1. Introduction
The residential building sector constitutes 27.9% of the final energy consumption within the European Union, ranking as the third most significant sector in terms of energy demand [
1]. For eight out of the twenty-seven European Union member states, the residential sector is characterized as the largest national energy consumer. For these member states, the energy consumption share of residencies varies from 29.3%, for the case of Germany, to 35.1% for the case of Croatia [
2]. In Greece, residential energy consumption accounts for 28.8% of the nation’s total final energy use [
3]. Heating, cooling, and domestic hot water (DHW) preparation make up approximately 79% of households’ energy usage in buildings [
4].
The electrification of the building sector is strengthened by the adoption of electrically driven energy systems, for instance, heat pumps, to satisfy space heating and cooling and DHW needs [
5]. Currently, only a quarter of Europe’s residential sector total energy demand is electrically satisfied [
1]. The contribution of renewables to electricity generation amounts to 44.2% for the European continent and 43.7% for Greece [
6]. The increasing proportion of heat-pump-served buildings [
7] combined with the implementation of deep retrofit initiatives aiming at the thermal performance upgrading of buildings act as key enablers for the decarbonization of Europe’s energy mix [
8]. Additionally, the incorporation of on-site renewable energy systems can aid in the transformation of the existing residential building stock into net-zero-energy and energy-self-sufficient buildings [
9]. Zero-energy buildings are characterized by on-site renewable energy production that adequately covers its energy demand for space heating and cooling, DHW production, and electricity for appliances and lighting. Through the use of energy storage systems or the net-metering facility of the electrical grid, zero-energy buildings consume as much as or less than they produce.
Greece is characterized by a high solar potential and an available area for rooftop PV system installation of 128 km
2 [
10]. The available PV rooftop installation area could potentially cover more than 30% of the country’s final electricity consumption [
11]. However, one of the primary obstacles hindering solar development, particularly in administrative processes such as rooftop solar installations in Greece, is the issue of grid availability [
12]. Many regions in Greece are experiencing rejections of rooftop solar PV applications due to insufficient electricity grid capacity [
12]. Additionally, the adoption of residential rooftop PV systems is interrupted by the relatively high investment cost [
13], which according to Sagani et al. [
14] is financially viable only over a minimum installed power of 5 kW. Nonetheless, the anticipated decrease in investment costs projected by the scientific community in the upcoming years, along with stable electricity prices, will gradually enhance the profitability of PV roof system installation [
15]. According to Greece’s energy regulation authority [
16], for 2022, the installed photovoltaic power was equal to 5288 MW, while from 2011 to 2022, the PV installed capacity of Greece increased by 687%. The electricity production capability, especially at a residential level, is a driving force in the positive energy transformation of buildings and cities [
17]. Regarding domestic electricity production by PVs, in 2023, Greece ranked first in Europe with a percentage (18.4%) more than double the European Union average (8.6%) and more than three times the global average (5.4%) [
18].
Figure 1 illustrates the annual electricity production by PVs as well as the CO
2 emissions abated by PVs in Greece from 2013 to 2023. According to the greenhouse gas emission factors for electricity consumption given by the European Commission, for Greece, the most recent calculated emission factor for electricity consumption is 0.411 kg CO
2/kWh [
19].
Building height is a pivotal determinant of the potential of PV systems, impacting both energy consumption and the available area for photovoltaic deployment. The transformation of multi-story residencies into zero-net-energy buildings is a challenge that can be successfully addressed through a holistic energy efficiency improvement plan. For instance, according to the study of Bellos et al. [
21], the transformation of a four-story multifamily building in Athens into a zero-energy building first requires a holistic energy renovation of the building thermal envelope and systems to restrict the overall energy demand. In another case, Dermentzis et al. [
22] monitored for consecutive years two multifamily buildings in Austria equipped with electrically driven heat pumps and a rooftop PV system. They also underlined that a zero-net-electricity balance is feasible for high-efficiency building constructions, such as the Passive House standard.
Following that, Thebault and Galliard [
23] investigated the integration of residential photovoltaic systems on low-, mid-, and high-rise buildings in France. They drew the conclusion that the installation of rooftop PV systems is the financially and energetically optimal strategy for low- and mid-rise buildings, whereas, for high-rise buildings, the incorporation of photovoltaics on the building façade is essential for building self-consumption and self-sufficiency. Solar potential for electrical production significantly determines the possibility of zero-energy classification in buildings. For this reason, Feng et al. [
24] examined solar electricity production for the various climatic categories of Chinese cities and evaluated the integration of photovoltaic systems and the potential of achieving zero net electricity demand for multi-story family buildings. According to their analysis, the available solar irradiation and household electricity requirements are decisive factors affecting the PV rooftop performance and its sufficiency to fully serve mid-rise buildings up to seven stories in height.
Regarding the climatic conditions of Greece, researchers have investigated the incorporation of photovoltaic systems as a part of a building energy retrofit strategy. More specifically, Pallis et al. [
18] examined various energy retrofit measures of a five-story office building and parametrically assessed their energy and economic performance for the four climatic categories of Greece. They concluded that the installation of photovoltaic systems is considered a cost-effective energy retrofit solution, regardless of the fuel type of building energy systems. The aforementioned conclusion is also drawn by a study by Gaglia et al. [
19] regarding the national residential building sector. Other studies have focused on the energy renovation of public buildings through the integration of renewable energy systems coupled with energy storage systems [
25], and the establishment of effective retrofit strategies based on the building typology [
26]. Additionally, another research study by Sougkakis et al. [
27] investigated the feasibility of near-zero or positive energy communities, through the examination of building-scale and community-scale retrofit action, for a case study in the city of Alexandroupolis.
The combinatorial use of photovoltaic systems and battery systems is considered a driving force toward a decarbonized and sustainable residential sector [
28]. Additionally, the installation of residential capacity stands as a driving force, and subsequently the absorbed energy from the grid, which may allow for securing public grid stability. In 2019, the total installed residential capacity of battery energy storage systems was equal to nearly 2 GWh, while residential battery energy storage systems in combination with PVs accounted for approximately 7% of the total residential PV systems in operation in Europe [
29]. Despite the increasing adaptation of battery energy storage systems, the market potential remains enormous, particularly considering that over 90% of European buildings still lack solar systems [
29].
The most popular and commercially available battery systems are the lithium-ion and sodium–nickel chloride batteries [
30]. Regarding lithium-ion batteries, according to Liu et al. [
31], their superiority over other battery configuration choices lies in their long lifespan, their ability to perform effective charge and discharge cycles, and the fact that they are an appropriate option for off-grid systems. For instance, Forrousso et al. [
32], parametrically investigated the sizing of a lithium-ion battery system for various climatic conditions in Morocco, to design a zero-net-energy building and maximize the load coverage factor and the energy autonomy. Additionally, Orth et al. [
30] examined 26 lithium-ion battery configurations to determine the sizing, conversion, control, and standby losses in residential-scale use. They identified a significant potential for enhancement in residential PV battery systems, especially with regard to conversion efficiency and standby power consumption. Moreover, Chreim et al. [
33] investigated the advantages of individual and shared renewable energy systems integrated with appropriate battery storage systems. They underlined that the sizing parameter is crucial for the optimum performance and economic efficiency of the system, which can be compromised due to the shortage of available solutions in the market.
The present analysis investigates the incorporation of PV roof systems in a typical building typology for a variable number of stories in four different locations. For this purpose, the DesignBuilder software [
34] is used for yearly building energy simulation, in combination with accurate weather data extracted from the PVGIS tool, namely, the incident solar irradiation on the PV modules, for the calculation of electricity production by the PV system [
35]. The aim of this study is to define the maximum number of stories that allow the transformation of a fully electrified building into a zero-energy building, an issue that has not been resolved for the residential building sector of Greece. The examined building locations are the cities of Chania, Athens, Thessaloniki, and Kastoria, which represent the four different Mediterranean climatic categories of Greece. The examined building is equipped with electrically driven air-to-air heat pumps that serve the building’s heating and cooling demands and an air-to-water heat pump for the satisfaction of DHW requirements. The conducted analysis allows for mapping the zero-energy building potential based on climatic data for Greece and introduces the possibility of positive electricity production through the integration of on-site renewable energy sources. The present study can serve as a valuable guideline for the identification of Greece’s potential to increase the number of zero-energy buildings and achieve European goals for the decarbonization and electrification of the residential sector. Additionally, the present study takes into consideration the versatility of Greek buildings, the wide range in the number of story levels, as well as the national energy standards. Lastly, the outputs of the present study can serve as a useful guideline for cities with similar climatic conditions and contribute to the correlation between zero-energy potential and climate types.
4. Discussion
Positive energy districts (PEDs) are urban areas of positive energy equilibrium, comprising highly efficient buildings and integrated renewable energy systems for energy production [
43]. The concept of positive energy buildings lies in the need to create sustainable cities, with a low carbon footprint, able to positively contribute to the energy supply of their surroundings. Addressing the challenges of converting a city to an energy-self-sufficient and sustainable urban environment requires the adoption of multiple, custom-made solutions that apply not only to city-scale restrictions but also to the national energy mix and energy strategy for security and resilience [
44]. Despite the indisputable advantages of positive energy districts, their development is restricted to a research level, with only a few case studies within projects with European funding being applied [
45].
Regarding the case of Greece, currently, three cities are engaged in international projects of developing exemplar energy districts utilizing concepts of sustainable energy deployment: Heraklion [
46], Trikala [
47], and Thessaloniki [
48]. The located efforts contribute to the country’s successful implementation of its national energy and climate plan goal, setting a cornerstone for future urban development goals. As in the majority of similar positive energy district projects [
45], the transformation of a city’s building stock into ZEBs is a pivotal element for the feasibility of a PED [
17]. It has been proven that the combinational adoption of passive or building envelope retrofitting techniques, as well as the installation of highly efficient active energy systems, supports PEDs as an achievable and economically viable target [
49]. In that direction, innovative building materials with advanced thermal and optical properties, highly efficient energy systems for heating, cooling, and domestic hot water production, renewable energy systems for energy production, as well as energy storage systems need to be appropriately utilized to enhance buildings and subsequently a city’s energy performance and footprint.
In this study, the parameter of a building’s height or number of stories was investigated concerning the transformation potential into zero-net-energy buildings. Greece is characterized by an old, energy-consuming building stock with immense potential for energy savings [
50]. According to the thermal analysis results, the mean annual specific energy demand of a residential building was calculated for heating at 20.92 kWh/m
2 (Chania), 24.29 kWh/m
2 (Athens), 41.21 kWh/m
2 (Thessaloniki), and 70.35 kWh/m
2 (Kastoria), and for cooling at 21.95 kWh/m
2 (Chania), 35.32 kWh/m
2 (Athens), 22.62 kWh/m
2 (Thessaloniki), and 9.48 kWh/m
2 (Kastoria). These results are corroborated by various studies that examine the thermal performance of residential buildings in the Mediterranean climate. For instance, in a study by Ascione et al. [
49], two three-floor villas equipped with a reversible air-source electric heat pump with a nominal COP/EER equal to 2.2 and 2.0 are examined for the climatic conditions of Greece and South Italy. For the newly constructed villa in Greece, the annual specific electricity demand for heating/cooling is 20.0 kWh/m
2/34.0 kWh/m
2, while for the ancient villa in South Italy, the corresponding values are 60.0 kWh/m
2 and 17.0 kWh/m
2. Furthermore, in a previous study concerning a flat roof single-family property in Athens, the specific energy demand for heating was found to range between 18.09 kWh/m
2 and 31.02 kWh/m
2, while for cooling, it was between 31.65 kWh/m
2 and 37.21 kWh/m
2 [
39]. Regarding the colder Mediterranean climate of Barcelona, the yearly specific heating demand for a mid-rise multifamily building ranges between 28.0 kWh/m
2 and 68.0 kWh/m
2, while the specific cooling energy demand is between 1.0 kWh/m
2 and 6.0 kWh/m
2 [
51]. Finally, the specific heating/cooling energy demands for a deeply renovated four-story multifamily building in Athens are found to be 11.0 kWh/m
2 and 24.4 kWh/m
2, respectively [
21]. In the latter study, a photovoltaic system is installed on an available rooftop area of 160 m
2, converting the multifamily building into an energy-positive building with a yearly electricity production of 24.4 MWh or 152.4 kWh per m
2 of available rooftop area. In the present study and for the case of Athens, the respective figures were found to be 25.5 MWh and 159.5 kWh per m
2 of available rooftop area.
Despite the fact that low-rise buildings are the most energy-consuming type of residential buildings [
52], they are characterized by an analogically larger available rooftop area for the installation of photovoltaic systems for energy production in comparison to mid- or high-rise buildings. In other words, the analogy of available rooftop area for electricity-production exploitation in comparison to the total treated floor area of a building is a key factor for its successful transformation into a zero-energy building. This aspect poses a question and a serious challenge of whether cities of high population and building density can be successfully converted into PEDs. Comprehensive research regarding the role of the number of building stories in impacting the feasibility of creating positive energy districts needs to be conducted.
Developing the equipment of the building sector with electrically driven heat pumps is proven to be a necessary step towards the decarbonization of the sector and the increase in its energy efficiency. In contrast to carbon-fueled energy systems, heat pumps, characterized by a high coefficient of performance, when combined with the enhancement of a building’s thermal behavior, result in the restricted consumption of electrical energy. The necessary electricity can be derived from on-site electricity production through the installation of a renewable photovoltaic system on a building rooftop, securing a building’s energy autonomy and low energy footprint.
The present work focused on four main cities of Greece, which represent the country’s four distinct climatic categories. According to the present study’s results, the coverage of the available rooftop area with a photovoltaic system can satisfy the yearly electricity demand of a variable number of stories of a multifamily building. The critical number of stories is directly connected to the meteorological conditions of a building’s location, and the findings of the present study can be useful inputs for a future study in which a correlation between the zero-energy potential of multi-story residencies and climate types could be identified. Regarding the outputs of the present study, the maximum number of stories is six for the moderate winter and summer conditions of the city of Chania, characterized by a mean yearly ambient temperature of 18.3 °C and an annual PV in-plane solar irradiation of 1999.3 kWh. For cooling-dominant Athens, where the mean yearly ambient temperature and annual PV in-plane solar irradiation are equal to 18.0 °C and 2070.3 kWh, respectively, the maximum height of a zero-energy building is five stories. For the colder climates of Thessaloniki and Kastoria, where the mean ambient temperature and annual PV-incident irradiation are 15.0 °C and 11.6 °C and 1905.9 kWh and 1789.9 kWh, respectively, the maximum multifamily building height is calculated at four or two stories. These results can be used as a guideline for cities that demonstrate similar climatic characteristics, namely, similar cooling and heating degree days, yearly solar irradiation, and ambient temperature, as well as the PV electricity production potential.
The widespread and successful integration of photovoltaic renewable systems in Greece’s national energy mix concerns both smaller- as well as larger-scale installations. In this direction, the Greek Ministry of Environment and Energy issued a public call with its EUR 238 million subsidy program (from 20% to 100% of the investment cost) for rooftop photovoltaics that either support household applications combined with a storage system, or professional farmers with or without the combination of a storage system, for self-consumption with the application of net metering [
53]. Regarding large-scale electricity production and storage projects, Greece has committed to completing by mid-2025 two important constructions of photovoltaic units with an individual capacity of 252 MW, in combination with integrated molten-salt thermal storage units, an extra-high-voltage substation, and a 309 MW photovoltaic unit with an integrated lithium-ion battery energy storage system [
54]. These projects are predicted to increase the annual net renewable electrical energy in Greece by 1.2 TWh, increasing the country’s capacity in energy storage and thereby enhancing grid stability and availability [
54]. Additionally, two important European Union-funded projects of grid interconnections between Greece’s mainland and islands (Cyclades and Crete) are being realized to combat grid unreliability and high electricity costs [
55]. Lastly, two important cornerstones in the substantial improvement of Greece’s electrical grid should inarguably involve upgrade works to the current infrastructure as well as the liberation of the national electricity market [
56].