1. Introduction
Since the heating and cooling sectors contribute to approximately 50% [
1,
2] of the European Union (EU) energy consumption, it is commonly recognized that they have to be decarbonized in order to meet climate change prevention goals. This is particularly important considering the fact that 75% [
3] of heating and cooling comes from fossil fuels, of which 69% [
4] are imported.
The advent of large-scale weather- and climate-driven renewable energy sources—in brief, variable renewable energy sources (VRESs)—has significantly altered the energy landscape. The most dramatic changes are being observed on the supply side, but the demand side is also evolving due to the emergence of new loads such as electrical vehicles. Consequently, national power systems are undergoing a process of rapid and mandatory transformation. At the same time, the non-dispatchable characteristic of solar and wind sources makes their integration into the power system a challenging and multifaceted task [
5].
In recent years, multiple approaches have been proposed in the literature to facilitate their integration [
6], such as spatial interconnection of geographically dispersed variable sources; complementary operation of non-dispatchable and dispatchable generators; demand response and flexible load shifting; electricity storage; oversizing peak renewable capacity; vehicle to grid; forecasting the weather to improve power system dispatch.
Many of them are already being implemented. From the above, of particular interest for this paper is the concept of power to heat conversion. A natural sector coupling [
7] where, in theory, available and cheap renewable energy is absorbed by various devices (mostly resistive heaters and heat pumps) to supply the heat demand. Conversion of renewable power to heat not only facilitates VRES integration and smoothens the market and power system operation but also reduces conventional fuels’ consumption, which otherwise would have to be used to cover the heat demand.
The decarbonization of the heat sector via heat pumps undoubtedly has many benefits. From the perspective of countries which experience very low air quality (as shown in
Figure 1), heat pumps can significantly contribute to solving this problem. The distributed emissions of pollutants (individual fossil fuel-based space heating) are replaced by emissions on the level of power plants. If the latter are mostly renewables or low-emission ones, the corresponding emissions from the heating sector will most likely be significantly lower.
Figure 1.
Low air quality index in Poland—carbon-based heating and power (source: [
8]).
Figure 1.
Low air quality index in Poland—carbon-based heating and power (source: [
8]).
Furthermore, large-scale power stations are heavily regulated, also from the environmental point of view. Even if they emit significant amounts of CO₂ and other greenhouse gases (GHGs) (as in case of hard or lignite powered ones) which impact the climate, it is much easier to control the emissions of harmful substances to human beings through regulatory decisions. Taking into account the above, it is clear that heat pumps have a significant potential to improve the local air quality. However, to consider them as a clean energy source, one has to take a detailed look at the country-specific CO
2 emissions associated with each kWh of electricity generated. This is of particular importance in countries such as Poland in which the process of decarbonizing the power system is still in the early stages and, therefore, the majority of electricity comes from combustion of fossil fuels, as shown in
Figure 2. In summary, it is obvious that despite heat pumps’ ability to convert electricity into useful heat with high efficiency, the heat they generate has some associated emissions linked directly to the power system emissions.
Figure 2.
Fuel mix and CO₂ emission intensity of the Polish power system (data sources [
9,
10,
11,
12,
13]).
Figure 2.
Fuel mix and CO₂ emission intensity of the Polish power system (data sources [
9,
10,
11,
12,
13]).
In recent years, multiple studies have been dedicated to the analysis of heat pumps’ role in shrinking the heating sector’s carbon footprint. A review article by Bayer et al. presented an overview on how ground source heat pumps can contribute to reducing the greenhouse gas emissions across European countries [
14]. Their findings indicate that with fully saturated markets, a reduction by 30% could be expected compared to present heating practices. In the case of Poland, the benefits of residential heat pumps in terms of GHG reduction are expected to be very low due to the high carbon intensity of the electricity generation sector. The above findings are in line with those reported by Gajewski et al., who analyzed the emissions associated with heating in various European countries [
15]. Their findings highlight that the “greenness” of a heat pump depends on the country’s electricity mix. Furthermore, they indicate that if fossil fuels are the main source of electricity in a given power system, then condensing gas boilers are a more ecological solution than heat pumps (again, ground source heat pumps were considered); such findings can be also found in Dzikuć and Adamczyk [
16]. As a sort of a follow-up work, Sewastianik and Gajewski analyzed the emissions associated with different types of heat pumps operating in different cities representing typical climate conditions in Poland [
17]. Their findings indicate that air-to-water heat pumps are, energy-wise, a non-profitable investment from the perspective of the Polish energy mix. Some contrary findings have, however, been reported by Nemś et al. when heat pumps (air-to-water and ground source) were analyzed from economic and environmental perspectives for a greenhouse heating system [
18]. Nemś et al. reported that both types of heat pumps are economically viable and contribute to significant reductions in environment pollution. It has to be noted that the reference heating system was based on a coal boiler. When a condensing gas boiler is used as a reference heat source, according to Gajewski et al., the air-to-water heat pump is neither an economically viable nor environmentally beneficial solution [
19]. Another more comprehensive (considering more heat sources) study by Hałacz et al. revealed that a condensing boiler with natural ventilation in case of single-family houses is, pollutant emission-wise, the most favorable solution [
20]. The above-mentioned studies support the argument that the country-specific energy mix has a significant impact on the “greenness” of heat pumps. The currently ongoing changes in the structure of the Polish energy mix indicate a promising trend (decarbonization), but the question emerges of whether this will be the only factor that will shape the “greenness” of heat pumps in the Polish context.
As shown earlier in
Figure 2, the structure of the energy mix in Poland is fairly stable during the year, with a clear and dominant share of large-scale conventional fossil fuel (mostly lignite and hard coal) power stations and a marginal contribution of VRESs. At the same time, the majority of heat pumps in Poland operate as small-scale, decentralized units powered by a centralized power station. The most common are air-to-water heat pumps, which provide domestic hot water and space heating (
Figure 3). Their high popularity is mostly due to their low investment costs, fast installation process and adaptability to local conditions—which makes them a perfect fit in urban environments.
Figure 3.
Heat pump sale in Poland by type of heat pump (source: [
21]).
Figure 3.
Heat pump sale in Poland by type of heat pump (source: [
21]).
To analyze the environmental impact of air source heat pumps in terms of CO₂ emissions per kWh of heat delivered, one has to analyze which factors determine those emissions. Awareness of their role and long-term change will not only enable a better understanding of the role of heat pumps (HPs) in heat and power systems’ transformation but will also facilitate the process of developing decarbonization actions/policies, which, in the long term, should enable a carbon-neutral energy sector.
In the Polish context, the electrification of the heating sector via air-to-water (A/W) heat pumps has two sides, a positive and a negative one. In general, HPs are able to eliminate local emissions and lead to air quality improvement. However, at the same time, their electricity demand puts additional stress on the power system operation. This is particularly important in countries such as Poland with a winter-peaking power system, which is dominated by conventional, highly emitting generators.
Considering the above, the objective of this work can be formulated in the form of the following research questions:
Do heat pumps in the context of the present Polish power system support the process of heating sector decarbonization and achieving EU targets?
What is the main factor shaping heat pump-derived heat emissivity? Is it climate change, technology progress or the power system energy mix?
3. Results and Discussion
This section presents and discusses the results obtained from the conducted analysis. As indicated in the earlier sections, the proposed approach for analyzing the factors shaping the CO₂ emissions associated with heat pump operation has been implemented on a case study from Poland.
Figure 9 shows the long-term changes in EI
HP calculated according to four alternative scenarios, where the grey area reflects the range of available CO₂ emission intensities dependent on the heat sink parameters. The lines reflect common heat sinks, where solid blue represents a space heating line with a sink temperature of 55 °C, dashed blue is domestic hot water preparation (55 °C according to Polish law), and the grey, low-temperature space heating with a sink temperature of 35 °C.
S0 reflects the historical emissions, EIHP, of working HP A/W including the 1995–2019 COP development, EIE decrease and ambient temperature increase. The HP unit operating in space heating mode at the supply temperature of 35 °C has the lowest EIHP due to the high efficiency achieved (high COP) and, thus, the low electricity demand to produce 1 kWhth (grey solid line). The highest EIHP is characteristic for a HP operating in heating mode at 55 °C (blue solid line). Low heat source temperatures in heating season (low ambient temperatures in Poland in winter) result in low HP efficiency (low COP) and high electricity consumption to produce 1 kWhth. A heat pump operating in domestic hot water preparation mode (supply 55 °C) reaches high EIHP values, but lower than for heating 55 °C, due to high-efficiency operation in the summer months when the heat source temperature is favorable for the heat pump, which affects the seasonal result.
The value and slope of the 1995–2019 long-term EIHP change show a favorable reduction in emissions and unfavorably high values of emissions associated with HP operation in Poland, even contrary to the idea of using heat pumps.
The S1 scenario with fixed EIE confirms the beneficial effects of heat pump technology development (COP increase) and climate change (heat source temperature increase) on EIHP by the generation of 1 kWhth. The lack of advances in fossil fuel power generation technologies does not stop the long-term reduction in EIHP. Increasing the efficiency of the heat pump (higher COP) gives a favorable result. The large difference in the slope compared to S0 indicates a large impact of many years of EIE changes, although the Polish energy system is still based on coal combustion and has a high emissivity.
The chart of the S2 scenario presents EIHP lines with COP fixed at the 1995 level. The emission decrease illustrates a beneficial reduction in HP emissions despite the lack of improvements in heat pump technology. In all analyzed HP operating modes, mitigation is driven only by long-term improvements in the national fossil fuel energy system (EIE) and 1995–2019 climate warming. In 25 years, the HP installed in 1995 produces fewer and fewer emissions, and its users passively support the decarbonization of energy and heating systems. They are the beneficiaries of positive changes beyond the boundaries of their house. This phenomenon confirms the thesis about the unused environmental potential of HP units in the Polish energy mix. Despite the lack of heat pump development, the EIHP decreases in all analyzed operating modes. HP emission mitigation is caused by the Polish power sector decarbonization and climate change.
In the climate-fixed S3 scenario, the decarbonization is driven only by technical improvements in central power plants and in individual HP units’ efficiency in all operation modes. Smooth lines, free of disturbances from weather data, illustrate the consistent development in both areas and the effect of planned activities both at the national level and at the level of distributed heat sources.
In the investigated period of 1995–2019, the global efficiency of heat pumps increased significantly (
Figure 6), and the mitigation actions in the Polish national energy mix were moderate. The national energy mix is still dominated by the combustion of fossil fuels, with high GHG emissions and low RES use (
Figure 1).
The influence weight of the three investigated HP emission factors in all analyzed operation modes is presented in
Figure 10. In each mode, the long-term decarbonization of the power system has the biggest impact on EI
HP emission by 53.0%, 55.2% and 55.4%, respectively. This shows and confirms the high impact and potential of decarbonization actions in central power plants and RES increase in the national energy system.
Development in HP technology, defined by long-term COP increase, is the second force in long-term mitigation. With a share of 33%, it is an important factor in supporting the reduction in CO₂ emissions, with all the advantages and disadvantages of electrification of the heating sector in Poland. HP heat sink temperature and operation mode only slightly increase this share from 32.2% to 33.8%.
Observed long-term ambient air increase (1995–2019) affects the heat production efficiency in the analyzed air source heat pump technology. Apart from the sink, ambient air temperature is a factor shaping the COP. Climate change impact was weighted at 14.8%, 12.2% and 10.8% for 35 °C, 55 °C heating mode and domestic hot water (DHW) 55 °C, respectively. For the heating sector, global warming decreases the heating needs in buildings and increases the air source HP efficiency. The strongest impact concerns HP units in low-temperature heating mode (35 °C). Due to HP technology, low sink temperature systems are the greatest beneficiaries of unfavorable climate warming.
In the Polish fuel mix, all HP units powered from the national energy system are not zero or near-zero emission devices. Their operation still has a negative impact on the environment. Despite the EU’s climate obligations, the technology of power generating from fossil fuels will dominate in Poland for many years.
The high power system emission intensity (EIE) paradoxically increases the HP impact on mitigation. At high system EIE, HP operation, even with low COP, results in a large CO₂ reduction in g CO₂/kWhth. Low EIE, despite increasing the COP, weakens the HP’s influence on absolute emission reduction in g CO2/kWhth.
The evaluated period (1995–2019) corresponds to the declared technical lifetime of a conventional heat pump. The observations made for the Polish energy sector show that the climate change mitigation potential of heat pumps has been wasted. The changes in the energy mix did not offer environmentally beneficial working conditions for heat pumps. In Poland and the EU, we are on the verge of a massive replacement of the heat pumps that were installed in 1990s. The technology has made significant progress efficiency-wise, but again, this potential might not be fully utilized in Poland. Although, there is still hope that the development of HPs will be accompanied by an increase in local, distributed renewable energy sources which will be the main or supplementary power source for them. However, such solutions might be only applicable in rural or sub-urban areas, as the densely populated urban areas usually have limited potential for VRES development that will enable a year-long HP power supply.
Proper design of a HP-based heating system should aim at supporting the decarbonization of national power and heating sectors. The improvement depends both on the public power sector and central power stations (55%) as well as individual investors and users (34%). The results have shown that the climate change in Poland is beneficial for A/W heat pumps (11%) in terms of their CO₂ emissions. A decreasing heat demand and increasing temperatures of heat source (here understood as ambient air temperature) during winter increase the effectiveness of the supply system. It is a paradox of benefits resulting from unfavorable changes.
4. Conclusions
In this paper, the environmental performance of air-to-water heat pumps has been investigated. Special attention has been paid to the impact of factors such as electrical sector energy mix, climate change and heap pump efficiency expressed as coefficient of performance. The presented method has been applied to a case study in Poland where the energy system is undergoing a rather slow process of decarbonization.
The analysis has been conducted based on annual data covering the period 1995–2019. From the obtained results, we conclude that considering the current electricity mix of the Polish energy sector, air-to-water heat pumps are unable to use their pro-environmental advantages. Heat pumps can contribute to improving local air quality but their indirect emissions are not competitive with alternative heating sources—which is in line with the available literature. This study builds upon those results and adds to the existing body of knowledge by providing evidence about how individual factors contribute to heat pumps’ indirect emissions. The findings indicate that the major role is played by the power sector energy mix and its emissions, followed by technology improvement (COP of heat pumps) and, lastly, by climate change (increasing temperatures).
The work presented has its limitations as it focuses only on air-to-water heat pumps and does not take into account the cooling sector. In future works, it is important to not only consider different heat pump technologies but also investigate, in more detail, the electricity sector emissions (both in temporal and spatial scales—line dynamic hourly emissions, or local energy mix). The study could also be extended to other countries as the presented method is easily transferable, and could also include the impact of climate change on the future performance of heat pumps.