Reduction of Heating Consumption in the School Building During Distance Teaching

. Thermal comfort is one of the basic human needs. There are many options for transforming primary energy sources into heat and, in our countries today, into cold. The heat/cooling supply system needs to be optimised. The criterion is to minimise the final consumption of heat, primary energy sources and emissions, including CO 2 , while ensuring individual human thermal comfort.


Introduction
The paper evaluates the heat consumption for the heating of the main building of the Faculty of Mechanical Engineering in Bratislava (FME) at Námestíe slobody 17 in the period 2018-2021. The heat consumption was influenced by the full-time and distance form of teaching. During this period, the objective was to operate compact heat transfer stations (CHTS) and heating system circuits in such a way as to ensure the thermal comfort of staff and students while minimising heat consumption.

Description of the faculty of mechanical engineering building and the heat supply system
The total area of the seven-storey Main Building with an auditorium and gymnasium is 25 168 m2. The loadbearing structure of the building consists of a reinforced concrete monolithic skeleton and ceiling slabs. The building envelope consists of a system of vertical monolithic reinforced concrete columns filled with ceramic masonry made of perforated bricks. In the period from 2009 to 2015, the following measures were implemented to reduce heat consumption for heating the FME building: complete replacement of glazed structures, thermal and waterproofing of the roof, thermostatic and hydraulic regulation of the central heating system. FME is supplied with heat from centralised heating system Bratislava -East of Bratislavská teplárenská, a.s. The original heat transfer station was replaced in 2012 by two compact heat transfer stations (CHTS) for the Main Building and the Laboratory Pavilion. In the primary circuit, the CHTS are connected to the hot water network at temperatures up to 130/70°C with a design overpressure of 2.5 MPa. In the secondary DHW circuit the heating water is heated at a temperature gradient of 90/60°C by plate heat exchangers connected in parallel. In the Main Building, the DHW distributor distribute the hot water for heating to four DHW circuits, the auditorium + gymnasium circuit, two HVAC circuits auditorium + gymnasium, and the extended lecture theatres. In both CHTS, the hot water (HW) preparation is provided by flow-through plate heat exchangers and HW storage tanks are installed.
A control system is used to manage the operation of the CHTS and their technologies, with the return water temperature being controlled equithermally. Quantitative control in the DHW circuits is provided by circulating water pumps with frequency converters, with control dependent on the air temperature in the four reference rooms.

Heat consumption in the main building of SJF
The heat consumption (CH) for heating QCH and the preparation of hot water QHW in the main building and the pavilion of the FME STU laboratories in the years 2018 to 2021 were analysed. BAT, Inc. provided data measured by a remote control billing metre on heat consumption at intervals of 1 hour at the FME sampling point in the periods 1.10.2018 to 31.5.2021 and 1.10.2021 to 19.1.2022 [1]. Monthly heat consumption and heat cost were also available [2]. In the CHTS installed at the FME Main Building and the Laboratory Pavilion, the daily consumption for heating and the consumption of HW are depreciated by the secondary heat metres [3].
The heat consumption of QHW for HW preparation in the evaluated period accounted for 6.0% to 7.6%. Figure  1 shows the CH in the Main Building and Laboratory Pavilion of the FME STU in each month of 2018 to 2021.
For each area j of the building envelope FME, the heat loss Pq is the heat transfer which depends on the summed heat transfer coefficient kj, the area Sj, and the difference between the indoor air temperature ti, and the outdoor air temperature te.
Simplistically, it can be stated that the heat loss due to heat transfer varies from day to day during the heating period, depending on the temperature difference ti -te. Assuming that the indoor temperature is ti = 20 °C, the temperature difference ti -te can be replaced by the number of day-degrees D20. The number of D20 during the months January 2018 to December 2021 is shown in Figure 2. Between 2018 and 2020, the number of day-degrees was 2,856 to 2,964 K.day, 3,299 K.day in 2021. The consumption of heat for heating QH is directly proportional to the number of day-degrees D20. The efficiency of heating control can be evaluated by the ratio QH / D20. Figure  3 shows the QCH/D20 ratio during the heating periods January 2018 to December 2021.
The QCH/D20 ratio was related to the form of teaching in FME STU, full-time or distance learning. In the period of the COVID-19 pandemic, the activities of the FME were influenced by the guidelines of the Chief Hygienist and the orders of the Rector. From March 2020, teaching was at distance, as well as from October 2020 to May 2021. In the academic year 2021/2022, the first 7 weeks of teaching were face-to-face. Based on the worsening epidemiological situation of COVID-19, from 8 October 2021 until the end of the winter semester on 17 December 2021, teaching was carried out by distance teaching. Examinations from 3.1.2022 onwards were mostly by distance type. The staff during the semester and examination period worked mainly in the faculty.
The mode of activities at the FME during distance teaching revealed opportunities to save energy consumption. Individual heating circuits were quantitatively controlled to maintain a maximum air temperature of ti = 22 °C in reference rooms at comfort mode during the working days. During the distance teaching and examinations, thermostatic valves were set to temper classrooms. During the attenuation modes in the economy mode, the maximum air temperature were ti = 17 °C. The attenuation modes were set as follows in terms of start and end:   It is necessary to distinguish the months of full-time teaching, i.e., January and February months in 2018, 2019 and 2020, September 2018, October, November and December in 2018 and 2019. In these months, the average values of the QCH/D20 ratio, denoted Φ(QCH/D20) [5], were calculated. In the other months of 2020 and 2021, teaching was done by distance teaching. There was an opportunity to reduce the heating consumption. Defined were the monthly heat savings for central heating: Heat savings: Costs savings on heating: ℎ = .
From Fig. 3 it can be seen that during the distance form of teaching at the FME STU the heating consumption was reduced in the months of October, November and December in 2020 and in March 2021. In 2021 the heating consumption of the QCH was reduced by 443 930 kWh and the saving of the variable component of the heating costs excluding VAT was 15 449 € (Fig. 4). Heat costs were reduced by 9.2%.

Modes of heating in the main building of FME
The purpose of adjusting the heating modes in the Main Building of the FME was to ensure the thermal comfort of staff and students and at the same time to reduce the consumption of heat for heating. Thermal comfort is mainly influenced by the temperature of the outside air, the way of controlling the central heating circuits including the damping modes, and the thermal capacity of the building. To investigate thermal comfort in more detail, thermocouples -sensors for indoor air temperature ti232, outdoor air temperature te, and wall temperature tw232were installed in reference room 232. Temperatures were sensed from 4.12.2021 to date at 15 minute intervals. Figure 5 shows the traces of the average daily air temperature values in the reference rooms ti232, exterior te, the day-degrees D20, the heat output of PCH to heat the Main Building and the Laboratory Pavilion, the ratio PCH / D20, and the ratio of heat output to the number of day-degrees [4]. The data are shown for the period from 4.12.2021 to 19.1.2021, including working days, weekends, and the Christmas and New Year period. Noticeable are the decreases in the PCH heat output and the PCH / D20 ratio during weekend lulls from 19:00 on Friday to 02:00 on Monday, and the subsequent increase in the values of these variables during Mondays. During the weekends, the air temperature ti232 decreased by 0.47 ° C to 0.69 ° C in room 232, with wall temperature decreases of 0.35 °C to 0.58 °C.
In the period 23.12.2021 to 30.12.2021, all the circuits of the central heating system were set to economy mode and air temperature was ti232 = 17 °C. The air temperature ti232 decreased by 3.40 °C and the wall temperature by 3.81 °C.
The offices of the employees who started to work on 3. In the room 232 the average daily air temperature was changed by ti232 = 4.35 °C and the wall temperature by tw232 = 4.51 ° C during the assessment period 4.12.2021 to 17.1.2022.
In Fig. 6, the time histories of the characteristic variables from 7.1.2022 to 12.1.2022 are plotted at intervals of 1 h.  The selected period is the period in which the Christmas and New Year lulls end and the working days begin. On 10.1.2022, the air temperature ta in the reference rooms gradually increased from 20.7 °C to 21.6 °C. It was necessary to heat the building intensively. The thermal output of the PCH varied from 273 kW to 1642 kW and the QHC/D20 ratio from 287 kWh.K -1 .day -1 to 1684 kWh.K -1 .day -1 . During other working days, decreases in night heating are noticeable.

Conclusion
During the 2021 heating season, teaching was conducted by distance. By regulating the CHTS and the heating circuits in the main building of the FME, the heating consumption was reduced by 443 930 kWh and the savings of the variable component of the heating costs excluding VAT was 15 449 €. Heat costs were reduced by 9.2%. The measurements of indoor and outdoor air temperature and wall temperature in the reference room demonstrated the thermal comfort of the FME building. The night, weekend, Christmas and New Year's damping modes were tested.