Supercritical CARbon dioxide/Alternative fluids Blends for Efficiency Upgrade of Solar power plant

Almost all concentrated solar power plants in operation worldwide adopt a conventional steam cycle for the conversion of thermal power into electricity, with just a few exceptions that are based on organic Rankine cycle (ORC) technology; nevertheless, these exceptions have a minor share of the total installed capacity since they have typical outputs in the order of a megawatt. More recently, supercritical CO2 (sCO2) technology has been identified as a potential major breakthrough in CSP applications, enabling a leap towards lower costs of solar thermal electricity. The potential of sCO2relies on the following features: (i) liquid or liquid-like (i.e. low compressibility) state of the working fluid during compression, which drastically reduces the associated work, and (ii) low expansion ratio of the cycle and low isentropic exponent of the working fluid (CO2), which largely increase the potential to recuperate heat within the cycle. The implementation of this concept would boost the performance of CSP plants but, at the same time, it poses significant technical challenges, the most relevant of which is achieving the very low temperature required at the start of the compression phase (to take the working fluid to liquid or liquid-like state). Indeed, the need to get close to the critical temperature of CO2 (31ºC) in a Brayton cycle embodiment, or even below it if a Rankine cycle is adopted (20ºC), makes it very difficult to attain and fully exploit the potential of the sCO2 cycle, particularly in typical CSP locations characterized by high ambient temperature and scarce water availability for cooling purposes.

To overcome this hurdle, some research groups proposed the addition of small quantities of selected compounds to the standard working fluid (pure CO2), yielding the so-called blended CO2, with the aim to raise the corresponding critical temperature and enable condensation at temperatures of 50°C to 60°C. The investigation of CO2 blends has mostly been carried out for geothermal and biomass applications to date, with maximum temperatures around 350-400°C. For these cases, results showed the potential of CO2-blends to increase the conversion efficiency by 30%.

Utilising these concepts, SCARABEUS moves the CO2 blend concept forward by developing mixtures which not only enable dry-cooled condensation in warm environments but can also withstand operating temperatures (at turbine inlet) up to 700ºC, hence tackling the two most influential factors affecting cycle efficiency.

Once the most appropriate CO2 blend is selected, the pseudo-supercritical temperature of the resulting working fluid can be increased notably (above 60ºC), which brings about the following key advantages over either steam or standard sCO2 cycles in parabolic trough and solar tower CSP plants:

Condensing (Rankine) sCO2 cycles are enabled in typical CSP locations, thus boosting the net heat to electricity efficiency to above 50%;
The complexity of the power block is largely reduced since only one recuperator and one primary heat exchanger are necessary as opposed to more than ten heat exchangers (six feed-water preheaters, one economizer, one evaporator, one super-heater, one re-heater) typically adopted in a steam cycle (see Figure 2, left).
The size of turbomachinery is significantly reduced compared to steam turbines of similar power output (lower volumetric flowrate as shown in figure), bringing about a large reduction in capital costs.

A preliminary comparison between state-of-the-art solar tower plants using Solar Salts as heat transfer fluid (HTF) in the receiver and TES and other plant configurations based on supercritical CO2 is shown in the table below. Amongst the CO2-based layouts, results are given for pure and blended CO2 and for two different HTFs: Solar Salts and Sodium. The results show the outstanding performance achievable for blended-CO2

The large potential for CAPEX and OPEX reduction targeted by SCARABEUS has been demonstrated by a preliminary economic assessment of the impact of adopting CO2 blends on the specific costs of the power block and the overall plant. The accompanying figure shows an economic comparison between the steam, pure CO2 and blended CO2 technologies for a reference power plant with 100 MWel output, obtained by well-referenced methodologies available in literature.