Performance assessment of residential building envelopes enhanced with phase change materials

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Performance Assessment of Residential Building Envelopes Enhanced with Phase Change Materials Viven Sharma , Aakash C. Rai PII: DOI: Reference:

S0378-7788(19)31682-2 https://doi.org/10.1016/j.enbuild.2019.109664 ENB 109664

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Energy & Buildings

Received date: Revised date: Accepted date:

31 May 2019 29 November 2019 1 December 2019

Please cite this article as: Viven Sharma , Aakash C. Rai , Performance Assessment of Residential Building Envelopes Enhanced with Phase Change Materials, Energy & Buildings (2019), doi: https://doi.org/10.1016/j.enbuild.2019.109664

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Highlights

  

PCM/insulation-enhanced envelopes greatly reduced the building’s summer heat gain. Summer heat gain shifted from peak to off-peak hours by using PCM or insulation. PCM’s thickness, melting point and latent heat were important design parameters.

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Performance Assessment of Residential Building Envelopes Enhanced with Phase Change Materials Viven Sharma1 and Aakash C. Rai1* 1

Department of Mechanical Engineering, Birla Institute of Technology and Science Pilani, Rajasthan, India - 333031.

*Corresponding author: Phone: +91-7259532487, Email: [email protected] Abstract Residential buildings in India account for ~22% of the national electricity consumption, of which one-third is used for space cooling; however, they are rarely constructed with energy-efficiency considerations. This presents an opportunity for reducing the energy consumption and associated greenhouse gas (GHG) emissions by design and construction of energy-efficient houses. Therefore, this investigation assessed the potential of PCM-enhanced building envelopes for reducing the cooling energy requirements of residential buildings in Delhi (capital of India). Through numerical simulations, we studied the impact of key PCM design parameters such as its thickness, position, melting point temperature and latent heat capacity on the proposed energy benefits, and compared them with those obtained with insulation-enhanced envelopes. We found that, applying a PCM layer on the roof reduced the summer heat gain by 12.6–36.2%, whereas an insulation layer of the same thickness reduced heat gains by 41.0–71.4% over the baseline construction. PCM-enhanced walls were also found to reduce the heat gain by 10.4–26.6%, while insulated walls led to a heat gain reduction of 32.4–64.0%. By extrapolating these results to city-scale, it appears that PCM/insulation-enhanced envelopes could reduce Delhi’s annual electricity consumption and GHG emissions by 0.3–1.5% and 0.2–1.0%, respectively. Keywords: Phase change material, insulation, energy saving, building envelope, greenhouse gas emissions 1. Introduction Globally residential buildings account for around 24% of the energy consumption, and 12% of the greenhouse gas (GHG) emissions (Edenhofer et al., 2014). Of this energy consumption, space heating and cooling is responsible for 32% and 2%, respectively (Edenhofer et al., 2014). Similar trends can also be seen in India, where residential buildings account for about 22% of the total electricity consumption (CEA, 2013), and 7% of the total GHG emissions (MoEF, 2010). However, in India space cooling requirements dominate over heating requirements (MoEF, 2018), and cooling accounts for about one third of the electricity used in

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residences (Chunekar et al., 2016), which translates to about 7% of the national electricity consumption. Due to the large energy consumption and GHG emissions associated with air-conditioning (heating and cooling) of residential buildings, there has been a global thrust towards design and construction of energy-efficient houses (Harvey, 2013). Recently researchers have started assessing the potential of building envelopes enhanced with phase change materials (PCM) for reducing the building energy consumption (de Gracia and Cabeza, 2015; Soares et al., 2013). PCMs can store large amount of latent heat by undergoing phase change (typically solid to liquid), which essentially adds thermal mass to the building envelope; and thus, reduces the air-conditioning (AC) requirements, and decreases and delays the peak AC load (Kalnæs and Jelle, 2015; Kuznik et al., 2011). However, for their efficient use, it is important to design the PCM layer appropriately, including its position in the building envelope (Jin et al., 2017; Lee et al., 2015; Zwanzig et al., 2013), melting point temperature (Izquierdo-Barrientos et al., 2012; Sun et al., 2014), and thickness (Kuznik et al., 2008). Thus, several experimental and numerical investigations have been conducted to assess and optimize the performance of PCM-enhanced building envelopes. The experimental studies concerning PCMs have used test chambers (Ahmad et al., 2006; Bontemps et al., 2011; Evers et al., 2010), scale model rooms/buildings (Lee et al., 2018; Lee and Medina, 2016; Medina et al., 2008), and full-scale rooms/buildings (Nghana and Tariku, 2016; Voelker et al., 2008). For example, Medina and co-workers (Evers et al., 2010; Jin et al., 2014; Sun et al., 2018) compared the performance of PCMenhanced walls with standard walls through a test chamber that could reproduce indoor and outdoor summer conditions for residential buildings. It was reported that, compared to the baseline wall, the peak heat flux reduced by as much as 9.2–36.5% for the PCM-enhanced wall, depending on the PCM layer configuration. Sun et al. (2019) constructed two identical scale models of typical houses in Kansas, USA, and compared the energy performance of PCM-enhanced walls with respect to the baseline construction. They found that incorporating PCMs in walls led to a reduction in heat gain during summer, and maximum energy savings could be achieved by installing PCMs within the west-facing walls. Kuznik and Virgone (2009) compared the thermal performance of a PCM-enhanced wallboard to that of a regular wallboard by conducting controlled experiments in a fullscale test room. It was found that depending on the season, PCM-enhanced wallboard led to comfort improvements in the room in summers by lowering the maximum air temperatures by 2.3–4.2°C.

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Since the experimental investigation are typically expensive and time consuming, several researchers have used numerical simulations to study the energy performance of PCM-enhanced building envelopes. For instance, Lee and Medina (2016) used EnergyPro software to study the performance of PCM-enhanced wallboards for use in residential buildings located in coastal and transitional climates in California, USA. It was found that PCM-enhanced wallboards not only reduced the peak wall heat flux, but also decreased the space cooling requirement and annual energy consumption by 10.3–10.4% and 6.7–7.6%, respectively, depending on the climate zone. Thiele et al. (2015) used numerical simulations to examine the benefits of adding microencapsulated PCMs to concrete walls subjected to diurnal sinusoidal outdoor temperature and solarradiation. They also found that the PCM-enhanced walls could reduce and delay the building’s AC requirements. Another numerical study by Izquierdo-Barrientos et al. (2012) assessed the performance of PCMenhanced building walls with different wall orientations, PCM layer locations, and PCM melting point temperatures (TPCM) for summer and winter seasons in Madrid, Spain. They found that, when T PCM was properly selected, the PCM layer helped to diminish the peak wall heat flux; however, the space heating or cooling requirements was not always reduced. A similar conclusion was also obtained by Zwanzig (2013), who studied the energy saving potential of PCM-enhanced residential buildings located in different climate zones of USA. Baniassadi et al. (2016) performed economic optimization of PCM layer thickness incorporated in roofs of residential buildings located in different climate zones of Iran. Their results showed that insulation boards are far more cost-effective than PCM based solutions in reducing the building energy consumption for Iran’s economic and climatic conditions. From the above discussion, it seems that PCM-enhanced building envelopes are generally effective in reducing and delaying the peak AC requirement; though, their efficacy in reducing the overall energy consumption depends on the climatic conditions. However, despite the tremendous potential that PCMenhanced building envelopes possess for improving the energy performance of residential buildings, only a handful of studies have explored their suitability for Indian homes. Pasupathy and co-workers (Pasupathy et al., 2008; Pasupathy and Velraj, 2008) constructed two identical test rooms for studying the thermal performance of PCM-enhanced roofs in Chennai, India. They recommend that double-layer PCM (two layers of PCMs with different physical properties) incorporated in the roof could reduce the indoor air temperature swings in all seasons. Another recent study by Singh and Bhat (2018) used experimental and numerical techniques to compare the thermal performance of a double-layer PCM gypsum board to that of a conventional board in the

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composite climate zone of India. It was found that PCM gypsum board attached to the roof reduced the room’s summer cooling load as well as the indoor temperature swings in both summer and winter seasons. Note that building energy-efficiency measures, including PCM-based enhancements, should be properly designed and analyzed for the local building construction practices and climatic conditions (Rodriguez-Ubinas et al., 2012; Zwanzig et al., 2013). However, as evident from the above discussion, only a few studies have been conducted to evaluate the potential of PCM-enhanced envelopes for reducing the AC energy consumption in Indian residences. Furthermore, the existing Indian studies have not considered how important PCM design parameters such as its TPCM, latent heat capacity, PCM layer location and thickness affect the anticipated energy benefits. Thus, this investigation was aimed to provide a comprehensive assessment of the energy performance of PCM-enhanced envelopes in summers for typical residential buildings in India, including the effect of key PCM design parameters as well as their comparisons with insulation-enhanced envelopes. 2. System Description To assess the potential of PCM-enhanced envelopes for reducing the AC energy consumption, this investigation studied typical residential buildings in Delhi, the national capital territory of India, with and without integration of a PCM layer. Delhi was chosen for this investigation since it is home to more than 25 million people (almost equal to that of Texas, USA), and has a composite climate (BEE, 2017) for which PCMs are deemed most suitable (Rodriguez-Ubinas et al., 2013, 2012). In Delhi, residential buildings typically have masonry walls that are made of clay-fired bricks, while the roof is generally made of reinforced cement concrete (RCC). Usually cement plaster is applied on the interior as well as exterior sides of the roof and walls for finishing. The typical roof and wall constructions (Case 0: baseline) are shown in Figure 1, and thermo-physical properties of the construction material are given in Table 1.

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Figure 1: Schematics of roof and wall constructions.

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Table 1: Thermo-physical properties of the construction material Thermal Specific Thickness Density conductivity heat Material (mm) (kg/m3) (W/m-°C) (J/kg-°C) a) Roof construction 15 1300 0.5 1000 Cement plaster 120 2100 1.4 840 RCC Insulation (EPS) 10–30 24 0.035 1340 PCM (from 10–30 750 0.15 1500 Rubitherm) b) Wall construction 15 1300 0.5 1000 Cement plaster 200 1700 0.84 800 Clay-fired bricks Insulation (EPS) 10–30 24 0.035 1340 PCM (from 10–30 750 0.15 1500 Rubitherm)

Latent heat (kJ/kg) 60 or 240 60 or 240

We compared the energy performance of PCM-enhanced building envelopes (walls and roof) with that of a baseline envelope as well as envelopes enhanced with insulations, which is a cheap alternative as compared to PCMs. Following are the different wall and roof configurations that were studied: 

Case 0 (Baseline): This is the typical three-layer roof (outside plaster-RCC-inside plaster) and wall (outside plaster-brick-inside plaster) configuration in the test region, as shown in Figures 1 a) and b), respectively.



Case 1 (Insulation-Out) and Case 2 (Insulation-In): Case 0 is modified by placing expanded polystyrene (EPS) insulation beside the outer plaster layer, and referred to as Case 1; whereas in Case 2, insulation is placed next to the inner plaster.



Case 3 (PCM-Out) and Case 4 (PCM-In): Case 0 is modified by placing a PCM layer on the outer side and referred to as Case 3; whereas in Case 4 (PCM-In), the PCM layer is placed on the inner side.



Cases 3a (No-PCM-Out) and 4a (No-PCM-In): They are identical to Cases 3 and 4, respectively, except that the latent heat storage effect has not be considered. Thus, Cases 3 and 4 can be compared with their “NoPCM” counterparts to identify how the latent heat storage by the PCM affects its performance. The thermo-physical properties of the PCM (Rubitherm, 2015) and insulation (Kumar and Suman, 2013)

used in this investigation are also given in Table 1. It has been assumed that the PCM in its solid or liquid phase has the same properties (Izquierdo-Barrientos et al., 2012). The PCM studied (GR from Rubitherm) is a boundPCM with an organic phase change material (RT from Rubitherm) bound to an inorganic carrier matrix, which helps to alleviate the problems related to handling of liquids and volume changes during phase transformation. This PCM is available over a wide range of TPCM. The PCM was assumed to be attached directly to the walls and roof surfaces, and the effect of any container holding the PCM is ignored (Huang et al., 2006).

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To study the effect of insulation and PCM layer characteristics on the envelope’s energy performance, we varied their thicknesses from 10–30 mm in steps of 10 mm. Additionally, TPCM was also varied (from about 24– 50 °C) such that the PCM undergoes melting and solidification. All these variations in PCM layer parameters were studied with the bound-PCM described above, which had a latent heat of 60 kJ/kg. Additionally, to quantify the effect of latent heat variation on the PCM layer’s performance, we also studied the performance of the pure PCM (RT from Rubitherm) without any carrier matrix with a latent heat of 240 kJ/kg and a thickness of 20 mm with TPCM ranging from about 24–50 °C. The other thermo-physical properties of the pure PCM were assumed to be identical to the bound-PCM. Transient hourly energy simulations through all the constructions for a typical summer season were performed, as described in the following section. 3. Methodology To investigate the heat-transfer through the cases described previously, a 1-dimensional transient heat conduction analysis was performed by using EnergyPlus software, as discussed in the following sub-sections. 3.1

Mathematical formulation EnergyPlus is a well-known building-energy simulation program developed by the US Department of

Energy (DoE), and has been extensively tested by researchers. This research used the CondFD algorithm of EnergyPlus, based on fully implicit solver with the capability to model the phase change phenomenon (DoE, 2010). It solves the following one-dimensional heat conduction equation:

( where,

)

(Eq. 1)

denotes the temperature, x the space coordinate, t the time, and ,

and

are the material

density, specific heat and thermal conductivity values, respectively. This equation is discretized using the fullyimplicit scheme that has second order accuracy in space and first order accuracy in time as: (

where is the modeled node, one.

)

and

is the calculation time step,

between nodes and

(

)

the adjacent nodes, the node spacing,

and between and

and

(Eq. 2)

the simulation time step and the previous the thermal conductivities for the interfaces

, respectively.

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For the exterior nodes (x = x0), the following boundary condition is applied:

| where is the incident solar radiation,

(

)

the absorptivity, and

(Eq. 3)

the outdoor air dry bulb temperature (DBT).

Equation 3 is discretized as follows:

( where

)

(

refers to the exterior surface node,

)

(Eq. 4)

is the combined convective and radiative heat transfer

coefficient modelled using the DOE2 algorithm For the interior surface nodes (x = xn), the boundary condition is:

| where

)

(Eq. 5)

is the combined convective and radiative heat transfer coefficient. This equation is discretized as:

( where

(

)

(

)

(Eq. 6)

refers to interior surface node.

For simulating cases with PCM (Cases 3 and 4), an enthalpy-temperature formulation was used in the CondFD algorithm. In this formulation, a variable specific heat function,

, is used for the PCM

according to the equation:

(Eq. 7) where h is the PCM enthalpy. This investigation solved Equations 2, 4 and 6 (for Cases 0, 1, 2, 3a and 4a) or 2, 4, 6 and 7 (for Cases 3 and 4) in EnergyPlus, subjected to the inputs discussed in the following sub-section. 3.2

Simulation inputs The outdoor conditions (DBT, solar radiation, etc.) for summer season were taken from Delhi’s Typical

Metrological Year 3 (TMY3) data file, while the indoor conditions were kept at a constant DBT of 26 °C. Note that the beginning of “summer” season was defined as the time of the year when the daily-averaged ambient

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DBT starts to exceed 26 °C, while the season ends with the arrival of monsoon. This definition meant that the summer season extended from 29 March to 25 June (89 days). 3.3 Model validation The EnergyPlus software used in this investigation has been extensively validated for energy simulation in buildings (Tabares-Velasco et al., 2012). However, to verify our simulation methodology, we solved the Stefan’s problem using the software, and compared the simulation results with the exact analytical solution (see “Stefan’s Problem” section in SI for details). Close agreement was obtained between our simulation results and the exact solution; thus, validating our methodology. 3.4

Energy performance metrics. To compare the energy benefits obtained from the different configurations (Cases 1, 2, 3, 3a, 4, and 4a)

with respect to the baseline construction (Case 0), we defined the following two performance metrics: i) Reduction in heat gain ( ): In order to estimate the cooling energy savings that can be obtained with insulation/PCM-enhanced building envelopes, the total percentage reduction in the envelope heat gain was computed as:

(Eq. 8) (kWh/m2) is the total summer heat flux across the building roof or walls, and the subscript denotes the

where

roof/wall configuration (baseline or enhanced with PCM/insulation layer). hourly envelope heat flux ( ̇ ) during the entire season as

∫ ̇

was calculated by integrating the

.

ii) Distribution of heat gain: To assess the performance of insulation/PCM enhanced envelopes in shifting the cooling energy requirements from peak electricity demand hours (defined as 14:00–17:00 and 22:00–01:00) to normal/off-peak hours, we first computed the envelope heat gains during peak ( normal (

), off-peak (

), and

) hours. These heat gains were then divided by the total heat gain ( ) to compute the corresponding

percentages of heat gains. For example, we calculated the percentage heat gain during peak hours (

) as:

(Eq. 9) where ∫

̇

is the total peak hour heat gain through the building roof or walls, given by . Similarly, the percentage heat gains during off-peak (

) and normal (

∫

̇

) hours were calculated.

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The peak (14:00–17:00 and 22:00–01:00), off-peak (04:00–10:00) and normal hours (all times of the day, excluding peak and off-peak hours) were defined according to the electricity tariff rates in Delhi (DERC, 2018). We note that this investigation only quantified the envelope heat gains and did not account for the lag between the heat gains and cooling load. The shifts in envelope heat gains were used as a proxy for estimating the shifts in cooling energy and electricity requirements. For example, a time shift in the heat gain from peak to off-peak hours was assumed to produce a similar shift in cooling energy and electricity requirements from peak to off-peak hours. 3.5

Life-cycle cost analysis To further assess the economic viability of the proposed envelope enhancement techniques, we also

conducted a Life-Cycle Cost (LCC) analysis by quantifying the Net Present Value (NPV) of the proposed techniques by using the following equation:

∑

(Eq. 10)

where C0 is the initial investment, k the number of years in the future, Ck the net cash flow (inflow minus outflow) in the kth year, and i the discount rate. The LCA analysis was conducted for 30 and 50 year periods for a typical dwelling with insulation/PCM enhanced envelopes. The bound-PCM (GR from Rubitherm) was used for this analysis, since it is chemically inert and without an active liquid phase, and thus does not show performance deterioration during its life cycle (Rubitherm, 2019). The price was taken as INR 300,000 /m3 (~USD 4300/m3) for the PCM (Kosny et al., 2013) and INR 5,000/ m3 (~ USD 70/m3) for the insulation (Singh et al., 2015). The discount rate was taken as 6%, which is a representative value for India (RBI, 2019), and the current electricity prices were taken as INR 8/kWh (~0.1 USD/kWh, as per DERC, 2018) with an estimated annual price increase of 4% (CEEW, 2017). 4. Results This section presents the performance assessment of PCM-enhanced roofs and the walls (east, west, north, and south facing) with different configurations, as described in Section 2, for the summer season as per the above-mentioned energy performance metrics. 4.1

Reduction in envelope heat gain Figures 2 a), b), and c) show the heat gain reduction ( ) in summers for roofs enhanced with PCM (bound-

PCM with latent heat equal to 60 kJ/kg) or insulation layers of 10, 20, and 30 mm thickness, respectively. For

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brevity, cases with insulation outside (Case 1) or inside (Case 2) were denoted as “Insulation-Out” or “Insulation-In”, respectively, in the figure. The cases with PCM (Cases 3 and 4) were represented using their TPCM (PCM melting point) and location. For example, Case 3, which had PCM layer on the outer side and TPCM = 36 °C was denoted as “T36-Out”. Finally, the No-PCM cases (Cases 3a and 4a), wherein the PCM was assumed to have no latent heat storage, were denoted as “No-PCM-Out” or “No-PCM-In”, depending on whether the PCM layer was outside or inside, respectively. A similar notation was followed in all subsequent figures.

Figure 2: Reductions in summer heat gain ( ) obtained with insulation or PCM (bound-PCM with latent heat equal to 60 kJ/kg) enhanced roofs. From figure 2, it can be seen that insulation on the outer side (Case 1) was the most effective in reducing the roof heat gain (

= 46.0–71.4%) when compared to all other cases. It can also be seen that putting

insulation layer on the inner side (Case 2) was the second most effective method for heat gain reduction (

=

41.0–67.1%). The reduction in heat gain obviously increased with the insulation layer thickness. A similar conclusion was also drawn for the four walls (see Tables SI 1–3) with their

values ranging between 34.7% to

64.0% for Case 1 and between 32.4% to 61.5% for Case 2. For both cases,

values increased with insulation

thickness and were found to be largely independent of the wall orientation. Figure 2 also shows that In), respectively. Clearly,

for roof was 15.3–36.2 % and 12.6–30.9 % in Cases 3 (PCM-Out) and 4 (PCMincreased with PCM layer thickness but remained largely unaffected by T PCM.

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Similar conclusions were also obtained for all the PCM-enhanced walls, and their

values ranged from 10.5–

29.0% in Case 3 and 10.4–26.6% in Case 4, as tabulated in Tables SI 1–3. Once again

increased with PCM

layer thickness, but was largely unaffected by T PCM and wall orientation. Thus, PCM-enhanced envelopes also led to a reduction in the summer heat gain as compared to the baseline construction (Case 0), with the PCM-Out configurations slightly outperforming the PCM-In configurations. Figure 2 also displays the heat gain reductions obtained in Cases 3a (No-PCM-Out) and 4a (No-PCM-In). As described in Section 2, the No-PCM cases can be compared with their corresponding PCM counterparts to determine how the latent heat storage by the PCM affects the heat gain reduction. In Cases 3 (PCM-Out) and 4 (PCM-In), depending on TPCM, the reductions in summer heat gains were marginally lower than their corresponding No-PCM counterparts, which means that the latent heat stored by the PCM led to a slight deterioration in the PCM layer’s performance. Similar results were obtained for all the four walls, and have been tabulated in Tables SI 1–3. To investigate the effect of latent heat capacity of the PCM on the reduction in the summer heat gain (Er), we compared the energy performance of the bound-PCM (latent heat equals 60 kJ/kg) with that of the purePCM (latent heat equals 240 kJ/kg). Their other thermo-physical properties were the same, as described in Section 2. Figure 3 shows that

values reduced with an increase in the latent heat capacity of the PCM; thus,

deteriorating the PCM layer’s performance. Note that the No-PCM cases consistently outperformed their corresponding PCM counterparts, and this outperformance clearly increased with the latent heat capacity of the PCM, which reaffirmed that the latent heat stored by the PCM in fact deteriorated its energy performance for the studied region, construction practices, and indoor conditions. Similar results were obtained for all the four walls, as given in Tables SI-2 and SI-4 with PCM latent heats equal to 60 kJ/kg and 240 kJ/kg, respectively.

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Figure 3: Comparison between summer heat gain reductions ( ) obtained with roofs enhanced with PCMs of different latent heat capacities. To further explore the effect of incorporating insulation/PCM layers in the building envelope on the reduction in summer heat gain, we compared the hourly heat gain through the baseline roof with those enhanced with 20 mm insulation or PCM (bound-PCM with latent heat of 60 kJ/kg) layers. This comparison is illustrated for two summer days (18–19 June) in Figure 4. Clearly, the hourly heat gain was generally the lowest in Case 1 (Insulation-out) followed by Case 2 (Insulation-in) due to the added thermal resistance. This led to a large reduction in the total summer heat gain (

= 62.7% for Case 1 and

= 57.7% for Case 2, as given in Table SI-

2) over the baseline construction (Case 0), as discussed previously. For roofs enhanced with PCMs (PCM-In or PCM-Out), the hourly heat gains were lower than those for the baseline construction from around 10 am in the morning to midnight due to the added thermal resistance and storage offered by the PCM layer. However, from midnight to 10 am in the morning, the heat gains were higher for the PCM-enhanced roofs when compared to the baseline roof because the energy stored by the PCM layer is partly released indoors. The overall effect of this phenomena (thermal resistance and energy storage and release of PCM) is a net reduction in the summer heat gain as compared to the baseline construction, as discussed previously.

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Figure 4: Summer heat gain through baseline, insulated, and PCM-enhanced (bound-PCM with latent heat equal to 60 kJ/kg) roofs.

Figure 4 also shows that roofs enhanced with insulation or PCM (latent heat equals 60 kJ/kg) led to a significant reduction in the peak heat gain, as compared to the baseline construction (~70% peak reduction with insulation and ~40% peak reduction with PCM-In and PCM-Out). Similar trends were also seen for all the four walls as given in Figures SI-3–6. This means that not only do the insulation/PCM-enhanced envelopes led to reductions in the total heat gains, but also helped to reduce the peak gains and thereby the AC equipment can be downsized by using such envelopes. To further study the effect of latent heat storage by the PCM on its energy performance, we computed the hourly PCM melt fractions and heat gains through the roof for Cases 3 (PCM-Out) and 4 (PCM-In) at TPCM = 36 °C and compared them with their corresponding No-PCM counterparts (Cases 3a and 4a). As shown in Figure 5, when PCM (latent heat equals 60 kJ/kg) is on the outer side of the roof, it melts completely during the day and stores latent heat, which subsequently led to a reduction in the hourly heat flux for Case 3 (PCM-Out) as compared to Case 3a (No-PCM-Out). Furthermore, the PCM-enhanced roof had about 6% lower peak heat gain, which can also be attributed to the latent heat stored by the PCM. The PCM then solidified during lateevening/night, which subsequently led to an increase in the indoor heat flux for Cases 3 as compared to its NoPCM counterpart since the latent heat released during solidification was partially transferred indoors.

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Figure 5: Comparison of diurnal variation in summer heat gain through the PCM-enhanced (bound-PCM with latent heat equal to 60 kJ/kg) roof for Cases 3 (PCM-Out) and 3a (No-PCM-Out). A similar behavior was seen when comparing Case 4 (PCM-In) with Case 4a (No-PCM-In); however, the melting and solidification times were shifted to later portions of the day as illustrated in Figure 6 since the PCM was located on the inner side. The net effect of this phenomenon (latent heat storage by PCM) was a marginal increase or decrease in the daily heat gains and a slight reduction in the peak heat flux values in Cases 3 and 4 as compared to their respective No-PCM counterparts.

Figure 6: Comparison of diurnal variation in summer heat gain through the PCM-enhanced (bound-PCM with latent heat equal to 60 kJ/kg) roof for Cases 4 (PCM-In) and 4a (No-PCM-In).

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To study the effect of increase in PCM’s latent heat on the peak heat gain reduction, the above-mentioned analysis was also conducted with a pure PCM that had a much higher latent heat (240 kJ/kg) as compared to the bound-PCM (latent heat equals 60 kJ/kg) discussed above. Qualitatively similar results were found as illustrated in Figures SI-7–9. 4.2

Distribution of heat gain during peak, off-peak and normal hours In addition to quantifying the reduction in the summer heat gain that can be obtained with insulation/PCM

enhanced building envelope, this study also computed the distribution of the summer heat gain during peak, offpeak and normal hours. Figures 7 a), b) and c) show the

,

, and

values for roofs enhanced with 10, 20

and 30 mm of insulation and PCM layers (latent heat equals to 60 kJ/kg), respectively, together with those for baseline construction. For the baseline roof, 36.5% heat gain happened during peak hours ( during off-peak hours (

= 36.5%), 4.1%

= 4.1%), and the rest during normal hours; whereas the insulated roofs experienced

31.5–34.5% heat gain during peak, 7.9–14.1% gain during off-peak, and the rest during normal hours, depending on the insulation layer thickness and location. This means that the fraction of cooling-energy (or electricity) required for the insulated roof would likely be less than that for the baseline roof during the peak hours, while the opposite is true during off-peak hours. A similar tendency was also found for all the four walls, as tabulated in Tables SI 5–7. Thus, the insulated envelopes seem to favorably shifts the AC requirements from the peak demand hours to the off-peak hours.

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Figure 7: Percentage of heat gain during peak (FP), normal (FN) and off-peak (FO) hours with baseline and enhanced (10–30 mm thick insulation or PCM with latent heat equal to 60 kJ/kg) roofs. In case of roofs enhanced with PCMs (latent heat equal to 60 kJ/kg),

ranged from 29.4–35.0% for Case 3

(PCM-Out) and from 27.0–35.0% for Case 4 (PCM-In), depending on the PCM layer thickness and T PCM. Generally,

decreased with increasing thickness; and the latent heat stored by the PCM also contributed

towards reducing

,

which is evident upon comparing Cases 3 and 4 with their corresponding NO-PCM

counterparts as shown in Figure 7. An opposite trend can be seen for

values, i.e.,

increased with

increasing PCM thickness and the latent heat storage also contributed towards increasing

. A similar trend

was also observed for the four walls, and their

,

, and

are given in Tables SI 5–7.

We also analyzed the effect of the latent heat capacity of the PCM on the heat gain distribution obtained by comparing roofs enhanced with bound-PCMs (latent heat equal to 60 kJ/kg) to those enhanced with pure PCMs (latent heat equal to 240 kJ/kg), as illustrated in Figure 8. It is clear that an increase in the latent heat capacity of the PCM led to a more favorable heat gain distribution by shifting the roof heat gain from peak to off-peak hours. A similar effect of latent heat was found for all the four walls by comparing their corresponding and

,

,

values given in Tables SI-6 and SI-8.

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Figure 8: Comparison between the percentage of heat gain during peak (FP), normal (FN) and off-peak (FO) hours with 20 mm thick PCM enhanced roofs of latent heats equal to a) 60 kJ/kg and b) 240 kJ/kg. Clearly, the heat gain distribution obtained with the pure PCM was even more favorable that those obtained with insulated roofs (Insulation-In and Insulation-Out), as shown in Figure 8. Thus, it would seem that PCMenhanced envelopes with high latent heat capacity could be more suitable to shift the cooling load from peak to off-peak hours than insulated envelopes. However, the favorable/unfavorable load distribution is also dependent on the electricity tariff rates (peak, normal and off-peak), which in turn depend on external factors such as the city’s overall electricity demand and supply variation with time of day. Thus, the results should be used with caution, and a case-by-case assessment of the proposed energy efficiency solution needs to be undertaken. From our results, it is evident that insulation/PCM-enhanced envelopes are not only effective in reducing the summer heat gain, but also seem to favorably shift the cooling-energy requirements from the peak to offpeak hours. 5. Discussions Our results showed that insulation/PCM based envelope enhancements can significantly reduce the cooling energy requirements of residential buildings in Delhi. Thus, it is worthwhile to discuss the dwelling-scale and city-scale reductions in electricity consumption and associated CO2 (carbon dioxide) emissions that can be obtained. Furthermore, the financial implications for the building owners are also discussed by comparing the NPVs of the proposed solutions.

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We first estimated the electricity savings and CO2 reductions that can be obtained by retrofitting a standalone house with insulation or PCM. The bound-PCM (latent heat equal to 60 kJ/kg) was considered for this analysis since it showed higher reduction in the envelope heat gain values, as discussed in Section 4.1. It was assumed that each household had at least one room with its roof (area = 10 m2) and a wall (area = 10 m2) exposed to ambient conditions, which required cooling throughout summers. The envelope heat gain was converted into air-conditioning electricity consumption by dividing it with the typical seasonal energy efficiency ratio (SEER) for India with the assumption that the envelope heat gain is equal to corresponding cooling energy requirement. Under those assumptions, we computed that insulated roofs could lead to 95–165 kWh electricity savings during summers, depending on the insulation layer thickness and position as given in Table 2 (see SI for sample calculations). The PCM-enhanced roofs led to 29–84 kWh electricity savings depending on the PCM layer position, thickness and T PCM. Similarly, insulated walls led to an estimated 21–78 kWh electricity savings, while the PCM-enhanced walls can save 7–35 kWh electricity as shown in Table 2. Thus, overall about 129–228 kWh or 40–112 kWh electricity (sum of the roof savings plus the average wall savings) can be saved by retrofitting an existing stand-alone house with insulation or PCM, respectively. This would translate to an associated reduction of 105.8–187.0 kg or 32.8–91.8 kg of CO2 emissions during summers for the insulation or PCM enhanced envelopes, respectively. Table 2: Electricity savings obtained with insulation/PCM-enhanced roof and walls per household in summers Roof (kWh)

East wall (kWh)

West wall (kWh)

North wall (kWh)

South wall (kWh)

Total envelope (kWh)

Insulation-Out (10 mm)

106

45

46

23

33

143

Insulation-Out (20 mm)

145

65

66

34

49

199

Insulation-Out (30 mm)

165

77

78

40

58

228

Insulation-In (10mm)

95

43

42

21

32

129

Insulation-In (20mm)

134

63

62

32

48

185

Insulation-In (30mm)

155

75

74

39

57

217

PCM-Out (10 mm)

35–37

13–15

14–15

7

10

46–48

PCM-Out (20 mm)

60–64

23–26

24–26

13

17–19

80–85

PCM-Out (30 mm)

77–84

31–34

31–35

17–18

23–26

103–112

PCM-In (10 mm)

29–30

13

13

7

10

40–42

PCM-In (20 mm)

52–53

23

24

13

18

71–74

PCM-In (30 mm)

70–71

32

32

17

24

97–101

Note: Total envelope electricity saving estimated as the sum of roof saving plus the average of wall savings. From the dwelling-scale results, we also estimated the city-scale results by multiplying the dwelling-scale results with the number of stand-alone dwellings in Delhi, which was estimated to be about 2.0 million (NSSO, 2014). Thus, city-wide 258–456 GWh or 80–224 GWh electricity could be saved during summers by retrofitting existing stand-alone residential buildings with insulation or PCM, respectively, which would lead to about 0.3–

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1.5% reduction in the annual electricity consumption of Delhi. Simultaneously, the city-wide CO2 emissions would be reduced by 212–374 kt or 66–184 kt during summers for the insulation or PCM enhanced envelopes, respectively, which amounts to 0.2–1.0% reduction in the annual CO2 emissions. In addition to aforementioned energy and environmental benefits of insulation/PCM enhanced envelopes, we also evaluated their financial implications for the building owner by calculating their NPV values (see SI for sample calculations). Table 3 shows that NPVs of insulated or PCM (latent heat equal to 60 kJ/kg) enhanced roofs ranges from 229 to 389 USD or −1,112 to −337 USD, respectively, for a 30-year period. The NPVs ranged from 72 to 246 USD for the insulated walls, while the NPVs were between −1,227 to −381 USD for the PCMenhanced walls. The NPV values for the roof and walls, translated to a total NPV of 331 to 568 USD for the insulated envelope and between −2,314 to −728 USD for a PCM-enhanced envelope. This clearly shows that the insulated envelope is a much better financial investment as compared to the PCM-enhanced envelope. Similar results were obtained, when the LCC analysis was conducted for 50-years, as given in Table SI-9. Note that we calculated the NPV values without considering the installation cost of insulation or PCM, which would be substantially variable depending on whether the insulation/PCM layer is being included in the initial build or as a retro-fitment. If the initial installation cost is known, it can directly be subtracted from the values given in Table 3 or SI-9 to recalculate the revised NPVs. Table 3: Net Present Values (NPVs) of insulation/PCM-enhanced roof and walls for a typical household for a 30-year period. Roof (USD)

East wall (USD)

West wall (USD)

North wall (USD)

South wall (USD)

Total envelope (USD)

Insulation-Out (10 mm)

257

109

151

72

106

366

Insulation-Out (20 mm)

346

154

212

102

154

502

Insulation-Out (30 mm)

389

178

246

116

177

568

Insulation-In (10mm)

229

104

137

65

103

331

Insulation-In (20mm)

319

149

198

95

150

467

Insulation-In (30mm)

364

173

232

112

174

537

PCM-Out (10 mm)

−342 to −337

−395 to −390

−381 to −377

−405

−394

−735 to −728

PCM-Out (20 mm)

−708 to −698

−798 to −790

−775 to −768

−813

−799 to −792

−1504 to −1489

PCM-Out (30 mm)

−1094 to −1077

−1206 to −1198

−1179 to −1166

−1227 to −1224

−1207 to −1197

−2299 to −2273

−356 to −354

−395

−384

−405

−394

−751 to −748

PCM-In (10 mm) PCM-In (20 mm)

−728 to −725

−798

−775

−813

−795

−1523 to −1520

PCM-In (30 mm)

−1112 to −1109

−1203

−1176

−1227

−1203

−2314 to −2312

Note: Total envelope NPV was estimated as the sum of roof saving plus the average of wall savings. The above discussion revealed the energy, environmental and economic benefits of insulation/PCM enhanced envelopes for reducing the cooling energy consumption in typical residential constructions in the megacity of New Delhi. The results are also useful for other locations in the composite climate zone (hot

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summers with daily maximum temperatures routinely exceeding 35 °C, and a diurnal temperature variation of about 10 °C) of India, which have a similar construction. This region includes major Indian cities such as Allahabad, Amritsar, Lucknow, etc. and a total population of about 0.4 billion people. Note that New Delhi also experiences a cold winter season (minimum temperatures can reach as low as 5 °C in winters), which means that heating would be required to maintain comfortable indoor temperatures. It is obvious that a PCM layer designed for summer conditions will mostly remain in solid state during winters. However, it would still be expected to slightly reduce the heating energy consumption since it would provide additional thermal mass and resistance to the building envelope. Similarly, adding insulation to the envelope will also aid in reducing the heating energy consumption. Thus, it is evident that the insulation/PCM enhanced envelopes discussed here will also be beneficial in reducing the heating energy consumption, and thus provide additional benefits over and above those obtained in summers. Future investigations could explore these aspects by conducting year-round analysis with single or multiple insulation and PCM layers. 6. Conclusions This investigation studied the energy performance of PCM-enhanced envelopes (roof and walls) in summers for residential buildings in Delhi, India, and compared it with those of baseline and insulationenhanced envelopes. It was found that, applying a PCM layer (latent heat capacity equal to 60 kJ/kg) on the roof reduced the summer heat gain by 12.6–36.2% over the baseline construction, depending on the layer thickness and position, but largely independent of the PCM’s melting point temperature; whereas an insulation layer of the same thickness reduced the heat gain by 41.0–71.4%. Similarly, PCM-enhanced walls were also found to reduce the heat gain by 10.4–26.6%; whereas insulated walls led to a much larger reduction (32.4– 64.0%) in the heat gain. The results also showed that an increase in the latent heat capacity of the PCM (from 60 kJ/kg to 240 kJ/kg) led to a net increase in the envelope heat gains; thereby, deteriorating the PCM layer’s performance. Overall, both PCM and insulation based envelope enhancements helped to reduce the summer heat gain; however, the insulation based enhancements outperformed those with PCMs. In addition to a large reduction in the heat gain, PCM/insulation based enhancements were also found to reduce the peak heat gains; and shift the cooling energy requirements from peak electricity demand hours to off-peak hours. In these aspects, an increase in the latent heat capacity of the PCM was found to have a favorable effect, i.e., lower peak heat gains and a further reduction in the peak electricity demand.

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By extrapolating those results to the city-level, we found that 258–456 GWh or 80–224 GWh electricity could be saved during summers by retrofitting existing homes with insulation or PCMs, respectively, which means 0.3–1.5% reduction in the annual electricity consumption of Delhi. Simultaneously, 212–374 kt or 66– 184 kt CO2 emissions could be reduced with the insulation or PCM enhanced envelopes, respectively, which amounts to 0.2–1.0% reduction in the annual CO2 emissions. In addition to the energy and environmental benefits of proposed insulation and PCM enhanced envelopes, financial implications for the building owners were also assessed. The NPVs ranged from 331 to 568 USD for insulated envelopes, and from −2,314 to −728 USD for PCM-enhanced envelopes for a typical dwelling in New Delhi for a 30-year period. Acknowledgements This study was supported by Birla Institute of Technology and Science (BITS), Pilani through its Outstanding Potential for Excellence in Research and Academics (OPERA) award.

Author Statement Viven Sharma: Methodology, Software, Validation, Formal analysis, Writing - Original Draft, Visualization. Aakash C. Rai: Conceptualization, Validation, Formal analysis, Writing - Review & Editing, Visualization, Supervision.

Conflict of Interest and Authorship Conformation Form Please check the following as appropriate:  All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.  This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue.  The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript

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