PCM thermal storage design in buildings: Experimental studies and applications to solaria in cold climates

Applied Energy 185 (2017) 95–106

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PCM thermal storage design in buildings: Experimental studies and applications to solaria in cold climates Francesco Guarino a,⇑, Andreas Athienitis b, Maurizio Cellura a, Diane Bastien b a b

University of Palermo, Dept. of Energy, Information Engineering and Mathematical Models, Italy Concordia University, Dept. of Building, Civil and Environmental Engineering, Canada

h i g h l i g h t s
This paper analyzes the performance of a building-integrated thermal storage system.
A wall opposing a glazed surface serves as phase change materials thermal storage.
The study is based on both experimental and simulation studies.
Heat is stored and released up to 6–8 h after solar irradiation.
Yearly heating requirements are reduced by 17% in a cold climate.

a r t i c l e

i n f o

Article history: Received 13 July 2016 Received in revised form 5 October 2016 Accepted 16 October 2016

Keywords: Phase change materials PCM Solaria Building design Building simulation Experimental studies Cold climates applications

a b s t r a c t As energy availability and demand often do not match, thermal energy storage plays a crucial role to take advantage of solar radiation in buildings: in particular, latent heat storage via phase-change material is particularly attractive due to its ability to provide high energy storage density. This paper analyzes the performance of a building-integrated thermal storage system to increase the energy performances of solaria in a cold climate. A wall opposing a highly glazed façade (south oriented) is used as thermal storage with phase change materials embedded in the wall. The study is based on both experimental and simulation studies. The concept considered is particularly suited to retrofits in a solarium since the PCM can be added as layers facing the large window on the vertical wall directly opposite. Results indicate that this PCM thermal storage system is effective during the whole year in a cold climate. The thermal storage allows solar radiation to be stored and released up to 6–8 h after solar irradiation: this has effects on both the reduction of daily temperature swings (up to 10 °C) and heating requirements (more than 17% on a yearly base). Coupling of the thermal storage system with natural ventilation is important during mid-seasons and summer to improve the PCM charge-discharge cycles and to reduce overheating. Results also show that cooling is less important than heating, reaching up to 20% of the overall annual energy requirements for the city of Montreal, Canada. Moreover, the phase change temperature range of the material used (18–24 °C) is below typical summer temperature levels in solaria, but the increase in thermal capacity of the room alone can reduce annual cooling requirements by up to 50%. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Energy storage is expected to play a key role in moving to a lowcarbon electricity system. It can supply more flexibility and balancing to the grid, providing a backup to intermittent renewable energy; locally, it can improve the management of distribution ⇑ Corresponding author at: Dept. of Energy, Information Engineering and Mathematical Models, University of Palermo, Viale delle Scienze, Building 9, 90128 Palermo, Italy. E-mail address: [email protected] (F. Guarino). http://dx.doi.org/10.1016/j.apenergy.2016.10.046 0306-2619/Ó 2016 Elsevier Ltd. All rights reserved.

networks, reducing costs and improving efficiency. In this way, it can ease the market introduction of renewables, accelerate the decarbonisation of the electricity grid, improve the efficiency of electricity transmission and distribution (reduce unplanned loop flows, grid congestion, voltage and frequency variations), stabilise market prices for electricity, while also ensuring a higher security of energy supply [1–7]. As solar accessibility and demand often do not match, thermal energy storage plays a crucial role to take advantage of solar radiation in buildings. Building-integrated thermal energy storage

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Nomenclature i i+1 i1 j+1 j Dt Dx Cp kw

node being modeled adjacent node to interior of construction adjacent node to exterior of construction new time step time step calculation time step finite difference layer thickness (always less than construction layer thickness) specific heat of material thermal conductivity for interface between i node and i + 1 node

systems [8,9] cover a wide range of materials, techniques and designs depending on the applications and aims. They however all have in common this underlining concept: being able to store excess energy for later use in order to reduce the time mismatch between energy availability and demand. Effective utilization of thermal energy storage for ambient renewable energy (e.g. solar heat for heating and cool outdoor air for free cooling) with proper design and control has proven promising in reducing peak demand and energy costs associated with space conditioning. Buildingintegrated thermal energy storage systems have recently attracted significant research interest. Savings in room space and material is achievable in comparison with conventional centralized and thermally isolated storage systems (e.g. water/ice tanks). One unique characteristic of building-integrated thermal energy storage systems is their thermal coupling with thermal zones due to large exposed surface areas. Latent heat storage via phase-change materials (PCMs) [10–16] is particularly attractive due to its ability to provide high energy storage density. Several studies have demonstrated that the use of PCMs in well-insulated buildings can reduce heating and cooling energy in residential buildings by as much as 25% and obtain similar reductions in the peak power required for air conditioning [17–19]. Such applications are of interest since they can have lower heating and cooling requirements for a given volume than most sensible systems and may be used in contexts where the application of standard solutions would be difficult, such as in renovation of historical buildings. On the other hand, daily charge-discharge cycles must be carefully planned. PCMs represent a potential solution for reducing peak heating and cooling loads and heating, ventilation, and air conditioning (HVAC) energy consumption in buildings for both new buildings and retrofits. The use of latent energy storage systems may be one of the solutions to the energy mismatches in Net-Zero Energy Buildings [20–29] when renewable energy production and building energy demand are out of phase. A building-integrated and distributed thermal storage could shift and reduce part of the load of residential air conditioners at peak to off-peak periods. As a result, capital investment in peak power generation equipment could be reduced for power utilities and then the savings could be passed on to customers. In areas where power utilities are offering time of day rates, building-integrated thermal storage could enable customers to take advantage of lower utility rates during off peak hours. Literature published on PCMs over the last two decades covers a broad area. The target of this paper is to study the use of wallboard incorporated PCMs to be used passively in high performance buildings with high window-to-wall ratios in cold climates, particularly in retrofit applications where the PCM is only applied to the surface which receives most of the solar radiation. Some relevant

ke

thermal conductivity for interface between i node and i  1 node q density of material CV(RMSE) coefficient of variation of the root mean square error MBE mean bias error n number of measures Tm,i monitored temperature T m;i mean monitored temperature Ts,i simulated temperature

previous experiences on building-integrated PCMs are briefly described below. Most existing studies focus on the design and optimization of PCM layers into vertical walls or in the roof mostly located on the inside walls in various positions. In [30], Chen et al. propose the modeling of a simple room, aiming at determining the best positioning of PCM in the envelope for improving all-year performance. At the optimal locations, the peak heat flux reductions were 51.3% and 29.7% for the south wall and the west wall, respectively. The maximum time delays in the peak heat flux were 6.3 h for the south wall and 2.3 h for the west wall. During winter, energy savings in comparison to non-PCM rooms can reach 10%. In [31] a light envelope test cell was equipped with 25 mm thick PCM on all internal surfaces of a test cell (cubical, around 1 m on all dimensions) to increase the thermal inertia of the envelope and reduce indoor temperature fluctuations. An identical test cell was built and operated on the same weather conditions but without PCM in the envelope. This study showed that PCM allowed a reduction of the indoor temperature range of approximately 20 °C in the test-cell. The influence of the PCM wall thickness was also studied through numerical simulation, from 10 mm to 35 mm. It was found that for a wall thickness higher than 20 mm the indoor temperature variation amplitude would not significantly decrease further. Kuznik and Virgone [32] carried out an experimental research in a full-scale test cell under controlled thermal and radiative conditions to evaluate the performance of walls with and without PCMs during a summer day. In a subsequent study, authors used the same PCM composite [13] to show that PCM wallboards reduce air temperature fluctuations in a room and overheating. In order to assess the potential of a PCM wallboard with 60% of microencapsulated paraffin within a copolymer (the melting and freezing temperatures are 13.6 °C and 23.5 °C respectively), a renovated office building in Lyon was monitored during one year by Kuznik et al. [18]. A room was equipped with PCM wallboards in the lateral walls and in the ceiling, and another room, identical to the first one, was not equipped with PCM wallboards but also monitored. The results showed that the PCM wallboards enhance the thermal comfort of occupants during the whole year. Evers et al. [33] evaluated the thermal performance of enhanced cellulose insulation with paraffin and hydrated salts for use in frame walls. The thermally enhanced frame walls were heated and allowed to cool down in a dynamic wall simulator that replicated the sun’s exposure in a wall of a building on a typical summer day. The results showed that the paraffin-based PCMenhanced insulation reduced the average peak heat flux by up to 9.2% and reduced the average total daily heat flow up to 1.2%. Other studies focused specifically on PCM application to the floor. In [34], authors discuss the application of PCM below

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different tile, wood and metal floor coverings aiming to absorb the solar radiation energy in the daytime and release the absorbed heat in the evening. Therefore, in winter, the indoor climate can be improved and the energy consumption for space heating may be greatly reduced. It is found that: (1) for the purpose of narrowing indoor air temperature swing, for a given position or weather condition, the suitable melting temperature of PCM is roughly equal to the average indoor air temperature of sunny winter days; (2) the heat of fusion and the thermal conductivity of PCM should be larger than 120 kJ/kg and 0.5 W/(m K), respectively; (3) the thickness of shape- stabilized PCM plate used under the floor should not be larger than 20 mm; (4) the performance of PCMs covered by tile or metal floor cover is higher than for a wooden floor cover; (5) the air-gap between PCM plates and the floor should be as small as possible. Passive solar solutions include several different applications that can benefit as well from the use of thermal energy storage in the form of PCM. Many applications are available in literature, some of which are reported below. In [35] De Gracia et al. investigated the effect of PCM on double skin façades. An experimental test was performed with PCM in the air channel of a double skin façade, during the heating season in the Mediterranean climate. Two identical house-like cubicles located in Puigverd de Lleida (Spain) were monitored during winter 2012, and in one of them, a ventilated facade with PCM was located in the south wall. The experimental results conclude that the use of the ventilated facade with PCM improves significantly the thermal behavior of the whole building. In [36] Diarce et al. evaluated the thermal performances of an active façade in Spain that included a phase change material in its outer layer. Experimental results showed that the phase change processes that took place in the façade led to increased heat absorption in comparison to similar architectural solutions without PCM. Simulation results showed that thermal inertia of the wall was higher in the PCM configuration than in all the standard no-PCM alternatives simulated. Most literature discussed previously focused on a uniform distribution of PCM panels, aiming to create a uniformly more inertial room. Many studies focused on optimizating the PCM layer positioning among the envelope layers or thickness while others focused on the use of PCM in integration with specific existing solar design solutions. This study proposes PCM energy storage only on one wall of the indoor environment, directly irradiated by solar radiation, which is suitable option for retrofit applications. The idea is to maximize the energy storage potential by adopting several layers of PCM facing large glazed openings on the south façade. The concept aims at applications mostly in the residential sector with solaria. The optimal applications would be solaria or, more in general, lightweight structures with high window-to-wall ratios with the possibility of performing natural ventilation to ease the discharge of the thermal storage. This solution would also be of interest for Net Zero Energy Buildings, as the potential to shave and delay thermal loads is paramount to align generation and loads. Also in retrofit applications where thermal storage needs to be added it is more easily done on a vertical interior surface. This paper describes the results of the experimental tests and is primarily oriented to building engineers, mechanical engineers and architects.

The main objective of this study is to investigate the potential of using PCMs on interior walls of solaria or rooms with high solar gains as a mean to save energy and reduce the room temperature fluctuations. The main concept of the design is to create a passive PCM wall, oriented towards a window and able to absorb solar radiation and slowly release it in the following hours. This application is mainly directed towards cold climates, since it does not aim – like many PCM applications – to ‘‘shield” the whole room from the exterior, but instead it aims to increase the effective use of solar gains by absorbing them in the storage system for later use. PCM layers were placed on the interior surface of a test room wall facing a large window and were tested under different indoor and outdoor conditions. The installed PCM panels are identical to those used in [32]; each panel measures 1.2  1.0 m with a 5.2 mm thickness. Five layers are used to cover around 80% of the back wall surface area. The main specifications given by the manufacturer are listed in Table 1. The test room (Figs. 1and 2a, c) is a raised parallelepiped with interior dimensions of 2.80 m width  1.30 m depth  2.44 m height. A large 2.2 m by 2.2 m window is installed on the front façade. It has a U-value of 1.3 W/(m2 K) at the center and 1.9 W/ (m2 K) for the frame. Its Solar Heat Gain Coefficient is 0.262. The test room is located inside the climatic chamber with dimensions of 8.9 m  7.3 m  4.7 m to test the performance of the room under different conditions. Solar radiation was simulated by means of a solar simulator (Fig. 2b) equipped with six special metal halide lamps as source of radiation. In combination with glass filters, the lamp system provides a spectral distribution very close to natural sunlight, which fulfills the specifications of the relevant standards EN12975:2006 and ISO 9806-1:1994. Monitoring was performed with thermocouples placed at different heights in the back wall at every layer interface and at a distance of 50 mm from the front surface and in the middle of the room for measuring the air temperature. Readings from thermocouples were stored every three minutes for each thermocouple for the whole test. The different experimental conditions that were tested are briefly summarized next. The numbering and the nomenclature adopted is described in Fig. 3. 2.1. Test n.1 Test n.1 aimed at obtaining monitored data to be used for performing numerical model validation. The test lasted around 60 h; the environmental chamber was set to a constant temperature equal to 16 °C while the indoor environment of the test room was heated at a constant rate (350 W). The heating phase lasted for 20 h and 51 min; after that, the test room was free floating inside the environmental chamber. Fig. 4 presents the results for temperature registered at different depths on a mid-height section of the PCM wall. Readings for thermocouple 1 (front layer), 6 (back layer) and 3 and 4 are presented together with the air temperature; they are considered representative of the whole dataset.

Table 1 Properties of the PCM modules [31].

2. Experimental setup This section describes the results of an experiment performed in the Solar Simulator and Environmental Chamber (SSEC) research facility located at Concordia University Montreal, Canada [37].

Latent heat of fusion Specific heat capacity (average) Density Dimensions Thickness

70,000 J/kg (18–24 °C) 2500 J/(kg °C) 855.5 kg/m3 1000 mm  1198 mm  5.2 mm 5.26 mm

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and faster thermal response than the layer at the back. The melting process lasts for around 7–8 h considering all the different depths; 2.2. Test n°2 The same experimental setup was used to model conditions representative of sunny days in mid-seasons in moderately cold climates. Boundary conditions for test n.2 are: – The temperature in the chamber was set to 10 °C at the beginning of the test; – A sinusoidal chamber temperature is set up with maximum at 20 °C (At 11 a.m.) and minimum at 10 °C after 12 h; – The solar radiation set point for the solar simulator was 500 W/m2 and the average radiation incident on the glass was measured as 496.11 W/m2.

Fig. 1. Schematics of the test room with exterior dimensions.

The most important observations are as follows: (1) Temperature will rise progressively slower, the deeper the PCM layer considered. The phase change is also identifiable at around 10–15 h in the air temperature where the slope of the curves suddenly changes (circled in Fig. 4). (2) The innermost PCM panel shows a clearer phase change. There is a significant delay of 5–6 h between the beginning of the slope change of the air temperature and the phase change of the innermost PCM layer. This means that the phase change of the front layer would allow for a stronger

The test has been running for 3 consecutive days (Fig. 5). Solar radiation is simulated initially for 3.5 h in the morning. During the first day, the solar simulator is switched on from 9:30 a.m. until 1:00 p.m. allowing indoor air temperature to reach nearly 35 °C and a surface temperature of the PCM of around 21 °C. Fig. 5 includes data for each thermocouple, with the exception of 4 and 5 since they had a trend intermediate between layers 3 and 6. On the second test day after solar irradiation, PCM temperatures values were higher (around 19 °C) than the day before at similar time after the solar simulator was switched off, and still in the phase change range. The minimum temperature of the sinusoidal air temperature of the climatic chamber was set to 6 °C for the last day trying to allow a better solidification of the PCM. From the analysis of Fig. 5, the following observations arise: (1) During the three and a half hours of solar radiation incident on the PCM wall, the temperature of the front layer (1) reacts quickly, rising by 17 degrees. The temperature of the back layer (6) grows with a much lower slope, reaching only a 3 °C increase during the irradiation time;

Fig. 2. (a) Back view of the test room; (b) Solar simulator working; (c) Front-side view of the test room.

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Fig. 3. Schematic and nomenclature of the thermocouple placements in the PCM wall.

Air

T1

T3

T4

T6

35 33

Temperature [°C]

31 29 27 25 23 21

grow for more than 4 °C during around ten hours. The maximum temperature of the front layer is higher than 30 °C: therefore, heat would flow from the front layer to the deeper ones. When the temperatures of the front and back layers reach an equilibrium, the slope of the curves changes and also the temperature of the back layer starts to decrease; (3) The melting process for the deeper layers occurs 5–6 h later than the front one. At this step, the melting process starts, however it would probably not be a complete melting, since the temperature of the PCM layer before the third irradiation phase is around 19 °C. This has several implications and may be a starting point for further analysis: without active ventilation, after a high radiation period, the system does not complete a charge-discharge cycle and therefore, the starting temperatures for the following irradiation cycle would be higher by 4–5 °C than those of 24 h before; (4) The second irradiation phase lasts for 3 h (half an hour less than the first one) since it is stopped as the temperature of the indoor air reaches the same maximum value of the previous day. This time, however, although the external temperatures are nearly the same of the previous day, the internal layers temperatures are nearly 5 °C higher (close to 26 °C, above the melting range). This means that the PCM would not probably even start melting with the current setup. For this reason the lower limit of the sinusoidal environmental chamber temperature was fixed for day 2–6 °C. As expected the cooling process is much faster than on day 1.

19

3. Modeling

15 0

5

10

15

20

25

30

35

40

45

50

Hours Fig. 4. Temperatures at different depths from the inside to the outside.

(2) When solar irradiation ends at around 15 h, the front PCM layer and the air temperature drop fast even though the concavity of the curve of the PCM layer is reversed at around 25 h due to the melting process. However, it is worth noting that the temperature of the contiguous layers continue to

In order to perform further studies, a model was created in EnergyPlus [38] by using a customized weather file to model the environmental chamber conditions. The phase change materials properties employed are those declared by the manufacturer. Enthalpy and conductivity curves as a function of temperature are described in [39]. The variability of conductivity with temperature was taken in consideration, ranging between 0.17 and 0.22 W/(m K). The simulation used the finite difference conduction model and Thermal Analysis Research Program (TARP) convection calculations. PCMs were modeled through the enthalpy and conductivity

35

25

33

23

31

21

Temperature [°C]

29

19

27

17

25 15 23 13 21 11

19 17

9

15

7

13

Chamber temperature [°C]

17

0

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

5

Hours

Fig. 5. Thermocouples temperature readings for Test n.2.

T1 T2 T3 T6 Air Chamber

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curves described in [39,40]. A third order backward difference solution for the room air heat balance was adopted. EnergyPlus allows for modeling of PCM using implicit finite difference scheme coupled with an enthalpy-temperature function to account for phase change energy accurately. The implicit formulation for an internal node is shown in Eq. (1).

C p q Dx

" ðT jþ1  T jþ1 T jþ1  T ij 1 ðT jþ1  T jþ1 t i Þ i Þ kw tþ1 ¼ þ ke t1 2 Dt Dx Dx # ðT j  T ij Þ ðT j  T ij Þ þ kw tþ1 þ ke t1 Dx Dx

each timestep was calculated and arranged in ascending order ranging from the minimum to the maximum absolute error. Table 3 includes some more statistical metrics to assess the error in modeling: Mean Bias Error (MBE) and the coefficient of variation of the root mean square error (CV(RMSE)). The mean bias error (MBE) of a sample of n measurements is defined as in Eq. (2):

MBE ¼ ð1Þ

where all symbols are described in the nomenclature. Model validation was performed by comparing experimental and simulated data (Test n.1). Different datasets have been analyzed: air average temperatures and the PCM superficial temperatures at different depths. The chosen Heat Balance Method modeling is based on the assumption that air temperature is constant in a zone volume. This specific feature has required the aggregation of some of the most detailed monitored data into less detailed averages. Air temperature was recorded on five different vertical heights, in the center of the room. Temperature difference between the top and the bottom level is always below 1 °C, as shown in Fig. 6, which justifies the uniform indoor temperature assumption. The simulation time step was adjusted to match the timeframe of the temperature readings (3 min). Monitored results have been compared with the simulated ones and the absolute errors are presented in Table 2. This table represents the distribution of the absolute error between measurements and simulation results, the percentage values are the percentile of exceedance, or in other words, the percent of a distribution that is equal to or below it. Absolute error for

n 1X ðts;i  t m;i Þ n j¼1

ð2Þ

where tm,i is monitored values and ts,i is simulated values at time i. The coefficient of variation of the root mean square error (CV (RMSE)) of a sample of n measurements is defined as in Eq. (3):

CVðRMSEÞ ¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi Pn 2 1 j¼1 ðt s;i  t m;i Þ n

ð3Þ

tm

where tm,i are monitored values and ts,i are simulated values at time j, while t m is the mean of the monitored data. 4. Parametric analysis: results 4.1. Configurations definition In order to explore the energy performance of PCM applications in the indoor environment, a parametric analysis has been performed. The system investigated is the one discussed in Section 2. A PCM wall composed of five layers of PCM is located on the wall opposite to the large south oriented glazed opening (60% window to wall ratio - WWR). The main concept behind this design is to allow for a direct solar radiation on the latent energy storage in order to maximize solar heat gains and store them for later use during winter. Two scenarios are discussed in the following: the

Vertical temperature difference

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

0

5

10

15

20

25

30

35

40

45

50

Hours Fig. 6. Vertical temperature difference.

Table 2 Absolute errors in simulation [°C]. Percentile errors

MIN 10% 25% 50% 75% 90% MAX

Air

1

2

3

4

5

6

0.0016 0.041 0.223 0.638 0.78 0.912 0.98

0.056 0.188 0.426 0.52 0.743 0.88 0.923

0.0011 0.13 0.192 0.53 0.795 0.872 0.937

0.0014 0.068 0.155 0.511 0.755 0.859 0.929

0.0021 0.259 0.288 0.702 0.765 0.812 0.897

0.0013 0.087 0.151 0.539 0.733 0.844 0.967

0.0013 0.087 0.151 0.539 0.632 0.758 0.832

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F. Guarino et al. / Applied Energy 185 (2017) 95–106 Table 3 Mean bias errors and root mean square errors.

MBE Cv(RMSE)

Air

1

2

3

4

5

6

0.3683 1.96%

0.0032 0.49%

0.2679 1.82%

0.2253 1.66%

0.1594 2.04%

0.1250 1.51%

0.1383 1.50%

Heat storage charge and discharge [W]

600 500 400 300 200 100 0 -100 -200 -300

01/01 01/11 01/22 02/01 02/12 02/23 03/05 03/16 03/27 04/06 04/17 04/27 05/08 05/19 05/29 06/09 06/20 06/30 07/11 07/21 08/01 08/12 08/22 09/02 09/13 09/23 10/04 10/14 10/25 11/05 11/15 11/26 12/07 12/17 12/28

01:00:00 16:00:00 07:00:00 22:00:00 13:00:00 04:00:00 19:00:00 10:00:00 01:00:00 16:00:00 07:00:00 22:00:00 13:00:00 04:00:00 19:00:00 10:00:00 01:00:00 16:00:00 07:00:00 22:00:00 13:00:00 04:00:00 19:00:00 10:00:00 01:00:00 16:00:00 07:00:00 22:00:00 13:00:00 04:00:00 19:00:00 10:00:00 01:00:00 16:00:00 07:00:00

-400

Scenario 2

Scenario 1

Fig. 7. Heat storage charging and discharging power for the PCM – north wall.

test room with no PCM layers (Scenario 1, from now on S1) and with PCM panels in the north wall (S2). Natural ventilation is included in both models through the ‘‘Zone: Wind and stack object” in EnergyPlus. Such scenarios use empirical formulations to correlate wind angle and speed, fenestration areas and opening factors to calculate the air change rate. In order to avoid overcooling in the room, natural ventilation is operated through a minimum indoor temperature set-point (23 °C) and a maximum outdoor temperature set-point (18 °C). Models are simulated as both free floating and conditioned, with 18–26 °C set-points for heating and cooling. These heating set-point were chosen to be outside the phase change temperature range. Weather data used is for Montreal, Canada (latitude 45 N). 4.2. Results In this section hourly and yearly data are presented. The main aim of the design is to increase the utilization of solar gains by storing excess heat for later use. Fig. 7 clarifies this concept by showing the energy stored and released in both S1 and S2 during the whole simulated year. Heat charge and discharge in the storage wall is shown in both S1 and S2. It can be seen that the presence of PCMs significantly improves the energy storage potential. The effectiveness in charging and discharging the wall is higher for S2 from roughly September to April than in the summer months, while it is maximum for S1 during summer. This trend is mirrored in the yearly data set included in Fig. 8, showing the solar beam radiation that reaches the north wall during the year. The highest values are reported for winter due to solar geometry; with the other walls working as solar shadings in summer, the PCM wall does not receive direct solar radiation in this season.

More insight is gained when selecting the first 10 days of the year taken as example in Fig. 9 and analyzing the trends in more detail. Fig. 9 shows the first days of the simulated year in the case of a conditioned building is. It demonstrates that in addition to the much larger heat storage rate in the PCM application scenario compared to S1, the discharge phase is also prolonged 6–8 h after the end of solar irradiation. Some more detailed trends are described in the following, considering different weather conditions both in the case of a conditioned building and a free floating (passive) building. Figs. 10a and 10b show trends for temperature and heating loads on some winter days. The simulated results in Fig. 10a show clearly the impact that the PCM can have on a conditioned building. When solar gains are high, the system is able to store energy and give it back in the following hours, as seen during the night between January 1st and 2nd. Peak heating load of S2 are lower by nearly 100 W during January, 2nd at night, in a range between 5 and 15% of the total S1 peak. Overall, the use of the PCM reduces the heating power in nearly all the hours shown. In Fig. 10b, the building is free floating. Although the phase change temperatures are not reached, the sensible storage of the PCM wall allows to reduce the air temperature variations from 15° and 15 °C, to 7 °C and 7 °C. Figs. 11a and 11b describe similar trends for the selected cold sunny and cloudy days of late March. The conditioned building (Fig. 13a) show results dependent on the solar radiation regime: during the cold cloudy days, scenario S2 reports only a slight reduction in heating loads, while during the last, sunny days, the evening heating requirements are reduced by around 40%. Cooling requirements are also relevant during the high solar radiation days and are roughly reduced by 50% in scenario S2. The particular configuration of the simulated room causes indoor free floating tem-

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600

400 300 200 100

01/01 01/11 01/22 02/01 02/12 02/23 03/05 03/16 03/27 04/06 04/17 04/27 05/08 05/19 05/29 06/09 06/20 06/30 07/11 07/21 08/01 08/12 08/22 09/02 09/13 09/23 10/04 10/14 10/25 11/05 11/15 11/26 12/07 12/17 12/28

01:00:00 16:00:00 07:00:00 22:00:00 13:00:00 04:00:00 19:00:00 10:00:00 01:00:00 16:00:00 07:00:00 22:00:00 13:00:00 04:00:00 19:00:00 10:00:00 01:00:00 16:00:00 07:00:00 22:00:00 13:00:00 04:00:00 19:00:00 10:00:00 01:00:00 16:00:00 07:00:00 22:00:00 13:00:00 04:00:00 19:00:00 10:00:00 01:00:00 16:00:00 07:00:00

0

Beam radiation reaching the PCM wall Fig. 8. Beam solar radiation incident on the PCM wall during a simulated year.

Heat storage charge/discharge [W]

900 750 600 450 300 150 0 -150 -300

Scenario 2

Scenario 1

Windows transmitted solar radiation

Fig. 9. Heat storage charge and discharge rate for the PCM (S2) or North wall (S1).

0

700

-2

600

-4 -6

500

-8

400

-10 -12

300

-14

200

-16 100

-18 -20 01/01 01:00:00

0 01/02 02:00:00

01/03 03:00:00

01/04 04:00:00

Window Transmitted Solar Radiation Rate [W]

Outdoor Air Drybulb Temperature [C]

S2 Heating [W]

S1 Heating [W]

Fig. 10a. Cold sunny days, January in Montreal: conditioned buildings with setpoints (S2).

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Solar radiation [W]

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103

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10

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F. Guarino et al. / Applied Energy 185 (2017) 95–106

0 01/02 02:00:00

01/03 03:00:00

01/04 04:00:00

Window Transmitted Solar Radiation Rate [W]

S2 Zone Air Temperature [C]

Outdoor Air Drybulb Temperature [C]

S1 Zone Air Temperature [C]

Fig. 10b. Cold sunny days, January in Montreal: free-floating temperatures (S2).

600 4 500

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-2

Power [W]

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-8 -10 03/25 09:00:00

0 03/26 10:00:00

03/27 11:00:00

Window Transmitted Solar Radiation Rate [W] S2 Heating [W] S1 Heating [W]

03/28 12:00:00 Outdoor Air Drybulb Temperature [C] S2 Cooling [W] S1 Cooling [W]

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-10 03/25 03/25 03/26 03/26 03/27 03/27 03/27 03/28 03/28 03/29 09:00:00 19:00:00 05:00:00 15:00:00 01:00:00 11:00:00 21:00:00 07:00:00 17:00:00 03:00:00 Window Transmitted Solar Radiation Rate [W]

S2 Zone Air Temperature [C]

Outdoor Air Drybulb Temperature [C]

S1 Zone Air Temperature [C]

Power [W ]

Temperature [°C]

Fig. 11a. Cold sunny/cloudy days, March in Montreal: conditioned building (setpoints: 18–26 °C).

0

Fig. 11b. Cold sunny/cloudy days, March in Montreal: free-floating temperatures.

peratures to rise up to 40 °C in S1 (Fig. 13b) even when external temperature is around 0 °C. The use of the PCM in S2 allows reducing the indoor maximum temperature by 10 °C during the sunniest days.

Figs. 12a and 12b present results during typical days in August. Some cooling loads reductions are obtained (Fig. 12a) when compared to S1 (a peak cooking load reduction of 120 W– around 17%), while the already limited heating load in S1 is zero in S2.

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800

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100 0 0 08/19 08/19 08/19 08/20 08/20 08/21 08/21 08/22 08/22 08/22 08/23 04:00:00 14:00:00 24:00:00 10:00:00 20:00:00 06:00:00 16:00:00 02:00:00 12:00:00 22:00:00 08:00:00

Window Transmitted Solar Radiation Rate [W] Outdoor Air Drybulb Temperature [C] S2 Heating [W] S2 Cooling [W] S1 Heating [W]

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Temperature [°C]

Fig. 12a. Summer days in Montreal, August – conditioned buildings with the setpoints.

0 08/22 23:00:00

Window Transmitted Solar Radiation Rate [W]

Zone Ventilation Air Change Rate [ach]

S1 Zone Air Temperature [C]

Outdoor Air Drybulb Temperature [C]

S2 Zone Air Temperature [C]

S2 PCM Temperature

Fig. 12b. Summer days in Montreal, August – Free floating.

Natural ventilation has a relevant role in reducing air and PCM temperatures and in helping the freeze-melting cycle. With comparable incident solar radiation as in the three days described in Fig. 12b, it can be seen that the role of natural ventilation is very important: during August 19th and 20th the zone temperature reaches 30 °C, acceptable for a solarium. On the 21st, natural ventilation was not performed during the hottest hours of the day: this causes indoor temperature to reach 45 °C in the case of S1. The use of PCM allows to lower this value by nearly 7 °C. However, the PCM system will not revert to solid after August, 22th and thus its latent storage capabilities would not be used at its best. It is worth noting that the PCM melting range (18–24) is not appropriate for cooling conditions optimization, since during mildly hot external conditions the PCM superficial temperature is above the phase change boundaries. The highest temperature of the PCM wall would be reached after the irradiation phase: this means that during the night, the PCM will often have higher temperatures than the air and will contribute to the overheating of the room. In summer PCM will need to be carefully discharged during the night if outdoor temperatures allow it. 4.3. Parametric study, annual summary The annual results of the study are presented in Fig. 13. It describes monthly contributions to heating and cooling requirements for the two scenarios.

Annual S1 heating requirements are the most relevant contributions to the total, amounting to 1374 kW h while cooling is equal to 326 kW h in the case of S1. Heating is lower in the case of S2 when compared to S1 during the whole year, varying from around 5% in January to 30% in April and reaching nearly 100% in summer. Cooling has a reverse trend during the year and shows savings that are not lower than 30% when comparing S2 to S1. Overall, heating is reduced by 17.4%, while cooling is lower in S2 by roughly 50% during the whole year. Furthermore, cooling could be reduced even more if different ventilation strategies would be adopted, by further lowering the maximum external temperature allowed indoor.

5. Discussion The paper discusses experimental studies and a set of calibrated building simulations that aim at proposing solutions and applications to solaria environments with application to one wall that receives most of the solar radiation. The two tests performed with different conditions represent two different situations in a typical year in Montreal, and as such can be representative of the climate in Northern Europe, Northern United States and Canada, as well as many more cold weather sites in the world. The results in their current state are still local and the findings reported cannot be generalized at this stage for any other climate conditions.

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Heating and cooling energy requirements [kWh]

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-60%

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100

S2 Cooling variation

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-10% Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

0%

Fig. 13. Monthly aggregated heating requirements for all scenarios.

The experiments have shown that five PCM layers placed on a back wall, directly irradiated by the sun, have a strong impact on the temperature fluctuations of the indoor environment. The exact number of layers depends on the amount of stored energy that is desired and the time period of the storage. The dynamics of the inter-layer heat exchange and temperature fluctuation allow for a delayed and persistent heat delivery effect after irradiation had stopped. This defines this particular solution set as one suited for implementation in colder climates (or at least with heating energy storage purposes) to store solar energy and re-deliver it later to the indoor environment. The experiments have shown that under mild external temperatures (10 °C) the indoor temperature would not be lower than 18 °C when PCMs are present. One of the tests represented some optimal conditions for the storage to work effectively. External temperature was not too low and solar radiation was high enough to reach the melting temperature of the PCM. The heat storage worked well since the temperature of PCM layers would remain inside the melting range up to 5–8 h after the end of the solar irradiation phase. Since the minimum PCM temperature was not low enough to complete the solidification process the environmental chamber minimum (external) temperature was decreased from 10 °C to 6 °C. This concept highlights the need for a good control of the room: either natural ventilation, solar shadings or a combination of the two are needed to ensure effective use of the PCM. Cooling and heating energy and peak power are always reduced during the whole year. The proposed design solution is practical, particularly for retrofit applications in cold climates since the storage system is able to discharge heat up to 8 h after irradiation has stopped. The optimal thickness of the PCM layers however is variable and depends on the design of the room and on the thermal loads of the building. The system is also effective for reducing cooling needs during the hottest months but it is only an indirect result of increasing the inertia of the room: the PCM is often completely melted during summer due to the melting range (18–24 °C) being too low for cooling applications. Moreover, since the main aim of a bioclimatic design of a building in cooling dominated climates is to shield the envelope from solar radiation, the solution itself, although it obtains good results during summer, is not the best for such applications. For cooling dominated applications, it could be useful to use a uniform distribution of PCM on the other surfaces of the room and materials with higher phase change temperature ranges. The system needs a finely tuned control to work at its best. Indoor air temperature set-points should be carefully adjusted to match also dynamically the melting/freezing states and the outdoor conditions to activate and discharge the storage system.

Moreover, compared to conventional sensible heat storage, PCM storages allow for a high energy density operating at lower temperature variation ranges. Thus, PCMs are usually more compact than sensible storages and can therefore be particularly effective in the case of building retrofits. Lastly, it is worth highlighting the importance of the passive approach largely discussed in this paper, from the design concept to all the applications and simulations described. Sensible heat storage systems were not reviewed in this paper. As a completely passive solution set, the design described can be probably characterized by higher first costs, in comparison to active systems solutions, but it would eliminate need for maintenance and be suitable for retrofit on walls.

6. Conclusions This study has analyzed the impacts of a PCM thermal energy storage system on the performance of a solarium in a cold climate. All the results point towards a reduction in daily temperature swings and of the overall heating requirements. The experiment showed that the PCM wall worked as intended under moderate cold weather conditions, since the PCM could provide an appreciable time lag with the solidification process that took around 5–8 h. However, without ventilation and after a high irradiation period, the system would not undergo a complete charge-discharge cycle and therefore, the starting temperatures for the following irradiation cycle would be around 19 °C, inside the melting range. This has implications for the use of such systems during shoulder months in moderately cold climates: natural ventilation is advised and also active solutions (fans blowing air through the PCM panels) could be beneficial. In solaria and highly glazed buildings, the room dynamics can be highly improved according to the simulation results when using PCM as a thermal storage: – Temperature air swings were decreased by up to 10 °C for daynight cycles, – Heating peak loads were lowered significantly (up to 40%) during high irradiation days in winter; Simulations defined the PCM solaria solution as clearly beneficial for cold regions applications. A concentrated PCM thermal storage proved able to reduce heating requirements by 17.4% in a year, largely increasing the storage capability of the wall and delaying heat discharge for 6–8 h during the night. It is advisable to include natural ventilation in the design of solaria, since it can

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