Heat Gains influence on Balance Point Temperature and Thermal Comfort

Transcription

1 Heat Gains influence on Balance Point Temperature and Thermal Comfort F. Kalmár, PhD University of Debrecen, Faculty of Technical Engineering, Department of Building Services, Hungary; KEYWORDS: heat gains, balance point temperature, energy consumption, control. SUMMARY: The length of the heating season is determined based on the balance point temperature of the building using the specific degree-day curve. The orientations of the facades and the glazed surfaces have an important influence on the balance point temperature value. With higher average thermal resistance of the building envelope the influence of the heat gains on the central heating system operation will increase. At the same time the appropriate control of the heating system is necessary otherwise needless energy consumption can occur which has a negative influence on the internal thermal comfort beside the economical and environmental aspects. In this paper the influence of heat gains on the balance point temperature is presented for different building types and glazed ratio of the facades using the Hungarian specific degree-day curve. Also the heat gains influence on the heating system operation is analysed in retrofitted buildings taking into account the limits of the central or/and local control systems. 1. Introduction Analysing the energy consumption of most European countries it can be observed that the building sector is the bigger consumer with a share of 28 32% (Mantzos et al., 23). In East European countries with a temperate climate from the total energy consumption of a building heating represents more than 5%. This fact results from the low thermal characteristics of the building envelope. Taking into account the actual energy policy of the European Union and the measures related to environment protection different national projects are launched in order to improve the thermal efficiency of these buildings. In Hungary there are approximately 4 millions of households and about 3,5 millions are situated in buildings with an average heat transfer coefficient of the envelope higher than,8 W/(m 2 K) (Csoknyai, 24). In such conditions when a building is retrofitted from energy point of view important energy savings could be obtained. Due to the financial limits there are cases when a complex building rehabilitation (building envelope and heating system) could not be realised. In these situations intermediate solutions have to be found in order to obtain appropriate thermal comfort at minimal energy consumption. Thousands of buildings/flats are connected to district heating systems. In Hungary at these systems the heating process is started when the mean outdoor temperature is lower then 12 o C three consecutive days, (Halász, 21). That means comfort problems in buildings where the real balance point temperature differs from the assumed value. Improving the thermal resistance of the building envelope beside the lower U value a shorter heating period is obtained (Kalmár, 24). If the heating system is not redesigned and completely changed the existent one has to be controlled properly otherwise the expected energy savings could not be obtained. There are cases when the output of the heating system radiators is controlled only by thermostatic valves. Practice has shown (Petitjean, 1997) that in such situations the thermostatic valves could not operate properly because a stable degree of opening cannot be found. It then works in on/off mode with oscillations in the room temperature. Changing the heat carrier temperature such situations may be avoided. If the temperature drop is kept constant depending on the rehabilitation level the optimal forward temperature could be determined (Kalmár, 23). At the same rehabilitation level in function of the room position the heat load reduction is different. Thus, in rooms with different heat loss coefficient the required forward temperature differs from the optimal value. If the required forward temperature is higher the radiator surface has to be increased. If the required temperature is lower the difference is controlled by the thermostatic valves. As it could be seen in the following if the thermostatic valves are not chosen and set correctly the operation will be unstable.

2 2. Balance point temperature of a building Neglecting the variation of stored heat, during the heating system operation, the condition of thermal balance is: Q Q + Q + Q + Q + Q = (1) t + tb v s i h where: Q t are the transmission heat losses; Q tb heat losses by thermal bridges; Q v heat losses by ventilation; Q s solar gains; Q i internal gains; Q h heat delivered by heating system. The heat to be delivered by the heating system depends on the climatic conditions and the internal set point temperature. The main goal is the minimization of the heat load and the energy consumption. From energy point of view the achievement of the internal set point temperature without auxiliary heating is favourable. When the heating system does not operate due to the solar and internal gains the internal temperature would exceed the external one. The balance point temperature is the external temperature when the heat gains are equal to the heat losses: t b Qs + Qi = ti (2) K where: K is the heat loss coefficient of the building; t i internal set point temperature. Using the balance point temperature value and the specific degree day curve the length of a heating season could be determined (fig. 1) tem, [oc] days FIG. 1: Determination of the number of days in a heating season. Using the geometrical interpolation method the degree-day curve can be approximated with a function, (Kalmár, 22): t e +,3835 = 15 3,55x (3) In this equation if the external temperature is equal to the balance point temperature the number of days in a heating season could be obtained: 2,6 t = b t N e (4) 3,55 where t e is the external design temperature. After building rehabilitation the heat loss coefficient will decrease considerable (K ) whereas the heat gains remain the same. Thus, the new value of the balance point temperature could be calculated with the following relation: ' ti tb tb = ti (5) ' K K

3 Having the value of the balance point temperature after thermal rehabilitation the number of days in the new shorter heating season is: ti tb ti te ' K ' N = N K (6) tb te The variation of balance point temperature and number of days in the new heating season in function of the rehabilitation level when the original value of balance point temperature is 12 o C is presented in Figure 2. (tb-tb')/tb; (N-N')/N, [%] (tb-tb')/tb 5 (N-N')/N (K-K')/K, [%] FIG. 2: Number of days in a heating season and balance point temperature. The heating energy demand for a building could be written as: N E = K ( ti te ) dx (7) So that using the (5)-(7) equations the ratio of the energy consumption before and after rehabilitation could be determined as follows:,3835 [( ti te ) 2,566N' ],3835 ( t t ) 2,566N E' K' N' = (8) E K N [ ] i e If the original value of the balance point temperature was 12 o C the variation of the energy consumption ratio depending on the rehabilitation level is presented in Figure 3. 1,8 Q'/Q; E'/E,6,4,2 Q'/Q E'/E 1,95,9,85,8,75,7,65,6 K'/K FIG. 3: Heat load and energy demand variation.

4 3. Heat gains and heat demand Heat gains of a building are constituted from solar gains and internal gains. The solar gains are influenced by the orientation of the facades, by season and meteorological conditions, by the glazed surfaces area and its gain factor. The internal gains are the sum of heat emitted by occupants, lighting, household appliances etc. So that the internal gains could be assumed to be constant but the bigger the flat/detached house area is the lower the specific heat gain values are. At different building types to obtain the balance point temperature equal to 12 o C the calculus have to be made assuming different specific heat gain values even if the building elements have the same thermal properties. The variation of heat gains in function of A/V value taking into account different glazed ratio of the external walls is presented in Figure Uwall=1,5 W/m2K; Uwin=2,5 W/m2K Qgains, [W/m2] % 1 3% 5,2,3,4,5,6,7,8,9 1 1,1 A/V FIG. 4: Specific heat gains for t b =12 o C. As it could be seen at a detached house with an A/V value equal to 1 the necessary specific heat gains have to be almost twice that in the case of a block of flats (A/V=,4). This is practically impossible, so that at the same value of the specific heat gains the balance point temperature will be higher at buildings with higher values of the A/V factor. Also for the studied interval the influence of the glazed area variation (15 3%) could be neglected. In Figure 5 the variation of the balance point temperature is presented assuming 2 o C the internal set point temperature. tb, [oc] Uwall=1,5 W/m2K;Uwin=2,5 W/m2K A/V=,2 A/V=,4 A/V=,6 A/V=,8 A/V=1, Qgains, [W/m2] FIG. 5: Balance point temperature for different building types (3% glazed area of the facades). The calculus was made for the worst values of the external building elements. As it could be seen there are important differences between balance point temperature values which results in shorter or longer heating season. The length of the heating season depends also on the thermal properties of the building envelope. Assuming that before thermal rehabilitation, for a building, the balance point temperature was 12 o C, in Figure 6 the variation of the heat demand is presented during the heating season.

5 Q/Qmax A/V=,4; Uwall=1,5 W/m2K; Uwin=2,5 W/m2K 1,9,8 Q/Qmax,7 (Q-Qgains)/Qmax,6,5,4,3,2, N, [days] FIG. 6: Heat demand variation before thermal rehabilitation. After thermal rehabilitation the heat demand ratio is the same but as it could be seen the heating season will be shorter taking into account the heat gains (fig. 7). Q/Qmax A/V=,4;Uwall=,5 W/m2K; Uwin=1, W/m2K 1,9,8 Q/Qmax,7 (Q-Qgains)/Qmax,6,5,4,3,2, N, [days] FIG. 7: Heat demand variation after thermal rehabilitation. Qgains/Q, [%] A/V=,4 2 Uwall=1,5 W/m2K; Uwin=2,5 W/m2K] 1 Uwall=,5 W/m2K;Uwin=1, W/m2K N, [days] FIG. 8: Heat gains ratio from the total heat demand.

6 When the heating system is dimensioned the heat gains are neglected so that the system will operate at partial capacity during the whole heating season. Furthermore, as it could be seen in Figure 8, 6 8% of the heating season the heat gains cover more than 5% of the heat demand. 4. Control of the heating system If a building envelope is retrofitted from thermal point of view and the heating system is not redesigned and totally changed the existent capacity has to be adjusted to the new heat load values. Depending on the room position the heat loss coefficient reduction is different. Having the original radiator surfaces and the new heat demand values the new forward temperatures could be determined assuming that the temperature drop is kept constant. Thus, in different rooms different forward temperature values will be obtained. Analysing these values for the qualitative central control will be selected a value which covers the heat demand in most of rooms. It is possible that in a few rooms, where the required new forward temperature is higher than the set value the radiator surface have to be increased. In other rooms the temperature differences will be compensated by thermostatic valves. In Hungary most central heating systems were designed for a forward/return temperature of 9/7 o C. The new forward temperature value depends on the rehabilitation level and on the original thermal characteristics of the building envelope. In Figure 9 the forward temperature variation is presented for a building depending on the thickness of the additional insulation of the external walls tf, [oc] 75 7 Uwall=1,5 W/m2K Uwall=,9 W/m2K 65 Uwall=,5 W/m2K 6,2,4,6,8,1,12,14,16,18,2,22 δ, [m] FIG. 9: Forward temperature after rehabilitation. Considering the internal temperature set point value 2 o C and the set value of the forward temperature 75 o C the variation of the necessary mass flow reduction depending on the required mean logarithmic temperature difference is presented in Figure 1. m'/mo 1,9,8,7,6,5,4,3, tln, [oc] FIG. 1: Necessary mass flow reduction at 75/55 o C.

7 At the central control system the forward temperature will follow the ever changed heat demand in function of the external temperature. In the rooms situated in the building corner the required forward temperature at the same rehabilitation level will be lower. In Figure 11 the control curves of the heating system are presented. tf, tr, tf', [oc] tf tr tf' tb 15 2 te, [oc] FIG. 11: Control curves at 75/55 o C and required forward temperature in the corner rooms. It could be observed that at design value of the external temperature the heat carrier temperature difference which has to be controlled by thermostatic valves is ca. 13 o C. Using the diagram from Figure 1 results the necessary flow ratio equal to.46. This ratio is almost constant during the heating season as it could be seen in Figure 12.,5 m/mo,48,46,44,42, te, [oc] FIG. 12: Necessary flow reduction during the heating season. Depending on the orientation and the glazed surface ratio the heat gains of the rooms are different. In Figure 13 the heat gains and heat demand ratio values are presented for a glazed surface ratio of the external walls equal to 3%. Qg/Q FIG. 13: Heat gains/heat demand ratio. 3% days N E W S NE, NW SE, SW

8 Actually the thermostatic valves, as local control elements, are used for energy saving exploiting these local gains. All thermostatic valves have a total lift of several millimetres. Starting from the shut position, a decrease in the room temperature of 2K will open the valve of about.5 mm. This part of the lift is called nominal lift. If the adjustment of the forward temperature is realised by thermostatic valves the nominal lift will be reduced during the whole heating season. If the thermostatic valve works in the proportional band the nominal lift will be reduced in our case with 54%. So that, the control lift will be practically ca..23 mm. If the heat gains ratio is higher than 78% the thermostatic valves opening will be lower than.5 mm which represents 1% from the nominal lift. Analysing the heat gains ratio it could be observed that for orientations different from N, NE, NW the thermostatic valves will work near the closed position 31 41% from the heating season. In such situations the control could become unstable (on/off operation mode) which means that the heat gains are not exploited properly and the internal temperature could not be kept constant. 5. Conclusions When a building is retrofitted from energy point of view about 3% of the total energy saving is obtained due to the shorter heating season. The new balance point temperature value depends on the rehabilitation level and on the original thermo-physical state of the building envelope. The influence of heat gains will increase significantly after rehabilitation. If the central heating system elements are not redesigned and changed the output of the existent system has to be adjusted properly to the new energy demand. Because the reduction of heat demand in different rooms depends on the position the required value of the new forward temperature is different. Choosing a value which covers the heat demand in most of rooms, there will be other rooms where the set value will be higher. The difference should be controlled by a mass flow reduction. If these differences are not taken into account presetting the thermostatic valves there will be rooms where the control will be unstable. The unstable control will lead to oscillations of the internal temperature and also the heat gains cold not be exploited properly. Presetting the right value of the mass flow at the thermostatic valves the system balancing has to be revised. In the calculus only heat gains were taken into account as disturbing factors. If the radiators are over dimensioned or the system is not balanced properly etc. the right operation of thermostatic valves will be affected more and more. The existent heating system capacity could be adjusted to the new energy requirements of the retrofitted building but to obtain the expected energy savings a complex energy and economically analysis have to be done choosing the most economical solution. 6. References Csoknyai T. (24). Energy conscious retrofit of residential buildings made with industrialized technology, PhD thesis, Budapest University of Technology and Economics, 15 pages. Halász E. (21). Analysis of district heating systems using mathematical models taking into account the optimal control of the occupants needs, PhD thesis, Budapest University of Technology and Economics, 17 pages. Kalmár F. (22). Energy analysis of building thermal insulation, Proc. 11 th Symposium for Building Physics, Dresden, Germany, September 26-3, p Kalmár F. (23). Optimal forward temperature in retrofitted buildings, Proc. of 2 nd Int. Conference on Building Physics, Leuven, Belgium, September, p Kalmár F. (24). Adjustment of central heating systems to reduced energy needs of retrofitted buildings, PhD thesis, Budapest University of Technology and Economics, 14 pages. Mantzos L. et al. (23). European Energy and Transport Trends to 23, European Commission, Directorate-General for Energy and Transport. Petitjean R. (1997). Total hydronic balancing, Ljung: Tour&Andersson AB.