Techno-economic assessment of a high-density fluid pumped hydro system: modelling, market revenues, and benchmarking against water-based configurations

High-density fluids have recently emerged as a promising strategy to increase the energy density of pumped-hydro storage (PHS) and unlock installations in sites where conventional water-based systems are unfeasible. Yet, no integrated assessment exists quantifying their realistic technical performance, costs, and economic viability under actual electricity market conditions. This study develops a comprehensive hydraulic and techno-economic model of a 500-kW high-density PHS system using R-19, a non-Newtonian fluid with density 2.5 times that of water, based on the specifications of the RheEnergise DEMO plant. The system is benchmarked against two equivalent water-based configurations designed to deliver the same power. Results show that the R-19 system reduces the total installation cost by 43–46% compared with the water-based configurations, owing to smaller reservoirs, penstocks and hydraulic machines, but exhibits a lower round-trip efficiency (53.5%), mainly due to the lower pump efficiency assumed for the R-19 fluid and, to a lesser extent, to auxiliary consumption and higher hydraulic losses. Annual revenues from the Italian DAM and IDM markets are therefore lower (7.7 k€/year), yielding a long payback period. However, this payback period is similar to that of the water-based systems, which also suffer from the small scale of the considered case study. A sensitivity analysis on the site topography further shows that the mean terrain slope is an economic parameter of the first order: the advantage of the water-based configuration vanishes as the terrain steepens, and above a mean slope of approximately 80% the R-19 configuration becomes the most favourable of the three, identifying steep sites and disused mining areas as its natural application domain. Overall, while high-density fluids currently face efficiency limitations, the study highlights their potential for compact, site-constrained storage applications and identifies machinery optimization as a key requirement for future viability.