Influence of heating elements dynamics on energy savings

© 2018 The Author(s) ISSN 2062-0810 Int. Rev. Appl. Sci. Eng. 9 (2018) 2, 169–173 DOI: 10.1556/1848.2018.9.2.13 This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International License (https://creativecommons.org/licenses/by-nc/4.0/), which permits unrestricted use, distribution, and reproduction in any medium for non-commercial purposes, provided the original author and source are credited, a link to the CC License is provided, and changes – if any – are indicated. 1. Introduction Pressure to reduce the energy performance of buildings, embodied in current regulations or technical standards, forces the professional public to look for heat savings in all areas of energy management in buildings and their energy systems [10, 11]. Basic dynamic behavior of heating surfaces, or heating bodies are dealt with by a number of publications based on experiments or detailed CFD simulations [6–9]. These researches are mainly focused on basic transient phenomena and not on energy savings. Another publication presents an experimental validation of a numerical model of transient behaviour of heating surfaces by experiments [5]. Some researches dealing with energy savings achieved by the proper use of Thermostatic Valve (TRV) [2, 4] but do not treat the thermal inertia of heating bodies. There are other publications dealing with room, radiator and TRV simulations, simplifying a thermal behaviour of a heating body to a lumped model – [1], state space and thermal resistance models – [3], but are not based on up-to-date detailed CFD simulations. The present contribution focuses on the quantifi - cation of energy savings in the dynamic operation of heating radiators (HR) using TRSYS simulations of room and radiator thermal behaviour based on CFD simulations. This paper compares two heating bodies in a heated room under chosen dynamic operation conditions. 2. Theory of dynamic heating of buildings In general, the need for heat output for room heating varies over time due to an infl uence of changing in heat losses, heat gains, and a user control. If the entire heating system is capable of responding accurately to these changes, the interior heating temperature is always exactly the required one and there is no temporary insuffi cient heating or overheating of the heated room. It is obvious, that every overheating is also automatically associated with a certain increased heat consumption. INFLUENCE OF HEATING ELEMENTS DYNAMICS ON ENERGY SAVINGS O. ŠIKULA 1,*, O. LADJEDEL2, P. CHARVÁT 3, L. ADJLOUT 2, S. A. REFFAS 2 1 Faculty of Civil Engineering, Brno University of Technology, Veveří 331/95, Brno 602 00, Czech Republic 2 Mechanical Engineering Faculty, Université des Sciences et de la Technologie d’Oran Mohamed Boudiaf, Algeria 3 Faculty of Mechanical Engineering, Brno University of Technology, Czech Republic * E-mail: [email protected] Dynamic heating computer simulations of one model typical living room heated alternatively by two types of heating bodies are presented in this paper. This contribution describes a numerical model of two heating elements (plate radiator and a new type of convector) showing different thermal inertia by using the TRNSYS software. The results show energy savings approximately of 10% for the new tested convector, where the thermal comfort is better in terms of reaching the required room temperature. Keywords: heating radiators, thermal inertia, energy savings, software TRNSYS Unauthenticated | Downloaded 01/18/23 12:23 PM UTC 170 Int. Rev. Appl. Sci. Eng. 9, 2018 O. ŠIKULA et al. From the physical point of view, all the heating surfaces exhibit, differently, the heat inertia rate during heating up and shut-down, and thus show a certain degree of under-heating or over-heating in a room. It is obvious that the greater the thermal inertia of the heating surface is, the more signifi cant the above mentioned phenomena are. 3. Defi nition of the case The effect of hot water radiators thermal inertia was assessed by computer simulation of the thermal behavior for a typical room using the TRNSYS computer simulation code. 3.1. Specifi cation of simulated radiators Two different types of radiators – plate radiator [13] and a convector, [14] – of, about the same, heating output were chosen. The basic specifi cation and comparison are shown in Table 1. It is worth noting, it is clearly seen that there a twice bigger compactness of the convector (heating output per m3 of radiator volume). Quantitative regulation of heating output is envisaged by a thermostatic valve for both radiators. The thermostatic valve is, for simplifi cation, considered to be responding extremely quickly to the changing required temperature of the interior. It is assumed that we have a heating water at a constant temperature of 75 °C at the radiator inlet. These simplifi cations are taken to exclude the control system infl uence and to be considered only the infl uence of the radiator thermal inertia/dynamics. 3.2. Specifi cations of the heated room The chosen room today is a common living room with 2 chilled walls and a window with a fl oor area of 20 m2. The thermal and technical properties meet the requirements of the thermal resistance standard, and ventilation with infi ltrations with a constant air exchange rate of n = 0.5 h–1 is considered. 3.3. Specifi cation of boundary conditions Simulations of 2 cases were performed – further [17]. Initially, a simplifi ed shutdown simulation (jumpoff of the heating water to the radiators) was performed to compare the maximum unwanted room overheating, due to their different heat capacity, at a constant room temperature of 20 °C. Subsequently, a thermal behavior and heat consumption simulations were performed for the typical winter day. A sunny January day with an average outdoor air temperature of +2.2 °C in South Moravia was chosen to simulate thermal behavior. In the room the desired interior temperature was selected as shown in Table 2. The initial temperature of the room has been set to 19 °C. The dynamics of the thermal behavior of the compared radiators were determined by the detailed non-stationary CFD simulation by their heating up and shut-down. The obtained results in this CFD simulation were used to create simplifi ed 1D models of both OTs in the TRNSYS software. The simplifi ed model can, among other things, distinguish thermal energy supplying by heating water to the radiator and the heating energy supplied by the radiator to the room. 4. Results and discussion A comparison of the heat capacity of the radiators and the thermal energy delivered to the heated room during shutdown (jump off of the heating water supply) at a constant ambient temperature of 20 °C is shown in Table 3. The obtained value of 77% is the theoretical, highest achievable heat savings when the convector is Table 1. Specifi cation of simulated radiators Specifi cation Type Heating output at 75/65/20 °C Dimensions (height / length / depth) Compactness Connecting way [W] [mm] [kW/m3] RADIK 20S CLEAN 80/90 cm Plate radiator 1170 900/800/100 16.3 double-sided, diagonal Tomton R1 with forced convection Convector 1216 570/502/135 31.5 bottom connection Table 2. Distribution of required temperature during a typical day Activity Sleep Morning activity Non occupied Evening activity Time [h] 21:00 – 4:00 4:00 – 6:00 6:00 – 16:00 16:00 – 21:00 Required temperature [°C] 19 °C 20 °C 17 °C 20 °C Unauthenticated | Downloaded 01/18/23 12:23 PM UTC Int. Rev. Appl. Sci. Eng. 9, 2018 171 INFLUENCE OF HEATING ELEMENTS DYNAMICS ON ENERGY SAVINGS However, the most signifi cant result is the heat savings of ~10% when the convector is used instead of the plate radiator. The course of all signifi cant variables for both heating modes of the room is shown in Fig. 1. The green areas present the heat savings using the convector, the red areas present the heat savings using the plate radiator. 5. Conclusion The achieved results of the selected room by a more dynamic radiator (convector) show in this case: – Lower heat loss of the room by transmission; – Lower heat losses of the room by infi ltration; – Higher degree of utilization of solar gains of the room of ~3%; – Lower average room overheating by ~0.2 K; – Heat energy savings for heating ~10 %. used instead of the plate radiator during one shutdown course (theoretical). The saving will always depend on the dynamics of changes in the required heating output in the reality. The smaller the changes in reality become, the smaller the savings will be. The results of the simulations for the typical winter day are shown in the following tables and graph. Table 4 shows both the desired and achieved temperatures and the average room overheating rate. It can be seen that a room heated by a more dynamic radiator – convector shows a more precise adherence to the desired room temperature and also a lower average unwanted overheating. Table 5 summarizes the essential components of the room’s thermal balance over the test day. The results show that, although plate radiator has slightly lower heating output (compared to the convector), due its higher thermal inertia, it supplies more heat to the room, hence increases its overall heat loss. Because of the higher average room temperature obtained, when we are using the plate radiator, a lower degree of utilization of solar gains of the room is observed. Table 3. Heat capacity of radiators and the corresponding amount of heat delivered to the room Thermal capacity Duration of the shutdown 3 000 s Duration of the shutdown 10 000 s CE E [J/K] [kWh] [kWh] Plate radiator 38 259.04 0.082 0.600 Convector 8 845.90 0.020 0.148 Energy saving [%] 77% 76% 75% Table 4. Temperatures and overheating during the test day OT Room temperature at 0 h Room temperature at 24:00 Room temperature real average per day Room temperature required average per day Average room overheating per day TT T T ∆T [°C] [°C] [°C] [°C] [K] Plate radiator 19.00 18.69 18.74 18.33 0.41 Convector 19.00 18.24 18.55 18.33 0.21 Table 5. Heat energy balance for the test day Heat delivered to the radiator Heat delivered to the room Transmission heat losses of the room Ventilation heat losses of the room Solar heat gains Room energy change per day EE E EEE [kWh] [kWh] [kWh] [kWh] [kWh] [kWh] Plate radiator 12.64 12.40 9.07 2.00 0.073457 1.40 Convector 11.16 11.16 8.97 1.98 0.075702 0.29 Saving [%] 11.7% 10.1% 1.2% 1.2% 3.0% 79.4% Unauthenticated | Downloaded 01/18/23 12:23 PM UTC 172 Int. Rev. Appl. Sci. Eng. 9, 2018 O. ŠIKULA et al. 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