J. Cent. South Univ. (2012) 19: 709-714

DOI: 10.1007/s11771-012-1061-y

Effect of thermal storage performance of

concrete radiant cooling room on indoor temperature

XIN Ya-juan(辛亚娟), WANG Zhi-qiang(王志强), TIAN Zhe(田喆)

School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2012

Abstract: A building model with radiant cooling system was established and the cooling load, indoor temperature, surface temperature of the walls and other parameters in non-cooling and radiant cooling room were calculated by TRNSYS. The comparative analysis of the characteristics of attenuation and delay proves that the operation of radiant cooling system increases the degree of temperature attenuation of the room and reduces the inner surface temperature of the wall significantly, but has little effect on the attenuation coefficient and delay time of wall heat transfer. The simulation results also show that the inner surface temperature of the walls in the radiant cooling room is much lower than that in non-cooling room in the day with the maximum cooling load, which reduces the indoor operation temperature largely, and improves the thermal comfort. Finally, according to the analysis of indoor temperature of the rooms with different operation schedules of cooling system, it can be derived that the indoor mean temperature changes with the working time of radiant cooling system, and the operation schedule can be adjusted in practice according to the actual indoor temperature to achieve the integration of energy efficiency and thermal comfort.

Key words: concrete radiant cooling; thermal storage performance; cooling load; operating schedule; TRNSYS

1 Introduction

Radiant floor heating system has been widely applied in many countries and regions with its advantages of energy efficiency and comfort [1-3], and radiant cooling system has also become the major concern in heating, ventilation and air conditioning (HVAC) field according to the same strong points when compared to traditional air conditioning systems. FEUSTEL and CORINA’s research showed that the radiant cooling system has better thermal comfort and an overall electrical cooling energy savings of more than 40% compared with a constant volume air system [4]. OLESEN [5-6] pointed out the limitations of radiant floor cooling and analyzed the various parameters affecting the cooling capacity by conducting a large number of studies on thermal comfort, cooling capacity, control and design of floor radiant cooling [5-6]. LEHMANN et al [7] presented application range and functionality of thermally activated building systems (TABS), and it was found that depending on the maximum permissible daily room temperature amplitude with TABS, typical heat gain profiles with peak loads up to around 50 W/m² floor area can be managed based on a simulation study for a typical office building. RIJKSEN et al’s research showed that reductions up to 50% of the cooling capacity for a chiller can be achieved using TABS, when the required cooling capacity was compared to the cooling capacity of a system without energy buffering, and the results can be used as design guidelines in the first stage of a design process [8]. GWERDER et al [9] presented a comprehensive TABS zone control strategy with a modular concept consisting of maximally several parts: outside air temperature compensated supply water temperature control, algorithm for room temperature feedback control, and pulse-width modulation (PWM) module for intermittent operation of the recirculation pump. SOURBRON  et al [10-11] evaluated the impact of the TABS control strategy on both energy consumption and thermal comfort, and conclusions are drawn regarding the configuration of the heat and cold production and distribution system. GWERDER et al [12] outlined a method allowing for automated control of TABS in intermittent operation with pulse width modulation (PWM). In China, WANG [13] also carried on a lot of intensive researches on floor radiant cooling system. LIU et al [14] proposed a method to define these two thermal properties in geometric way, and modified RC-network model was established in order to validate the method. Then, application scope of the modified RC-network model was investigated and defined. LING et al [15] established a heat transfer model and developed a calculation program based on the features in heat transfer and layout mode of the system, then the thermal performance and the basic law in operation of the system using the program were analyzed.

At present, researches on radiant cooling system are mainly conducted for system performance, power consumption and thermal comfort, but studies on thermal storage performance of buildings with radiant cooling system and the effect of start-stop time of air conditioning on indoor temperature are limited. In this work, a building model with concrete radiant cooling system was established, the cooling load, room temperature, wall inner surface temperature and other parameters were calculated using TRNSYS simulation software, and the delay and attenuation characteristics between radiant cooling room and non-cooling room were compared. Finally, by the analysis of the room and wall surface temperatures in the modes with different operation schedules of cooling system, some suggestions about start and stop time of concrete radiant cooling system was put forward, and it will become the basis of ascertainment of reasonable running control strategy for achieving the integration of energy efficiency and thermal comfort.

2 Model construction

The typical meteorological parameters in    Beijing were used in this model and a general office for two persons was identified as the study objects, with a size of 4 m(length)×2 m(width)×3 m(height). In addition,   the indoor design temperature is ascertained to be     26 °C.

The person-density, the equipment power and the fresh air volume are listed in Table 1, the structure of each layer of the building envelope and their thermal conductivity are listed in Table 2, and the cooling parameters of radiant cooling system in the model are listed in Table 3.

Table 1 Person-density, equipment power and fresh air volume

Table 2 Thermal parameters of building envelope

Table 3 Cooling parameters of radiant cooling system

3 Simulation and analysis

3.1 Overview of TRNSYS

TRNSYS [16-17] is a complete and extensible simulation environment for the transient simulation of systems, including multi-zone buildings. It is used by engineers and researchers around the world to validate new energy concepts, from simple domestic hot water systems to the design and simulation of buildings and their equipment, including control strategies, occupant behavior, alternative energy systems (wind, solar, photovoltaic, and hydrogen systems), etc. Its applications include solar systems (solar thermal and PV), low energy buildings and HVAC systems with advanced design features (natural ventilation, slab heating/cooling, double fa?ade, etc), renewable energy systems, cogeneration, etc.

For the above building model, a simulation platform established in TRNSYS simulation studio is shown in Fig. 1.

Fig. 1 Simulation platform

3.2 Characteristics analysis of cooling load

The cooling load in air-conditioning season of the building model was calculated by TRNSYS, and the result is shown in Fig. 2. It can be seen that the cooling load reaches the maximum of 853 W at 14:30 on July 30th from Fig. 2, and the day with the maximum load is defined as the typical day which becomes the main study object. The hourly cooling load of the building model in the typical day with the maximum load is shown in   Fig. 3. Figure 3 shows that the cooling load during 8:00-18:00 is significantly higher than that at other times, which is consistent with the schedule of personnel, equipments and fresh air. Moreover, the sum of personnel load, equipment load and fresh air load accounts for more than 50% of the total load, as shown in Fig. 4. Thus, personnel, equipments, and fresh air play an important role in the formation in the indoor cooling load.

Fig. 2 Hourly cooling load of building model from June 15th to September 15th

Fig. 3 Hourly cooling load of building model in day with maximum load

Fig. 4 Percentage of each part of cooling load in typical day

Indoor sensible heat gains can be divided into convective heat and radiant heat. The radiant heat includes solar radiant heat into the room through the outside window and the radiant section of heat gains from personnel, equipments, and the convective heat gains consist of fresh air heat and the convection section of heat gains from personnel and equipments.

Radiant heat is absorbed and stored by the surfaces of various objects through the indoor air, and the temperature of these objects will increase. Then, the stored heat is to distribute to the air by convection when the surface temperature is higher than the indoor temperature [18]. As shown in Fig. 5, the radiant heat occupies 47.03% of the total heat gains, nearly the half of the total, thus it is clear that the effect of the radiant heat gain on the formation of cooling load cannot be ignored. Consequently, the impact of radiant cooling system, which eliminates the cooling load by reducing the inner surface temperature of the wall, on the indoor temperature and the wall surface temperature, is worth studying further.

Fig. 5 Percentage of each category of cooling load in typical day

3.3 Temperature attenuation and delay

Sol-air temperature [19] fluctuates cyclically, and the thermal storage performance of building envelope, furniture and other objects has an effect about attenuation and delay on the fluctuation of sol-air temperature. The sol-air temperature was calculated using the components and functions in TRNSYS, and the indoor temperatures and the wall surface temperatures of radiant cooling room and non-cooling room were simulated, respectively. According to the simulation results, as shown in Fig. 6 and Fig. 7, the attenuation coefficient and delay time in the two modes are compared and analyzed.

Fig. 6 Indoor temperatures and sol-temperatures in typical day

Fig. 7 Wall surface temperatures in typical day: 1-Inner surface of south wall_non-cooling; 2-Outer surface of south wall_non-cooling; 3-Outer surface of north wall_non-cooling; 4-Inner surface of south wall_radiant cooling; 5-Outer surface of south wall_radiant cooling; 6-Inner surface of north wall_radiant cooling; 7-Cold surface _radiant cooling

3.3.1 Sol-air temperature and indoor temperature

From Fig. 6, it can be seen that the indoor temperature of the radiant cooling room is significantly lower than that of the non-cooling room, the indoor temperatures in the two modes reach the maximum and the minimum at the same time, respectively, and the delay times of the peak value of indoor temperature relative to the peak of sol-air temperature are both 2.25 h. Attenuation coefficient of air temperature is defined as the ratio of the indoor temperature amplitude to the sol-temperature amplitude, and the attenuation coefficients of non-cooling room and radiant cooling room are 0.214 and 0.258, which proves that the operation of radiant cooling system increases the degree of temperature attenuation of the room.

3.3.2 Outer surface temperature and inner surface temperature of wall

The attenuation coefficient of wall heat transfer is defined as the ratio of the inner surface temperature amplitude to the outer surface temperature of the wall, and the delay time of wall heat transfer is defined as the delay time of the peak value of the inner surface temperature relative to the peak of outer surface temperature of the wall. Then, the attenuation coefficients and the delay time of the wall heat transfer are identified from Fig. 7, as listed in Table 4.

Table 4 Attenuation coefficient and delay time of south wall heat transfer

Similarly, according to the analysis of the attenuation coefficients and the delay time of the wall heat transfer during the whole air conditioning season, the operation of radiant cooling system reduces the inner surface temperature of the wall significantly, but has little effect on the attenuation coefficient and delay time of wall heat transfer. Thus, the thermal storage performance referring to the attenuation coefficient and delay time is considered to be the inherent characteristics of the walls, and has little to do with whether the radiant cooling system is working.

Figure 7 shows that the inner surface temperature of the walls in the radiant cooling room is 10-12 °C lower than that in non-cooling room in the day with the maximum cooling load, which reduces the indoor operation temperature largely, and improves the indoor thermal comfort.

3.4 Effect of different operating schedules of radiant cooling system on indoor temperature

Figures 8 and 9 show that the change trends of the temperatures of indoor air and the radiant cooling surface are different in the situations with the same time period but different start-stop time. First, when the cooling system works during 6:00-18:00, the mean indoor temperature in the daytime is the lowest. Second, the mean temperature of the room with the cooling system starting working after 12:00 is higher, and the indoor temperature rises rapidly in the daytime, which proves that it is inappropriate to turn off the cooling system prematurely. In addition, although the indoor temperatures at night are lower and decrease rapidly when the cooling system works at night, the temperatures in the morning from 8:00 to 10:00 are the same, and the cold surface temperatures are mostly below 20 °C in the night, lower than the dew point temperature, easily resulting in the condensation. Consequently, the radiation cooling system is not recommended to work for a long time during the night when the room has low cooling load.

Fig. 8 Indoor temperatures with radiant cooling system having different operation schedules in typical day

Fig. 9 Cold surface temperatures with radiant cooling system having different operation schedules in typical day

Figure 10 shows the indoor air temperatures and the operation temperatures of the building models with the radiant cooling systems having the same start time and different stop times. It is clear that the mean indoor temperature changes with the working time of radiant cooling system, and when the working time reduces by 1 h, the mean operation temperature increases by 0.24 h. Therefore, the operation schedule can be adjusted in practice according to the actual indoor temperature to achieve the integration of energy efficiency and thermal comfort.

Fig. 10 Indoor temperatures and operation temperatures with radiant cooling system having different operation schedules in typical day

4 Conclusions

1) The radiant heat occupies 47.03% of the total heat gains, nearly the half of the total. Thus, it is clear that the effect of the radiant heat gain on the formation of cooling load cannot be ignored.

2) The operation of radiant cooling system increases the degree of temperature attenuation of the room and reduces the inner surface temperature of the wall significantly, but has little effect on the attenuation coefficient and delay time of wall heat transfer.

3) In the day with the maximum cooling load, the inner surface temperature of the walls in the radiant cooling room is 10-12 °C lower than that in non-cooling room, which reduces the indoor operation temperature largely, and improves the indoor thermal comfort.

4) The radiation cooling system is not recommended to work for a long time during the night when the room has low cooling load, because the cold surface temperature is mostly lower than the dew point temperature, easily resulting in the condensation.

5) The mean indoor temperature changes with the working time of radiant cooling system, and when the working time reduces by 1 h, the mean operation temperature increases by 0.24 h. Therefore, the operation schedule can be adjusted according to the actual indoor temperature in practice to achieve the integration of energy efficiency and thermal comfort.

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(Edited by YANG Bing)

Foundation item: Project(2010DFA72740) supported by the International Science & Technology Cooperation Program of China

Received date: 2011-07-26; Accepted date: 2011-11-14

Corresponding author: TIAN Zhe, Associate Professor, PhD; Tel: +86-22-27407800; E-mail: tianzhe@tju.edu.cn