*3.3. Northern Bu*ff*er Space*

Analysis of solar energy availability in buildings also requires designers to pay attention to that part of the building envelope, which is not exposed to solar radiation, especially in winter. It is the northern part of the building enclosure which requires this thoughtful approach. In the considered climate, with cold winters and warm (or even hot) summers, a highly opaque insulated buffer zone should be created at the northern part of the building to reduce the effect of the severe climatic conditions in winter. The external north partitions must be characterized by very high thermal resistance (0.5 m thick walls, which include 0.25 m thick insulation) and no windows or any other transparent elements.

Figure 5 presents such a north façade, which is a façade of the considered low energy solar house.

The northern walls and roof are elements of the buffer space. Another important feature is to create usable spaces which do not require heating energy, because they are not for permanent residence of the inhabitants. The air temperature in that space can be, or even is required to be, lower than in the living space of the house. It is possible to plan a cold store, pantry, wardrobe, garage or boiler room there. Northern fully opaque and highly insulated buffer zones can significantly reduce the influence of severe climatic conditions in winter and have a positive impact on the energy balance of the building.

Planning cold and solar buffer spaces in a building allows introduction of temperature zones into the building in a natural way. Due to the architecture of the building, natural passive control of thermal comfort is created. In this way the following air temperature zones are designed:

• A cold northern zone with a seasonally variable indoor air temperature, daily changes of temperature are very small;


**Figure 5.** A view of the northern buffer space of the house.

Thus, the architecture of the building creates a smart concept for building structure focused on the planned use of the environment to reduce the need for heating and cooling energy for the living spaces. The temperature zones go from the north, always with the lowest indoor air temperature, to the south, with most variable indoor air temperature during daylight hours, the highest in summer, high or moderate in spring and autumn and high or low in winter. The thermal state of the southern buffer space is directly impacted by the solar radiation and by ambient air temperature. However, the radiation is the dominant component. In such a way a smart solar low energy concept of a house can be created. When the energy needs of a house are accomplished through the smart and energy efficient design of a building then highly energy efficient devices and installations can be introduced.

#### **4. Building Energy Needs and Final and Primary Energy Consumption**

#### *4.1. Space Heating Energy Needs of the Considered House*

Usually, to reduce building energy consumption, two main measures are applied. The first is focused on improvement of the thermal performance of the building envelope by adding insulation and reducing infiltration rates, and the second is on improvement of energy efficiency of the devices and installations used in buildings, including lighting, heating, ventilation and air-conditioning [22]. However, to reduce the energy consumption significantly the design of the building cannot be based only on reduction of heat and mass flow through the building envelope (through improvement of insulation and reduction of infiltration). The architecture of a building is crucial and it should take into account specific climatic and environmental conditions with a focus on solar energy availability specific for the given climate and location of the building. The single family house presented in this paper was designed using this wide holistic approach to reduce energy needs, as has been described in the previous section. In this section the results of calculations of the energy balance of the building and the final and primary energy consumption are presented. Simulation studies have been performed using our own simulation code. The availability of solar radiation and its impact on the energy needs of the building have been determined. Modeling of the space heating and DHW needs have been based on the methodology on determination of energy performance of buildings [23].

The floor area of the heated space of the house is 350 m2. In addition to two floors, the building has an unheated attic (there is no basement). The roof directed to the south and north is inclined at 30 degrees. The northern slope is larger than the southern one, required by the specially designed north unheated buffer space. The south façade of the building is transparent in 70% of its total surface area, which in turn is related to the design of the south solar buffer space. There are two types of windows. Large windows form the main part of the façade and their heat transfer coefficient *U* is equal to 1.2 W/(m2K). The *U* value of the other regular windows and internal windows of the south buffer space is 1.4 W/(m2K). The main idea of the building design was to use standard construction materials, typical nowadays for such a climate, to make the building envelope energy efficient and reduce heat transfer with ambient surroundings. External walls are 0.5 m thick (two layers: mineral wool from outside and bricks from inside). The heat transfer coefficient *U* is equal to 0.14 W/(m2K). The *U* value for the ceiling over the second floor, under the unheated attic, is lower and accounts for 0.12 W/(m2K), but the heat transfer coefficient for the floor on the ground is larger and is equal to 0.17 W/(m2K).

The energy balance of the building was formulated and space heating energy needs were calculated. The results are shown in Figure 6. This figure presents monthly space heating energy demand with all components of the energy balance of the building. Thus all heat losses through the building envelope and ventilation (positive values), as well as heat gains: internal and solar (negative values), are presented. It can be noticed that ventilation and heat losses through windows dominate among all others, but it is also evident how large the solar gains are and their impact on the energy balance of the building.

**Figure 6.** Monthly space heating demands with energy balance components: heat losses and gains.

The main observations on heat losses from Figure 6 are confirmed by the diagrams in Figure 7 which presents the seasonal share of different heat loss components in the total heat loss of the building. As could be expected for a low energy house, the largest losses occur through ventilation and then through windows, and they account for 34% and 29% of the total losses, respectively. In the third place there are heat losses through walls (15%) and all the others take nearly the same share (6–7%) (heat losses through doors are lower at 3%).

It can be noticed that the heat energy demand for the ventilation system results from the natural necessity to exchange air in the building, including exchange for hygienic purposes in rooms such as kitchens and bathrooms. In the building under consideration, the heat demand is significantly reduced due to the use of heat recuperation. In the calculations of energy demand the use of a recuperative unit was taken into account when determining the final energy consumption. Heat recuperation requires a forced ventilation system to be used. The design of the forced ventilation system is usually taken into

account at the design stage of the building and its interior, and this was the case with the building under consideration. Ventilation ducts supplying fresh air, which is preheated in a recovery unit, as well as ducts discharging used air outside through the recovery unit, were planned at the time of creating the architectural design of the building. It can be mentioned that the heat recovery ventilation unit is not used all the time, but only when inhabitants are at home. For the remaining months outside the heating season only short-term morning and evening ventilation is used.

**Figure 7.** The seasonal share of heat losses in the total amount of heat losses of the building.

The calculated index of the annual space heating energy needs amounts to 36.16 kWh/m<sup>2</sup> of the heated floor area. The low space heating energy demand results from the architectural concept of the building, the introduction of temperature zones resulting from the existence of buffer spaces at the northern and southern sides of the building, including the passive use of solar radiation energy. Low energy demand also results from the use of appropriate building materials of high thermal insulation and thermal capacity. The high thermal capacity is demonstrated by a building time constant, equal to 222 h.

According to existing regulations [23], when the energy performance of a residential building is calculated the energy consumption for domestic hot water (DHW) heating is also taken into account. For the considered house, the annual total heat demand index (for DHW and space heating, which is only seasonal) is equal to 45.6 kWh/m2. The annual DHW head demand is equal to 3011 kWh (heat demand index for DHW is equal to 9.44 kWh//m2). Up to now there have not been any official regulations introduced to limit these energy-need indices of buildings; this is a problem which does not help in significantly reducing the energy consumption of buildings, as the authors try to present in this paper. There are limits only for the heat loss coefficients of walls, e.g., for walls *U* was equal to 0.3 W/(m2K), now it is 0.25 W/(m2K). It can be mentioned, that 10 years ago when the house was constructed according to the obligatory regulations, buildings (of similar compact shape) required a maximum of 90 kWh/m<sup>2</sup> of final energy and 69 kWh/m<sup>2</sup> of primary energy consumption (regulation [10], before amendments in 2013). Nowadays, these indices are even higher, because the official limits stated for primary energy consumption for a single family house is equal to 95 kWh/m2. Since the beginning of the 2021, even if a new house is to be called "nearly zero energy" the index for primary energy consumption will be at a level of 69 kWh/m2.

#### *4.2. Final Heating Energy Consumption of the Building*

In order to determine the final energy consumption, it is necessary to take into account the energy efficiency and effectiveness of energy devices and installations used to cover heating needs, as well as their time of operation. In addition, the work of auxiliary devices necessary for the operation of heating systems, such as circulation pumps in liquid circuits and fans in air circuits also have to be taken into account.

Operation of heating systems in the low energy house is of course based on using energy efficient devices and installations. Heating demand is met by a ground source heat pump with vertical heat exchangers coupled to a solar thermal systems with flat plate solar collectors via a buffer storage tank. The buffer storage tank has a smaller DHW tank inside. In this way water in the main volume of the buffer tank not only serves as a storage medium, but also as thermal insulation for the water in the internal DHW tank. DHW is preliminarily heated in the internal tank and then it flows to another DHW tank with an auxiliary heat source (electric heater). Solar collectors are integrated into part of the south roof surface. A low temperature underfloor space heating system is used. The flow is forced by a pump into every loop of the system. As has been mentioned, the heat recuperation unit is also applied. Fans are used to force the flow of fresh and used air through the ventilation ducts. The main characteristics of the heating system are presented in Table 1.


**Table 1.** Main (elements) devices of the heating system of the house and their main parameters.

The operation of the solar thermal collectors and the heat pump is not directly connected. Both devices operate in parallel. They can operate at different times of the day, but they can also supply heat at the same time. Flat plate solar collectors supply heat to the main buffer storage tank (via a heat exchanger). An antifreeze mixture circulates in a solar collector loop. There is another loop with an antifreeze mixture circulating in vertical ground heat exchangers, which are coupled with the evaporator of the heat pump. Heat can be sent to the buffer storage. It is also possible to supply heat directly from the heat pump to the underfloor heating system or DHW tank (without charging the storage tank). The buffer storage tank with water as a storage medium contains a small tank inside. The small tank is used as a buffer tank for the DHW system. Cold water is supplied to the small tank and when the water is heated up it flows out of the tank and is transferred to the other tank, which is the main DHW storage tank (50 l volume) equipped with an auxiliary electric heater. The ground source heat pump is used only during heating season. Domestic hot water out of the space heating system is accomplished via the solar thermal system operation, which operates very effectively in Polish conditions [18,24]. From May until the end of September the thermal solar energy system can cover all DHW heating demand. In March, April and October, solar energy provides about 60–70% of demand, in winter the share of solar energy is very small and it does not exceed 10% for the DHW and space heating. Figure 8 presents the print screen of a display showing the operation of the ground heat pump system coupled with the solar heating system for space heating and DHW heating.

**Figure 8.** The print screen displaying the configuration and operation of the ground heat pump system coupled with solar heating system.

A micro-scale energy managemen<sup>t</sup> system is used in the house. Operation of the heating systems is controlled by a central system that continuously monitors the operation of all heating devices and systems. Several variants of operation are possible depending on the availability of the given renewable energy source in time and its adherence with the energy demand in that time. The system is equipped with a number of sensors enabling on-line observation of the system's operation as well as its control. Through a dedicated computer application, it is possible to remotely change the temperature settings in the rooms. It is also possible to change the parameters of the system operation, mainly temperatures and flows, and even completely turn off the operation of individual devices or installations. Operation priorities are set according to the efficiency of energy conversion from a given energy source and the effectiveness of using that energy at a given time. The automatic control system helps in a smart way to ensure the highest energy efficiency in gaining the available renewable energy and consuming it in an effective way.

Figure 9 presents the distribution of monthly space heating final and primary energy consumption for comparing the distribution of monthly space heating needs. It can be noticed that monthly space heating needs are presented in two graphical forms. The highest bars show total energy demand with standard ventilation needs, as shown in Figure 4 (where ventilation is a dominant factor of energy demand). The lower bars (colored red also show the total energy needs, but the demand is much reduced due to application of the heat recuperation ventilation system. The smallest bars represent final energy consumption. It is evident that final energy consumption is really very low thanks to the highly energy efficient energy systems and mainly because of using a heat pump that has been well selected for the given operating conditions and operates with high energy performance (SCOP (Seasonal Coefficient of Performance) nearly equal to 5 after nearly 10 years of operation).

The seasonal index of the final energy demand for space heating accounts for 4.61 kWh/m2, which is very low. As has been mentioned, determination of the energy performance of any residential building requires taking into account only the heat consumption of the building for space heating and DHW. The so called annual index of energy consumption includes the annual DHW heating and space heating, while the space heating takes place only during the heating season, and for the considered house it lasts only four months. Thus the final annual energy demand index for space heating and domestic hot water is 11.58 kWh/m2, which is still a very low value even if the electric energy consumption by the auxiliary devices of the heating loops (like pumps and fans) is included. It can be mentioned here, that in Poland a building can be classified as a low energy building, when its final annual energy consumption (for all heating needs) amounts to 30–60 kWh/m2. Such a range of indices was proposed in 2007 [25] and is still used [26].

**Figure 9.** Monthly final and primary energy consumption and energy demand for space heating (bars for ventilation with heat recuperation and without are shown).
