Experimental evaluation of PCM embedded radiant chilled ceiling for efficient space cooling

. Because of climate change, together with rapid urbanisation and continuous population growth, the global demand for space cooling is increasing dramatically. Under a business-as-usual trajectory, there will be a more than threefold rise in the number of in-use air conditioners worldwide by 2050. A radical shift to innovative space cooling technologies is therefore essential, ones that can sustainably meet the growing requirements. Phase change material embedded radiant chilled ceiling, called PCM-RCC, offers an emerging alternative for more sustainable space cooling provision. This system provides a range of benefits to end-users, in terms of efficiency and indoor environmental quality, together with demand-side flexibility. PCM-RCC, however, is still under development, and further research is needed to realise its full capabilities. The present work experimentally analyses the thermal-energy performance of a PCM-RCC system using a full-scale test cabin equipped with PCM ceiling panels. Here, the transient thermal behaviour of the panels besides the cooling energy delivered in charging-discharging cycles are examined. Additionally, the indoor thermal comfort and peak energy demand reduction enabled by the present PCM-RCC are discussed. Based on the results, typically 4–5 hours of chilled water circulation overnight could sufficiently be able to fully recharge the panels in the morning. Over 80% of the occupancy time was found within Class B thermal comfort defined in ISO 7730. About 70% of the system’s daily electricity usage time was during off-peak hours. The significance of implementing optimal predictive operating schedules was also highlighted to fully utilise PCM-RCC’s potentials.


Introduction
Since 1990, the energy consumed for space cooling has tripled, significantly impacting the electricity infrastructures.In 2021, the demand for space cooling experienced the largest annual increase among all building end-uses, accounting for around 16% of the global final energy consumption in the building sector (~2000 TWh) [1].This is mainly driven by the growing global demand for air conditioners (ACs) as a result of continuous climate change and population growth [2].The number of in-use ACs worldwide jumped from 1.6 billion units in 2020 to 2.2 billion in 2021 [1,3].On a business-as-usual trajectory, this number is expected to more than triple by 2050 [4].
The data signifies how vital it is for the building and construction sector to firmly adopt not only effective policies but also more advanced, energy-efficient space cooling technologies [5].Over the past several years, academia and industry have searched for potential technologies to enhance the cooling efficiency of AC units.One such technology involves the use of phase change material (PCM) embedded radiant chilled ceiling (RCC), herein called PCM-RCC.In this system, the PCM acts as a thermal battery and is charged overnight via the chilled water flowing through the ceiling panels (freezing cycle).It is then discharged during the daytime by absorbing the interior sensible heat (melting cycle).Thanks to the radiant cooling and thermal energy storage mechanisms, PCM-RCC offers end-users a variety of advantages in terms of efficiency and indoor environmental quality (IEQ), together with demand-side flexibility.Lower energy consumption, better space utilisation, improved thermal comfort, quiet operation, enhanced IEQ with limited recirculation of pathogens, as well as peak load shifting are the benefits of the PCM-RCC technology, compared to classic AC systems [2].
Notwithstanding broad interest in PCM-RCC applications among the scientific and industry communities, the technology has not yet transitioned to its 'limited availability' stage.Several studies investigated PCM-RCC's cooling potentials (for example, see [6][7][8][9]); however, concerns about how this technology should be designed and operated in real-life scenarios need to be credibly addressed.
This study carries out a field study on a full-scale test cabin equipped with a new PCM-RCC system to explore the capabilities of the system and potential challenges that it might encounter in real-world settings.Here, the transient thermal behaviour of PCM ceiling panels plus the cooling energy delivered in charging-discharging phases are evaluated.Then, the level of interior thermal comfort as well as the peak electricity demand reduction provided by the present PCM-RCC are discussed.

Experimental setup 2.1.1. Test cabin.
The current experiments were carried out on a full-scale test cabin equipped with a PCM-RCC system (Figure 1), located at the University of Melbourne.This test cabin, which is of a conditioned volume of 96.7 m 3 , is fully exposed to real-world climatic conditions.R4.3 and R4.8 sandwich panels were utilised for the cabin structure.Advanced floor-to-ceiling glazing with interstitial Venetian blinds and a vision area of 13.5 m 2 was also used to create a seamless connection to the natural surroundings, as is common in today's modern architectural design.More details of the cabin structure (e.g., layers thickness, conductivity, capacity, and density) can also be found in [10].
Figure 1.PCM ceiling panels used for the proposed PCM-RCC system.

Shape-stabilised PCM composite boards.
As seen in Figure 1, organic-based shape-stabilised PCM boards placed in a 1.5 mm-thick aluminium tray were used for the present PCM-RCC system.These PCM boards with dimensions of 121.5 cm (L) × 65.2 cm (W) × 2.5 cm (D) have a phase change range of 15.6-19.6˚C,a density of 765 kg/m 3 , and a latent heat capacity of 900 kJ/m 2 .Thermal conductivities of the PCM board in solid and liquid phases were also measured using a C-Therm conductivity analyser, and the values were found to be 1.02 and 0.68 W/m.K (with ±5% uncertainty), respectively.
Approximately 60% of the cabin's ceiling area was covered by 24 PCM panels installed in eight parallel rows.For the current panel design, as shown in Figure 1, capillary tube mats were used for chilled water circulation during PCM recharge cycle.These thin capillary tubes are flexible and easyto-assemble by plastic welding [2,8].Based on lessons learnt in the authors' previous study [3], a 3mm-thick plaster layer was applied on top of PCM boards to improve the thermal contact between the capillary tubes and the PCM.Panels were also covered with additional insulation layers to decrease energy losses caused by heat transfers to the roof.Additionally, the panel surfaces were painted using high-emissivity paint to increase the ceiling emissivity rate.
2.1.3.Chilled water supply and distribution system.The hydronic distribution unit, as well as the arrangements of PCM ceiling panels, is shown in Figure 2.Each row, consisting of three PCM ceiling panels in series, is independently connected to the chilled water supply and return lines.Chilled water, provided by an air-source heat pump, is pumped and circulated within the ceiling panels to extract heat from PCM until the controlling setpoints of the recharge schedule are met.The supply and return lines, plus all fittings, were properly insulated to avoid heat losses from the surroundings.The operation of equipment (circulating pumps, heat pump, fans, heater) was remotely controlled via a programmable logic controller.In this configuration, the panels and the cabin air temperature can be easily monitored, and different control strategies can be implemented remotely.

Dedicated outdoor air system (DOAS).
A dedicated outdoor air system (DOAS) supplies cooled, dehumidified outdoor air to the indoor space, principally to handle the internal latent loads.DOAS units are often used in conjunction with a parallel cooling system.Radiant ceiling panels are the optimum choice as a parallel system to be coupled with a DOAS unit [11].Here, a DOAS system was installed in the cabin to deliver fresh air and supplementary cooling, when needed.With this parallel cooling system design, the potential for indoor air quality problems can be further minimised.

Data acquisition and monitoring.
Multiple sensors and internet-of-things (IoT) devices were installed in the cabin to read and collect essential variables for the present PCM-RCC performance evaluation.These variables are classified into five main categories as presented in Table 1.

Measurement category
Measured variable PCM ceiling panel

Operational schedule
The PCM-RCC operating schedule, shown in Table 2, was planned based on the current ceiling layout and PCM panel design, findings of the authors' previous field testing, as well as recommendations given in the IEQ-related standards/guidelines.

Occupancy period
Run the pump during DOAS fan operation if PCM panels are mostly liquid (TPCM,middle layer > 18.5˚C) AND Tindoor > 24˚C 3 .

Occupancy period
Run DOAS fan based on 0.5-hr ON/1-hr OFF schedule 5 .AND Run DOAS secondary pump during fan operation 6 if Toutdoor > 26˚C AND Tindoor > 24˚C 3 .

Un-occupied period
Run the heat pump when the ceiling pump or DOAS secondary pump is activated, to supply chilled water with a setpoint of 10°C.

Occupancy period
Run the heat pump if the ceiling pump or DOAS secondary pump is activated.
1 9:00 am -7:00 pm was referred to as occupancy time.Outside of this period was considered as un-occupancy time. 2 According to the preliminary assessments of PCM-RCC operation in different weather condition, maximum of 4-5 hours was required to fully recharge PCM panels. 3Indoor temperature limits were defined based on ISO7730 standard. 4DOAS operation overnight effectively prevents cabin overheating and regulates indoor relative humidity. 5With averagely 90 L/s of airflow rate, DOAS fan operation with 0.5-hr-ON/1-hr-OFF schedule provides minimum fresh air for occupants during the occupancy time. 6Secondary DOAS pump operation aimed to provide supplementary cooling during occupancy time, if required. 7This schedule was set up to manage the possible issue of overcooling in early mornings after the recharge cycle.

Charging-discharging performance of PCM ceiling panels
Figure 3 shows the measured indoor (globe) temperature, PCM ceiling temperatures, and ceiling heat flux (HF) during system operation in three consecutive summer days.Figure 4 displays the hourly estimated partial enthalpy in both melting and freezing paths plus the liquid fraction of PCMs in the ceiling panels.As observed, the active recharge cycle on the first day started with the initial ceiling temperature and PCM liquid fraction of approximately 18.5°C and 71%, respectively.After 4 hours of chilled water circulation with an average water flow rate of 470 L/hr, the PCM panels were sufficiently recharged.Positive HF values during the daytime confirm that the PCM ceiling panels steadily absorbed heat from the cabin interior as the PCM phase was changing (solid → mushy → liquid).Furthermore, the DOAS unit and ceiling pump were intermittently operated to provide supplementary cooling and maintain the desired comfort condition.This supplementary cooling also added a residual cooling capacity to the PCM, extending the active time of the panels in late afternoon.
For the first day, the ceiling panels remained active until 07:30 pm.A sharp increase in panel temperatures was then observed, indicating that the PCMs had almost fully melted.
As the data shows, the second day started with cabin and ceiling temperatures of around 22°C and a PCM liquid fraction of 100% (fully liquid).In accordance with the defined operating schedule, the DOAS was activated before the ceiling pump operation overnight to ventilate and cool the cabin using cool night-time ambient air.The estimated values of partial enthalpy and liquid fraction for PCM panels in the early morning demonstrate that they were almost fully recharged.The cooling storage capacity of the PCM ceiling almost covered the entire occupancy time, thanks to the extra cooling periodically provided by the DOAS and ceiling operations.The same initial state of temperatures and PCM liquid fraction was also observed for the third day.However, since the ambient temperature on this day dropped significantly in the afternoon, no supplementary cooling was required after 02:00 pm, and the PCM ceiling's storage capacity, despite being exhausted around 04:00 pm, was sufficient to maintain an acceptable level of thermal comfort.
Focusing on the PCM ceiling recharge period overnight, it was obtained that the efficiency of the recharge cycle followed a gradual decline during the next hot days.It was uncertain whether fullyrecharged PCM panels could be achieved with the same active recharge schedule since the PCMs were completely melted, and heat accumulated in indoor space after consecutive hot days.The HF patterns also confirm that the first 1 -2 hours of the active recharge period dealt with removing indoor sensible  heat, thus slowing the freezing rate of the PCM.Although the current operating schedule aimed to respond to the transient thermal behaviour of indoor space and PCM ceiling panels, a more dynamic, optimal predictive schedule with precise weather forecasting is still required to carefully control uncertainties in the system's performance.

Ceiling cooling energy
Using the heat transfer balance method at the radiant ceiling surface, the heat flux between the indoor space and PCM ceiling panels can be formulated as follows: where the subscripts s, ir, c, and r denote, respectively, transmitted solar radiation absorbed by the radiant ceiling surface, radiative flux with internal heat sources, convective heat flux with the interior environment, and radiative heat flux with other indoor surfaces.Besides, the heat extracted from PCM ceiling panels by chilled water during the recharge period is equal to: Here, ṁw is the water flow rate [kg/s], Cp,w is the specific heat capacity of water [4182 J/kg.K], Apanels is the PCM ceiling area [m 2 ], and Trw and Tsw are the return and supply water temperatures [°C], respectively.
During the three-day experiment, the average total heat transfer coefficient from the PCM ceiling panels to the indoor space was calculated to be 8.48±0.97W/(m 2 .K), taking into account both radiative and convective heat transfer paths.This value is consistent with the relevant standards, e.g., REHVA, EN1264-5, and EN15377-1.Meanwhile, comparing the total daily energy absorbed by PCM ceiling panels from the indoor space with the total heat extracted from the panels by chilled water during the active recharge period reveals almost equal values of cooling energy absorbed and extracted (Figure 5).The small difference observed on the first day could be due to the residual cooling capacity existed initially in the PCM panels with the liquid fraction of 71%, resulting in less heat being extracted during the chilled water circulation time.The differences in the extracted-absorbed values for the second and third days might also be due to the accumulated indoor heat being removed directly by the hydronic system.As mentioned in Section 3.1, the first 1 -2 hours of the active recharge overnight dealt with removing indoor sensible heat before extracting heat from the PCM panels.

Thermal comfort assessment
Low quality and deteriorated indoor thermal comfort conditions lead to individual dissatisfaction and negatively influence their well-being, performance, and productivity [2].This highlights that thermal comfort must be cautiously monitored, such that any issues can be urgently addressed.Heating, ventilation, and air-conditioning (HVAC) systems are the key elements in delivering desired indoor thermal comfort; thus, an inappropriate design and implementation of these systems raise the occurrence probability of thermal comfort issues during daily building operations.
The predicted mean vote (PMV) is a well-known, commonly-used index in thermal comfort assessments [12].As listed in Table 3, ISO 7730 and EN 16798-1 specify different categories for indoor thermal comfort depending on PMV levels.Here, the PMV method was used to evaluate the thermal comfort conditions provided by the present PCM-RCC system.PMV values were determined using a developed program, and the results were validated by comparison with those obtained from the CBE thermal comfort tool [13].The measured data for air temperature, globe temperature, and indoor relative humidity were used for PMV calculations.The metabolic rate of 1.2 met (for reading, typing, and filing activities) and the minimum air velocity of 0.05 m/s (due to the limited air movement in radiant cooling systems [3]) were assumed.Also, a clothing factor of 0.65 clo was considered for the summer season based on the analyses of the RP-884 database for HVAC buildings.Assuming an occupancy time of 9:00 am -07:00 pm, the results reveal that the PCM-RCC system is capable of maintaining an acceptable level of indoor thermal comfort.The "Out of Comfort Category" and/or "Category IV", indicated in Figure 6, was mostly due to overcooling in the mornings caused by the active recharge.To address this issue, which was already predicted by the authors, a small electric air heater was installed and operated in the early mornings following the defined operating schedule (see Table 2).However, its heating capacity was not adequate as expected.In addition, temperature measurements were taken at heights of 0.1 m (for a person's ankle), 1.1 m (for a seated person's head), and 1.7 m (for a standing person's head) to confirm that the vertical temperature difference was always less than 2°C, as the ISO 7730 standard specifies within "Class A".

Total electricity usage
Considering the Time of Use (ToU) tariff in Victoria, Australia, the daily time is typically divided into off-peak (22:00-7:00), shoulder (7:00-15:00 & 21:00-22:00), and peak (15:00-21:00) periods.Figure 7 shows the electricity usage of PCM-RCC components during different Victoria's ToU periods.The most electricity consumed belongs to the off-peak time with 58.5% of daily usage on the first day, 59.1% on the second day, and 80.2% on the third day.This confirms the capability of PCM-RCC systems in shifting a major part of peak cooling load to off-peak hours.
One limitation of this study was the absence of a similar second test cabin equipped with only radiant ceiling panels, without PCM.Such a setup would have allowed us to compare the energy usage of both RCC and PCM-RCC systems across different ToU periods.However, a preliminary analysis using a validated simulation model revealed that removing PCM from the radiant ceiling panels would increase the system's operating time by 45-50% during the day.This additional load is mostly observed during shoulder hours (~65%) and the rest during peak time.

Conclusions
This work introduced a new PCM-RCC technology developed for space cooling and discussed whether the design is functioning as expected.Using a full-scale experimental setup, the transient thermal behaviour of the proposed PCM-RCC in addition to the indoor thermal comfort and peak electricity demand reduction provided were examined.Overall, the system demonstrated satisfactory performance in terms of space cooling provision during the daytime, load shifting from peak to offpeak time, as well as providing thermal comfort.The findings and concluding remarks are: • Applying a thin plaster layer to the top surface of PCM panels sufficiently resolved the issue of low thermal contact between the PCM composite board and water capillary tubes.Typically, 4-5 hours of chilled water circulation overnight could fully recharge the PCM panels.• The average total heat transfer coefficient from the PCM ceiling surface to the interior space was estimated to be 8.48±0.97W/(m 2 .K).This meets the requirements of the relevant standards.• Over 80% of the whole occupancy time am -07:00 pm) was found within ±0.5 for PMV.
The cabin overcooling in the early morning following the active recharge cycle was shown to be the cause of the thermal discomfort.Despite considering a heating provision, the capacity of the electric air heater was not adequate as expected.• About 70% of the daily electricity usage time of the PCM-RCC system was during the off-peak hours as in Victoria's ToU.• The operating schedule presented here responds to the transient thermal behaviour of the indoor environment and the PCM ceiling panels.The results nevertheless highlight the importance of a more dynamic, optimal predictive schedule that considers real-time weather forecasting.Modelling and simulations are crucial in identifying the optimum operating strategy of the PCM-RCC system.

Figure 2 .
Figure 2. PCM-RCC hydronic system and the arrangements of PCM panels in the test cabin.

Figure 3 .
Figure 3. Measured cabin (globe) and PCM ceiling temperatures plus PCM ceiling heat flux.

Figure 4 .
Figure 4. Partial enthalpy and liquid fraction of PCM ceiling panels.

Figure 5 .
Figure 5.Total energy absorbed by PCM ceiling panels from indoor environment and energy extracted from the ceiling during chilled water circulation periods.

Figure 6 .
Figure 6.PMV-based thermal comfort level during occupancy hours in each day.

Figure 7 .
Figure 7. Measured electricity usage of PCM-RCC components in different Victoria's ToU periods.

Table 2 .
PCM-RCC operating schedules for the experiments.