Energy efficiency by use of automated energy-saving windows with heat-reflective screens and solar battery for power supply systems of European and Russian buildings

The new energy saving windows with heat-reflecting shields have been developed, and for their practical use they need to be integrated into the automated system for controlling heat supply in buildings and the efficiency of their use together with the existing energy-saving measures must be determined. The study was based on the results of field tests of windows with heat-reflective shields in a certified climate chamber. The method to determine the minimum indoor air temperature under standby heating using heat-reflective shields in the windows and multifunctional energy-efficient shutter with solar battery have been developed. Annual energy saving for the conditions of different regions of Russia and France was determined. Using windows with heat-reflecting screens and a solar battery results in a triple power effect: reduced heat losses during the heating season due to increased window resistance; lower cost of heating buildings due to lowering of indoor ambient temperature; also electric power generation.


Introduction
Pursuant to Russian legislation, annual specific consumption of energy in buildings as of 1 January 2020 shall be reduced by 40% of the basic level. In France, the thermal regulations RT, which set reduction of energy consumption in buildings, were adopted. Annual energy consumption per 1 square meter of building Q, kWh/(m 2. year) is the regulated value. The requirements of the Act Grenelle (Loi Grenelle), dated August 03, 2009 and the thermal regulation RT 2012 allowed to build buildings with low energy consumption (BBC, Q <50 kWh / (m 2. year) depending on region of France) with January 1, 2013 and with January 1, 2020 -only buildings with "positive energy» (Bâtiment à énergie positive, BEPos), i.e. with positive balance, such as the production of electricity (often due to photovoltaic method). Energy efficiency class "A" is assigned to such buildings.
Existing European and Russian energy efficiency regulations stipulate strict requirements regarding annual energy consumption, and particularly, the heat transfer resistance coefficient of translucent structures. Thus, European Union legislatures stipulate a coefficient of heat transfer resistance for windows by 2020 of 1 use of screens is desirable during nighttime or in the absence of people. Screens may be placed inside or outside buildings, or between window panes. The use of screens not only reduces losses related to heat transfer but also permits ambient temperature reduction in setback heating mode. There is a need for creating a system of automated window screen control, and also for methods of determining ambient temperature in setback heating mode.

Determining Minimal Ambient Temperature in Setback Heating Mode
A certified environmental chamber belonging to "Ivanovostroiispytaniya" (an independent non-profit making organization engaged in construction engineering) was used [1] to study the impact of using heat reflecting window screens on raising heat transfer resistance in windows and reducing heat losses. For control purposes, two double-glazed windows, made according to formula 4М1х10х4М1х10х4М1 and 4М1х10х4М1х10х4i, respectively, with low-emission coating, were used. According to the given data (refer with Table   The current regulations (SP 60.13330.2012. "Heating, ventilation and air conditioning") with reference to buildings during the cold season when not in use or during non-working hours permit reducing indoor ambient temperature below the regulation value, but not below 15 0 C in residential buildings, 12 0 C in public buildings and 5 0 C in the case of industrial premises. In France, according to the regulations it is not allowed to decrease indoor ambient temperature below 16 0 C for residential, public and administrative and service buildings.
Indoor temperature reduction in the setback heating mode creates a high energy saving potential. Minimal ambient temperature in setback heating mode is to a large extent determined by the conditions necessary to prevent condensation on enclosing structure surfaces. We note the high probability of condensation on windows, because translucent structures are the weak spot in heat insulation of buildings.
It should be noted that the appearance of moisture on glass surfaces is not only an esthetic defect, as continuing condensation may eventually result in moistening of the structures [2], with possible formation of fungi and mildew on window sills. Particular attention should be paid to horizontally positioned and slanting windows, also skylights, as pursuant to Current regulations (SP 50.13330.2012 "Buildings Heat Insulation"), the inner surface temperature should not be lower than indoor ambient dew-point temperature at the assumed outdoor ambient temperature during the cold season.
Relative humidity inside buildings RH is a regulated value (30 % to 65 % for public and residential buildings), while air humidity, window heat transfer resistance and outdoor ambient temperature are the factors determining dew-point temperature along internal glazing, and, therefore, the minimal ambient temperature in setback heating mode (at a given heat transfer coefficient for inner window surfaces). As shown by calculations and experimental data (refer to Table 1), the use of external heat-reflecting screens increases heat transfer resistance up to 1.76 m 2.0 С/W with a significant increase in internal glass temperature, thereby enabling us to further lower indoor ambient temperature (depending on indoor humidity) in the absence of people. We were interested to know how to determine the value (refer with Fig. 1) to which an automated window control system may lower the ambient temperature in the setback heating mode while excluding condensation on internal window glass surfaces with enhanced heat insulation properties (using heat-reflecting screens).  In the course of mathematical transformations, it was deduced that minimal indoor ambient temperature in setback heating mode may be determined by: Thus, knowing the window resistance value window R during non-working hours, outdoor ambient temperature, ambient temperature and humidity during working hours we can determine minimal ambient temperature in setback heating mode.
In accordance with formulae obtained in Mathcad and Excel computing environments, we prepared a programme for calculating the minimal ambient temperature in setback heating mode and using windows with the heat-reflecting screens. We simulated changes in the resistance of the translucent part of Window R from 0.5 to the resulting experimental value of 1,757 м 2.0 С/W, while setting outdoor ambient temperature t out from -10 to -30 0 С, and ambient temperature during working hours at 20 0 С. The calculation was made for relative humidity RH equal to 35 and 50%. The results of these calculations may be seen in Fig. 2. As it is expected, the in b s t . will be significantly higher with higher indoor humidity. A significant difference also results from the use of screens. Thus, according to calculations, with relative humidity of 35%, outdoor ambient temperature of -30 0 С and the use of I-glass windows with two heatreflecting screens, minimum permissible ambient temperature in setback heating mode falls from 16.4 0 С to 7.8 0 С, that is, by more than 8 0 С, while with relative humidity of 50% the difference in temperature is more than 10 0 С.
The authors also developed and patented a panel design featuring heat-reflecting screens using a solar battery on the external side, consisting, for example, of thin-film photocells. The use of a solar battery permits generation of electric power which not only ensures the independent operation of windows with heat-reflecting screens but also partially or completely serves the consumer's electrical load, and in the event of large-scale generation may be fed into the exterior electrical network. Moreover, the conversion efficiency of solar batteries reaches 20%.
Thus, using windows with heat-reflecting screens and a solar battery results in a triple power effect: reduced heat losses during the heating season due to increased window resistance; lower cost of heating buildings due to lowering of indoor ambient temperature; also electric power generation.

Determination of Thermal Energy Saving
After processing data received from the Russian and French Meteorological Services, the authors determined the duration of daylight hours for each month in various cities of the Russian Federation. As it is well known, it is desirable to use screens at night and in the absence of people. Hours of daylight per day (24 hours) during a calendar year in the city of Norilsk are presented in diagram form in Fig. 3. Efficiency is determined without the inertial component of walls, windows, etc, as well as power supply systems.
Staff working hours are assumed to be from 8:00 up to 17:00. Significantly, in Norilsk, December marks the onset of polar night, when the sun does not rise above the horizon, hence screens are used around the clock.  We also analyzed the use of screens in Russian and French cities representing different climatic zones.Determining thermal energy saving also required additional city data, including geographical latitude S, number of heating period days heat.per n , hours during which screens were used and not used during the heating period ( 1 n and 2 n , respectively), design ambient temperature tdes and mean ambient temperature during the heating period tmean heat. Data for the cities under consideration are given in Table 2. Despite the fact that France is located south of 52 degrees north latitude (south of the Russian Belgorod) and the climate is much warmer, heating period longer than many cities of the Central Federal District in Russia. Calculations were made for a window comprising a multiple heat reflecting I-glass unit (4М1х10х4М1х10х4I), using a panel screen consisting of two metal sheets separated by a low heat conductivity frame (Table 1). Design indoor temperature during working hours was assumed to be 20 0 С, with relative humidity of 40%. For non-working hours (nighttime) in setback heating mode two levels of ambient temperature reduction were assumed -to standardized 12 0 С (16 0 С -for France), and to the minimal permissible ambient temperature satisfying the requirement of preventing condensation on translucent structures. Heat losses for the heating period were determined per 1 m 2 of window surface. Calculation data are given in Tables 3 and 4. Thus, the maximum thermal energy saving was achieved in all cities using panel heat reflecting screens and automation systems intended to maintain setback heating mode during non-working hours at the minimal permissible ambient temperature, and in the absence of condensation. Maximum saving (0.235 Gcal) was actually achieved for conditions in the city of Norilsk.
For the southern cities of Russia and most of French cities in the case of using these proposed energy-saving measures annual consumption of thermal energy for compensating transmission losses will be minimal (from 0.11 to 0.28 Gcal / m 2 ).