A new study led by researchers at the German Aerospace Center (DLR) and Siemens Energy has delivered a sobering verdict on the future of supercritical carbon dioxide (sCO₂) power cycles in next-generation solar thermal power plants. Despite the theoretical advantages of sCO₂ — higher efficiency, compact design, and faster ramp-up—the research concludes that steam still reigns supreme when it comes to cost.
The Promise and the Problem
Supercritical CO₂ cycles have long been considered a potential game-changer for power generation, particularly in Concentrated Solar Power (CSP) plants. These systems operate at extremely high pressures and temperatures, allowing for compact turbine systems and the promise of thermal efficiencies exceeding 50%.
But turning that promise into commercial reality has proven difficult. According to lead author Lukas Heller, even under optimistic assumptions, sCO₂-based CSP plants had at least 9% higher levelised cost of electricity (LCOE) than a reference steam-based plant. No matter which cycle configuration or turbine inlet temperature (550°C or 650°C) was used, steam remained the cheaper option.
Methodology: Comparing Apples to Apples
The team modeled six sCO₂ cycle designs — simple recuperated, recompression, and partial cooling variants — against a state-of-the-art subcritical steam system. All systems were integrated with an advanced particle-based solar receiver system using CentRec® technology, which allows temperatures up to 900°C and efficient thermal storage.
Each model included:
- Hourly performance simulations over a full year in sunny Northern Cape, South Africa.
- Detailed component-level costing, including heat exchangers, compressors, turbines, and thermal energy storage (TES).
- A sensitivity analysis varying component costs by ±50% to test economic robustness.
What Drives sCO₂’s Higher Cost?
Several key factors drove up the cost of sCO₂ systems:
- Expensive heat exchangers, especially the particle-to-sCO₂ primary heat exchangers (PHX), which must withstand extreme temperatures and pressures.
- Complex compressors, more numerous and costly than steam pumps.
- Lower temperature spreads in high-efficiency sCO₂ cycles, which increased TES costs and required larger particle inventories and transport systems.
Even under best-case cost reduction scenarios, sCO₂ could not match the LCOE of the steam-based design.
High Efficiency, But Not High Value
One surprising finding was that even high thermal efficiencies didn’t translate into economic gains. The simple recuperated sCO₂ cycle, while the least efficient, often had the lowest cost among sCO₂ options due to its simplicity. Meanwhile, higher-efficiency recompression and partial cooling cycles couldn’t overcome their additional complexity and cost.
Off-Design and Operational Reality
sCO₂ systems also showed more volatile off-design performance under changing ambient conditions—common in CSP sites—while the steam plant handled temperature fluctuations more gracefully, improving performance during cool periods.
Moreover, sCO₂’s much-touted operational flexibility (e.g., faster start-up) contributed negligibly (<1%) to annual energy yield, undermining one of its key selling points.
No Silver Bullet Yet
The authors stress that these findings don’t doom sCO₂ entirely. In smaller-scale plants, medium-temperature applications, or future “Carnot battery” storage systems, sCO₂ might still prove competitive. But for large, next-generation solar thermal power plants, steam remains the more cost-effective and mature technology—at least for now.
Verdict
“Even if the costs of most sCO₂-specific components were cut in half, the technology still doesn’t reach cost parity with modern steam systems,” the study concludes.
Given the urgent need for affordable and scalable renewable energy, this research offers a crucial checkpoint for developers, investors, and policymakers. Rather than chase unproven efficiency, the authors suggest that innovation may be better spent improving what already works: steam.
Source
Cost benefit analysis of supercritical CO₂ cycles in next-generation solar thermal power plants, Renewable Energy, 256, 123613, 2025-06-27
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