Thesis title: Air/Water Heat Pumps in Existing Heating and Hot Water Systems for Better Urban Air Quality: building stock, replacement scenarios, case studies
A significant contribution of different pollution agents in urban areas is due to fuel-fired building heating systems. The substitution of the current boilers for space heating and domestic hot water (DHW) production, with air-to-water heat pumps, is proposed a measure to improve urban air quality.
Urban-scale interventions can contribute to a long-term decrease in air pollution by significantly cutting emissions in cities and lowering both energy use and environmental impact, marking a crucial step toward more sustainable urban development.
This thesis evaluates replacement scenarios in two Italian cities that differ both in climate and in the characteristics of the building stock. An industrial and a residential case study are also presented and analysed. The focus of the evaluation is on the avoided pollutant emissions, the primary energy savings and the reduction of CO2 emissions.
To proceed with the study, it is essential to know the thermal energy need at the city scale in the case of replacement scenarios and at the building scale in the case of substitution for the case studies.
The building sector is a major global consumer of energy, accounting for 34% of the final energy demand, mainly for operational needs like heating and cooling, and 37% of energy and process-related CO2 emissions [1].
A significant portion of Italy's existing building stock was built before the introduction of energy performance legislation, and this, combined with limited renovation rates, highlights the need to address energy inefficiencies. Targeted interventions in these buildings are essential to achieve decarbonisation targets and transition to a building stock predominantly composed of Near Zero Energy Buildings (nZEBs).
A detailed assessment of the building stock is crucial for identifying potential energy-saving measures. However, obtaining data on residential buildings across multiple Italian cities proved challenging, as many databases are either not publicly accessible or lack key information needed to assess transmittance, dispersing surfaces, and overall energy performance. Among the cities examined (Rome, Milan, Salerno, Genoa, Torino, Rimini, Pisa, Palermo, and Naples), only Milan and Salerno had complete and accessible datasets, allowing, with a proposed procedure, for an in-depth analysis. These cities, located in different climatic zones of Italy and therefore related to different outdoor temperatures (Milan is the colder city), present distinct thermophysical characteristics and architectural typologies, making them a representative sample of the broader Italian housing stock.
The evaluation of the residential buildings by period of construction showed that, in both cities, most buildings were constructed before 1970. In Milan, 66% of buildings have four or more floors above ground, and only 7% are single-storey. This is confirmed by the fact that more than half of the buildings (55%) contain more than eight housing units. In contrast, in Salerno, most buildings (46%) have two or three floors, and 37% include one or two units.
The energy characterisation of these cities relied on different data sources, resulting in a methodological approach that is not fully generalizable. Specifically, Milan analysis was based on the database of energy performance certificates, Salerno’s study relied on the Municipal Energy Plan. The Milan database is more precise and comprehensive, as it provides thermal transmittance data for each flat subject to energy certification. However, it lacks information on building types. Conversely, the Salerno database offers average transmittance values and building surfaces rather than data on individual flats, but it provides detailed insights into recurring building typologies. A comparison of the two datasets revealed few differences in the transmittance values for opaque surfaces, while for glazed surfaces, apartments in Milan had lower transmittance values compared to those in Salerno. Additionally, in Milan, the proportion of glazed to opaque surfaces increased from 12.5% in buildings built before 1930 to 18% in post-2006 constructions. In Salerno, according to the Municipal Energy Plan, glazed surfaces consistently represent 25% of opaque surfaces across all construction periods. The analysis further revealed that the heating demand of buildings constructed after 2006 in Milan is 2.5 times higher than in Salerno, due to the less favourable climatic conditions in Milan. Given the discrepancies in data sources and methodologies between the two cities, future research should focus on developing a standardized approach using homogeneous datasets. This would enable a more accurate and comprehensive assessment of the energy performance of urban building stocks.
Carella et al. [2] previously demonstrated that fuel-based heating systems are a major source of air pollution in urban environments. As a result, replacing traditional boilers used for space heating and domestic hot water (DHW) with high-temperature air-to-water heat pumps—capable of operating with existing radiators, which are widespread in the current building stock—is proposed as a minimally invasive solution to enhance urban air quality.
The study focused on analysing substitution scenarios across the entire residential building stock in Milan and Salerno. The study primarily aimed to evaluate whether replacing the approximately 480,000 individual methane gas boilers in Milan and 74,000 in Salerno—an action that would lead to a significant reduction in urban pollutant emissions—might also result in increased energy and environmental costs. Replacing these boilers would remove numerous localized sources of emissions, displacing emissions at thermal power plants located in suburban areas, characterized by the highest generation efficiencies and equipped with trapping and reaction systems (for sulfur, nitrogen oxides, particulate matter, CO2), releasing emissions into the atmosphere at significantly higher altitudes compared to those in urban zones. The results indicate that the reduction in primary energy consumption, assessed at the design outdoor temperature and based on the current generation mix, ranged from 34% to 54% in Milan and from 43% to 60% in Salerno, depending on two levels of renewable energy integration in electricity generation. CO2 emissions were reduced by 30–52% in Milan and 39–58% in Salerno. The only unfavourable outcome was observed in Milan under a scenario with entirely non-renewable electricity generation. These findings support incentive policies for heat pump adoption, given the minimal disruption for end users, as existing radiator-based heat distribution systems remain unchanged. However, the study does not constitute a feasibility assessment for individual installations. The assumptions made, such as average boiler efficiency and standard heat pump selection, represent a first approximation and should not be used for case-specific evaluations. It is also worth noting that design conditions are not the most representative of real-world operation. Under milder temperatures, heat pumps operate at partial loads with higher coefficients of performance (COP), leading to greater efficiency gains. A more detailed analysis will be conducted in future work to evaluate the performance of these systems under real operating conditions, in particular considering the COP profiles provided by manufacturers as a function of the external temperature profiles for each city scenario using the EnergyPlus [3] dynamic simulations. Future works will also investigate more favourable/unfavourable climatic conditions related to different Italian and European cities.
As mentioned before, two case studies are then presented, one industrial and one residential, for which a procedure to obtain information on the building behaviour is proposed. The first case involves replacing LPG boilers with air-to-water heat pumps at an industrial site in Cecchina, near Rome. These boilers are currently used to supply hot water for heating two workshops and supporting the manufacturing process (test benches). The energy benefits, in terms of reduced primary energy consumption and CO2 emissions, are analyzed. Specifically, the reduction in primary energy usage ranges from 51% to 64% across two evaluated scenarios, one based on design conditions and the other on real operating conditions. The latter scenario uses data from heat loads and outdoor air temperatures recorded in a small town in central Italy during 2022. Additionally, the results show a decrease in CO2 emissions between 58% and 68%. The proposal highlighted the benefits in terms of reduced pollutant emissions and the potential economic advantages that could be realized with public support under real operating conditions. The study was based on available monthly LPG consumption data, serving as an initial approximation of the intervention. A more precise analysis, utilizing weekly consumption patterns, would account for heat load peaks, which can be significant on industrial sites. Nevertheless, the current approach, relying on data typically available in an industrial setting, provides a valuable starting point for analysing various sites and scenarios. The analysed proposal will be of interest to the production management, which plans to re-equip its boiler rooms with air-to-water heat pumps. This replacement, carried out in a manufacturing context, represents a case study that can also be applied in different industrial sites. In the second case, the substitution of gas boiler used for heating and DHW preparation with high–temperature air/water heat pump in a residential flat in Rome, was proposed. The study confirmed the effectiveness of the proposal in reducing primary energy consumption and CO2 emissions based on the current electricity generation mix. Specifically, primary energy consumption reductions range from 56.8% to 63% across two scenarios evaluated under design and real operating conditions. The real operating scenario, informed by heat load trends and outdoor air temperatures recorded in 2024, suggests improved heat pump (HP) performance in terms of COP due to more favourable outdoor air temperatures. Additionally, the results indicate a decrease in CO2 emissions between 56.2% and 62.5%. The study relies on commonly available data, such as monthly gas consumption, and provides an initial approximation of the intervention. Nevertheless, this approach could serve as a valuable starting point for analysing various cases and scenarios. The analysis highlighted the challenges of choosing high-temperature heat pumps, which can work with current radiators without changing the existing distribution system, due to the limited availability of such systems and the lack of available data made public by manufacturers. Furthermore, the feasibility assessment pointed out the critical issues in installing individual heat pumps in flats within condominiums. In particular, the lack of technical rooms, which are typically present in buildings with centralized systems, introduces complications. Different configurations in terms of size, weight, and installation space were carefully considered in evaluating the feasibility. In addition, a dynamic simulation was carried out using EnergyPlus [3] software, analysing real operating conditions, considering the COP as a function of time, starting from the COP data provided by the manufacturers, as a function of external temperature profiles and supply water temperature.
In all cases analysed, both in terms of scenarios and in particular case studies, the proposed replacement of existing heating systems with moderately high temperature air-to-water heat pumps is advantageous. Primary energy savings range between 34% and 57% in design conditions and between 55% and 64% in real operating conditions. The reduction of CO2 emissions ranges between 30% and 58% in design conditions and between 52% and 68% in real operating conditions. The results relate to some specific climatic conditions that are representative of a wider part of the Italian territory and a good sample of the building stock. The feasibility of the intervention is related to the size, weight, and installation space, fuel costs and the availability of public support.
In any case, different urban scenarios and case studies could be analysed, starting from lacking information, by means of the procedures proposed in this work.