Potential benefits of TRI-HP systems around Europe

In the four years of duration of the TRI-HP project, two main systems have been developed with the aim of reducing installation costs compared to state-of-the-art systems with equivalent energetic performance. The two systems developed have been the solar-ice slurry system with the supercooling method and the dual source/sink system. Both systems made use of natural refrigerant heat pumps coupled with photovoltaics. The solar-ice slurry system has been developed using two heat pumps with supercoolers (acting as evaporators): i) a CO₂ heat pump with a transcritical cycle and a tri-partite gas cooler and ii) a propane heat pump. The dual source/sink system was developed using a propane heat pump with a reversible cycle to produce heating and cooling.

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 in the evaporator of the heat pump to provide heating, e.g. ground and ambient air, which are also used as heat sinks to provide cooling demands. By using two heat sources/sinks, it is possible to optimize heat pump operation's conditions and reduce the borehole length by up to 25 \%. 

The solar-ice slurry system is an alternative solution to ground source heat pump systems (GSHP) where the borehole field is substituted by solar thermal collectors and an ice storage, which acts as a low-temperature seasonal storage to shift summer solar heat to winter, while being partially charged/discharged on a daily basis in winter to cover periods without enough solar irradiation, e.g. nights. These continuous charge and discharge, the use of water as heat transfer medium, and the large latent heat of fusion, makes the levelized cost of storage very attractive. The solar-ice slurry system has several advantages compared to the GSHP systems: i) it does not need space to drill boreholes ii) is not affected by noise and ground water regulations and iii) it does not need to be regenerated. 

In the present deliverable, a cost-energetic analysis of the three TRI-HP systems for providing heating, cooling and electricity to multifamily residential buildings in different European locations is presented based on pytrnsys framework (https://pytrnsys.readthedocs.io/en/latest/) using TRNSYS as simulation core. 

In the previous deliverable, the pytrnsys / TRNSYS models were validated by means of experimental data obtained using the hardware-in-the-loop CCT method. 
Based on those simulation models, and with the capability to easily scale technologies and extrapolate simulations to other locations from pytrnsys, a parametric study for different Europe locations and demands has been carried out to evaluate the cost-energetic potential of TRI-HP systems around Europe. For this parametric study, 940 simulations have been carried out that lead to the conclusions summarized hereafter.

The solar-ice slurry system is able to reach SPF in the order of 4 for all locations and buildings with the exception of Warsaw and Helsinki, where the low irradiation limits the use of solar heat achieving SPF above 3.5. For the DSHP, the SPF is usually lower, in the range of 3.5 to 4, due to the lack of solar collectors that can provide solar direct heat in comparison to the solar-ice slurry system. The overall SPF includes the electrical consumption of the heat pump compressor, electric back-ups, circulation of all pumps, control and penalties for not reaching the set-points. Moreover, the multi-family buildings have a constant re-circulation loop for DHW where around 15 % of the DHW is lost and has to be supplied at a high temperatures of 55-58 °C. 

The natural refrigerant heat pumps have shown to work very efficiently when targeting their optimum  application, i.e. the CO₂ heat pump is best when supplying large DHW demands, and the propane is best when providing large SH demands. When not clear if DHW is high enough, then propane would be the recommended choice, since it is more flexible to provide demands at different temperature levels.

The TRI-HP systems include PV and an electrical battery reaching a renewable share above 80 % with a net zero energy concept. Thus, the PV field is dimensioned to provide all electricity needed for the provision of household electricity and to drive the heating system. However, only 80 % is used on-site and the remaining 20 % is purchased from the grid and provided as feed-in in other time periods (mostly in summer). The need for purchasing electricity from the grid is due to the lack of a seasonal storage which, with today's technologies and prices, would not be cost-economic.

The solar-ice slurry system using the CO₂ heat pump reaches energy cost for heating and electricity, including a battery and the PV field, in the range of 19-29 ct/kWh depending on the location achieving in most cases an SPF of 4. The upper range cost is obtained in cities with low irradiation where the solar-ice system is not meant to be used, such as Helsinki or Warsaw. The lower limit of 19 ct/kWh is reached in the location of Davos (alpine) with a very high irradiation in winter together with a large heating demand due to the low temperatures. However, for simplicity, results from Davos are obtained with the assumption that snow does not cover the collectors in winter. The second lowest cost of 19 ct/kWh is reached in Madrid, which similarly to Davos, has a high irradiation in winter and a large heat demand. In this case, however, results could be achieved in practice since snow will not be present on the collector field. On the other hand, the large cooling that would be necessary in Madrid could not be covered by this system unless an additional air heat exchanger for heat rejection would be used, which has not been studied within TRI-HP project. For the other cities, the SPF ranges from 23-27 ct/kWh with only 20~\% of electricity purchased from the grid and without considering any revenue for the 20 % of electricity feed into the grid. The large range of energy cost shows a significant influence on the weather conditions, which is clear since the system relies on solar thermal energy to drive the heat pump.

The solar-ice slurry propane system reaches energy (including heat and electricity) cost in the range of 16- 27 ct/kWh for the MFB90 depending on the location. Removing the exceptions of Warsaw, Helsinki and Davos, the energy cost ranges between 18-25 ct/kWh achieving an SPF of 4 with only 20 % of electricity purchased from the grid and without considering any revenue for the 20 % of electricity feed into the grid. Energy cost are lower for the MFB90 building since the heat demand is more distributed in shoulder seasons compared to the MFB30, where most of the energy demand is concentrated in winter, when solar irradiation is lower.

As an example, the solar-ice slurry system would reach an SPF of 4 with a renewable share of 80~\% and an energy cost of 24 ct/kWh for a new building with 43.6 kWh/(m²,y) in Brussels with a yearly heat demand of 36~MWh for space heating, 17~MWh for DHW and 16~MWh for household electricity (appliances, lighting, cooking, etc.). To achieve this large efficiency, it is necessary to install 82 m² of collector area (1.6 m²/MWh per annual SH and DHW demands) and 61 m³ (1.2 m³/MWh) of ice storage volume, which could be easily integrated into the building's basement (equivalent to parking needs for two cars), as well as a PV field of 29 kWp with a battery of 29 kWh.

To achieve the target of SPF of 4 for a refurbished building with 110 kWh/(m²,y) in Brussels with a yearly heat demand of 116 MWh for space heating, the necessary collector area is 195 m² (1.5 m²/MWh) and the ice storage volume is 146 m³ (1.1 m³/MWh, which equals to parking needs for five cars). To achieve a renewable share of 80 %, a PV field of 47 kWp with a battery of 47 kWh was used leading to an energy cost of 21 ct/kWh. From the absolute values needed here one will have to think if this large collector area will fit into the roof if PV will be there too. Solutions to overcome roof competition between solar thermal and PV could be the use of hybrid PVT collectors or to use facade panels.

The performance of the DSHP system is less dependent on the location and on the parametric variation (borehole length) compared to the solar-ice slurry case. For most locations the energetic efficiency SPF is in the range of 3.2 to 4. The borehole length has been sized to reach a minimum average temperature on the borehole close to 0 °C after 5 years for heating dominated locations and to close 2 °C for non-heating dominated locations. This leads to extraction powers on the borehole field between 20-80 W/m depending on the location. The energy cost of the DSHP system is in general very good due to the possibility of providing heating and cooling, due to the large size of the building and the reduced needs of the borehole length, with energy cost ranging from 15-20 ct/kWh.