Feasibility Study on the Spread of NZEBs Using Economic Incentives

Abstract

Nowadays, environmental and energy issues attract a lot of attention in the civil buildings sector, leading to the emergence of new technologies and new targets, which include Net Zero Energy Buildings (NZEBs). However, despite the great response in scientific research, the spread of NZEBs in Europe is quite limited. This is due not only to the lack of transposition of the related European Directives into the various national legislations, but also to the high initial cost of such high-performance buildings. The aim of this paper is to demonstrate how different energy retrofit strategies on existing buildings can lead to the achievement of the NZEB target if encouraged by tax incentives, at zero or almost zero cost. The introduction of tax incentives by individual EU member states would allow the spread of NZEBs that are still underdeveloped, especially in highly urbanized contexts. A suitable building energy dynamic simulation software has been used. The case study refers to a villa located in Southern Italy and for which different energy retrofit strategies are proposed to reach the NZEB target. For each case, an energy and economic evaluation is carried out to evaluate the feasibility of the interventions, exploiting the so-called “Super-Eco-Bonus 110%” incentive. The main results highlight that among the various solutions, the greatest energy cost reductions are obtained with the use of heat pump generators. Furthermore, the solution with the biomass boiler allows the use of a smaller number of photovoltaic panels to meet the yearly energy balance of the NZEB.

1. Introduction

Nowadays, it is well known that energy consumption in the building sector accounts for 40% of worldwide energy use (Figure 1) [1]. This data is very worrying and leads to the search for solutions to reduce the energy demand of buildings, especially residential ones.

Over the years, the growing attention to environmental issues has led to the development of research all over the world aimed at improving the building system, generating almost or completely self-sufficient building models.

The evolution of studies on energy efficiency in buildings led the EU, in 2002, to formulate the concepts of the nearly Zero Energy Building (nZEB) and Net Zero Energy Building (NZEB) [2], to reduce the energy and environmental impact of buildings using high efficiency systems and renewable energy sources.

The concept of the NZEB starts before 2002. In fact, in 1977, the NZEB concept was proposed by some Danish researchers [3]. They designed a house built at the Technical University of Denmark to obtain energy self-sufficiency for the building’s heating. In that case, the building envelope’s opaque components were equipped with mineral wool insulation, while the windows and doors were equipped with two glasses with insulating shields. The building was provided with a solar thermal system coupled to its water storage and heat distribution circuit.

In 1995, W. Gilijamse [4] highlighted the difference between electrical consumption and electrical production by introducing the connection between renewable energy systems and electrical national grid.

A year later, these aspects were applied to an existing house built by the Fraunhofer Institute for Solar Energy Systems in Freiburg, Germany [5] by using different strategies. The authors demonstrated that there is a considerable reduction in the energy demand of the building through the combination of new technologies and procedures suitable for improving the building envelope.

Subsequently, other studies also proposed the model of a zero energy building “on-grid” [6,7,8], focusing the attention on the possibility of producing, through renewable sources, more energy than that required by the building, to avoid the use of fossil fuels. These types of building are commonly known as Plus Zero Energy Buildings (PZEBs).

Over time the importance of Net Zero Buildings has grown more and more, so numerous papers have been proposed in the international literature. Torcellini et al. [9] highlighted fundamental principles, which concern the reduction of environmental impact, the use of available resources in the long term, and the use of renewable sources. To address the lack of a common definition in this field, the authors proposed a classification of ZEBs according both to the type of balance and to the accounting methods commonly used, i.e., Net Zero Site Energy, Net Zero Source Energy, Net Zero Energy Costs, and Net Zero Energy Emissions. The same authors a few years later in [10] proposed different NZEB definitions based on energy production, that could be on-site or off-site. In the first case the energy from renewables is obtained from the control volume that overlaps with the physical boundary of the building, while in the second case the energy from renewable sources is imported from outside, i.e., in the appurtenances of the building. The best solution is on-site because this type is more efficient, especially for reducing losses due to energy transport.

Considering the definition reported before, some researchers [11] emphasized the possibility of protecting the environment by reducing CO₂ emissions into the atmosphere; this does not directly imply that an emission-neutral building is also classified as a zero-energy building.

What emerges from other studies and from the Report of the International Energy Agency (IEA) [12] is the connection between the building and the grid, since all types of balance are obtained by equaling the energy produced with that supplied by the grid. The balance is usually conducted on yearly basis, but according to Hernandez et al. (2010) [13] the entire life span of a building and of its components must be considered, hence the idea of extending the evaluation to the life cycle (LC-ZEB) and considering the energy incorporated by the materials.

The wide variety of terms and expressions leads to distinct approaches to a commonly accepted definition of NZEB even at the international level, which has drawn attention to how “zero balance” is calculated. The critical points on which the researchers have formulated their assessments concern the following aspects [14]:

-

Method and type of the balance. It may depend on the final energy supplied, primary energy, costs and CO₂ emissions.

-

Type of energy consumption. Several research papers mention the consumption due to heating, cooling, domestic hot water and lighting, as well as the energy incorporated by materials during the life phase [15,16], and consumption due to user behavior [17].

-

System boundary. The boundary can be the footprint of the building, the project area, or you can extend the balance to the source of supply [18].

-

Sources for energy production. To meet the needs of the building, only renewable energy sources must be used, so that they can balance the consumption of fossil fuels.

-

Connection to the network. The building can be connected to the grid to give up the energy surplus at peak times or draw on when production from renewable sources is not sufficient to meet the needs, or it may not be connected to the grid becoming completely self-sufficient.

Although many states are supporting the concept of a sustainable self-sufficient building in their national regulations, there are still no standardized procedures, and most of the proposals stem from considerations made with reference to individual case studies.

As for the legislation in Europe, with the EPBD Directive [2], later supplemented by EPBD Recast [19], European states were directed towards the use of renewable sources and the certification of the building performances, imposing specific requirements, especially in the case of new construction. To implement these objectives, differently from the NZEBs, the concept of nearly Zero Energy Buildings (nZEB) is therefore established, which each Member State has transposed into its legislative regulations. Another European Directive, 218/844/EU [20], leads to strengthening energy efficiency, environmental sustainability policies, decarbonized development and the use of renewable sources, which increased by more than 32%. Several differences are found in the legislative regulations of European Countries [21], so in this paper reference will be made to what is reported in REHVA Task Force [22], for the evaluation of the energy performance of the building through the so-called “zero balance”. This means that the sum of the differences between energy supplied and exported for each energy carrier, using the appropriate conversion factors, must be zero (Equation (1)). The calculation is carried out considering energy flows mainly related to heating, cooling, domestic hot water and lighting.

$$E={\displaystyle \sum }_i\left(E_{del,i}-E_{exp,i}\right)f_i=0$$

(1)

Considering that the large building stock is responsible for a large amount of CO₂ emissions into the atmosphere, it is useful to apply the principle of zero energy balance even in the case of existing buildings, to promote the conscious use of energy resources and reduce the environmental impact significantly. Although some decades have passed since the introduction of the NZEB concept, their diffusion remains rather poor.

This issue was also analyzed by Zhang et al. [23] which collected some existing policies with the aim to evaluate their impact on the development of nZEBs in China.

In this regard in Italy, numerous financial incentives were implemented by the government aimed at improving the energy efficiency of the existing building stock. In fact, the main obstacle to the actual implementation of the interventions is the high cost to obtaining energy performing buildings. Magnani et al. [24] analysed the issue of energy retrofitting of existing buildings in Italian cities and highlighted how incentives promote energy requalification.

Therefore, with the so-called “Relaunch Decree” of May 2020 [25], the legislator has defined the terms for obtaining the “Super-Eco-Bonus”, which provides for an increase in the tax deduction rate up to 110% of the expenditure, if an improvement of two energy classes is achieved [26]. The energy class is identified by calculating the global non-renewable performance index (Figure 2) of the building (EP_gl,nren) expressed in kWh/m² and takes into account the primary energy consumption from non-renewable energy sources for heating, cooling, domestic hot water and possible mechanical ventilation. Only in the case of non-residential buildings, it also includes artificial lighting and the transport of persons, i.e., elevators. This parameter is evaluated assuming a continuous use of HVAC system (24/24 h) referring to the usable area of the building. The energy class is obtained through multiplicative coefficients of reduction or increase of EP_gl,nren.

From these important innovations, the aim of this work comes. Considering that energy refurbishment of existing buildings is hampered by the considerable economic resources required, it is possible to apply the tax incentives to the case of a single-family dwelling to obtain a Net Zero Energy Building and at the same time guarantee the contractor an energy retrofit at almost zero cost.

The incentives considered by the “Super-Eco-Bonus” are aimed at:

• the thermal improvement of the building envelope by reducing the transmittance of opaque walls or windows;
• the replacement of obsolete heating and cooling generators with others characterized by higher efficiencies;
• the insertion of on-site photovoltaic solar panels.

Other energy efficiency interventions, such as those on lighting systems and household appliances, are not eligible for incentives.

The aim of the paper is to demonstrate how different energy retrofit strategies on existing buildings can lead to the achievement of the NZEB target if encouraged by tax incentives, at zero or almost zero cost (Figure 3). This represents the innovative contribute of the paper in the current scientific-technical context.

The introduction of tax incentives by individual EU member states would allow not only to make existing buildings energy efficient but also to spread Net Zero Energy Buildings that are still underdeveloped, especially in highly urbanized contexts.

An existing building of the Southern Italy is analyzed as a case study, with different energy retrofit strategies in order to obtain an NZEB by using the so-called “Super-Eco-Bonus 110%” tax incentives.

A building energy performance dynamic simulation software is used.

2. Methodology

The methodology is based firstly on the study of an existing building located in South Italy. Through various inspections, the geometric, technical and thermo-energetic characteristics of the existing building were found in order to propose retrofit strategies compatible with the current state of the building and with the incentives considered by the “Super-Eco-Bonus”. Initially the building was simulated dynamically using software called DesignBuilder in order to obtain the thermal loads and the energy consumption of the existing building related to different energy uses. Subsequently, based on the interventions that can be incentivized by the Super-Ecobonus 110% Decree (building envelope, HVAC systems, renewable energy sources) [27], different energy retrofit strategies were proposed. The scenarios were simulated dynamically, both by means of DesignBuilder and through software called Termolog, to evaluate the improvement in energy class with respect to the current state of the building. Lastly, photovoltaic and solar thermal systems were dimensioned in order to obtain the NZEB target. Finally, the total costs of the proposed interventions were calculated and compared to the maximum values fixed by the rule. Figure 4 shows a workflow of the methodology used in this paper.

The stationary thermal transmittance U [W/m²K] for opaque components of the building envelope has been calculated as follows:

$$\mathrm{U}=\frac{1}{\frac{1}{{\mathrm{h}}_{\mathrm{i}}}+{\displaystyle \sum }\frac{\mathrm{s}}{\mathsf{\lambda }}+\mathrm{Rn}+\frac{1}{{\mathrm{h}}_{\mathrm{e}}}}$$

where:

• h_i is the internal superficial conductance, usually considered equal to 7.7 W/m²K
• s is the thickness of each layer [m]
• λ is the thermal conductivity of the material for each layer [W/mK]
• Rn is the thermal resistance of non-homogeneous layers [m²K/W]
• h_e is the external superficial conductance, usually considered equal to 25 W/m²K.

The stationary thermal transmittance U [W/m²K] for windows has been calculated as follows:

$$\mathrm{U}=\frac{{\mathrm{A}}_{\mathrm{g}}{\mathrm{U}}_{\mathrm{g}}+{\mathrm{A}}_{\mathrm{f}}{\mathrm{U}}_{\mathrm{f}}+{\mathrm{L}}_{\mathrm{g}}{\mathsf{\psi }}_{\mathrm{g}}}{{\mathrm{A}}_{\mathrm{g}+}{\mathrm{A}}_{\mathrm{f}}}$$

where:

• A_g is the area of the glass [m²]
• U_g is the thermal transmittance of the glass [W/m²K]
• A_f is the area of the frame [m²]
• U_f is the thermal transmittance of the frame [W/m²K]
• L_g is the perimeter of the glazed profile [m]
• ${\mathsf{\psi }}_{\mathrm{g}}$ is the linear thermal transmittance related to the presence of the spacer between glass layers [W/mK]

The formulas used are the same for the existing building and after the proposed retrofit interventions, while obviously the thermal characteristics of some materials change.

2.1. Building Modeling

The energy consumption of the existing building and the proposed energy retrofit strategies are implemented through the dynamic simulation software DesignBuilder [28]. The U.S. Department of Energy with ANSI/ASHRAE have performed validation test results [29] for DesignBuilder v.6.1 with EnergyPlus v.8.9. referring to:

• Building Thermal Envelope and Fabric Load Tests
• Space-Heating Equipment Performance Tests HE100 to HE230
• Space-Cooling Equipment Performance Comparative Tests CE300 to CE545
• Space-Cooling Equipment Performance Analytical Verification Tests AE101 to AE445
• Space-Cooling Equipment Performance Analytical Verification Tests CE100 to CE200.

The climatic data used in DesignBuilder are taken from ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) [30].

Regarding the DesignBuilder software, the CTFM (Conduction Transfer Function Module) is selected based on the algorithm “conduction transfer function”. Moreover, the algorithm DOE-2 [31] is set for the outside convection and the algorithm TARP [32] for the inside convection.

To obtain the energy classifications for both existing building and proposed scenarios, as indicated in Figure 2, the building is analyzed through the software TERMOLOG [33]. A three-dimensional reproduction of the building is obtained. Moreover, the intended use of each area, the indoor air temperature and humidity set-points, the schedules for lighting, appliances and Domestic Hot Water (DHW) are defined. The building envelope components are thermally and technologically defined into the software, and then a HVAC system is implemented to obtain the energy consumption for different scenarios.

The current state of the building is assessed for the evaluation of its critical issues and for the consequent determination of the improvement hypotheses. Three different scenarios are evaluated, which have in common the interventions carried out on the building envelope, i.e., insertion of thermal insulation layer on the external walls and replacement of the existing windows. These interventions on the building envelope are the same for all the scenarios, which are distinguished by the different HVAC systems proposed. In any case, in order to access to the economic incentives it is mandatory both the improvement of two energy classes and to the verification of technical and economic requirements imposed by the so-called “Minimum Requirements Decree” of 26 June 2015 [26] and the Ministerial Decree of 6 August 2020 [27]. In particular, the transmittance of the existing building envelope and of the proposed components must be greater and lower, respectively, than that reported in [27]. Furthermore, the Coefficient of Performance (COP) and the Energy Efficiency Ratio (EER) of the proposed heating and cooling generators must be greater than the referring value reported in the same Decree. Lastly, a check of the retrofitting costs must be carried out to consider the limits of expenditure for each category of intervention, as their sum defines the maximum admitted expenditure for the whole redevelopment operation.

2.2. Obtaining the NZEB Target

In Italy, there are still no legislative prescriptions for obtaining the NZEB target, but only in the case of nearly Zero Energy Building, by introducing several energy parameters on building-plant system to verify [26]. To obtain a NZEB, several paths can be followed as reported by Bourelle et al. [34], but in this study, with reference to the energy flows of the building, it was verified that the production of energy from renewable sources was greater than or equal to the energy required by the building [35] for heating, cooling, domestic hot water and lighting systems.

To quantify the amount of photovoltaic (PV) panels to be installed on-site [36] to entirely balance the energy consumption of the building, a dynamic energy simulation is carried out for each case proposed. The energy consumptions depend on the characteristics of the building envelope, internal heat gains, artificial lighting, type of generators for cooling and heating, home appliances, etc. Therefore, the proposed interventions have resulted from a careful observation of the energy analysis of the existing building. Each energy improvement solution is analyzed through the dynamic simulation software, and at the same time the verification of the criteria established by the recent regulations on access to the tax incentive of the “Super-Eco-Bonus” is performed. At the end of the checks, the solutions are compared each other to evaluate which is the best in terms of cost-benefit.

3. Case Study

The case study refers to a single-family building located in Portici (near Naples, Southern Italy). The building is a typical civil building present in the outskirts of Italian metropolitan cities, as well as a very widespread type of building in Europe and for which this type of incentive is particularly suitable. The city of Portici, according to the classification reported by DPR 412/93 [37], belongs to climatic zone C, with a value of Heating Degree Days (HDD) equal to 1028. Figure 5 and Figure 6 show some information about annual solar radiation and precipitation for the city of Portici.

For the climatic zone C, the operating period of the heating system is from 15th of November to 31st of March for a maximum of 10 h per day. Furthermore, there is not a mandatory operation period for the cooling system.

The building was built in the early 1900s but, in the nineties, it underwent a profound renovation that defined its current configuration. The building envelope externally has a yellow color and has a medium air tightness.

The building stands on four levels, with a total area of about 370 m² and a volume equal to 1000 m³. Figure 7 clearly shows how the first level is underground. The existing building envelope is made of tuff masonry (sedimentary rock typical of Southern Italy), with blocks of variable thicknesses (30 cm, 40 cm and a basement wall of 50 cm) without thermal insulation. The roof has a concrete structure, and it has a natural ventilation system. All the windows are made of single glass and wooden frame, equipped with an external iron shielding. In

Table 1. Stationary thermal transmittance of the case study building envelope (W/m²K).

Uwall,1.	Uwall,2	Uwall,3	Uroof	Upitched roof	Uwindows
0.756	0.965	0.912	1.348	0.443	3.6–5.1

, the stationary thermal transmittances of the building envelope components are shown. The values are quite high and, as consequence, the thermal loads, shown in

Table 2. Thermal loads of the building.

Volume	Thermal Loads
	Heating Load		Cooling Load
m3	kW	W/m3	kW	W/m3
1000	20	19.85	33.4	33.1

, are quite high, too.

The heating and cooling thermal loads are evaluated considering specific thermohygrometric conditions. These are referred to an indoor air winter set point temperature equal to 20 °C and to an indoor summer set point temperature equal to 26 °C, as indicated by the legislator in DPR 74/2013 [38]. The design outdoor temperature reference is 2 °C for winter (taken from the DPR 1052/77 [39]) and 32 °C for summer (from UNI 10339 [40]).

As the building presents several rooms, two thermal zones have been identified, i.e., the sleeping area and the living area, with different operating conditions during the day for the heating and cooling system, following the limits set by [37].

The HVAC system is based on three obsolete standard natural gas boilers that serve both heating and domestic hot water production. Furthermore, there are four air-to-air heat pumps (split-systems) for cooling.

In order to carry out the dynamic energy simulation correctly, it was necessary to establish timesheets, identifying temperature and humidity set-points, and also the power and use schedules of appliances, lighting and domestic hot water, as shown in

Table 3. Representative schedules for the living area.

Living Area
Employment	Occupancy (People/m2)	People	Metabolic Rate	Schedules (Hours)
Married couple—two children	0.01	4	Moderate physical activity (≈ 1.7 met)	7–10 19–22
Other Contributions		Power Density (W/m2)	Fuel	Schedules (Hours)
Appliances		5	Electricity from the grid	19–22
Food preparation		0	Electricity from the grid	11–12 18–19
Lighting (500 lux)				7–10 19–22
			Indoor Air Temperature Setpoint (°C)	Schedules (Hours)
Cooling system			26	13–16 19–21
Heating system			20	6–8 18–21
Natural ventilation (8 l/s per person)				On 24/7

and

Table 4. Representative schedules for the bedroom area.

Bedroom
Employment	Occupancy (People/m2)	People	Metabolic Rate	Schedules (Hours)
	0.01	2	Sleeping (≈ 0.7 met)	14–16 22–7
Other Contributions		Power Density (W/m2)	Fuel	Schedules (Hours)
Appliances		5	Electricity from the grid	14–16 22–23
Lighting (200 Lux)				22–7
			Indoor Air Temperature Setpoint (°C)	Schedules (Hours)
Cooling system			26	13–16 19–21
Heating system			20	19–24
Natural ventilation (8 l/s per person)				On 24/7

.

The existing building belongs to an Energy Class type E, calculated according to the indications of the Ministerial Decree of 2015 [26] and obtained through the software Termolog [33].

After evaluating the energy criticalities of the existing building, it was possible to formulate specific hypotheses to improve the building envelope and the efficiency of the HVAC system.

For the external walls, a 12 cm thick layer of rock wool is proposed. In this way the limit on the maximum value of stationary thermal transmittance is verified. Furthermore, this type of thermal insulation respects the Minimum Environmental Criteria (CAM) [41], as required by the Italian rule. Based on these criteria the thermal insulation must respect the following main characteristics:

• thermal or acoustic insulation materials must not be produced using flame retardants subject to restrictions or prohibitions provided for by the applicable national or community regulations;
• thermal or acoustic insulation materials must not be produced with blowing agents with an ozone reduction potential greater than zero;
• thermal or acoustic insulation materials must not be produced or formulated using lead catalysts when sprayed or in the course of plastic foaming;
• if produced from an expandable polystyrene resin, the blowing agents must be less than 6% of the weight of the finished product.

The existing windows are replaced with others with a PVC frame and triple glass camera. The glasses are low emissivity type with a thickness of 3 mm, separated by two interlayer spaces of 13 mm each, filled with argon.

Interventions on the roof are not considered since the transmittance of the pitched roof has a low value. Even if it would be better to improve the transmittance of the terrace, this intervention was considered negligible due to the reduced surface area of the terrace, also to fall within the spending limits of the “Superbonus”. Furthermore, it should be noted that the required increase of at least two energy classes (for the whole building) occurs even without this intervention.

In addition to the described measures on the building envelope, three are the hypothesis of intervention, as follows.

• The first scenario analyzed presupposes the replacement of the existing generators with three high efficiency condensing boilers, with an efficiency η equal to 98%. Furthermore, the installation of a thermostatic valve for each radiator is hypothesized.
• The second scenario involves the replacement of the existing generators with three biomass boilers powered by pellets, with a thermal capacity of 24 kW, and equipped with kits for the production of domestic hot water; also in this case on each radiator a thermostatic valve is installed.
• Finally, the third scenario involves the replacement of the existing generators with an air-to-water heat pump (heating capacity of 21 kW and COP of 4.5; cooling capacity of 24.4 kW and EER equal to 3.55), to serve the main rooms of the house, and a multi-split system with two internal units with a total heating capacity of 4.7 kW. To produce DHW two heat pump water heaters with thermal capacity of 1.2 kW and 2 kW are inserted.

4. Results

4.1. Energy Results

First, the dynamic simulation of the existing building is carried out through DesignBuilder [28]. The validation of the model is performed by comparing the obtained results with the real consumption derived from the energy bills of gas and electricity, with the difference between simulation data and bills not exceeding 10% (

Table 5. Comparison between real consumption and consumption results from simulation.

Service	Simulation Results		Real Consumption		Δ
	kWh	kWh/m2	kWh	kWh/m2	%
Auxiliary appliances, lighting, cooling (electricity)	8351	22.5	9317	25.1	10
Heating and DHW (natural gas)	16,997	45.8	18,542.02	49.9	8

).

The absence of insulation system in the existing building has a significant impact on the thermal performance of the building. For this reason, the insertion of an insulation layer on the external walls and the replacement of the existing windows lead to a considerable reduction (60–79%) of the stationary thermal transmittance of the building components (Figure 8). Therefore, the building thermal loads are also reduced (−37% in winter and −29% in summer), as shown in Figure 9.

The reduction of the thermal loads causes the consequent reduction of energy consumption necessary to satisfy the building energy needs, making easier to obtain an NZEB. In fact, from

Table 6. Energy comparison between the current state and the three proposed scenarios.

	Existing Building		First Scenario		Second Scenario		Third Scenario
	kWh	kWh/m2	kWh	kWh/m2	kWh	kWh/m2	kWh	kWh/m2
Electricity	8351	21	6095	16.4	6445	17.4	9847	26.5
Natural gas	16,997	42.8	9923	26.8	-	-	-	-
Biomass	-	-	-	-	10,090	27.2	-	-
Photovoltaic system	-	-	11,828	31.5	8764	23.6	9857	26.6
Solar thermal system	-	-	4463	12	-	-	-	-

it can be seen how energy consumption changes in each case analyzed, while

Table 7. Use of renewable energy sources to achieve the NZEB target for the three proposed scenarios.

	Type Property	n° of Panels	Surface	Yearly Energy Production	Power/Thermal Capacity
	-	-	m2	kWh/m2	kW
First Scenario
PV system	silicon monocrystalline (330 W for each panel)	30	47.1	31.5	9.9
Solar thermal collector	vacuum tube collectors	10	20	12.0	-
Second Scenario
PV system	silicon monocrystalline (330 W for each panel)	22	34.5	23.6	7.3
Biomass	Pellets	-	-	27.2	24
Third Scenario
PV system	silicon monocrystalline (330 W for each panel)	25	39.2	26.6	8.3

shows the different use of renewable energy sources needed to achieve the target NZEB.

In the first scenario, the energy vectors related to the yearly energy consumption are electricity and natural gas (Table 6). To balance the energy consumption, solar PV and thermal systems are considered. The PV system is based on silicon monocrystalline panels (330 W for each panel), while the solar thermal system is composed by vacuum tube collectors and three storage tanks. The tilt angle and the orientation of solar panels are the same for each scenario and equal to 30° and south orientation. The panels for all the proposed scenarios have the same inclination, but for each proposed strategy the number of panels is different in order to always achieve the NZEB target.

With the solar thermal system, it is possible to only partially compensate the rate of thermal energy spent to meet the building’s needs. Therefore, as reported in Marino et al. [42], the remaining amount of gas which is converted into primary energy through an appropriate conversion factor (1.05 for natural gas) will be compensated by the photovoltaic system (Table 7). In this way, the energy balance of a NZEB is satisfied, as shown in Equation (2).

$$E={\displaystyle \sum }_i\left(E_{del,i}{f}_i-E_{exp,i}{f}_i\right)=(\left(9923-4463\right)kWh,t)\times 1.05 kWh,pr+\left(6095-6095\right)kWh,e+\left(-5733\times 1\right)kWh,pr=0$$

(2)

In the second scenario, both the electricity and the biomass consumption are obtained from the simulation (Table 6). The installation of a photovoltaic system is required that will be responsible for balancing electricity requirements and the non-renewable rate of biomass, equal to 20% of the related energy consumption. Therefore, 22 single crystalline silicon panels are needed (Table 7) to obtain a zero primary energy balance, as shown in Equation (3).

$$E={\displaystyle \sum }_i\left(E_{del,i}{f}_i-E_{exp,i}{f}_i\right)=\left(10090 kWh,t\times 0.2\times 1.05-2120 kWh,e\times 1\right)+\left(6445-6445\right) kWh,e\cong 0$$

(3)

In the third scenario, the only energy vector used in order to cover the building energy needs is the electricity (Table 6 and Table 7); therefore, only a photovoltaic system manages to meet the energy balance (Equation (4)) and there is no need to convert the terms of equation in primary energy.

$$E={\displaystyle \sum }_i\left(E_{del,i}-E_{exp,i}\right)=\left(9847-9857\right)kWh,el\cong 0$$

(4)

In addition, with all the solutions analyzed, it is possible to obtain an improvement of at least two energy classes (

Table 8. Energy performance index and energy classes for all the cases.

		Current State	First Scenario		Second Scenario		Third Scenario
EPgl,nren	kWh/m2	139.84	47.37	−66%	11.95	−91%	15.32	89%
Energy class	-	E	A2	Improvement of five energy classes	A4	Improvement of seven energy classes	A4	Improvement of seven energy classes

), as required by the Italian legislation [25], to access to the economic incentives. The energy class is determined by the global non-renewable energy performance index (EP_gl,nren) which indicates the total primary energy consumed by the building in each year for heating, cooling, domestic hot water and mechanical ventilation (for residential buildings lighting and transport are not considered) per unitary surface.

The difference found in the three cases hypothesized, in addition to the type of fuel or energy carrier, is the number of photovoltaic panels required on the roof of the building. As can be seen from Table 7, the photovoltaic system will be assigned the burden of balancing the energy needs of the building, while the solar thermal system will be used, to a limited extent, only in conjunction with the condensing boiler (scenario 1).

Furthermore, a Net Zero Energy Building should have a primary energy requirement less than 57 kWh/m² [18]. This last criterion is not met in all the three scenarios proposed. In fact, the primary energy consumption is in the range of about 60–65 kWh/m². This probably happens as, despite the proposed interventions aimed at reducing energy demand, it is an existing building that inevitably limits the choice of the most appropriate procedures to obtain the NZEB target. However, the result can be considered in any case more than satisfactory.

4.2. Economic Performance

The economic aspect is one of the highlights for the access to the “Super-Eco-Bonus” [25]; therefore, for each proposed intervention it is necessary to carry out an estimative metric computation to verify the congruity of the expenditure with respect to the maximum limit laid down by the legislation. For the proposed scenarios, in order to evaluate the cost of the interventions, the expenditure of the different interventions is taken from the price list of the Campania Region for Public Works of 2020 [43], also taking into account the costs related to labor and professional services (

Table 9. Economic evaluation by category of intervention.

	Wall Insulation	Replacement of Windows	Replacement of Heat Generator	Insertion of PV System	Total
	€	€	€	€	€
First scenario	64,200	41,251	24,240	12,000	146,700
Second scenario	64,200	41,251	11,695	8800	130,960
Third scenario	64,200	41,251	15,545	10,000	136,000

). The maximum expenditure limit allowed for the “Super-Eco-Bonus” [25] consists of the sum of the amounts provided for each category of intervention, such as wall insulation, generator replacement (including solar thermal system), photovoltaic system and window replacement.

It is important to observe from

Table 10. Costs and cost limits.

	Wall Insulation	Replacement of Windows	Replacement of Heat Generator + Insertion of Solar Collectors	Insertion of PV System	Global Cost Limit for First Scenario	Global Cost for the First Scenario	Global Cost Limit of Second Scenario	Global Cost for the Second Scenario	Global Cost Limit of Third Scenario	Global Cost for the Third Scenario
	€	€	€	€/kW	€	€	€	€	€	€
Single-family house	50,000	54,545	30,000	2400	158,305	146,700	151,970	130,960	154,345	136,000

that in each hypothesis carried out it is possible to comply with the spending limit set out by the Italian Decree [27], and this is a very important aspect since the legislation allows the completed use of the incentive.

Moreover, the proposals made allow for a relevant saving on energy costs. In fact, a saving of 3214 €/year, 3564 €/year and 3963 €/year can be obtained in the first, second and third scenario, respectively. Apparently, the best solution should be the third one because it allows the owner to have the greatest savings on energy bills, thus having immediate feedback of the benefits resulting from the transformation, but it does not represent the most efficient solution since the emission terminals for heating are radiators, for which replacement is not foreseen as they are still able to fully guarantee their operation, so the possible replacement would involve a useless waste of economic resources. As known, the operation of radiators is not optimal when served by a heat pump, due to the low temperature level of the water.

5. Conclusions

A NZEB is a powerful solution to reduce CO₂ emissions, energy consumption and to improve the thermal comfort of users. Furthermore, it represents a concept still not widespread, mainly in urbanized contexts and when the energy retrofit regards an existing building. This overall depends on the huge economic resources required to reach the NZEB target (obtaining a very low value of primary energy consumption, by means of a very high performing building-plant system).

This work shows in an ambitious way that it is possible to transform an existing building into a NZEB, at a relatively low cost, by using the incentives made available by national legislations (the so-called “Super-Eco-Bonus 110%” in the case of Italy).

At this aim, three scenarios of intervention are analyzed which correspond to different generators for heating and cooling, i.e., condensing boiler in the first scenario, biomass boiler in the second, and heat pump in the third. Specifically, it is demonstrated that the use of a condensing boiler involves higher energy expenditure than a biomass boiler or a heat pump, which are known to be more performing systems, and also requires a greater number of PV panels in order to obtain the NZEB target. The results also show the greater convenience of the heat pump generator from an energy cost point of view, since it allows significant savings on energy costs (almost 4000 €/year). However, this solution is not the best from a technological point of view, since radiators already present in the house require a temperature water higher than that obtainable through the heat pump. The choice could therefore fall on the replacement of old boilers with biomass boilers, since they allow to reach the highest energy class (A4) with the lowest number of PV panels and an economic saving on energy consumption of about 79% compared to the existing building.

Lastly, it is important to highlight that, although the “Super-Eco-Bonus” requires a careful feasibility study and a meticulous design for the verification of technical and economic law parameters, the NZEB target has been obtained in each scenario analyzed, with interventions at zero global retrofit cost. These results are therefore encouraging, considering that this process should be extended to a significant part of the existing building stock to make progress towards saving energy, protecting the environment and obtaining better thermal comfort for the occupants of the buildings.