Supercritical CO₂ for concentrating solar power

Dispatchability is still the Achilles heel of energy obtained from intermittent renewable sources. It is in this context where new developments capable of providing affordable, safe energy with high availability and dispatchability gain importance, such as Concentrated Solar Power (CSP) with storage.

Despite the efforts made in the development of renewable energy to combat climate change, dispatchability continues to be the Achilles heel of energy obtained from intermittent renewable sources. Available technologies can provide clean energy at low cost, but its production is intermittent and not very predictable. For this reason, institutions, governments and companies are working together in an effort to develop and improve energy sources capable of providing affordable, safe energy with high availability and dispatchability, such as CSP with storage.

“The use of supercritical CO₂ in concentrating solar energy has multiple advantages not only for the dispatchability of the plants but also for cost reduction.

One of these lines of research involves the use of supercritical CO₂ cycles for both conventional and solar thermal power plants. Carbon dioxide (CO₂) is a simple molecule basic for human life (photosynthesis, respiration, etc.) and it has physicochemical characteristics relevant for several uses. The supercritical state is a state of the matter in which the limits between the liquid state and the gaseous state are blurred, the substance behaves with mixed properties, diffusing and spreading like a gas, but dissolving substances as if it were a liquid.  In the case of the CO₂, the supercritical state (s-CO₂ or sCO₂) occurs when CO₂ is at temperatures higher than 31ºC and pressures greater than 73 atmospheres.

The sCO₂ cycle has a higher performance than conventional steam cycles. The increment, from around 40% in steam to around 50% in sCO₂, is strongly related to the higher working temperature of the new cycle. Furthermore, CO₂ has a specific volume much lower than steam, which has the fundamental advantage of reducing the size of the equipment and interconnections, significantly reducing the cost of the plant. Finally, the smaller size in pipelines presents a second benefit. And it is that the impact of increasing the thickness when operating at high pressure and high temperature has a lower cost in sCO₂ cycles than in steam cycles, where it works with a larger diameter and therefore the thickness of the pipeline has bigger economic impacts.

“The use of supercritical CO₂ implies an increase in performance, a reduction in the size of the equipment and a reduction in water consumption.

The increase in performance, the reduction in the size of the equipment and the reduction in the use of such an important resource as water are a great incentive to make sCO₂ technology a mature and viable technology. In this sense, there is great pressure at the international level to achieve this. The United States is betting heavily on sCO₂, with one of the most ambitious projects for its integration into CSP: Gen3; Europe, under the Horizon 2020 program, is financing research and development projects such as SCO2Flex, SCO2hero, Scarabeus or SolarsCO2OL; China, for its part, aspires to be the first to start up a sCO₂ project in Shouhang with a power level of 10 MW.

Aligned with these international developments, Abengoa is currently participating in two European projects centered around sCO₂: Scarabeus and SolarsCO2OL. These projects try to solve different technical challenges in order to quickly introduce sCO₂ technology into the concentrated solar energy market.

Cristina Prieto Ríos,director of Solar Thermal Innovation at Abengoa