Experimental study on the dynamic thermal response of a radiant floor system in an office building

The dynamic thermal performance of radiant terminal plays an important role in the design and control of radiant cooling system, which is shown as the dynamic thermal response of radiant floor system (RFS) under imposed control. In this paper, the field measurement method was used to study the dynamic thermal response of RFS. The RFS was activated in summer and the supply water temperature was regulated in winter to make dynamic change of thermal performance. The floor surface temperature was selected as the characteristic parameter to describe the dynamic heat transfer performance of the system, and response time τ95 and time constant τ63 were used to quantify the dynamic thermal response. The maximum τ95 was 13.5 h and τ95/τ63 was greater than or equal to 2 in the cooling mode, while τ95 and τ63 were both less than 10 h and τ95/τ63 was 1.6 in the heating mode. As a result, there was no significant lessening of temperature change rate, and the thermal response of RFS was faster under intermittent control of supply water temperature in winter. Therefore, the study aims at providing reference for making intermittent control strategy by using the dynamic thermal performance of radiant system.


Introduction
Since the 21st century, an increasing amount of attention has been given to energy saving due to emission reduction goals, and the rapid development of  In this paper, the field measurement method was used to study the dynamic thermal response of RFS.
The experimental room created a real-world scenario in which changes in thermal and humidity parameters in the room were influenced by outdoor weather conditions. The dynamic thermal response of RFS was caused by different imposed control, which were supply water temperature regulation in summer and start-up control in winter respectively. The change of floor surface temperature during the process of heat exchange between radiant floor and buried pipe water was analyzed. In addition, the τ95, τ63 and τ95/τ63 of the RFS under different imposed controls in winter and summer were compared, and the differences along with the effect of two imposed controls on thermal response were obtained.

Experimental scenario description
The field test was conducted in a south facing room with a floor area of 50 m 2 on the fifth (top) floor of an office building in Jinan. Jinan is located in the north of China, which is in a cold region as shown in Figure 1.

Experimental setup
The RFS is depicted in Figure 3. During summer the cooling energy supplied to the radiant terminal comes entirely from the ground heat exchanger and the heat pump unit was by-passed. Therefore, the underground pipe water was directly supplied to the room without cooling by the heat pump unit. During winter the supply water temperature in the buried pipe was controlled by the heat pump unit. The supply water temperature was heated to the appropriate temperature based on indoor requirement and sent to the room to provide heating for indoor environment.       Jan. 14 when it decreased to 23~24°C, and accordingly

Experimental schemes
Tf increased. Tsw on Aug. 16 and Jan. 13 were described in Figure 6.  Tsw on Jan. 13.

Thermal response evaluation index
In this paper, the radiant surface temperature was  (1).
where, Q is cooling/heating provided by buried pipe water, W/m 2 ; Ts is radiant surface temperature,°C,  Tw is supply and return water temperature,°C, R is thermal resistance between radiant surface and buried pipe water, (K·m 2 )/ W, and qstore is the cooling energy used to remove the heat accumulated in radiant floor, W/m 2 .

Thermal response under imposed control in
the cooling mode When approaching 95% of the total temperature change, Tf decreased slowly, and essentially stabilized after a small decrease. The RFS was combined with displacement ventilation system for cooling in summer.
After the RFS was started, Tf was higher due to shutdown of displacement ventilation system so that the insufficient cooling caused Tin to increase in a short time. When Tf decreased by about 1°C, cooling capacity of RFS could counter indoor heat gain and achieve cooling effect, and therefore Tin began to decrease. With the decrease of heat transfer between chilled water and radiant floor, Tin gradually remained a stable level.

Thermal response under imposed control in the heating mode
The changes of Tf, Tin and Qh after Tsa of RFS increased in winter are illustrated in Figure 8. Tsw decreased by 2~3°C after working period, and

Conclusion
Under the start-up control of RFS in the cooling mode and Tsa regulation in the heating mode, thermal performance of radiant terminal dynamically changed.
(1) With the decrease of the temperature difference between pipe water and radiant floor, the heat exchange lessened, and the change of the Tf came to stability.
While Tin began to decrease/increase with the effect of cooling/heating 0.5~1 h after imposing control to RFS.
(2) In the cooling mode, τ95/τ63 was greater than or equal to 2, and the maximum τ95 was 13.5h. In the heating mode, τ95/τ63 was 1.6, and the maximum τ95 was 8h. Both indicated the long response time of RFS.
(3) Compared with the start-up control in summer, Tsa regulation in winter had a greater impact on the thermal performance of the RFS, which made the thermal response faster.

Conflicts of interest
The authors declare no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. Indicators of thermal response and output of