Cooling Capacity and Energy Performance of Open-Type Ceiling Radiant Cooling Panel System with Air Circulators

energies

Article Cooling Capacity and Energy Performance of Open-Type Ceiling Radiant Cooling Panel System with Air Circulators

Mi-Su Shin 1, Ji-Su Choi 2 and Kyu-Nam Rhee 1,*

1 Department of Architectural Engineering, Pukyong National University, Busan 48513, Korea; [email protected] 2 Division of Architectural and Fire Protection Engineering, Graduate School, Pukyong National University, Busan 48513, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-51-629-6090

Abstract: Ceiling radiant cooling panel (CRCP) systems are being increasingly applied to commercial buildings due to their high thermal comfort level and energy efficiency. It is recommended that CRCP systems should be operated at a relatively high chilled water temperature to prevent condensation and save energy. However, even though a high chilled water temperature is effective for achieving condensation-free operation and high chiller efficiency, it can lead to insufficient cooling capacity. In this study, a method of enhancing the cooling capacity of CRCP systems was investigated through mock-up chamber tests. The open-type installation of CRCPs and the combination of air circulators were used to enhance the cooling capacity and energy performance of CRCP systems. Experimental results showed that compared to a conventional CRCP system, the cooling capacity of an open-type CRCP system with air circulators increased by up to 26.2%, and its cooling energy consumption decreased by up to 26.4%. Additionally, the open-type CRCP system with air circulators reduced the difference between the room air temperature and mean chilled water temperature. Thus, the proposed system can operate at a relatively high chilled water temperature, which is effective for reducing condensation risk and cooling energy consumption. Keywords: ceiling radiant cooling panel; cooling capacity; open-type installation; air circulators; Citation: Shin, M.; Choi, J.; Rhee, K. cooling energy consumption Cooling Capacity and Energy Performance of Open-Type Ceiling Radiant Cooling Panel System with Air Circulators. Energies 2021, 14, 5. 1. Introduction https://dx.doi.org/10.3390/en140 10005 Ceiling radiant cooling panel (CRCP) systems have been widely applied to space cooling due to their high thermal comfort level, energy-saving potential, and harmonization Received: 29 November 2020 with architectural design [1,2]. Closed-type CRCPs are generally installed, where an entire Accepted: 17 December 2020 ceiling surface is covered with CRCPs. As closed-type CRCP systems separate a plenum Published: 22 December 2020 and an occupied space, it is difficult to utilize cooled plenum air to cool the occupied space. This separation does not lead to problems for the top-insulated panels that minimize the Publisher’s Note: MDPI stays neu- heat transfer at the upper surface of CRCPs [3,4]. However, metal ceiling panels without tral with regard to jurisdictional claims top insulation, which are commonly installed in office buildings, can absorb heat from in published maps and institutional the plenum space [5,6]. If metal ceiling panels are installed with closed-type CRCPs, affiliations. the occupied space is cooled by only the lower surface of the CRCPs only. This might lead to insufficient cooling capacity and a consequent increase in cooling energy. In contrast, in open-type CRCPs, openings can be created between two adjacent CRCPs or between

Copyright: © 2020 by the authors. Li- CRCPs and walls. Cooled plenum air can move down to the occupied space through censee MDPI, Basel, Switzerland. This these openings. This movement of plenum air can enhance not only the air mixture in the article is an open access article distributed occupied space but also the heat transfer rate at the CRCP surface [2]. under the terms and conditions of the Shin et al. reported that the cooling capacity of CRCP systems can be enhanced by Creative Commons Attribution (CC BY) promoting the movement of plenum air through the openings in open-type CRCPs [7]. license (https://creativecommons.org/ However, the limitation of the study was that results were obtained using only a numerical licenses/by/4.0/). method. In the follow-up study, the numerical model of open-type CRCPs was veriﬁed

Energies 2021, 14, 5. https://dx.doi.org/10.3390/en14010005 https://www.mdpi.com/journal/energies Energies 2021, 14, 5 2 of 15

against mock-up test results, and it was shown that open-type CRCPs enhanced cooling capacity by 54–80%, depending on the opening area and panel layout [2]. However, the enhancement of cooling capacity considered only the natural convection effect of the movement of the plenum air. Another method of enhancing the cooling capacity of CRCP systems is to cause forced convection by increasing the air speed at the panel surface. Jeong and Mumma showed that CRCPs combined with mechanical ventilation could benefit from mixed (natural + forced) convection. Mixed convection was generated by air flow from a nozzle diffuser. Cooling capacity was enhanced by 5–35% compared to CRCPs with only natural convection [8]. Kim and Leibundgut proposed a CRCP system combined with an airbox convector, which could generate air flow on the panel surface. The mixed convection effect increased cooling capacity by 32% [9]. Novoselac et al. utilized a high-aspiration diffuser to create a specific air flow pattern along cooling panels to increase the convective heat transfer coefficient. Full-scale test results showed that the high-aspiration diffuser increased the heat transfer coefficient by 4–17% [10]. Karmann et al. [11] demonstrated that in a thermally activated building system (TABS), a ceiling fan increased cooling capacity by up to 22%, and small fans installed between the ceiling and acoustic clouds increased cooling capacity by up to 26%. The abovementioned studies on the effect of forced convection on cooling capacity examined the enhancement of the cooling capacity of closed-type CRCP systems. This study aims to investigate the impact of forced convection on the cooling capacity of open-type and closed-type CRCP systems. In addition, previous studies increased the forced convection effect using heating, ventilation, and air conditioning (HVAC) systems such as nozzle diffusers [8,10], convectors [9], and ceiling fans [11]. As these systems are designed and installed for the specific purpose of generating air flow on a panel surface, additional space and cost are required to install them. In addition, it is difficult to modify the air flow direction or flow rate because HVAC systems are installed at a specific location in a room. In this context, the present study applied air circulators to enhance the effect of forced convection on CRCP surfaces. Air circulators are becoming well known as an efficient method of increasing the overall air velocity in a conditioned space. This can contribute to the improvement of air diffusion and thermal uniformity. In addition, they reduce cooling energy consumption because the cooling set-point temperature can be decreased due to the elevated air velocity in an occupied space [12–14]. If an air circulator is configured to transport room air to CRCP surfaces, the enhancement of cooling capacity can be expected owing to the increased heat transfer rate at a panel surface. Considering that an air circulator can move air over a long distance, it was selected as an air circulation device to enhance the heat transfer even in a CRCP located far from the circulator. Based on this assumption, this study was conducted to improve cooling capacity and energy performance by utilizing the air circulator, which would be used as a low-cost and affordable auxiliary cooling system. Previous studies on the enhancement of cooling capacity have mainly focused on the evaluation of cooling capacity. Few studies have investigated the energy consumption of open-type CRCPs combined with air circulation devices. Therefore, the present study evaluated not only the cooling capacity but also the energy performance of an open-type CRCP system with air circulators for forced convection. For this purpose, a mock-up test chamber was constructed to analyze the cooling capacity and energy consumption depending on the CRCP installation types and the application of air circulators. The closed-type CRCP system was realized by covering an entire ceiling surface with CRCPs, and the open-type CRCP system was realized by providing an opening area between perimeter CRCPs and the chamber wall. Two air circulators were deployed to create air flow at the CRCP surfaces. Cooling capacity was evaluated based on the heat absorption by the chilled water in the CRCPs in accordance with EN 14240 [15]. Energy consumption was evaluated by adding the electricity consumed by the chiller and that consumed by the chilled water circulation pump. The outdoor air system used to han- Energies 2021, 14, 5 3 of 15

dle the ventilation requirement and latent load was not included in the scope of the present study, because the outdoor air system generates a non-isothermal air ﬂow, which makes it difﬁcult to distinguish the impact of air velocity on the cooling capacity. As addressed in test standards such as EN 14240 [15] and ASHRAE 138 [16], internal (sensible) cooling load was only considered to investigate the impact of air circulation on cooling capacity and energy consumption. This paper is organized as follows. Section2 describes the development of the mock-up test chamber for the CRCP systems and performance-evaluation methods. Section3 describes the cooling capacity and energy consumption of the CRCP systems and the application of air circulators. Section4 discusses the implications and limitations of this study. Section5 presents the conclusions of the study.

2. Research Methods 2.1. Development of Mock-Up Test Chamber A mock-up test chamber was constructed to investigate the impact of the open- type installation on the cooling capacity and energy consumption of the CRCP system. The chamber could be equipped with open-type or closed-type CRCPs, as illustrated in Figure1. The dimensions of the test chamber were 3.8 m (W) × 3.7 m (L) × 2.9 m (H), which represent the typical condition of an office space. The cooling load was represented by electrical dummies in accordance with EN 14240 [15]. The simulated cooling load was approximately 900 W (64.0 W/m2), which corresponds to the internal cooling load of conventional office buildings [17]. To minimize the heat gain or loss through the test chamber, it was constructed inside an existing air-conditioned building, and its envelope was composed of metal panels with 100 mm thick insulation. In addition to the thermal insulation, an electrical heat pump system was used to control the temperature outside the test chamber to minimize the heat transfer through the test chamber envelope. Twenty-five CRCPs were installed on the ceiling of the test chamber. The size of each CRCP was 0.6 m × 0.6 m, and it was composed of a perforated aluminum panel (thickness = 3 mm), a chilled water pipe, and a heat conduction plate. The areas of the ceiling surface and CRCP surface were 14.06 m2 and 9.0 m2, respectively. The ceiling surface that was not covered by CRCPs was blocked with gypsum panels when conducting experiments for the closed-type CRCP system. For the open-type CRCP system, the gypsum panels were removed to represent the opening through which plenum air can move down to the conditioned space. The CRCP system was designed with five hydronic subcircuits for the chilled water supply. An air-cooled chiller was used to supply chilled water. The temperature of chilled water was accurately controlled by a water storage tank and 3-way mixing loops, as shown in Figure1c. Figure2 shows the images of the developed test chamber. Temperature sensors for supply and return water and flow meters were installed in each subcircuit to analyze the cooling capacity of the CRCPs. Room air temperature and air velocity in the conditioned space were measured 1.1 m above the floor. The information on measurement devices is summarized in Table1. Energies 2021, 14, 5 4 of 15 Energies 2020, 13, x FOR PEER REVIEW 4 of 16

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(a) (b)

(c)

FigureFigure 1. Conﬁguration 1. Configuration of the of mock-upthe mock-up test test chamber chamber for for the the evaluation(c )eval uation of of ceiling ceiling radiant radiant cooling cooling panelpanel (CRCP)(CRCP) perfor- performance: mance: (a) floor/ceiling plan view; (b) sectional view; (c) mechanical system. (a) ﬂoor/ceilingFigure 1. Configuration plan view; ( bof) sectionalthe mock-up view; test (chamberc) mechanical for the system. evaluation of ceiling radiant cooling panel (CRCP) performance: (a) floor/ceiling plan view; (b) sectional view; (c) mechanical system.

(a) (b)

Figure 2. Developed test chamber:(c) (a) inside view; (b) mechanical system; (c) CRCPs; (d) plenum(d) space and upper side of CRCPs. FigureFigure 2. Developed 2. Developed test test chamber: chamber: (a )(a inside) inside view; view; (b)) mechanical mechanical system; system; (c) (CRCPs;c) CRCPs; (d) plenum (d) plenum space and space upper and side upper of side CRCPs. of CRCPs.

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2.2. Performance Evaluation Method Four cases were considered to investigate the impact of the open-type installation and air circulators on the cooling performance of the CRCP system, as shown in Figure 3. The closed-type CRCPs without air circulators were designated as a reference case (Case 1). The closed-type CRCPs with air circulators (Case 2) were configured to investigate the Energies 2021, 14, 5 5 of 15 impact of elevated air velocity on the cooling performance of the closed-type CRCP system. The open-type CRCPs without air circulators (Case 3) were used to investigate the impact of the open-type installation on the enhancement of the cooling capacity and en- ergyTable performance 1. Specifications of the of measurement CRCP system. device. The open-type CRCPs with air circulators (Case 4) were utilized to examine the further enhancement of cooling performance by applying the elevated air velocityItem to the open-type Type CRCP system. Range Accuracy Room air temperature NTC thermistor −20–+70 ◦C ±0.2 ◦C ◦ ◦ TablePlenum 1. Specifications air temperature of measurement T-type device. thermocouple −250–+350 C ±0.5 C Room air velocity Hot wire 0–5 m/s ±0.03 m/s Chilled waterItem temperature RTD PT100Type −70–+500Range◦C ±Accuracy0.3 ◦C ChilledRoom waterair temperature flow rate Volumetric NTC thermistor 0.2–42 L/min−20–+70 °C ±0.2 2% °C Plenum air temperature T-type thermocouple −250–+350 °C ±0.5 °C 2.2. PerformanceRoom air Evaluationvelocity Method Hot wire 0–5 m/s ±0.03 m/s ChilledFour water cases temperature were considered to investigate RTD PT100 the impact of−70–+500 the open-type °C installation ±0.3 °C andChilled air circulators water flow on the rate cooling performance Volumetric of the CRCP system,0.2–42 L/min as shown in 2% Figure 3. The closed-type CRCPs without air circulators were designated as a reference case (Case 1). TheFor closed-typeCases 2 and CRCPs4, two air with circulators air circulators were (Caseused to 2) increase were configured the air velocity to investigate at the CRCPthe impact surfaces. of elevated The specifications air velocity of on the the air cooling circulator performance are summarized of the closed-type in Table 2. CRCPThey weresystem. located The at open-type a height CRCPsof 2.1 m without above the air floor circulators to create (Case air 3)flow were as usedclose toto investigatethe CRCP surfacethe impact as possible, of the open-type as shown installation in Figure 4. on The the air enhancement flow rate from of the each cooling air circulator capacity was and approximatelyenergy performance 284 m of3/h, the which CRCP generated system. The an open-typeair velocity CRCPs of 4.5 withm/s at air the circulators outlet of (Case the circulator.4) were utilized A multichannel to examine anemomaster the further enhancement with ten velocity of cooling probes performance was deployed by applying to ana- lyzethe elevatedthe impact air of velocity the air tocirculator the open-type on the CRCPair velocity system. at the CRCP surface.

(a) (b)

FigureFigure 3. 3. EvaluationEvaluation cases: cases: (a) ( aclosed-type) closed-type CRCPs; CRCPs; (b) (closed-typeb) closed-type CRCPs CRCPs with with air circulators; air circulators; (c) open-type (c) open-type CRCPs; CRCPs; (d) open-type(d) open-type CRCPs CRCPs with with air circulators. air circulators.

For Cases 2 and 4, two air circulators were used to increase the air velocity at the CRCP surfaces. The specifications of the air circulator are summarized in Table2. They were located at a height of 2.1 m above the floor to create air flow as close to the CRCP surface as possible, as shown in Figure4. The air flow rate from each air circulator was approximately 284 m3/h, which generated an air velocity of 4.5 m/s at the outlet of the circulator. A multichannel anemomaster with ten velocity probes was deployed to analyze the impact of the air circulator on the air velocity at the CRCP surface. EnergiesEnergies2021 2020,, 14, 13 5 , x FOR PEER REVIEW 6 of6 15of 16

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TableTable 2. Speciﬁcations2. Specifications of theof the air air circulator. circulator. Table 2. Specifications of the air circulator. ItemItem PerformancePerformance Data Data Shape FanItem stepFan * step * IPerformance I II Data II III III Shape Fan speed 1200 rpm 1700 rpm 2000 rpm Fan speedstep * 1200 rpm I 1700 II rpm III 2000 rpm 3 3 3 AirFan ﬂow Airspeed rateflow rate 212 1200 m3 rpm/h 212 m /h 1700 284 rpm284 m3 m/h/h 2000 rpm357 357 m m3/h/h MaximumMaximum air air velocity 3 3 7.5 m/s 3 Air flow rate 212 m /h 2847.5 m m/s/h 357 m /h MaximumvelocityAir air travel velocity range 7.5 m/s 20 m Air travel range 20 m Air travel range 20 m 2 2 AreaArea covered covered 20 m 20 m Area covered* Fan step II (1700 rpm and 284 m 203/h) m2 was used in this study. * Fan step II (1700 rpm and 284 m3/h) was used in this study. * Fan step II (1700 rpm and 284 m3/h) was used in this study.

(a) (b) Figure 4. Concept(a) of air circulator test: (a) plan view; (b) section(b )view. FigureFigure 4. Concept 4. Concept of air ofcirculator air circulator test: ( test:a) plan (a) view; plan view; (b) section (b) section view. view. In each case, the cooling performance of the CRCP systems was investigated in terms Inof eachcoolingIn each case, capacity case, the cooling the and cooling electricity performance performance consumptio of the ofCRCPn. the Cooling systems CRCP capacity systems was investigated was was evaluated investigated in terms by meas- in of coolingtermsuring of capacity the cooling heat andremoved capacity electricity andby the electricityconsumptio CRCPs under consumption.n. Cooling steady-state capacity Cooling conditions, was capacity evaluated while was bya evaluatedcooling meas- load uringbyof the measuring 900 heat W removed was the applied heat by removedthe by CRCPs five electrical by under the CRCPssteady-state dummies. under Duringconditions, steady-state the whiletest, conditions, the a cooling average whileload surface a of 900coolingtemperature W was load applied of 900of the W by wasCRCPs five applied electrical was bycontrolled fivedummies. electrical at 17 During dummies.°C to removethe test, During thethe thecoolingaverage test, theload. surface average Cooling ◦ temperaturesurfacecapacity temperature of was the determined CRCPs of thewas when CRCPs controlled the was 60 minat controlled 17 stan °Cdard to atremove deviation 17 C the to removeofcooling mean thewaterload. cooling Coolingtemperature, load. capacityCoolingwall was surface capacity determined temperature, was when determined theand 60 reference min when stan the (roomdard 60 deviation min air) standardtemperature of mean deviation watermet the temperature, of steady-state mean water cri- walltemperature, surfaceteria defined temperature, wall in EN surface 14240. and temperature,reference Cooling capacity(room and air) referencewa temperatures calculated (room metby air) dividing the temperature steady-state the total met cri- heat the re- teriasteady-state definedmoved byin criteriaENthe 14240.CRCP defined Coolingarea, in as EN capacityformulated 14240. Coolingwa sin calculated Equations capacity by was(1) dividing and calculated (2). the The total by amount dividing heat re-of the heat movedtotalremoved by heat the removed wasCRCP calculated area, by the as CRCP basedformulated area, on the as in formulateddifference Equations between in (1) Equations and the (2). temperatures (1)The and amount (2). The of of the amountheat chilled removedofwater heat was removed in thecalculated CRCP was hydroniccalculatedbased on subcircuits.the based difference on the differencebetween the between temperatures the temperatures of the chilled of the chilled water in the CRCP hydronic subcircuits. water in the CRCP hydronic subcircuits.Q = q = c m (T -T ) T  i  p i r,i s,i (1) i i QQ == q q== c cmpm(T(T-T−)T ) (1) T T ∑i i ∑p i i r,i r,i s,i s,i (1) i i i i Q P = T (2) Q QTA P =T T (2) P = AT (2) AT where QT is the total heat removed [W], qi is the heat removed in the i-th circuit [W], cp is where QT is the total heat removed [W], qi is the heat removed in the i-th circuit [W], cp is the specific heat of chilled water [J/kgK], mi is the mass flow rate for the i-th circuit [kg/s], wherethe specificQT is the heat total of heat chilled removed water [J/kgK],[W], qi is m thei is heat the massremoved flow in rate the for i-th the circuiti-th circuit [W], c [kg/s],p is Tr,i and Ts,i are the return and supply water temperatures for the i-th circuit, respectively theT specificr,i and T heats,i are of the chilled return water and [J/kgK], supply watermi is the temperatures mass flow rate for thefor ithe-th i circuit,-th circuit respectively [kg/s], ◦ [°C], P is cooling capacity [W/m2 2], and AT is the total CRCP area [m2 2]. Tr,i [andC], TPs,iis are cooling the return capacity and [W/m supply], water and A temperaturesT is the total CRCP for the area i-th [m circuit,]. respectively [°C], P isEnergy coolingEnergy performance capacityperformance [W/m was was2], and evaluated evaluated AT is the byby total operating CRCP the thearea CRCP CRCP [m2]. system system at ata set-point a set-point tem- temperatureEnergyperature performance of of26 26 °C◦ Cand and was applying applyingevaluated a cooling a by cooling operating load load of the900 of 900 CRCPW to W the system to thetest test chamber.at a chamber.set-point Simple tem- Simple on/off peratureon/offcontrol of control 26 was °C andimplemented was applying implemented ato cooling operate to operateload the ofCRCP 900 the W CRCPsystem to the system totest clearly chamber. to clearlycompare Simple compare the on/off operation the controloperationtime was and implemented time energy and consumption. energy to operate consumption. The the circulatio CRCP The systemn circulation pump to wasclearly pumpstopped compare was when stopped the the operation room when air the tem- timeroom andperature airenergy temperature reached consumption. 26 reached °C and The 26restarted circulatio◦C and ifn restarted pumpthe room was if theair stopped roomtemperature when air temperature the exceeded room air exceeded 27 tem- °C. The perature27electricity◦C. reached The electricityconsumption 26 °C and consumption restartedof the chiller, if of the thecirculation room chiller, air circulationpump,temperature and pump,air exceeded circulator and 27 air was °C. circulator measured The electricity consumption of the chiller, circulation pump, and air circulator was measured

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was measured while operating the CRCP system in each case under the abovementioned conditions for 10 h.

3. Results 3.1. Cooling Capacity Analysis Table3 and Figure5 show the evaluation results of cooling capacity for the four cases. The cooling capacity of the closed-type CRCP system (Case 1) was 96.1 W/m2, while that of the open-type CRCP system (Case 3) was 104.3 W/m2. The open-type CRCP system reduced the room air temperature by 1.6 ◦C compared to the closed-type CRCP system under the same operating conditions. The open-type CRCP system also reduced the temperature difference between room air temperature and plenum air temperature, which implies that the mixture of room air and plenum air was improved compared to the closed-type CRCP system. This can be attributed to the movement of cooled plenum air, as reported in a previous study [2]. Thus, the open-type CRCPs can effectively enhance cooling capacity under the same operating conditions as those of the closed-type CRCPs.

Table 3. Evaluation results of cooling capacity.

Room Air Plenum Air Temperature Air Velocity Heat Removed Cooling Capacity Case Temperature Temperature Difference * [m/s] [W] [W/m2] [◦C] [◦C] [◦C] Case 1 24.2 20.8 3.4 0.11 865 96.1 (Baseline) Case 2 23.4 19.8 3.6 0.19 1013 112.5 (+17.0%) Case 3 22.6 22.3 0.3 0.07 940 104.3 (+ 8.5%) Energies 2020, 13, x FOR PEER REVIEW 8 of 16 Case 4 22.2 21.0 1.2 0.21 1091 121.2 (+26.2%) * Difference between room air temperature and plenum air temperature.

FigureFigure 5. 5.MeasurementMeasurement result result of ofcooling cooling capacity capacity for for different different cases. cases. The cooling capacity of the closed-type CRCPs (Case 2) with air circulators was higher than that of the closed-type CRCPs without air circulators (Case 1) by 17.0%. Figure6 shows the air velocity measured at the axis of air ﬂow from the air circulator, as described in Figure4. Although the air velocity gradually decreased, it was much higher than the typical air velocity (e.g., 0.2 m/s [14]) in the conditioned space. It was considered that the elevated air velocity at the CRCP surfaces promoted convective heat transfer between the panel surfaces and room air, which led to the increased cooling capacity and decreased room air temperature.

Figure 6. Air velocity measured in the direction of air flow by air circulator.

Regarding the surface condensation problem, it is of importance to maintain the panel surface temperature above the dew-point temperature in the conditioned space. Ta- ble 4 shows the range of panel surface temperatures (min, max and mean values), room air temperature, relative humidity and dew-point temperature in each case. It was found that condensation was prevented because the minimum surface temperature was always higher than dew-point temperature.

Table 4. Panel surface temperature and dew-point temperature.

Panel Surface Temperature [°C] Room Air Relative Dew-Point Case Min Max Mean Temperature [°C] Humidity [%] Temperature [°C] Case 1 16.3 19.0 17.2 24.2 42 10.5 Case 2 16.4 18.6 17.2 23.4 42 9.7 Case 3 16.0 18.6 16.9 22.6 60 14.4 Case 4 16.5 18.9 17.4 22.2 57 13.3

In addition, the use of openings or air circulators reduced the difference (Δθ) between the mean chilled water temperature and room air temperature, which is formulated as Equation (3). T + T Δθ = − r s Ta (3) 2

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Energies 2021, 14, 5 8 of 15 Figure 5. Measurement result of cooling capacity for different cases.

Figure 6. Air velocity measured in the direction of air flow by air circulator. Figure 6. Air velocity measured in the direction of air flow by air circulator. When forced air flow was generated on the surface of the open-type CRCPs using air circulatorsRegarding (Case the 4), coolingsurface capacitycondensation increased problem, up to 121.2it is of W/m importance2, which to was maintain the largest the amongpanel surface all cases. temperature This cooling above capacity the wasdew-point higher temperature than that of the in the closed-type conditioned CRCP space. system Ta- (Caseble 4 shows 1) and the range open-type of panel CRCPs surface without temper airatures circulators (min, (Casemax and 3) by mean 26.2% values), and 16.2%, room respectively.air temperature, Thus, relative cooling humidity performance and dew-po can beint further temperature enhanced in ifeach the case. open-type It was CRCP found systemthat condensation is operated was with prevented a supplementary because air th circulatione minimum device. surface temperature was always higherRegarding than dew-point the surface temperature. condensation problem, it is of importance to maintain the panel surface temperature above the dew-point temperature in the conditioned space. TableTable4 shows 4. Panel thesurface range temperature of panel and surface dew-point temperatures temperature. (min, max and mean values), room air temperature, relative humidity and dew-point temperature in each case. It was Panel Surface Temperature [°C] Room Air Relative Dew-Point Case found that condensation was prevented because the minimum surface temperature was Min Max Mean Temperature [°C] Humidity [%] Temperature [°C] always higher than dew-point temperature. Case 1 16.3 19.0 17.2 24.2 42 10.5 Case 2 16.4 18.6Table 4. Panel 17.2 surface temperature 23.4 and dew-point temperature. 42 9.7 Case 3 16.0 18.6 16.9 22.6 60 14.4 Panel Surface Temperature [◦C] Room Air Relative Dew-Point CaseCase 4 16.5 18.9 17.4 22.2 57 13.3 Temperature [◦C] Humidity [%] Temperature [◦C] Min Max Mean Case 1 16.3 19.0In addition, 17.2 the use of openings 24.2 or air circulators reduced 42 the difference 10.5(Δθ) between Case 2 16.4the 18.6 mean chilled 17.2 water temperature 23.4 and room air temperature, 42 which is formulated 9.7 as Case 3 16.0Equation 18.6 (3). 16.9 22.6 60 14.4 Case 4 16.5 18.9 17.4 22.2 57 13.3 T + T Δθ = T − r s (3) a 2 In addition, the use of openings or air circulators reduced the difference (∆θ) between the mean chilled water temperature and room air temperature, which is formulated as Equation (3). T + T ∆θ = T − r s (3) a 2 ◦ where Ta, Tr, and Ts denote the temperatures [ C] of room air, supply chilled water, and return chilled water, respectively. A smaller ∆θ implies that the CRCP system can operate at a chilled water temperature that is close to the room air temperature. Figure7 shows the relationship between cooling capacity and temperature difference (∆θ). In the open-type CRCP system with air circulators, a higher cooling capacity was achieved at a smaller value of ∆θ compared to the other cases. This was because in the open-type installation, a part of the cooling load was removed by cooled plenum air and the air circulators increased the heat transfer rate between the CRCP surfaces and room air. Therefore, a high chilled water temperature that is close to the room air temperature can be used to obtain the required cooling capacity when the open-type installation and air circulators are applied. This can not only improve the energy efficiency of the chiller unit but also mitigate the condensation risk at panel surfaces. Energies 2020, 13, x FOR PEER REVIEW 9 of 16

where Ta, Tr, and Ts denote the temperatures [°C] of room air, supply chilled water, and return chilled water, respectively. A smaller Δθ implies that the CRCP system can operate at a chilled water temperature that is close to the room air temperature. Figure 7 shows the relationship between cooling capacity and temperature difference (Δθ). In the open-type CRCP system with air circulators, a higher cooling capacity was achieved at a smaller value of Δθ compared to the other cases. This was because in the open-type installation, a part of the cooling load was removed by cooled plenum air and the air circulators increased the heat transfer rate between the CRCP surfaces and room air. Therefore, a high chilled water temperature that is close to the room air temperature can be used to obtain the required cooling capacity when the open-type installation and Energies 2021, 14, 5 9 of 15 air circulators are applied. This can not only improve the energy efficiency of the chiller unit but also mitigate the condensation risk at panel surfaces.

Figure 7. Relationship between ∆θ and cooling capacity. Figure 7. Relationship between Δθ and cooling capacity. 3.2. Energy Consumption Analysis 3.2. EnergyFigure Consumption8 shows the Analysis measurement results of the room air temperature, pump operation, andFigure chiller 8 operation.shows the measurement These results wereresults used of the to room analyze air thetemperature, energy consumption pump opera- for tion,each and case. chiller As shownoperation. in FigureThese 8resultsa, the we roomre used air temperature to analyze the was energy relatively consumption stable in forCase each 1 becausecase. As theshown CRCPs in Figure removed 8a, heatthe room at the air same temperature rate at which was the relatively cooling stable loadwas in Caseapplied. 1 because The pumpthe CRCPs operated removed continuously heat at th duringe same the rate test at periodwhich the because cooling chilled load waterwas applied.was continuously The pump suppliedoperated tocontinuously the CRCP systemduring tothe maintain test period the because steady-state chilled condition. water wasThe continuously chiller operated supplied intermittently to the CRCP because system it repeatedly to maintain performed the steady-state on/off operations condition. in Theorder chiller to maintain operated a intermittently constant chilled because water temperatureit repeatedly in performed the water tank.on/off operations in order toIn maintain Case 2, the a constant room air chilled temperature water temperature was not stable in the because water tank. cooling capacity was enhancedIn Case by 2, thethe airroom circulators, air temperature as described was inno Sectiont stable 3.1because. The enhancedcooling capacity cooling was capacity en- ◦ hancedreduced by thethe roomair circulators, air temperature as described to less in than Section 26 3.1.C,causing The enhanced the circulation cooling capacity pump to ◦ reducedstop. Then, the room the room air temperature air temperature to less increased than 26 °C, until causing the CRCP the circulation system restarted pump to at stop. 27 C. Then,Owing the to room this air on/off temperature operation increased of the CRCP until the system, CRCP the system pump restarted operation at time27 °C. in Owing Case 2 towas this less on/off than operation that in Case of the 1. This CRCP led system, to the energy-saving the pump operation effect of time the CRCP in Case system; 2 was this less is thandiscussed that inin Case subsequent 1. This led paragraphs. to the energy-saving effect of the CRCP system; this is dis- cussedThe in subsequent fluctuation ofparagraphs. room air temperature was also found in Case 3, as shown in Figure8c. However, the room air temperature decreased more rapidly compared to Case 2 because cooled plenum air contributed to the cooling of room air. For this reason, it is expected that Case 3 can provide higher energy-saving potential compared to Case 2 because of the reduced operation time of the chiller and circulation pump. In Case 4, cooling capacity was further enhanced by the air circulators. Hence, the room air temperature decreased more rapidly compared to Case 3. Thus, the on–off cycle time and pump operation time were reduced. This could lead to more energy savings for the CRCP system. Power meter data showed that the reduced operation time could reduce the energy consumption of the overall system. Figure9 and Table5 show the electricity consumption of the chiller, pump, and air circulators, in addition to the operation time of the chiller and pump. In Case 2, the application of the air circulators reduced the energy consumption of the chiller and pump by 1.14 kWh and 3.19 kWh, respectively, compared to Case 1, even though the air circulators consumed 0.72 kWh. The total reduction in energy consumption in Case 2 was 3.61 kWh (10.2%) compared to Case 1. Energies 2020,2021, 1314, x 5 FOR PEER REVIEW 1010 of of 16 15

(a) (b)

Energies 2020, 13, x FOR PEER REVIEW 11 of 16

was reduced by 2.70 kWh and 7.33 kWh, respectively, compared to Case 1. As the air circulators consumed 0.72 kWh, the total reduction in energy consumption was 9.30 kWh (26.4%). Therefore, it can be concluded that the open-type installation and air circulators effectively enhance not only cooling capacity but also energy performance. In this study, a simple on/off control was implemented to compare energy performance depending on the CRCP installation type and the application of air circulators. The reason for selecting on/off control was to investigate how the enhanced cooling capacity affects system behaviors and energy consumption. However, much fluctuation of room air temperature was observed in Cases 2–4, owing to the inherent characteristics of the on/off control. Although its impact on thermal comfort may not be significant, it can lead to the short-cycling of pump and chiller, which can deteriorate the system efficiency. To solve(c) this problem, it needs to mitigate the room temperature(d) fluctuation by modulating the chiller water flow rate (e.g., PID control). A further study is suggested to investigate Figure 8. Measurement results of room air temperature, pump operation, and chiller operation: (a) Case 1; (b) Case 2; (c) Figure 8. Measurement resultsthe energy of room performance air temperature, when pump the advanced operation, co andntrol chiller method operation: is used (a) for Case the 1; CRCP (b) Case system 2; Case 3; (d) Case 4. (c) Case 3; (d) Case 4. with air circulators. The fluctuation of room air temperature was also found in Case 3, as shown in Figure 8c. However, the room air temperature decreased more rapidly compared to Case 2 because cooled plenum air contributed to the cooling of room air. For this reason, it is expected that Case 3 can provide higher energy-saving potential compared to Case 2 because of the reduced operation time of the chiller and circulation pump. In Case 4, cooling capacity was further enhanced by the air circulators. Hence, the room air temperature decreased more rapidly compared to Case 3. Thus, the on–off cycle time and pump operation time were reduced. This could lead to more energy savings for the CRCP system. Power meter data showed that the reduced operation time could reduce the energy consumption of the overall system. Figure 9 and Table 5 show the electricity consumption of the chiller, pump, and air circulators, in addition to the operation time of the chiller and pump. In Case 2, the application of the air circulators reduced the energy consumption of the chiller and pump by 1.14 kWh and 3.19 kWh, respectively, compared to Case 1, even though the air circulators consumed 0.72 kWh. The total reduction in energy consumption in Case 2 was 3.61 kWh (10.2%) compared to Case 1. In Case 3, the energy consumption of the chiller and pump was reduced by 2.69 kWh Figure 9. Comparison of energy consumption in each case. Figureand 6.30 9. Comparison kWh, respectively, of energy consumptioncompared to in Case each 1. case. The total reduction in energy consumption was 8.99 kWh (25.5%). In Case 4, the energy consumption of the chiller and pump Table 5. System operation time and electricity consumption.

Operation Time [min] Electricity Consumption [kWh] Case Chiller Pump Chiller Pump Air Circulator Total Case 1 195 600 20.62 14.64 0 35.3 Case 2 178 468 19.48 11.45 0.72 31.6 (−10.2%) Case 3 157 339 17.93 8.34 0 26.3 (−25.5%) Case 4 157 296 17.93 7.31 0.72 26.0 (−26.4%)

Energies 2021, 14, 5 11 of 15

Table 5. System operation time and electricity consumption.

Operation Time Electricity Consumption [kWh] Case [min] Chiller Pump Chiller Pump Air Circulator Total Case 1 195 600 20.62 14.64 0 35.3 Case 2 178 468 19.48 11.45 0.72 31.6 (−10.2%) Case 3 157 339 17.93 8.34 0 26.3 (−25.5%) Case 4 157 296 17.93 7.31 0.72 26.0 (−26.4%)

In Case 3, the energy consumption of the chiller and pump was reduced by 2.69 kWh and 6.30 kWh, respectively, compared to Case 1. The total reduction in energy consumption was 8.99 kWh (25.5%). In Case 4, the energy consumption of the chiller and pump was reduced by 2.70 kWh and 7.33 kWh, respectively, compared to Case 1. As the air circulators consumed 0.72 kWh, the total reduction in energy consumption was 9.30 kWh (26.4%). Therefore, it can be concluded that the open-type installation and air circulators effectively enhance not only cooling capacity but also energy performance. In this study, a simple on/off control was implemented to compare energy performance depending on the CRCP installation type and the application of air circulators. The reason for selecting on/off control was to investigate how the enhanced cooling capacity affects system behaviors and energy consumption. However, much fluctuation of room air temperature was observed in Cases 2–4, owing to the inherent characteristics of the on/off control. Although its impact on thermal comfort may not be significant, it can lead to the short-cycling of pump and chiller, which can deteriorate the system efficiency. To solve this problem, it needs to mitigate the room temperature fluctuation by modulating the chiller water flow rate (e.g., PID control). A further study is suggested to investigate the energy performance when the advanced control method is used for the CRCP system with air circulators.

4. Discussion 4.1. Research Implications The experimental results presented in Section3 show that the cooling capacity and energy performance of the CRCP systems can be enhanced by utilizing cooled plenum air for space cooling and/or applying air circulators to increase the heat transfer rate at the panel surfaces. It was found that the open-type installation increased the cooling capacity by 8.5% and air circulators increased the cooling capacity by 16.2% for the open-type CRCP and 17.1% for the closed-type CRCP system. Table6 shows the summarized result of the increase in cooling capacity, compared with the previous studies on the cooling capacity enhancement of ceiling radiant cooling systems. Shin et al. [2] reported that the cooling capacity can be enhanced by 54–80%, which is much higher than this study. As their study assumed a much higher cooling load (135 W/m2) than that of this study (64 W/m2), strong thermal plumes could be generated from the cooling load (electrical dummies), which promoted the natural convection effect in the whole space. Therefore, the difference in the cooling load condition seemed to cause the difference between the cooling capacity of the two studies. Jeong and Mumma [8] claimed that the cooling capacity could be enhanced by 5–35% when the discharge air velocity from a nozzle diffuser was 2–6 m/s. In the present study, discharge air velocity from the air circulator was 4.5 m/s, and the cooling capacity was increased by 16.2–17.1%. Thus, the increased rate of cooling capacity seems to fall within the range of the previous study. Kim and Leibundgut [9] showed that air ﬂow by air convector increased the cooling capacity by up to 32%, which is higher than that of the present study. This is because the cooling capacity was also affected by the cooling coil in the air convector. Thus, the additional enhancement of cooling capacity can be expected if the pre-cooled (non-isothermal) air ﬂow is distributed at CRCP surfaces. Novoselac et al. [10] reported that the isothermal jet from Energies 2021, 14, 5 12 of 15

the high-aspiration diffuser increased the cooling capacity by 4–17% depending on the air ﬂow rate. This result is quite similar to that of the present study, because air circulators were used to generate isothermal air ﬂow by recirculating the room air. Karmann et al. [11] demonstrated that the cooling capacity can be increased by 22% and 26% when operating a ceiling fan and small fans, respectively. This increase in cooling capacity is slightly higher than that of the present study. This is because their study was conducted for the TABS with acoustic clouds, wherein air movement can be highly promoted in the plenum (space between ceiling surface and acoustic clouds).

Table 6. Comparison with previous studies on cooling capacity enhancement.

Method to Increase Cooling Literature Air Flow Characteristic Increase in Cooling Capacity Capacity This study Natural convection by opening area Isothermal 8.5% (1) This study Forced convection by air circulator Isothermal 16.2–17.1% (2) Shin et al. [2] Natural convection by opening area Isothermal 54–80% Jeong and Mumma [8] Forced convection by nozzle diffuser Isothermal 5–35% Kim and Leibundgut [9] Forced convection by air convector Non-isothermal 32% Novoselac et al. [10] Forced convection by nozzle diffuser Isothermal 4–17% Energies 2020, 13, x FOR PEER REVIEW 13 of 16 Karmann et al. [11] Forced convection by ceiling fan Isothermal 22% Karmann et al. [11] Forced convection by small DC fans Isothermal 26% (1) 8.5% = 104.3 W/m2 (Case 3) compared to 96.1 W/m2 (Case 1). (2) 16.2% = 121.2 W/m2 (Case 4) compared to 104.3 W/m2 (Case 3), 2 2 17.1% = 112.5 W/m (Case 2) comparedEven to though 96.1 W/m the(Case open-type 1). CRCP system effectively enhances cooling capacity, its applicationThe comparison might be limited with because previous of studies the dust showed migration that from the the enhancement plenum to ofthe cooling occu- piedcapacity space. was Thus, appropriately appropriate achieved maintenance in this or study. cleaning Thus, is thesenecessary results for could utilizing be applied cooled to plenumdevelop air an for energy-efficient space cooling. designIn addition, or operation sufficient method thermal of insulation CRCP systems. should For be example,applied tothe the cooling perimeter capacity wall that of CRCP surrounds systems plenum is frequently air (e.g., insufficient spandrel wall) in hot because and humid the plenum climates airbecause temperature the chilled can increase, water temperature owing to the should heat be transmitted increased tothrough prevent the condensation. spandrel wall. If a ThisCRCP increases system the is used cooling with load the open-typeand energy installation consumption or with of the supplementary open-type CRCP air circulation system. Therefore,devices, the the required appropriate cooling thermal capacity insulation can be achievedof the spandrel at a high wall chilled is important water temperature. for suc- cessfullyFigure using 10 theshows open-type the relationship CRCP system. between ∆θ and energy consumption. The open-type installationIn the cases and where air circulators it is difficult reduce to notadopt only the energy open-type consumption CRCP system but also because∆θ. A smallerof the abovementioned∆θ indicates that issues, the chilledthe closed-type water temperature CRCP system is closerwith air to circulators the room aircan temperature.be another solutionBased onfor this, increasing the proposed cooling CRCP capacity systems and energy (Cases 2efficiency. to 4) can operateAs the air at acirculators higher chilled in- creasewater the temperature; air velocity, thisas shown increases in Table chiller 3, thermal efficiency comfort [18–21 can]. Thus,be improved the proposed while main- CRCP tainingsystems the can same save room more air energy temperature compared as that to the in conventionalthe conventional CRCP CRCP system system. (Case 1).

FigureFigure 10. 10. RelationshipRelationship between between Δθ∆ θandand energy energy consumption. consumption.

4.2. ResearchEven though Limitations the open-type CRCP system effectively enhances cooling capacity, its application might be limited because of the dust migration from the plenum to the occupied In this study, room air temperature and air velocity at a representative point (center of the test chamber) were measured to evaluate the general comfort depending on the CRCP installation type and the application of air circulators. The room air temperature at the representative point was also used as a reference value for the system control, which was necessary to analyze the cooling energy consumption. Nonetheless, air movement through the opening or the operation of air circulators may cause excessive air velocity and local discomfort (e.g., cold draught) at specific locations. To evaluate this local discomfort and thermal uniformity, air temperature and velocity need to be measured at multiple points simultaneously. Multi-point measurement with a sensor grid or CFD simulation is necessary to evaluate the thermal uniformity and local discomfort for the CRCP system with air circulators. The thermal uniformity can also be deteriorated because the entire CRCP surfaces may not be provided with the air flow by air circulators. This is a limitation of air circulator that is designed to generate a directional air flow, not a diffusive air flow. To overcome this limitation, the cooling capacity and thermal uniformity depending on the air flow rate and arrangement of air circulators need to be investigated. The application of multiple small fans can be another solution to generate the diffusive air flow, which is effective to increase the air velocity at the entire panel surfaces while preventing excessive air velocity. This study has the limitation that the cooling capacity of the CRCP system was investigated with regard to total cooling capacity, wherein the radiant and convective heat

Energies 2021, 14, 5 13 of 15

space. Thus, appropriate maintenance or cleaning is necessary for utilizing cooled plenum air for space cooling. In addition, sufficient thermal insulation should be applied to the perimeter wall that surrounds plenum air (e.g., spandrel wall) because the plenum air temperature can increase, owing to the heat transmitted through the spandrel wall. This increases the cooling load and energy consumption of the open-type CRCP system. Therefore, the appropriate thermal insulation of the spandrel wall is important for successfully using the open-type CRCP system. In the cases where it is difficult to adopt the open-type CRCP system because of the abovementioned issues, the closed-type CRCP system with air circulators can be another solution for increasing cooling capacity and energy efficiency. As the air circulators increase the air velocity, as shown in Table3, thermal comfort can be improved while maintaining the same room air temperature as that in the conventional CRCP system.

4.2. Research Limitations In this study, room air temperature and air velocity at a representative point (center of the test chamber) were measured to evaluate the general comfort depending on the CRCP installation type and the application of air circulators. The room air temperature at the representative point was also used as a reference value for the system control, which was necessary to analyze the cooling energy consumption. Nonetheless, air movement through the opening or the operation of air circulators may cause excessive air velocity and local discomfort (e.g., cold draught) at specific locations. To evaluate this local discomfort and thermal uniformity, air temperature and velocity need to be measured at multiple points simultaneously. Multi-point measurement with a sensor grid or CFD simulation is necessary to evaluate the thermal uniformity and local discomfort for the CRCP system with air circulators. The thermal uniformity can also be deteriorated because the entire CRCP surfaces may not be provided with the air flow by air circulators. This is a limitation of air circulator that is designed to generate a directional air flow, not a diffusive air flow. To overcome this limitation, the cooling capacity and thermal uniformity depending on the air flow rate and arrangement of air circulators need to be investigated. The application of multiple small fans can be another solution to generate the diffusive air flow, which is effective to increase the air velocity at the entire panel surfaces while preventing excessive air velocity. This study has the limitation that the cooling capacity of the CRCP system was investigated with regard to total cooling capacity, wherein the radiant and convective heat transfer were not separated. To quantify the impact of air circulators on the cooling capacity, convective heat transfer needs to be distinguished from the total cooling capacity. Although many studies on ceiling radiant cooling systems have suggested radiant/convective heat transfer coefficients [1,6,8,22–24], these studies focused on the ceiling radiant cooling system made of top-insulated panels. Thus, heat transfer at the upper panel surface was not considered to determine the radiant/convective heat transfer coefficients. As this study applied the ceiling panels without top-insulation, radiant/convective heat transfers also occurs at the upper panel surfaces. This makes it difficult to adopt the radiant/convective heat transfer coefficients suggested by previous studies. Therefore, a further study is proposed to determine the radiant/convective heat transfer coefficients for the CRCP system without top insulation. In this study, the cooling capacity of the CRCP system was enhanced by recirculating room air. Outdoor air can be utilized to enhance cooling capacity if it is appropriately processed by a dedicated outdoor air system (DOAS), which is typically combined with radiant cooling systems to address ventilation and latent load. Cooling capacity can be further enhanced by utilizing an air distribution system, such as an active chilled beam, to generate widespread air flow on the CRCP surface. Thus, the cooling performance of the CRCP system combined with various air distribution systems should be further investigated. Energies 2021, 14, 5 14 of 15

5. Conclusions The impact of the open-type installation and air circulators on the cooling capacity and energy performance of the CRCP system was investigated through mock-up chamber experiments. Compared to the reference case (Case 1: closed-type CRCP), cooling capacity increased by 17% when air circulators were used (Case 2: closed-type CRCP with air circulators), by 8.5% in the open-type installation (Case 3: open-type CRCP), and by 26.2% when the open-type installation and air circulators were combined (Case 4: open- type CRCP with air circulators). The experimental results showed that in the open-type CRCP system, the utilization of cooled plenum air enhanced cooling capacity. In addition, air circulators effectively increased the cooling capacity of the closed-type and open-type CRCP systems. Regarding the energy performance, the increase in cooling capacity due to the open- type installation and air circulators reduced the operation time of the chiller and circulation pump. Compared to the reference case (Case 1), the open-type installation and/or air circulators reduced cooling energy consumption by 10.2–26.4%. The open-type installation and air circulators also reduced the difference between the room air temperature and mean chilled water temperature. This implies that the open-type CRCP system with air circulators can operate at a relatively higher chilled water temperature, which effectively reduces the condensation risk on panel surfaces. A high chilled water temperature provides additional energy-saving potential owing to increased chiller efﬁciency. The detailed impact of the open-type CRCP system with air circulators on condensation risk and chiller efﬁciency should be further examined. In this study, cooling capacity was enhanced by utilizing cooled plenum air or the recirculation of room air. Considering that a radiant cooling system is typically combined with a DOAS to address ventilation and latent load, the processed outdoor air from the DOAS can be utilized to enhance the cooling capacity of the CRCP system. A future study will be conducted to improve the cooling capacity and energy performance by supplying processed outdoor air through air distribution systems such as an active chilled beam system.

Author Contributions: Conceptualization, M.-S.S. and K.-N.R.; methodology, K.-N.R.; formal analysis, J.-S.C.; investigation, J.-S.C.; resources, M.-S.S.; data curation, J.-S.C.; writing—original draft preparation, M.-S.S.; writing—review and editing, K.-N.R.; visualization, J.-S.C.; supervision, K.-N.R.; project administration, K.-N.R.; funding acquisition, K.-N.R. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by a grant (19CTAP-C151980-01) from infrastructure and trans- portation technology promotion research program funded by the Ministry of Land, Infrastructure and Transport of Korean government. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References 1. Rhee, K.N.; Kim, K.W. A 50 year review of basic and applied research in radiant heating and cooling systems for the built environment. Build. Environ. 2015, 91, 166–190. [CrossRef] 2. Shin, M.S.; Rhee, K.N.; Park, S.H.; Yeo, M.S.; Kim, K.W. Enhancement of cooling capacity through open-type installation of cooling radiant ceiling panel systems. Build. Environ. 2019, 148, 417–432. [CrossRef] 3. Park, S.H.; Kim, D.W.; Joe, G.S.; Ryu, S.R.; Yeo, M.S.; Kim, K.W. Establishing boundary conditions considering inﬂuence factors of the room equipped with a ceiling radiant cooling panel. Energies 2020, 13, 1684. [CrossRef] 4. ASHRAE. ASHRAE Handbook—HVACSystems and Equipment; Atlanta: ASHRAE, GA, USA, 2020. 5. Zhang, L.; Liu, X.H.; Jiang, Y. Experimental evaluation of a suspended metal ceiling radiant panel with inclined ﬁns. Energy Build. 2013, 62, 522–529. [CrossRef] 6. Jeong, J.; Mumma, S. Practical cooling capacity estimation model for a suspended metal ceiling radiant cooling panel. Build. Envi- ron. 2007, 42, 3176–3185. [CrossRef] Energies 2021, 14, 5 15 of 15

7. Shin, M. Cooling Capacity Estimation and Design Process for the Open-type Installation of Ceiling Radiant Cooling Panels. Ph.D. Dissertation, Seoul National University, Seoul, Korea, 2016. 8. Jeong, J.; Mumma, S.A. Ceiling radiant cooling panel capacity enhanced by mixed convection in mechanically ventilated spaces. Appl. Therm. Eng. 2003, 23, 2293–2306. [CrossRef] 9. Kim, M.; Leibundgut, H. A case study on feasible performance of a system combining an airbox convector with a radiant panel for tropical climates. Build. Environ. 2014, 82, 687–692. [CrossRef] 10. Novoselac, A.; Burley, B.J.; Srebric, J. New convection correlations for cooled ceiling panels in room with mixed and stratified airflow. HVAC&RRes. 2006, 12, 279–294. [CrossRef] 11. Karmann, C.; Bauman, F.; Raftery, P.; Schiavon, S.; Koupriyanov, M. Effect of acoustical clouds coverage and air movement on radiant chilled ceiling cooling capacity. Energy Build. 2018, 158, 939–949. [CrossRef] 12. ASHRAE. ANSI/ASHRAE Standard 55: Thermal Environmental Conditions for Human Occupancy; ASHRAE: Atlanta, GA, USA, 2017; pp. 9–13. 13. Yang, B.; Schiavon, S.; Sekhar, C.; Cheong, D.; Tham, K.W.; Nazaroff, W.W. Cooling efficiency of a brushless direct current stand fan. Build. Environ. 2015, 85, 196–204. [CrossRef] 14. Mun, S.H.; Kwak, Y.; Kim, Y.; Huh, J.H. A comprehensive thermal comfort analysis of the cooling effect of the stand fan using questionnaires and a thermal manikin. Sustainability 2019, 11, 5091. [CrossRef] 15. CEN. EN 14240: Ventilation for Buildings–Chilled Ceilings–Testing and Rating; CEN: Brussels, Belgium, 2004. 16. ASHRAE. ANSI/ASHRAE Standard 138: Method of Testing for Rated Ceiling Panels for Sensible Heating and Cooling; ASHRAE: Atlanta, GA, USA, 2013; pp. 6–10. 17. Zhang, Q.; Yan, D.; An, J.; Hong, T.; Tian, W.; Sun, K. Spatial distribution of internal heat gains: A probabilistic representation and evaluation of its influence on cooling equipment sizing in large office buildings. Energy Build. 2017, 139, 407–416. [CrossRef] 18. Seshadri, B.; Rysanek, A.; Schlueter, A. High efficiency ‘low-lift’ vapour-compression chiller for high-temperature cooling applications in non-residential buildings in hot-humid climates. Energy Build. 2019, 187, 24–37. [CrossRef] 19. Pieskä, H.; Ploskić,A.; Wang, Q. Design requirements for condensation-free operation of high-temperature cooling systems in mediterranean climate. Build. Environ. 2020, 185, 107273. [CrossRef] 20. Saber, E.M.; Tham, K.W.; Leibundgut, H. A review of high temperature cooling systems in tropical buildings. Build. Environ. 2016, 96, 237–249. [CrossRef] 21. Bruelisauer, M.; Chen, K.W.; Iyengar, R.; Leibundgut, H.; Li, C.; Li, M.; Saber, E.M. BubbleZERO—design, construction and operation of a transportable research laboratory for low exergy building system evaluation in the tropics. Energies 2013, 6, 4551–4571. [CrossRef] 22. Shinoda, J.; Kazanci, O.B.; Tanabe, S.I.; Olesen, B.W. Review on the Surface Heat Transfer Coefficients of Radiant Systems. In E3S Web of Conferences; EDP Sciences: Bucharest, Romania, 2019; Volume 111. [CrossRef] 23. Karada˘g,R. New approach relevant to total heat transfer coefficient including the effect of radiation and convection at the ceiling in a cooled ceiling room. Appl. Therm. Eng. 2019, 29, 1561–1565. [CrossRef] 24. Causone, F.; Corgnati, S.P.; Filippi, M.; Olesen, B.W. Experimental evaluation of heat transfer coefficients between radiant ceiling and room. Energy Build. 2009, 41, 622–628. [CrossRef]