Twelve partners joined forces in the EU-funded TRI-HP project to develop innovative trigeneration systems. Their goal was to develop eco-friendly, affordable energy systems for apartment buildings by combining heat pumps, solar power and energy storage, using 80 % of the 100 % self-produced renewable energy onsite.
The shift from synthetic to natural refrigerants
Aware of the damaging effects of synthetic refrigerants – used since the 1930s and linked to ozone depletion, global warming and health issues – the team sought eco-friendly alternatives. As the core components of heat pumps, sustainable natural refrigerants are essential for minimising the system’s environmental impact. Project coordinator Daniel Carbonell stresses that natural refrigerants (hydrocarbons, water, ammonia and carbon dioxide) provide the most reliable, long-term solution for heat pumps. With this in mind, TRI-HP developed two heat pumps with propane and one with carbon dioxide. Project members conceived two groundbreaking concepts to provide renewable heat to the heat pumps: the solar-ice slurry system with the supercooling method and the dual source/sink system.
Heat pump system tapping into ground and ambient air
The dual source/sink reversible heat pump system utilises propane as a refrigerant. It is designed to harness two renewable heat sources (ground and ambient air) in the evaporator of the heat pump to provide heating. The same sources also serve as heat sinks for meeting cooling requirements. In simpler terms, this heat pump system can efficiently switch between using heat from the ground and heat from the ambient air, depending on which source is more suitable at a given time. A key benefit of this dual system is that it has proved to reduce the need for extensive drilling in central Europe.
Harnessing the sun and storing heat in the frost
The solar-ice slurry system serves as a sustainable solution for heating buildings with the capability to provide some free cooling, addressing certain limitations of traditional ground source heat pump (GSHP) systems. By using solar thermal collectors and ice storage, the system captures solar energy during the summer months and stores it melting the ice to provide heating in the winter. The system also partially charges and discharges daily in the winter to compensate for periods without sufficient solar irradiation such as night-time. “A notable breakthrough was the development of the ice slurry concept that utilises a supercooling method and ice repellent coatings. This innovation allows removing internal heat exchangers from the ice storage, leading to significant cost savings in system installation,” remarks Carbonell. “For the first time in Europe, a supercooling ice slurry heat pump system was successfully demonstrated in the laboratory, marking a substantial advancement in this technology,” adds Carbonell. “Our simulations reveal that solar-ice slurry systems can achieve higher efficiencies at comparable costs to state-of-the-art GSHP systems, offering several additional benefits.”
Setting the stage for a sustainable residential energy future
Researchers developed an open-source simulation framework called pytrnsys to evaluate the cost and energy potential of their TRI-HP systems across Europe. The propane-based solar-ice slurry system achieved high annual system performance factors around 4, improving GSHP energetic efficiencies by 5-8 % in certain cities. The levelised cost of energy ranged from EUR 16 to 26.7 ct/kWh, depending on the location. TRI-HP’s innovative trigeneration systems, harnessing natural refrigerants and renewable sources, will pave the way for a greener future in residential energy management.