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Modelling for performance prediction of highly insulated buildings with different types of thermal mass
Applied Thermal Engineering 122 (2017) 139–147
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Modelling for performance prediction of highly insulated buildings with different types of thermal mass O. Siddiqui a, R. Kumar a,⇑, A.S. Fung a, D. Zhang a, M.A. White b, C.A. Whitman b a b
Department of Mechanical and Industrial Engineering, Ryerson University, Toronto, ON M5B 3K3, Canada Department of Chemistry and Institute for Research in Materials, Dalhousie University, Halifax, NS B3H 4J3, Canada
h i g h l i g h t s TRNSYS modelling for performance prediction of highly insulated buildings. The impact of thermal mass on the energy saving and occupant comfort was studied. The thermal masses were found to contribute to energy savings of 10–15%. The thermal mass was adding considerably to the comfort of the occupants.
a r t i c l e
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Article history: Received 27 September 2016 Revised 15 April 2017 Accepted 5 May 2017 Available online 6 May 2017 Keywords: Phase change material Thermal mass Thermal comfort Net zero energy house TRNSYS simulation Energy savings
a b s t r a c t Thermal performances of two phase change materials (PCM) in the Toronto’s Net-Zero Energy House are compared and contrasted with commonly available forms of thermal mass. TRNSYS simulations show that the use of thermal mass was found to contribute to energy savings of 10–15% when different types of thermal mass were mixed into the building envelope. The results exhibit that the performance of a novel solid-solid phase PCMs, recently developed by researchers at Dalhousie University and known as DalHSM-1, could be comparable to a commercially available PCM from BASF (Micronal) in the heating mode. The cooling mode performance revealed that DalHSM-1 provided lower energy savings when compared to Micronal, due to a lower phase transition temperature and latent heat. The impact of thermal mass on the occupant comfort was also investigated by considering the total number of hours where the temperature exceeded the heating set point of 21 °C. The results showed that the total number of hours where the temperature exceeded 21 °C could be reduced significantly with different types of thermal mass, thus contributing considerably to the comfort of the occupants. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction The rising price of fossil fuel energy along with a major concern of the environment over the last few years has generated significant interest in the energy conservation, system/process efficiency, and renewable energy technologies. Efforts have been initiated on different scales, depending on the government programs and policy, to promote energy conservation and efficiency in all aspects of daily energy uses such as transportation and buildings. It is evident that even a small modification to existing buildings could dramatically reduce their energy consumptions [1,2]. The use of thermal mass in the building’s foundation, floors, and walls provides a better strategy for energy management in the building envelope and also provide an improved indoor environment for ⇑ Corresponding author. E-mail address: [email protected] (R. Kumar). http://dx.doi.org/10.1016/j.applthermaleng.2017.05.021 1359-4311/Ó 2017 Elsevier Ltd. All rights reserved.
the occupants [3]. In the absence of any heat storage solution in the building foundations/walls, most of the passive thermal energy from the sun would go towards increasing the indoor temperature inside the building, making it uncomfortable for the occupants and also increasing the air-conditioning load [4,5]. One of the solutions for heat storage that has shown an enormous potential is an inclusion of phase change material (PCM) in building materials [6–8]. A variety of building materials has been investigated for the purpose of incorporating PCMs. The most recognition has been given to gypsum and concrete/Portland cement since these are readily available and inexpensive to produce. Extensive research has been conducted to use of gypsum/PCM composite as a means for storage in the building envelope. Darkwa and Kim [9] have investigated the dynamics of energy storage in gypsum wallboards and found that the laminated PCM board provided improved thermal characteristics when compared to randomly distributed PCM. Kedl and Stovall [10] used paraffin wax incorporated into gypsum
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wallboards using the immersion process. Two methods were suggested in which the PCM wallboard could be manufactured. It could either be melted into a liquid and then absorbed into the porous gypsum wallboard, or it could be added during the plasterboard manufacturing process, where it is added to the wet plaster. Hawes et al. [11] results of PCM/gypsum composite found that wallboards can absorb up to 50% of their weight. However, optimal thermal performance was obtained at the mass percentage of between 25 and 30%. Flexural strength of PCM integrated drywall was found to be comparable to the conventional wallboard. The absorption of moisture which is undesirable was found to be only one-third that of conventional wallboard which makes it well suited for building applications. Considerable research has been done to examine the mixing of phase change materials into building fabrics, for instance, the use of PCM composites in buildings as a means for heat storage and residential cooling [12,13]. Stoval and Tomlinson [14] have analyzed the use of PCM wallboard as a load management device for passive solar applications and estimated the energy savings. Peippo et al. [15] have shown energy reduction potential of almost 4 GJ/year (or 15% of the annual energy cost) for a 120 m2 house in Madison, Wisconsin (43°N). They have also shown that a melting temperature that is 1–3 °C above the average room temperature provides the most optimal heat storage. Darkwa and O’Callaghan [16] carried out thermal simulations of the phase change drywalls in a passive solar building. They concluded that wallboard with a small phase change material would moderate the overnight temperature more efficiently than a passively-designed room. Neeper [17] investigated the use of gypsum wallboard with PCM for newly constructed buildings and found that it creates an opportunity for passive solar heating as well as ventilated cooling and time shifting of mechanical loads. In terms of cost effectiveness, it was established that the PCM wallboards were economically viable when compared to ordinary wallboards. Another notable research on PCM-gypsum composite wallboard was carried out at Fraunhofer Institute [18] and Concordia University [19]. In both these studies, it was concluded that the presence of PCMs stabilizes and reduces the indoor air room temperature by 3 °C. The contemporary research in the area of energy-efficient buildings has produced a variety of techno-economic solutions that have led to the development of alternative materials and technologies [20–22]. Significant possibilities exist in the building construction, one of the solutions involves the design of buildings incorporated with thermal masses and phase change materials. The use of PCMs in buildings holds significant potential to reduce the overall energy consumption, which can also contribute to improvement in the occupant comfort. The current research provides an insight and thorough understanding of the scope of different types of the thermal storage medium on the performance of Net-Zero Energy Townhouse in the climatic conditions of Toronto, Canada. Thermal performance of house was established with two commercially available solid-to-liquid PCM (Micronal) and solidto-solid PCM (DalHSM-1). The performances of the two PCMs were compared with commonly available forms of thermal mass (concrete slab). In addition, analysis of the energy savings and reduction in daily thermal fluctuations for different parts of buildings in diverse seasons will be made, and an interpretation of results has been provided in terms of occupant comfort.
tigation of the effect of thermal mass in a building must take into account the unique properties of the building envelope. Buildings that are highly insulated behave in a significantly different manner with respect to heat transfer and storage than buildings that are of light construction, manifesting as differences in indoor temperature fluctuations and comfort levels [23]. The Toronto Net-Zero Energy House represents an award-winning design initiative that represents the collaboration between the Sustainable Urbanism Initiative Toronto (SUI) and a host of architectural and engineering firms, with the objective of increasing public awareness and adoption of energy-efficient homes in Canada [24,25]. The main purpose of this research is to evaluate the impact of implementation of PCMs in the proposed Toronto’s Net-Zero Energy House. Considering the uniqueness of this project and integration of PCM with the building envelope, a model was developed in TRNSYS. 2.1. Description of SUI Net Zero Energy Townhouses The model of the townhouse has a total heated area of 210 m2 (heated volume 685 m3) and the orientation of the house is 37° west of south. The orientation and location of the houses have been optimized to ensure that a maximum amount of solar energy can be captured to operate the roof integrated photovoltaic and solar thermal panels for the generation of electricity and hot water respectively for the house. A ground source heat pump is also utilized during the winter to provide a reliable and efficient source of heating. Fig. 1 shows a computer-generated 3-D model of the house. The building envelope of the townhouse is designed with the intention of minimizing the heat transfer between interior and exterior, thereby, saving energy in maintaining a comfortable environment for occupants. However, one of the possible drawbacks with a highly insulated building, such as Toronto Net Zero Energy Townhouse, is the potential for overheating, especially in winter, as a result of solar gain [26]. This fact should be taken into consideration in the design of the building envelope to ensure that there is a enough thermal storage capacity (in terms of PCM or concrete slab or combination of both) within the building envelope to absorb any surplus solar gain. For the present simulation the external load was taken 4 kW h/day (1460 kW h/y). This can be estimated in terms of CFL Lighting, Garage door opener, Standby losses of the garage door opener, power tools. The Toronto Net-Zero Energy Townhouse is a light-weight construction and due to the large glazing area, is highly susceptible to over-heating during both the winter and summer seasons. The external walls have been insulated with sprayed polyisocyanurate foam insulation, which provides an overall insulation value of R-60
2. Model development The development of a precise model for a house or a building along with relevant parameters that describe its unique characteristics is essential for the evaluation of the impact of factors such as the use of thermal mass and phase change materials (PCMs). Inves-
Fig. 1. 3-D computer representation of the three townhouses of Toronto Net-Zero Energy unit [10].
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(RSI-10.6). The roof assembly consists of drywall on 19 mm 19 mm furring and 0.15 mm polyethylene vapour retarder attached to the bottom of the 294 mm pre-engineered Ijoists. Sprayed polyisocyanurate foam is applied between joists as roof insulation. The roof has an insulation value of R-76 (RSI13.4). The windows used in the house have low emissivity and are argon-filled with a fiberglass frame and have an overall insulation value of R-4 (RSI-0.7). Walls below grade are of the insulating concrete form and have a 6.3 cm of rigid polystyrene board with a waterproof membrane. The overall insulation value of the below grade wall is R-35 (RSI-6.27). 2.2. PCM implementation in the model of house TRNSYS is a complete simulation program for the transient simulation of energy systems, including multi-zone buildings [27]. It is used by engineers and researchers around the world to validate new energy concepts, from domestic water heating to the design and simulation of buildings and their equipment, including control strategies, occupant behaviour, alternative energy systems (wind, solar, photovoltaic, etc.) and thermal comfort. TRNSYS is typically setup by connecting components graphically in the Simulation Studio and each Type of component is described by a mathematical model in the TRNSYS simulation engine. TRNSYS provides a simulation model classified as Type 204 for the study of PCMs. Before the Type 204, the only methodology in which the effect of PCMs in buildings could be analyzed was through the development of an active layer within the building envelope. Kuznik et al. [28] developed a Type 260 and connected with TRNSYS multi-zone building Type 56. The inside surface temperature calculated by Type 56 is linked as an input in Type 260. The conductance of layers between the thermal zone and PCM is included into the Type 260 and was validated by experimental results [29]. Recently, a new PCM Type 399 was developed by Dentel and Stephen [30] for TRNSYS 17 and can be applied to the modelling of temperature-depended thermal characteristics. However, the accuracy of this new Type is yet to be validated by experimental data. To examine the impact of PCMs on the thermal performance and occupant comfort in the Toronto Net-Zero Energy Townhouse, Type 204 was used. The following input parameters were entered into the model to predict the PCM impact on the building load accurately: (1) Number of iterations – this parameter can give any value between one and infinity and is used primarily for the sake of accuracy; (2) Melting temperature – the initial temperature during which the PCM undergoes phase transition; (3) Crystallization temperature – the temperature where the PCM changes back to the solid phase; (3) Range of crystallization temperature – this parameter is used to define the phase change temperature range; (4) Latent heat of PCM; (5) PCM specific heat capacity; (6) PCM density; (7) Specific heat capacity of other materials incorporated with the PCM; and (8) Volume fraction of PCM in node – the overall concentration of PCM in a particular specimen can be entered through this parameter. For each scenario (materials and amount of thermal mass), the simulations were run for one year with a time step of one hour, which is the standard value for the evaluation of seasonal and annual building simulation. The simulations were run with the house unconditioned, which means that other than the solar gain and heat loss, there is no artificial heating or cooling of the house. To ensure that the there is no interference with the heating/cooling equipment of the thermal mass effect provided by the building envelope. The house consists of five zones that represent the Garage, 1st Floor, 2nd Floor, 3rd Floor and Mezzanine (Fig. 1). As a means of comparison, all of the results shown are on the 3rd Floor of the house, since this is the location where the temperature peaks and fluctuations are expected to be the
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greatest because of the large glazing area. Fig. 2 illustrates the complete TRNSYS model of the Toronto Net-Zero Energy Townhouse with the Type 204 model integrated within the building model. In the first part of the study, a commercial phase change material product, Micronal DS 5001, was utilized. As per ASHRAE Standard 55-2013, thermal environmental conditions for human occupancy, the temperature could range 19.4–27.8 °C (67–82 °F). These numbers also fluctuate, depending on the level of relative humidity, season, clothing, activity levels, etc. Micronal 5001, used in this study, has a melting temperature between 23 and 26 °C and a latent heat of 110 kJ/kg [31–33]. The use of Micronal has allowed modern buildings of light-weight construction a similar level of thermal comfort to buildings constructed from heavier materials such as concrete and brick. The thermal storage capacity of a half-inch drywall with 30% Micronal by weight has been found to be similar to a six-inch brick wall. When utilized as part of an interior wall in a building, the PCM begins absorbing additional heat from the room, as the indoor temperature of the room rises beyond the melting point. The large latent heat of the material would therefore keep the room temperature at a more uniform level. As the temperature drops during the night, the excess heat stored within the material is released to the room. The use of adequate ventilation during the night ensures that this heat can be released to the outdoor environment. This introduces a time delay or lag in the peak indoor temperature, which has a beneficial economic impact, since typically during summer residential airconditioning equipment consumes the most energy during the time of day when the outdoor temperatures are at their peak. The reduction of the indoor thermal fluctuations also contributes greatly to the thermal comfort of the occupant. In the simulation, the Micronal product was integrated with gypsum wallboard, having a thickness of 13 mm (0.5 in.), to ensure a uniform distribution of PCM throughout the building material. The energy storage capacity of PCM/concrete block was found to be more than 200–230% when compared to pure concrete blocks [11]. So, concrete is utilized on the outside of buildings and can directly capture any resulting solar gain. A four inch concrete slab with PCM was placed at each of the floors for this purpose. Table 1 presents the material properties of Micronal DS 5001.
3. Results and discussion The TRNSYS simulations are performed for the 3rd floor room of designed net zero townhouses for a typical day of winter (February 1) and summer (July 15). It is apparent from Fig. 3 that as the concentration of PCM within the building envelope is increased; the indoor temperature profile is slightly elevated in winter. Since the building is unconditioned, the indoor temperatures are below the PCM transition temperature. Thus any improvements in the indoor temperature profile are attributable to the increased sensible storage capacity of the PCM/drywall in comparison with the ordinary building envelope containing no PCM. This would translate into an overall reduction in the heating load requirement of the building during the winter period. On the other hand, Fig. 4 illustrates the impact of PCM during a typical summer day. The key difference in Figs. 3 and 4 is the relative magnitude of the room’s temperature change, which is significantly higher during summer. This can be interpreted in terms of melting point of PCM (26 °C). During the winter months, this temperature would rarely be achieved, and the only heat transfer mechanism for the PCM would be sensible heat transfer, which produces only a small change in temperature magnitude. However, during summer, complete melting of the PCM occurs as most of the time the indoor temperatures are above the melting point of the
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Fig. 2. TRNSYS model of the Toronto Net-Zero Energy Townhouse.
divergence exists between the day and night temperatures, the use of a concrete slab as thermal mass would provide results similar to what could be achieved using the PCM. The use of a 2-in. concrete slab as thermal mass produces a sizable reduction in the peak indoor temperature during the summer. This can be contrasted with the results for the 10% and 20% of PCM. Depending on the particular design requirements, the option of selecting a particular thermal mass depends on the occupant comfort requirements and economic considerations. Although the thermal performance of the 5% PCM was similar to a 2-in. concrete slab (Fig. 5), however, when the economic factors (saving in the energy consumptions) are taken into account (Fig. 6), the advantage of a concrete slab over the PCM is significant. On the other hand, if the heating and cooling loads of the building are significantly higher, then it might be economically practical to implement PCMs with higher loading (either heating or cooling) into the building, rather than increasing the thickness of the concrete slabs. Fig. 6 illustrates the overall impact of using PCM in terms of heating and cooling loads reduction. The yearly heating and cooling loads of the Toronto Net-Zero Townhouse are presented. It
Table 1 Material properties of Micronal DS 5001 [31]. PCM model number Melting point Specific heat capacity (including sensible energy) Latent heat Density
DS 5001 23–26 °C 145 kJ/kg 110 kJ/kg 250 to 350 kg/m3
PCM. It is evident from Fig. 4 that not only there is a reduction in the peak temperature, but also a time delay of approximately seven hours between the outside peak temperature (No PCM) and inside peak temperature. Under no PCM case, the indoor temperature registers an earlier peak, closer to the time of the peak of outside temperature. It is also obvious from Fig. 4 that the appropriate selection of the type and amount of thermal mass depends strongly upon the climate of a particular location. Depending on the diurnal temperature range of a location, the type and amount of thermal mass to be used can be readily determined. For places where a significant
Temperature (ºC)
15 14 13 12 No PCM
11
10% PCM 20% PCM
10 1
3
5
7
9
11
13
15
17
19
21
Time (Hours) Fig. 3. Temperature profiles on a typical winter day (February 1) in Toronto.
23
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35
Temperature(ºC)
33 31 29 27 25 No PCM 2 Inch Concrete Slab
23
10% PCM 20% PCM
21 1
4
7
10
13
16
19
22
Time (Hours) Fig. 4. Temperature profiles on a typical summer day (July 15) in Toronto.
2 inch concrete slab
35
5% PCM
Temperature (ºC)
33 31 29 27 25 23 21 1
5
9
13
17
21
Time (Hours) Fig. 5. Temperature profiles on a typical summer day (July 15) in Toronto when 5% PCM is contrasted with the 2-in. concrete slab.
Heating Load (kWh) Cooling Load (kWh) Total Load (kWh)
10000 9000
Load (kWh/yr)
8000 7000 6000 5000 4000 3000 2000 1000 0 No PCM
2 Inch Concrete Slab
10% PCM
20 % PCM
Fig. 6. Yearly heating/cooling load requirements of the Toronto Net-Zero Energy Townhouse for different heat-storage constructions and combinations.
shows that compared to the base case where no PCM is used, total load reductions of almost 15% are achievable when 20% PCM is used. Increasing the amount of the PCM from 10% to 20% produces a small reduction (5%) in the load. As before, the impact of using a 2-in. concrete slab as thermal mass is contrasted with PCM and it is determined that the overall impact is equivalent to what would be achieved by using PCM with a concentration of 5%, as was shown in the temperature profile presented in Fig. 5. It should be stated that,
since design of the Toronto Net-Zero Energy Townhouse is dictated by the overall goal of the minimizing the energy consumption through the use of high resistance insulation, the heating and cooling loads are already significantly lower than an average home in Canada. The determination of the optimum PCM concentration in terms of the total energy consumption within the building envelope is important not only because of the economic considerations but
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also for the structural integrity of the building envelope. Simulations were performed to ascertain the maximum proportion of PCM that could be integrated into the building envelope so that a sizeable reduction in the total energy consumption is achieved. Fig. 7 presents the results of the analysis. While it was found that a definite relationship exists between the concentration of PCM used and the total load reduction, but, for 40% or more PCM amounts, the impact in energy saving is relatively very small.
Table 2 Properties of DalHSM-1, as used in current simulation.
a
3.1. Performance comparison of different storage materials All the above simulations were carried out for Micronal PCM and 2-in. concrete slab. The model is also applied further to the simulations of a variety of materials as thermal mass including PCMs to analyze the relative performance effectiveness in terms of energy saving in Toronto Net Zero Townhouse. The additional simulations were carried out with a heat storage material, DalHSM-1, identified by the teammates of this research at Dalhousie University [34–36]. The key objective of this segment of research was to identify materials that possess most desired characteristics that would cause a reduction in the building heating and cooling loads and would create an improvement in the thermal comfort to the building occupants. 3.1.1. DalHSM-1 material properties DalHSM-1 belongs to the solid-solid class of phase change materials [34–36]. The DalHSM-1 material, which has a chemical composition (C6H13)2NH2Br, has been synthesized in the form of coarse particles, which can be grounded into a very fine powder, to assist with the integration of the PCM into building materials directly without expensive encapsulation process. DalHSM-1 possesses excellent phase change characteristics in the temperature range of interest for building applications. Because it stays as a solid, it can be integrated more easily than PCMs that undergo solid-liquid phase change. Some of the fundamental material properties of the PCM used for the simulation are given in Table 2. DalHSM-1 has a significant advantage for building materials over many potential solid-solid heat storage materials as its transition temperature is in the range of room temperature. 3.1.2. Comparison results and analysis The impact of using different thermal masses (different compositions and amounts) on the annual heating and cooling loads was analyzed and is presented in Fig. 8. The performance of 20% Micronal PCM is comparable to that of a 4-in. concrete slab. The performance of DalHSM-1 is found to be better than the base case (no PCM) but inferior to Micronal. As can be expected, due to the
Property
Value
Density (q) Thermal conductivity (k) Specific heat (Cp) Onset phase change temperature upon heating/cooling (To)a End phase change temperature upon heating/cooling (Te)a Latent heat of transition on heating (L)
800 kg/m3 0.3 W/m/K 1.6 kJ/kg/K 20/17 °C 21/15 °C 80 kJ/kg
Data for heating or cooling rates typical for a building application.
slightly lower latent heat, 30% DalHSM-1 performs comparably with 20% Micronal for the heating season. However, DalHSM-1 does not have much effect on cooling load. The total cooling load is 10% higher when 30% DalHSM-1 is used as compared to 20% Micronal PCM. This is understandable due to its phase transition on cooling being below the heating set point. Since the cooling set point is higher than the phase transition temperature of DalHSM-1, any thermal energy stored by the PCM does not have opportunity to be released. The peak heating and cooling loads for each combination are shown in Fig. 9. A similar trend has been seen for the peak heating and cooling loads as observed for the annual loads (Fig. 8), where the performance of DalHSM-1 is better than the base case but inferior to Micronal (as modeled) at the same loading, especially during the summer when cooling is required. A detailed analysis of the occupant thermal comfort was also conducted for each of the above case by analyzing the total number of hours the indoor temperature would exceed the heating set point of 21 °C. The analysis was performed to determine the total number of hours where the temperature exceeds 25 °C, which is considered to be an uncomfortable temperature for indoor occupants, and cooling is required. These results are shown in Tables 3 and 4, respectively. As evident from Tables 3 and 4, the use of thermal mass significantly reduces the total number of hours where the temperature exceeds the heating set point. During the winter months, the combination of an active heating system and admittance of solar radiation during the day creates a condition of over-heating which is further exacerbated by the highly insulated building envelope, which traps the thermal energy inside. This phenomenon typically occurs during the daytime on a sunny winter day, when the peak level of radiation is incident on the building. This causes the temperature of the building to rise. The use of thermal mass in such a situation would enable this unnecessary solar gain during the daytime to be stored and released during the evening when the temperature drops. Analyzing Tables 3 and 4, it is clear that the use
10000
Annual Load (kWh)
9000 8000 7000 6000 5000 4000
0%
10%
20%
30%
40% 50% 60% 70% PCM concentration (%)
80%
90% 100%
Fig. 7. Total annual heating/cooling load with varied concentration of PCM is varied.
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Total load (kWh) 6000
Heating
Cooling
5000 4000 3000 2000 1000 0 Base Case 2-Inch Slab4-Inch Slab
20% 5% 10% 20% 30% Micronal DalHSM-1 DalHSM-1 DalHSM-1 DalHSM-1
Fig. 8. Annual heating and cooling loads for the Toronto Net-Zero Energy Townhouse using different types and amounts of thermal mass.
7
Total load (kWh)
Heating
6
Cooling
5 4 3 2 1 0 Base Case 2-Inch Slab 4-Inch Slab
20% 5% 10% 20% 30% Micronal DalHSM-1 DalHSM-1 DalHSM-1 DalHSM-1
Fig. 9. Peak heating and cooling loads for the Toronto SUI Net-Zero Energy house using different types and amounts of thermal mass.
Table 3 Total number of hours during the winter season that the indoor temperature for the Toronto Net-Zero Energy Townhouse would exceed the heating set point of 21 °C. Scenario
1st Floor
2nd Floor
3rd Floor
Mezzanine
Base case 2-in. concrete slab 4-in. concrete slab 20% Micronal PCM 5% DalHSM-1 10% DalHSM-1 20% DalHSM-1 30% DalHSM-1
477 394 368 383 464 441 407 387
653 577 523 527 640 598 564 542
1603 1186 1039 998 1564 1335 1209 1066
358 299 255 237 344 321 289 271
Table 4 Total number of hours during the winter season that the indoor temperature for the Toronto Net-Zero Energy Townhouse would exceed 25 °C. Scenario
1st Floor
2nd Floor
3rd Floor
Mezzanine
Base case 2-in. concrete slab 4-in. concrete slab 20% Micronal PCM 5% DalHSM-1 10% DalHSM-1 20% DalHSM-1 30% DalHSM-1
290 234 207 199 277 254 221 209
454 377 323 311 445 413 374 355
602 386 273 256 577 443 379 334
161 109 92 92 158 127 111 101
of thermal mass is most effective on the 3rd floor, where the glazing area is the highest compared to other areas of the house. Temperature profiles were also developed for typical summer and winter weeks, to analyze the variation of temperature with time. The typical winter and summer weeks are defined as occur-
ring between February 1 to February 7 and July 15 to July 21, respectively. Fig. 10 shows the temperature profile of the 3rd floor of the Toronto Net-Zero Energy Townhouse for a winter week. As shown in Fig. 10, the indoor temperature rises dramatically in the absence of thermal mass. During the winter months, the
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34 No Thermal Mass 4 Inch Concrete Slab 30% DalHSM-1 20% Micronal PCM
Temperature (ºC)
32 30 28 26 24 22 20 1
13
25
37
49
61
73
85
97
109
121
133
145
157
Time ( Hours) Fig. 10. Temperature profile of the 3rd floor of the Toronto Net-Zero Energy Townhouse for a typical winter week (where hour 1 is 1 am and so on).
27
Temperature (ºC)
26
No Thermal Mass
4-Inch Concrete Slab
30% Dal HSM-1
20% Micronal PCM
25 24 23 22 21 20 1
13
25
37
49
61
73
85
97
109
121
133
145
157
Time (hours) Fig. 11. Temperature profile of the 3rd floor of the Toronto Net-Zero Energy Townhouse for a typical summer week (where hour 1 is 1 am and so on).
temperature of the air is a combination of input heat from an active heating system and a passive solar gain due to the low declination angle of the sun. Therefore, a fluctuation has been seen in the room air temperature for different days of the winter simulation. The rate of temperature rise is slowed by the introduction of different types of thermal masses. In this case, the peak temperature without the use of thermal mass is almost 8 °C higher than when thermal mass is used, but the addition of the thermal masses keep the maximum temperature within the occupant comfort zone. With PCM and different types of thermal mass, the floor temperature is more flatten as the energy absorbed by the PCM/thermal mass during the sunshine is released to the room during off-sunshine hours. In a highly insulated house of a lightweight construction such as the one modelled in this research, one of the key factors impacting the comfort of the occupants is the fluctuations in the indoor temperature caused by interplay of a variety of factors such as the incident solar radiation, HVAC system, and heating/cooling set points, etc. During the winter months, overheating of the indoor environment is a major concern, especially on a bright, sunny day, where the house is exposed to direct incident radiation. These solar gains, when combined with the regular winter heating capability, dramatically increase the degree of discomfort for the occupant. A
strategically placed unit of thermal mass, such as a PCM or concrete slab, within the building envelope, can not only minimize the indoor temperature fluctuations but also contribute to a reduction in the cooling load of the building. Fig. 11 shows the temperature profile of the 3rd floor of the house for a typical summer day. In this case, the degree of temperature reduction is not as great as for the winter case. This can be explained by the complex interaction of air-conditioning equipment that dictates the degree of temperature rise based on the set temperature. In the summer simulation conditions, once the set point of 25.5 °C is reached, the cooling equipment is activated. If the temperature were allowed to rise beyond the cooling set point, a similar trend as the winter case would be observed and would create extremely uncomfortable conditions for the indoor occupants. On the other hand, Fig. 11 shows that all three thermal masses significantly reduce the diurnal temperature range and the need for air conditioning.
4. Conclusions The buildings constitute a large portion of overall energy consumption (30%) in Canada, the use of PCMs in buildings holds sig-
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nificant possibilities to make them more energy efficient. The objective of this research project was the development of a fundamental understanding of the technical aspects of incorporating PCMs into building envelope. It was seen that there was a 15% reduction in the overall energy consumption as a result of using 20% Micronal PCM. The performance of other types of thermal mass was also determined to contribute to energy savings, and thus economic savings. The performance of 20% Micronal PCM is comparable to that of a 4-in. concrete slab. The novel PCM, DalHSM-1, was found to provide a significant thermal mass, but showed performance inferior to Micronal PCM, especially when the cooling loads are taken into consideration. Due to the slightly lower latent heat, 30% DalHSM-1 performs comparably with 20% Micronal for the heating season. However, DalHSM-1 does not have as much effect on cooling as Micronal PCM. It was also noted that the presence of PCM/thermal mass reduces the total number of hours when the temperature exceeded 21 °C, thus contributing considerably in the peak air-conditions loads and in the comfort of the occupants. Acknowledgement This work was supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada through the Solar Buildings Research Network (SBRN). Funding from Ontario Graduate Scholarship (OGS) for Mr. Omar Siddiqui and Mr. Dahai Zhang is gratefully acknowledged. References [1] J. Cuddihy, C. Kennedy, P. Byer, Energy use in Canada: environmental impacts and opportunities in relationship to infrastructure systems, Can. J. Civ. Eng. 32 (2005) 1–15. [2] M. Dabaieh, O. Wanas, M.A. Hegazy, E. Johansson, Reducing cooling demands in a hot dry climate: a simulation study for non-insulated passive cool roof thermal performance in residential buildings, Energy Build. 89 (2015) 142–152. [3] N. Ekrami, R.S. Kamel, A.S. Fung, Effectiveness of a ventilated concrete slab on an air source heat pump performance in cold climate, Presented in eSim Conference, Ottawa, May 8–9, 2014. [4] R. Mora, G. Bitsuamlak, M. Horvat, Integrated life-cycle design of building enclosures, Build. Environ. 46 (2011) 1469–1479. [5] R. Pacheco, J. Ordóñez, G. Martínez, Energy efficient design of building: a review, Renew. Sustain. Energy Rev. 16 (2012) 3559–3573. [6] D. Zhang, A.S. Fung, O. Siddiqui, Numerical studies of integrated concrete with a solid-solid phase change material, in: Proceedings of the 2nd Annual Solar Building Network conference, Calgary, Alberta, June 07, 2007. [7] K. Darkwa, P.W. O’Callaghan, D. Tetlow, Phase-change drywalls in a passivesolar building, Appl. Energy 83 (2006) 425–435. [8] H.E. Feustel, C. Stetiu, Thermal performance of phase change wallboard for residential cooling application, Report LBL-38320, Lawrence Berkeley National Laboratory, US, 1997. [9] K. Darkwa, J.S. Kim, Dynamics of energy storage in phase change drywall systems, Energy Res. 29 (2005) 335–343. [10] R.J. Kedl, T.K. Stovall, Activities in support of the wax-impregnated wallboard concept, US Department of energy thermal energy storage researches activity review, New Orleans, LA, USA, 1989. [11] D.W. Hawes, D. Banu, D. Feldman, Latent heat storage in concrete, Sol. Energy Mater. 19 (1989) 335–348. [12] R.A. Taylor, N. Tsafnat, A. Washer, Experimental characterization of subcooling in hydrated salt phase change materials, Appl. Therm. Eng. 93 (2016) 935–938.
147
[13] H. Tse, O. Siddiqui, A.S. Fung, F.M. Rad, Simulation and analysis of a net-zero energy townhome in Toronto, in: 3rd Canadian Solar Buildings Conference, Fredericton, NB, August 20–22, 2008. [14] T. Stoval, J. Tomlinson, Potential benefits of including latent storage in common wallboard, in: Proceedings of the 26th Intersociety Energy Conversion Engineering Conference, Boston, Aug. 4–9, 1991. [15] K. Peippo, P. Kauranen, P.D. Lund, A Multicomponent PCM wall optimized for passive solar heating, Energy Build. 17 (1991) 259–270. [16] K. Darkwa, P.W. O’Callaghan, Simulation of phase change drywalls in a passive solar building, Appl. Therm. Eng. 26 (2006) (2006) 853–858. [17] D. Neeper, Benefits of Wallboard Impregnated with Phase-Change Material for Residential Heating and Cooling, Solar Energy Research Institute, 1990. [18] P. Schossig, H.M. Henning, S. Gschwander, F. Haussmann, Microencapsulated phase change materials integrated into construction materials, Sol. Energy Mater. Sol. Cells 89 (2005) 297–306. [19] A.K. Athienitis, C. Liu, D. Hawes, D. Banu, D. Feldman, Investigation of the thermal performance of a passive solar test-room with wall latent heat storage, Build. Environ. 32 (1997) 405–410. [20] M.M. Shapiro, D. Feldman, D. Hawes, D. Banu, PCM thermal storage in wallboard, in: Proceedings of the 12th Passive Solar Conference, Portland, Orlando, 1987, pp. 48–58. [21] J. Jokisalo, P. Lamberg, K. Sirén, Suitability of building construction materials in short-term energy storage, Part 2: thermal simulation of phase change materials in concrete products, Helsinki University of Technology, HVACLaboratory, Report B64, 1999. [22] M. Ahmad, A. Bontemps, H. Sallée, D. Quenard, Thermal testing and numerical simulation of a prototype cell using light wallboards coupling vacuum isolation panels and phase change material, Energy Build. 38 (2006) 673–681. [23] D. Roeleveld, R. Kumar, D. Naylor, A.S. Fung, A CFD study of natural convection inside a BIPV/T system, in: Proceedings of the eSim Conference 2016, Hamilton, Ontario, Canada, May 03–06, 2016. [24] O. Siddiqui, A. Fung, H. Tse, D. Zhang, Modelling of the Net Zero town house in Toronto using TRNSYS, and an analysis of the impact using thermal mass, in: Proceedings of the 2nd International Conference on Energy Sustainability, August 10–14, 2008, Jacksonville, Florida, USA, 2008. [25] E. Poulad, A. Fung, D. Naylor, Effect of PCM thermal conductivity on energy demand and temperature on the net-zero energy building in Toronto, in: Proceeding of the 23rd CANCAM Conference, June 5–9, 2011, Vancouver, BC, Canada, 2011. [26] J.S. Sage-Lauck, D.J. Sailor, Evaluation of phase change materials for improving thermal comfort in a super-insulated residential building, Energy Build. 79 (2014) 32–40. [27] TRNSYS and Solar Energy Laboratory, University of Wisconsin-Madison, TRNSYS User Manual Version 16.0 (accessed on Mar. 12, 2017). [28] F. Kuznik, J. Virgone, K. Johannes, Development and validation of a new TRNSYS type for the simulation of external building walls containing PCM, Energy Build. 42 (2010) 1004–1009. [29] M. Ibanez, A. Lazaro, B. Zalba, L.F. Cabeza, An approach to the simulation of PCMs in building applications using TRNSYS, Appl. Therm. Eng. 25 (2005) 1796–1807. [30] L.J. Claros-Marfil, A. Dentel, J.F. Padial, B. Lauret, Active and passive PCM walls simulation – a new TRNSYS PCM-Type, CON TECH 2014, June 11–13, Madrid, Spain, 2014. [31] BASF Energy Efficiency/Functional Polymers Group, Micronal Phase Change Material (PCM), (accessed 01.08.2016). [32] A.H. Mosaffa, C.A. Infante Ferreira, M.A. Rosen, F. Talati, Thermal performance optimization of free cooling systems using enhanced latent heat thermal storage unit, Appl. Therm. Eng. 59 (2013) 473–479. [33] A. Eddhahak-Ouni, S. Drissi, J. Colin, J. Neji, S. Care, Experimental and multiscale analysis of the thermal properties of Portland cement concretes embedded with microencapsulated Phase Change Materials (PCMs), Appl. Therm. Eng. 64 (2014) 32–39. [34] N. Zhu, Z. Ma, S. Wang, Dynamic characteristics and energy performance of buildings using phase change materials: a review, Energy Convers. Manage. 50 (2009) 3169–3181. [35] M.J.M. Oort, Studies of polymorphic substituted ammonium salts PhD Thesis, Department of Chemistry, Dalhousie University, Halifax, Canada, 1987. [36] M.J.M. Van Oort, M.A. White, Polymorphism in dialkylammonium chlorides. An adiabatic calorimetric study, Berichte der Bunsen-Gessellschaft fur physikalische Chemie 92 (1988) (1988) 168–176.
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Modelling for performance prediction of highly insulated buildings with different types of thermal mass O. Siddiqui a, R. Kumar a,⇑, A.S. Fung a, D. Zhang a, M.A. White b, C.A. Whitman b a b
Department of Mechanical and Industrial Engineering, Ryerson University, Toronto, ON M5B 3K3, Canada Department of Chemistry and Institute for Research in Materials, Dalhousie University, Halifax, NS B3H 4J3, Canada
h i g h l i g h t s TRNSYS modelling for performance prediction of highly insulated buildings. The impact of thermal mass on the energy saving and occupant comfort was studied. The thermal masses were found to contribute to energy savings of 10–15%. The thermal mass was adding considerably to the comfort of the occupants.
a r t i c l e
i n f o
Article history: Received 27 September 2016 Revised 15 April 2017 Accepted 5 May 2017 Available online 6 May 2017 Keywords: Phase change material Thermal mass Thermal comfort Net zero energy house TRNSYS simulation Energy savings
a b s t r a c t Thermal performances of two phase change materials (PCM) in the Toronto’s Net-Zero Energy House are compared and contrasted with commonly available forms of thermal mass. TRNSYS simulations show that the use of thermal mass was found to contribute to energy savings of 10–15% when different types of thermal mass were mixed into the building envelope. The results exhibit that the performance of a novel solid-solid phase PCMs, recently developed by researchers at Dalhousie University and known as DalHSM-1, could be comparable to a commercially available PCM from BASF (Micronal) in the heating mode. The cooling mode performance revealed that DalHSM-1 provided lower energy savings when compared to Micronal, due to a lower phase transition temperature and latent heat. The impact of thermal mass on the occupant comfort was also investigated by considering the total number of hours where the temperature exceeded the heating set point of 21 °C. The results showed that the total number of hours where the temperature exceeded 21 °C could be reduced significantly with different types of thermal mass, thus contributing considerably to the comfort of the occupants. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction The rising price of fossil fuel energy along with a major concern of the environment over the last few years has generated significant interest in the energy conservation, system/process efficiency, and renewable energy technologies. Efforts have been initiated on different scales, depending on the government programs and policy, to promote energy conservation and efficiency in all aspects of daily energy uses such as transportation and buildings. It is evident that even a small modification to existing buildings could dramatically reduce their energy consumptions [1,2]. The use of thermal mass in the building’s foundation, floors, and walls provides a better strategy for energy management in the building envelope and also provide an improved indoor environment for ⇑ Corresponding author. E-mail address: [email protected] (R. Kumar). http://dx.doi.org/10.1016/j.applthermaleng.2017.05.021 1359-4311/Ó 2017 Elsevier Ltd. All rights reserved.
the occupants [3]. In the absence of any heat storage solution in the building foundations/walls, most of the passive thermal energy from the sun would go towards increasing the indoor temperature inside the building, making it uncomfortable for the occupants and also increasing the air-conditioning load [4,5]. One of the solutions for heat storage that has shown an enormous potential is an inclusion of phase change material (PCM) in building materials [6–8]. A variety of building materials has been investigated for the purpose of incorporating PCMs. The most recognition has been given to gypsum and concrete/Portland cement since these are readily available and inexpensive to produce. Extensive research has been conducted to use of gypsum/PCM composite as a means for storage in the building envelope. Darkwa and Kim [9] have investigated the dynamics of energy storage in gypsum wallboards and found that the laminated PCM board provided improved thermal characteristics when compared to randomly distributed PCM. Kedl and Stovall [10] used paraffin wax incorporated into gypsum
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wallboards using the immersion process. Two methods were suggested in which the PCM wallboard could be manufactured. It could either be melted into a liquid and then absorbed into the porous gypsum wallboard, or it could be added during the plasterboard manufacturing process, where it is added to the wet plaster. Hawes et al. [11] results of PCM/gypsum composite found that wallboards can absorb up to 50% of their weight. However, optimal thermal performance was obtained at the mass percentage of between 25 and 30%. Flexural strength of PCM integrated drywall was found to be comparable to the conventional wallboard. The absorption of moisture which is undesirable was found to be only one-third that of conventional wallboard which makes it well suited for building applications. Considerable research has been done to examine the mixing of phase change materials into building fabrics, for instance, the use of PCM composites in buildings as a means for heat storage and residential cooling [12,13]. Stoval and Tomlinson [14] have analyzed the use of PCM wallboard as a load management device for passive solar applications and estimated the energy savings. Peippo et al. [15] have shown energy reduction potential of almost 4 GJ/year (or 15% of the annual energy cost) for a 120 m2 house in Madison, Wisconsin (43°N). They have also shown that a melting temperature that is 1–3 °C above the average room temperature provides the most optimal heat storage. Darkwa and O’Callaghan [16] carried out thermal simulations of the phase change drywalls in a passive solar building. They concluded that wallboard with a small phase change material would moderate the overnight temperature more efficiently than a passively-designed room. Neeper [17] investigated the use of gypsum wallboard with PCM for newly constructed buildings and found that it creates an opportunity for passive solar heating as well as ventilated cooling and time shifting of mechanical loads. In terms of cost effectiveness, it was established that the PCM wallboards were economically viable when compared to ordinary wallboards. Another notable research on PCM-gypsum composite wallboard was carried out at Fraunhofer Institute [18] and Concordia University [19]. In both these studies, it was concluded that the presence of PCMs stabilizes and reduces the indoor air room temperature by 3 °C. The contemporary research in the area of energy-efficient buildings has produced a variety of techno-economic solutions that have led to the development of alternative materials and technologies [20–22]. Significant possibilities exist in the building construction, one of the solutions involves the design of buildings incorporated with thermal masses and phase change materials. The use of PCMs in buildings holds significant potential to reduce the overall energy consumption, which can also contribute to improvement in the occupant comfort. The current research provides an insight and thorough understanding of the scope of different types of the thermal storage medium on the performance of Net-Zero Energy Townhouse in the climatic conditions of Toronto, Canada. Thermal performance of house was established with two commercially available solid-to-liquid PCM (Micronal) and solidto-solid PCM (DalHSM-1). The performances of the two PCMs were compared with commonly available forms of thermal mass (concrete slab). In addition, analysis of the energy savings and reduction in daily thermal fluctuations for different parts of buildings in diverse seasons will be made, and an interpretation of results has been provided in terms of occupant comfort.
tigation of the effect of thermal mass in a building must take into account the unique properties of the building envelope. Buildings that are highly insulated behave in a significantly different manner with respect to heat transfer and storage than buildings that are of light construction, manifesting as differences in indoor temperature fluctuations and comfort levels [23]. The Toronto Net-Zero Energy House represents an award-winning design initiative that represents the collaboration between the Sustainable Urbanism Initiative Toronto (SUI) and a host of architectural and engineering firms, with the objective of increasing public awareness and adoption of energy-efficient homes in Canada [24,25]. The main purpose of this research is to evaluate the impact of implementation of PCMs in the proposed Toronto’s Net-Zero Energy House. Considering the uniqueness of this project and integration of PCM with the building envelope, a model was developed in TRNSYS. 2.1. Description of SUI Net Zero Energy Townhouses The model of the townhouse has a total heated area of 210 m2 (heated volume 685 m3) and the orientation of the house is 37° west of south. The orientation and location of the houses have been optimized to ensure that a maximum amount of solar energy can be captured to operate the roof integrated photovoltaic and solar thermal panels for the generation of electricity and hot water respectively for the house. A ground source heat pump is also utilized during the winter to provide a reliable and efficient source of heating. Fig. 1 shows a computer-generated 3-D model of the house. The building envelope of the townhouse is designed with the intention of minimizing the heat transfer between interior and exterior, thereby, saving energy in maintaining a comfortable environment for occupants. However, one of the possible drawbacks with a highly insulated building, such as Toronto Net Zero Energy Townhouse, is the potential for overheating, especially in winter, as a result of solar gain [26]. This fact should be taken into consideration in the design of the building envelope to ensure that there is a enough thermal storage capacity (in terms of PCM or concrete slab or combination of both) within the building envelope to absorb any surplus solar gain. For the present simulation the external load was taken 4 kW h/day (1460 kW h/y). This can be estimated in terms of CFL Lighting, Garage door opener, Standby losses of the garage door opener, power tools. The Toronto Net-Zero Energy Townhouse is a light-weight construction and due to the large glazing area, is highly susceptible to over-heating during both the winter and summer seasons. The external walls have been insulated with sprayed polyisocyanurate foam insulation, which provides an overall insulation value of R-60
2. Model development The development of a precise model for a house or a building along with relevant parameters that describe its unique characteristics is essential for the evaluation of the impact of factors such as the use of thermal mass and phase change materials (PCMs). Inves-
Fig. 1. 3-D computer representation of the three townhouses of Toronto Net-Zero Energy unit [10].
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(RSI-10.6). The roof assembly consists of drywall on 19 mm 19 mm furring and 0.15 mm polyethylene vapour retarder attached to the bottom of the 294 mm pre-engineered Ijoists. Sprayed polyisocyanurate foam is applied between joists as roof insulation. The roof has an insulation value of R-76 (RSI13.4). The windows used in the house have low emissivity and are argon-filled with a fiberglass frame and have an overall insulation value of R-4 (RSI-0.7). Walls below grade are of the insulating concrete form and have a 6.3 cm of rigid polystyrene board with a waterproof membrane. The overall insulation value of the below grade wall is R-35 (RSI-6.27). 2.2. PCM implementation in the model of house TRNSYS is a complete simulation program for the transient simulation of energy systems, including multi-zone buildings [27]. It is used by engineers and researchers around the world to validate new energy concepts, from domestic water heating to the design and simulation of buildings and their equipment, including control strategies, occupant behaviour, alternative energy systems (wind, solar, photovoltaic, etc.) and thermal comfort. TRNSYS is typically setup by connecting components graphically in the Simulation Studio and each Type of component is described by a mathematical model in the TRNSYS simulation engine. TRNSYS provides a simulation model classified as Type 204 for the study of PCMs. Before the Type 204, the only methodology in which the effect of PCMs in buildings could be analyzed was through the development of an active layer within the building envelope. Kuznik et al. [28] developed a Type 260 and connected with TRNSYS multi-zone building Type 56. The inside surface temperature calculated by Type 56 is linked as an input in Type 260. The conductance of layers between the thermal zone and PCM is included into the Type 260 and was validated by experimental results [29]. Recently, a new PCM Type 399 was developed by Dentel and Stephen [30] for TRNSYS 17 and can be applied to the modelling of temperature-depended thermal characteristics. However, the accuracy of this new Type is yet to be validated by experimental data. To examine the impact of PCMs on the thermal performance and occupant comfort in the Toronto Net-Zero Energy Townhouse, Type 204 was used. The following input parameters were entered into the model to predict the PCM impact on the building load accurately: (1) Number of iterations – this parameter can give any value between one and infinity and is used primarily for the sake of accuracy; (2) Melting temperature – the initial temperature during which the PCM undergoes phase transition; (3) Crystallization temperature – the temperature where the PCM changes back to the solid phase; (3) Range of crystallization temperature – this parameter is used to define the phase change temperature range; (4) Latent heat of PCM; (5) PCM specific heat capacity; (6) PCM density; (7) Specific heat capacity of other materials incorporated with the PCM; and (8) Volume fraction of PCM in node – the overall concentration of PCM in a particular specimen can be entered through this parameter. For each scenario (materials and amount of thermal mass), the simulations were run for one year with a time step of one hour, which is the standard value for the evaluation of seasonal and annual building simulation. The simulations were run with the house unconditioned, which means that other than the solar gain and heat loss, there is no artificial heating or cooling of the house. To ensure that the there is no interference with the heating/cooling equipment of the thermal mass effect provided by the building envelope. The house consists of five zones that represent the Garage, 1st Floor, 2nd Floor, 3rd Floor and Mezzanine (Fig. 1). As a means of comparison, all of the results shown are on the 3rd Floor of the house, since this is the location where the temperature peaks and fluctuations are expected to be the
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greatest because of the large glazing area. Fig. 2 illustrates the complete TRNSYS model of the Toronto Net-Zero Energy Townhouse with the Type 204 model integrated within the building model. In the first part of the study, a commercial phase change material product, Micronal DS 5001, was utilized. As per ASHRAE Standard 55-2013, thermal environmental conditions for human occupancy, the temperature could range 19.4–27.8 °C (67–82 °F). These numbers also fluctuate, depending on the level of relative humidity, season, clothing, activity levels, etc. Micronal 5001, used in this study, has a melting temperature between 23 and 26 °C and a latent heat of 110 kJ/kg [31–33]. The use of Micronal has allowed modern buildings of light-weight construction a similar level of thermal comfort to buildings constructed from heavier materials such as concrete and brick. The thermal storage capacity of a half-inch drywall with 30% Micronal by weight has been found to be similar to a six-inch brick wall. When utilized as part of an interior wall in a building, the PCM begins absorbing additional heat from the room, as the indoor temperature of the room rises beyond the melting point. The large latent heat of the material would therefore keep the room temperature at a more uniform level. As the temperature drops during the night, the excess heat stored within the material is released to the room. The use of adequate ventilation during the night ensures that this heat can be released to the outdoor environment. This introduces a time delay or lag in the peak indoor temperature, which has a beneficial economic impact, since typically during summer residential airconditioning equipment consumes the most energy during the time of day when the outdoor temperatures are at their peak. The reduction of the indoor thermal fluctuations also contributes greatly to the thermal comfort of the occupant. In the simulation, the Micronal product was integrated with gypsum wallboard, having a thickness of 13 mm (0.5 in.), to ensure a uniform distribution of PCM throughout the building material. The energy storage capacity of PCM/concrete block was found to be more than 200–230% when compared to pure concrete blocks [11]. So, concrete is utilized on the outside of buildings and can directly capture any resulting solar gain. A four inch concrete slab with PCM was placed at each of the floors for this purpose. Table 1 presents the material properties of Micronal DS 5001.
3. Results and discussion The TRNSYS simulations are performed for the 3rd floor room of designed net zero townhouses for a typical day of winter (February 1) and summer (July 15). It is apparent from Fig. 3 that as the concentration of PCM within the building envelope is increased; the indoor temperature profile is slightly elevated in winter. Since the building is unconditioned, the indoor temperatures are below the PCM transition temperature. Thus any improvements in the indoor temperature profile are attributable to the increased sensible storage capacity of the PCM/drywall in comparison with the ordinary building envelope containing no PCM. This would translate into an overall reduction in the heating load requirement of the building during the winter period. On the other hand, Fig. 4 illustrates the impact of PCM during a typical summer day. The key difference in Figs. 3 and 4 is the relative magnitude of the room’s temperature change, which is significantly higher during summer. This can be interpreted in terms of melting point of PCM (26 °C). During the winter months, this temperature would rarely be achieved, and the only heat transfer mechanism for the PCM would be sensible heat transfer, which produces only a small change in temperature magnitude. However, during summer, complete melting of the PCM occurs as most of the time the indoor temperatures are above the melting point of the
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Fig. 2. TRNSYS model of the Toronto Net-Zero Energy Townhouse.
divergence exists between the day and night temperatures, the use of a concrete slab as thermal mass would provide results similar to what could be achieved using the PCM. The use of a 2-in. concrete slab as thermal mass produces a sizable reduction in the peak indoor temperature during the summer. This can be contrasted with the results for the 10% and 20% of PCM. Depending on the particular design requirements, the option of selecting a particular thermal mass depends on the occupant comfort requirements and economic considerations. Although the thermal performance of the 5% PCM was similar to a 2-in. concrete slab (Fig. 5), however, when the economic factors (saving in the energy consumptions) are taken into account (Fig. 6), the advantage of a concrete slab over the PCM is significant. On the other hand, if the heating and cooling loads of the building are significantly higher, then it might be economically practical to implement PCMs with higher loading (either heating or cooling) into the building, rather than increasing the thickness of the concrete slabs. Fig. 6 illustrates the overall impact of using PCM in terms of heating and cooling loads reduction. The yearly heating and cooling loads of the Toronto Net-Zero Townhouse are presented. It
Table 1 Material properties of Micronal DS 5001 [31]. PCM model number Melting point Specific heat capacity (including sensible energy) Latent heat Density
DS 5001 23–26 °C 145 kJ/kg 110 kJ/kg 250 to 350 kg/m3
PCM. It is evident from Fig. 4 that not only there is a reduction in the peak temperature, but also a time delay of approximately seven hours between the outside peak temperature (No PCM) and inside peak temperature. Under no PCM case, the indoor temperature registers an earlier peak, closer to the time of the peak of outside temperature. It is also obvious from Fig. 4 that the appropriate selection of the type and amount of thermal mass depends strongly upon the climate of a particular location. Depending on the diurnal temperature range of a location, the type and amount of thermal mass to be used can be readily determined. For places where a significant
Temperature (ºC)
15 14 13 12 No PCM
11
10% PCM 20% PCM
10 1
3
5
7
9
11
13
15
17
19
21
Time (Hours) Fig. 3. Temperature profiles on a typical winter day (February 1) in Toronto.
23
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35
Temperature(ºC)
33 31 29 27 25 No PCM 2 Inch Concrete Slab
23
10% PCM 20% PCM
21 1
4
7
10
13
16
19
22
Time (Hours) Fig. 4. Temperature profiles on a typical summer day (July 15) in Toronto.
2 inch concrete slab
35
5% PCM
Temperature (ºC)
33 31 29 27 25 23 21 1
5
9
13
17
21
Time (Hours) Fig. 5. Temperature profiles on a typical summer day (July 15) in Toronto when 5% PCM is contrasted with the 2-in. concrete slab.
Heating Load (kWh) Cooling Load (kWh) Total Load (kWh)
10000 9000
Load (kWh/yr)
8000 7000 6000 5000 4000 3000 2000 1000 0 No PCM
2 Inch Concrete Slab
10% PCM
20 % PCM
Fig. 6. Yearly heating/cooling load requirements of the Toronto Net-Zero Energy Townhouse for different heat-storage constructions and combinations.
shows that compared to the base case where no PCM is used, total load reductions of almost 15% are achievable when 20% PCM is used. Increasing the amount of the PCM from 10% to 20% produces a small reduction (5%) in the load. As before, the impact of using a 2-in. concrete slab as thermal mass is contrasted with PCM and it is determined that the overall impact is equivalent to what would be achieved by using PCM with a concentration of 5%, as was shown in the temperature profile presented in Fig. 5. It should be stated that,
since design of the Toronto Net-Zero Energy Townhouse is dictated by the overall goal of the minimizing the energy consumption through the use of high resistance insulation, the heating and cooling loads are already significantly lower than an average home in Canada. The determination of the optimum PCM concentration in terms of the total energy consumption within the building envelope is important not only because of the economic considerations but
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also for the structural integrity of the building envelope. Simulations were performed to ascertain the maximum proportion of PCM that could be integrated into the building envelope so that a sizeable reduction in the total energy consumption is achieved. Fig. 7 presents the results of the analysis. While it was found that a definite relationship exists between the concentration of PCM used and the total load reduction, but, for 40% or more PCM amounts, the impact in energy saving is relatively very small.
Table 2 Properties of DalHSM-1, as used in current simulation.
a
3.1. Performance comparison of different storage materials All the above simulations were carried out for Micronal PCM and 2-in. concrete slab. The model is also applied further to the simulations of a variety of materials as thermal mass including PCMs to analyze the relative performance effectiveness in terms of energy saving in Toronto Net Zero Townhouse. The additional simulations were carried out with a heat storage material, DalHSM-1, identified by the teammates of this research at Dalhousie University [34–36]. The key objective of this segment of research was to identify materials that possess most desired characteristics that would cause a reduction in the building heating and cooling loads and would create an improvement in the thermal comfort to the building occupants. 3.1.1. DalHSM-1 material properties DalHSM-1 belongs to the solid-solid class of phase change materials [34–36]. The DalHSM-1 material, which has a chemical composition (C6H13)2NH2Br, has been synthesized in the form of coarse particles, which can be grounded into a very fine powder, to assist with the integration of the PCM into building materials directly without expensive encapsulation process. DalHSM-1 possesses excellent phase change characteristics in the temperature range of interest for building applications. Because it stays as a solid, it can be integrated more easily than PCMs that undergo solid-liquid phase change. Some of the fundamental material properties of the PCM used for the simulation are given in Table 2. DalHSM-1 has a significant advantage for building materials over many potential solid-solid heat storage materials as its transition temperature is in the range of room temperature. 3.1.2. Comparison results and analysis The impact of using different thermal masses (different compositions and amounts) on the annual heating and cooling loads was analyzed and is presented in Fig. 8. The performance of 20% Micronal PCM is comparable to that of a 4-in. concrete slab. The performance of DalHSM-1 is found to be better than the base case (no PCM) but inferior to Micronal. As can be expected, due to the
Property
Value
Density (q) Thermal conductivity (k) Specific heat (Cp) Onset phase change temperature upon heating/cooling (To)a End phase change temperature upon heating/cooling (Te)a Latent heat of transition on heating (L)
800 kg/m3 0.3 W/m/K 1.6 kJ/kg/K 20/17 °C 21/15 °C 80 kJ/kg
Data for heating or cooling rates typical for a building application.
slightly lower latent heat, 30% DalHSM-1 performs comparably with 20% Micronal for the heating season. However, DalHSM-1 does not have much effect on cooling load. The total cooling load is 10% higher when 30% DalHSM-1 is used as compared to 20% Micronal PCM. This is understandable due to its phase transition on cooling being below the heating set point. Since the cooling set point is higher than the phase transition temperature of DalHSM-1, any thermal energy stored by the PCM does not have opportunity to be released. The peak heating and cooling loads for each combination are shown in Fig. 9. A similar trend has been seen for the peak heating and cooling loads as observed for the annual loads (Fig. 8), where the performance of DalHSM-1 is better than the base case but inferior to Micronal (as modeled) at the same loading, especially during the summer when cooling is required. A detailed analysis of the occupant thermal comfort was also conducted for each of the above case by analyzing the total number of hours the indoor temperature would exceed the heating set point of 21 °C. The analysis was performed to determine the total number of hours where the temperature exceeds 25 °C, which is considered to be an uncomfortable temperature for indoor occupants, and cooling is required. These results are shown in Tables 3 and 4, respectively. As evident from Tables 3 and 4, the use of thermal mass significantly reduces the total number of hours where the temperature exceeds the heating set point. During the winter months, the combination of an active heating system and admittance of solar radiation during the day creates a condition of over-heating which is further exacerbated by the highly insulated building envelope, which traps the thermal energy inside. This phenomenon typically occurs during the daytime on a sunny winter day, when the peak level of radiation is incident on the building. This causes the temperature of the building to rise. The use of thermal mass in such a situation would enable this unnecessary solar gain during the daytime to be stored and released during the evening when the temperature drops. Analyzing Tables 3 and 4, it is clear that the use
10000
Annual Load (kWh)
9000 8000 7000 6000 5000 4000
0%
10%
20%
30%
40% 50% 60% 70% PCM concentration (%)
80%
90% 100%
Fig. 7. Total annual heating/cooling load with varied concentration of PCM is varied.
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Total load (kWh) 6000
Heating
Cooling
5000 4000 3000 2000 1000 0 Base Case 2-Inch Slab4-Inch Slab
20% 5% 10% 20% 30% Micronal DalHSM-1 DalHSM-1 DalHSM-1 DalHSM-1
Fig. 8. Annual heating and cooling loads for the Toronto Net-Zero Energy Townhouse using different types and amounts of thermal mass.
7
Total load (kWh)
Heating
6
Cooling
5 4 3 2 1 0 Base Case 2-Inch Slab 4-Inch Slab
20% 5% 10% 20% 30% Micronal DalHSM-1 DalHSM-1 DalHSM-1 DalHSM-1
Fig. 9. Peak heating and cooling loads for the Toronto SUI Net-Zero Energy house using different types and amounts of thermal mass.
Table 3 Total number of hours during the winter season that the indoor temperature for the Toronto Net-Zero Energy Townhouse would exceed the heating set point of 21 °C. Scenario
1st Floor
2nd Floor
3rd Floor
Mezzanine
Base case 2-in. concrete slab 4-in. concrete slab 20% Micronal PCM 5% DalHSM-1 10% DalHSM-1 20% DalHSM-1 30% DalHSM-1
477 394 368 383 464 441 407 387
653 577 523 527 640 598 564 542
1603 1186 1039 998 1564 1335 1209 1066
358 299 255 237 344 321 289 271
Table 4 Total number of hours during the winter season that the indoor temperature for the Toronto Net-Zero Energy Townhouse would exceed 25 °C. Scenario
1st Floor
2nd Floor
3rd Floor
Mezzanine
Base case 2-in. concrete slab 4-in. concrete slab 20% Micronal PCM 5% DalHSM-1 10% DalHSM-1 20% DalHSM-1 30% DalHSM-1
290 234 207 199 277 254 221 209
454 377 323 311 445 413 374 355
602 386 273 256 577 443 379 334
161 109 92 92 158 127 111 101
of thermal mass is most effective on the 3rd floor, where the glazing area is the highest compared to other areas of the house. Temperature profiles were also developed for typical summer and winter weeks, to analyze the variation of temperature with time. The typical winter and summer weeks are defined as occur-
ring between February 1 to February 7 and July 15 to July 21, respectively. Fig. 10 shows the temperature profile of the 3rd floor of the Toronto Net-Zero Energy Townhouse for a winter week. As shown in Fig. 10, the indoor temperature rises dramatically in the absence of thermal mass. During the winter months, the
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34 No Thermal Mass 4 Inch Concrete Slab 30% DalHSM-1 20% Micronal PCM
Temperature (ºC)
32 30 28 26 24 22 20 1
13
25
37
49
61
73
85
97
109
121
133
145
157
Time ( Hours) Fig. 10. Temperature profile of the 3rd floor of the Toronto Net-Zero Energy Townhouse for a typical winter week (where hour 1 is 1 am and so on).
27
Temperature (ºC)
26
No Thermal Mass
4-Inch Concrete Slab
30% Dal HSM-1
20% Micronal PCM
25 24 23 22 21 20 1
13
25
37
49
61
73
85
97
109
121
133
145
157
Time (hours) Fig. 11. Temperature profile of the 3rd floor of the Toronto Net-Zero Energy Townhouse for a typical summer week (where hour 1 is 1 am and so on).
temperature of the air is a combination of input heat from an active heating system and a passive solar gain due to the low declination angle of the sun. Therefore, a fluctuation has been seen in the room air temperature for different days of the winter simulation. The rate of temperature rise is slowed by the introduction of different types of thermal masses. In this case, the peak temperature without the use of thermal mass is almost 8 °C higher than when thermal mass is used, but the addition of the thermal masses keep the maximum temperature within the occupant comfort zone. With PCM and different types of thermal mass, the floor temperature is more flatten as the energy absorbed by the PCM/thermal mass during the sunshine is released to the room during off-sunshine hours. In a highly insulated house of a lightweight construction such as the one modelled in this research, one of the key factors impacting the comfort of the occupants is the fluctuations in the indoor temperature caused by interplay of a variety of factors such as the incident solar radiation, HVAC system, and heating/cooling set points, etc. During the winter months, overheating of the indoor environment is a major concern, especially on a bright, sunny day, where the house is exposed to direct incident radiation. These solar gains, when combined with the regular winter heating capability, dramatically increase the degree of discomfort for the occupant. A
strategically placed unit of thermal mass, such as a PCM or concrete slab, within the building envelope, can not only minimize the indoor temperature fluctuations but also contribute to a reduction in the cooling load of the building. Fig. 11 shows the temperature profile of the 3rd floor of the house for a typical summer day. In this case, the degree of temperature reduction is not as great as for the winter case. This can be explained by the complex interaction of air-conditioning equipment that dictates the degree of temperature rise based on the set temperature. In the summer simulation conditions, once the set point of 25.5 °C is reached, the cooling equipment is activated. If the temperature were allowed to rise beyond the cooling set point, a similar trend as the winter case would be observed and would create extremely uncomfortable conditions for the indoor occupants. On the other hand, Fig. 11 shows that all three thermal masses significantly reduce the diurnal temperature range and the need for air conditioning.
4. Conclusions The buildings constitute a large portion of overall energy consumption (30%) in Canada, the use of PCMs in buildings holds sig-
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nificant possibilities to make them more energy efficient. The objective of this research project was the development of a fundamental understanding of the technical aspects of incorporating PCMs into building envelope. It was seen that there was a 15% reduction in the overall energy consumption as a result of using 20% Micronal PCM. The performance of other types of thermal mass was also determined to contribute to energy savings, and thus economic savings. The performance of 20% Micronal PCM is comparable to that of a 4-in. concrete slab. The novel PCM, DalHSM-1, was found to provide a significant thermal mass, but showed performance inferior to Micronal PCM, especially when the cooling loads are taken into consideration. Due to the slightly lower latent heat, 30% DalHSM-1 performs comparably with 20% Micronal for the heating season. However, DalHSM-1 does not have as much effect on cooling as Micronal PCM. It was also noted that the presence of PCM/thermal mass reduces the total number of hours when the temperature exceeded 21 °C, thus contributing considerably in the peak air-conditions loads and in the comfort of the occupants. Acknowledgement This work was supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada through the Solar Buildings Research Network (SBRN). Funding from Ontario Graduate Scholarship (OGS) for Mr. Omar Siddiqui and Mr. Dahai Zhang is gratefully acknowledged. References [1] J. Cuddihy, C. Kennedy, P. Byer, Energy use in Canada: environmental impacts and opportunities in relationship to infrastructure systems, Can. J. Civ. Eng. 32 (2005) 1–15. [2] M. Dabaieh, O. Wanas, M.A. Hegazy, E. Johansson, Reducing cooling demands in a hot dry climate: a simulation study for non-insulated passive cool roof thermal performance in residential buildings, Energy Build. 89 (2015) 142–152. [3] N. Ekrami, R.S. Kamel, A.S. Fung, Effectiveness of a ventilated concrete slab on an air source heat pump performance in cold climate, Presented in eSim Conference, Ottawa, May 8–9, 2014. [4] R. Mora, G. Bitsuamlak, M. Horvat, Integrated life-cycle design of building enclosures, Build. Environ. 46 (2011) 1469–1479. [5] R. Pacheco, J. Ordóñez, G. Martínez, Energy efficient design of building: a review, Renew. Sustain. Energy Rev. 16 (2012) 3559–3573. [6] D. Zhang, A.S. Fung, O. Siddiqui, Numerical studies of integrated concrete with a solid-solid phase change material, in: Proceedings of the 2nd Annual Solar Building Network conference, Calgary, Alberta, June 07, 2007. [7] K. Darkwa, P.W. O’Callaghan, D. Tetlow, Phase-change drywalls in a passivesolar building, Appl. Energy 83 (2006) 425–435. [8] H.E. Feustel, C. Stetiu, Thermal performance of phase change wallboard for residential cooling application, Report LBL-38320, Lawrence Berkeley National Laboratory, US, 1997. [9] K. Darkwa, J.S. Kim, Dynamics of energy storage in phase change drywall systems, Energy Res. 29 (2005) 335–343. [10] R.J. Kedl, T.K. Stovall, Activities in support of the wax-impregnated wallboard concept, US Department of energy thermal energy storage researches activity review, New Orleans, LA, USA, 1989. [11] D.W. Hawes, D. Banu, D. Feldman, Latent heat storage in concrete, Sol. Energy Mater. 19 (1989) 335–348. [12] R.A. Taylor, N. Tsafnat, A. Washer, Experimental characterization of subcooling in hydrated salt phase change materials, Appl. Therm. Eng. 93 (2016) 935–938.
147
[13] H. Tse, O. Siddiqui, A.S. Fung, F.M. Rad, Simulation and analysis of a net-zero energy townhome in Toronto, in: 3rd Canadian Solar Buildings Conference, Fredericton, NB, August 20–22, 2008. [14] T. Stoval, J. Tomlinson, Potential benefits of including latent storage in common wallboard, in: Proceedings of the 26th Intersociety Energy Conversion Engineering Conference, Boston, Aug. 4–9, 1991. [15] K. Peippo, P. Kauranen, P.D. Lund, A Multicomponent PCM wall optimized for passive solar heating, Energy Build. 17 (1991) 259–270. [16] K. Darkwa, P.W. O’Callaghan, Simulation of phase change drywalls in a passive solar building, Appl. Therm. Eng. 26 (2006) (2006) 853–858. [17] D. Neeper, Benefits of Wallboard Impregnated with Phase-Change Material for Residential Heating and Cooling, Solar Energy Research Institute, 1990. [18] P. Schossig, H.M. Henning, S. Gschwander, F. Haussmann, Microencapsulated phase change materials integrated into construction materials, Sol. Energy Mater. Sol. Cells 89 (2005) 297–306. [19] A.K. Athienitis, C. Liu, D. Hawes, D. Banu, D. Feldman, Investigation of the thermal performance of a passive solar test-room with wall latent heat storage, Build. Environ. 32 (1997) 405–410. [20] M.M. Shapiro, D. Feldman, D. Hawes, D. Banu, PCM thermal storage in wallboard, in: Proceedings of the 12th Passive Solar Conference, Portland, Orlando, 1987, pp. 48–58. [21] J. Jokisalo, P. Lamberg, K. Sirén, Suitability of building construction materials in short-term energy storage, Part 2: thermal simulation of phase change materials in concrete products, Helsinki University of Technology, HVACLaboratory, Report B64, 1999. [22] M. Ahmad, A. Bontemps, H. Sallée, D. Quenard, Thermal testing and numerical simulation of a prototype cell using light wallboards coupling vacuum isolation panels and phase change material, Energy Build. 38 (2006) 673–681. [23] D. Roeleveld, R. Kumar, D. Naylor, A.S. Fung, A CFD study of natural convection inside a BIPV/T system, in: Proceedings of the eSim Conference 2016, Hamilton, Ontario, Canada, May 03–06, 2016. [24] O. Siddiqui, A. Fung, H. Tse, D. Zhang, Modelling of the Net Zero town house in Toronto using TRNSYS, and an analysis of the impact using thermal mass, in: Proceedings of the 2nd International Conference on Energy Sustainability, August 10–14, 2008, Jacksonville, Florida, USA, 2008. [25] E. Poulad, A. Fung, D. Naylor, Effect of PCM thermal conductivity on energy demand and temperature on the net-zero energy building in Toronto, in: Proceeding of the 23rd CANCAM Conference, June 5–9, 2011, Vancouver, BC, Canada, 2011. [26] J.S. Sage-Lauck, D.J. Sailor, Evaluation of phase change materials for improving thermal comfort in a super-insulated residential building, Energy Build. 79 (2014) 32–40. [27] TRNSYS and Solar Energy Laboratory, University of Wisconsin-Madison, TRNSYS User Manual Version 16.0 (accessed on Mar. 12, 2017). [28] F. Kuznik, J. Virgone, K. Johannes, Development and validation of a new TRNSYS type for the simulation of external building walls containing PCM, Energy Build. 42 (2010) 1004–1009. [29] M. Ibanez, A. Lazaro, B. Zalba, L.F. Cabeza, An approach to the simulation of PCMs in building applications using TRNSYS, Appl. Therm. Eng. 25 (2005) 1796–1807. [30] L.J. Claros-Marfil, A. Dentel, J.F. Padial, B. Lauret, Active and passive PCM walls simulation – a new TRNSYS PCM-Type, CON TECH 2014, June 11–13, Madrid, Spain, 2014. [31] BASF Energy Efficiency/Functional Polymers Group, Micronal Phase Change Material (PCM),