1. Introduction
In Europe, buildings are responsible for about 36% of total CO
2 emissions. Thus, European Directives have been issued to reduce these emissions by improving energy performance of buildings, i.e., the Energy Performance Building Directive, so-called “EPBD” [
1] and “EPBD recast” [
2]; successively, the European Directive 2018/844 [
3] aims to completely decarbonise the building stock by 2050.
This can be achieved by transforming current buildings into nearly Zero Energy Buildings (nZEBs) and employing mainly renewable energy sources [
4]. However, the goal of Net Zero Energy Buildings (NZEBs) is still far from being achieved.
An NZEB is a building characterized by opaque and transparent components with optimal values of thermal parameters (for example, a low unitary thermal transmittance), in which the position of the openings helps natural ventilation and the most energy efficient systems are installed; for these reasons, this building has a very low energy requirement, completely satisfied by renewable energy sources [
1,
5,
6]. In practice, to obtain this target, many parameters must be fixed, such as the control volume for applying the energy balance, and the time base of this balance, i.e., monthly, seasonal, or annual [
7]. Berggren and Wall [
8] analyse the importance of normalizing the balance referring to both internal and external boundary conditions and highlighting the importance to keep the indoor temperature under control. Moreover, it is important to understand if solely on-site renewable energy sources are sufficient; in fact, D’Agostino et al. [
9] highlight how in densely urbanized cities the roof of a building is the main useful space if the use of on-site renewable sources is desired. In this case, therefore, the satisfaction of the energy balance is greatly influenced by the surface-to-volume ratio of the building. Other factors concern the type of renewable energy source to be used. Among renewable energy systems, those that use solar energy are common, such as photovoltaic systems, solar thermal systems, and photovoltaic/solar collectors. Many different parameters relating to solar energy have been investigated, such as the choice of the type of system [
10,
11] and the drop in performance of different systems due to cell degradation [
12,
13]. To enhance the use of renewable energy sources for obtaining a ZEB, Bae et al. [
14] identify an innovative solution based on a trigeneration system consisting of a geothermal heat pump and solar thermal-photovoltaic system. Furthermore, Ferrante et al. [
15] and D’Agostino et al. [
16] underline the importance of using passive solutions, integrated design and other systems, such as improvement of the thermal characteristics of the building envelope or using solar and wind energy micro-generation.
Despite the widespread use of renewable energy sources and innovative systems, achieving an energy balance equal to zero (i.e., yearly energy consumption equal to yearly energy production from renewables) is not easy and this is not sufficient to characterize an NZEB; it can often cause additional stress on the existing energy infrastructure when the plant powered by renewable energy sources is oversized to satisfy the balance and exchanges with the grid. [
17]. Franco and Fantozzi [
18] analyse the operating performance of a small-size photovoltaic system and its possible utilization for promotion of self-consumption policies of nZEBs, reaching the conclusion that in this type of building the interaction with the national electricity grid remains high. Similar results were found by Rey-Hernández et al. [
19] who found that the primary energy consumption and energy production from renewable sources of an existing LEED-certified (Leadership in Energy and Environmental Design) nZEB in Spain were higher than the standards recommended in the EPBD [
1], due mainly to the national energy conversion factors. Thus, it is important to consider a budget to implement and improve control strategies.
Given this, to achieve the NZEB target, it is first essential to reduce the energy needs of the building. In Ascione et al. [
20], the boundary conditions are analysed, as well as the design criteria and fundamental concepts for an NZEB; it is highlighted that a good design leads to drastically reduced energy needs for heating and cooling. In Buonomano et al. [
21], for a non-residential NZEB in Mediterranean climates the obtained energy demand is 3.9 kWh/m
2y for heating and 6.7 kWh/m
2y for cooling. Feng et al. [
22] highlight how, for the New Building Institute of the U.S., the target NZEB is obtained when the ratio between renewable energy production intensity and energy use intensity is equal to 1 and the building has a primary energy consumption equal to or less than 57 kWh/m
2.
The ground source heat pump (GSHP) is one of the various high efficiency systems that are suitable for reducing energy consumption. The GSHP usually has a higher coefficient of performance (COP) than the more common air source heat pump, due to the more favourable and stable thermal level of the soil compared to the outside air [
23]. In fact, particularly for the great depths at which geothermal heat pumps with vertical probes work, the soil temperature is higher in winter and lower in summer than that of the outside air. This improves the ideal COP of the GSHP and therefore also the achieved COP (see also Equations (6) and (7)). Although it is a system that exploits the soil and may have considerable surface requirements, there are innovative systems that exploit the foundations of buildings for the implementation of geothermal probes. Despite the difficulty often encountered in finding an available surface on which to lay the probes on site, Kotarela et al. [
24] highlight that the combined use of a photovoltaic system and geothermal heat pump is one means of reducing dependence on fossil fuels while simultaneously avoiding overloading of the electricity grid. Indeed, in Carotenuto et al. [
25,
26], innovative ground systems are analysed using the finite elements method. In this geothermal configuration, freezing probes are inserted in foundation pylons to exploit the low enthalpy of the ground. It is also important to bear in mind that geothermal heat pumps, both in summer and winter, exploit the renewable energy of the soil. Revesz et al. [
27] investigate the GSHP in urbanized cities where the heat from an underground tunnel (the ground surrounding the infrastructure) could be exploited to improve the heating mode operation of a GSHP. Thus, the advantage of this technology can be used to satisfy the need to use renewable energy sources for civil and non-civil use. In the literature, this system is coupled with several innovative complementary systems. In Buonomano et al. [
28], geothermal energy coupled with solar energy is used and investigated in a trigeneration plant. Huang et al. [
29] investigate the optimization of a large scale solar-assisted ground source heat pump for district heating, finding an optimal match between the size of the ground heat storage, the collector area, and the tank volume. In [
30,
31], the coupling between the GSHP and the photovoltaic/thermal system is analysed, demonstrating how the use of these systems together increases the thermal efficiency of the collectors; the electrical efficiency of the cells remains high without the risk of cell damage due to overheating. Due to its characteristics and high efficiency in terms of energy saving, CO
2 emissions reduction, and renewable energy source use, this system can easily be implemented in buildings to achieve more easily the NZEB target. However, little research has been undertaken about this system in the literature.
In Fedajev et al. [
32], with reference to an nZEB, the influence of different earth heat exchangers linked to a GSHP and thermal storage is examined, for the very harsh climatic conditions of Finland; the importance of the thermal storage to achieve the goal of an nZEB is shown.
In a review article, Gao et al. [
33] highlight the different potentialities of the technologies that can be coupled to the geothermal heat pump, suggesting this technology can be used in zero energy buildings.
In the technical-scientific literature to date, little examination has been made of the contribution of GSHPs to obtain a zero-energy balance in an NZEB and their influence in reducing the photovoltaic surface to be used in densely urbanized cities. Often these systems are analysed separately without any reference to the reduction of photovoltaic panels to be installed. Therefore, this paper analyses the aforementioned contribution by comparing the GSHP to other systems, such as an invertible (”reversible”) air source heat pump or a condensing boiler coupled to a chiller, in order to meet the NZEB target in different climatic conditions and to evaluate the photovoltaic surface needed to achieve the status of an NZEB compared to that available on the roof. To this aim, a case study building used as a bed and breakfast (B&B) and virtually located in two Italian towns with different climatic conditions is analysed. Using the dynamic energy simulation software, DesignBuilder, a detailed energy analysis is carried out. The procedure is partially validated by comparing the building energy requirements with literature data. Moreover, other important results are obtained regarding the primary energy consumption, the reduction of CO2 emissions, and the percentage of renewable energy used for the various solutions. Finally, a technical-economic analysis using the discounted payback is performed.
2. Methodology
2.1. Model Description
Referring to a case study building, the seasonal energy consumption and CO2 emissions of the building’s energy systems (in particular, in the case with GSHP) are evaluated.
First, the building to be designed should be characterised by low energy needs. Therefore, optimal thermal characteristics for the building envelope must be chosen to minimize the thermal losses in winter, while the free heat gains should be maximized in winter and minimized in summer. To this end, optimal thermal insulation, compact form, and innovative technologies should be selected [
16].
Subsequently, the HVAC (heating, ventilation and air-conditioning) system and other building energy systems must be chosen and designed to minimize the primary energy requirements [
22] by using energy-efficient solutions.
Therefore, the use of a GSHP is considered, and it is subsequently determined if this choice makes it easier to reach the NZEB target (i.e., the energy balance equal to zero between energy required by the building-system complex and energy produced from renewable sources).
The building of the case study is virtually located in two different Italian climatic zones and the energy analysis is performed through dynamic building energy performance simulation software, i.e., DesignBuilder [
34], based on the EnergyPlus calculation engine. The U.S. Department of Energy has performed several validation tests referring to EnergyPlus [
35,
36]. The climatic data are taken from ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) [
37].
The focus of the paper is an energetic, environmental, and technical-economic comparison between various generators of thermal energy: a GSHP characterized by vertical probes, an air source heat pump, and a condensing boiler plus an air-cooled chiller.
Regarding the DesignBuilder software, the CTFM (Conduction Transfer Function Module) is selected based on the algorithm “conduction transfer function”. Moreover, the algorithm DOE-2 [
38] is set for the outside convection and the algorithm TARP [
39] for the inside convection [
40].
The heat pumps are simulated by DesignBuilder using an equation-fit based model [
41,
42] to predict the heat pumps’ performance for cooling and heating modes. Equations (1)–(4) characterize the load curves of the heat pumps:
where:
A1–D5: Equation fit coefficients for the cooling and heating mode;
Tref: Reference temperature;
TL: Load side entering water temperature, K;
TS: Source side entering water temperature, K;
: Load side volumetric flow rate, m3/s;
: Source side volumetric flow rate, m3/s;
: Reference load side volumetric flow rate, m3/s;
: Reference source side volumetric flow rate, m3/s;
Qc Load side heat transfer rate (cooling mode), W;
Qc,ref: Reference load side heat transfer rate (cooling mode), W;
Power,c: Power needed (cooling mode), W;
Powerc,ref: Reference power needed (cooling mode), W;
Qh: Load side heat transfer rate (heating mode), W;
Qh,ref: Reference load side heat transfer rate (heating mode), W;
Power,h: Power needed (heating mode), W;
Powerh,ref: Reference power needed (heating mode), W.
The coefficients A1–D5 are entered in the heat pump model to simulate the performances. They refer to the performance at partial loads of heat pumps present in the DesignBuilder database. After inputting the dimensions of the HVAC system and choosing the heat pump of the required size, the software chooses suitable values of these coefficients basing on information from the manufacturer. A library of templates containing pre-defined manufacturers’ heat pump data is provided to allow an early stage analysis to be carried out.
In addition, the undisturbed temperature of the ground is needed in order to evaluate the seasonal efficiency of a GSHP. To this end, the equation of Kusuda [
43] is applied:
where:
Tg (D,t): ground temperature at a depth D after t days (starting from 1 January), °C;
Tav: yearly average temperature of the outdoor environment on the basis of statistical information, °C;
A: amplitude of the temperature annual oscillation, °C;
t: sequential number of the day (1 refers to 1 January);
tTmin: sequential number of the day corresponding to the minimum ground temperature, according to statistical data (1 refers to 1 January);
D: depth of the ground, m;
αg: daily equivalent thermal diffusion of the ground (m2/day).
For these parameters, the values reported in
Table 1 are used.
The seasonal energy performance of the heat pumps and chiller (seasonal coefficient of performance (SCOP) for heating operation, seasonal energy efficiency ratio (SEER) for cooling) are calculated by means of Equations (6) and (7):
where:
θh is the absolute temperature of the hot source;
θc is the absolute temperature of the cold source;
is the second law efficiency.
DesignBuilder cannot automatically derive the size of the GSHP, thus the following procedure was applied:
The building design thermal loads (both for heating and cooling) are evaluated;
A heat pump type from the DesignBuilder database is selected, taking into account the design thermal loads for both heating and cooling. Alternatively, it is possible to model a different kind of heat pump;
The number of vertical probes is evaluated.
For the evaluation of the correct size of the heat pump, the thermal power necessary for handling the treated ventilation external air (“primary air”) was also considered. In the examined case study, this is about 6 kW in summer; moreover, considering a ground that dissipates 50 W/m of thermal energy, the number of 4 vertical U-bend probes was obtained for both Milan and Palermo.
Regarding energy analysis, the primary energy is evaluated: the considered conversion factors are 2.2 for electricity and 1.05 for natural gas. To calculate the CO
2 emissions, the used procedure is explained in
Section 4 (Results and discussion—CO
2 emissions) [
44].
2.2. Procedure to Obtain an NZEB
Regarding the satisfaction of the energy balance, there are currently no specific requirements for the NZEB target. The Italian law strictly defines only nearly zero energy building (nZEB), by means of the D.M. 26 June 2015 [
45]. Therefore, according to this decree, the following requirements have been satisfied for the building-system complex examined (the first three refer only to the building envelope, the fourth refers to only the building energy systems, the last to the entire building-system complex):
Building average heat transfer coefficient: H′T ≤ reference value (W/m2K);
The ratio between the summer equivalent solar area of the windows and the useful walking surface: Asol,equiv,summer/Awalking surface ≤ reference value (-);
Energy performance indexes for the building envelope in winter and summer: EPH,nd, EPC,nd ≤ reference values (kWh/m2y);
Efficiencies of the building energy systems (H for heating, C for cooling, DHW for domestic hot water): ηH, ηC, ηDHW ≥ reference values (-);
Global energy performance index of the building-systems complex: EPglob,tot ≤ reference value (kWh/m2y).
In this way it was verified that the case study building is a nZEB; subsequently, in order to obtain an NZEB, an electricity production system from renewable sources (photovoltaic solar modules) is designed and inserted on the roof of the building. The global capturing surface of these modules is evaluated for the following different cases: GSHP, air source heat pump, condensing boiler plus chiller. Thereby, it is possible to estimate whether the GSHP would allow to achieve the NZEB target with a photovoltaic surface significantly smaller compared to the other solutions.
The derivation of the energy balance of a NZEB can be performed in different ways [
6,
46,
47]. In this work, the control volume that coincides with the complete building is chosen, and therefore the building-systems energy demand and the energy produced by renewable sources (kWh/m
2y) are considered in the energy balance:
This energy balance must be satisfied to obtain a NZEB. To this end, an accurate design of the building-system complex is required to minimize the energy demands. Therefore, it was determined which of the analysed solutions (GSHP, air source heat pump, condensing boiler plus chiller) has the minimum energy requirement to facilitate the achievement of the NZEB objective.
The
Figure 1 shows the general design and computational workflow followed in this study for obtaining the NZEB target.
3. Case Study
The pilot building is used as a bed and breakfast and is virtually located in two Italian towns having different climates (
Table 2): Palermo (South Italy), with mild winters and very hot summers; Milan (North Italy), with cold winters and hot summers.
The building is characterised by two floors, global area of 310 m2, and global volume of 900 m3.
Accurate design of the building and passive design rules are applied in order to achieve the NZEB target: considering the sun position, the living zone is placed on the south side in order to maximize solar gains in winter, while the service rooms and sleeping zone are placed on the north side (
Figure 2 and
Figure 3). Moreover, the windows are designed in order to optimise natural ventilation. A compact form of the building is chosen, so the surface-to-volume ratio is equal to only 0.20 m
−1. Low values of unitary thermal transmittance of the building envelope components are used (
Table 3), particularly for the coldest town (Milan), in order to minimise the thermal losses in winter.
Table 4 reports the design thermal loads of the building (for both heating and cooling conditions), calculated by the software, while
Table 5 shows the values of the seasonal average energy efficiency for the three compared systems.
Each of the analysed thermal energy generators is connected to fan-coil units located inside the rooms and to an air handling unit (AHU) for primary air.
The design thermo-hygrometric conditions are as follows:
indoor air: temperature of 20 °C for winter and 26 °C for summer, relative humidity of 50% for both winter and summer;
supply primary air: temperature of 20 °C for winter and 12 °C for summer.
Referring to the heating mode, different working programs are selected for the two analysed towns, according to DPR 412/93 [
48]:
Palermo (climatic zone: B): from 1 December to 31 March (6:00–9:00 a.m., 6:00–11:00 p.m.—total of 6 h per day);
Milan (climatic zone: E): from 15 October to 15 April (5:00–9:00 a.m., 12:00 a.m.–3:00 p.m., 5:00–12:00 p.m.—total of 14 h per day).
Referring to the cooling mode, the operational programs are fixed as follows:
Palermo: from 1 June 1 to 30 September (11:00 a.m.–4:00 p.m., 6:00–10:00 p.m.—total of 9 h per day);
Milan: from 1 June to 30 September (11:00 a.m.–3:00 p.m., 6:00–10:00 p.m.—total of 8 h per day).
The production of domestic hot water (DHW) is obtained by means of solar thermal collectors (five for Milan, four for Palermo) and a dedicated air-to-water heat pump (COP = 2.5), while the consumption rate is considered to be 2.5 L/m2day.
LED devices with a linear control system are used as the lighting system (2.5 W/m2-100 lux). The design values of the illuminance are 300 lux in bedrooms and 500 lux in common spaces.
Few other electrical devices exist, so a range of only 1–4 W/m2 is considered.
5. Conclusions
This paper investigates through an energy assessment the contribution that the geothermal heat pump can have on the reduction of the surface to be used for photovoltaics in order to obtain an NZEB with only on-site renewable sources. These assessments are carried out by comparing the GSHP with a common system—that is, is a condensing boiler coupled to a chiller—and with a more energy efficient system, namely, an air source heat pump.
It was shown that when using the GSHP, compared to the other systems, the NZEB target is obtained by using a smaller photovoltaic surface. Furthermore, although the comparison is also carried out with a performing technology such as the air-to-water heat pump, the GSHP is the only system that allows annual primary energy consumption equal to or lower than 57 kWh/m2 to be obtained for both of the analysed climatic zones. This is an important result since this value in the current scientific community is taken as a reference to define an NZEB. These are the main innovative aspects obtainable from this paper, that is, the optimal coupling of a GSHP with a photovoltaic system to obtain the NZEB target, minimizing the number of photovoltaic panels.
In more detail, it was found that the use of the GSHP, compared with the two other systems, allows considerable energy savings for heating and cooling (15–38% for Palermo and 23–55% for Milan).
Moreover, a significant reduction of CO2 emissions is obtained when using a ground source heat pump instead of a condensing boiler (plus chiller). This reduction
is 60 and 67% when considering only heating energy requirements for Milan and Palermo, respectively;
is instead 33% for Milan and 16% for Palermo in reference to global energy consumption.
The CO2 emissions reduction comparing the GSHP and the air source heat pump is also significant (19–23%) regarding to heating only consumption. Conversely, when considering the global energy consumption, this reduction is only 5–11%.
Regarding the photovoltaic surface to obtain a NZEB, the use of a GSHP instead of a condensing boiler coupled to a chiller leads to reductions of 92% for Palermo and 61% for Milan. These reductions are 5% for Palermo and 10% for Milan when comparing the GSHP with a more efficient technology such as the air source heat pump. The obtained results in the context of the NZEB is very important, as in highly urbanized contexts the space to allocate the production energy systems from renewable sources is often small and even 10% of surface savings can make the difference to obtain the NZEB target. In fact, the results showed that in the case of a continental climate such as Milan, in which the winter energy demand is predominant, it is not always possible to satisfy the NZEB target due to insufficient roof surface.
Beyond satisfying the energy balance, an energy demand limit value was considered to classify a building as an NZEB: it is suggested by the New Building Institute of the U.S. and is equal to 57 kWh/m2. If this limit value is considered, the building in question can be considered an NZEB for both climatic conditions only by using the GSHP.
Finally, referring to the energetic-economic analysis, the discounted payback (DPB) period for the GSHP is about 3 years for Milan and 14 years for Palermo when compared to the configuration with boiler plus chiller, while it is more than the useful life when compared to an air source heat pump.
Therefore, the air source heat pump seems more convenient than the GSHP from an economic point of view. However, it should be noted that the air source heat pump, especially in cold climates (Milan), can be affected by malfunctions in the case of very low external temperatures, when it may sometimes not satisfy the user.
Clearly, for the analysed cases, in order to obtain an NZEB, the ground source heat pump (compared to the common configuration with a condensing boiler coupled to a chiller and to the solution with the air source heat pump) has the highest performance from an energy point of view, but is still too expensive. Therefore, from an economic point of view (discounted payback period), it is really convenient only for continental climates (Milan) when compared to the solution with a condensing boiler coupled to a chiller.