Demonstrated energetic performance and cost competitiveness of TRI-HP systems

Within the TRI-HP project, we have developed trigeneration integrated solutions based on natural refrigerant heat pumps with multiple renewable energy sources (air, ground and solar) to provide high-efficiency heating, cooling and electricity to new and refurbished multifamily residential buildings with renewable share 80%. ThecTRI-HP solutions are the solar-ice slurry system and the dual-source heat pump system.
The solar-ice slurry system consists of a natural refrigerant heat pump (either propane-R290 or CO2-R744), able to supercool water to produce ice slurry, which makes use of solar thermal collectors to regenerate the ice slurry storage used as the heat source for the heat pump. The solar thermal collectors that supply mostly heat to the ice slurry in winter can also provide direct heat to Domestic Hot Water (DHW) and Space Heating (SH) in the other seasons. The solar-ice slurry system is a meaningful concept to supply heating demands to buildings in heating-dominated climates with enough solar radiation in winter, as in most Central Europe regions. More specifically, the system using the CO2 heat pump is targeting new buildings with a high share of DHW demands due to the characteristics of the supercritical refrigerant cycle that undergoes a large temperature glide in the CO2 gas cooler. The other system employing the propane heat pump targets refurbished buildings with a higher share of SH.
The dual-source/sink heat pump (DSHP) system, which uses a natural refrigerant propane (R290) heat pump, is able to use two renewable heat sources and sinks, e.g. ground and ambient air in the heat pump, in order to
optimize heat pump operation conditions, and reduce the borehole length by up to 25%.
The present deliverable compares the energetic performance and cost competitiveness of TRI-HP systems and reference systems for heating (DHW and SH), cooling and electricity based on experimental and numerical analyses. There are two main reference systems to which TRI-HP systems are compared: the conventional solar-ice "ice-on-coil" system and the Ground Source Heat Pump (GSHP) system.
After having performed static tests with the heat pump prototypes to verify their performance in standard, steady-state conditions, more realistic tests of the whole TRI-HP systems have been carried out to evaluate their performance in relevant dynamic boundary conditions. For this purpose, the hardware-in-the-loop Concise Cycle Test (CCT) has been applied using several representative days as the methodology to represent the whole year or a season. With this methodology, dynamic experiments of only a few days can be performed in the laboratory, allowing us to accurately characterize the system performance during the year for a specific climate and demand profile. These tests also included the electrical part, emulating the PV field and the household electricity demand, and physically installing the battery, its management and the inverter in the laboratory.
The tests for the solar-ice slurry system for SH and DHW applications have been performed at the Institute for Solar Technology (SPF-OST) in Switzerland. The Swiss company HEIM AG designed and manufactured the two natural refrigerant heat pumps. The solar-ice slurry systems achieved an annual System Performance Factor (SPF) between 4 and 4.8 with the climate of Bern (Switzerland) for the CO2 and propane heat pumps slurry systems for a new and refurbished building, respectively. These energetic performances are higher than the typical values achieved by GSHP systems for this climate and equivalent demands.
The tests of the dual source/sink system were performed in the SEILAB laboratory of IREC in Spain and resulted in an overall SPF of 3.7 in a Mediterranean climate where both heating and cooling were necessary. Although the SPF was expected to be higher, it was achieved with the first dual source/sink heat pump prototype.
With the current results of the CCT test presented in this deliverable, we have demonstrated the autonomous operation of the complete TRI-HP systems in a relevant environment using the accelerated hardware-in-the-loop approach, including the integration of both thermal (heat pump, storage tanks, hydraulics and control) and electrical elements (battery, inverter, PV). Regarding the solar-ice slurry system, we can state that the supercooling slurry heat pump has reached a TRL of 6, even higher than the TRL 5 expected at the proposal phase. For the complete system, we have reached a TRL of 5, as anticipated at the proposal phase. A TRL 6 was not achieved at a complete system level since the ice crystallizer and storage vessel was not physically installed. The coupling of the supercooling ice slurry heat pump with an ice crystallizer and an ice slurry storage would be the next step in future projects to bring the complete system to a TRL 6 and beyond. Regarding the dual source/sink system, a TRL of 6 has been achieved for the dual source/sink heat pump, successfully testing the whole system using the CCT method.

Experimental results of the CCT have been used to validate the TRNSYS simulation models implemented in the pytrnsys framework1. For the energetic performance and cost competitiveness analysis of the TRI-HP system, different pytrnsys/TRNSYS detailed models of the TRI-HP systems and their references to compare with have been developed.
The cost-competitive advantage of solar-ice slurry TRI-HP systems over the conventional solar-ice with a synthetic refrigerant has been proven. The investment costs of the solar-ice slurry TRI-HP systems are between 8% to 11% lower than the reference ice-on-coil system in Bilbao and 14% to 17% lower in Zurich. The heat generation costs in cte/kWh are between 7% to 12% lower for the slurry TRI-HP version. Furthermore, the potential for full
integration of ice slurry storage into the building’s construction envisages a significant cost reduction which, not being reachable to the conventional ice storage would bring even a higher cost competitive advantage for the TRI-HP system.
Regarding the cost-efciency competitive advantage of the solar-ice slurry TRI-HP system over the GSHP system with a synthetic refrigerant, the following can be concluded after comparing two climates and two buildings:
• The energetic efficiency of the solar-ice slurry system as a dimension here is always higher than the GSHP systems, with improvements in SPF in the range of 5 % to 22 %. This might seem counterintuitive since the ground source is supposed to be a better heat source than ice. However, one needs to take into account that i) solar thermal collectors can provide heat for DHW and SH directly ii) the ice storage is typically only in ice form for three months in winter, and iii) the typical dimension of boreholes using Swiss SIA-Norm 384/6 leads to temperatures on the ground heat exchanger around 0 °C after five years, which is the same as the ice slurry source.
• The investment costs of solar-ice slurry TRI-HP systems are between 13% to 29% lower compared to GSHP, and the heat generation cost are between 5% to 17% lower. However, the cost comparison results with the GSHP should be taken with caution. The cost of GSHP can vary a lot depending on the ground type, country and sizing procedure; therefore, these results can be inverted depending on conditions. What is clear is that the solar-ice slurry system developed in TRI-HP has the potential to be cost-competitive even with well-established GSHP systems that have existed on the market for decades. However, GSHP can be used for heating and cooling, while solar ice systems can only provide some free passive cooling from the ice storage in its current form. To include active cooling, one would need to add an additional air heat exchanger for heat rejection, which has not been considered within the TRI-HP project but is technically possible.
The dual-source/sink heat pump system has been compared to a pure GSHP system for the climate of Zurich and the refurbished building MFB90. In this case, the borehole length could be reduced by 25%, which leads to an investment cost of the GSHP, which is 9% higher than the DSHP system. These cost reductions are expected to be less important for climates where both heating and cooling are possible due to the regeneration of the borehole field by the cooling demands.
More results of both systems are provided in the last deliverable of the TRI-HP project [2], where we have studied the benefits of all TRI-HP systems around Europe.