Possible Development of Efficient Local Energy Community on the Example of the City of Žilina in Slovakia

Abstract

Reducing the energy demand in the housing sector is one of the current topics in the EU. Slovakia, as an EU member, is also trying to lower the dependence on the import of energy raw materials used for heating. While new buildings reflect the technical requirements of applicable standards, buildings built in the past usually do not meet any technical requirements. The basis of efficient operation is not only satisfactory building structures, but also technological equipment of the buildings. The heating system is often in an unsatisfactory state, and an outdated heat source disproportionately reduces the overall efficiency of energy conversion. Complex restoration is, therefore, in most cases, necessary and often financially costly. The presented article analyzes the current state of housing stock in the example of a selected city district. In the next step, the current state and energy consumption are identified. Subsequently, needed retrofit measures are identified and the possibilities of renewal are analyzed. The use of RES in buildings is proposed, while selected city districts could create an independent energy community. The main goal of this article is to show the necessary steps to achieve efficient energy use and, using the example of a Zilina City district, show the possible benefits of such community creation in Slovakia. The article also discusses the correlation between the number of sunny days and possible energy generation in winter months.

1. Introduction

Building renovation plays a very important role in increasing operational efficiency and is also necessary to achieve carbon neutrality goals [1]. Current EU standards and regulations support this renewal trend. The second trend is the intensive introduction of renewable sources to supply buildings with energy or the obligation to install RES in newly built buildings. The main characteristics of fossil fuels used today are emissions of CO₂ and other pollutants, uneven distribution of sources, and expensive production costs [2]. On the other hand, renewable energy sources are uniformly available and environmentally friendly and can significantly compensate for the need for fossil fuels in the production of electricity and heat [2,3].

The first step in increasing the efficiency of building operations is to reduce the need for energy [1,2]. The need for energy depends mainly on the condition of the building constructions, i.e., perimeter walls, ceiling, windows and doors. Buildings in Slovakia built from 1950 to 1990 partially respected the minimum technical requirements valid at the time of construction [4]. This changed only after the legislation was harmonized with EU legislation. This fact is very well illustrated, for example, by the number of issued energy certificates, when the newest buildings are classified in classes A and B, while old buildings are often classified in classes E and worse [5,6].

Recently, many studies have been carried out to address the synergistic effect of building renovation and the application of renewable resources, such as [5,6,7,8,9,10]. In his study, Brown [5] confirmed the importance of in-depth renovation of the building, which contributed to the reduction of the carbon footprint. Authors Szalay and Zold [6] progressed from the evaluation of a single building to the creation of a group of buildings, which are considered as a single entity that achieved zero-energy building status in the summary evaluation.

In general, the terms community, neighborhood, and district are used to denote a set of buildings in this sense [11]. The use of a specific term depends on the objectives of the study [12,13] (reference) and also on the evaluated area, whether it is a part of the city, a separate unit, or a differently defined area [14,15,16,17].

The definitions of energy community are also different. Carlisle [18] defines a net zero community as an area where the need for energy has been reduced, and the consumption has been covered by RES. The US Department of Energy defines an energy community as an area where the annual amount of energy delivered is less than or equal to the amount of energy produced [19].

The European Union issued regulation 2010/31/EU—Energy Performance of Buildings Directive, which defines the term nearly zero energy district. This guideline was adapted, for example, by Amaral et al. [20] in their study. The update of this directive from 2018 established the objectives of the EU policy and set the goal of achieving highly efficient and decarbonized buildings by 2050 [21,22].

A very important factor that affects the possible development of energy communities is primarily the geographical location, which is related to climatic conditions. The outside temperature and humidity have an effect on the heating, ventilation, or air conditioning of the interior spaces. The very location, orientation, and arrangement of buildings have an impact on the passive solar gains of the building, which directly reduce the need for heat for heating [16,22,23].

The orientation and arrangement of buildings are closely related to the possibility of installing PV. Several studies report the use of PV as the main means of achieving an energy-self-sufficient community [24]. Many of the very important studies carried out [12,20,21,22] point to the possible coverage of heat consumption up to 70% by photovoltaic panels. If PV panels are combined with solar thermal collectors, it is possible to cover up to 90% of heat consumption. When the system is expanded to include heat storage, excess heat will be produced [24]. In the case of electricity production, the situation is more complex, as each building has different generation and storage options [25]. Diverse load profiles are also different, depending on the type of building or the intensity of use of a particular building. Recent studies [12,13,20,21,22] have shown that it is possible to achieve a zero-energy concept in renovated residential buildings. If there are no renovated buildings in the selected area, the need for energy increases significantly, which can no longer be covered by PV energy production alone. And if there are administrative or hotel-type buildings in the given area, it is necessary to increase the total production of other buildings [24]. Energy sector coupling is also important [23]. It is possible to achieve a transition to purely renewable energy by appropriately connecting individual heating and electricity production systems in combination with energy storage. Another benefit of creating such energy units is also the reduction of the total costs of operating energy networks, which is positively reflected in the final price for the consumer [13,14,15,16,17]. Yet, the important question that is present in countries with lower amounts of solar energy during winter months is, what should be the installed power output of solar photovoltaic power plants to cover the energy demand? For Slovakia, with only 94 sunny days in a year, the higher installed power can lead to an energy surplus during summer days, when most of the sunny days are present.

2. Materials and Methods

Modeling energy consumption and production in a building is a major part or basis for developing possible future scenarios. Over the past decades, several modeling methods have been developed that assume certain simplifications. Top-down models enable the determination of energy consumption in the building, and bottom-up models enable more detailed calculations of energy flows in the building [26,27]. Dynamic modeling is an important approach when it is possible to simulate the behavior of a building during the day or in selected time periods [28,29].

In this study, part of Zilina City in Slovakia was selected as an example of the creation of an energy-self-sufficient community. For selected city districts, a survey was performed, where the actual condition of every house was examined [30] as the next step was to process the calculation of heat demand for selected houses. The standards valid in Slovakia are in accordance with [31] and define the calculation procedure and marginal conditions for calculating energy consumption in the building.

The community includes 63 houses that form a separate neighborhood of the city. The time of construction of houses is roughly from the mid-1970s until today, while the construction of new houses, or the reconstruction of existing ones, is still ongoing. Some houses have undergone renovation; they have been insulated, and windows and doors have been replaced; some of the houses are in their original condition from the time of construction. The use of RES is on a small scale, and some houses use solar thermal collectors to heat water, a few buildings use photovoltaic panels to produce electricity. From the point of view of the future use of solar energy, the houses differ mainly in orientation and type of roof. The three-dimensional model of one reference house is shown in Figure 1, and the total summary of the parameters for the houses is shown in

Table 1. Houses in the selected district.

Houses	Count
Total amount	63
Original condition or only partial reconstruction	29
Overall renovated and new houses	34
Using solar energy	7

.

The next step was determining the energy class for selected houses. The reference building method [28] with houses of average dimensions was used for the determination. For this study, 3 reference buildings were simulated. One of them, house no. 1, is shown in Figure 2, and the dimensions of these houses are in

Table 2. Dimensions of the model houses—reference buildings.

	House No. 1	House No. 2	House No. 3
Construction type	area [m2]
Perimeter wall	165.03	159.17	234.50
Ceiling to outside	102.27	104.16	138.06
Floor	90.28	89.6	129.85
Windows	12.59	25.37	21.4
Doors	4.7	1.83	6.7
Construction material for walls	bricks	bricks	precast concrete
Wall insolation	NO	Mineral wool 5 cm	Mineral wool 16 cm
Roof insolation	NO	Mineral wool 10 cm	Mineral wool 30 cm
Windows and doors	Uw = 3	Uw = 1.4	Uw = 0.9
Energy class	F	C	A

. House no. 3 belongs to energy class A, and house no. 2 belongs to class C. Other variations of houses were created by changing the structures of house no. 1. Different insolation thicknesses allowed the creation of a house from classes D and B. Natural gas was considered the main fuel for heating for all houses.

With this assumption, the whole variety of houses from different energy classes was simulated. For the simulation of heat demand, a monthly calculation method was used as described in valid Slovak technical standard [31]. The calculation maintained methodology described in [32], which is based on the building thermal envelope method.

The specific heat loss H is determined using the sum of the specific heat loss by heat transfer H_T and the specific heat loss by ventilation H_V.

H = H_T + H_V,

where H—specific heat loss, H_T—specific heat loss through transition, H_V—specific heat loss through ventilation

The specific heat loss through the transition is calculated according to the following relationship:

H_T = ∑b_xi × u_i × A_i + ∆u × ∑A_i

The specific heat loss through ventilation is calculated as follows:

H_V = ρ_a × c_p × n × V

where the coefficient n is based on the geometric properties of the building

$$\mathrm{n}=3600\times \frac{\sum \left({\mathrm{i}}_{\mathrm{l}\mathrm{v}}\times \mathrm{l}\right)\times \mathrm{B}\times \mathrm{M}}{{\mathrm{V}}_{\mathrm{m}}}$$

Average temperatures for the city of Zilina were used, as shown in

Table 3. The average temperature in individual months for Zilina City [°C].

January	February	March	April	May	June
−3.2	−1.2	3.2	8.7	13.0	15.9
July	August	September	October	November	December
17.4	16.8	13.7	8.5	3.1	−1.5

. Solar in-plane irradiation was calculated at a value of 1309.55 kWh/m² [4]. As mentioned in the first chapter, an important parameter is the number of sunny days during the year. In Figure 3, the number of sunny days is shown.

Solar thermal gains are gains that depend on the solar radiation available at a given location, the orientation of the collection surfaces, permanent shading, solar transmittance, and absorption and thermal transmittance of the collection surfaces.

Q_sol = ∑_k [ I_sol × t × ∑_n A_sol,n]

After expressing the specific heat loss, it is possible to express the need for heat for heating without heat gains:

Q_B = H × (ϴ_i − ϴ_e) × n × 24.10⁻³

where (ϴ_i − ϴ_e) is the difference between inside (ϴ_i) and outside temperature (ϴ_e). Coefficient n represents the number of days for a specific month.

With this, the current energy needed for heating was calculated. The result of this calculation was the heat demand for every reference building. Since the electricity consumption for family houses is not evaluated in terms of valid technical standards in Slovakia, the average electricity consumption for individual houses was determined on the basis of [4], where average energy consumption is presented from current statistical data. Results are shown in

Table 4. Energy demand.

Average electricity demand per house	0.6 kW
Heat demand—house class E	26 kW
Heat demand—house class E	20 kW
Heat demand—house class D	16 kW
Heat demand—house class C	12 kW
Heat demand—house class B	7 kW
Heat demand—house class A	4 kW

.

After heat demand calculation, energy classes for houses in selected city districts were assigned according to made survey [4]. In

Table 5. Houses in the selected community.

Energy Class	Houses in Selected Community
A	4
B	23
C	21
D	12
E	2
F	1

, the number of houses in each energy class is evaluated.

This formed the basis for modeling the whole community in terms of energy demand and possible energy generation.

3. Results

The simulation of energy consumption in the energy community was performed using the EnergyPlan software, version 16.22 [33]. The goal was to determine the heat and electricity demand during the whole year based on the determined heat needs. The following Figure 4 shows the course of heat and electricity demand for the entire community. The average values per house and total values for all houses in the community are shown in

Table 6. Summary of calculated heat and electricity demand.

Electricity demand per house	0.6 kW
Heat demand per house	13.35 kW
Yearly electricity demand per house	3500 kWh/a
Yearly heat demand per house	44,735.1 kWh/a
Electricity demand for community	38 kW
Heat demand for community	607.3 kW
Total electricity demand for the whole community	220,500 kWh/a
Total heat demand for the whole community	1,877,766.3 kWh/a

.

Covering the energy needs in the selected energy community in a form without the use of fossil fuels would be possible in several ways. The first is the installation of photovoltaic panels to cover electricity consumption. This would be possible, as the input power is not that high. The available roof area depends on the type of building. Figure 5 shows one model house with different options for installing FTVE panels. The possible installed power ranges from 5.3 kW to 15.8 kW per house, considering the available roof area. The summary of produced energy is shown in

Table 7. The power output of the photovoltaic system.

Roof area used	22.1 [m2]	67 [m2]
Installed power per house	5265 [W]	15,795 [W]
Angle of inclination	39	39
Energy produced per house per year	3249.43 [kWh]	9748.28 [kWh]
Energy produced in the community per year	204,714.09 [kWh]	614,141.64 [kWh]
Share of energy consumption coverage	92.8%	278%

. Such a technical solution would make it possible to achieve sufficient energy production for the needs of households, while the excess output of one house would be used in another house from the community. The total possible amount of electricity produced by the photovoltaic system in the entire community is shown in Table 7. Here, it is obvious that even a smaller installed capacity can cover a large share of the entire energy consumption. Detailed simulation in the next step showed also expected power output during winter days.

However, covering the heat demand is more difficult, as the only option is a heat pump. The heat input is high, and the total amount of heat reaches almost 1 MW, which would represent an estimated investment at the level of 1 million Euros. Therefore, the second step of the simulation was started, where saving measures were proposed for individual houses from the worst energy classes. The main measure is the restoration of the perimeter structures, the replacement of windows, and the replacement of the heating system. A summary of the proposed measures is presented in

Table 8. Recommended insolation thickness for selected houses.

	Energy Class/Recommended Insolation Thickness [cm]
Construction type	F	E	D
Perimeter wall	16	16	14
Ceiling	38	30	25
Floor	10	10	10
Windows	Replacement
Entrance doors	Replacement

.

The result of the proposed measures is, first of all, a reduction in the heat demand for heating. This will contribute to reduced fuel consumption for heating. The summary results are shown in

Table 9. Summary of average heat and electricity demand.

Electricity demand per house	0.6 kW
Heat demand per house	4.8 kW
Yearly electricity demand per house	3500 kWh/a
Yearly heat demand per house	10,331.1 kWh/a
Electricity demand for community	38 kW
Heat demand for community	306.3 kW
Total electricity demand for the whole community	220,500 kWh/a
Total heat demand for the whole community	671,959.55 kWh/a

. Graphically, the energy saving is shown in Figure 6, where a considerable saving is visible during the winter months. In the case of the installation of RES energy sources, especially photovoltaic panels, it is possible to cover a significant part of the energy consumption in the community precisely because of the increased production of the community as a whole, as shown in Figure 6.

Heat demand was reduced significantly. This outlined that even smaller installed photovoltaic systems could cover energy demand. Yet, in the yearly simulation, as seen in Figure 7, it is clear to see, that in winter months, from November to February, due to a lower number of sunny days, there is an energy deficit, which means that energy has to be purchased.

During spring and autumn, as there are more sunny days, more energy is available. To compare all possible options, one week in March was selected. In Figure 8, all four cases are presented. In cases A and C, the whole community has installed photovoltaic panels with a summary power output of 331 kW, or 5 kW per building. In cases B and D, the summary power output is 995 kW or 15 kW per building. Cases A and B are in the current state, and cases C and D are with houses after retrofit. As can be seen, if the buildings are not restored, even a larger installed power output is not enough to cover the energy consumption. On the contrary, if the buildings have been renovated, a photovoltaic system with lower power output is sufficient to cover most of the energy consumption. In all cases, there is an energy surplus during the summer months, which opens the question about a long-term energy storage system that will be necessary for countries where there are more sunny days during summer, as is in the case of Slovakia.

4. Discussion and Conclusions

The creation of energy communities is a new trend in the energy sector and leads to the creation of decentralized energy networks. Such independent units are consequently more resistant to power supply failures. This situation is also confirmed by several recent studies [1,5,12,14,15,16,20].

The presented community is located in the city of Zilina in Slovakia, part of the EU. As it was presented, in the community, there are houses in their original condition, houses partially reconstructed, and there are also new houses. The simulation of such a larger number of buildings can be performed in several ways, as shown by studies [30,32,34]. It is also possible to use the sampling method, whereby the buildings in the city are replaced by some representative sample, as presented in the study [35]. Finally, for this study, the method described in [36] was used, where the authors describe the possibilities of building renovation and possible savings. To simulate the energy community, three reference buildings were selected based on the survey, for which the energy consumption was simulated.

The simulation pointed to a considerably high heat demand. This high price showed the necessity of restoring family homes in this community. Therefore, in the second step, the impact of retrofit measures was simulated. The known initial state and state after retrofit made it possible to calculate the expected energy savings for heating for each house in the respective energy class. The simulation results showed a reduction in energy consumption similar to studies presented in [36].

The reduction of heat demand will also enable the installation of a renewable energy source and a heat pump for heating. According to study [37], the use of RES is associated with several limitations, while it is appropriate if the energy is consumed directly at the place of production. For this reason, two alternatives of the installed power of photovoltaic panels were simulated. The subsequent simulation of the use of solar energy pointed to the reduction of dependence on the supply of energy from external sources and the coverage of a significant part of energy consumption. Such a procedure would make it possible to significantly reduce CO₂ emissions, which is consistent with studies [38,39]. Even the smaller installed power of the photovoltaic system is sufficient to cover the energy needs of most houses after the restoration of the houses, which would have a direct impact on the return of the system [40,41].

The different installed power options for individual houses are ultimately an advantage of the created community, as the surplus energy produced in one house is consumed in another house from the community while not burdening the transmission lines. The creation of such small communities can be very beneficial and represents a possible next development in energy supply with regard to efficient and environmentally acceptable operation. [42] Creating such communities would also lead to a reduction in overall fuel consumption, which would ultimately contribute to the creation of fully renewable energy systems [43]. As simulation showed, in the case of Slovakia, a country with a lower number of sunny days, even smaller photovoltaic installations can cover a significant part of energy demand. During winter months, there is still a need to purchase energy or use long-term energy storage systems.