For a technology once hailed as a breakthrough in clean energy, the supercritical carbon dioxide (sCO₂) power cycle is facing an uncomfortable truth: it may simply not be worth it.
A new study, led by researchers at the German Aerospace Center (DLR) in partnership with Siemens Energy, delivers the most comprehensive cost-benefit analysis yet of sCO₂ cycles integrated into solar thermal power plants. The verdict is sobering. Even under generous assumptions, sCO₂ systems remain significantly more expensive than the steam-based technologies they are designed to replace.
The findings matter. The European Union and other industrial blocs have made high-temperature solar thermal energy a pillar of their net-zero transition plans. These systems can store and dispatch renewable energy long after the sun has set — unlike solar photovoltaics, which rely on batteries. And for years, engineers and policymakers alike have treated sCO₂ as the next logical step: more compact, more efficient, more modern.
Yet the research suggests that, in this case, chasing efficiency may be a strategic misstep.
A Technology Caught in Its Own Complexity
The principle behind sCO₂ systems is straightforward. When carbon dioxide is compressed beyond its critical point — a state in which it behaves like both a gas and a liquid — it becomes an exceptionally effective working fluid for transferring heat and generating power. Turbines can be smaller, cycle efficiencies higher, and system response times sharper.
But the reality is more complicated. The researchers evaluated six sCO₂ cycle configurations, each integrated with a next-generation solar receiver that uses falling ceramic particles to capture and store heat at up to 900°C. The comparison point was a state-of-the-art subcritical steam cycle. Both systems were tested under identical conditions using an hourly simulation over an entire year at a solar-rich site in South Africa.
The outcome was unequivocal: across all sCO₂ cycle types, the levelised cost of electricity (LCOE) was at least 9% higher than for the steam reference case — a gap that widened with more complex designs. Even when the researchers halved the assumed cost of key sCO₂ components such as heat exchangers and compressors, the technology still could not compete on cost.
Efficiency Isn’t Everything
The result is counterintuitive. The sCO₂ cycles — particularly the recompression and partial cooling variants — consistently achieved higher thermal efficiencies than the steam system. But they also introduced greater cost and engineering complexity.
The key culprits were threefold:
- Heat exchangers that must operate at extremely high temperatures and pressures — particularly the primary heat exchanger linking the particle receiver to the sCO₂ loop.
- Compressors, which are not only more expensive than steam pumps but also more numerous.
- Thermal storage systems, which became bulkier and costlier in the sCO₂ case because the cycles required narrower temperature differences to operate efficiently.
In other words, the gains in efficiency were real — but so were the costs required to achieve them.
Flexibility Without Payoff
Another often-cited advantage of sCO₂ systems is their operational agility. Unlike steam plants, which can be slow to start up or respond to changes in grid demand, sCO₂ systems can ramp quickly and operate efficiently across a range of loads.
But in this study, that advantage proved largely academic. The dynamic simulations showed that sCO₂’s faster response had negligible impact on annual energy output. Moreover, the systems showed greater performance volatility at off-design conditions, especially during cooler ambient temperatures — a particular challenge for regions with wide daily temperature swings.
Rethinking the Innovation Pipeline
It would be tempting to interpret these findings as a rejection of sCO₂ altogether. That would be premature. The authors acknowledge that sCO₂ still holds promise — particularly for smaller-scale or modular applications, industrial heat recovery, or as a component in long-duration energy storage systems.
Yet for large-scale, next-generation solar thermal plants — the kind needed to support decarbonised grids in the 2030s and beyond — steam remains not only the devil we know, but the devil we can afford.
There is also a larger lesson here: innovation must be evaluated not only by what it promises, but by what it delivers in context. High thermal efficiency may be a marvel of engineering, but it is not a synonym for economic viability. A holistic view — including material costs, system integration, performance stability, and market readiness — is what ultimately determines whether a technology moves from lab to landscape.
As Europe and others continue to invest in the green transition, this research offers a timely warning against seductive but costly detours. In the race to reinvent how we generate power, not every upgrade is an improvement.
Source
Cost benefit analysis of supercritical CO2 cycles in next-generation solar thermal power plants, Renewable Energy, 2025-06-27
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