Energetic and Environmental Aspects of Individual Heat Generation for Sustainable Development at a Local Scale—A Case Study from Poland

: The housing sector, especially with respect to energy generation to provide heating and domestic hot water, has been identiﬁed, after transport, as contributing the most to air pollution and the occurrence of low emissions in Poland. In particular, this applies to areas where there is a lack of heating and gas networks. This paper presents the results of calculations relating to the emission of atmospheric pollutants (TSP—total suspended particles as particulate matter PM10 and PM2.5, SO x —sulphur dioxide, NO x —nitrogen dioxide, CO—carbon monoxide) from individual sources of heat. The fact that a commune that does not have the network infrastructure, noted above, was taken into consideration, and the structure of heat generation was estimated on the basis of coal, fuel oil and biomass. The analysis was carried out taking into account the variable heat generation structure in households depending on the fuels used, including the heating values of fuels and the e ﬃ ciency of heating devices. Based on the calculations carried out, an ecological e ﬀ ect was obtained by assuming the replacement of heat sources by devices with higher e ﬃ ciency and also by considering the possibility of using heat pumps as a zero-emission solution in the households. This article attempts to answer the question posed by municipal authorities on how to limit the negative impact on the environment of individual heating devices in order to achieve sustainable development, including the speciﬁc conditions resulting from limited infrastructural opportunities. with a 5% share of fuel oil. The results obtained indicate about half of the reductions seen in the case of Model 2, that is, in the range 5.62%–29.72%.


Introduction
Energetic and environmental aspects of heat generation play an important role in the context of the sustainable development of each country. In the energy context, access to the energy carriers used for heat generation (heating of buildings and production of domestic hot water) and electricity production is crucial. In the ecological context, it becomes important to maximise the reduction of pollutant emissions into the atmosphere arising from the use of renewable energy sources. The consequences of energy pollution are increasingly felt in the modern world. Poland is a country that has struggled with the problem of high concentrations of pollutants in the air for many years. In the heating season, high concentrations of suspended particulate matter-PM10 (with a diameter of no more than 10 µm) and PM2.5 (with a diameter of no more than 2.5 µm) and benzo(a)pyrene (organic highly carcinogenic compound), which are highly carcinogenic compounds, SO x (sulphur dioxide-inorganic compound, a heavy, colourless, poisonous gas), NO x (nitrogen dioxide-inorganic compound, at room temperature it is a brown, highly toxic gas with a pungent odour reminiscent of chlorine gas) and CO (carbon monoxide-inorganic compound with strong toxic properties)-are recorded in the air. The reason for this is primarily the energy sector (based on coal), as well as transport and industrial Air quality control in Poland has been carried out for several decades, and since 2004 it has been adapted to European Union regulations and requirements. The air quality monitoring system consists of various types of suitably equipped measuring stations. Research is also conducted based on other monitoring techniques, such as modelling, passive methods and additional tests [31]. Tests and assessment of air quality in Poland, in accordance with the Act on Environmental Protection [32], are carried out by the Inspectorates of Environmental Protection as part of the State Environmental Monitoring (SEM) system. The average annual norms for individual pollutants are, respectively: for PM10-40 μg/m 3 , for PM2.5-25 μg/m 3 , for benzo(a)pyrene-1 ng/m 3 , for SO2 and NOx-20 μg/m 3 (applicable acceptable levels for plant protection), for NO2-40 μg/m 3 and for CO-10,000 μg/m 3 (concentration averaging period, 8 h).
The results of the annual air quality assessment for 2017 in Poland do not significantly differ from the results obtained in 2016 and in previous years. There is a slight increase (compared to 2016) in the number of exceedances set for PM2.5 and PM10 for the criterion on average annual concentrations and an improvement of these parameters compared to 2015. For benzo(a)pyrene, the highest average annual values were obtained as a result of measurements carried out in 2017 in the following provinces: Małopolska (22.7, 14. [27]. These data prove that the air quality standards in Poland are permanently exceeded. This is also demonstrated in data published by the European Environment Agency [28]. The problem of exceeding the permissible standards applies mainly to PM10, PM2.5 and benzo(a)pyrene, and to a lesser extent SOx, NOx and CO. Figures 2 and 3 show data on the concentration of PM10 and PM2.5 in 2016 (annual limit value) in Europe [28], where Poland is characterised by one of the highest exceedances of permissible PM10 standards (locally above 50 μg/m 3 ) and PM2.5 (locally exceeding 30 μg/m 3 ).  [27]).
Air quality control in Poland has been carried out for several decades, and since 2004 it has been adapted to European Union regulations and requirements. The air quality monitoring system consists of various types of suitably equipped measuring stations. Research is also conducted based on other monitoring techniques, such as modelling, passive methods and additional tests [31]. Tests and assessment of air quality in Poland, in accordance with the Act on Environmental Protection [32], are carried out by the Inspectorates of Environmental Protection as part of the State Environmental Monitoring (SEM) system. The average annual norms for individual pollutants are, respectively: for PM10-40 µg/m 3 , for PM2.5-25 µg/m 3 , for benzo(a)pyrene-1 ng/m 3 , for SO 2 and NO x -20 µg/m 3 (applicable acceptable levels for plant protection), for NO 2 -40 µg/m 3 and for CO-10,000 µg/m 3 (concentration averaging period, 8 h).
The results of the annual air quality assessment for 2017 in Poland do not significantly differ from the results obtained in 2016 and in previous years. There is a slight increase (compared to 2016) in the number of exceedances set for PM2.5 and PM10 for the criterion on average annual concentrations and an improvement of these parameters compared to 2015. For benzo(a)pyrene, the highest average annual values were obtained as a result of measurements carried out in 2017 in the following provinces: Małopolska (22.7, 14.6 and 10.0 ng/m 3 ), Silesia (16.0, 14.5 and 12.3 ng/m 3 ) and Lower Silesia (15.9 ng/m 3 ). In the case of PM10 dust, the highest average annual concentration in 2017 was recorded at urban background stations in the Małopolska (64.3 µg/m 3 ) and Silesia (55.6 µg/m 3 ) zones. In previous years, the value of this parameter was the highest at the communication station in the Kraków Agglomeration (56.7 µg/m 3 in 2016 and 67.8 µg/m 3 in 2015) [27].
These data prove that the air quality standards in Poland are permanently exceeded. This is also demonstrated in data published by the European Environment Agency [28]. The problem of exceeding the permissible standards applies mainly to PM10, PM2.5 and benzo(a)pyrene, and to a lesser extent SO x , NO x and CO. Figures 2 and 3 show data on the concentration of PM10 and PM2.5 in 2016 (annual limit value) in Europe [28], where Poland is characterised by one of the highest exceedances of permissible PM10 standards (locally above 50 µg/m 3 ) and PM2.5 (locally exceeding 30 µg/m 3 ).   Access to the infrastructure of heating and gas networks is also a problem. Statistical data on the population using gas from the network and gas consumption per capita indicate that 52.1% of the population used gas in Poland in 2017 ( Figure 4). This value should be considered as low. In addition, taking into account the fact that it was 71.2% in urban areas and only 23.3% in rural areas, there is an obvious conclusion of a link to limited access to the gas network [25]. The lack of local heating   Access to the infrastructure of heating and gas networks is also a problem. Statistical data on the population using gas from the network and gas consumption per capita indicate that 52.1% of the population used gas in Poland in 2017 ( Figure 4). This value should be considered as low. In addition, taking into account the fact that it was 71.2% in urban areas and only 23.3% in rural areas, there is an obvious conclusion of a link to limited access to the gas network [25]. The lack of local heating Access to the infrastructure of heating and gas networks is also a problem. Statistical data on the population using gas from the network and gas consumption per capita indicate that 52.1% of the population used gas in Poland in 2017 ( Figure 4). This value should be considered as low. In addition, taking into account the fact that it was 71.2% in urban areas and only 23.3% in rural areas, there is an obvious conclusion of a link to limited access to the gas network [25]. The lack of local heating networks Energies 2020, 13, 454 5 of 16 was recognised as the second element excluding municipalities from the possibility of sustainable development and improvement in air quality. Statistical data on the density of heating networks in 2017 in Poland indicate an average length of 8.1 km per 100 km 2 . The most important issue, however, is that 98.3% of the heating networks occur in urban areas ( Figure 5). Unfortunately, the fuel used in existing heating plants is usually solid fuel, at 66.2% [25].
Energies 2020, 13, x FOR PEER REVIEW 5 of 16 networks was recognised as the second element excluding municipalities from the possibility of sustainable development and improvement in air quality. Statistical data on the density of heating networks in 2017 in Poland indicate an average length of 8.1 km per 100 km 2 . The most important issue, however, is that 98.3% of the heating networks occur in urban areas ( Figure 5). Unfortunately, the fuel used in existing heating plants is usually solid fuel, at 66.2% [25].  There are 1555 rural communes in Poland, which in relation to the above data shows the scale of the problem. Lack of access to the network infrastructure necessitates the use of other energy carriers. Additionally, statistical data published by the Central Office of Building Control [33] indicate that the number of building permits issued in Poland in 2018 was 196,322 (226,469 facilities) and these should be taken into account. In this overall figure, residential buildings accounted for 98,915, of which, 94.7% were single-family houses. Unfortunately, 65% of newly built houses are equipped with solid fuel boilers [34]. The above statement is confirmed by data on fuels used to generate energy for space heating and domestic hot water purposes, as presented in Figure 6.  networks was recognised as the second element excluding municipalities from the possibility of sustainable development and improvement in air quality. Statistical data on the density of heating networks in 2017 in Poland indicate an average length of 8.1 km per 100 km 2 . The most important issue, however, is that 98.3% of the heating networks occur in urban areas ( Figure 5). Unfortunately, the fuel used in existing heating plants is usually solid fuel, at 66.2% [25].  There are 1555 rural communes in Poland, which in relation to the above data shows the scale of the problem. Lack of access to the network infrastructure necessitates the use of other energy carriers. Additionally, statistical data published by the Central Office of Building Control [33] indicate that the number of building permits issued in Poland in 2018 was 196,322 (226,469 facilities) and these should be taken into account. In this overall figure, residential buildings accounted for 98,915, of which, 94.7% were single-family houses. Unfortunately, 65% of newly built houses are equipped with solid fuel boilers [34]. The above statement is confirmed by data on fuels used to generate energy for space heating and domestic hot water purposes, as presented in Figure 6. There are 1555 rural communes in Poland, which in relation to the above data shows the scale of the problem. Lack of access to the network infrastructure necessitates the use of other energy carriers. Additionally, statistical data published by the Central Office of Building Control [33] indicate that the number of building permits issued in Poland in 2018 was 196,322 (226,469 facilities) and these should be taken into account. In this overall figure, residential buildings accounted for 98,915, of which, 94.7% were single-family houses. Unfortunately, 65% of newly built houses are equipped with solid fuel boilers [34]. The above statement is confirmed by data on fuels used to generate energy for space heating and domestic hot water purposes, as presented in Figure 6.

Materials and Methods
Analytical work was carried out for a theoretical rural commune with a population of 8000. Taking into account the statistical data [35,36], the density of population in rural communes in Poland is from 100 to 149 people per square kilometre (78.4% of all rural communes.) In addition, the average area of a rural commune in Poland is 126 km 2 . Multiplying these values gives a range of 12,600-18,774 inhabitants. Considering the data on access to the gas network in Poland (52.1%) and assuming an uneven distribution of the size of municipalities in terms of population, it was decided to adopt the value of 8000 inhabitants as representative.
The average number of people in a household was adopted as 3.4 [25], which allowed us to calculate the total number of households in the commune as being 2353. According to data published by the Central Statistical Office [25], the average area of a household in Poland in a rural region is 108.3 m 2 . The average energy demand is 216 kWh/m 2 year [37]. This information allowed for the calculation of the total heat demand of a commune for space heating and domestic hot water production, at 198,150.776 GJ. The first stage was to determine the structure of the energy carriers and heating devices used. As a baseline, it was assumed that 70% of the inhabitants of the commune use coal to produce heat, 25% use biomass and 5% use fuel oil (Model 1). Simulations of the potential ecological effect resulting from the exchange of the energy source were carried out in six variants (Model 2-7), assuming different shares of energy carriers in the energy balance of the commune.
The analysis was carried out for six models: Model 2 is included in the models calculated above due to a desire to compare the base state pollutant emissions with other models, assuming that renewable energy provision is predominantly provided by solid fuel boilers fired with biomass. The assumptions for models 4-6 result from the technical and financial possibilities of a total departure from solid fuels towards renewable energy sources, hence a balanced share of solid fuel boilers fired with coal and biomass was assumed, with a constant share of fuel oil, however with the assumption of an increase in the share of heat pumps,

Materials and Methods
Analytical work was carried out for a theoretical rural commune with a population of 8000. Taking into account the statistical data [35,36], the density of population in rural communes in Poland is from 100 to 149 people per square kilometre (78.4% of all rural communes.) In addition, the average area of a rural commune in Poland is 126 km 2 . Multiplying these values gives a range of 12,600-18,774 inhabitants. Considering the data on access to the gas network in Poland (52.1%) and assuming an uneven distribution of the size of municipalities in terms of population, it was decided to adopt the value of 8000 inhabitants as representative.
The average number of people in a household was adopted as 3.4 [25], which allowed us to calculate the total number of households in the commune as being 2353. According to data published by the Central Statistical Office [25], the average area of a household in Poland in a rural region is 108.3 m 2 . The average energy demand is 216 kWh/m 2 year [37]. This information allowed for the calculation of the total heat demand of a commune for space heating and domestic hot water production, at 198,150.776 GJ. The first stage was to determine the structure of the energy carriers and heating devices used. As a baseline, it was assumed that 70% of the inhabitants of the commune use coal to produce heat, 25% use biomass and 5% use fuel oil (Model 1). Simulations of the potential ecological effect resulting from the exchange of the energy source were carried out in six variants (Model 2-7), assuming different shares of energy carriers in the energy balance of the commune.
The analysis was carried out for six models: • Model 2 is included in the models calculated above due to a desire to compare the base state pollutant emissions with other models, assuming that renewable energy provision is predominantly provided by solid fuel boilers fired with biomass. The assumptions for models 4-6 result from the technical and financial possibilities of a total departure from solid fuels towards renewable energy Energies 2020, 13, 454 7 of 16 sources, hence a balanced share of solid fuel boilers fired with coal and biomass was assumed, with a constant share of fuel oil, however with the assumption of an increase in the share of heat pumps, as well as an increase in the nominal efficiency of heating devices resulting from the modernisation of the heat source. Model 7, based on the use of biomass and heat pumps, was adopted as the most ecological solution because of the need to strive to increase the share of renewable energy sources when generating heat for space heating and domestic hot water.
Each of the energy carriers analysed was assigned lower heating values (LHW) and the nominal efficiency of heating devices (in the case of oil boilers, including traditional and condensing boilers) on the basis of the catalogues of heating equipment manufacturers and the typical lower heating values (LHW) of conventional fuels and the efficiency factor of the heat pump in Polish conditions (Table 1) [38]. In total, six types of biomass were analysed (pellets, dry wood, straw briquettes, wood briquettes, rape straw, corn straw), three types of coal (eco-pea coal, nut coriander, culm), two types of fuel oil (light and heavy) and three types of heat pumps (air/water, brine/water with vertical heat exchanger, brine/water with horizontal heat exchanger). In the case of heat pumps, it was decided to consider three variants because of their widespread use in Polish conditions. The temperature stability of the ground as a heat source is important from the point of view of using a heat pump for heating and domestic hot water preparation, hence it was decided to consider devices with vertical and horizontal heat exchangers. However, taking into account the changing standards for new buildings (decreasing demand for heat), an air/water heat pump for central heating and domestic hot water preparation has also been taken into account. This type of device is increasingly used due to the investment costs, which are lower because of the lack of the need to carry out work related to the implementation of the heat source.
Arithmetic means were calculated for individual groups (biomass, coal, fuel oil and heat pumps) and then the percentage share of the model analysed was assigned.
In addition, an increase in nominal efficiency has been assumed for solid-fuel boilers (coal and biomass). For biomass boilers, the efficiency increase was assumed to be from 80%-88% to 90%, while for coal boilers from 75% to 80%. The emission of pollutants into the atmosphere was calculated on the basis of the National Centre for Emissions Management (KOBIZE) guidelines, on emission factors for where: E-emission of pollutants (g/MJ), B-amount of fuel, (m 3 ) or (Mg), W-emission factor (Mg).

Results
The results of the calculations are shown in Figures 7-13. Model 1, with a 70% share of coal, a 25% share of biomass and a 5% share of fuel oil, was adopted as the baseline, against which further variants of the calculations were compared. The results obtained (Figure 7) showed the quantities of pollutants emitted into the atmosphere during the year to be: SO x -35,191 kg, NO x -11,394 kg, CO-234,280 kg and TSP (particulate matter PM10 and PM2.5)-55,639 kg.

Results
The results of the calculations are shown in Figures 7-13. Model 1, with a 70% share of coal, a 25% share of biomass and a 5% share of fuel oil, was adopted as the baseline, against which further variants of the calculations were compared. The results obtained (Figure 7) showed the quantities of pollutants emitted into the atmosphere during the year to be: SOx-35,191 kg, NOx-11,394 kg, CO-234,280 kg and TSP (particulate matter PM10 and PM2.5)-55,639 kg. The results obtained in Model 2 (Figure 8), assuming the opposite situation in relation to the base state-70% share of biomass, 25% share of coal and 5% share of fuel oil-indicate a significant reduction in emissions, depending on the pollutant concerned, ranging from 11.23% to 59.45%. By far the largest emissions reduction concerns SOx (59.45%) and TSP (24.23%). The smallest reduction is CO, at 11.23%. In Model 3 (Figure 9), an equal share of coal and biomass utilisation was assumed at 47.5%, with a 5% share of fuel oil. The results obtained indicate about half of the reductions seen in the case of Model 2, that is, in the range 5.62%-29.72%. The results obtained in Model 2 (Figure 8), assuming the opposite situation in relation to the base state-70% share of biomass, 25% share of coal and 5% share of fuel oil-indicate a significant reduction in emissions, depending on the pollutant concerned, ranging from 11.23% to 59.45%. By far the largest emissions reduction concerns SO x (59.45%) and TSP (24.23%). The smallest reduction is CO, at 11.23%. In Model 3 (Figure 9), an equal share of coal and biomass utilisation was assumed at 47.5%, with a 5% share of fuel oil. The results obtained indicate about half of the reductions seen in the case of Model 2, that is, in the range 5.62%-29.72%.  The last model, which does not take into account the share of heat pumps using renewable energy sources, was Model 4 (Figure 10), based on the same assumptions as Model 3, but with the additional condition that the efficiency of heating devices would increase by 5%. The assumed value of the increase in efficiency is relatively low, but this is because the increase in the efficiency of the entire space heating and domestic hot water installation (transmission, regulation and accumulation efficiency) is taken into account, and not just the efficiency of heat generation. Nevertheless, the results obtained show that the replacement of solid fuel boilers with class-5 boilers that meet the requirements of EcoDesign, even without a comprehensive modernisation of the installation, brings the desired results. Compared to Model 4, the reduction in SOx increased from 29.72% to 33.80%, for NOx from 8.69% to 14.19%, for CO from 5.62% to 11.87% and for TSP from 12.12% to 18.71%. It is worth noting that in the case of the reduction of CO emissions, the value of 11.87% obtained in Model 3 is slightly higher than in the case of Model 1, where it was 11.23%.  The last model, which does not take into account the share of heat pumps using renewable energy sources, was Model 4 (Figure 10), based on the same assumptions as Model 3, but with the additional condition that the efficiency of heating devices would increase by 5%. The assumed value of the increase in efficiency is relatively low, but this is because the increase in the efficiency of the entire space heating and domestic hot water installation (transmission, regulation and accumulation efficiency) is taken into account, and not just the efficiency of heat generation. Nevertheless, the results obtained show that the replacement of solid fuel boilers with class-5 boilers that meet the requirements of EcoDesign, even without a comprehensive modernisation of the installation, brings the desired results. Compared to Model 4, the reduction in SOx increased from 29.72% to 33.80%, for NOx from 8.69% to 14.19%, for CO from 5.62% to 11.87% and for TSP from 12.12% to 18.71%. It is worth noting that in the case of the reduction of CO emissions, the value of 11.87% obtained in Model 3 is slightly higher than in the case of Model 1, where it was 11.23%. The last model, which does not take into account the share of heat pumps using renewable energy sources, was Model 4 (Figure 10), based on the same assumptions as Model 3, but with the additional condition that the efficiency of heating devices would increase by 5%. The assumed value of the increase in efficiency is relatively low, but this is because the increase in the efficiency of the entire space heating and domestic hot water installation (transmission, regulation and accumulation efficiency) is taken into account, and not just the efficiency of heat generation. Nevertheless, the results obtained show that the replacement of solid fuel boilers with class-5 boilers that meet the requirements of EcoDesign, even without a comprehensive modernisation of the installation, brings the desired results. Compared to Model 4, the reduction in SO x increased from 29.72% to 33.80%, for NO x from 8.69% to 14.19%, for CO from 5.62% to 11.87% and for TSP from 12.12% to 18.71%. It is worth noting that in the case of the reduction of CO emissions, the value of 11.87% obtained in Model 3 is slightly higher than in the case of Model 1, where it was 11.23%. Models 5 and 6 contain a 15% share of heat pumps. This is an optimistic and cautious option, which results from the development of the heat pump market in Poland and the decreasing price difference between class-5 solid fuel boilers and air/water heat pumps. On the other hand, it is difficult to foresee a heat pump in buildings with high heat demand and possessing a heating system operating at high supply temperatures. This is due to the need to modernise the installation to work at the lower temperatures required by heat pumps for efficient operation. The results of the calculations for models 5 ( Figure 11) and 6 ( Figure 12) show a higher level of pollution reduction for NOx, CO and TSP than in all the other models investigated. Only in the case of SOx are the values better than those in Model 2. Emissions of pollutants are reduced by: 39.99%-43.43% for SOx, 21.88%-26.5% for NOx, 20.51%-25.77% for CO and 25.93%-31.49% for TSP.  Models 5 and 6 contain a 15% share of heat pumps. This is an optimistic and cautious option, which results from the development of the heat pump market in Poland and the decreasing price difference between class-5 solid fuel boilers and air/water heat pumps. On the other hand, it is difficult to foresee a heat pump in buildings with high heat demand and possessing a heating system operating at high supply temperatures. This is due to the need to modernise the installation to work at the lower temperatures required by heat pumps for efficient operation. The results of the calculations for models 5 ( Figure 11) and 6 ( Figure 12)   Models 5 and 6 contain a 15% share of heat pumps. This is an optimistic and cautious option, which results from the development of the heat pump market in Poland and the decreasing price difference between class-5 solid fuel boilers and air/water heat pumps. On the other hand, it is difficult to foresee a heat pump in buildings with high heat demand and possessing a heating system operating at high supply temperatures. This is due to the need to modernise the installation to work at the lower temperatures required by heat pumps for efficient operation. The results of the calculations for models 5 ( Figure 11) and 6 ( Figure 12) show a higher level of pollution reduction for NOx, CO and TSP than in all the other models investigated. Only in the case of SOx are the values better than those in Model 2. Emissions of pollutants are reduced by: 39.99%-43.43% for SOx, 21.88%-26.5% for NOx, 20.51%-25.77% for CO and 25.93%-31.49% for TSP.  The conclusions drawn from the analysis of the results obtained in models 2-6 indicate that the optimal solution would be the replacement of coal-fired solid-fuel boilers with solutions based on the use of renewable energy sources. For this reason, a model in which only biomass boilers and heat pumps are used (each technology providing 50%), was also investigated. When compared to the baseline, Model 7, to which these assumptions refer, indicates the highest reduction of pollutant emissions to the atmosphere, as shown in Figure 13. In the case of SOx, it is a reduction of 403 kg, which is 87.32% less than in the base state. For NOx, it is 3636 kg (68.09%), for CO, 94,439 kg (59.69%) and for suspended dust TSP, 16,344 kg (70.63%). In addition, the possibilities of achieving the proposed changes over a 10-year period were also analysed. The calculation was based on the above models, where the baseline was dominated by coal in addition to models which make use of heat pumps and the increase in efficiency of solid-fuel boilers (Figure 14), as well as a model which only uses renewable energy sources-biomass boilers and heat pumps ( Figure 15). The analysis was conducted for the period 2019-2029. The values for individual years were calculated based on the assumption of a regular change in the proportions in the structure of devices and fuels used. The difference in the percentage of the baseline and target state was divided by the value of ten years, which for each group of devices and fuels gave an annual The conclusions drawn from the analysis of the results obtained in models 2-6 indicate that the optimal solution would be the replacement of coal-fired solid-fuel boilers with solutions based on the use of renewable energy sources. For this reason, a model in which only biomass boilers and heat pumps are used (each technology providing 50%), was also investigated. When compared to the baseline, Model 7, to which these assumptions refer, indicates the highest reduction of pollutant emissions to the atmosphere, as shown in Figure 13. In the case of SO x , it is a reduction of 403 kg, which is 87.32% less than in the base state. For NO x , it is 3636 kg (68.09%), for CO, 94,439 kg (59.69%) and for suspended dust TSP, 16,344 kg (70.63%).
The conclusions drawn from the analysis of the results obtained in models 2-6 indicate that the optimal solution would be the replacement of coal-fired solid-fuel boilers with solutions based on the use of renewable energy sources. For this reason, a model in which only biomass boilers and heat pumps are used (each technology providing 50%), was also investigated. When compared to the baseline, Model 7, to which these assumptions refer, indicates the highest reduction of pollutant emissions to the atmosphere, as shown in Figure 13. In the case of SOx, it is a reduction of 403 kg, which is 87.32% less than in the base state. For NOx, it is 3636 kg (68.09%), for CO, 94,439 kg (59.69%) and for suspended dust TSP, 16,344 kg (70.63%). In addition, the possibilities of achieving the proposed changes over a 10-year period were also analysed. The calculation was based on the above models, where the baseline was dominated by coal in addition to models which make use of heat pumps and the increase in efficiency of solid-fuel boilers (Figure 14), as well as a model which only uses renewable energy sources-biomass boilers and heat pumps ( Figure 15). The analysis was conducted for the period 2019-2029. The values for individual years were calculated based on the assumption of a regular change in the proportions in the structure of devices and fuels used. The difference in the percentage of the baseline and target state was divided by the value of ten years, which for each group of devices and fuels gave an annual In addition, the possibilities of achieving the proposed changes over a 10-year period were also analysed. The calculation was based on the above models, where the baseline was dominated by coal in addition to models which make use of heat pumps and the increase in efficiency of solid-fuel boilers (Figure 14), as well as a model which only uses renewable energy sources-biomass boilers and heat pumps ( Figure 15). The analysis was conducted for the period 2019-2029. The values for individual years were calculated based on the assumption of a regular change in the proportions in the structure of devices and fuels used. The difference in the percentage of the baseline and target state was divided by the value of ten years, which for each group of devices and fuels gave an annual change. The results obtained clearly show that moving away from hard coal combustion to renewable energy sources results in a significantly faster means of achieving reduced emissions and achieving the Energies 2020, 13, 454 12 of 16 target thresholds. This is evidenced by achieving better values than the target threshold from Model 6 for the variant with 100% share of renewable energy sources in 2024, i.e., five years earlier.
Energies 2020, 13, x FOR PEER REVIEW 12 of 16 change. The results obtained clearly show that moving away from hard coal combustion to renewable energy sources results in a significantly faster means of achieving reduced emissions and achieving the target thresholds. This is evidenced by achieving better values than the target threshold from Model 6 for the variant with 100% share of renewable energy sources in 2024, i.e., five years earlier.

Discussion
In the context of the results obtained, it is important to plan specific actions to improve the quality of the environment and increase energy efficiency in the region. In the context of energy planning, the methodology presented in this article and the results obtained can certainly complement activities at the regional level. This is particularly important in order to understand the potential of renewable energy sources to improve the quality of the environment, primarily by using heat pumps as a zero-emission technology in the households. The results obtained during the research clearly indicate that the optimal model of those proposed is the one based on the use of biomass and heat pump technology. By analysing the dynamics of change, it can be seen that the rate of reduction of pollutant emissions, when compared to the other models, is at least twice as high. change. The results obtained clearly show that moving away from hard coal combustion to renewable energy sources results in a significantly faster means of achieving reduced emissions and achieving the target thresholds. This is evidenced by achieving better values than the target threshold from Model 6 for the variant with 100% share of renewable energy sources in 2024, i.e., five years earlier.

Discussion
In the context of the results obtained, it is important to plan specific actions to improve the quality of the environment and increase energy efficiency in the region. In the context of energy planning, the methodology presented in this article and the results obtained can certainly complement activities at the regional level. This is particularly important in order to understand the potential of renewable energy sources to improve the quality of the environment, primarily by using heat pumps as a zero-emission technology in the households. The results obtained during the research clearly indicate that the optimal model of those proposed is the one based on the use of biomass and heat pump technology. By analysing the dynamics of change, it can be seen that the rate of reduction of pollutant emissions, when compared to the other models, is at least twice as high.

Discussion
In the context of the results obtained, it is important to plan specific actions to improve the quality of the environment and increase energy efficiency in the region. In the context of energy planning, the methodology presented in this article and the results obtained can certainly complement activities at the regional level. This is particularly important in order to understand the potential of renewable energy sources to improve the quality of the environment, primarily by using heat pumps as a zero-emission technology in the households. The results obtained during the research clearly indicate that the optimal model of those proposed is the one based on the use of biomass and heat pump technology. By analysing the dynamics of change, it can be seen that the rate of reduction of pollutant emissions, when compared to the other models, is at least twice as high.
However, it should be noted that the barrier to achieving the assumed goals may be social acceptance of the proposed solutions resulting from, among other influences, the wealth of society. In a similar context, namely the essence of social acceptance of renewable energy, Schumacher, Krones, McKenna and Schultmann [40] made the following comments. They note that this is largely dependent on a given technology and the researcher's overall experience in renewable energy, as well as the level Energies 2020, 13, 454 13 of 16 of experience in the field of energy autonomy. The last of the terms, i.e., energy autonomy, may be important in the future because the interesting vision is the role of rural areas as renewable energy exporters to cities in order to meet their growing energy demand [41].
Although the purpose of this study was to address individual heat sources and focus on the thermal management of buildings, it is worth bearing in mind that in an energy planning situation, decision makers should be aware of the potential consequences for public acceptance when setting framework conditions to support specific technologies or projects. In addition, one should agree with the statement [40] that opinion polls at the national level cannot be used as a basis for decisions for local projects in the field of renewable energy sources and sustainable development, because the subject of acceptance is simply different and the approval for specific projects associated with renewable energy sources must be assessed on a case-by-case basis.
Returning to the results obtained during the research, the need to decarbonise energy systems in both rural and urban areas is unambiguous. Kostevsek, Petek, Cucek, Kleme and Varbanov [42], as well as Benedek, Tihamér-Tibor and Bartók [43], agree that one of the most important issues relating to the current energy policy is the need to focus on local energy systems and the role of renewable energy in these systems. In the context of this article, it should be stated that this is the right approach, and therefore one should strive to do this, mainly in rural areas characterised by a lack of district heating systems or access to natural gas as a more ecological energy carrier than hard coal or heating oil. An additional advantage of this approach to energy systems, among other approaches proposed in rural areas, is that it is possible to use them as a tool to improve energy security and support economic development [43].
The calculations carried out showed large ecological benefits resulting from the use of renewable energy sources, although the use of only renewable sources in households in the coming years seems unrealistic in Poland. Systems based on renewable energy sources such as sun or wind, which are characterised by unpredictability, require the use of peak sources (e.g., gas or electric) or adequate energy storage technologies; therefore, real energy models are those that take into account both renewable and conventional sources.
It should be added that the production of energy from renewable sources is likely to be more decentralised in the future, which may lead to potentially significant changes in the regional economy. Undoubtedly, one should agree with the statement that in the context of the region and its energy transformation, reference should be made to such aspects as employment rates, supply chain analyses, input-output models or economic losses in conventional energy generation sectors due to the growing share of energy production from sources of renewable energy in the structure of local energy production [44]. Renewable energy systems are becoming a strong element of local sustainable development strategies. The answer to the question remains whether direct financial support for renewable energy systems has an impact on the increase in the use of renewable energy [45]. It seems that in Polish conditions, with a less prosperous society and a not very well-developed renewable energy sector, direct support may, however, be the main incentive to invest in renewable energy.

Conclusions
The occurrence of low-level emissions in Poland is a common phenomenon, which is confirmed annually by reports of the Chief Inspectorate for Environmental Protection in Poland and the European Environment Agency. The main reason for this is the individual combustion of solid fuels to provide space heating and domestic hot water. In the case of rural communes, the reasons for such a situation should be seen, first of all, in the lack of a heating and gas network infrastructure, and secondly, in the wealth of society. The calculation results presented in this article clearly showed that the current structure of heat generation based on fossil fuels is unfavourable from the point of view of environmental protection.
One cannot indicate one utilitarian model of conduct for each rural commune in Poland, because each of them has different geographical, topographic, spatial and economic conditions. However, it is Energies 2020, 13, 454 14 of 16 possible to analyse the situation in municipalities using the calculation model presented in the article to indicate specific actions to improve air quality through a planned change in the heat generation structure. It should be clearly stated that the results obtained in individual models indicate the legitimacy of increasing the proportion of renewable energy sources used. In addition, building a sustainable energy mix on a local scale, at the same time as reducing the share of carbon, undoubtedly contributes to reducing emissions of pollutants into the environment.
The models presented in this paper show that, although there are already municipalities in Poland that prohibit the burning of fossil fuels (excluding natural gas), the most advantageous solution there is Model 7. The first results of these activities will be known after the end of the heating season, 2019/2020. Model 7 is characterised by the largest decrease in pollutant emissions compared to the basic model, which undoubtedly proves the need to take action to replace old boilers with new equipment and promoting solutions based on renewable energy sources.
Key Conclusions: • The problem of low emissions in rural areas is primarily due to individual heat sources, which results from, among other reasons, a lack of infrastructure for district heating networks and gas networks.

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The classification of the individual heat sources analysed in this article on the basis of the emissions they produce, from the most to the least, is as follows: coal, biomass, heating oil, heat pumps.

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It should be noted that the heat pump in fact does not emit any pollution in the place of heat delivery, and the resulting emissions derive from the structure of electricity generation in Poland (majorly based on coal), which is necessarily used to drive the compressor and circulation pumps.

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Although heat pumps are a renewable energy solution to reduce heat emissions due to air pollution, it should be noted that from the socioeconomic aspect, the procurement of these devices is expensive for rural communities.

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The results obtained indicate that the structure of the energy sources used is the main problem. The effectiveness of the devices used does not significantly affect the results.