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The Influence of a Radiant Panel System with Integrated Phase Change Material on Energy Use and Thermal Indoor Environment.

INTRODUCTION

High quality of the indoor environment in office buildings is essential for comfort, health and productivity of the occupants. This can be related to considerable economic costs for installation, operation and maintenance of the climatic systems and may hamper the goal of achieving energy efficient buildings. Climate change, equipment heat gains, and modern building design cause a continuously increasing cooling demand of office buildings. Thus, high performance cooling systems are necessary to meet both indoor environment and energy requirements.

Another parameter influencing the cooling demand is the thermal mass of the building, affecting the thermal storage capacity. Conventional heavy buildings can create thermal energy storage of sensible heat in the construction, decreasing the energy demand and enhancing the thermal environment. Modern light buildings do not benefit from this. However, with the implementation of phase change material (PCM) in the building construction or elements, considerable thermal storage of latent heat can be obtained. This is achieved through the phase change of the PCM with thermal energy charge and discharge during heat absorption and release, respectively (Kosny 2015).

In this study a numerical model of an office containing radiant ceiling panels with and without PCM was investigated during the cooling season. The purpose of the study was to quantify the effects related to thermal comfort and energy use when integrating phase change material to panels for cooling under various conditions.

METHODOLOGY

Model Construction

A two-person office was modelled in a commercially available software. The systems were investigated in the climates of Copenhagen, Denmark and Rome, Italy with a basic and a night cooling strategy. The model was based on the requirements for new buildings of the 2020 Danish Building Regulation (Building Regulations 2015). This regulation sets, among others, demands regarding thermal transmittance of the building construction and energy use of technical installations. The overall energy frame of 2020 buildings requires that the yearly primary energy demand for heating, cooling, ventilation, lighting, and domestic hot water does not exceed 25 kWh/[m.sup.2] (7925 Btu/[ft.sup.2]) of the heated floor area.

Control of the indoor climatic systems was designed to comply with indoor environment Category II of EN 16798-1 (EN 2016). Simulations were run during the Danish cooling season, namely, from 1st of May until 30th of September, using test reference year (TRY) weather files.

The office had an internal floor area of 22.7 [m.sup.2] (244.3 [ft.sup.2]) and a total height of 3.0 m (9.8 ft), of which 0.5 m (1.6 ft) was suspended ceiling, bounded by the ceiling panels. Figure 1 shows a plan and section of the office. The office had one external wall with a window of 3 [m.sup.2] (32.3 [ft.sup.2]), corresponding to a window-to-floor ratio of 0.13.

Two people were seated in the office with an activity of 1.2 met and dressed equivalent to a clothing insulation of 0.5 clo. The equipment consisted of two computers each of 100 W (341.2 Btu/h) and electrical lighting with a total power of 200 W (628.4 Btu/h), corresponding to 8.8 W/[m.sup.2] (2.8 Btu/h*[ft.sup.2]). The total internal heat load was 24.7 W/[m.sup.2] (7.8 Btu/h*[ft.sup.2]). The time schedule of internal gains was Monday to Friday at 08:00 - 17:00, except for a lunch break from 12:00-13:00. Additional details of the model can be found in (Nielsen 2016). The window had a U-value of 0.8 W/[m.sup.2]K (0.14 Btu/h*[ft.sup.2]*[degrees]F), g-value of 0.5, and a solar transmittance of 0.48, of which 0.74 was visible transmittance. External blinds were integrated for solar shading and drawn when solar radiation on the window surface exceeded 100 W/[m.sup.2] (31.7 Btu/h*[ft.sup.2]). The office was orientated with the external wall facing south. The infiltration was set as a fixed air change of 0.06 [h.sup.-1].

The floor, ceiling and internal walls were adiabatic. The external wall was constructed of concrete, insulation and brick, with a U-value of 0.1 W/[m.sup.2]K (0.02 Btu/h*[ft.sup.2]*[degrees]F). The floor slab consisted of concrete, acoustic insulation and screed. The ceiling panels were modelled as a suspended ceiling in the office. The panels were constructed of two clay board layers separated by cardboard for structural support. The ceiling panels not containing PCM are denoted radiant panels in spite of radiant panels usually referring to aluminum panel plates. When investigating the effect of PCM integration, microencapsulated paraffin was added to the clay boards corresponding to 26% of the mass. The PCM panel construction corresponded to PCM panels described by Pavlov (2014). Table 1 shows the construction and properties of the panels. The PCM properties only relate to the PCM panels.

The paraffin had a fusion temperature of 23[degrees]C (73.4[degrees]F) and a specific latent heat of 110 kJ/kg (47.3 Btu/lb). Pavlov (2014) made a theoretical and experimental investigation of the entire PCM panel. The results regarding the specific heat capacity of the PCM panel as a function of the temperature are illustrated in Figure 2. The data of Figure 2 were used for both charging and discharging of the PCM. Both measured and theoretical properties of the PCM are used for the investigation.

Model Control Strategy

The systems were subject to air quality control regulated by a set-point of maximum C[O.sub.2] concentration of 1200 ppm and the cooling control was based on the operative temperature. The temperature set-point for the panels was set to achieve indoor thermal conditions with 90% of the occupied hours within the operative temperature range of 23-26[degrees]C (73.4-78.8[degrees]F), in compliance with Category II of EN 16798-1 (EN 2016).

Two cooling strategies were examined: a basic cooling strategy and a night cooling strategy. With the basic cooling strategy, a constant cooling set-point of 25.9[degrees]C (78.6[degrees]F) was set in both Copenhagen and Rome, and cooling was supplied to meet this set-point whenever there was a demand. The scope of the night cooling strategy was to move the heat removal to night time, namely outside the occupancy period. This was regulated by lowering the set-point from 00:00-08:00 to 23.4[degrees]C (74.1[degrees]F) in Copenhagen and 23.0[degrees]C (73.4[degrees]F) in Rome.

The air quality was regulated by mechanical ventilation consisting of a VAV air handling unit. The heat exchanger had an efficiency of 75% and the two fans a total SFP of 1.5 kJ/[m.sup.3] (0.71 kW/1000 cfm). The supply temperature followed the outdoor temperature within the range of 16-20[degrees]C (60.8-68.0[degrees]F). Cooling of the air was provided through a cooling coil when the outdoor temperature exceeded 20[degrees]C and heated through the heat exchanger when below 16[degrees]C. Additional information regarding the simulation model can be found in Nielsen (2016).

RESULTS

Figure 3 shows the percentage of occupied hours within comfort Category I and II of EN 16798-1 (2016) for Copenhagen (CPH) and Rome for the three panel systems with basic and night cooling strategy (NC). Figure 4 shows the primary energy use for the corresponding systems, locations, and cooling strategies.

Figure 3 illustrates an enhancement of thermal comfort when implementing PCM to the ceiling panels. In Copenhagen with night cooling strategy, the occupied hours in comfort Category II increased by 3% and 10% when applying PCM with measured and theoretical properties, respectively, and correspondingly 9% and 43% for Category I. Greater enhancements were observed in Rome with a Category II increase of 6% and 10% with measured and theoretical properties of the PCM, respectively, and a Category I increase of 18% and 57%, respectively. Negligible effects were observed with the basic cooling strategy in both Copenhagen and Rome.

Application of PCM to the ceiling panels had a negligible effect on the energy use of the cooling systems. For the panels without PCM, the night cooling strategy increased the energy use by 12% in Copenhagen and 8% in Rome. Comparable results were observed for the PCM panels. The energy use included ventilative cooling, water based cooling for the panels, and the auxiliary energy use for fans and pumps. Detailed results can be found in Nielsen (2016).

Figure 5 and 6 show the day of peak cooling power of the panels with basic and night cooling (NC) strategy in Copenhagen and Rome, respectively.

In Copenhagen, Figure 5, very similar results were obtained with the three panel characteristics when using the basic cooling strategy. With application of the night cooling strategy, the cooling supply during night was sufficient to avoid cooling during daytime for all panels. The integration of PCM with measured and theoretical properties decreased the peak cooling power by 15% and 21%, respectively compared to the panel without PCM.

In Rome, Figure 6, comparable cooling power profiles to Copenhagen were observed. However, cooling during night time could not eliminate completely the demand of cooling during the occupied hours for the panels without PCM and the panels with measured properties. The application of PCM with measured and theoretical properties decreased the peak cooling power by 17% and 20%, respectively compared to the panel without PCM.

Figure 7 illustrates the weekly variation of operative temperature with the different panel specifications combined with night cooling strategy during the warmest week of the year in Copenhagen.

Figure 7 shows that in spite of having the same initial temperature at the beginning of occupancy, decreased temperature fluctuations were observed causing a peak temperature 0.3[degrees]C (0.5[degrees]F) lower with the implementation of the theoretical PCM properties. Implementation of the measured PCM properties caused a peak temperature difference less than 0.1[degrees]C (0.2[degrees]F).

Figure 8 and Figure 9 illustrates the PCM temperature in the panel with measured PCM properties and the corresponding heat capacity of the PCM, respectively, with basic and night cooling (NC) strategy during the warmest week of the year in Copenhagen.

Figure 8 shows a PCM temperature variation during occupied days between 24[degrees]C (75.2[degrees]F) and 25[degrees]C (77.0[degrees]F) with the basic cooling strategy, whereas the application of night cooling brought PCM temperatures below 22[degrees]C (71.6[degrees]F). This difference affected the specific heat capacity of the PCM, Figure 9, which with night cooling was higher and more varying compared to the basic strategy. The average PCM layer temperature and specific heat capacity over the entire cooling season was 25.0[degrees]C (77.0[degrees]F) and 1.373 kJ/kgK (0.33 Btu/lb*[degrees]F) in Copenhagen and 23.8[degrees]C (74.8[degrees]F) and 2.083 kJ/kgK (0.50 Btu/lb*[degrees]F) in Rome.

DISCUSSION

Negligible effect was observed when applying PCM to the ceiling panels when using the basic cooling strategy, both regarding thermal comfort, energy use, and peak cooling power of the systems. The minimum PCM temperature for the panel with measured PCM properties was above 24[degrees]C (75.2[degrees]F) thus preventing utilization of the PCM thermal storage capacity. The conditions obtained with the basic cooling strategy showed that implementation of PCM has no effect, unless a suitable correlation between the PCM properties and system control is created.

With night cooling strategy, the objective was to shift the time of heat removal to night time with a lower limit of the operative temperature of 23[degrees]C (73.4[degrees]F). This could be obtained in Copenhagen for all systems and in Rome with PCM panels with measured specifications. The time shift of energy use is particularly beneficial for systems with cooling sources enabling utilization of the colder climate during night. For the radiant panels, the night cooling strategy increased the energy use by 12% in Copenhagen and 8% in Rome. However, the strategy had significant effect on thermal comfort as the majority of the occupied hours then fell within Category I.

With night cooling strategy, PCM temperature variations between 22[degrees]C (71.6[degrees]F) and 25[degrees]C (77.0[degrees]F) were observed, activating the PCM and enabling heat to be absorbed and released with change of phase. Charge and discharge of the PCM is essential for the efficiency of the material and with PCM temperatures closer to the melting point temperature, the thermal storage potential increased. This led to smaller operative temperature variations and more hours within Category I and II. No effect was observed for the total energy use. However, higher thermal storage capacity decreased the peak cooling power by 15% in Copenhagen and 17% in Rome for the panel with measured PCM properties. This decrease could enable a smaller cooling plant for the system leading to lower investment costs and possibly lower operation costs.

Application of PCM to the panels was found to be most beneficial for Rome. The warmer outdoor climate in Rome required a night cooling set-point of 23.0[degrees]C (73.4[degrees]F), contrary to Copenhagen where a night cooling set-point of 23.4[degrees]C (74.1[degrees]F) was sufficient to avoid overheating during daytime. The lower cooling set-point of Rome caused the average PCM temperature to be closer to the PCM heat of fusion temperature, providing a greater utilization of the PCM.

The difference between the measured and theoretical properties of the PCM had a significant effect on the thermal indoor environment and peak cooling power of the systems. With night cooling strategy, an increase of hours within comfort Category I of 43 % and 58 % was observed for Copenhagen and Rome, respectively. The considerable difference between the theoretical and measured PCM panel specifications could require additional experimental investigations of other PCM enhanced elements to examine the efficiency of PCM application.

The effect of PCM implementation, even with theoretical properties, was limited compared to findings in previous studies with energy savings of 7% (Ascione et al. 2014) up to 57% (Becker 2014) in addition to improvement of thermal indoor conditions. An important factor is the latent heat capacity of the PCM which was 165 kJ/kg (70.94 Btu/lb) in Becker (2014) compared to 110 kJ/kg (47.3 Btu/lb) in this study, when examining the theoretical properties. The results in Becker (2014) were with implementation of 40 mm (1.57 inch) pure PCM, whereas the 26% micro encapsulated PCM in this study corresponds to a thickness of 6.4 mm. The effective PCM integration to the panels in this study is low, compared to the conditions found in literature, showing more beneficial results of PCM implementation.

CONCLUSION

Thermal indoor conditions were improved when microencapsulated PCM was integrated to the radiant ceiling panels. With a night cooling strategy in Copenhagen, PCM application decreased the peak cooling power by 15% and increased the occupied hours in Category I by 8%. Clearer effects were observed in Rome, decreasing the peak cooling

power by 17% and increasing the occupied hours in Category I by 18%. The increase of thermal mass with PCM application decreased temperature fluctuations during occupancy.

The control strategy was essential for the efficiency and utilization of the PCM and negligible effect was observed with the basic cooling strategy preventing activation of the PCM. Equivalent to this were the most significant effects observed for Rome where lower set point temperatures were required due to the warmer outdoor climate compared to Copenhagen.

Insignificant effects were observed for the energy use of the PCM panel systems. Low effect of the PCM application was found to be caused by the cooling control preventing full capacity utilization and low PCM content in the panels. Additionally, low measured properties of the PCM panels compared to the theoretical specifications decreased the effects of PCM integration to the ceiling panels.

REFERENCES

Ascione, F., Bianco, N., Masi, R., Rossi, F., Vanoli, G. 2014. Energy refurbishment of existing buildings through the use of phase change materials: Energy savings and indoor comfort in the cooling season. Applied Energy 113, 990-1007.

Becker, R. 2014. Improving thermal and energy performance of buildings in summer with internal phase change material. Journal of Building Physics 37(3) 296-324.

Building Regulations, 2015. Danish building regulations 2015. Building class 2020.

EN 2016. EN 16798-1, Energy performance of buildings--part 1: Indoor environmental input parameters for design and assessment of energy performance of buildings addressing indoor air quality, thermal environment, lighting and acoustics.

Kosny, J. 2015. PCM enhanced building components. An application of phase change materials in building envelopes and internal structures. Springer.

Nielsen, L. F. 2016. Energy and indoor environment comparison of conventional and novel cooling systems in buildings. Master thesis, Department of Civil Engineering, Technical University of Denmark.

Pavlov, G. K. 2014. Building thermal energy storage. Ph.d. Thesis, Department of Civil Engineering, Technical University of Denmark

Lin Flemming Nielsen

Student Member ASHRAE

Bjarne W. Olesen, Ph.D.

Fellow ASHRAE

Eleftherios Bourdakis

Student Member ASHRAE

Ongun Berk Kazanci, Ph.D.

Associate Member ASHRAE

L. F. Nielsen is a master student, Technical University of Denmark, Denmark. E. Bourdakis is a Ph.D. candidate at the ICIEE, Technical University of Denmark, Denmark. O.B. Kazanci is a Postdoctoral Researcher at the ICIEE, Technical University of Denmark, Denmark. B.W. Olesen is Professor, Technical University of Denmark, Denmark.
Table 1. Construction and properties of the ceiling panels.

Panel       Thickness      Thermal Conductivity
            m (ft)         W/mK (Btu*ft/h*[ft.sup.2]*[degrees]F)

Clay board  0.01(0.033)    0.47 (0.27)
Cardboard   0.005 (0.016)  0.055 (0.03)
Clay board  0.01 (0.033)   0.47 (0.27)
PCM         -              0.153 (0.09)

Panel       Specific heat capacity      Density
            kJ/kgK (Btu/lb*[degrees]F)  kg/[m.sup.3] (lb/[ft.sup.2])

Clay board  1.000 (0.239)               1300 (81.25)
Cardboard   2.000 (0.478)                270 (16.88)
Clay board  1.000 (0.239)               1300 (81.25)
PCM         [Figure 2]                   980 (61.25)
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Article Details
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Author:Nielsen, Lin Flemming; Olesen, Bjarne W.; Bourdakis, Eleftherios; Kazanci, Ongun Berk
Publication:ASHRAE Conference Papers
Article Type:Report
Date:Jan 1, 2018
Words:3052
Previous Article:Simulation Study of Performance of Active Ceilings with Phase Change Material in Office Buildings under Extreme Climate Conditions.
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