An international research team has conducted the first empirical study on retrofitting PV-assisted two-stage heat pump (HP) systems in existing multi-family buildings.
The research is part of the EU-funded Happening project, which aims to develop solutions that integrate both centralized and decentralized HPs. In those solutions, the centralized system preheats the water, which is then heated in a second heat pump in each dwelling.
“The selected case study sites are located in Spain, Italy, and Austria, each operating under different climate conditions and featuring varying building characteristics and heating temperature requirements,” explained the team. “This diversity ensures a comprehensive assessment for the integration of these systems.”
The scientists compared, in particular, the energy performance of the proposed systems to standard solutions in existing multi-family buildings, focusing on primary energy use (PEU) and CO2-equivalent emissions through the analysis of field measurement data.
The Spanish case study (CASE_ES) is located in Pasaia, in the Basque Autonomous Community of northern Spain; the Italian case study (CASE_IT) in Verzuolo, in the northern region Piedmont; and the Austrian case study (CASE_AT) in Liezen, in the federal state of Styria.
According to the Köppen–Geiger climate classification, these locations are classified as having a temperate oceanic climate (Cfb), a humid subtropical climate (Cfa), and a warm-summer humid continental climate (Dfb), respectively. The heating degree days are 925 for Pasaia, 1,717 for Verzuolo, and 2,523 for Liezen.
The building in the Spanish case was built in 2008 and has never been renovated. In Italy, it was built in the 16th century and renovated between 2006 and 2008. The multi-family building in Austria was built between 1940 and 1945 and renovated in recent decades. In all three cases, only two floors are counted as inhabitable.
The Austrian case has a heated area of 980 m², the Italian has 400 m², and the Spanish has 620 m². The Austrian building has 18 dwellings and 31 residents, while the Italian one has 10 houses and 14 residents. The Spanish one has 8 dwellings and 24 residents.
In CASE_ES, two centralized air-to-water heat pumps (AWHPs) use propane (R-290) as a refrigerant, providing a total heating capacity of 36 kW. They are connected to a thermal energy storage (TES) system, consisting of a 2,000-liter water tank, which preheats water in a low-temperature distribution loop. Each dwelling is equipped with a decentralized propane water-to-water heat pump (WWHP), providing a total heating capacity of 48 kW. The electricity used by the HPs is partially supplied by PV panels with a peak power output of 15 kW.
CASE_IT comprises three centralized AWHPs with refrigerant R-32 and a total heating capacity of 45 kW. These AWHPs are linked to two TES totaling 1,300 liters. The total thermal capacity of the decentralized WAHPs amounts to 39 kW for heating and 33 kW for cooling. Additionally, a PV system with a peak output of 10 kW was installed on the roof, alongside a battery system with a capacity of 22 kWh.
In CASE_AT, four centralized AWHPs using refrigerant R-410A provide a total heating capacity of 124 kW with a backup electric heater of 6 kW. Those are connected to four TES, totaling 3,200 liters and 18 decentralized WWHPs, one for each dwelling. The decentralized HPs use refrigerant R-32 to provide a total heating capacity of 132 kW. A 28 kW PV system powers the HP installed on the building’s roof.
The assessment period for the research was 12 consecutive months. It lasted from November 2023 to October 2024, for CASE_ES and from August 2023 to July 2024 for both CASE_IT and CASE_AT.
“The overall seasonal performance factors of the heat pump systems are similar, averaging around 2.0 across all three case studies. Between 63.5% and 76.3% of the primary energy used for heating and cooling comes from renewable energy sources,” the scientists said. “Compared to previous heating supply systems, greenhouse gas emissions from the heat pump systems are reduced by 65.4% to 84.3%, fulfilling the European Union’s target of 60% reduction by 2030. Between 60.9% to 86.4% of the emission savings are attributed to the installation of the two-stage heat pump systems, while 13.6% to 39.1% of the savings result from photovoltaic electricity self-consumption.”
They concluded that retrofitting photovoltaic-assisted two-stage heat pump systems in existing multi-family buildings is technically feasible across various climate and energy demand conditions.
The experiments were presented in the paper “Energy performance of photovoltaic-assisted two-stage heat pump systems in existing multi-family buildings: Three case studies from Europe,” published in Energy. Researchers from Germany’s Fraunhofer Institute for Solar Energy Systems (ISE), Spain’s Tecnalia Research and Innovation, Italy’s Tecnozenith, and Austria’s AEE – Institute for Sustainable Technologies contributed to the research.
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