Residential space heating is a significant energy consumer and contributor to CO₂ emissions globally. Heat pumps (HPs), particularly air-to-air heat pumps (AAHPs), offer a sustainable alternative to fossil fuel-based systems. While previous research has shown the potential of HPs for emission reductions, their effectiveness depends on factors like system type, operational practices, and electricity generation sources. Studies have demonstrated varying results depending on the regional energy mix and climate conditions; for example, HPs are less carbon-intensive than fossil fuel alternatives in many regions but can increase emissions where electricity generation relies heavily on fossil fuels. This study investigates the impact of switching to AAHPs in Toulouse, France, a city with a mixed heating energy source (electric and gas), using an integrated modeling approach combining HVAC models, building energy models (BEMs), urban canopy models (UCMs), and numerical weather prediction (NWP) models. This approach allows for a comprehensive assessment of energy consumption, urban climate dynamics, and CO₂ emissions under real-world conditions, offering insights into AAHP efficiency in various environmental settings and their transient behavior under changing building and meteorological conditions.

Existing literature highlights the potential of heat pumps to reduce energy consumption and greenhouse gas emissions in buildings. However, the effectiveness of heat pumps varies depending on factors such as system type, operational practices, and the carbon intensity of electricity generation. Studies in various regions, including 53 out of 59 regions analyzed by Knobloch et al. (2020), have shown that heat pumps are generally less carbon-intensive than fossil fuel alternatives. Conversely, studies in regions with high fossil fuel reliance, such as the United States (Vaishnav & Fatimah, 2020), have shown that switching to electric heat pumps could increase both heating costs and CO₂ emissions. Empirical evidence from Arizona (Liang et al., 2022) challenges the assumption of universal energy savings from heat pump adoption, revealing no significant electricity savings during summer and increased electricity demand in winter. A study on UK dwellings (Kelly & Cockroft, 2011) showed a modest reduction in CO₂ emissions but a slight increase in operational costs after retrofitting with air-to-water heat pumps. These studies emphasize the importance of considering regional energy sources and climate conditions when assessing the environmental and economic impacts of heat pumps.

This study uses an integrated modeling approach to assess the impact of transitioning from fossil fuel and resistive heating to AAHPs in Toulouse, France. The approach combines offline and online simulations using various models: 1. **Numerical Urban Climate Model:** The Town Energy Balance (TEB) model, incorporated into SURFEX-TEB version 8.2, is used to calculate the exchange of energy between the city and the atmosphere. It considers meteorological forcing, building characteristics, and human behavior to simulate indoor air temperature, humidity, and building energy consumption. 2. **Numerical AAHP Model:** The HVAC model MinimalDX version 0.3.0, a simplified direct-expansion (DX) coil model from EnergyPlus, is coupled to SURFEX-TEB to simulate AAHP performance. It uses bivariate quadratic fits to model capacity and electric input ratio (EIR = 1/COP) as functions of indoor and outdoor temperatures. Five scenarios with different rated COP (RC) values (2.5, 3.0, 3.5, 4.0, and 4.5) are investigated. 3. **Simulated AAHP Scenarios:** A baseline scenario, based on previous high-resolution simulations of Toulouse's building energy consumption (Schoetter et al., 2017), is compared with the five AAHP scenarios. Offline SURFEX-TEB simulations are conducted for the entire year to assess energy consumption, while online simulations, coupled with the atmospheric model Meso-NH version 5.3, are used to evaluate the impact on microclimate during a cold spell (January 22-30, 2005). 4. **Design Temperature:** The heating and air conditioning design temperatures are adjusted for baseline and AAHP scenarios based on building type, efficiency, and occupancy. Modifications are made for AC to account for potential rebound effects (increased AC usage). Internal heat gains from electricity and gas are also considered. 5. **Domain of Investigation:** The simulation domain covers a 15 x 15 km area encompassing most of Toulouse. Data on urban morphology, building materials, and heating system types are obtained from various sources, including the French Institute for Statistics and Economics (INSEE). 6. **Coupled Online Simulations:** The impact of AAHPs on meteorological conditions during the cold spell is investigated through coupled Meso-NH-SURFEX-TEB-MinimalDX simulations. ECMWF high-resolution data are used for initialization and lateral forcing.

The study's key findings include: 1. **Significant Reduction in Energy Consumption:** Transitioning to AAHPs resulted in a substantial reduction in annual building energy consumption (BEC) for heating (57-76%) and electric energy consumption (EEC) for heating (6-47%), depending on the AAHP's efficiency (RC values). 2. **Near-Zero Local CO₂ Emissions:** The large reduction in heating energy consumption, especially in Toulouse's heating mix which includes a significant portion of electricity, leads to virtually zero local CO₂ emissions related to heating. 3. **Slight Microclimate Impact:** AAHPs caused a slight reduction in near-surface air temperature (up to 0.5 °C) during the cold spell due to reduced sensible heat flux. This effect is localized and unlikely to affect AAHP operational efficiency. 4. **Potential for Increased Electric Energy Consumption:** In cities with a higher reliance on fossil fuel-based heating, switching to AAHPs may lead to increased overall electric energy consumption, highlighting the importance of sustainable electricity generation. 5. **Rebound Effect:** The study observes a rebound effect, with increased energy consumption for cooling in summer (54% increase for the median scenario, RC3.5) due to the use of AAHPs for cooling and a lack of energy saving restrictions by occupants. 6. **Variable AAHP Efficiency:** The efficiency of AAHPs (COP) varies with both internal and external conditions. This dependency impacts energy consumption and introduces complexities for grid management during cold spells. The observed COP fell below 2.5 on several occasions during the coldest winter days in the median RC3.5 scenario. 7. **Spatial Variation in Impacts:** The impact of AAHPs on near-surface temperature varies spatially; the effect is larger in dense urban areas (e.g., Saint Cyprien) compared to peripheral areas (e.g., L'Union), reflecting variations in heating energy consumption and sensible heat fluxes. 8. **Anthropogenic and Sensible Heat Flux Reduction:** Significant reductions in both anthropogenic and sensible heat fluxes are observed in the city center when AAHPs replace conventional heating systems. These findings are quantitatively detailed in tables and figures presented in the original paper, showing annual and daily energy consumption data broken down by heating and cooling, for the city center and the whole simulation domain, and showing spatial distributions of heat fluxes and temperature variations.

The findings highlight the significant potential of AAHPs for reducing energy consumption and CO₂ emissions in urban areas, particularly in cities with a high proportion of electric heating, like Toulouse. The substantial reduction in heating energy consumption and near-zero local CO₂ emissions demonstrate the efficiency of AAHPs in mitigating climate change. However, the potential for increased electricity consumption in cities relying on fossil fuel heating underscores the crucial need for a transition to sustainable electricity generation. The observed rebound effect, particularly the increase in cooling energy consumption, emphasizes the importance of considering behavioral changes and the dual role of electricity in both heating and cooling when assessing the overall impact of AAHP adoption. The variability in AAHP efficiency due to temperature dependence highlights the complexities of grid management and the need for robust energy integration strategies. The spatial variations in the impacts of AAHPs underscore the importance of considering local context when implementing such transitions. These results contribute to a comprehensive understanding of the interplay between technology, energy consumption, microclimate, and broader sustainability goals.

The study concludes that AAHPs offer significant potential for reducing energy consumption and improving environmental sustainability in cities like Toulouse, but their effectiveness depends heavily on existing heating infrastructure and the carbon intensity of electricity generation. While the transition from electric resistive heating to AAHPs presents a clear path toward greater efficiency, transitioning from other fossil fuel-based systems requires careful consideration of trade-offs. Future research should focus on further investigating the rebound effect, optimizing AAHP design and control strategies for cold climates, and developing comprehensive policy frameworks that integrate the deployment of AAHPs with broader energy transition initiatives. The substantial increase in HP installations observed in Occitania in 2021 reinforces the growing interest in these technologies and their role in achieving sustainability goals.

The study's limitations include the reliance on modeling and simulations, which may not perfectly capture the complexities of real-world scenarios. The models employed use assumptions about building characteristics, occupancy patterns, and AAHP performance, which may introduce some uncertainty. The study focuses on a specific city (Toulouse) with a particular energy mix, limiting the generalizability of the findings to other urban contexts. While the rebound effect is considered, its quantification may be subject to further refinement. The assessment of refrigerant leaks and end-of-life recycling is simplified and warrants more detailed investigation. Finally, the study does not explicitly address socio-economic factors affecting adoption and integration challenges, including aesthetic concerns, acoustic emissions, and regulatory constraints.