greatly reduce levelized cost of energy (LCOE) by: 1) increasing the power-block cycle efficiency significantly, and 2) decreasing solar collector cost with innovative designs and optical materials.

  • High-Temperature, High-Pressure, Closed-Cycle CO2 Brayton Cycle: Develop supercritical, 20–80 bar, 600°–800°C Brayton cycle with >50% efficiency (100 kW to 1 MW).
  • Low-Temperature Organic Rankine Cycle: Develop organic Rankine cycle with operating at <330°C and with efficiency >20% (25 kW to 1 MW).
  • Thermal Storage and Hybridization: Develop hybridized storage systems for the diverse temperature ranges of the Brayton and Rankine converters in the first two activities.

High-temperature, high-pressure, closed-cycle CO2 Brayton cycle

A typical large-scale CSP plant based on the steam Rankine cycle has a cycle efficiency of about 35% (for >50 MWe plants). The cycle efficiency falls to as low as 20% for plants of about 1 MWe capacity. Therefore, we will explore other thermodynamic cycles and working fluids for significantly higher cycle efficiencies.

For example, air Brayton cycles with very high receiver temperatures (∼800°–1000°C) can yield an efficiency >40%. The cycle efficiency can be >50% with closed-cycle, high-pressure CO2 Brayton cycles, even with lower receiver temperatures (∼600°–700°C). With a supercritical CO2 Brayton cycle, the cycle efficiencies can even reach >55% with regeneration, even with moderate receiver temperature.

Our main challenge for closed-cycle CO2 Brayton (100 kW–1 MW) is to develop a cost-effective heliostat field and high-temperature storage, plus hybridization that is common to all these cycles. In general, systems operating at higher pressures will have high efficiency even at lower receiver temperatures. Our projects are designed to address critical materials and technology gaps for high-pressure receivers, high-pressure turbo-expanders, and high-pressure heat exchangers. Detailed engineering and economic analyses will also be a part of our work scope.

The table summarizes the research projects and their tasks in the two consortium projects (CSP-1, CSP-2) in this activity.

Projects for High-T, High-P, Closed-Cycle CO2 Brayton Cycle Activity
CSP-1:  High-temperature, pressurized CO2 receiver
Task 1: Modeling, engineering design, and prototyping
Task 2: Testing, validation, and optimization
CSP-2:  Low-cost heliostats for Brayton cyclesTask 1: Heliostat design, development, and testing with control systems

Low-temperature organic Rankine cycle

Organic Rankine cycle (ORC)-based CSP plants are fairly well established at the 500 kW to 1 MW level. The main advantage of these cycles at this scale is that isentropic efficiencies of turbines for moderate-scale plants (few hundred kW) are fairly high (∼70%–80%) compared to steam cycles, which have such high efficiencies only at tens of MW or higher scales.

However, even for ORC, the isentropic efficiency drops drastically at the smaller scales, which makes small-scale ORC economically unviable. For distributed ORC, a huge market exists in the range of 25–500 kW because of ease of land availability at that scale and the nature of electricity needs at the village/district level.

Hence, our major focus is to develop 1) efficient small-scale expanders for ORC, and 2) appropriate fluids to enhance efficiencies at the small scale. Because of low operating temperatures, stand-alone ORC systems cannot be very efficient thermodynamically. But cost-effective collector/tracking systems can be developed for this temperature range to bring down the LCOE significantly.

Our projects are targeted at ameliorating any scale-down penalty due to low isentropic efficiency of turbines in the small scale for ORC cycles ranging from 25 kW to 1 MW (operating at temperatures <300°C).

The table summarizes the research projects and their tasks in the two consortium projects (CSP-3, CSP-4) and one core project (CSPCore-1) in this activity.

Projects for Low-Temperature Organic Rankine Cycle Activity
CSP-3:  ORC collector and optical materialsTask 1: ORC collector and optical materials
CSP-4:  Small-scale positive displacement ORC expanderTask 1: Small-scale positive displacement ORC expander
CSPCore-1: Small-scale turbo-expanders
Task 1: Small-scale turbo-expanders

Thermal storage and hybridization

Our objective for distributed CSP is to supply stable power. Hence, we aim to develop auxiliary heating systems (hybridization) and effective thermal storage systems for Brayton cycle and ORC plants.

For the Brayton cycle plants, we will develop technology related to a biomass hybridization subsystem. For the closed-cycle CO2 Brayton cycles, we will develop appropriate gas-to-gas heat exchangers for transferring the auxiliary heat to the working fluid. We will design similar hybridization systems for lower-temperature ORC; the difference is that heat will be exchanged with the heat-transfer fluid, instead of direct heat transfer with the ORC working fluid.

The table summarizes the research project and its tasks in the one consortium project (CSP-5) in this activity.

Projects for Thermal Storage and Hybridization Activity
Task 1: High-temperature molten salt storage and hybridization for Brayton cycles
Task 2: Low-temperature ORC storage and hybridization