Possibilities of Upgrading Warsaw Existing Residential Area to Status of Positive Energy Districts

This paper analyses possibilities of refurbishment of Warsaw’s residential buildings towards standards of the Positive Energy District. The annual final energy consumption in the city in 2019 for the district heating was 8668 GWh, gas (pipelines) was 5300 GWh, electricity from the grid was 7500 GWh, while the emission of the carbon dioxide was 5.62 × 109 kg. The city consists of 18 districts, which are heterogeneous in terms of typology and structure of buildings. The great variety of buildings can be seen, for example, by the annual final energy demand for space heating and hot water preparation per unit of room area. This annual index ranges from over 400 kWh/m2 in historic buildings to 60 kWh/m2 in modern buildings. A reduction in the consumption of non-renewable energy sources and carbon dioxide emissions can be achieved by improving the energy standard of residential buildings and by using renewable energy sources: solar energy, geothermal energy and biogas. The potential barriers for achieving the status of a positive energy district, for example, problems connected with ownership, financing new investments and refurbishment and legal boundaries, have been identified. Moreover, changing the existing electrical grid and district heating systems in urban areas in Warsaw requires comprehensive modernization of practically the entire city’s infrastructure.


Introduction
The first intensive measures aimed at improving energy security appeared in response to the fuel crisis of 1973 [1]. Indirectly, this global shock also contributed to increasing the importance of environmental protection to preserve the environment for future generations. There are several standard actions aimed at reducing the burden of harmful substances in the environment: rationalization of needs, improvement of broadly understood efficiency, and increased use of "clean" technologies to meet the needs.
In the residential buildings sector, these activities are mentioned: -rationalizing the behaviour of building users (residents), -rationalizing the internal systems operation, -reduction of heat losses in buildings by improving the thermal insulation of the envelope, -recovery of heat discharged to the environment, e.g., from ventilation air, domestic sewage, -improving the energy efficiency of all technical devices and systems. Nevertheless, it is not enough just to try to reduce the energy demand to meet the needs. Hence the idea of using "clean" technologies that do not burden the environment, especially renewable energy ones, have consequently led to the creation of "nearly zeroenergy", "zero-energy" and "energy-positive" buildings in terms of final energy and/or primary energy. The assessment also concerns the balance of emissions of harmful solid and gaseous substances, e.g., carbon dioxide (CO 2 ).
The "greenest" districts are Wawer (1.43 × 10 7 km 2 ) and Białołęka (1.17 × 10 7 m 2 ). Additionally, taking into account the percentage of parks, lawns and green areas in housing estates in the district's area, Wawer (18%), Rembertów (17%) and Włochy (15%) can be distinguished in this regard. The districts with the largest share of transport routes are located in the outskirts of Warsaw, Wawer (1.35 × 10 7 m 2 , 17%) and Białołęka (1.17 × 10 7 m 2 , 15%) [17]. This is mainly due to the location of the main transport connections: internally within in the city and externally outside of the city.
The structure of the land by use is as follows: 28% residential areas, 28% green areas, 12% agricultural areas, 10% development and communication areas, 7% service areas, 3% surface water areas, 1% technical infrastructure areas and 6% other [18]. It can therefore be concluded that Warsaw has a relatively large biologically active area, which constitutes 42%.
The final energy consumption structure in the year 2014 [9], averaged for the long-term weather conditions, is shown in the Figure 2. Most important is the heat from the district heating network (41.87%) and electricity from the national grid (32.52%). The category "Others", which includes renewable energy systems, was still marginal (5.82%) that year.  According to the data [9], buildings constructed in the period of 1945-1970 have the largest share of the total number of buildings in Warsaw (26.39%). The share of relatively new buildings constructed after 2002 accounts only for 4.60%.
The final as well as primary energy consumption indices depending on the construction period show how different buildings in terms of the energy efficiency are in Warsaw (Table 2) [9,19,20]. Low thermal energy buildings can be improved through the typical retrofit methods: reduction of thermal transmission of the building external partitions (e.g., windows, walls, roofs), elimination of the existence of thermal bridges, improvement of efficiency of heating systems as well as hot water preparation (e.g., exchange of heat source, introduction and optimization of energy control strategy), improvement of ventilation system and heat recovery from the exhausted air. The possible effects of the thermo-modernization in two variants, depending on the construction period and type of a building, of two variants are presented in the Table 3 [20]. Table 3. Reduction potential of the energy demand in houses [20]. The first variant "standard" means reaching the level of the reduction when current requirements are fulfilled [3], while "deep" means the application of more intensive technical upgrading actions, up to reaching the maximal possible effects assuming use of new but conventional technologies. It is interesting that the application of the current technical possibilities, even in buildings that have recently been put into use, allows for reducing the demand for final energy in residential buildings by approximately 35% to almost 40%. It is possible mainly due to the reduction of the heat demand for heating the ventilation air, e.g., replacing gravity ventilation with a mechanical ventilation system with heat recovery.

Construction
It is characteristic for old buildings that no changes in their external appearance are possible if they are under conservator's protection. Therefore, in such buildings, the thermal insulation of the external partitions can be added only from the inside of the building. The effectiveness of such treatments is low, and reduction of energy consumption does not exceed 20%. In many cases there is no possibility to install solar collector systems or PV systems due to heritage protection, so the old quarters of Warsaw are not so attractive in terms of generating energy on-site (locally). The most interesting in terms of retrofit are groups of buildings built in the 1970s and later, with the annual final energy consumption index for space heating and hot water preparing at a level of 220 kWh/m 2 . In such cases the possible reduction of energy demand reaches 40%, but it is still too low for reaching positive energy building status. Figure 3 presents maximum annual primary non-renewable energy indices for heating, ventilation and hot water preparation [3]. The strongest requirements are foreseen for Energies 2021, 14, 5984 6 of 17 educational buildings, while for residential buildings the requirements are less restrictive. Nevertheless, in all cases the limit values are constantly lowered, which helps to reduce the consumption of non-renewable primary energy and hence CO 2 emissions. systems or PV systems due to heritage protection, so the old quarters of Warsaw are not so attractive in terms of generating energy on-site (locally). The most interesting in terms 173 of retrofit are groups of buildings built in the 1970s and later, with the annual final en-174 ergy consumption index for space heating and hot water preparing at a level of 220 175 kWh/m 2 . In such cases the possible reduction of energy demand reaches 40%, but it is still 176 too low for reaching positive energy building status. 177 Figure 3 presents maximum annual primary non-renewable energy indices for 178 heating, ventilation and hot water preparation [3]. The strongest requirements are fore-179 seen for educational buildings, while for residential buildings the requirements are less 180 restrictive. Nevertheless, in all cases the limit values are constantly lowered, which helps 181 to reduce the consumption of non-renewable primary energy and hence CO2 emissions. 182 183 Figure 3. Maximum annual primary non-renewable energy indices for heating, ventilation and hot water preparation [3]. 184 The annual final energy consumption in the year 2019 [9] for district heating was 185 8668 GWh, gas (pipelines) was 5300 GWh, electricity from the grid was 7500 GWh, while 186 the emission of the carbon dioxide was 5.62 × 10 9 kg [9]. 187 In the year 2019, the co-generation heat and power plants had a thermal capacity of 188 2245 MW [9]. Old coal-fired plants are expected to be decommissioned in the near future. 189 They will be replaced by modern gas-steam units [21]. During peak demand, the system 190 can use thermal energy of 1 GWh stored in a water tank [9]. 191 The capacity of all electricity generation sources in the year 2019 was 8.529 GW,192 while electricity consumption was 7500 GWh, of which households consumed 2141 GWh 193 while the electricity storage in the electricity grid was 2 MWh [9]. 194 In the year 2014 [9], taking into account the type of heating source (system and fuel), The annual final energy consumption in the year 2019 [9] for district heating was 8668 GWh, gas (pipelines) was 5300 GWh, electricity from the grid was 7500 GWh, while the emission of the carbon dioxide was 5.62 × 10 9 kg [9].
In the year 2019, the co-generation heat and power plants had a thermal capacity of 2245 MW [9]. Old coal-fired plants are expected to be decommissioned in the near future. They will be replaced by modern gas-steam units [21]. During peak demand, the system can use thermal energy of 1 GWh stored in a water tank [9].
The capacity of all electricity generation sources in the year 2019 was 8.529 GW, while electricity consumption was 7500 GWh, of which households consumed 2141 GWh while the electricity storage in the electricity grid was 2 MWh [9].
In the year 2014 [9], taking into account the type of heating source (system and fuel), the situation was as follows: district heating supplying heat to buildings of 80,789,204 m 2 of heated area (71.0%) and gaseous fuel for 23,395,279 m 2 (20.6%). There were other energy carriers used in small amounts: electrically driven heaters for 2,599,475 m 2 (2.3%), propanebutane mixture for 1,316,459 m 2 (1.2%), light fuel oil for 3,432,031 m 2 (3.0%), hard coal for 1,843,420 m 2 (1.6%) and renewable energy sources for 411,743 m 2 (0.4%).
Possibilities of the reduction of final energy consumption in residential buildings has been estimated by assuming average efficiency of the heating systems for all types of residential buildings in the specific period of their construction due to lack of detailed data. Additionally, it was assumed that historical buildings would not be refurbished. As the final energy reduction was decreased by almost 30% in the standard and 40% in the deep retrofit, relevant emissions in the residential sector are possible.
A characteristic feature of the demand for electric power is the lower demand in summer, but due to the increase in cooling demand, these relations will change ( Figure 4). Although the available capacity of renewable energy sources is constantly increasing, it is still lower than 16% of the actual needs.
Energies 2021, 14, 5984 7 of 17 the final energy reduction was decreased by almost 30% in the standard and 40% in the 205 deep retrofit, relevant emissions in the residential sector are possible. 206 A characteristic feature of the demand for electric power is the lower demand in 207 summer, but due to the increase in cooling demand, these relations will change ( Figure   208 4). Although the available capacity of renewable energy sources is constantly increasing, 209 it is still lower than 16% of the actual needs.  [9]. 212 Electricity consumption in the last three years has remained stable despite the con-213 tinuous development of the city ( Figure 5). Energy consumption in households in the 214 following years has been similar and accounts for less than 29% of total energy con-215 sumption in the city. Unfortunately, the coverage of energy demand from renewable 216 energy systems is less than 2%. However, the real share of renewable energy sources is 217 higher because the official data do not take into account the energy produced by 218 prosumers for their own needs [9]. Electricity consumption in the last three years has remained stable despite the continuous development of the city ( Figure 5). Energy consumption in households in the following years has been similar and accounts for less than 29% of total energy consumption in the city. Unfortunately, the coverage of energy demand from renewable energy systems is less than 2%. However, the real share of renewable energy sources is higher because the official data do not take into account the energy produced by prosumers for their own needs [9].
deep retrofit, relevant emissions in the residential sector are possible. 206 A characteristic feature of the demand for electric power is the lower demand in 207 summer, but due to the increase in cooling demand, these relations will change ( Figure   208 4). Although the available capacity of renewable energy sources is constantly increasing, 209 it is still lower than 16% of the actual needs.  [9]. 212 Electricity consumption in the last three years has remained stable despite the con-213 tinuous development of the city ( Figure 5). Energy consumption in households in the 214 following years has been similar and accounts for less than 29% of total energy con-215 sumption in the city. Unfortunately, the coverage of energy demand from renewable 216 energy systems is less than 2%. However, the real share of renewable energy sources is 217 higher because the official data do not take into account the energy produced by 218 prosumers for their own needs [9]. The basic rate of thermal energy necessary to cover the needs in Warsaw is generated in heat and power plants and heating plants ( Figure 6). The available thermal installed capacity of renewable energy systems accounts for about 6% of the total demand. The basic rate of thermal energy necessary to cover the needs in Warsaw is generated in heat and power plants and heating plants ( Figure 6). The available thermal in- 23 stalled capacity of renewable energy systems accounts for about 6% of the total demand. 24 25 Figure 6. Thermal installed capacity of energy plants in Warsaw, [MW] [9]. 26 The heat generated in renewable energy systems ( Figure 7) accounts for less than 5% 27 of the heat supplied to end users by the district heating network. However, it should be 28 noted that the heat generated by individual users for their own needs is not included in 29 the balance. The heat generated in renewable energy systems ( Figure 7) accounts for less than 5% of the heat supplied to end users by the district heating network. However, it should be noted that the heat generated by individual users for their own needs is not included in the balance.
The basic rate of thermal energy necessary to cover the needs in Warsaw is gener-222 ated in heat and power plants and heating plants ( Figure 6). The available thermal in-223 stalled capacity of renewable energy systems accounts for about 6% of the total demand.

234
The city of Warsaw is characterized by a very specific situation in terms of supply-235 ing heat to end users: the residential and public sector, services and industry. Heat is 236 supplied centrally, ensuring cheap and reliable collection of this heat by users located in 237 all parts of the city. Warsaw's district heating network is one of the largest in Europe, 238 with a length of almost 1800 km. It supplies heat to over 80% of buildings in the city. Heat 239 is produced in co-generation, and coal is the main energy fuel [22]. Polish district heating

District Heating Network
The city of Warsaw is characterized by a very specific situation in terms of supplying heat to end users: the residential and public sector, services and industry. Heat is supplied centrally, ensuring cheap and reliable collection of this heat by users located in all parts of the city. Warsaw's district heating network is one of the largest in Europe, with a length of almost 1800 km. It supplies heat to over 80% of buildings in the city. Heat is produced in co-generation, and coal is the main energy fuel [22]. Polish district heating systems are high temperature systems since they can supply steam with temperatures of 120 • C. The present state and planned development of Warsaw's district heating system is presented in Figure 8 and it is mainly for residential purposes.  However, when analysing Figure 8, a question can be asked: why should the new residential districts (in yellow) be connected to the central district heating network? Extending the old network of poor quality (poor thermal insulation, corrosion of pipelines, leaks of heating medium (water or steam)) to new urban areas raises concerns not only in terms of energy efficiency, but also of the economic efficiency of such projects. The construction of new distributed energy systems focused on the use of own local energy resources, in particular renewable energies, seems to be a much more rational solution of energy and economic efficiency. Moreover, the construction of new buildings in newly emerging districts with the use of modern building materials and technologies enables a significant reduction in energy requirements for heating buildings. Newly created housing estates are usually characterized by low indices of energy needs. If, moreover, modern installation technologies are used, then, as a consequence, the final energy consumption can be significantly reduced and can easily meet modern building energy codes. Thanks to the use of renewable energy, the consumption of primary energy from fossil fuels can decrease significantly or even not be used at all. When using their own local and renewable energy resources, new estates can not only cope with their own small energy load but can also share energy with districts in the neighbourhood, becoming Positive Energy Districts. The next section presents and analyses the potential for using renewable energy in Warsaw. The different subchapters refer to the different renewable energies that can be used.

Geothermal Energy
In Poland, geothermal energy potential is rather low. There are resources of low enthalpy geothermal water which can be applied for heating and balneology needs. There are thermal plants where geothermal waters are effectively applied for heating, however their utilization is combined with heat pumps and gas boilers.
Geothermal water resources in Warsaw have been assessed as average [23], i.e., at a depth of 1000 m the temperature reaches 30-35 • C, at a depth of 2000 m it exceeds 55 • C, and at a depth of 3000 m it reaches even 85 • C [23,24]. Economic profitability, understood as competition with the currently used technologies, requires temperatures of a resource (geothermal water) to be at least 65 • C at a depth of 2000 m. As the prices of "conventional" energy and heat increase, the competitiveness of this technology increases.
The districts of Wawer, Białołęka and Ursynów have the best opportunities to use geothermal energy for heating buildings (Figure 9), but so far geothermal energy is not used.
In Poland, geothermal energy potential is rather low. There are resources of low 269 enthalpy geothermal water which can be applied for heating and balneology needs. 270 There are thermal plants where geothermal waters are effectively applied for heating, 271 however their utilization is combined with heat pumps and gas boilers. 272 Geothermal water resources in Warsaw have been assessed as average [23], i.e., at a 273 depth of 1000 m the temperature reaches 30-35 °C, at a depth of 2000 m it exceeds 55° C, 274 and at a depth of 3000 m it reaches even 85° C [23,24]. Economic profitability, understood 275 as competition with the currently used technologies, requires temperatures of a resource 276 (geothermal water) to be at least 65 °C at a depth of 2000 m. As the prices of "conven-277 tional" energy and heat increase, the competitiveness of this technology increases. 278 The districts of Wawer, Białołęka and Ursynów have the best opportunities to use 279 geothermal energy for heating buildings (Figure 9), but so far geothermal energy is not

Solar Energy
Warsaw is located at 51 • N latitude in a moderate climate zone with influences of Atlantic and Continental climate, as is true of the whole country. Such a location causes the city (and Poland) to be affected by different atmospheric fronts that result in frequent heavy cloud formation. The averaged mean yearly temperature is equal to 7.9 • C and average annual global solar irradiation accounts for 1100 kWh/m 2 and solar hours are on average equal to 1600 [25]. The winters are relatively severe. The coldest months are January, February and December and solar irradiation is lowest in those months, which can be seen in Figure 10. In winter the ambient air temperature can even drop to −20 • C on extremely cold days. The average hourly ambient air temperature for averaged days of January and February (the coldest months in a year) varies during the daytime from −6 • C to −2 • C (January) or to 0 • C (February). tube solar collectors and water storage tanks. Anti-freezing mixture circulates in a solar 14 collector loop. The annual share of the solar heating system supplying heat to DHW 15 (domestic hot water) system can be at a level of 50%-65%; in the case of space heating the 16 solar share is lower and accounts for 20%-30% (even when the surface area of solar col- 17 lectors is at least two times bigger than in the case of a system only for DHW heating) 18 [25]. 19 It should be underlined that to determine the effective use of solar energy it is nec- The highest solar irradiation occurs in June and in Warsaw the average monthly irradiation is nearly 150 kWh/m 2 . The lowest solar irradiation occurs in December and the average monthly irradiation is equal to about 11 kWh/m 2 . From October to April only about 20% of annual total radiation is available. The structure of solar radiation is characterized by a very high share of diffuse radiation. An average annual percentage of direct radiation amounts to only 46%. In summer, the share of direct radiation is higher and accounts for approximately 56%. However, from November to the end of February the percentage of diffuse radiation varies from 65% to 80%. This situation recommends solar systems for applying both direct and diffuse solar radiation; systems with concentrators are not recommended. With such a low share of direct solar radiation, the effect of concentrating solar beams is very small and can be seen only in summer. The small increase in efficiency is too expensive and the solar concentration technology (solar thermal power plants, including solar power tower plants) is not an economically efficient solution in Polish climate. The solar thermal heating systems consist of flat plate or vacuum tube solar collectors and water storage tanks. Anti-freezing mixture circulates in a solar collector loop. The annual share of the solar heating system supplying heat to DHW (domestic hot water) system can be at a level of 50%-65%; in the case of space heating the solar share is lower and accounts for 20%-30% (even when the surface area of solar collectors is at least two times bigger than in the case of a system only for DHW heating) [25].
It should be underlined that to determine the effective use of solar energy it is necessary to analyse the availability of solar energy. Estimation of distribution of solar radiation incident on any surface of solar energy receiver is very important. The surfaces of solar energy receivers, elements of active solar systems, i.e., solar collectors and PV modules, are tilted to the horizontal surface and can have different orientations (azimuth angle). To analyse solar energy availability, the appropriate solar radiation data are required. However, when we consider utilization of solar energy in cities, not only is access to reliable data important. The architecture of the buildings and the urban planning of the entire city is also very important. The arrangement of individual buildings and other construction structures in close neighbourhoods are also very important. The urban environment may limit the availability of solar radiation, which is related to the shading of the surface of solar receivers. In some locations, building walls and roofs can have very good exposure to incident solar radiation, which makes utilization of solar radiation very effective throughout the year. Figure 11 shows a solar map of the main campus of the Warsaw University of Technology. The figure shows the annual solar irradiation of the individual campus buildings, which enables efficient planning and dimensioning of the solar energy receiver locations. Consequently, it is possible to estimate the technical potential of using solar energy to generate heating and cooling energy and electricity in a given area or district of the city.  the Siekierki heat and power plant [29]. 366 What is important is that only the latest technologies will be applied in the n 367 reconstructed plants [10]. 368 Every city generates a lot of waste. This waste should be recycled, both for the recov 369 raw materials and for energy recovery. Each city may, or rather, should, strive t 370 Figure 11. Solar map (Warsaw University of Technology, Main Campus as an example) [8].
It seems quite interesting to use solar radiation energy to generate electricity in some of the unshaded areas of facilities and transport facilities, which account for about 10% of the city's area. It should be noted that in the current legal and ownership situation, this energy can be mainly used by the landowner.

Biomass and Biogas
In the year 2019, a total of approximately 668,600 tons of municipal waste were produced, including 610,700 tons from households [26]. This means that a statistical resident of Warsaw generated 342.4 kg of municipal waste, including 271.1 kg of mixed waste. The share of mixed waste in the total mass of municipal waste collected was as high as 71.9%. Collection of biodegradable waste amounted to 27,098 tons and 15,020 tons of paper and cardboard. The calorific value of municipal waste ranges from 7-16 MJ/kg [27].
The production of electricity and heat from biogas from sewage is carried out in the "Czajka" and "Południe" sewage treatment plants [28]. Czajka's annual heat production is about 40 GWh with similar amounts of electricity. Annual heat production of the second plant is about 5 GWh. Both plants use the heat and energy for their own purposes.
Interestingly, the owner-Municipal Water and Sewerage Company in the capital city of Warsaw S.A.-plans to invest in photovoltaic installations in the near future, for a total production of 6.7 GWh per year [28].
Electricity and heat are also generated from municipal waste in the Municipal Waste Disposal Plant (owner: Municipal Cleaning Enterprise in the Capital City of Warsaw, location: Targówek district). The reconstruction of this plant is planned in the near future [10].
PGNiG Termika's plans also include a modern municipal waste incineration plant at the Siekierki heat and power plant [29].
What is important is that only the latest technologies will be applied in the new or reconstructed plants [10].
Every city generates a lot of waste. This waste should be recycled, both for the recovery of raw materials and for energy recovery. Each city may, or rather, should, strive to implement a circular economy, which is one of the most important and even basic elements of sustainable development and gives the base for PEDs creation.

Wind Energy
The possibilities of using wind energy to generate electricity in urban areas are quite limited due to the legal requirements in force, i.e., the location of the wind farm requires appropriate provisions in the local spatial development plans. In the part of the city under conservation protection, the installation of even micro wind farms is practically impossible. Availability of wind energy in the area of the Capital City of Warsaw's electricity production is relatively low.
One of the important parameters to urban meteorological investigations is an aerodynamic roughness length (RL, Figure 12 [30]). In the centre of the town, that value is equal to 1 m or in some urbanized areas is even equal to 1.5 m, while is most attractive in terms of wind energy utilization in parts where the value is much lower, e.g., in agricultural areas or wastelands along the Vistula River the factor is equal to 0.1 m. The minimum value of this indicator is above the river surface. In this area, the value is 0.0002 m. plement a circular economy, which is one of the most important and even basic elem 371 of sustainable development and gives the base for PEDs creation. In Warsaw, for the 10% of the windiest surface at a height of 10 m above the ground, the average density of the available power is 87 W/m 2 with an average air flow velocity of 4.00 m/s [30]. At a height of 50 m, these values are, respectively, 204 W/m 2 and 5.63 m/s. At a height of 100 m, these values are, respectively, 299 W/m 2 and 6.60 m/s. The best conditions are on the outskirts of the city, in the north and south, and along the Vistula, i.e., where the roughness length (RL) is not greater than 0.1 m, as can be seen on Figure 12.
The northern region (at the top of Figure 12) does not seem to be attractive for the use of wind energy due to the planned housing development.
The most favourable and sparsely inhabited area of about 17.0 km 2 (south, where the roughness length is not greater than 0.1 m, at the bottom of Figure 12) at a height of 10 m is characterized by an average wind speed of 4.4 m/s, and an average power density of 117 W/m 2 . When at a height of 50 m the average wind speed is estimated as 5.7 m/s, and the average power density as 213 W/m 2 . For the height of 100 m these values are the following: 6.7 m/s and 300 W/m 2 , respectively. Based on the average power density and the assumed area of land, electricity production can be estimated: at a height of 10 m-63 GWh, at a height of 50 m-105 GWh, and at a height of 100 m-160 GWh.

Hydropower
Warsaw is located on the Central Masovian Lowland on the Vistula River, with no access to the sea. Although within the city limits the Vistula is 28 km long, the use of hydropower in Warsaw is not taken into account due to inadequate conditions, i.e., low slope of the land and high density of buildings [31]. The use of smaller rivers, streams or existing water channels with low and variable water flow is currently not considered.
It is worth adding that about 40 km to the north of Warsaw there is the Dębe Hydro Power Plant, which was established in the 1960s [32]. The installed capacity of this power plant is 20 MW, and the average annual energy production is about 91 GWh. This power plant directly supplies the Polish power system with energy.

Towards Positive Energy Districts in Warsaw and Risks to Achieving This Goal
As shown in the previous section, in Warsaw there is a theoretical potential for the use of renewable energies. It should be noted, however, that the use of renewable energies is a rational solution only if there is a reduction in energy demand for traditional methods, which in this case means reducing the energy demand of existing buildings or erecting new ones in accordance with the latest energy and environmental standards.
In Warsaw, buildings in terms of energy performance are very diverse. In many cases, it is possible to significantly improve their energy balance and reduce energy demand through refurbishment and thermo-modernization. Nevertheless, in the historic part of the city, as well as in densely built-up areas with too much energy demand for heating and ventilation, achieving the "positive energy" standard can be difficult even with the current state of technology.
In Polish climatic conditions, it is difficult to achieve energy self-sufficiency in urbanized areas based on only one type of renewable source because energy needs are too large in relation to the possibility of coverage from monovalent-source renewable systems.
The solar energy is easiest to acquire on non-shaded surfaces, e.g., roofs. South or south-west and south-east facades of buildings can also be used as long as they are not shaded.
Low-temperature geothermal energy cannot directly supply a high-temperature heating network. It may be necessary to create local or neighbourhood networks to distribute heat in a smaller area. Heat pumps will be required to raise the temperature. Later such local systems equipped with heat pumps can be connected, thus competing with the traditional district heating network.
The city of Warsaw "produces" large amounts of municipal waste. Some of them are not recyclable or biodegradable. However, they can be used in modern plants for the combined production of electricity and heat. Therefore, special attention can be put to developing modern landfill gas energy plants. Up until now none of these landfill plants operate in Warsaw or its surroundings.
Large areas of green spaces can also be a source of biomass. However, a traditional combustion procedure should not be applied. It is necessary to use highly effective gasification processes as a preliminary energy treatment of biomass residues.
Potential barriers for implementation of renewable energy technologies and achieving the status of a smart city with some positive energy districts should be identified and mentioned. These are mainly problems with ownership, financing new investments and refurbishment, and legal boundaries. Buildings may be private property, there may be business ownership issues, and buildings may belong to a cooperative, housing community or the city of Warsaw, or even a combination of the previously mentioned. This ownership structure determines the ways and possibilities of financing the retrofitting measures. It should be mentioned that land around buildings usually has a similar ownership structure. This, in turn, can result in problems with the placement of energy systems outside the building, and even applying extra thermal insulation to the building walls (the external size of the building increases). The use of renewable energy, rationalization of the energy efficiency of existing systems, proper energy, water and waste management are an important part of efforts to transform Warsaw into a smart city.

Conclusions
Sustainable development, carried out by taking into account the protection of the climate and the natural environment, is de facto a political, economic, technical and social problem even on the scale of the city.
Without the general awareness of the importance of these issues and, on the other hand, social pressure, activities in this area may not be accepted due to the high costs of implementing investments.
Moreover, the development of technology is still important in order to maximize the widely understood effectiveness of technical solutions. It is also important to prepare specialists who carry out comprehensive projects using renewable energy sources and take care of the proper operation and maintenance of existing systems.
Increased outlays for the implementation of the change of the heat and energy supply system require financial support, perhaps on preferential terms or from funds focused on renewable energy.

•
The Capital City of Warsaw supports all activities aimed at reducing the consumption of conventional energy, increasing the use of renewable energy sources and reducing carbon dioxide emissions. • Increasingly stringent requirements in the field of thermal and energy protection of buildings reduce both the demand for heating and ventilation and the required design power of heat sources.

•
Lowering the required design heating load per area below 60 W/m 2 (indicative value) allows the use of low-temperature surface (e.g., floor) heating, which means that the heat source can be a heat pump using, for example, low-temperature geothermal heat available in Warsaw.

•
Achieving a positive energy standard by parts of the city with buildings with unsatisfactory energy performance may be economically unprofitable with the current technical possibilities. • Due to the diversified ownership structure of land, buildings and accompanying infrastructure, as well as in view of further city expansion plans, the easiest way to implement the idea of energy-positive districts is to modernize the relatively recently established housing estates and design new ones with the "obligatory" use of renewable energy sources.

•
The use of renewable energy sources instead of conventional fuels means a significant reduction in carbon dioxide emissions and environmental protection. • Changing the existing electricity and heat supply system for urbanized urban areas in Polish conditions requires comprehensive modernization of practically the entire city infrastructure.
Summarizing, it is very difficult to modernize large energy systems, which for many years have been the only ones supplying energy to various types of energy users in the city. However, it does not mean that nothing can be done. Warsaw, like many world cities, is expanding to new areas and this causes opportunity for development of new local energy systems based on renewables. Such systems can assure self-energy generation and consumption, giving energy security for new districts.
Moreover, new districts can share their energy with other neighbourhoods. As a consequence, the new districts can become Positive Energy Districts which will increasingly interfere with the central energy systems of the city centre. New PEDs will absorb central areas step by step, replacing ineffective central energy systems with new ones based on renewable energy, giving energy independence to the whole city.
It should be underlined that the way to achieve complete energy independence for the city is very complicated and rather long, which is what the authors have tried to present in this paper. If thermal energy (a central district heating system) and electricity (a central power grid) are available everywhere in the city, why does the energy sector, developers, communities and inhabitants have to look for new "fuels" and innovative energy conversion technologies, mainly those based on renewables? This is a question and idea which should be answered by the municipalities as often as possible. Of course, academic people and scientists and local "green" NGOs should help and support the urgent need of the development of local efficient energy generation systems based on locally available energy sources, such as renewables. They should foster the idea of sharing energy in a modern way, which gives the basis for creating more Positive Energy Districts in the city.