**1. Introduction**

This paper deals with the analysis of the energy performance of a solar low-energy house, which can also be considered a smart house. The smartness of the house is based on its design and construction with the focus on using the ambient surrounding and energy sources, mainly solar energy, in a passive way to reduce energy demand for space heating and cooling, and then to provide the energy demand in an e ffective way through the well-planned operation of its energy systems. In other words, to achieve real smartness in a house it is necessary first to create the architectural concept, which from the beginning takes into account energy aspects as well as aesthetics, and assures passive and active utilization of renewable energy and waste heat available to the house. Then the well-designed integration of di fferent energy devices and installations assures their complementarity thanks to e ffective automatic control of the multisource energy system. It can be said that smartness is achieved through coupling passive and active methods of energy conservation, joining architecture, civil engineering and technical energy aspects. Avoiding energy demand is the best way to save energy. Thanks to the low energy demand concept of a house it is easier to consume less energy living there. The paper presents how di fficult it is to technically prove, using standard methods of determination of the energy performance of buildings, that a building is really both low energy and smart, and that all aspects of energy e fficiency have been treated equally.

In most countries in the world there are specific standards and regulations put into place to assure energy e fficiency in buildings. In EU countries the formal background for the development of energy e fficiency in buildings has been set out by the directive of the European Parliament and of the Council on the energy performance of buildings (Directive 2010/31/EU) [1]. The directive entered into force in 2010 and was a recast of the previous EU Directive on the Energy Performance of Buildings—EPBD, published in 2002. Further amendments have been made in 2018 (2018/844/EU) [2]. Since the beginning of the new millennium the EU Commission has been fostering the development of low energy buildings and the application of energy saving technologies, including utilization of renewable energy. Nowadays, thanks to Directive 2010/31/EU, measures to reduce the energy consumption in buildings have led to the evident improvement of thermal comfort in buildings and a reduction in environmental pollution. According to Article 9 of the directive, from 2021 all new buildings will have to be nearly zero-energy buildings, and in the case of public utility buildings the rule already came into force at the beginning of 2019. Thus, due to the o fficial EU legislative policy to adopt the rules of nearly zero-energy use in the construction and use of buildings, the idea of energy e fficiency in buildings is not only a concept of implementing general energy conservation principles and minimizing their environmental footprint, but it has become a national duty and legal necessity. The EU directive promotes the idea of nearly zero-energy for new buildings and stimulates the transformation of existing buildings to be refurbished into nearly zero-energy ones. Energy conservation in buildings has been supported by other EU regulatory frameworks, mainly by the EU directive on energy e fficiency [3].

Nearly zero-energy buildings (NZEBs) should have very high energy performance. The low amount of energy which these buildings require for their e ffective use comes mostly from the use of renewable energy sources. The idea of zero energy buildings and net zero energy buildings was discussed before the EPBD directive came into force [4]. The EU energy legislative policy (through directives) compels national member state regulations to introduce limits for final and primary energy consumption for buildings with regard to their typology. These indices are di fferent in di fferent EU countries depending on climate and energy mix in those countries. The EU policy leaves it open for the member states to make a decision on the quantification of the boundary indices for energy consumption, i.e., what is the maximum quantity of energy to be used in buildings with regard to their final and primary energy.

Thanks to the implemented regulations the energy needs of buildings have been much reduced in recent times, mainly due to the well-designed envelopes and structures of buildings (i.e., shape of the building, types of construction, one layer or multilayered, type of construction materials and insulation applied with the focus on their thermal parameters). Recently, focus has been put on the thermal quality of buildings, mainly on application of thick thermal insulation of very low conductivity, and on windows characterized by low heat loss (transmission) coe fficients ( *U*-values). However, it can be mentioned here, that using insulation in a warm climate can reduce the heat losses necessary in summer to limit overheating. Heat losses are necessary during summer nights to release the excess heat gained during the day, because of high solar irradiation and to keep the indoor air temperature at the required level. Therefore, application of insulation must always be adapted to the given climate conditions. In addition, highly e fficient and reliable equipment and energy installations, including ventilation systems with heat recovery, have been implemented. Such measures have caused significant drops in final energy demand. Application of renewable energy systems have also reduced the primary energy consumption based on fossil fuels.

Buildings are responsible for approximately 40% of EU energy consumption and 36% of the CO2 emissions [5]. For many years high energy consumption in buildings was caused by high heat losses through buildings' envelopes [6]. The space heating demand used to be the highest demand component of energy consumption in buildings and is still responsible on average for about 65% of the total energy needs of buildings in the residential sector [7]. Of course, the share of space heating demand was higher in high latitude countries than those of low latitudes. Energy consumed by space heating and domestic hot water systems now accounts on average for 80% of the total (according to EUROSTATS). Therefore, it is not surprising that measures undertaken to reduce energy consumption in buildings have been mainly focused on decreasing the space heating demand.

In many European countries, as in Poland, new regulations connected with the determination of energy performance of residential buildings take into account only heating energy demand for space heating and domestic hot water (DHW). Cooling energy demand and electricity consumption by lighting units and systems, and electrical appliances are not limited by any o fficial regulations. As electricity consumption was relatively small in residential buildings, it was believed that there was no reason to set limits for electricity consumption in houses. In the case of cooling energy it is arbitrarily assumed, that residential buildings in Poland do not require cooling, because of the relatively cold climate. Nowadays, it turns out that supplying cooling energy to residential buildings is sometimes necessary to maintain thermal comfort. Summer cooling demand can be seen especially for south and west facing rooms with large windows [8]. It turns out that cooling demand becomes a challenging issue for new buildings in moderate climates [9].

In Poland, according to existing regulations [10] since 2021 the indices of primary energy consumption for space heating and domestic hot water of all newly constructed residential buildings, called nearly zero energy buildings (NZEB), cannot exceed 70 kWh/m2a or 65 kWh/m2a for single family houses or multi-family apartment buildings, respectively. In addition, the heat transfer coe fficients for external walls must not be higher than 0.2 W/(m2K). As a result, external walls have thick insulation of high thermal quality (in the 1970s the recommended thickness of thermal insulation was 6–8 cm, at present it is 20–25 cm). What is more, the heat transfer coe fficients for windows will soon not be allowed to be larger than 0.9 W/(m2K) and currently they cannot be higher than 1.3 W/(m2K) (in the 1970s it was 3.2 W/(m2K)). Existing regulations on energy performance of buildings define the indices of maximal primary energy consumption considering only technical issues. They put the focus on energy efficiency, which results in reduction of final energy consumption and gives support for renewable energy sources, which utilize much less primary energy. Unfortunately, they do not show how crucial for energy consumption is the architectural concept of a building. Its shape, structure, location, the sizing of di fferent elements of the building envelope and their orientation to specific directions of the world, and surroundings are of grea<sup>t</sup> importance. Without analysis of all the architectural and local settlement conditions, it is like being halfway to the finish line, but with a slow first half and no chance of winning. When a building is designed and constructed without a real vision of maintaining low energy consumption throughout the whole year, then it will not be possible to reduce the final and primary energy consumption to the set limits relying only on the energy e fficiency of devices and installations applied. Therefore, such a building will not be a smart low-energy building. To ge<sup>t</sup> a real reduction in energy consumption of any building it is necessary to have a global interdisciplinary approach and look at the process of building design, construction and use in a holistic way.

Many energy simulation programs have been developed to determine building energy performance and they are used in many di fferent countries. Comparison of the features and capabilities of twenty major simulation programs determining the energy performance of buildings can be found in the literature [11]. The authors showed how contrasting the building energy performance approaches can be. So it is not surprising that it is di fficult to find such a method of determining energy performance of buildings, which would take into account all aspects of the actual energy e fficiency of buildings such as those described in this paper.

This paper presents the problem of how the existing regulations supporting the reduction of energy consumption in buildings through an engineering determination of energy performance cannot fully present a true and reliable assessment and evaluate the real impact on energy consumption. The paper shows how di fficult it is to prove technically that a building is really low energy and smart, and that all aspects of energy e fficiency mentioned above have been treated equally. An example of a single family house designed and constructed as a low energy solar house located in Warsaw's suburbs has been considered. Section 2 describes a general idea of a low energy building and a smart building and shows how some of their main features are similar to each other, whilst others are di fferent. Section 3 presents solar radiation conditions in Poland and a concept of solar passive architecture, which should be taken into account when a low energy smart solar house has to be designed and constructed. Section 4 describes the heating energy demand as well as final and primary energy consumption of the house under consideration. At the end, both the o fficially calculated and real energy performances of the building are discussed and general conclusions are formulated.

#### **2. Low Energy and Smart Buildings**

Low energy buildings are not usually equated with smart buildings. However, it seems that if a building is low energy it must be smart. However, the smartness of a building can be defined by many parameters that are distinct from those usually applied to low energy buildings. If we would like to consider a modern low energy building as a smart building, we should define what both 'low-energy' and 'smart' mean and whether a building can be both. A low energy building can be defined as a building which needs and consumes a small amount of energy during its life-time. It is a building needing a small amount of energy for space heating or cooling, thanks to its architectural and civil engineering design and construction. Low energy needs result from the specific concept of the building envelope, materials used, specific location of di fferent partitions such as opaque walls and transparent glazing in the structure of the building, as well as specific location of rooms of di fferent functions inside the building. It also needs the correct utilization of solar radiation for gaining energy in the winter and protecting from excessive solar gains in the summer. Of course the aesthetic values of the building envelope cannot be forgotten. A low energy building also consumes a small amount of energy because of the use of energy e fficient devices and systems applied to fulfill all its energy needs: space and water heating, cooling, ventilation, air conditioning, lighting, electricity for electrical appliances, etc. Moreover, renewable energy sources are used to reduce primary energy consumption of fossil fuels.

Di fferent means of construction and operation of low energy buildings have been developed in recent decades, with the best achievements in the last decade. Kivimaa and Martiskainen [12] conducted a systematic review of case studies on low energy innovations in the European residential building sector from the beginning of this century. They analyzed drivers important for systemic and architectural innovation in low energy buildings and pointed out how di fferent the low energy buildings can be in their main purpose. There are some key words used to describe buildings of low energy consumption, such as: energy e fficient, low energy, zero carbon, passive houses, etc. All these keywords express the main features of low energy buildings, i.e., energy, e fficiency, environment and architecture; however, according to the names of those buildings the focus can be put on di fferent aspects.

When we search through the Internet trying to find a definition of the smart building, the most common one is as follows: A smart building is any structure that uses automated processes to automatically control the building's operations including heating, ventilation, air conditioning, lighting, security and other systems [13]. This idea is certainly connected to the energy e fficiency aspect of low energy buildings, but it seems to be much more closely connected to energy e fficiency measures introduced (mainly to o ffice buildings) and known as BMS—building managemen<sup>t</sup> system [14]. BMS systems are also known as building automation systems (BASs). Such a system is a computer-based control system used in buildings to monitor and control the building's energy systems and other systems such as fire and security. Smart buildings use sensors, data monitoring and collecting systems, which give the information needed for e ffective operation of di fferent buildings' systems, including energy systems. Smart buildings use IT—Information Technology and IoT—Internet of Things. Such technology is usually used for office buildings, hospitals, health care and educational facilities, sport centers and sometimes for public buildings, but very rarely for residential houses. There are no standards for smart buildings. Low energy architecture is not one of determinants of the smartness of such buildings. Therefore it can be said, that in many smart buildings like office buildings, the important element of energy efficiency required to reduce energy needs is usually missed. However, without low energy or energy efficient architecture of a building it is really difficult to call any building a smart one.

One more aspect not analyzed in detail in the paper is very important, namely the smartness of building users. Energy consumption in buildings depends on user behavior. It can be said that the inhabitants of residential houses basically want energy savings because they relate to the user costs and directly affect them. The problem with these unthinking building users' behavior is particularly evident in office buildings [15]. Therefore, it should be stated that a smart building also requires smart users.

Taking into account what features should be common to both low energy buildings and smart buildings, it can be seen that energy efficiency is essential. However, it seems that the definition of a smart building should be much wider, especially in the case of residential buildings. The next section presents the concept of a smart low energy building realized at the micro scale, i.e., in a single family house in Polish climatic conditions.

#### **3. Influence of Solar Energy Availability on Architecture of a Building**

#### *3.1. Solar Radiation Conditions in Poland*

The relation between climate, and especially solar radiation conditions, and architecture of a building should be obvious [16,17]. Unfortunately, nowadays it is quite often forgotten. Any building is under the influence of solar radiation, but the solar building must pay very special attention to solar radiation conditions. In Poland the climate is moderate with the influence of continental climate. The annual average ambient air temperature, depending on the region can be around 8 ◦C to 11 ◦C. However, there are relatively large differences in ambient air temperature during the year and especially between summer and winter. Thus in summer, during the daytime, the temperature can be +30 ◦C or more, as has happened quite often recently. In winter, ambient air temperature can drop to −30 ◦C, however such a low temperature was last recorded almost 10 years ago. The annual global solar irradiation varies from 900 kWh/m<sup>2</sup> to 1200 kWh/m2. Annual solar hours are on average equal to 1600. Climate is characterized by relatively large differences in solar irradiation throughout the year. For example, in Warsaw in June, the average monthly solar irradiation is about 160–180 kWh/m2, but in December only 11–12 kWh/m2. What is also typical for the climate is the high share of diffuse radiation. The annual share of diffuse radiation usually accounts for 54–56% of the global and in winter this share is especially high and accounts for 70–80%. Only in summer is the share of direct radiation higher and can be on average equal to 60% of global radiation [18]. Figure 1 presents the averaged distribution of the average hourly global solar irradiance on averaged days of the all months of the average year. Figure 2 shows the distribution of average hourly ambient air temperature for averaged days of all months of the average year for Warsaw.

Relatively large differences in solar irradiation and ambient air temperature in summer and winter can easily be seen in Figure 1. In such climatic conditions not every solar passive system can be used in an effective way. Very uneven distribution of solar radiation during the whole year means that specific passive architectural solutions should be recommended [19].

**Figure 1.** Distribution of average global solar irradiance for every hour on averaged days of different months of the average year in Warsaw.

**Figure 2.** Distribution of average hourly ambient air temperature for averaged days of different months of the average year for Warsaw.

#### *3.2. Bu*ff*er Space Incorporated Into Interior of a House as a Specific Solar Passive Architectural Concept*

The single family solar house presented here has been designed and constructed with particular attention to passive and active utilization of renewable energy, mainly solar. Availability of solar energy with regard to the climatic conditions and specific location of the building has been considered. It can be mentioned here, that even if the climate is the same, the conditions in a city center are different than in the suburbs and in the country side in the vicinity. The considered house is located in the suburbs of Warsaw. The location of the house was specially selected so that in winter the southern facade of the house is fully exposed to solar radiation. Deciduous trees were planted on the south-east and south-west sides creating shade on these sides in the summer. The south is completely open, as it is beneficial in winter. Elements of building architecture provide shading in summer, as is described in the next section. A plan of the first floor of the house is presented in Figure 3. The main living space area is marked with the red lines.

**Figure 3.** A plan of the first floor of the considered house, main living space area is delimited by red lines.

The south transparent solar buffer space of a special design and north opaque buffer space are described below. Buffer spaces, as the name indicates, create a kind of a buffer between the outdoor and indoor climate. The southern transparent glazed solar buffer space allows solar radiation to penetrate the interior of the house in a planned way. Solar radiation can be fully utilized in winter and significantly reduce the space heating demand, whilst in summer, to reduce solar energy gains, the special architectural form of the southern buffer space is needed. A glazed solar buffer space is incorporated into the interior of the building. This specific architectural concept of a solar passive system is shown in Figure 4. It can be seen that the buffer space contains two theoretical cuboid sub-spaces. The external one is higher and the internal one lower. There is no partition between them. The solar buffer space has external and internal glazed vertical surfaces. External, four meter high glazed partitions are in direct contact with the ambient surroundings on one side, and with the interior of the buffer space, on the other. Internal glazed partitions (regular windows) are at one side in direct contact with the interior of the buffer space on the one side and with the interior of the main living space on the other.

The key architectural concept of the southern glazed façade of the house is to design and plan two overhangs at the south side of the building. The first external overhang is just a regular one being a part of the roof (marked with a symbol E). The internal overhang (marked with a symbol I) is a part of the internal construction of the building. The main point is to properly design (place and size) the internal overhang in accordance with the sizing of the external overhang to protect the interior of a building against too much solar energy gains in summer and to allow solar radiation to penetrate the interior of the house without any obstacles in winter. Of course the size of the external south glazed facade is taken into account. The external overhang of the roof (E) has been designed to shade the buffer space for a few noon hours (between 10 a.m. and 4 p.m.) in the warmest part of the year, i.e., from May to the end of August, but not to block the access of solar radiation in the rest of the year. The internal overhang (I) is formed by part of the floor on the second floor being, at the same time, a part of the ceiling of the first floor (over the lower part of the buffer space, as can be seen in Figure 2). The internal overhang has been designed to allow direct solar radiation to enter fully into the interior space of the house in winter, exactly from November to the end of February, and to fully block the direct solar radiation penetration from May to the end of August. In the remaining months

of the year, solar radiation reaches the interior directly in the morning and afternoon. As has been mentioned, the buffer space has full access to solar energy from October to the end of April and partly (mornings and late afternoons) from May to the end of August.

**Figure 4.** Architectural solar passive system: (**a**) A view of the south glazed buffer space incorporated into the house; (**b**) and a cross-section of a house along the south–north direction with apparent shading planes.

To have planned such access to solar radiation, the size of the buffer space, the size and position of different partitions of the buffer space, including the size of the external and internal overhangs, have been determined on the basis of the astronomical relationship between position of the Sun and the different partitions of the buffer space (glazed and opaque) [20]. Calculations of solar energy availability have been conducted with a time step equal to one hour throughout the whole year. The anisotropic diffuse solar radiation HDKR (Hay – Davis – Klucher – Raindl) model has been applied to determine solar irradiation of surfaces of different inclination and orientation [21].
