Periodic Reporting for period 1 - HYDROCOOL (Novel advanced hydraulic CO2 refrigeration system for multiple sectors)
Reporting period: 2024-09-01 to 2025-08-31
The HydroCool project addresses one of the fastest-growing sources of global energy demand: Cooling. Air-conditioning, refrigeration, and industrial chilling account for about 20% of global electricity use and significant greenhouse gas emissions. Most existing systems rely on high-impact fluorinated refrigerants and inefficient mechanical compression technologies.
HydroCool proposes a radical alternative: a hydraulic CO2 refrigeration system that uses liquid-based compression and expansion instead of conventional mechanical components. By switching from solid to fluid mechanics, the project aims to make CO2 (a natural refrigerant with zero ozone depletion and a global warming potential of one), a practical, affordable, and efficient option for all climates and applications.
The project’s overall goal is to develop, test, and validate a 17.6 kW pilot system that demonstrates hydraulic isothermal compression and energy recovery from expansion, achieving a step-change in performance and sustainability. The target applications range from air-conditioning to deep-freezing (–40 °C to +12 °C). In parallel, HydroCool aims to define circularity criteria, evaluate environmental and economic benefits, and prepare the pathway for market uptake.
HydroCool proposes a radical alternative: a hydraulic CO2 refrigeration system that uses liquid-based compression and expansion instead of conventional mechanical components. By switching from solid to fluid mechanics, the project aims to make CO2 (a natural refrigerant with zero ozone depletion and a global warming potential of one), a practical, affordable, and efficient option for all climates and applications.
The project’s overall goal is to develop, test, and validate a 17.6 kW pilot system that demonstrates hydraulic isothermal compression and energy recovery from expansion, achieving a step-change in performance and sustainability. The target applications range from air-conditioning to deep-freezing (–40 °C to +12 °C). In parallel, HydroCool aims to define circularity criteria, evaluate environmental and economic benefits, and prepare the pathway for market uptake.
During its first reporting period, HydroCool completed all initial research and design stages.
A comprehensive thermodynamic model of the hydraulic CO2 cycle was developed to simulate subcritical and transcritical operations. The simulations demonstrated that isothermal compression can raise the Coefficient of Performance (COP) from about 2.6 to nearly 9, showing the large efficiency potential of hydraulic compression.
Extensive CFD studies were performed to analyse gas–liquid mixing inside the pressure vessels and validate the feasibility of isothermal compression. The results confirmed up to 40% lower energy consumption compared to mechanical compressors.
A screening of ten potential liquid piston fluids (LPFs) was carried out, combining aqueous salts, organic solvents, and deep eutectic solvents. Laboratory measurements of melting point, viscosity, and CO2 solubility led to the selection of an aqueous solution of 1-butylpyrrolidin-2-one as the most promising candidate, stable down to –40 °C.
A life-cycle and circularity assessment quantified HydroCool’s environmental and economic advantages. The system could reduce climate-change impact by over 50% (0.144–0.184 kg CO2-eq/kWhcooling) and halve the levelised cost of cooling to 0.17–0.21 €/kWhcooling compared with current technologies.
On the engineering side, the consortium finalised the complete mechanical and control design of the 17.6 kW pilot unit, including high-pressure compressor, expander, and heat-exchange components. Manufacturing of core parts began early to secure materials and avoid supply delays.
All planned deliverables and milestones for the first project year were achieved, establishing a solid foundation for prototype testing in the next period.
A comprehensive thermodynamic model of the hydraulic CO2 cycle was developed to simulate subcritical and transcritical operations. The simulations demonstrated that isothermal compression can raise the Coefficient of Performance (COP) from about 2.6 to nearly 9, showing the large efficiency potential of hydraulic compression.
Extensive CFD studies were performed to analyse gas–liquid mixing inside the pressure vessels and validate the feasibility of isothermal compression. The results confirmed up to 40% lower energy consumption compared to mechanical compressors.
A screening of ten potential liquid piston fluids (LPFs) was carried out, combining aqueous salts, organic solvents, and deep eutectic solvents. Laboratory measurements of melting point, viscosity, and CO2 solubility led to the selection of an aqueous solution of 1-butylpyrrolidin-2-one as the most promising candidate, stable down to –40 °C.
A life-cycle and circularity assessment quantified HydroCool’s environmental and economic advantages. The system could reduce climate-change impact by over 50% (0.144–0.184 kg CO2-eq/kWhcooling) and halve the levelised cost of cooling to 0.17–0.21 €/kWhcooling compared with current technologies.
On the engineering side, the consortium finalised the complete mechanical and control design of the 17.6 kW pilot unit, including high-pressure compressor, expander, and heat-exchange components. Manufacturing of core parts began early to secure materials and avoid supply delays.
All planned deliverables and milestones for the first project year were achieved, establishing a solid foundation for prototype testing in the next period.
HydroCool delivers a fundamentally new refrigeration concept that goes beyond the limitations of today’s CO2 and fluorocarbon-based systems.
The project demonstrates, for the first time, that hydraulic isothermal compression and expansion can be applied to small- and medium-scale cooling systems. This innovation allows direct energy recovery from the expansion process, reducing compressor power needs by 40–60% and doubling energy efficiency compared with the best available vapor-compression systems.
The newly identified liquid piston fluid represents another scientific breakthrough: it maintains liquid stability and low CO2 solubility at pressures up to 80 bar and temperatures down to –40 °C, enabling CO2 use for deep-freezing applications — a long-standing challenge in the field.
HydroCool’s integrated approach also achieves a 53% reduction in greenhouse gas emissions and 50% lower lifetime costs, proving that high performance and environmental sustainability can be achieved simultanously.
To reach full market readiness, further steps are needed:
- long-term testing and optimisation of the 17.6 kW pilot (TRL 6–7);
- partnerships with industrial manufacturers for scale-up and certification;
-protection and licensing of the key intellectual property (compressor, expander, and LPF formulation);
- engagement with regulators and standards bodies to formalise testing procedures for hydraulic compression systems;
-and access to investment for demonstration and commercialisation.
Once demonstrated, HydroCool will enable a new generation of cost-effective, safe, and low-carbon refrigeration systems, applicable to food storage, industrial processing, data centres, and building air-conditioning. By doing so, it contributes directly to the EU Green Deal, the Energy Efficiency Directive, and global climate-mitigation goals.
The project demonstrates, for the first time, that hydraulic isothermal compression and expansion can be applied to small- and medium-scale cooling systems. This innovation allows direct energy recovery from the expansion process, reducing compressor power needs by 40–60% and doubling energy efficiency compared with the best available vapor-compression systems.
The newly identified liquid piston fluid represents another scientific breakthrough: it maintains liquid stability and low CO2 solubility at pressures up to 80 bar and temperatures down to –40 °C, enabling CO2 use for deep-freezing applications — a long-standing challenge in the field.
HydroCool’s integrated approach also achieves a 53% reduction in greenhouse gas emissions and 50% lower lifetime costs, proving that high performance and environmental sustainability can be achieved simultanously.
To reach full market readiness, further steps are needed:
- long-term testing and optimisation of the 17.6 kW pilot (TRL 6–7);
- partnerships with industrial manufacturers for scale-up and certification;
-protection and licensing of the key intellectual property (compressor, expander, and LPF formulation);
- engagement with regulators and standards bodies to formalise testing procedures for hydraulic compression systems;
-and access to investment for demonstration and commercialisation.
Once demonstrated, HydroCool will enable a new generation of cost-effective, safe, and low-carbon refrigeration systems, applicable to food storage, industrial processing, data centres, and building air-conditioning. By doing so, it contributes directly to the EU Green Deal, the Energy Efficiency Directive, and global climate-mitigation goals.