Evaluation of Building Energy Savings Achievable with an Attached Bioclimatic Greenhouse: Parametric Analysis and Solar Gain Control Techniques

Abstract

Bioclimatic solar greenhouses are passive solar systems of relevant interest in the building sector, as they allow the reduction of energy needs related to air-conditioning. The aim of this work is to analyze the thermal behavior of a bioclimatic solar greenhouse attached to a residential building. It is equipped with photovoltaic solar blinds (SPBs) to manage solar inputs and produce electricity. Automated control systems are implemented to activate the vents and SPBs. The parametric performance analysis conducted using the dynamic simulation software EnergyPlus allowed the evaluation of the influence of glass type, thermal mass, size, ventilation and location. The results show how the automation of the vents allows the maximization of heat exchange throughout the year, leading to a reduction in consumption even during the summer period. Analyses conducted for some cities in the Mediterranean area show that the maximum energy saving obtained is greater than 13%; in addition, photovoltaic solar shading contributes to the production of more than 1000 kWh/year of electricity.

1. Introduction

In developed countries, the residential sector is responsible for about 40% of total energy consumption, of which more than 50% is used for the air-conditioning of buildings, contributing to the release of 30% of total CO₂ emissions [1]. Over the years, the need to reduce CO₂ emissions in the building sector has led to the emergence of the concept of bioclimatic architecture. The benefits are linked to site and building geometry, with the aim of exploiting solar radiation as much as possible to reduce energy consumption [2,3]. The main systems used for this purpose are passive solar systems, which can help maintain indoor comfort without the aid of active devices powered by external energy sources. Passive solar systems are based on capturing solar energy and storing and/or distributing heat inside the building by means of thermal air circulation [4,5].

Bioclimatic solar greenhouses, consisting of a glassed-in environment adjacent to the building, are used to reduce the energy demand of adjacent air-conditioned rooms during the winter period [6,7]. The high temperature achieved in the greenhouse allows the air-conditioned room to be heated [8] by directly supplying warm air through vents [9]. The attached solar greenhouses, which are an open thermodynamic system with energy and mass exchange, receive heat from the sun’s rays passing through the glass envelope [10]. The installation of a solar greenhouse also provides additional benefits, not related to energy savings, such as the renovation and weather protection of the building’s exterior façade [11].

The attached greenhouse is the most common type; however, the large number of dispersing surfaces reduces the energy gain and also increases the risk of overheating [12]. These drawbacks are mitigated in the built-in greenhouse, which is characterized by a smaller external surface area [13]. The benefits of using attached greenhouses are also closely dependent on parameters such as thermal storage capacity, type of glazing, size, and night shading. A careful analysis is needed to assess the influence of these factors in the design phase.

Various studies have been conducted to evaluate their performance in different climate zones and latitudes as the operation of solar greenhouses is dependent on outdoor conditions. Mihalakakou and Ferrante [9] simulated the thermal behavior of a south-facing attached greenhouse in four European cities (Milan, Dublin, Athens and Florence), concluding that in all the cities analyzed, the greenhouse led to a reduction in heating demand in the winter period, despite the occurrence of more or less pronounced overheating phenomena in the summer. Aelenei et al. [13] evaluated energy savings following the installation of an attached greenhouse in six climate zones in Portugal. They obtained energy savings during the winter period, against higher consumption during the summer period, with a total energy saving of between 15 and 55%. However, the role played by the greenhouse’s construction characteristics and thermal storage mass was not emphasized in these studies. Chiesa et al. [14] evaluated the impact on energy consumption of an attached solar greenhouse at 50 locations in central and southern Europe. In contrast to a significant reduction in winter consumption, they observed an increase in summer consumption in colder climates if no action was taken to prevent overheating.

Mihalakakou [15] demonstrated how it is possible to reduce the internal greenhouse temperature and limit the effect of overheating by adopting different passive cooling systems, such as night ventilation, solar shading systems and the use of underground ducts. Bataineh et al. [10] achieved a consumption reduction of 42% with an attached solar greenhouse in Jordan, minimizing overheating problems by adopting passive cooling systems. In the present work, considering the problems found in these articles, control logic will be implemented to move the blinds so as to optimize the inputs in winter and minimize them in summer to avoid excessively high temperatures.

Ulpiani et al. [8] found that in Italy, a greenhouse with convective heat exchange with double glazing and reduced depth reduced daily consumption more than 27%, highlighting how the use of a greenhouse with reduced depth leads to greater savings. Grudzinska [11] achieved energy demand savings of 33% by using a double-glazed envelope with selective coating in Poland. These articles do not investigate the performance using low-e double glazing.

The design of a solar greenhouse must consider the geometric configuration: the rectangular shape allows for a larger area of south-facing glazing and the contact surface between the greenhouse and the indoor environment, increasing solar gains and heat exchange with the building [15].

Numerous studies have shown that a southern exposure maximizes the amount of incident solar radiation in the winter period and reduces it in the summer period [9,16,17]. Attention must be paid to the glazed surfaces constituting the sides and roof, as they may cause overheating in summer and a reduction in benefits during winter [8,16]. The roof or south façade of the greenhouse could be tilted to maximize winter gain by reducing the angle of incidence [18].

The optical properties of the glazing greatly influence the behavior of the greenhouse; for proper functioning, the glass chosen should allow an adequate level of insulation without an excessive penalization of solar gain [11]. In sites with cold climates or harsh winters, the use of double glazing is advisable, as opposed to sites with mild winters, where single glazing is sufficient [13,19]. Regardless of the climate, good results can also be achieved using single glazing if the building is not insulated, while otherwise the use of high-performance glass is necessary [14].

The presence of thermal mass brings the following benefits [20]: increased average and minimum temperatures, reduced daily temperature fluctuations and reduced maximum air temperature inside the greenhouse. Rempel et al. [21] found that the dimensioning of the thermal mass depends on the time of day in which the stored heat is to be made available. If heating is required in the daytime, the mass must be minimal. If it is required in the evening, it must be greater. Moreover, thermal mass can provide cooling benefits and can prevent overheating [22]. Chiesa et al. [14] found that thermal mass has little influence on the performance of an attached solar greenhouse. They argue that this system requires further parametric analysis depending on the type of glass adopted.

The purpose of solar shading is to reduce consumption in the summer and avoid overheating, ensuring thermal and visual comfort for occupants [23]. Shading systems are subdivided according to type (fixed or movable) and positioning with respect to the glazing (external, internal, integrated). The main types of fixed shading systems are: overhanging, horizontal slat, vertical slat and egg-crate [24]. Their effectiveness depends on the geometric characteristics of the shading elements, the climate and the latitude of the installation site [25]. Mobile shading systems, which can be placed either externally or internally, can be adjusted according to the user’s needs. The main types are Venetian blinds, vertical slat blinds and roller blinds [24]. Venetian blinds are the most widely used shading devices due to their versatility, and their effectiveness depends on the width of the slats, the ratio of the length to the distance between the slats, the angle of inclination and the place of installation [26]. The setup of mobile solar shading can be adjusted using three types of control: manual, motorized and automated. Various studies have shown that the use of automated solar shading contributes to improved indoor environmental conditions and increased energy savings [26,27]. In particular, Nicoletti et al. [26] evaluated the energy savings resulting from the implementation of a strategy to control the slat angle of Venetian blinds, obtaining 15% higher energy savings than Venetian blinds with fixed slat orientation. The inclination angle of solar shading and the reflection coefficient of their external surfaces influence energy savings in the summer [28]. There are no studies that consider the use of automated blinds to manage the performance of a solar greenhouse.

Building-integrated photovoltaic (BIPV) systems consist of photovoltaic cells integrated in the building that not only produce electricity but also form the building envelope [29]. Based on their location, BIPV systems are divided into three groups [30]: roof-integrated systems, façade-integrated systems and systems integrated into external architectural elements. Solar photovoltaic blinds (SPB) are part of façade-integrated BIPV systems, the function of which is the shielding of solar radiation and the production of photovoltaic energy [31]. For the estimation of producibility, it is necessary to consider the mutual shading between the louvers, the reflection coefficient of the back of the louvers and the type of photovoltaic cells used [32,33].

Indirect-gain solar greenhouses with an insulated separation wall and heat exchange by means of thermal air circulation constitute a configuration that has been poorly analyzed in the literature. Furthermore, the installation of photovoltaic solar shading moved by intelligent logic in a solar greenhouse has not been contemplated in any of the cited studies.

The influence of all these characteristics (type of glazing, thermal mass, size, shading and ventilation systems, location) are investigated for this paper. The aim of the work is to provide a general overview of the characteristics that a solar greenhouse attached to a building located in the Mediterranean area should have. In fact, this area is characterized by a hot summer climate and, therefore, attention must be paid to the temperatures obtained. An attached solar greenhouse is studied to evaluate the influence of all parameters on the reduction of energy consumption for a residential building. In the analyzed case, the separating wall is insulated and photovoltaic blinds are placed to screen the greenhouse glazing. The opening of the vents placed on the separation wall and the angle of inclination of the SPBs are controlled by appropriate control systems based on IoT (internet of things) technology. The adoption of this technology makes it possible to extend the savings achieved to the entire year, not limiting it to the winter period.

The study is conducted using the EnergyPlus simulation software, where a reference building with an attached solar greenhouse is modeled, implementing the algorithms related to the control techniques. The evaluation of the impact of the solar greenhouse on the building’s consumption is conducted with parametric analyses, studying the savings achieved as the following parameters varied: depth, type of glass constituting the envelope, thickness of the thermal mass, use of solar shading, ventilation during the summer period, location. Finally, the overall electrical energy produced by photovoltaic solar shading is quantified.

2. Materials and Methods

This section describes the reference building analyzed and how the attached sunspace and control techniques are modeled for the numerical calculations.

2.1. Reference Building

The study is conducted considering Rome (Italy) as the reference locality. Climate data from the Gianni De Giorgio database (IGDG) are used to perform the dynamic simulations. These data are recognized as reliable by the scientific community and are recorded in the period 1951–1970 by the weather stations of the Meteorological Service of the Italian Air Force. Monthly average values of diffuse solar radiation on the horizontal plane, of direct normal solar radiation and outdoor air temperature are shown in Figure 1. The annual average outdoor temperature, the annual average amount of diffuse solar radiation and direct normal radiation are 15.3 °C, 625.3 kWh/m² e 969.0 kWh/m², respectively.

The building is a semi-detached house consisting of a basement, ground floor raised from ground level and attic. The basement consists of a cellar, garage and tavern, while the ground floor consists of an entrance hall, kitchen, living room, three bedrooms and bathroom. The air-conditioned floor area of the building is 110 m². The following rooms are not air-conditioned: cellar, cellar, stairwell, attic, bathrooms. The height of the rooms is 2.7 m. The 3D model of the reference building and the layout of the rooms are illustrated in Figure 2.

The composition of vertical structures and the relative transmittance values are shown in

Table 1. Properties of the wall layers.

	Material	Thickness (m)	Conductivity (W/m K)	Spec. Heat (J/kg K)	Density (kg/m3)
Insulated external wall (U = 0.281 W/m2K)	External plaster	0.005	1.4	1000	2000
	EPS insulation	0.1	0.0385	1200	30
	Hollow bricks	0.3	0.39	840	866.67
	Internal plaster	0.015	0.7	1000	1400
Insulated basement wall (U = 0.268 W/m2K)	Pebbles	0.4	1.2	840	1700
	Synthetic material sheets	0.01	0.23	900	1100
	EPS insulation	0.1	0.0418	1200	30
	Hollow bricks	0.3	0.39	840	866.67
	Internal plaster	0.02	0.7	1000	1400
Insulated internal wall (U = 0.274 W/m2K)	Internal plaster	0.01	0.7	1000	1400
	Masonry (hollow bricks)	0.08	0.48	840	2000
	EPS insulation	0.1	0.0385	1200	30
	Masonry (hollow bricks)	0.08	0.48	840	2000
	Internal plaster	0.01	0.7	1000	1400
Adjacent units partition wall (U = 0.736 W/m2K)	Internal plaster	0.02	0.7	1000	1400
	Soundproofing bricks	0.3	0.265	1000	1200
	Internal plaster	0.02	0.7	1000	1400

. The internal walls consist of a 0.08 m thick layer of perforated brick, covered on both sides with 0.01 m of plaster. The composition of horizontal structures is shown in

Table 2. Properties of floors and roof layers.

	Material	Thickness (m)	Conductivity (W/m K)	Spec. Heat (J/kg K)	Density (kg/m3)
Insulated floor slab (U = 0.248 W/m2K)	Ceramic tiles	0.01	1.3	840	2300
	Concrete mortar screed	0.06	1.06	1000	2000
	EPS insulation	0.14	0.0418	1200	30
	Slab blocks	0.26	0.67	840	842.31
	Internal plaster	0.01	0,7	1000	1400
Roof cover (U = 0.257 W/m2K)	Stainless steel	0.002	17	460	7900
	EPS insulation	0.05	0.0418	1200	30
	Steel	0.002	50	450	7800
	Concrete	0.03	1.162	1000	2000
	EPS insulation	0.09	0.0418	1200	30
	Slab blocks	0.26	0.7429	840	1146.15
	Internal plaster	0.02	0.7	1000	1400
Ground floor (U = 0.270 W/m2K)	Ceramic tiles	0.01	1.3	840	2300
	Concrete mortar screed	0.08	1.08	1000	1600
	EPS insulation	0.09	0.034	1200	50
	Reinforced concrete	0.315	1.91	1000	2400
	Synthetic material sheets	0.005	0.23	900	1100
	Pebbles and crushed stones	0.4	0.7	840	1500

. In addition, values of the linear transmission coefficients characterizing the thermal bridges of the building envelope have been included for precautionary purposes.

The internal gains related to occupancy are obtained by setting the occupancy density, the metabolic rate according to the activity carried out in each zone (

Table 3. Occupancy density and metabolic rate.

Zone	Occupancy Density (People per m2)	Metabolic Rate (W per Person)
Living/rumpus room	0.0188	110
Kitchen	0.0237	160
Entrance—corridor	0.0196	180
Bedrooms	0.0229	90
Bathroom	0.0187	120

) and the hourly occupancy schedule. Figure 3 shows, for each hour of the day, the percentage of time the area is occupied.

The internal loads due to the equipment and lighting system are assumed to be 4.1 W/m², constant throughout the day. In each air-conditioned zone, 20, 26 and 24 °C are set as set-point temperatures for heating, cooling and ventilation, respectively. In the non-air-conditioned rooms, internal loads and heating and cooling systems are absent. The building’s heating system is active from 1 November to 15 April, as regulated by Italian regulations, while the cooling system is active from May to September.

The exchange of air in the rooms is by means of natural ventilation in order to activate the free-cooling regime in summer. In particular, natural ventilation is used if the air temperature inside the zone is higher than both the outside air temperature and the set point temperature for ventilation.

The windows are composed of a 70 mm thick softwood frame and a low-emission double glazing unit with argon in the cavity, with the thickness of the glass panes and the cavity being 3 and 13 mm, respectively. The frame has a transmittance of 1.258 W/m²K, while the double glazing has a solar gain of 0.649 and a transmittance of 1.512 W/m²K. During the summer (June–September), windows are protected by Venetian blinds placed outside the glazing. The solar shading consists of 0.025 m deep slats with a fixed 80° inclination, spaced 0.01875 m and characterized by a reflection coefficient of 0.8. The external entrance door on the ground floor of the building and the insulated internal doors separating the air-conditioned and non-air-conditioned areas have transmittance values of 1.551 W/m²K and 1.761 W/m²K, respectively.

2.2. Modelling of the Attached Solar Greenhouse

The solar greenhouse under study is attached on the south façade of the building near the living room (Figure 4). The greenhouse has the following gross dimensions: height 2.7 m and width 4.5 m. Since the separating wall with the living room is insulated and has a glazed opening, the greenhouse will be an insulated, direct-gain hybrid system. Heat exchange between the greenhouse and the interior of the building will take place due to the presence of four openings along the top and bottom of the partition, in order to ensure the thermo-circulation of air. The greenhouse has fully glazed east, south and west walls and roof, supported by a wooden frame. The thermo-circulation of air is ensured by four rectangular air vents with dimensions of 0.8 m × 0.2 m.

The thermal storage mass inside the greenhouse consists of concrete blocks with the characteristics shown in

Table 4. Properties of thermal storage material.

	Conductivity (W/m K)	Specific Heat (J/kg K)	Density (kg/m3)
Concrete block	1.63	1000	2300

. The thickness of the thermal mass varied parametrically and it is distributed on both the separating wall and the floor. The floor has a 5 cm thick layer of insulation material at its base, with the aim of preventing the loss of accumulated heat to the ground [20,34]. The surface of the storage material is covered with a layer of plaster, which is dark-colored with an absorption coefficient of 0.9.

The choice of making the absorption coefficient of the wall higher than that of the floor is justified by the monthly variability of the total absorption coefficient of the greenhouse, with a minimum value in June and a maximum value in December. This monthly variability is exploited to increase the absorption of solar radiation as much as possible during the winter period and reduce it in the summer, thus limiting overheating problems. The glass walls of the greenhouse are protected externally by automated photovoltaic blinds, characterized by the same geometry as the screens used to protect the building’s window elements. The roof is instead protected by a solar blind with a high reflection coefficient in the period between June and September. Natural ventilation of the greenhouse during the summer period is achieved by opening the east and west windows.

2.3. Control Techniques

One of the objectives of this study concerns the integration of IoT technology to improve greenhouse performance. Dynamic control systems are used to open the vents and manage solar radiation. The latter is controlled by the inclination of the SPBs. To this end, a method for opening the vents is proposed and a control method taken from the literature [26] for the Venetian blinds is implemented. The use of photovoltaic blinds on an attached solar greenhouse has never been analyzed in the literature. These control processes are modeled through the EMS (energy management system) functionality of EnergyPlus, in which the Erl programming language is used (EnergyPlus Runtime Language).

The algorithm for opening and closing the vents requirs two air temperature sensors: for the living room and for the solar greenhouse. The control system exploited the daily temperature excursion inside the greenhouse to enable summer night ventilation. The vents are opened if:

• The air temperature in the greenhouse is lower than in the living room and the air temperature in the living room is higher than 25 °C (cooling mode);
• The air temperature in the greenhouse is higher than in the living room and the air temperature in the living room is below 21 °C (heating mode).

With this configuration, the greenhouse is used not only for the reduction of consumption related to heating, but also for the reduction of consumption related to the cooling of the building (free cooling). The actuators are electric servomotors that allow the slats of the vents to be moved. The information would be managed by a local network in which the sensors and actuator are connected to a control unit that processes the simple algorithm. For energy simulations conducted in EnergyPlus, the algorithm is implemented in the EMS section in a dynamic manner.

The strategy used to control SPBs is the one developed by Nicoletti et al. [26], in which the angle of inclination of the lamellae ω is adjusted with the aim of maximizing solar gains while avoiding overheating phenomena. Furthermore, the slats are adjusted so that the occupants are not subject to glare phenomena, thus guaranteeing visual comfort. The latter condition is not considered in this case, since the solar greenhouse is conceived as a non-air-conditioned environment not subject to occupants.

The sensors detect the air temperature inside the living room T, where thermal comfort conditions must be maintained, and the global solar radiation G incident on the walls of the greenhouse (using a cheap PV cell which provides a voltage signal proportional to the incident radiation). Figure 5 shows the algorithm implemented to control the SPBs. The angle of inclination depends on the solar altitude $\alpha$ and solar azimuth evaluated with respect to the wall orientation ($\gamma -{\gamma }_w$). The sun horizontal angle profile $\beta$ also appears in the functional diagram and is defined as:

$$\beta =\arctan \left(\frac{\tan \alpha }{\cos \left(\gamma -{\gamma }_w\right)}\right)$$

(1)

Photovoltaic blinds consists of organic cells with an electrical efficiency of 10%. The electrical producibility of PV blinds is estimated in accordance with procedure shown in [35].

2.4. Parametric Analysis

The reference case study concerns a solar greenhouse located in Rome with the following characteristics: a depth of 1.5 m, low emissivity double glazing with argon in the cavity, a floor thickness of 5 cm, use of SPBs and ventilation in summer with east and west windows open 50% at night. The optimal greenhouse configuration is obtained through a series of parametric analyses and the results are compared to the reference building without sunspace, evaluating the energy savings achieved. In each analysis, the individual parameter of interest is varied, leaving the other characteristics belonging to the base case unchanged.

The parameters analyzed are listed below:

• Greenhouse depth:
  • 1.5 m;
  • 2 m;
  • 2.5 m.
• Type of glass constituting the greenhouse envelope (values for solar gain, direct solar transmission and transmittance (W/m²K) are given in brackets):
  • 6 mm single-glazing (0.819, 0.775, 5.778);
  • 3/13/3 mm double glazing with air in the cavity (0.764, 0.705, 2.716);
  • 3/13/3 mm double glazing with argon in the cavity (0.764, 0.705, 2.556);
  • 3/13/3 mm low-emission double glazing with argon in the cavity (0.649, 0.538, 1.512).
• Thermal storage mass inside the greenhouse:
  • 5 cm floor thickness, no thermal mass on the wall;
  • 10 cm floor thickness, no thermal mass on the wall;
  • 20 cm floor thickness, 10 cm floor thickness;
  • 30 cm floor thickness, 20 cm floor thickness.
• Use of SPBs:
  • With SPBs;
  • Without SPBs.
• Greenhouse ventilation methods in summer:
  • East and west windows open at 50% at night;
  • East and west windows open at 100% at night;
  • East and west windows open at 50% all day;
  • East and west windows open at 100% all day.
• Locality:
  • Genoa, Italy (latitude 44°23′);
  • Rome, Italy (latitude 41°54′);
  • Capo Palinuro, Italy (latitude 40°1′).

The climatic characteristics of the locations chosen for comparison are shown in the results section. The sites are chosen to be representative of northern, central and southern Italy. They all belong to the same climatic zone with the Italian classification so that the building transmittances do not need to be changed. Therefore, a comparison with the same building envelope is possible.

3. Results and Discussion

This section discusses the results obtained, considering the period of one year. The results for the reference building (without greenhouse) show that the annual thermal energy demand for heating was 1718 kWh and for cooling was 954.2 kWh.

3.1. Parametric Performance Analyses with Attached Solar Greenhouse

3.1.1. Influence of Sunspace Depth

Heating consumption increases as the depth of the greenhouse increases, in contrast to summer consumption, as shown in

Table 5. Building’s annual energy needs and percentage change relative to the reference case varying sun space’s depth.

Depth (m)	Energy Needs (kWh)		Variation from Reference Case
	Heating	Cooling	Heating	Cooling
1.5	1570.0	805.7	−8.6%	−15.6%
2.0	1580.3	790.9	−8.0%	−17.1%
2.5	1591.8	780.1	−7.3%	−18.2%

: a depth of 1.5 m results in greater savings in the winter period (8.6%), while a depth of 2.5 m results in greater savings in the summer period (18.2%).

The results are influenced by the low thermal resistance characterizing glass walls; consequently, heat losses to the outside increase as the envelope surface area increases. Figure 6a shows the average monthly heat gains per unit of glazed area, net of losses to the outside. In the winter months, due to lower insolation, the glazing is more susceptible to heat losses to the outside and the reduction of the dispersing surfaces increases the gains. In the intermediate periods, the greater amount of solar radiation incident on the glazing balances out the losses. This leads to an increase in gains as the glazed area increases. During the summer months, the gains tend to even out due to the activation of the solar shading; however, the absence of incident solar radiation favors heat losses as the depth of the greenhouse increases. Thermal gains influence the air temperature inside the greenhouse (Figure 6b). Consequently, they influence the heat exchanges with the living room (Figure 6c,d) with the thermo-circulation of air through the vents on the separation wall.

3.1.2. Influence of Glass Type

The results in

Table 6. Building’s annual energy needs and percentage change relative to the reference case varying the glazing type constituting the envelope of the sun space.

Glazing Type	Energy Needs (kWh)		Variation from Reference Case
	Heating	Cooling	Heating	Cooling
Single	1643.6	757.7	−4.3%	−20.6%
Double—Air	1608.8	776.7	−6.4%	−18.6%
Double—Argon	1604.5	778.7	−6.6%	−18.4%
Double LoE—Argon	1570.0	805.7	−8.6%	−15.6%

show that the use of low-e double glazing is the optimal solution during the winter period. It leads to a saving of 8.6% compared to 4.3% achieved with single glazing. However, in the summer period the situation is reversed: single glazing leads to a greater reduction in consumption (20.6%) than low-e glass (15.6%). The performance obtained in the case of double glazing is intermediate with respect to those discussed; moreover, the presence of argon in the cavity leads to negligible energy savings compared to the case where there is air.

The use of low-emissivity double glazing allows higher temperatures inside the greenhouse (Figure 7a), although the amount of solar radiation transmitted is penalized (Figure 7b). This behavior is caused by the low-emissivity coating being opaque to the high wavelength radiation emitted by the surfaces. The glass is able to retain heat inside the greenhouse and the low transmittance value results in less heat loss [36]. The opposite behavior is observed in the case of single-glazing: despite the greater amount of solar radiation transmitted into the greenhouse, the high transmittance characterizing the glass leads to greater dispersion with a consequent reduction in gains. By analyzing the temperatures inside the greenhouse, it is possible to evaluate the effects on heat gains and losses in the living room through the vents (Figure 7c,d).

3.1.3. Influence of Thermal Mass

From the values shown in

Table 7. Building’s annual energy needs and percentage change relative to the reference case varying the heat storage mass.

Thermal Mass	Energy Needs (kWh)		Variation from Reference Case
	Heating	Cooling	Heating	Cooling
Floor 5 cm	1570.0	805.7	−8.6%	−15.6%
Floor 10 cm	1581.8	817.2	−7.9%	−14.4%
Floor 20 cm, wall 10 cm	1630.7	847.1	−5.1%	−11.2%
Floor 30 cm, wall 20 cm	1649.9	849.2	−4.0%	−11.0%

, it is evident how the presence of storage was counterproductive: as the thickness increases, the consumption for both heating and cooling increases, resulting in a reduction of energy savings (from 8.6% to 4.0% for heating and from 15.6% to 11.0% for cooling).

The greenhouse analyzed is predominantly an insulated gain system, as the separating wall is insulated. Therefore, the temperatures of the thermal mass do not directly influence the conditions inside the building. The greenhouse only acts as a heat collector and not as a thermal buffer between the interior and exterior environment. In particular, the main gain is the thermal exchange through the vents when the air temperature inside the greenhouse is between 21 °C and 25 °C.

The presence of the accumulation mitigats daily temperature fluctuations, which is a counterproductive effect, as the lower temperature peaks during the winter period reduce heat exchange with the living room. The same is true for the summer period when, in order to have a greater heat exchange with the interior, night temperatures must be as low as possible. The days of 5 January and 5 July were chosen as representative for the heating and cooling periods, respectively, whose daily temperature trends inside the greenhouse are shown in Figure 8a,b. Figure 8c,d shows the daily trends of thermal gains (5 January) and losses (5 July) in the living room.

3.1.4. Influence of Solar Photovoltaic Blinds

The energy requirements with and without solar shading to protect the greenhouse are shown in

Table 8. Building’s annual energy needs and percentage change relative to the reference case with and without SPBs.

Presence of SPBs	Energy Needs (kWh)		Variation from Reference Case
	Heating	Cooling	Heating	Cooling
With SPBs	1570.0	805.7	−8.6%	−15.6%
Without SPBs	1510.0	1071.7	−12.1%	+12.3%

.

In the winter period, the absence of shading results in an increase in energy savings from 8.6% to 12.1%. Although SPBs are controlled in such a way as to let in as much solar radiation as possible, their design causes the glazing to be partially hidden from the sun’s rays. This effect can be seen in Figure 9a, which shows the monthly average trends of solar radiation transmitted into the greenhouse. Shading, on the other hand, is essential during the summer period. Their absence leads to a 12.3% increase in consumption compared to the building without a greenhouse. This phenomenon is caused by the overheating of the greenhouse (Figure 9b), which is in turn favored by the presence of low-emissivity glazing.

3.1.5. Influence of Greenhouse Summer Natural Ventilation

Table 9. Building’s annual energy needs and percentage change relative to the reference case varying ventilation strategy.

Ventilation	Energy Needs (kWh)		Variation from Reference Case
	Heating	Cooling	Heating	Cooling
50%—only night	1570.0	805.7	−8.6%	−15.6%
100%—only night	1570.0	775.7	−8.6%	−18.7%
50%—all day	1570.0	775.5	−8.6%	−18.7%
100%—all day	1570.0	738.8	−8.6%	−22.6%

shows the results with the different opening modes of the side windows of the solar space. Consumption during the cooling period decreased both as the percentage of window opening increased and as the duration of the ventilation period increased. The maximum saving during the cooling period was 22.6%. In the winter period, of course, consumption remained unchanged.

The reduction in consumption is caused by the lower temperatures inside the greenhouse (Figure 10a), which promote the removal of heat from the living room through increased thermo-circulation of air (Figure 10b).

3.1.6. Influence of Locality

The greenhouse performance analysis was evaluated by comparing the results obtained at three representative locations in Northern, Central and Southern Italy (Genoa, Rome, Capo Palinuro). The climate data for Genoa and Capo Palinuro are shown in Figure 11. The average annual outdoor temperature is 15.2 °C in Genoa and 16.3 °C in Capo Palinuro. Genoa has an average annual amount of diffuse solar radiation and direct normal radiation of 631.7 and 729.1 kWh/m², respectively; for Capo Palinuro these values are 644.5 and 957.8 kWh/m².

The results (

Table 10. Building’s annual energy needs and percentage change relatively to the reference case varying sun space’s location.

Locations	Energy Needs (Reference Building/Building with Sunspace) (kWh)		Variation from Reference Case
	Heating	Cooling	Heating	Cooling
Genoa	1897.9/1757.5	679.8/579.7	−7.4%	−14.7%
Rome	1718.0/1570.0	954.2/805.7	−8.6%	−15.6%
Capo Palinuro	862.9/735.2	1090.1/955.9	−14.8%	−12.3%

) show that for Genoa, the benefits are lower than for Rome during both the summer and winter periods. For cities with a warmer climate, such as Capo Palinuro, the winter percentage savings are greater than in Rome, while there is a slight reduction in the summer period. In all cases, however, it should be noted that the solar greenhouse, with proper controls, also leads to a reduction in cooling requirements in all locations.

3.2. Comparison with the Case Where the Separation Wall Is without Insulation

The results presented so far related to the performance of the greenhouse in the case of an insulated separation wall. In the following, the performance without the insulating layer in the separation wall will be analyzed. The new transmittance results show U = 1.037 W/m²K. The absence of insulating material makes the greenhouse a hybrid system with direct gain (due to the presence of a glazed component in the separation wall) and indirect gain, in which the wall acts as a thermal storage mass.

The results in

Table 11. Building’s annual energy needs and percentage change relative to the reference case for insulated and not insulated separation wall.

Separation Wall	Energy Needs (kWh)		Variation from Reference Case
	Heating	Cooling	Heating	Cooling
Insulated	1570.0	805.7	−8.6%	−15.6%
Not insulated	1617.4	849.8	−5.9%	−10.9%

show that, without insulation, there is an increase in energy requirements, resulting in reduced savings compared to the base case. This trend can be seen in both the winter and summer period. The saving for heating is 8.6% with the presence of insulation, and 5.9% without insulation. For cooling, the saving is 15.6% in the first case and 10.9% in the second case. The separating wall, by contributing to the accumulation of heat, helps to dampen the daily temperature fluctuations inside the greenhouse. This effect generates less heat exchange with the living room through the vents, as already discussed in the parametric analysis of the storage mass. In addition to the reduction in thermal exchange through the vents, it is necessary to take into account the counterproductive effect of the high transmittance value characterizing the non-insulated partition, which causes greater losses in the winter period and greater gains in the summer period.

3.3. SPBs Electricity Production

The methodology used to calculate the electrical energy produced by photovoltaic solar shading is the one developed by Nicoletti et al. [35]. The control system adopted penalizes photovoltaic production in the winter period, as the slats of the screens are arranged parallel to the sun’s rays to maximize solar gain entering the greenhouse. Figure 12 shows the monthly energy produced by the SPBs per unit of glass area. It can be seen that east- and west-facing blinds produce approximately the same amount of energy in all months. South-facing venetian blinds produce more energy throughout the year, with the exception of the months of May, June and July, when production is higher at the east and west orientations.

The method [35] consideres the solar radiation incident on the photovoltaic cells to be composed of the sum of direct, diffuse and reflected radiation from the ground and from the back of the lamellas. Consequently, these contributions affect the total electricity production, as shown in Figure 13. The contribution of solar radiation reflected from the back of the louvers has zero values during the summer months, since the slats are closed or partially closed most of the time. On the other hand, during the winter period, the reflected radiation contributes to energy production, since the high inclination angle means that the view factor between the back of the slats and the outside is non-zero.

In the winter months, the contribution of reflected solar radiation from the ground is approximately zero. This occurs because the control system adjusts the inclination of the lamellas so that they are parallel to the sun’s rays in the presence of direct radiation or inclined by 110° in the case of diffuse radiation only; thus, the lamellas are mainly facing upwards, with a view factor between the photovoltaic layer and the ground equal to zero. This contribution is non-zero during the summer period, when the blinds were partly or completely closed. Figure 14 shows the monthly energy produced by the three installed screens, analyzing the case where the greenhouse has a depth of 1.5 m, with the size of the south window being 4.5 m × 2.7 m and the size of the east and west windows being 1.5 m × 2.7 m. The highest energy production occurs in July (169 kWh) and the lowest in December (14.7 kWh). The electrical energy produced, taking into account the inverter efficiency, assumed to be 0.9, amounted to 1036 kWh.

3.4. Summary of Results

The analyses, which were conducted by orienting the greenhouse to the south, revealed the following main results:

• The solar greenhouse with reduced depth allows greater energy savings in winter, as the smaller amount of transparent surface area contributes to the reduction of heat loss;
• The use of low-emissivity double glazing in the construction of the greenhouse envelope results in greater gains in winter, while in summer their use is counterproductive in the absence of adequate measures to combat overheating;
• The presence of accumulation mass in the greenhouse is counterproductive in the case of an insulated separation wall, as temperatures inside the greenhouse are mitigated by reducing heat exchange through the vents;
• The use of sunscreens in summer is of paramount importance in reducing temperatures inside the greenhouse, as is an adequate level of ventilation;
• The analysis by location showed that energy savings are greater in southern Italy, as it is characterized by a higher level of solar radiation.

The use of automated control techniques, based on IoT technology, allows the isolated gain greenhouse to achieve greater energy savings than the indirect gain configuration. In particular, the automation of the vents allows the maximization of heat exchange throughout the year, leading to a reduction in energy consumption even during the summer period. The results show that cooling requirements can also be reduced. This is in agreement with studies in the literature that show that proper ventilation brings summer benefits [37,38]. In particular, many scientific articles [39,40] have emphasized how reductions in summer consumption can be achieved through shading and natural ventilation.

The analysis showed that the maximum reduction in energy needs for the city of Rome results in a thermal energy saving of 148 kWh for heating and 215 kWh for cooling. Percentage savings are 8.6% for heating and 22.6% for cooling. The optimized solar greenhouse has the following properties:

• Depth of 1.5 m;
• Floor with 5 cm thick accumulation material;
• 3/13/3 mm low-emission double glazing envelope with argon in the cavity;
• Solar shading system;
• East and west windows open 100% all day.

4. Conclusions

In this study, the impact of an attached solar greenhouse on the heating and cooling energy requirements of a residential building was evaluated. The study was conducted by means of parametric analyses, evaluating the performance of the greenhouse as the construction and operational characteristics changed.

In the winter period, the solar greenhouse contributes to greater energy savings in the case of reduced depth and the use of high-performance glass, such as low-emissivity double glazing. In the summer period, on the other hand, the greatest energy savings were found by favoring the ventilation of the greenhouse and activating solar shading to reduce the counterproductive effects of overheating. Furthermore, it appears that the presence of storage mass inside the greenhouse penalizes energy exchange in the case of insulated partition wall. Specifically, for the city of Rome, percentage savings are 8.6% for heating and 22.6% for cooling. In addition, photovoltaic solar shading contributes to the production of more than 1000 kWh/year of electrical energy.

The study, carried out on locations in the Mediterranean area, shows how a solar greenhouse constitutes an important structure for reducing energy consumption in the residential sector, contributing to the achievement of building energy self-sufficiency. The proposed system, in which the use of BIPV technologies is contemplated, can be used by planners not only in residential areas but also for offices and buildings with similar use. In addition to the reduction of energy needs for air-conditioning, this system favors the on-site production of electricity, especially if the space on the roof surfaces is insufficient for the installation of an adequate number of photovoltaic panels. Furthermore, the presence of IoT systems allows for customized management of control systems, combining the needs of maximizing energy exchange with the maintenance of comfort conditions.

The study, which was conducted by analyzing a solar greenhouse configuration that has not been extensively studied in the literature, shows that the energy savings obtained were not negligible, despite the high quality of the building envelope and the small south-facing surface area where the solar greenhouse of the reference building was assumed to be installed. Consequently, further developments could concern the analysis of the energy savings achieved by attaching an isolated gain solar greenhouse to buildings characterized by different configurations and different quality of the building envelope.