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Simulation Study of Active Ceilings with Phase Change Material in Office Buildings for Different National Building Regulations.

INTRODUCTION

Several efforts have been made to make both current and future buildings more sustainable and energy efficient. Among the research conducted on the subject is the implementation of Phase Change Materials (PCM) in indoor building components. The PCM increases the thermal mass of the building substantially without an increase in building mass or volume (Babiak et. al 2009). This attribute has made it an attractive alternative for conventional building thermal mass, e.g. solid wood, gypsum and concrete (Kosny 2015).

Research has shown that PCM has the capability to store energy both in commercial products (heat and cool pads, etc.) and potentially in buildings. Several studies have shown that heat storage of PCM was significantly larger compared to more conventional building materials (Dincer and Rosen 2002; Mehling and Cabeza 2008; Kosny 2015). PCM enhances thermal energy storage (TES) in building materials. The two main types are latent and sensible heat storage (Dincer and Rosen 2002). The subject of investigation in this paper has been the use of PCM in radiant ceiling panels. The system was considered an active application of PCM-enhanced ceiling systems. The structure of this system was PCM integrated into gypsum panels in the form of microcapsules with embedded pipes inside the panels. The panels were actively heated or cooled by water circulation in the embedded water pipes and/or ventilation. The purpose of the system was to meet both the heating and cooling demand of the room while discharging the PCM, which meant releasing the stored energy or absorbing excess energy from the room.

The energy use and the thermal indoor environment were analyzed, with the target of reducing the energy use while maintaining an acceptable, and possibly improving, thermal indoor environment. The requirements on building energy use were the Danish Building regulations (BR10, BR15 and BR20) from 2010, 2015 and 2020, addressing energy use in terms of heating, cooling, ventilation, and the auxiliary components. These regulations are similar to the building energy codes program provided by the United States Department of Energy (DOE). The building energy codes by DOE is more of a guideline for energy efficiency than requirement. Table 1 shows the different minimum required U-values for the relevant construction types for each building code represented as followed. Whereas the energy performance for each building code for the simulated construction was 71.3, 41 and 20 kWh/[m.sup.2] (22600, 13000 and 6300 Btu/[ft.sup.2]) per year for BR10, BR15 and BR20, respectively. The energy performance was based on heated surface area for BR10 and BR15.

The thermal indoor environment was analyzed in accordance with the European standard EN 15251 with primary focus on operative temperature, Relative Humidity (RH), Carbon Dioxide (C[O.sub.2]) concentration and Predicted Percentage of Dissatisfied (PPD). The desired thermal indoor environment was based on the Category II or better which meant that the objective was to keep the temperature between 20-26[degrees]C (68-78.8[degrees]F). The Relative Humidity range was desired to be between 25-60%, the C[O.sub.2] below 500 ppm above outside concentration (assumed to be 400 ppm) and the PPD to be below 10%. The results were gathered and presented in graphs and tables comparing the different building regulations.

SIMULATION STUDY

A model representing a two-person office room on an intermediate floor was created with a total floor area of 22.9 [m.sup.2] (246 [ft.sup.2]) and a total height of 3.2 m (10 ft) using a commercially available building simulation software. Two of the walls were defined as external. The wall facing south had a window with an area of 5.8 [m.sup.2] (62 [ft.sup.2]) and the other external wall was facing east. The top of the room was defined as a roof; these three construction elements were exposed to outside conditions. The remaining walls and floor were considered internal construction parts. The outside conditions were defined by applying the weather file ASHRAE IWEC 2 for Copenhagen Kastrup in the simulations.

The room-+ included two occupants with an activity level of 1.2 met and a clothing insulation of 0.85 [+ or -] 0.25 clo. The total heat gains from occupants, office equipment and lighting were assumed to be 17.3 W/[m.sup.2] (1.61 W/[ft.sup.2]) and only occurred during occupied hours, namely from 8am to 5pm weekdays.

The model was divided into two zones composing of an office room for the occupants and the other part being the space between the suspended ceiling and the roof - plenum. The fundamental part of the model was the active ceiling system with a total area of 19.1 [m.sup.2] (205 [ft.sup.2]) and a maximum power of 1911 W (6500 Btu) for both heating and cooling purposes. The space between the roof and the ceiling is referred to as plenum, had a distance of 0.5 m (1.6 ft) between them. The ceiling panels composed of 18 mm (0.7 in) thick PCM layer between two 1 mm (0.04 in) layers of aluminum. Due to its low thermal conductivity, 0.25 W/mK (0.145 Btu/hft[degrees]F) was a layer of aluminum inserted above and below the layer of PCM, to ensure that the stored energy was released homogeneously. The chosen PCM was simulating a paraffin wax with a melting temperature of 23[degrees]C (73.4[degrees]F) and a heat storage capacity of 0.227 kJ/kg (10200 lb/Btu). The PCM took form as solid wax or as liquid, with a density of 910 kg/[m.sup.3] (56.8 lbm/[ft.sup.3]) in liquid form and 830 kg/[m.sup.3] (51.8 lbm/[ft.sup.3]) as solid wax.

The water pipe system ran on two temperature signals with a deadband providing a range for which a signal was sent to either activate or stop circulating water in the embedded pipes, Figure 1.

The system was divided into two smaller systems, one addressing the cooling and the other one the heating part in the water pipes. In the cooling part the setpoint was 24.5[degrees]C (76[degrees]F) reflecting the maximum operative temperature, for which the system then would begin to cool the PCM and the room. The other temperature of 22[degrees]C (72[degrees]F) was the setpoint for the surface temperature of the PCM layer, both adjusted with a deadband of [+ or -]1.5[degrees]C (3[degrees]F). The setup of the system was designed to avoid operative temperatures above 26[degrees]C (79[degrees]F) as EN15251 prescribes, and avoid overheating of the PCM.

The heating part of the water pipe system was designed to provide enough heating to achieve a minimum operative temperature of 20[degrees]C (68[degrees]F). The system was always operating but with different setpoints depending on the season. The setpoint was always 20[degrees]C (68[degrees]F) but during summer the setpoint was lowered to 17[degrees]C (63[degrees]F) outside occupancy period. This setting would help reducing the heating use in the building.

The total energy use is calculated by using Primary Energy Factors (PEF) which has the purpose to uniform the different loads of the building that has a different energy source. In this way, it is possible to come up with a more accurate total energy use. Table 2 shows that the PEF change considerably among the different building codes. This is a result of the transition from fuel to renewable energy forms as main energy source in buildings. The heating/cooling source was designed to be district which is one of the most common energy sources in buildings in Denmark.

The models were designed to meet the requirements for U-values in Table 1 according to the building regulations, which in return gives models designed as following in Table 3.

The ventilation was designed to be a variable air volume (VAV) system with a constant inlet temperature of 17[degrees]C (63[degrees]F). The system had a minimum air flow of 1.7 ACH and maximum of 8.7 ACH, operating to prevent operative temperature reaching above 24.5[degrees]C (76[degrees]F) and maintain an acceptable Indoor Air Quality (IAQ). The system was only operating during occupancy period.

To analyze and estimate the impact of the model with PCM and active ceilings, a similar model (referred to as Vent. model) was created for comparison. The main difference between the two systems was that the model without active ceiling only relied on ventilation for heating and cooling purposes. The ventilation system had a different buildup which involved the inlet air temperature being dependent on the ambient temperature. If the ambient temperature was below -20[degrees]C (-4[degrees]F) the inlet temperature was set to 23[degrees]C (73[degrees]F). From -20[degrees]C (-4[degrees]F) to 10[degrees]C (50[degrees]F) the relation would decline linearly to 20[degrees]C (68[degrees]F). The inlet temperature would for ambient temperatures above 20[degrees]C (68[degrees]F) have an inlet temperature of 16[degrees]C (61[degrees]F). Further details regarding the simulation models can be found in (Stefansen and Farhan, 2017).

RESULTS AND DISCUSSION

All results were extracted from yearly simulations, occupancy period was assumed to be 2349 hours. The primary focus of the simulations was the Indoor Environmental Quality (IEQ) and the energy use of the simulated models. Results in IEQ included unoccupied hours which could give a false indication, this meant that periods of overheating and energy peaks outside occupancy period were included in the results.

Indoor Environment Quality

Both the operative temperature and relative humidity were analyzed for all six scenarios and set up as a cumulative distribution function (CDF). The results were sorted according to size in relation to the percentage of simulated time, 8760 H. The CO2 concentration never exceeded the maximum threshold of 500 ppm above the assumed outside concentration. This meant that the requirements on this area were met and therefore not presented.

Figure 2 shows that the operative temperature in the PCM models follow the same pattern with some minor differences, whereas the differences in the Vent. models are substantial.

The BR20 PCM model had the most stable and desirable temperature range of approximately 20.8[degrees]C (69[degrees]F) to 25.6[degrees]C (78[degrees]F). Both BR10 and BR15 experienced an operative temperature slightly above 26 [degrees]C (79[degrees]F), for less than 5% of the whole simulated year. In all cases the PCM models experienced lower and more stable operative temperature compared to the Vent. Models. The desired range of operative temperatures was met during all seasons.

The relative humidity was almost identical for both the PCM and Vent. Models although with some small variations. This was mostly obvious when the RH reached 35% or higher as illustrated in Figure 3. The results were influenced by the different set up of ventilation system and partly due to the incorporation of PCM in the ceilings.

For the major of time, Relative Humidity was in the desired range of 25% - 65% but the models did experience periods with lower Relative Humidity. However the implementation of a dehumidifier would solve it if necessary which means that the results for relative humidity should not be prioritized like those for the C[O.sub.2] concentration.

The overall thermal comfort was described by the PPD among the occupants. The results for PPD shown in Table 4 and 5 reflect satisfaction for the mentioned indoor environment with the criteria of Category I & II met more than 90% of the occupied time in the simulations. The satisfaction levels were higher among the PCM models compared to the Vent. models. This proved that the active ceiling concept could provide a satisfactory thermal indoor environment.

Peak heating and cooling demands

The peak demands for heating and cooling addressed the peak during a certain moment for a simulated year. The maximum time step was set to 1.5 hours, meaning the results were either data points in minutes or hours. The PCM models relied largely on water based heating/cooling but also in some occasions on the Air Handling Unit (AHU). Whereas the Vent. model relied mainly on AHU, as shown in Table 6. Some of the values in Table 6 were higher than the maximum power capacity which was explained by some of those values representing demands at a certain time. The peaks occurred at 5pm after the occupancy were scheduled to leave the office room.

The difference between the peaks experienced in the PCM respectively Vent. model was explained by the two model's systems. For the cooling part, the PCM model worked to remove heat from ceiling panels (discharging) before the next arrival of occupants. On the other hand, heat in the Vent. model was removed instantaneously, which explained the high cooling peaks.

Energy use

As the IEQ targets clearly were met, it was more important to investigate the annual energy use to achieve the desired thermal indoor environment.

For building regulations 2010 and 2015, the need for heating was smaller for the PCM model with a difference of 31 kWh/[m.sup.2] (9830 Btu/[ft.sup.2]) and 12.6 kWh/[m.sup.2] (3990 Btu/[ft.sup.2]) respectively. An influential factor to the difference between the PCM and Vent. models was the different medium of heat transfer. The PCM models relied mostly on water based heating and cooling but in some cases also on AHU. The Vent. models relied only on the AHU for both heating and cooling, with the energy source also being district. The combination of radiation and convection from the integrated water pipes in the ceiling proved to be a more efficient as heating system.

In contrast, the cooling demand for PCM models was larger than the energy in the Vent. Models, due to the discharging process. The internal heat gains were absorbed by the PCM panels, but the increasing amount of heat gains during summer period could cause overheating and thereby decrease the effectiveness of the PCM in the panels. This was complicated by the system running entirely on mechanical ventilation with closed windows the whole year. Which meant a higher cooling demand for the PCM models to cool down the PCM panels. As shown in Table 7 the cooling energy use was larger for all PCM models. Although it resulted in the ceiling system operating in accordance to the discharging system and resulted in a better utilization of the system.

The total energy used to provide the desired thermal indoor climate was 44.6% lower with a PCM model for BR10, 34.5% for BR15 but 16.4% higher for BR20 (Table 7). When addressing the energy use in relation to the requirements of the building regulations, it was indicated by the results in Table 7 that the requirements were met by all PCM models. Whereas the Vent. Models clearly had an energy use exceeding the requirements, except for the BR20 model. The main factor behind the difference between the systems were heating and use of HVAC auxiliary. The water-based heating and cooling systems are more efficient to transfer and transport heat compared to air, and these can be seen in the results.

The results in Table 7 illustrate that the PCM model for building regulation 20 used 16.4% more energy than its counterpart. Both the PCM and Vent. model was well-insulated and less dependent on active means, specifically mechanical heating and cooling to meet an acceptable indoor environment. The larger reliance on passive means that mechanical heating was reduced, resulting in smaller peaks and fewer operation hours compared to the cases with BR10 and BR15. As reflected by the results in Table 7, Vent. Model benefited by reducing significantly all demands, heating in particular. The active ceiling system was not designed in accordance to low-energy buildings as BR20 regulation aspires to. Although the system clearly met the energy requirements and provided an acceptable thermal indoor environment according to EN15251.

The investigation of PCM as a mean of cooling have both provided promising but also some unexpected results. Despite this, it was indicated by the results in Table 7 that the PCM models were more efficient in terms of heating. The heating demand dropped while the cooling demand increased when compared to the Vent. Models. The Danish climate is considered temperate and the outdoor temperatures are mainly considered cold for the majority of the year. This means that the heating demand is higher than the cooling demand in countries with a similar climate to Denmark. However as indicated by the results in Table 7, the heating demand for BR15 was 3 times the cooling demand. This is not a normal tendency found in buildings unless the climate is considered extreme. This could add to the question if it is worth it to invest in active ceilings with PCM for cooling purposes if the results indicate an increase in cooling instead of a decrease as projected. Though the results have shown that the system have a positive effect on indoor climate and also decreasing the heating demand.

FUTURE STUDIES

The increase in cooling demands meant that the system control was not entirely suitable for a scenario with low cooling demand. It is therefore recommended that the system operation is improved to be more adaptable to these scenarios. Another solution could be focusing more on heating purposes, to build on the results pointing out that PCM system could potentially decrease heating demand by up to 44.6%. PCM have in previous studies proved to be beneficial as cooling applications in offices. Nevertheless as indicated by the results could PCM also be useful for heating applications.

On the other hand, a change of PCM material in terms of melting temperature could be more suitable and better utilized for the temperature range in the models, particularly the BR20 model. Further it could be interesting to create a system with more reliance on passive means (non-mechanical energy sources). In that way the energy use could be reduced and the PCM system more utilized, making the system more sustainable. It could also be possible to implement PCM panels without embedded water pipes. Thereby creating a system similar to the Vent. model but with PCM panels. This would mean increasing the thermal mass while avoiding the additional energy use for operating the hydronic system, providing a satisfactory indoor environment and reducing the energy use. A similar but different change to the models could involve the comparison active ceiling systems with and without a PCM panel. These options could apply only to well insulated low-energy buildings, designed according to Danish Building code 2020.

CONCLUSION

All models met the indoor environment recommendations of EN15251 proving that the active ceiling concept involving PCM was capable of providing an acceptable indoor climate. It was proved that this was possible while having a significantly lower energy use. Although the PCM system was investigated for cooling purposes it was shown by the results that the system operation increased the cooling energy demand compared to the Vent. models and instead proved to be more efficient in decreasing the heating energy demand. Despite providing satisfactory indoor climate and meeting the energy requirement for BR20, it was considered that the system needed to be improved by buildup. Optimization of the system could help increase the utilization of passive means of energy. Whereas another interesting subject could be optimization of the system operation to have heating purposes instead of cooling. Although PCM are mostly used for cooling purposes in office buildings could it be beneficial to use PCM as heating as well. As the results showed the active ceilings with PCM to be influential in reducing the heating.

REFERENCES

Babiak, J., Olesen, B. W., & Petras, D. (2009). Low temperature heating and high temperature cooling. Rehva.

CEN, E. (2007). 15251, Indoor environmental input parameters for design and assessment of energy performance of buildings addressing indoor air quality, thermal environment, lighting and acoustics. European Committee for Standardization, Brussels, Belgium.

Dincer, I., & Rosen, M. (2002). Thermal energy storage: systems and applications. John Wiley & Sons.

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

Mehling, H., & Cabeza, L. F. (2008). Heat and cold storage with PCM. Berlin: Springer.

Stefansen, C & Farhan, H.R. (2017). Performance of active ceilings with PCM in buildings. Technical University of Denmark

Hajan Farhan

Student member ASHRAE

Eleftherios Bourdakis

Student Member ASHRAE

Bjarne W. Olesen, Ph.D.

Fellow ASHRAE

Casper Stefansen

Student Member ASHRAE

Ongun B. Kazanci, Ph.D.

Associate member ASHRAE

Hajan Farhan is a MSc. Student, Casper Stefansen a B.Eng. graduate, Eleftherios Bourdakis is a Ph.D. student, Ongun B. Kazanci is a post doc, and Bjarne W. Olesen is a professor, all at Technical University of Denmark, Lyngby
Table 1. Danish Building regulation U-Value requirements

Construction    BR10

Unit            [W/[m.sup.2]K] / [(Btu/([ft.sup.2] h [degrees]F))]
Roof            0.20 (0.035)
External walls  0.30 (0.053)
Internal walls  0.40 (0.070)
Floor           -
Window          -

Construction    BR15

Unit            [W/[m.sup.2]K] / [(Btu/([ft.sup.2] h [degrees]F))]
Roof            0.20 (0.035)
External walls  0.30 (0.053)
Internal walls  0.40 (0.070)
Floor           -
Window          1.80 (0.317)

Construction    BR20

Unit            [W/[m.sup.2]K] / [(Btu/([ft.sup.2] h [degrees]F))]
Roof            0.08 (0.014)
External walls  0.12 (0.021)
Internal walls  0.40 (0.070)
Floor           -
Window          0.80 (0.141)

Table 2. Primary Energy Factors

Primary Energy    BR10  BR15  BR20
Factors

District Heating  1.0   0.8   0.6
District Cooling  0.4   0.4   0.4
Electricity       2.5   2.5   1.8

Table 3. U-values for the simulation models

Construction    BR10

Unit            [W/[m.sup.2]K] / [(Btu/([ft.sup.2] h [degrees]F))]
Roof            0.20 (0.035)
External walls  0.30 (0.053)
Internal walls  0.40 (0.070)
Window          1.80 (0.317)

Construction    BR15

Unit            [W/[m.sup.2]K] / [(Btu/([ft.sup.2] h [degrees]F))]
Roof            0.18 (0.032)
External walls  0.22 (0.039)
Internal walls  0.40 (0.070)
Window          1.00 (0.176)

Construction    BR20

Unit            [W/[m.sup.2]K] / [(Btu/([ft.sup.2] h [degrees]F))]
Roof            0.08 (0.014)
External walls  0.09 (0.016)
Internal walls  0.40 (0.070)
Window          0.70 (0.123)

Table 4. PPD for PCM models

Category in  BR10         BR15         BR20
EN15251      [Hours]      [Hours]      [Hours]

I & II       2210 (94%)   2200 (94%)   2349(100%)
III          2349 (100%)  2349 (100%)  2349 (100%)

Table 5. PPD for Vent. models

Category in  BR10         BR15         BR20
EN15251      [Hours]      [Hours]      [Hours]

I & II       2150 (92%)   2170 (92%)   2330 (99%)
III          2349 (100%)  2349 (100%)  2349 (100%)

Table 6. Peaks in heating and cooling during Un-occupied hours

Delivered         AHU heating    AHU Cooling    Water based
energy                                          Heating

Units             [W]/[(BTU/H)]  [W]/[(BTU/H)]  [W]/[(BTU/H)]
BR10       PCM     649 (2210)    1981 (6760)    2166 (7390)
           Vent.  2646 (9030)    3074 (10490)      0
BR15       PCM     634 (2160)    2209 (7540)    1684 (5750)
           Vent.  2235/7630      3228/11010        0
           PCM     510 (1740)    1514 (5165)     615 (2100)
           Vent.   896 (3060)    1819 (6200)       0

Delivered  Water based
energy     Cooling

Units      [W]/[(BTU/H)]
BR10       3433 (11710)
              0
BR15       3167 (10800)
              0
           3019 (10300)
              0

Table 7. Energy use for simulation models

Energy use         Heating             Cooling
Unit               [kWh/[m.sup.2]]/    [kWh/[m.sup.2]]/
                   [(BTU/[ft.sup.2])]  [(BTU/[ft.sup.2])]

BR10        PCM    23.6 (7480)         13.2 (4180)
            Vent.  54.6 (17300)         6.7 (2120)
BR15        PCM    18.7 (5930)          5.6 (1780)
            Vent.  31.3 (9920)          2.7 (860)
BR20        PCM     7.4 (2350)          2.4 (760)
            Vent.   6.6 (2100)          1.6 (510)

Energy use  HVAC aux.           Lighting            Total
Unit        [kWh/[m.sup.2]]/    [kWh/[m.sup.2]]/    [kWh/[m.sup.2]]/
            [(BTU/[ft.sup.2])]  [(BTU/[ft.sup.2])]  [(BTU/[ft.sup.2])]

BR10         6.0 (1900)         2.7 (860)           45.5 (14400)
            18.2 (5770)         2.7 (860)           82.2 (26060)
BR15         6.2 (1970)         2.7 (860)           33.2 (10520)
            14.4 (4560)         2.7 (860)           51.0 (16170)
BR20         3.9 (1240)         1.9 (600)           15.6 (4950)
             3.3 (1050)         1.9 (600)           13.4 (4250)
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Author:Farhan, Hajan; Bourdakis, Eleftherios; Olesen, Bjarne W.; Stefansen, Casper; Kazanci, Ongun B.
Publication:ASHRAE Conference Papers
Article Type:Report
Geographic Code:4EUDE
Date:Jan 1, 2018
Words:4082
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