Field study and numerical investigation on heating performance of air carrying energy radiant air-conditioning system in an office

Energy & Buildings 209 (2020) 109712

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Field study and numerical investigation on heating performance of air carrying energy radiant air-conditioning system in an office Pei Peng, Data curationWriting - review & editing a,b, Guangcai Gong, SupervisionFunding acquisition a,b,∗, Xiaorui Deng a,b, Chun Liang a,b, Wenqiang Li a,b a b

College of Civil Engineering, Hunan University, Changsha 410082, China Key Laboratory of Building Safety and Energy Efficiency, Ministry of Education, Hunan University, Changsha 410082, China

a r t i c l e

i n f o

Article history: Received 31 July 2019 Revised 4 December 2019 Accepted 18 December 2019 Available online 19 December 2019 Keywords: Composite air carrying energy Radiant system CFD simulation Temperature distribution

a b s t r a c t A ceiling-sidewall composite air carrying energy radiant air-conditioning system (ACERS) is presented, and the heating performance of the ceiling-sidewall composite ACERS combined with air source heat pump is investigated by field experiment. The results show that the combined system can meet the heating requirement for office buildings in south-central China, and also have the energy-saving potential. Moreover, the indoor thermal environment under three different installation types, namely the ceiling, the sidewall, and the ceiling-sidewall composite type, is analyzed by using computational fluid dynamic (CFD) simulation. The comparisons of three types indicate that all of them can meet the temperature requirement, but the temperature is more uniform and higher in the composite ACERS. Meanwhile, the temperature gradient in the ceiling ACERS is smaller than that of the sidewall ACERS due to the further heat transfer caused by air penetration. Both the experimental and CFD simulated temperature distribution demonstrates that the shelter of furniture has an impact on the indoor thermal environment, and it should not be ignored in practical engineering. This research shows the feasibility of winter heating by using the ACERS in office buildings, and is helpful for HVAC engineers to design and apply different ACERS types and realize the integration of air conditioning in winter and summer in this area. © 2019 Elsevier B.V. All rights reserved.

1. Introduction The energy consumption of building accounts for almost 40% of the total energy usage and leads to 30% CO2 emissions. With the economic development and demand for improving indoor thermal comfort, energy consumption is still rising. In Europe, the annual growth rate of energy consumption is about 1.5%. In China, the proportion of building energy consumption in total energy usage ranged from 24.1% in 1996 to 27.5% in 2001, and the proportion will increase to about 35% in 2020 [1–4]. Among the building energy consumption, Heating Ventilation and Air Conditioning (HVAC) systems consume the most parts nearly 50%. Therefore, the appropriate HVAC system for buildings can greatly save energy. Compared to the convective air conditioning systems, radiant heating and cooling systems have been studied extensively. The latter systems can not only provide a better indoor thermal environment, but also save energy [5–9]. The origins of radiant heat-

∗

Corresponding author. E-mail address: [email protected] (G. Gong).

https://doi.org/10.1016/j.enbuild.2019.109712 0378-7788/© 2019 Elsevier B.V. All rights reserved.

ing and cooling systems can go back to thousands of years ago. As early as the Roman Empire, the Romans heated the floor with exhaust gases produced by burning wood. The first studied of radiant system was in the European laboratories in the 1990s [10]. The radiant heating and cooling systems have many forms, according to the position of radiant surfaces, the systems can be divided as radiant ceiling cooling, radiant floor heating, radiant floor cooling, radiant wall-ceiling heating and so on [11–15]. From the viewpoint of the thermal medium of radiant systems, they can be divided into three categories. The first is the hydraulic radiant systems, and the thermal medium is liquid usually the water. And when the medium is air, the system is called as Air Carrying Energy Radiation Air-conditioning System (ACERS). The third is non-load energy medium radiant system, like the electrical heater. In the past few decades, many scholars have investigated the radiant cooling and heating systems from various aspects [16–23]. Imanari et al. [24] compared the thermal comfort and energy consumption of radiant ceiling panel system and conventional allair system. The results showed that the energy-saving of radiant system was nearly 10% and the thermal environment of radiant system was more comfortable. Wu et al. [25] developed a new simplified model for radiant floor heating and cooling system

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Nomenclature ACERS CFD ASHP CCA HVAC Temp. RH COP Q E V

ρ Cp t1 t2 A q T SET∗ PMV PPD

air carrying energy radiation air-conditioning system Computational fluid dynamic air source heat pump cellulose fiber cement sheets, autoclaved heating ventilation and air conditioning Temperature (°C) Relative humidity (%) coefficient of performance the heating capacity of the ASHP system (kW) the power consumption per hour of the ASHP system (kW) water flowrate in the system (m3 /s) the density of water (kg/m3 ) the specific heat capacity of water (°C) the water return temperature (°C) the water supply temperature (°C) the heating area (m2 ) the cumulative average heating supply per unit area (W/m2 ) the time of system running (h) standard effective temperature (°C) Predicted Mean Vote Predicted Percentage of Dissatisfied

to calculate the surface temperature and heat transfer, which is helpful to design and control the systems. Miriel et al. [26] evaluated the heating and cooling performance of radiant ceiling panels by experiment and simulation, simultaneously they investigated the thermal comfort and yearly energy consumption of the system. The results further proved the energy-saving of radiant system. Lin et al. [27] put forward a new type of under-floor electric heating system with shape-stabilized phase change material (PCM) plates, and the results showed that the heating system was comfortable and energy-efficient. Jin et al. [28] proposed a calculation method to estimate the floor surface temperature in radiant floor heating/cooling system. Zhang et al. [29] analyzed the operating characteristics of lightweight radiant floor heating system by experiment and numerical method. The results indicated that the system has fine thermal stability and thermal comfort. The advantages of radiant cooling and heating systems in thermal comfort and energy consumption are obvious. However, the problems of risk condensation on the cooling panels and the overheating of floor heating system can influence the system performance. Many studies have been carried out from the perspective of system control and design to solve the problems [30–35]. Zhang and Niu [36] investigated the indoor moisture change when chilled ceiling and air dehumidification system were combined, and they compared the combined system with other systems such as all-air, all-air with total heat recovery and so on. The results showed that the chilled ceiling combined with proper air dehumidification could save energy and get good applications in hot and humid climates like Hong Kong. Causone et al. [37] found that the floor heating combined with displacement ventilation cannot increase draught risk at ankle level, and the combined system obtained high ventilation effectiveness values. These researches make it possible for the radiant systems to effectively application. According to the characteristics of airflow, the radiant ceiling system is usually designed for cooling, and the radiant floor system is for heating. However, occupants in some areas need heating in winter and cooling in summer, and the installation cost of the

two systems is high. Therefore, it is significant to realize the integration of air conditioning in winter and summer. Air carrying energy radiant air-conditioning system (ACERS) is a new type of radiation terminal proposed by Gong et al. in recent years, which can meet the dual needs of heating and cooling in hot summer and cold winter area. The ACERS as an energy-efficient radiant airconditioning terminal was patented in 2011, and the enterprise standard of ACERS was issued in 2012. Peng et al. [38] investigated the thermal environment and thermal comfort of ACERS in winter and summer by field experiment. The results reflected that this novel system can be used to meet the heating and cooling requirements in south-central China, and the energy-saving ratio of ACERS is 26.4% in winter and 24% in summer compared to the convective air-conditioning system. Meanwhile, the unit exergy cost of three radiant air-conditioning systems including ACERS, capillary radiant air conditioning system and a combined system of split air conditioning with floor radiant air conditioning was compared. The calculated results demonstrated that the overall exergy economic performance of ACERS is better that the other two radiant air-conditioning systems [39]. Gong et al. [40] also obtained the correction coefficient for the load calculation of ACERS by using the experimental and CFD simulation methods. The radiative heat transfer for the orifice plate of the ACERS was investigated and a new simplified equation of the radiative heat transfer of the orifice plate was developed [41]. Liu et al. [42] analyzed the operation performance of ACERS by the experiment and CFD simulation, and they studied three different summer operation conditions (without opening door and window, open-door only, and open-window only) in a residential building with ACERS. The results proved that the ACERS could create a good environment, and there is a lowtemperature boundary zone with a large temperature gradient under the orifice plate of ACERS to prevent condensation. Xu et al. [43] discussed the three factors affecting the condensation of orifice plates in ACERS by CFD simulation, and the three factors were orifice coefficient, air velocity and air temperature, respectively. The simulation data also demonstrated that the ACERS could effectively prevent condensation compared to the traditional radiant plate system. The thermal environment and energy transfer of the ACERS in a waiting hall of railway station was also simulated. The simulated results show that the indoor temperature and humidity are evenly distributed, and the radiant heat transfer accounts for 64.2%. Besides, the total heat transfer of the ACERS is about 10% higher than the metal flat radiant air-conditioning system. This study preliminarily verified the feasibility of the system in large space [44]. Because the previous research mainly focused on the ceiling type of ACERS, so this study conducts further research on the different installation types of ACERS to meet the building requirement. In hot summer and cold winter area of China like Changsha, there is no central heating in this area, and the convective air conditioners usually used in summer cannot meet the indoor thermal comfort demand in winter. People usually choose other heating methods such as electric heaters, which leads to further energy consumption. The purpose of this paper is to analyze the heating performance and thermal comfort of the ceiling-sidewall composite ACERS combined with air source heat pump in winter by experiment, and compare the indoor environment under three different installation types by CFD simulation. Besides, the impact of furniture on the indoor environment is also considered. This study is helpful for HVAC engineers to design and apply the different radiant installation types of ACERS according to the user requirement. Meanwhile, it can help to solve the current heating problem in winter in south-central China, and provide theoretical support for the integration of air conditioning in winter and summer in this area.

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2. Methodology 2.1. Experimental method 2.1.1. Description of ACERS combined with heat pump Air carrying energy radiant air-conditioning system is a new type of radiant air-conditioning terminal with unique design, and it has great advantages in terms of thermal comfort, energy efficiency and economic. Nowadays, this system has been applied to office buildings, residential buildings, and the clean rooms of hospitals in Hunan. The ACERS has three typical installation types, namely the ceiling, the sidewall, and the ceiling-sidewall composite type. In this paper, the heating performance of the ceiling-sidewall composite ACERS in winter is investigated by the experiment. Besides, the CFD methods is used to compare these three installation types. The principle diagram of the composite ACERS is illustrated in Fig. 1. The room equipped with ACERS is usually divided into the energy storage buffer zone and air-conditioning area, and the energy storage buffer zone is a place to exchange heat with radiant plates and stabilize pressure. For the composite ACERS, there are two energy storage buffer zones including the ceiling energy storage buffer and sidewall energy buffer zone. The test room in Fig. 1 is divided into three parts. The upper part divided by radiant orifice plates is the 300 mm ceiling energy storage buffer, the one side is the 300 mm sidewall energy storage buffer divided by radiant plates, and the rest is the air conditioning area. The ceiling orifice plates are made of thin aluminum with 7% opening ratio and 1.5 mm diameter. The sidewall plates are the cellulose fiber cement sheets, autoclaved (CCA) radiant panels. The contact parts between the energy storage buffer and enclosure structure are insulated by the polystyrene foam board. When the system is running, the heating air processed by the fan coil unit is firstly sent to the energy storage buffer zones. Most of the air circulates in the energy storage buffer and transfers heat with the radiant plate. Meanwhile, the air in the sidewall buffer can also flow into the ceiling buffer through the loop pipe in the sidewall. Due to the effect of pressure infiltration, a small part of the air can enter into the room through the ceiling orifice plates and further exchange heat with the indoor environment. The indoor air subsequently can return to the fan coil unit via the return air outlet of the side wall. In this way, the ceiling and sidewall radiant plates can constantly get cold or heat from the circulating

Fig. 1. Schematic of composite ACERS.

Fig. 2. Diagram of Air Source Heat Pump System.

air in energy storage buffer and transfer to the indoor objects. The difference from the current radiant plate systems is that the air in the ceiling buffer zone of ACERS can penetrate into the room and further heat exchange in room at a small velocity without causing draught. As shown in Fig. 2, there are three parallel air source heat pump (ASHP) units to provide hot and cold water for the air-conditioning terminal system. The hydraulic circuit is composed of two heat storage water tanks with a capacity of 350 L, an ultrasonic flow meter to measure the water flow rate and output the data to the automatic monitor system, two temperature sensors where measure the water supply and return temperature. The temperature and water flow rate data can be transmitted to the control system. The water flows through the pipes into the fan coil unit of ACERS then to heat the air. All pipes in the hydraulic system are well insulated. The experiment was carried out in an office building of 201 m2 floor areas including three rooms. Because three rooms share a hydraulic system, in order to test the performance of air source heat pump system, the operating conditions in three rooms are setting the same during the experiment. The building is in Shaoshan city of China located at longitude 112.29E and latitude 27.54N. The three rooms are all installed with the ceiling-sidewall composite air carrying energy radiant air-conditioning system. 2.1.2. Test room and measurements Fig. 3a is the layout the test room, and the dimension is 8 m × 4.05 m × 2.4 m. The upper ceiling plates and inner side of the east wall are both the energy storage buffer with a thickness of 300 mm. To measure the effect of furniture on the heat transfer of radiant plate in this experiment, a set of desk and chair was placed in the test room. The size of desk is 1.2 m × 0.45 m × 0.7 m, and the position of the desk and chair are displayed in Fig. 3a. The experiment was conducted from December 28, 2017, to January 1, 2018, and January 13 to 16, 2018. The experimental test contained two parts: the indoor thermal environment under the composite ACERS and the operating performance of air source heat pump. As shown in Fig. 3b, there are many measuring points are placed in different horizontal and vertical height directions to investigate the indoor thermal environment. Besides, the temperature around the desk was also measured to analyze the influence of furniture on the indoor environment. The indoor environment parameters like air temperature, relative humidity, air velocity were obtained by the instruments in

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Fig. 3. (a) Layout of the test room and (b) Measuring point distribution in room. Table 1 Measuring instruments and parameters. Instrument

Parameters

Accuracy

PT100 thermocouple testo H174 temperature and humidity recorder Ultrasonic flowmeter TES-1340 hot wire anemometer

Air and surface temperature Air temperature/humidity Water flowrate Air velocity

±0.1 °C ±0.5 °C /±3% ±1.0% ±3%

Table 1. The surface temperature was measured by the PT100 thermocouples and recorded by a X6032C data logger. The temperature of supply air and energy storage chamber was also measured by the PT100 thermocouples. These thermocouples calibrated by ice water mixture are very thin, and can be regarded as no effect on the temperature field. A temperature and humidity recorder was placed in the return air outlet to measure the properties of return air. The water flow rate and water temperature data for the hydraulic system were obtained by the control system. 2.2. Numerical method Different installation types of radiant system can directly affect the distribution of indoor thermal environment. Therefore, based on the experimental results of the composite ACERS, the thermal environment of three typical installation types was analyzed by using the CFD simulation software in this paper. The simulation results can help to evaluate the different installation of ACERS and provide guidance for the system design. 2.2.1. Physical model of CFD simulation The physical models of three ACERS types are depicted in Fig. 5, namely orifice ceiling, sidewall plate and the combination of two types. There are some assumptions in the simulation. (1) The external solar radiation and indoor heat source radiation were ignored. (2) Considering the limitation of mesh and computing ability, the pore size of orifice plate in the physical model was larger, but the whole open pore ratio was the same as the real model. (3) The cold air infiltration by the door and window was neglected. 2.2.2. Grid generation The grid is established by the ANSYS pre-processing software called ICEM. Considering the complex shape of orifice plate, it is inconvenient to adopt structured mesh, so the structuredunstructured mixed type mesh is used in the computational domain. The mesh is encrypted in some regions, such as the inlets

and outlets, orifice plate and near-wall surface with high gradients, to achieve good calculation accuracy. 2.2.3. Turbulence model The fluid flow process involved in the simulation is a low speed incompressible steady flow, thus the solver was based on a pressure solver. The SIMPLE pressure-velocity coupling algorithm was used for steady-state operation. The airflow process was governed by the conservation equation of mass, momentum, energy and the transport equation of component, the turbulent transport equation. Ponser [45] and Chen [46] investigated the indoor air flow by using different turbulence model, and they found that the renormalization group (RNG) k-ε turbulence model is most accurate to model the indoor flow. And based on our previous study, the RNG k–ε model was employed in the CFD simulation and the standard wall functions were chosen to handle near wall problems. Besides, the buoyancy effect of Boussinesq approximation was also considered in the model. 2.2.4. Radiation model ANSYS Fluent provides six radiation models to consider the radiation in heat transfer simulation, including the Discrete Transfer Radiation Model (DTRM), P-1 radiation model, Rosseland Radiation Model, Surface-to-Surface (S2S) Radiation Model, Discrete Ordinates (DO) Radiation Model, Monte Carlo (MC) Radiation Model. The DTRM is not compatible with parallel processing. Meanwhile, the P-1 and Rosseland radiation model are mainly used for the large optical thickness, since the indoor air can be considered transparent, with an optical thickness of almost zero. Besides, the Monte Carlo model now does not support the user to define material properties [47]. Therefore, the DO and S2S radiation model are mostly used for the indoor thermal environment simulation of radiant air-conditioning systems [48–50]. The DO method solves the radiative transfer equation (RTE) for a finite number of discrete solid angles, and each of them is associated with a vector direction fixed in the global Cartesian system. Besides, the coupling between

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Table 2 Properties of the materials.

Density (kg/m3 ) Heat capacity (J/kg•k) Thermal conductivity (W/m•k)

Orifice plate

CCA plate

Wood

Wall

Glass

Door

2719 871

1300 800

500 2510

2500 920

2500 840

8030 502

202.4

1.11

0.16

1.57

0.76

1.76

energy and radiation intensities at a cell can accelerate the convergence of the finite volume scheme for radiative heat transfer, when using the DO radiant model [51]. Considering the computational cost and memory requirements, the Do radiation model is used in this study, and all the surfaces are considered as grey and diffused. 2.2.5. Boundary conditions (1) Inlet and outlet boundary condition The inlet boundary conditions were adopted for the simulation, which included the velocity, temperature, turbulence intensity and hydraulic diameter of the inlet. The temperature and velocity of air supply were used the measured value, and the boundary condition of the orifice ceiling and sidewall plate was the same as that of the composite ACERS. The hydraulic diameter and turbulence intensity are calculated by the following formulas:

A S

(1)

ν dH μ

(2)

dH = 4 Re =

1

I = 0.16Re− 8

thermal comfort in winter. And in Fig. 6b, the temperature is about 18.1–18.8 °C and the relative humidity fluctuates at 35–40%, which can fully meet the requirement of human thermal comfort in winter. According to the ASHRAE Standard 55-2010, the temperature difference should not exceed 3 °C from the height of the ankle to the neck, namely t1.1 -t0.1 ≤ 3 °C. As shown in Fig. 7, the temperature difference between 0.2 m and 1.2 m height is within 3 °C, which is in the range of thermal comfort requirement. Besides, the floor surface temperature is a little higher than the air temperature of 0.2 m height. Besides, the floor surface temperature is slightly higher than the air temperature of 0.2 m height, which may be due to the cold radiation from the south outer window to lower the air temperature to some degree. But this temperature difference of about 0.1–0.2 °C is small and acceptable. As seen in Fig. 4, the window arrangement at the 0.2 m height may have a certain radiation effect on the indoor temperature distribution. Fig. 8 is the air temperature variation at different measuring points of the 1.2 m height from the east radiant wall to the west wall. The temperature near the radiant wall is higher, but the temperature

(3)

where dH is the hydraulic diameter (m), Re is the Reynolds number, I is the turbulence intensity, A is the flow across section area (m2 ), S is the wetted perimeter (m), ν is fluid kinematic viscosity (m2 /s), μ is the average flow rate of fluid (m/s). Due to the limitation of experiment, the velocity and pressure distribution at the outlet boundary were not measured. Consequently, the free outflow boundary was used as the outlet boundary condition, which could be calculated directly by Fluent. (2) Wall boundary condition The external room envelops like the south wall and south window were set as the first kind of boundary conditions. The internal wall and door of the adjacent air-conditioned room were regarded as adiabatic, and the partitions in the sidewall and the desk were also assumed as adiabatic. The properties of materials are summarized in Table 2.

Fig. 4. Real photo of the test room.

3. Results and discussion 3.1. Experimental results 3.1.1. Temperature distribution analysis Fig. 6 displays the air temperature distribution of different horizontal measuring points at the 1.5 m height when the indoor setting temperature is 16 °C and 18 °C, respectively. It is found that the temperature and humidity variation in the horizontal direction are relatively consistent and uniform. The maximum temperature difference is within 1 °C. The temperature of measuring point 3 and 6 is a little lower due to the effect of cold radiation from the outer window and convection from the return air outlet. Besides, from Fig. 6a, it can be seen that the average indoor temperature is about 16.8–17.4 °C when the indoor setting temperature is 16 °C, which can basically meet the requirement of human

Fig. 5. Physical model. (a) Composite ACERS. (b) Sidewall ACERS. (c) Ceiling ACERS.

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Fig. 6. Horizontal temperature and humidity variation at different setting temperature (a)16 °C and (b)18 °C.

Fig. 7. Vertical temperature variation at different setting temperature (a)16 °C and (b)18 °C.

Fig. 8. Middle Horizontal temperature variation at different setting temperature (a)16 °C and (b)18 °C.

difference is less than 1 °C. From Figs. 7 and 8, it can be concluded that the composite ACERS combined with the ceiling and sidewall does not cause indoor radiation non-uniformity. The surface temperature changes of different walls are displays in Fig. 9. The temperature fluctuations of the east wall and west wall are large due to the circulating air in the east wall. And the temperature fluctuations of the east wall affect the west wall temperature due to the direct radiation. Meanwhile, the fluctuation trend of the west wall is consistent with that of the east wall, indicating that the indoor temperature response is faster. Besides, the temperature fluctuation trend of the east wall reflects the thermal energy storage characteristic of the ACERS, and the intermittent operation can be considered to achieve energy saving.

The temperature variation inside and outside the desk are demonstrated in Fig. 10, it can be seen that the air temperature difference between the 0.35 m height in the desk and indoor air can reach about 2 °C, which indicates that the shelter of furniture has a great effect on heat transfer. So the placement of furniture should be considered for occupants’ thermal comfort in practical engineering. Besides, the fluctuation trend of air temperature at 0.35 m height in the desk is consistent with that on the surface of desk, which shows that the indoor temperature response in the composite ACERS is very fast. In addition, as shown in the figure, the floor temperature can be maintained at 15–15.5 °C and 15.5– 16.3 °C when the setting temperature is 16 °C and 18 °C, respectively, and the floor temperature difference between the inside and

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Fig. 9. Wall temperature variation at different setting temperature (a)16 °C and (b)18 °C.

Fig. 10. Temperature variation inside and outside the desk at different setting temperature (a)16 °C and (b)18 °C.

outside desk is about 0.2–0.5 °C. The results show that the radiation from the sidewall can effectively alleviate the cold feet of office occupants, when the furniture obscures the radiation from the ceiling. 3.1.2. Thermal comfort analysis The PMV (Predicted Mean Vote) and PPD (Predicted Percent Dissatisfied) indexes are established on the thermal comfort equation proposed by Fanger [52]. The two indexes can represent the average vote of occupants’ thermal sensation. According to the ASHRAE Standard 55-2010, the intrinsic clothing insulation and metabolic rate were calculated. The office workers in Hunan province usually wear thermal underwear, longsleeved shirts sweaters, long socks and trousers in winter. Therefore, the average clothing insulation value is estimated about 1.34clo, and 0.07clo thermal insulation of the chair is considered in the calculation. The metabolic rate is about 1.1met for the sitting office activity [53]. The ISO 7730 defines different levels of thermal environment classification based on the PMV and PPD values. When −0.2 ≤ PMV ≤ +0.2, PPD ≤ 6%, the thermal environment is A grade. When −0.5 ≤ PMV ≤ +0.5, PPD ≤ 10%, the thermal environment is B grade. When −0.7≤ PMV ≤ +0.7, PPD ≤ 15%, the thermal environment is C grade. Besides the PMV and PPD indexes, the SET∗ (standard effective temperature) is also used as the thermal comfort evaluation parameter in this study. The SET∗ is proposed by Gagge and described as the standard apparent temperature by ASHRAE [54].

The thermal comfort of different horizontal measuring points at the 1.2 m height in the room are displayed in Fig. 11. When the setting temperature is 16 °C in Fig. 11a, the PMV and PPD value are in C grade. And when the temperature is 18 °C in Fig. 11b, the thermal environment is regarded as B grade. The results show that these two setting conditions both can meet the thermal comfort of 80% occupants at least. Besides, the SET∗ of different measuring points are relatively uniform, except that the temperature of measuring point 3 and 6 is a little low. This is because point 3 and 6 are closed to the south exterior window and south exterior wall. 3.1.3. Energy performance In order to evaluate the performance of ACERS combined with air source heat pump, the coefficient of performance (COP) and the cumulative average heating supply per unit area (q) are calculated. The calculation of COP and q are given by the following formula:

COP =

Q E

V ρC p (t2 − t1 ) 3600  Q  q= 10 0 0A T Q=

(4)

(5)

(6)

where Q (kW) is the average heating capacity of the ASHP system and E (kW) is the power consumption per hour of the ASHP

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Fig. 11. Thermal comfort at different setting temperature (a)16 °C and (b)18 °C.

Table 3 Statistic summary of experimental data. Measured variables

Case1

Case2

Average outdoor temperature (°C) Average temperature of orifice plate (°C) Average temperature of indoor air (°C) Average temperature of floor surface (°C) Water supply temperature (°C) Water return temperature (°C) Water Flow rate (m3 /h) Average heating supply per unit area(W/m2 ) Cumulative average heating supply per unit area(W/m2 ) Power consumption per hour (kW) COP

5.7 17.8 17.1 15.3 40.5 38.8 4.22 41.64 15.2 3.19 2.71

6.8 19.3 18.5 16.2 41.1 38.2 4.3 71.03 39.3 5.56 2.62

system, V (m3 /s) is water flowrate in the system, ρ (kg/m3 ) is the density of water, Cp (°C) is the specific heat capacity of water, t2 (°C) is the water supply temperature, t1 (°C) is the water return temperature, A is the heating area (m2 ), T is the time of system running (h). Table 3 is the summary of experimental data when the setting temperature is 16 °C (Case1) and 18 °C (Case2). When the outdoor temperature is below 7 °C, the COP of the combined system is about 2.71 and 2.62. Comparing to the results in Ref. [55], the COP of air source heat pump in Chonqing area is about 2–2.5 when the outdoor temperature is 5–10 °C. Hence, the measured data indicate that the composite ACERS combined with air source heat pump can meet the heating demand for office buildings in winter. Moreover, the average heating supply per unit area is about 41.64 W/m2 and 71.03 W/m2 , and these values are lower than the current standard, indicating the energy saving potential of the composite ACERS. The cumulative average heating supply per unit area is about 15.2 W/m2 and 39.3 W/m2 during the experiment, which reflects the thermal storage characteristic of the composite ACERS. 3.1.4. Uncertainty analysis The uncertainties of experimental measurement consist of two categories. The first is the numerical values which can be estimated by statistical analysis. The second is complex and usually belongs to the random uncertainty [56]. In this study, the uncertainties in measurement and results were analyzed by using the method proposed by Holman. The value of R is the function of independent variables x1 , x2 …xn . If the uncertainty of independent variables w1 , w2 …wn has the same probability, the uncertainty of the result wR with these probabilities can be calculated by the equation as follows [57].

 wR =

∂R w ∂ x1 1

2



∂R + w ∂ x2 2

2



∂R + ...+ w ∂ xn n

2 1/2 (7)

Therefore, according to the measured data and instruments introduced in Table 1, the acceptable uncertainty of the COP is 3.4% and 5.5%, and the uncertainty of cumulative average heating supply per unit area (q) is 0.24% and 1.6%, when the setting temperature is 16 °C and 18 °C respectively. 3.2. CFD simulation results 3.2.1. Grid independency Chen and Srebic [58] recommended a verification of the model results by refining the grid size, so the grid independence verification of three different case was conducted to ensure the mesh does not affect the results. The simulation data are displayed in Fig. 12. The results show that the temperature field difference between mesh1 and mesh2 is acceptable. Therefore, the mesh1 is used considering the computer performance and computing time. 3.2.2. Validation of the CFD model The CFD model is further validated by experimental results. Figs 13 displays the air temperature comparison between the measured and the CFD simulated in the test room. Fig. 13a shows the vertical temperature gradient in the room, and Fig. 13b is the temperature variation of middle horizontal. Besides, the relative error between the experimental and CFD simulated results is shown in Fig. 14. Comparing the experimental and simulated data at the same measuring point, it can be seen that the maximum error is about 7.2% and the average error is about 2.39%, which can be acceptable for engineering applications. The reason for the error is mainly due to the simplification of the geometric model, the orifice of the geometric model is larger than the actual condition. So there is more air in energy storage buffer penetrating into the air-conditioning area through orifice plate, which may lead to the temperature of CFD results slightly higher than the experimental results as shown in Fig. 14. All in all, there is a good agreement between the measured and simulated results. 3.2.3. Analysis of three installation types (1) Air velocity fields Fig. 15 show the air velocity contours across the central plane in the X direction of the room in the composite ACERS and ceiling ACERS, because the radiant plate used in the sidewall ACERS is non-porous, and the indoor air velocity is almost 0 m/s, so the velocity contour is not given. As shown in Fig. 15, the velocity is very small in the feet and ankles’ zone and cannot cause discomfort for the occupants according to the velocity limit of 0.3 m/s in ASHERE 2010, since the buoyancy effect in the occupied zone is the main driving force for the airflow. The velocity is relatively higher near the ceiling due to the static pressure penetration of orifice plate, and this effect can effectively prevent the condensation of the orifice plate when cooling. Besides, it can also be noticed in

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Fig. 12. Grid independence verification. (a) Composite ACERS, (b) Sidewall AECRS, (c) Ceiling ACERS.

Fig. 13. Validation of the air temperature (a) Vertical temperature, (b) Middle horizontal temperature.

Fig. 15 that the desk and chair can influence on the indoor velocity field, the furniture shelter can reduce the effect of the velocity field. (2) Temperature field Figs. 16 and 17 present the temperature contours of X section in three installation types. The temperature distribution of X section without desk and chair is shown in Fig. 16. As seen in Fig. 16a, the room temperature is about 18.6 °C in the composite ACERS, and the temperature distribution is very uniform both in the vertical

and horizontal directions. Fig. 16b is the temperature distribution of the sidewall ACERS. The temperature is about 17.8–18.2 °C in the occupied zone. Although there is an obvious temperature gradient distribution in the horizontal direction of the room. While according to the ASHERE 2010, the horizontal temperature gradient in the room does not cause radiation inhomogeneity. Fig. 16c displays the temperature distribution of the ceiling ACERS, the temperature of the room maintains 17.4–17.8 °C. The bottom air temperature is relatively low due to the effect of thermal buoyancy, but the temperature gradient is still in the thermal comfort range.

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18.5–19 °C. The average temperature of the sidewall system and the ceiling ACERS is about 17–18 °C, and the temperature distribution is more uniform in the ceiling ACERS. Because the air in the energy storage area can penetrate into the room by the orifice plates, which can enhance the energy exchange. As shown in Fig. 18b, the temperature gradient is obvious from the radiation east wall to the no-radiant west wall in sidewall ACERS. The reason for this phenomenon is that the radiation is mainly heat transfer mode in this system, and there is almost no convective heat transfer. Besides, it is also observed in Fig. 18b that the gradient of temperature distribution appears in the north-south direction, which is caused by the lower temperature of the outer wall surface. Overall, although the temperature gradient is higher, it is still within the comfort range of radiation asymmetry. 4. Conclusions Fig. 14. Air temperature comparison of measured and simulated air temperature.

Besides, the vertical temperature gradient is relatively smaller and uniform compared to Fig. 16b. The reason is that in the ceiling system, the air in the energy storage area can penetrate through the orifice to further exchange heat with indoor air by convection. Fig. 17 describes the temperature distribution of X central plane with the desk and chair. The temperature of the occupied zone in the composite ACERS is higher and more uniform compared to the other two types. Meanwhile, it can also be found that the temperature inside the desk in Fig. 17b is higher in Fig. 16b, because the main heat transfer mode in the sidewall system is radiation, and the shelter of furniture enhances the heat transfer in the inner space of the desk. While in Fig. 17a, due to the convection, the temperature inside the desk is a little lower than that of sidewall type, but the overall temperature is about 0.8–1 °C higher than that of sidewall type. Besides, the shelter influence of desk and chair on indoor temperature distribution is obvious. As Fig. 17c shows, the floor temperature in the desk is even 1.2 °C lower than that outside the desk. And the temperature difference is the smallest about 0.4 °C in the composite ACERS, which is consistent with the experimental result. Therefore, the shelter effect of furniture should not be ignored in practical engineering considering the office workers’ thermal comfort. The temperature distribution of three systems at 1.2 m height is presented in Fig. 18. The figure indicates that the temperature in composite ACERS is highest, and the average temperature is about

In this paper, the heating performance of the ceiling-sidewall composite ACERS combined with air source heat pump has been investigated by field experiment. Meanwhile, the indoor thermal environment in three installation types of ACERS has been compared by CFD simulation. Besides, the shelter effect of a desk on the indoor thermal environment in these three systems has also been analyzed. The main conclusions are as follows: (1) When the outdoor temperature is below 7 °C and the indoor setting temperature of the room with the composite ACERS is 16 °C and 18 °C, the COP of the combined system is about 2.71 and 2.62, respectively. Besides, the cumulative average heating supply per unit area is about 15.2 W/m2 and 39.3 W/m2 during the experiment. These show that the combined system can meet the heating demand for office buildings in south-central China, and also have the energy saving potential. (2) The composite ACERS can provide a comfortable thermal environment in winter. When the indoor setting temperature is 16 °C and 18 °C, the indoor temperature at the occupied zone is about 16.8–17.4 °C and 18.1–18.8 °C and the humidity fluctuates at 35–40%. The calculated thermal comfort index shows that these two setting conditions both can meet the thermal comfort of 80% occupants at least. (3) The temperature distribution characteristics of the three installation types are different, but they all can meet the temperature requirements. Under the same boundary setting conditions, the average temperature of the ceiling and sidewall ACERS is about 17.8–18.2 °C, and the temperature is higher at nearly 18.6 °C in the composite ACERS, Meanwhile the temperature distribution

Fig. 15. Velocity contours across the X-central plane of the room. (a) Composite ACERS and (b) Ceiling ACERS.

P. Peng, G. Gong and X. Deng et al. / Energy & Buildings 209 (2020) 109712

Fig. 16. Temperature distribution of X section without desk and chair. (a) Composite ACERS, (b) Sidewall AECRS, (c) Ceiling ACERS.

Fig. 17. Temperature distribution of X central plane with desk and chair. (a) Composite ACERS, (b) Sidewall AECRS, (c) Ceiling ACERS.

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CRediT authorship contribution statement Xiaorui Deng: Investigation. Chun Liang: Software. Wenqiang Li: Data curation. Acknowledgments This research was supported by Hunan Provincial Innovation Foundation for Postgraduate Studies (Grant no. CX20190287) and the National Key Technology Support Program (Grant no. 2015BAJ03B01). Pei Peng also acknowledges support from the China Scholarship Council (No. 201906130075). Meanwhile, the Shaoshan tourism development group company and the China Academy of Building Research help us to complete the work. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.enbuild.2019.109712. References

Fig. 18. Temperature distribution at 1.2 m height. (a) Composite ACERS, (b) Sidewall ACERS, (c) Ceiling ACERS.

is more uniform in the composite ACERS. Besides, the temperature gradient in the ceiling ACERS is smaller than that of the sidewall ACERS due to the further heat transfer caused by air penetration. (4) Both of the experimental and CFD simulated temperature distributions demonstrate that the shelter of furniture has an impact on the indoor thermal environment, which can lead to a 1.2 °C temperature difference between inside and outside the desk. So the shelter effect of furniture cannot be ignored in office buildings, and the ACERS with sidewall can improve this shelter effect. The present research provides fundamental and useful information for HVAC engineers to design and apply the ACERS. Meanwhile this study is also helpful to solve the current heating problem in winter in south-central China, and realize the integration of air conditioning in winter and summer in this area.

Declaration of Competing Interest None.

[1] R. Yao, B. Li, K. Steemers, Energy policy and standard for built environment in China, Renew. Energy 30 (2014) 1973–1988. [2] J. Wang, Z. Zhai, Y. Jing, C. Zhang, Influence analysis of building types and climate zones on energetic, economic and environmental performances of BCHP systems, Appl. Energy 88 (2011) 3097–3112. [3] Y. Liu, H. Yan, J.C. Lam, Thermal comfort and building energy consumption implications – a review, Appl. Energy 115 (2014) 164–173. [4] L. Pérez-Lombard, J. Ortiz, C. Pout, A review on buildings energy consumption information, Energy Build. 40 (2008) 394–398. [5] X. Dong, W. Yun, H. Wang, S. Mo, M. Liao, Numerical analysis of temperature non-uniformity and cooling capacity for capillary ceiling radiant cooling panel, Renew. Energy 87 (2016) 1154–1161. [6] W.H. Chiang, C.Y. Wang, J.S. Huang, Evaluation of cooling ceiling and mechanical ventilation systems on thermal comfort using CFD study in an office for subtropical region, Build. Environ. 48 (2012) 113–127. [7] B.W. Olesen, Radiant floor heating in theory and practice, ASHRAE J. 44 (2002) 19–26. [8] Z. Wang, M. Luo, Y. Geng, B. Lin, Y. Zhu, A model to compare convective and radiant heating systems for intermittent space heating, Appl. Energy 215 (2018) 211–226. [9] B. Lin, Z. Wang, H. Sun, Y. Zhu, Q. Ouyang, Evaluation and comparison of thermal comfort of convective and radiant heating terminals in office buildings, Build. Environ. 106 (2016) 91–102. [10] C.K. Wilkins, R. Kosonen, Cool ceiling system: a European air-conditioning alternative, ASHRAE J. (1992) 41–45. [11] T. Catalina, J. Virgone, F. Kuznik, Evaluation of thermal comfort using combined CFD and experimentation study in a test room equipped with a cooling ceiling, Build. Environ. 44 (2009) 1740–1750. [12] J.H. Lim, J.H. Jo, Y.Y. Kim, M.S. Yeo, K.W. Kim, Application of the control methods for radiant floor cooling system in residential buildings, Build. Environ. 41 (2006) 60–73. [13] P. Weitzmann, J. Kragh, P. Roots, S. Svendsen, Modelling floor heating systems using a validated two-dimensional ground-coupled numerical model, Build. Environ. 40 (2005) 153–163. [14] K. Nagano, T. Mochida, Experiments on thermal environmental design of ceiling radiant cooling for supine human subjects, Build. Environ. 39 (2004) 267–275. [15] R. Hu, J.L. Niu, A review of the application of radiant cooling & heating systems in Mainland China, Energy Build. 52 (2012) 11–19. [16] C. Stetiu, Energy and peak power savings potential of radiant cooling systems in US commercial buildings, Energy Build. 30 (1999) 127–138. [17] S. Oxizidis, A.M. Papadopoulos, Performance of radiant cooling surfaces with respect to energy;consumption and thermal comfort, Energy Build. 57 (2013) 199–209. [18] E.Z.E. Conceição, M.M.J.R. Lúcio, Evaluation of thermal comfort conditions in a classroom equipped with radiant cooling systems and subjected to uniform convective environment, Appl. Math. Model. 35 (2011) 1292–1305. [19] K. Kitagawa, N. Komoda, H. Hayano, S.I. Tanabe, Effect of humidity and small air movement on thermal comfort under a radiant cooling ceiling by subjective experiments, Energy Build. 30 (1999) 185–193. [20] X. Sui, X. Zhang, X. Han, Performance analysis on a residential radiant chilled ceiling system and evaluation on indoor thermal environment in summer: an application, Build. Serv. Eng. Res. Technol. 34 (2013) 317–331. [21] M. Jradi, S. Riffat, Experimental and numerical investigation of a dew-point cooling system for thermal comfort in buildings, Appl. Energy 132 (2014) 524–535. [22] S.J. Rees, P. Haves, An experimental study of air flow and temperature distribution in a room with displacement ventilation and a chilled ceiling, Build. Environ. 59 (2013) 358–368.

P. Peng, G. Gong and X. Deng et al. / Energy & Buildings 209 (2020) 109712 [23] Jeong J.W, S.A. Mumma, Ceiling radiant cooling panel capacity enhanced by mixed convection in mechanically ventilated spaces, Appl. Therm. Eng. 23 (2003) 2293–2306. [24] T. Imanari, T. Omori, K. Bogaki, Thermal comfort and energy consumption of the radiant ceiling panel system.: comparison with the conventional all-air system, Energy Build. 30 (1999) 167–175. [25] X. Wu, J. Zhao, B.W. Olesen, F. Lei, F. Wang, A new simplified model to calculate surface temperature and heat transfer of radiant floor heating and cooling systems, Energy Build. 105 (2015) 285–293. [26] J. Miriel, L. Serres, A. Trombe, Radiant ceiling panel heating–cooling systems: experimental and simulated study of the performances, thermal comfort and energy consumptions, Appl. Therm. Eng. 22 (2002) 1861–1873. [27] K. Lin, Y. Zhang, X. Xu, H. Di, R. Yang, P. Qin, Experimental study of under-floor electric heating system with shape-stabilized PCM plates, Energy Build. 37 (2005) 215–220. [28] X. Jin, X. Zhang, Y. Luo, A calculation method for the floor surface temperature in radiant floor system, Energy Build. 42 (2010) 1753–1758. [29] D. Zhang, C. Ning, Z. Wang, Experimental and numerical analysis of lightweight radiant floor heating system, Energy Build. 61 (2013) 260–266. [30] A. Lipczynska, J. Kaczmarczyk, A.K. Melikov, Thermal environment and air quality in office with personalized ventilation combined with chilled ceiling, Build. Environ. 92 (2015) 603–614. [31] M. Younis, K. Ghali, N. Ghaddar, Performance evaluation of the displacement ventilation combined with evaporative cooled ceiling for a typical office in Beirut, Energy Convers. Manag. 105 (2015) 655–664. [32] M. Itani, K. Ghali, N. Ghaddar, Increasing energy efficiency of displacement ventilation integrated with an evaporative-cooled ceiling for operation in hot humid climate, Energy Build. 105 (2015) 26–36. [33] X. Hao, G. Zhang, Y. Chen, S. Zou, D.J. Moschandreas, A combined system of chilled ceiling, displacement ventilation and desiccant dehumidification, Build. Environ. 42 (2007) 3298–3308. [34] S.G. Hodder, D.L. Loveday, K.C. Parsons, A.H. Taki, Thermal comfort in chilled ceiling and displacement ventilation environments: vertical radiant temperature asymmetry effects, Energy Build. 27 (1998) 167–173. [35] D. Zhang, X. Xia, C. Ning, A dynamic simplified model of radiant ceiling cooling integrated with underfloor ventilation system, Appl. Therm. Eng. 106 (2016) 415–422. [36] L.Z. Zhang, J.L. Niu, Indoor humidity behaviors associated with decoupled cooling in hot and humid climates, Build. Environ. 38 (2003) 99–107. [37] F. Causone, F. Baldin, B.W. Olesen, S.P. Corgnati, Floor heating and cooling combined with displacement ventilation: possibilities and limitations, Energy Build. 42 (2010) 2338–2352. [38] P. Peng, G. Gong, X. Mei, J. Liu, F. Wu, Investigation on thermal comfort of air carrying energy radiant air-conditioning system in south-central China, Energy Build. 182 (2019) 51–60. [39] G. Gong, D. Yin, Algorithm of room input exergy and application analysis of exergy cost for air carrying energy radiant air-conditioning system, J. Hunan Univ. Nat. Sci. (2019) 46.

13

[40] G. Gong, L. Jia, M. Xiong, Investigation of heat load calculation for air carrying energy radiant air-conditioning system, Energy Build. 138 (2017) 193– 205. [41] G. Gong, H. Yang, H. Su, C. Wang, C. Xu, The research on simplified algorithm of radiant heat transfer for air carry energy radiant air-conditioning terminal system, J. Hunan Univ. Nat. Sci. 40 (2013) 31–38. [42] J. Liu, G. Gong, R. Liu, P. Peng, Investigation of operation performance of air carrying energy radiant air-conditioning system based on CFD and thermodynamic model, Build. Simul. 7 (2018) 1–15. [43] C. Xu, G. Gong, H. Yang, S. Gong, Numerical study of moisture condensation on pore panels of air-carrying energy radiation air-conditioning system, Build. Sci. 30 (2014) 79–84. [44] J. Han, G. Gong, H. Yang, J. Liu, Study on thermal environment and energy tranfer on air carrying energy radiant air-conditioning terminal in large space, Build. Sci. 33 (2017) 113–119. [45] J.D. Posner, C.R. Buchanan, D. Dunn-Rankin, Measurement and prediction of indoor air flow in a model room, Energy Build. 35 (2003) 515–526. [46] Q. Chen, Comparison of different k-$\varepsilon$ models for indoor air flow computations, Numer. Heat Transf. Part B Fundam. 28 (1995) 353–369. [47] A. Fluent, User’s Guide Release 16.1, Ansys Inc., 2015. [48] X. Zheng, Y. Han, H. Zhang, W. Zheng, D. Kong, Numerical study on impact of non-heating surface temperature on the heat output of radiant floor heating system, Energy Build. 155 (2017) 198–206. [49] B. Ning, Y. Chen, H. Liu, S. Zhang, Cooling capacity improvement for a radiant ceiling panel with uniform surface temperature distribution, Build. Environ. 102 (2016) 64–72. [50] C. Zhang, P.K. Heiselberg, Q. Chen, M. Pomianowski, Numerical analysis of diffuse ceiling ventilation and its integration with a radiant ceiling system, Build. Simul. 10 (2017) 203–218. [51] S.R. Mathur, J.Y. Murthy, Coupled ordinates method for multigrid acceleration of radiation calculations, J. Thermophys. Heat Transf. 13 (1999) 467–473. [52] P.O. Fanger, Thermal Comfort, Analysis and Applications in Environmental Engineering, McGraw-Hill, New York, 1972. [53] ASHRAE Standard 55-2010, Thermal Environmental Conditions for Human Occupancy, American Society of Heaing Refigerating and Air-Conditioning Engineers, Atlanta, 2010. [54] A.P. Gagge, J.A.J. Stolwijk, J.D. Hardy, Comfort and thermal sensations and associated physiological responses at various ambient temperatures, Environ. Res. 1 (1967) 1–20. [55] L. Meng, X. Kai, Z. YU, L. Cong, Nighittime performance analysis of air source heat pump heating system in Chongqing rural residential, J. Civ. Arch. Environ. Eng. 37 (2015) 87–97. [56] L. Kirkup, R.B. Frenkel, An Introduction to Uncertainty in Measurement Using the Gum (Guide to the Expression of Uncertainty in Measurement), (2006). [57] J.P. Holman, W.J. Gajda, Experimental Methods for Engineers, McGraw-Hill, New York, 2001. [58] Q. Chen, J. Srebric, A procedure for verification, validation, and reporting of indoor environment CFD analyses, Hvac R. Res. 8 (2002) 201–216.