Analysis of heat pump systems for decarbonisation in the residential sector

According to the 2023 IPCC Report, global emissions must be reduced by 45 percent by 2030, compared to 2010 levels, and achieve net zero emissions by 2050, to keep global warming below the critical 1.5°C threshold. The European Green Deal aims to make Europe climate neutral by 2050, with a 55 percent reduction in emissions by 2030. Energy transition is one of the pillars of this strategy. In fact, according to the International Energy Agency (IEA), about 80 percent of global CO₂ emissions come from the energy sector for electricity generation, heating, transportation, and industry. Of these, the building sector plays a crucial role in the energy transition, being responsible for about 36 percent of global energy consumption and 39 percent of energy-related CO₂ emissions. The main sources of emissions in the building sector come from the use of energy for heating, cooling, lighting, and other operational functions of buildings, which are powered mainly by fossil fuels. The high energy consumption and high environmental impact of buildings are closely linked to the composition of the existing building stock. According to the European Building Stock Observatory, about 85 percent of buildings In Europe were built before 2001, thus predating the introduction of the first Energy Efficiency of Buildings Directive (EPBD) in 2002, and more than half date from before 1970. Most of the buildings currently in use were built at a time when energy and environmental standards were less stringent, resulting in low energy efficiency for many of them. The renovation of the building stock is therefore a key strategy for the decarbonization of buildings. In these terms, Europe has adopted the Renovation Wave as a strategy in order to double the rate of energy refurbishment of buildings by 2030. Within this context is the present of this thesis. The research focused on the analysis of innovative plant systems within the built environment, with a particular emphasis on the residential sector and public housing. In more detail it studies, within the ongoing decarbonization process, the plant systems that are upstream of the main energy end uses, such as heating, cooling and domestic hot water production. Getting more specific, research is focusing on the study and optimization of different plant configurations, at different scales and contexts, whose recurring elements are: Heat pump (air-to-water, water-to-water, dual source, hybrid), thermal reservoirs and cooled/hybrid photovoltaic panels. With the aim of identifying the plant configuration that, for that given load profile, maximizes the performance of the individual components and the system, as well as the exploitation of renewable sources by the plant and minimizing its emissions during operation. The work aims to analyze the effectiveness of innovative plant systems for decarbonization of the residential sector at different scales: single-family, multifamily, and building complexes. In particular, the thesis focuses on the analysis of three plant systems. The first system studied is a very high-efficiency plant and part of the larger European RESHeat project with application on a small-to-medium size multifamily public housing building. The second system studied analyzes the hybrid heat pump technology as a plant solution with reduced cost and construction site but with high potential in the energy transition phase, this system is studied on two different case studies, the same previously mentioned multifamily building and a single-family building representative of the most energy-intensive slice of the Italian and European building stock. Finally, the last analysis goes up in scale by focusing on the role of three main plant components (photovoltaic panels, heat pumps and thermal storage) in the implementation of Energy Communities (ECs), specifically the study focuses on mass linear building and applying the analysis to a case study located in Rome. Analyses were conducted through the use of dynamic energy modeling software such as TRNSYS and/or Simulink.For each plant system, the different components were validated through the use of laboratory measurements, as in the case of the photovoltaic panels, air-water heat pump, gas boiler and hybrid heat pump, or through direct on-site measurements as in the case of the building model, or through benchmarks or data provided by the manufacturers themselves. For each plant configuration, the evaluation of energy, economic, environmental, and social aspects is carried out, particularly in terms of achievable energy and economic savings, the related decrease in pollutant emissions, the improvement of indoor comfort as well as the quality of life of tenants, relating potential improvements to the related costs. RESHeat (Renewable Energy System for Residential Building Heating and Electricity Production) is designed to heat and cool residential buildings with renewable energy. The proposed plant system was created for colder climates than in Italy and consists of a water source heat pump, thermal photovoltaic modules and two seasonal buried thermal tanks. One of the challenges of the project, within which part of the thesis work is included, is to study a different plant conformation for a Mediterranean climate such as Italy, which, in addition to the need for heating, have a significant need for summer cooling. The RESHeat system thus modified presents the same main components (water heat pump, photovoltaic/thermal modules and thermal tanks), but duly adapted both in sizing and plant layout and in the very logic of system operation. The main goals set for the development and optimization of the system are high system efficiency, with a minimum color pump sCOP of 5, and a minimum 70% coverage from renewable sources, with the idea of pushing optimization up to 100%. One of the innovations of the RESHeat system and a key point in the high efficiency of the system is the use of waste thermal energy from the photovoltaic cooling system to ensure constant energy levels in the heat pump cold source. The European project as a whole involves the analysis and installation of the system on three different demo sites, two of which are located in Poland and one in the province of Rome owned by the Territorial Agency for Residential Housing (ATER). The present study focused on the Italian demo site. This is a three-story reinforced concrete building from the 1980s with 13 apartments. The work related to this first plant system is spread over several phases. Having to apply such a plant system to a specific demo site as a first phase, work was done on the analysis of the building itself by means of several inspections, surveys and instrumental analyses that allowed to know and characterize in detail the existing building-plant system and build its energy model. As a second phase, the main components of the plant system to be developed were identified and the respective mathematical models were implemented and validated in Simulink and Trnsys Come seconda fase sono stati individuati i componenti principali del sistema impiantistico da sviluppare e sono stati implementati e validati i rispettivi modelli matematici in ambiente Simulink e Trnsys. The work continued with the simulation and analysis of the preliminary design. At the end of this third phase, a dynamic simulation was performed to check whether the designed system is sufficient to cover at least 70% of the annual energy demand of the buildings at the demonstration site. As the next step, optimization was carried out with the aim of achieving 100% coverage of the buildings' heating and cooling energy needs and minimizing system costs. Special attention was paid to optimizing the use of the thermal and electrical energy produced by the PV/T panels and the maximum utilization of renewable resources. The studies examined several variables, including the inlet temperature of the heat pump and the volume of the thermal storage tank. The optimization of the system, conducted through parametric analysis, led to the definition of the optimized system through the application of the multi-criteria decision maker. Specific studies on energy generation and primary energy requirements were then conducted, and the optimized system was characterized from an energy, environmental and economic perspective. The results show that the RESHeat system is able to guarantee an adequate coverage of the annual energy demand of the demonstration site, with significant reductions in non-renewable consumption and consequent benefits for the end users. Eth,st,pvt Eth,hp,s Eth,DHW Eel,pvt Eel,cons Ep,nren PEnren,red f,sol f,sc sCOP 31 MWh/y 16 MWh/y 25 Mwh/y 30 Mwh/y 28 MWh/y 56 MWh/y 63 % 98% 15% 6 One of the key objectives of the European project was the realization of the plant on the three demo sites, after which it was possible to collect and analyze the first experimental and real-life performance data on the plant and evaluate their deviation from the expected data. The results highlight how innovative high-efficiency systems can have a significant impact and play a key role in achieving climate neutrality by 2050. Sometimes system complexity and economic constraints can be an obstacle to large-scale deep retrofits in particular in the short term. One of the difficulties / one of the most frequently encountered constraints in the literature is the economic one, especially given the initial investment. In this respect, tax incentives certainly play a crucial role. From a technical/technological point of view, one strategy is to identify components at the plant level that allow a significant reduction in the building's primary energy requirements at reduced initial costs. The role of which can be crucial in the transition phase. If the first system investigated was a high/very high efficiency 100% renewable system but which would require significant intervention on the building from a plant engineering point of view, the second system analyzed is diametrically opposed, going to investigate a solution which may require minimal intervention on the building with the aim of achieving the best possible performance through the use of a hybrid heat pump system. The analysis on the second type of system was carried out by considering two different hybrid heat pump systems, the first is a minimum intervention, punctual, involving the coupling of an air source heat pump with an existing gas boiler system, the second focuses instead on the characterization of a pre-assembled heat pump from an energy, economic and environmental point of view. In the first case, eight different scenarios with three different types of heating generation were analyzed and compared: natural gas boiler only (s0, s1.1 and s1.2), air-water heat pump only (s2.1 and s2.2) and a bivalent system with the heat pump connected in series with the natural gas boiler (s3.1, s3.2 and a3.3). In order to evaluate the optimal configuration to increase primary energy savings, the monovalent, with and without heat pump, and bivalent scenarios were compared. With the aim of increasing the efficiency of the plant system without carrying out any intervention on the distribution and supply system, the previously described multi-family social housing case study was selected for this purpose. The results indicate that although the existing building is characterized by low supply efficiency and high thermal conductivity of the external walls, significant results in terms of primary energy savings can be achieved with the introduction of a hybrid system (especially where it is not possible to intervene on the building envelope). Achieving a reduction in primary energy demand of up to 28%. The second hybrid heat pump study similarly starts from the need to identify an effective technical strategy in the current transition phase of the decarbonization of the building sector, where deep retrofits are not possible in the short to medium term. In particular, reference is made to buildings with high-temperature heating systems whose design makes them difficult to couple with a heat pump system alone. In this work, a hybrid heat pump was studied in detail as a possible solution to this specific problem. The work was developed in two different phases. The first consisted of laboratory measurements on the main components of the system and the detailed realization of the mathematical energy-environmental model in Simulink. The second phase, on the other hand, starting from the model calibrated and validated in the previous phase, involved the application of the technology to a standard case study, representative of residential buildings constructed between 1960 and 1980 in reinforced concrete in Europe. The laboratory analyses will focus on the main transients during operation of the two generators under different operating conditions and control parameters. Emphasis was placed on the control and integration part of the two machines, and on CO2 and NOx emissions during operation. In addition to providing a better understanding of the machine, the measurements made allowed the calibration and validation of the mathematical model. To this end, seven different measured and simulated quantities were compared, with an average deviation between the two of ±2.2. In the second application phase, 6 descriptive quantities of the hybrid system behavior were evaluated, such as: the primary energy demand (PE) in kWh/m2y of the two generators and of the whole, the average seasonal efficiency of the individual machine in terms of sCOP and efficiency (sηcNGB), as well as the primary energy efficiency (PEE) of the two generators and of the whole, and the annual emissions of CO2 and NOx expressed as gCO2/m2y and gNOx/m2y and their ratio with respect to required non-renewable primary energy (gCO2/kWh PEnREN, mgNOx/kWh PEnREN), with respect to fuel consumption (gCO2/kWh fuel and mgNOx/kWh fuel) and with respect to heat produced (gCO2/kWh heat and mgNOx /kWh heat). The values are summarized in the table. PEgl PEEgl gCO2/kWhPEnREN gCO2/kWhHeat mgNOx/kWhPEnREN mgNOx/kWh Heat kWh/m2y - 74.0 1.65 0.18 0.14 38.8 40.8 The performance of the hybrid system (HHP) was then compared with that of the systems consisting of individual generators, the gas boiler (NGB), and the air-water heat pump. (ASHP). In particular, the comparison was evaluated based on 5 additional metrics: the primary energy savings (PES), the percentage of annual emissions avoided (%CO2,avd/y), the decarbonization cost (DC €/CO2,avd/y), and the fuel cost savings (FCS %). From the HHP-NGB comparison, thanks to the use of the hybrid heat pump, there is a 30% reduction in primary energy demand (PED) corresponding to a 35% reduction in CO2 emissions. With an estimated initial cost (IC) of 15,175 euros, a DC of 7.13 €/CO2,avd/year is obtained. Finally, the cost-effectiveness ratio was evaluated with respect to the fuel cost in three different scenarios, taking into account the current fuel cost (Cf,ac), the average from 2016 to 2023 (Cf,av), and the maximum (2023). (Cf,max). The analysis of the results highlighted the greater flexibility of the HHP in operating under optimal conditions even from an economic standpoint, being able to switch from one energy carrier (electricity or natural gas) to the other based on the cost of energy. In fact, considering the Cf,max scenario and evaluating its FCS% (fuel cost saving %) in the two systems HHP and ASHP, this is negative (FSC%HHP =-0.26) due to the high cost of electricity. In conclusion, from the analysis conducted, the hybrid heat pump proves to be a valid tool for reducing the energy-environmental impact of the construction sector. Furthermore, thanks to the high flexibility of the system, it can be economically advantageous even in critical energy market conditions. The research finally focused on the impact that the heat pump system can have on energy and environmental behaviour at urban scale. The focus is on on the implementation of EC within linear mass housing in Rome, with particular attention given to the Tor Bella Monaca district. The study proposes and simulates six energy community distinct scenarios using the Urban Modelling Interface (UMI) and Simulink in order to advance understanding of this topic. These scenarios evaluate the integration of photovoltaic systems, heat pumps, and energy storage systems to determine their comprehensive effect on renewable energy production, CO2 emission reduction, and the enhancement of self-consumption. The research finally focused on the impact that the heat pump system can have on energy and environmental behaviour at urban scale. The focus is on on the implementation of EC within linear mass housing in Rome, with particular attention given to the Tor Bella Monaca district. The study proposes and simulates six energy community distinct scenarios using the Urban Modelling Interface (UMI) and Simulink in order to advance understanding of this topic. These scenarios evaluate the integration of photovoltaic systems, heat pumps, and energy storage systems to determine their comprehensive effect on renewable energy production, CO2 emission reduction, and the enhancement of self-consumption. The research is structured on three different macro-sections. The first sees the characterisation of the building archetype and its validation, in the second macro-section, a parametric analysis is developed that correlates the installed photovoltaic capacity, the degree of electrification of consumption, the distribution of prosumers and consumers within the REC itself, the inclusion of thermal-electric storage tanks, and different operating strategies to the variation of shared energy in the REC. The last macro-section sees the detailed analysis of sex specific case. The parametric analysis involved the variation of four different parameters. As a first analysis, the installed power of the photovoltaic field was varied from a minimum of 10 kW, with an increment of 10 kW, up to the maximum installable for access to incentives equal to 1 MW. The degree of electrification of thermal consumption was then varied from 0, production of heat from fossil or non-electric sources only, to 1, total production of heat from electric energy, with a gradual increase of 10%. Next, 10 different scenarios were considered in order to analyse the energy dynamics within the REC by considering differently the share of consumers and prosumers constituting the energy community itself. This was done by considering a dedicated POD (Point of Delivery) in the base scenario and a prosumer share (Pdg) equal to 0, up to considering the entire prosumer user itself (Pdg=1). Different thermal reservoirs were then sized using two different approaches, the first according to the maximum thermal load to be electrified and thus the degree of electrification, and the second according to the maximum electrical production and thus the installed photovoltaic power. For the purpose of the analysis, the fraction of energy shared by the REC (fsh), the fraction of self-supply (fss) and the fraction of self-consumption (fsc) were taken into account as KPIs. Defining respectively fsh as the ratio between the energy produced by the PVs and absorbed/consumed within the ERC itself and the entire annual photovoltaic production, while fsc as the ratio between the energy produced and self-consumed by the prosumer and the annual photovoltaic production, finally, fss as the ratio between all the energy produced by the PVs and consumed within the REC perimeter (both by prosumers and consumers) and the annual electricity demand of the REC. The size of the photovoltaic systems and the integration of other technologies were carefully evaluated to reduce CO2 emissions. Investment effectiveness was assessed through annual costs (ACs), where CAPEX varies from €357,630 to €1,155,238 over 25 years. Optimal ACs-PES results are for scenarios with maximum plant capacity, while halved PV scenarios require low annual costs for balanced primary energy savings. In conclusion, the results show that the implementation of plant system integrated into building and appropriately optimized, can be a key solution for decarbonization of the building sector, significantly reducing primary energy consumption and GHG emissions and contributing to climate neutrality.