Effect Of Thermal Insulation Of Envelope On Heating Energy Consumption Of Intermittently Heated Rooms

Outdoor temperature of Hot Summer and Cold Winter(HSCW)zone in China is low in winter and central heating system is not provided in this zone according to Chinese government policy.This lead to a poor indoor thermal environment in this zone in winter.With the development of economy,more and more households have been equipped with individual air conditioners for heating.Due to the habit of thrift and convenience of operating heating devices,the most common way of using heating devices is “part-time and part-space” in the HSCW zone rather than “full-time and full-space”;namely,occupants only heat the room in which they are staying.However,with the wide installation and utilization of heating devices,the residential heating energy consumption has been rapidly increased.Effective thermal insulation and proper configuration of residential building envelopes play vital roles to reduce energy consumption for heating.Most of existing studies have been performed on this subject only considered the buildings with continuous heating.Even some studies have been conducted the dynamic thermal behavior of intermittent building,they all focused on the “full-space” heated.For buildings with heating operation of “part space”,energy absorbed by intenral wall during heating duration should not be neglected due to a lower temperature of adjoining room.Moreover,it may be the most influential factor affecting the heating energy consumption.In the present study,the characteristics of room heating load and dynamic heat transfer process of different internal wall configurations under intermittent heating system are investigated and analyzed in detail to provide the theoretical support for engineers when design proper envelope configurations of residential buildings in the lower reaches of the Yangtze River zone in China or other areas with similar climate features.CFD simulations were used to evaluate the effect of heating time ration on the heating energy consumption for intermittent heating system.A series of experiments were carried out in a test chamber equipped with warm-air heating system.The CFD techniques used in the present study were validated by comparing the indoor air temperature and temperature distribution in thermal mass between simulation and experiment data.Based on this,the temperature fields and dynamic heat transfer process of building envelope were simulated by using the validate numerical method.Also,the effect of heating time ratio on the room heating load and its five components have been investigated.The numerical results show that with the increase of heating time ratio,the inner surface temperature of envelope and furniture(indoor thermal mass)is increasing,and the transmission load between thermal mass and indoor air is decrease.Thus,a smaller heating load index is achieved.The ratio of heating load of internal wall to room heating load is ranging from 65 % to 75 % by comparing the five components of room heating load and the ratio of external wall is only about 17 %.This is due to the fact that the inner surface area of the internal wall is approximately 5 times larger than that of the external wall,and more energy is absorbed during heating period.Aiming at the characteristics of a higher heating load of internal wall in intermittent heating room,the dynamic thermal behavior and heating load of internal wall is investigated in detail in this paper.The results show that the energy absorbed by internal wall during heating period comes from three parts,namely: energy stored in internal wall,transferring to adjoining room and releasing to indoor during heating cession period.More than 70 % of energy releases to indoor room during heating cession period when heating time ratio is less than 33 %.This is due to the fact that the just a thin layer of internal wall close to the indoor side is heated and few energy transfers to the inside of internal wall.When heating time ratio is larger than 33 %,more energy transfers to the insider of internal wall.This leads to a higher energy stored and transferring to adjoining room,and the energy stored accounts to 50 % of the heating load of internal wall.To reduce the heating load of internal wall in intermittently heated rooms,thermal insulation and proper configuration of internal wall is a key factor.Therefore,four typical internal wall commonly used in HSCW zone are considered in the present work.Moreover,the effects of heating durations are thoroughly studied.The results show that for the intermittently heated room with short or long heating duration of each operation,different indicators should be used separately to evaluate or guide the energy efficiency design of internal walls.For an intermittently heated room with heating time ratio less than 50 %,the heating load of the internal walls increases linearly with the increase of the surface thermal absorption coefficient,which can be reduced by placing thermal insulation on the surface of the internal walls.When the heating time ratio is larger than 50 %,the heating load of the internal walls grows linearly with the inside thermal storage coefficient,which can be reduced by increasing the thermal resistance of the thermal insulation or decreasing heat capacity of the internal walls.A threshold value of heaing duration exists by comparing the daily heating energy consumption between continuous heating and intermittent heating rooms,and the threshold value is affected by the configuration of internal wall.The daily heating energy consumption of intermittent heating room will be higher than that of continuous heating when the daily operation hours is larger than a threshold value.In this paper,the threshold value of operation hours is 18.0 h,15.8 h,14.6 h and 13.5 h,respectively,for Wall 1,Wall 2,Wall 3,and Wall 4 in the present paper.With the operation hours lower than the threshold value,the daily heating load is clearly less than that of continuous heating.Nevertheless,it seems to be more economical to use continuous heating mode when the operation hour is more than the threshold value.Placing thermal insulation on the inner or outside surface of external wall is a popular way to reduce heating energy consumption due to the larger temperature difference between indoor and outdoor.Some studies considered that a smaller transmission load of external wall with inside insulation is achieved than that of outside insulation due to a lower temperature difference of inner surface of external wall and indoor air.However,the heat transfer though thermal bridge is evident by inside insulation,which will result in more energy transferring to the outdoor through thermal bridge.Moreover,the transmission load of thermal bridge is increasing with the increase of heating duration and the decrease of outdoor temperature.Therefore,a comprehensive analysis is performed to investigate the effect of heating duration on the dynamic heat transfer process of external wall and thermal bridge.The results show that for heat transfer of external wall,the energy saving of inside insulation is decreasing with the increase of heating duration,while the energy saving of outside insulation is increasing with the increase of heating duration for heat transfer of thermal bridge.Therefore,a threshold heating duration exists by comparing the room heating load between external wall with inside insulation and outside insulation.The threshold heating duration are 9.5 h and 8.5 h,respectively,for two different outdoor temperature.When heating duration is less than the threshold value,placing insulation on the inner surface of external wall can obtain a lower heating load.For an intermittent room with heating duration longer than the threshold vale,it is more economical to use outside insulation.