Implementing Cool Roof and Bio-PCM in Portable Cabins to Create Low-Energy Buildings Suitable for Different Climates

Abstract

The building sector’s energy consumption has significantly increased due to climate change, emphasizing the need to develop sustainable low-energy buildings using experimental and computational tools. As a joint project between Kuwait and Australia, two portable cabins with internal sizes of 2 m × 2 m × 2.80 m, made from 75 mm thick sandwich panels and metal frames, were built in Kuwait to investigate their thermal and power consumption characteristics under various energy-saving techniques and different climates. This is the first attempt to analyze the energy-saving aspects of portable cabins made from sandwich panels for future sustainable cities. Each cabin has an indoor air-quality-sensing device and a novel power-monitoring system to measure their thermal and power consumption characteristics. First, shingles and novel finned metal cool roof (CR) techniques were experimentally investigated. Then, a new multi-zone SketchUp model of the portable cabins was created and simulated in TRNSYS. Next, the energy saving data of the portable cabins were investigated by adding PCM on the walls and the roof using built-in models of bio-phase change materials (Bio-PCMs) in TRNSYS. The annual energy performance index (EPI) as an important sustainability index was explored for determining heating/cooling/total demands of the portable cabins in the desert climate of Kuwait and in various climate regions of Australia. The findings reveal that both shingles and finned metal roofs contributed to higher power consumption. Meanwhile, the use of sustainable Bio-PCMs in Kuwait demonstrates a significant energy-saving potential of 30%, with variations ranging from 25% to over 45% across different climate regions in Australia.

1. Introduction

The demand for reducing cooling energy consumption in Kuwait’s buildings is steadily increasing for developing sustainable cities. Despite the existence of energy savings codes dating back to 1983, implementation has been suboptimal. With the rapid rise in electricity demand in the building sector, many residential buildings lack energy reduction strategies [1]. Non-renewable hydrocarbons are heavily relied upon for electricity generation and water desalination, making up about 50% of the country’s economy, and these are not sustainable. Air-conditioning services alone account for over 70% of residential electricity demand due to rising temperatures [1]. Residential buildings consume 90% of Kuwait’s electricity production by unsustainable resources [2,3]. With an estimated 128,000 new houses expected to be developed by 2035, there is significant potential to introduce more stringent energy conservation codes and regulations.

Climate change presents significant challenges for future sustainability in Australia and the world. Residential building cooling in Australia has shown the most rapid growth rate of 16.1% per annum since 1990, increasing consumption from 3 PJ to 17.7 PJ in 2020 [4,5]. Houses in Australia contribute 30% to total CO₂ emissions [4], making it urgent to develop low-energy sustainable buildings in both Kuwait and Australia [5,6]. Solar window films were analyzed in [7,8,9], but these only saved 1–3% of energy in office buildings of Kuwait [10]. Sustainable green rooftops and bio-phase change materials (Bio-PCM) have shown a higher potential of up to 20% in a container without insulation in Australia [11,12]. Alternative innovative sustainable materials and techniques, such as insulation, Bio-PCM, cool roof (CR) materials in house envelopes, and the application of solar-assisted air-conditioning (SAC) methods, are viable options to address this issue.

1.1. Cool Roof Techniques

Direct sunlight causes roof and house heating during sunny hours. Cool roofs are passive sustainable techniques that decrease roof temperature and extend service life [13]. They save energy and reduce air conditioning, electricity bills, and CO₂ emissions [14]. Energy savings depend on factors such as weather, insulation, residents’ energy use, electrical energy costs, and heating/cooling equipment. Cool roofs reflect sunlight better due to their surface properties, solar-reflectance (τ) and thermal-emittance (ε), combined in the SRI (solar reflectance index). Cool roofs require minimum SRI values of 64 and 16 for 3-year-old low- and steep-sloped surfaces, respectively. However, the USGBC [15] uses SRI values of 78/29 for low- and steep-sloped roofs, respectively. Sustainable roof insulation significantly reduces heat gain/loss, so cool roofs are not an appropriate substitute. For more information on thermal insulation, refer to the DOE resources [16,17,18].

Cool roof white paints are a common coating option with unique reflective properties that can reflect sunlight, reduce heat gain/loss, save energy used for air conditioning, protect against UV damage, and increase the longevity of sustainable roofs. There are three types of cool roof paints available: solvent-based paints that are generally used for spray polyurethane roofs; water-based acrylic cool roof paints that have good adhesion and are easy to apply; and urethane solvent-based cool roof paints that have stronger bonding capabilities (3–10 times stronger than acrylic paints) and become even stronger when exposed to the atmosphere. However, they are more difficult to work with and are more expensive.

Research conducted by Seifhashem et al. [19] found that the use of an acrylic cool roof paint on a retail warehouse building in Australia led to a decrease in roof temperature and energy savings, although no comparison was made since only one warehouse was tested. In another study, Antonaia et al. [20] applied three paints commonly used in the automotive sector to an insulated roof in a Mediterranean climate and found an annual energy saving of up to 3%. Conversely, Bozonnet et al. [21] found no significant changes in indoor operative temperatures when cool roof paint was used on buildings in France, which they attributed to France’s strict insulation standards. Finally, De Masi et al. [22] studied the aging of white acrylic paint and discovered that its solar reflectance degrades by about 20–25% over time.

Several studies have investigated the impact of sustainable cool roofs and other building envelope materials on thermal comfort and energy efficiency. Kolokotroni et al. [23] found that using TRNSYS in an office room in the United Kingdom resulted in a 2.5 °C increase in thermal comfort. Romeo and Zinzi [24] reported a 54% reduction in cooling using organic paint. Shittu et al. [25] studied cool roof paints for two houses in Sicily and Jamaica, finding significant energy savings in both locations, particularly in Jamaica. Yew et al. [26] used a combination of passive and active cool roof techniques, reporting a 7 °C reduction in roof temperature using a high-SRI paint in Singapore. Zingre et al. [27] studied modelling of cool roof in tropical climate. Costanzo et al. [28] found that, in an Italian office room, higher solar reflectance may not be practical for overall annual energy savings due to the need for heating during winter. Sahoo [29] monitored the effects of sustainable cool roofs on two identical houses in Ahmedabad, India, observing a maximum temperature reduction of 1 °C–4.8 °C. Roman et al. [30] investigated the use of sustainable phase change materials and cool roofs to mitigate urban heat islands in 7 climate zones in the USA using EnergyPlus simulation. They found that PCM roofs can save up to 54%, while cool roofs can offer up to 40% energy savings on rooftops. Stavrakakis et al. [31] found no significant differences in two rooms of a school building in Athens, Greece, when using a cool roof with either ceiling fans or split units. Kim et al. [32] monitored two buildings with and without cool roofs in Changown city, South Korea, for a year and found that the effects of cool roofs were more pronounced in winter than summer, which contradicts previous studies. He et al. [33] compared the application of sustainable green roofs with cool roofs in the climate of Shanghai, China and found that cool roofs reduced surface temperature by 3.3 °C, while green roofs only reduced it by 2.9 °C in the summer. Chen and Lu [34] developed a cool roof model and validated it for five different cities in China, reporting energy savings of less than 10% for the studied cities. Androutsopoulos et al. [35] investigated cool roof coatings in a school building in Athens, Greece, and found 1.3–2.3 °C indoor temperature differences, as well as a 20% annual AC energy reduction and up to 30% increased AC savings in the summer. Lie et al. [36] demonstrated that adaptive shape-morphing materials in building envelopes can enhance energy efficiency and sustainability of buildings, while Habibi et al. [37] found that carefully selected roofing materials, such as a three-layer roofing system from a PV panel, an EPDM membrane, and an insulation layer, can lead to energy savings.

Composite sustainable roofs can be constructed using various materials and methods. One common type is the built-up roof (BUR), which typically consists of several layers of ply sheets and asphalt bonding agents applied to a deck surface. To enhance the roof’s reflective properties, reflective marbles or painted slags can be added to the surface, or white mineral granules can be used to cover the roof [13,38]. Another option is the modified bitumen roofing system, which utilizes multiple layers of polymer materials and reinforcing fabrics, with a protective surfacing layer to delay aging and increase sustainability. This layer can be comprised of aggregate, minerals, a smooth surface coating, or a metal foil laminate. Spray polyurethane foam (SPF) is another popular roofing material, which is made of polyurethane and provides excellent sustainable insulation properties. Acrylic can also be used for SPF, although it may result in different color options than white depending on the desired appearance of the building [3].

Shingled roofs are created by overlapping flat or curved sustainable pieces made from various materials such as wood, ceramic, and asphalt [39]. Fiberglass asphalt shingles are commonly used in residential buildings in the USA and can include coated granules to improve solar reflectance. Other shingles are factory-coated for better reflective properties. Roof tiles can be made from sustainable rocks, concrete, metal, plastic, or clay. Concrete tiles can be colored with cool roof paints. However, a study by Alchapar and Correa [40] found that 16 shingle and tile materials, as well as 3 cool roof coatings, increased roof temperatures by 3.5 °C–24 °C and were therefore not recommended for urban or building heat reduction.

Metal roofs, including those made of zinc, copper, and galvanized steel alloys, are another type of roofing material [41]. While unpainted metal roofs tend to have high reflectance, their low thermal emittance makes them unsuitable for use as cool roofs in low-slope applications. To achieve sustainable cool roof performance, metal roofs are often coated with silicone–polyester- or fluoropolymer-based paints.

For the purposes of this study, two cool roof methods are compared: ceramic shingles, which are commonly used in residential buildings in Kuwait, and an innovative finned metal roof made from aluminum.

1.2. Phase Change Materials (PCMs)

Phase change materials (PCMs) are commonly used in the sustainable building sector and can undergo a phase change from solid to liquid at temperatures ranging from 20 to 29 °C [42,43]. PCMs have a high heat absorption capacity and can absorb up to 14 times more heat per unit volume compared to materials like brick, sandstone, and concrete. The function of PCMs in sustainable building envelopes involves absorbing heat during the day and releasing it to the surrounding environment at night. This process helps maintain a good thermal balance while conserving energy. PCMs are classified into three groups: organic, inorganic, and eutectic. Organic PCMs include paraffin (e.g., C_nH_2n+2), non-paraffin, such as fatty acids (e.g., CH₃(CH₂)_2nCOOH), and polyalcohols [44]. Inorganic materials are salt hydrates with a general formula A.B._nH₂O and metals with low melting points. A eutectic is a composite PCM made from two or more components. However, some of these PCM types cannot be used directly in building envelopes due to their flammability or significant volume changes. Recent developments in sustainable PCM applications for heating/cooling in buildings have led to the creation of Bio-PCMs, a new organic-based PCM that is nonflammable [45,46,47,48,49]. There are 12 Bio-PCM built-in models in Type1270 in the TRNSYS library based on the products of Phase Change Energy Solutions (a PCM manufacturer [50]) that can be investigated for their suitability and adaptability in building envelopes.

Further research on Bio-PCMs is necessary, as limited data are currently available in the public domain, both in Australia and Kuwait. In our previous work [51], TRNSYS simulations as a sustainability tool were reported for a similarly sized room as the portable cabins using 10 commonly used building materials and 12 Bio-PCMs in three different hot climates: Kuwait (desert), East Australia (sub-tropical), and South India (tropical).

This paper models two cabins with 12 sustainable Bio-PCM in TRNSYS for the desert climate of Kuwait and six different climates in Australia, including equatorial (Darwin), tropical (Normanton), subtropical (Rockhampton), desert (Meekatharra), grassland (Alice Springs), and temperate (Sydney). These locations provide major climates in Kuwait and Australia and the impact of the climate on the studied energy savings can be analyzed. The novelty of this work is the comparison of both AC power consumption through measurements and simulations, as well as the effects of sustainable sandwich panel materials (rather than commonly used materials) and Bio-PCM in different climates of Kuwait and Australia using portable cabins.

Section 1 provides a literature review on cool roof techniques and PCMs. The manufacturing of the two portable cabins and measurement equipment are briefly discussed in Section 2. The developed six-zone model in SketchUp and TRNSYS solver with/without Bio-PCM model Type 1270 are presented in Section 2. Section 3 reports results from measurements for the two cool roofs. TRNSYS simulations are validated against experimental results for the two portable cabins and discussed in Section 4. TRNSYS simulations are presented for six locations in Australia and Kuwait using 12 Bio-PCMs on the interior, middle, and outer layers of the portable cabin walls. Additionally, an overall comparison is made between the insulation wall and the insulation wall with added Bio-PCM based on the energy performance index. Finally, in Section 5, conclusions of this study are drawn.

2. Materials and Methods

2.1. Studied Climates

Kuwait is a West Asian country situated on the northeastern edge of the Arabian Peninsula. It shares borders with Iraq to the north and the Kingdom of Saudi Arabia to the south. The country has a 500 km long coastline and shares water borders with Iran. Its latitudinal and longitudinal coordinates range between 28° and 31° N and 46° and 49° E, respectively. Kuwait comprises ten islands and features a flat terrain, with the highest point standing at 306 m above sea level [52]. Kuwait lies in the desert region (refer to Figure 1) and experiences a continental climate that is hot, dry, and marked by long summers and short, mild winters. Occasional rainfall occurs during winters, which are also characterized by very humid days. In summer, dusty storms are common, and the temperature may soar up to 50 °C in the shade. The winter season lasts only a few months, during which the average temperature may occasionally touch 18 °C, while the minimum temperature can drop as low as 0 °C. Rainfall during winters and springs is erratic and varies in quantity from year to year. Kuwait experiences short periods of autumn and spring seasons.

Australia’s climate is diverse due to the continent’s vast size, with six climate zones shown in Figure 2. The equatorial zone is hot and wet year-round, with high rainfall and constant temperatures. The tropical zone (e.g., Normanton) has a wet and dry tropical climate with a wet season and a dry season lasting over two months [53]. The subtropical zone (e.g., Rockhampton) has hot, humid summers and mild–cold winters. The arid and semi-arid desert zone (e.g., Meekatharra) has very high temperatures in summer and cool winters. The grasslands (e.g., Alice Springs) are surrounded by other zones and have cold winters and hot summers with little rain. The temperate zone (e.g., Sydney) has four seasons, with mild winters and summers [53].

2.2. Experimental Equipment

The flowchart in Figure 3 illustrates the overall methodologies employed in the present study. The left cabin serves as the base case, where continuous measurements of indoor air and AC power are recorded and stored in an internet cloud. The right cabin is designated as the testing cabin, where two sustainable cool roofs are implemented, and similar measurements are conducted. Computational simulations are also utilized to assess the effects of sustainable Bio-PCMs in the right cabin. The two portable cabins are identical and manufactured from sandwich panels measuring 2 m × 2 m × 3 m, as depicted in Figure 4. The internal dimensions are 2 m × 2 m (wall–wall) and 2.8 m (ground–false ceiling). The frame structure of the cabins consists of 120 mm I-beams, 40 mm × 40 mm square hollow sections at the bottom, and purlins at the roof from 50 mm × 50 mm × 2 mm hollow sections to support the frame. The columns are made from 50 mm × 50 mm × 2 mm hollow sections with angles, and the leg height is 300 mm from the ground. PVC skirting with a height of 100 mm and a thickness of 3 mm is used. The walls and roof are constructed using 75 mm polyurethane sandwich panels, which consist of 74 mm insulation material and 1 mm galvanized-steel sheets. The sloped roof is covered with a gypsum ceiling to provide a consistent internal height of 2.8 m. The flooring consists of 18 mm thick plywood finished with a vinyl sheet. Each cabin is equipped with one aluminum external door and one aluminum window. The door is a single leaf measuring 2 m × 1 m with a heavy-duty shutter, while the window is a double leaf measuring 1 m × 1 m with weatherproofed glass. All wirings and the main distribution board (MDB) for electrical power sockets/switch sockets run through PVC conduits fixed in the wall sandwich panels. Three multiple plugs are installed on three sides of the room. An LED light is installed on the back wall (refer to Figure 4a), with an electrical switch next to the door wall. For air conditioning, an 18,000 BTU split unit is installed in each cabin. Both split units were preset to 22 °C for indoor temperature while the air conditioning system was operational. Throughout the tests, no occupancy, equipment, or air ventilation rates were applied.

Previous TRNSYS simulations [55] indicated that the north-facing buildings consume less energy compared to other orientations. Therefore, the two cabins were installed facing north (N), with a width gap of 3 m in between the two to avoid shade effects (see Figure 4b).

Three Davis Airlink devices [56] were installed to monitor indoor and outdoor weather conditions and indoor air quality in the portable cabins. Two devices were placed inside the cabins, and one was mounted outside the window of the left cabin. The Airlink device measures temperature, humidity, and particle matter (PM) to provide a sustainable environment with real-time readings of AQI (air quality index) and HI (heat index). To comply with South Holland Building Codes 602.5, a wooden stand was placed with the Airlink device at the center of each cabin to monitor zonal indoor weather and air quality conditions. The Airlink device detects particle matter sizes of PM1 (1 µm), PM2.5 (2.5 µm), and PM10 (10 µm) and records readings in the weather link iCloud, which can be viewed on a mobile app.

Table 1. Accuracy of Davis Airlink measurements.

Particulate matters
Particle range of measurements	0.3–1 µm; 1–2.5 µm; 2.5–10 µm
Particle resolution	1 µg/m3
Accuracy	±10 µg/m3
Update interval	1 min
Indoor/outdoor relative humidity
Resolution and unit	0.1%
Range	0.1% to 100% RH
Accuracy	±2%
Update interval	1 min
Indoor/outdoor temperature
Resolution and unit	Current data: 0.1 °C
Range	−40 °C to +60 °C
Accuracy	± 0.3 °C (typical)
Update interval	1 min

shows the precision of the Airlink measurement sensors for particulate matter, relative humidity, and temperature.

Emporia’s Vue Energy Monitor [57] can monitor the sustainable energy consumption/generation of up to 16 home appliances in real-time. The Vue sensor uses a 50 A bundle for single-phase appliances and a 200 A bundle for 3-phase appliances. Emporia can monitor the real power consumption of various sustainable appliances, including air conditioners, solar energy systems, water heaters, washers, dryers, etc. The device requires 2.4 GHz WIFI for transferring 1 s energy data with an accuracy of ±2% through an internet link (https://web.emporiaenergy.com/ (accessed on 1 October 2023)). The device stores per-second data for up to three hours, per-minute data for up to seven days, and one-hour data indefinitely. Users can choose an optional data interval and export data to a registered email address in units of W, kW, and kWh. Emporia Vue sensor 50 A details and accuracy are shown in

Table 2. Technical specification of the Vue 50 A sensor.

Details	Values
Frequency	50–60 Hz
Max Current	50 A
Output	0.0–0.333 V (Measuring 0–50 A)
Accuracy	±2%
Dimensions	1.6″ × 0.9″ × 1″ (41 mm × 23 mm × 26 mm)
Window Diameter	0.39″ (10 mm)
Cable Length	39″ (1 m)
Connector	2.5 mm right-angled two-pole audio connector

.

2.3. Simulation Technique

TRNSYS models were developed to determine annual temperature/humidity and cooling/heating of the sustainable portable cabins (see Figure 5a). TRNSYS uses a library of weather stations worldwide and TRNBuild constructs a full 3D model of the room. For this study, natural air infiltration and latent heat gain are considered. The output of the model is annual solar, temperature/humidity, and heating/cooling of the portable cabins. The TRNSYS model in Figure 5a uses Type1270 PCM model with 12 built-in commercially available sustainable Bio-PCMs. A novel six-zone explosion model of the room was created in SketchUp2022 (Figure 5b) to model a Bio-PCM using Type1270 in TRNSYS for the portable cabins in Figure 4. Five extra zones in the room model in Figure 5b are used to provide boundary conditions that are needed in the Bio-PCM Type1270 model. The Bio-PCM layers are added on the inside-facing, middle, and outside-facing walls, as well as the roof, as shown in Figure 6.

One external aluminum door (0.9 m × 2.0 m) and one aluminum window (1.0 m × 1.0 m) were installed, and the roof is 2.60 m × 2.60 m with an overhang of about 0.25 m on each side. Wall thickness is 0.2 m with two layers of cement plasters from inside and outside. The energy efficiency of ten representative wall materials was compared with/without Bio-PCM, based on an extensive study of 43 wall material manufacturers [58,59] and some innovative energetic materials studied in [60].

The present study focuses on the sustainable energy efficiency of the two manufactured portable cabins. The thermal properties of wall materials, roof, and ground floor of the portable cabins made from 75 mm sandwich panel are listed in

Table 3. Thermal properties of materials used in the portable cabins.

Materials	U-Value (W/m2K)	ρ (kg/m3)	k (kJ/hmK)	Cp (kJ/kgK)
Sandwich Roof	0.218
Plywood [60]: Floor	2.064
Sandwich Panels (75 mm): Walls	0.2534	47.18	0.07236	0.8
Gypsum Board [62]		752	0.077	1.017

. Plywood is a massless layer with the thermal resistance of 0.04335 hm²K/kJ [61]. The 18 mm plywood and an airgap were used for the floor in TRNSYS simulation. For the sandwich roof, 10 mm gypsum board [62], airgap, and 75 mm sandwich panel are considered. For the walls here, just a 75 mm sandwich panel is used; later, a Bio-PCM layer was added to the inside face of the walls, the outside face of the walls, and in the middle of the walls (see Table 3).

In TRNSYS, Type1270 PCM model comes with 12 predefined sustainable Bio-PCM-type materials that are commercially available by a company named Phase Change Energy Solutions [50]. The PCM is selected by setting a single parameter, as shown in

Table 4. Preset sustainable phase change materials in Type1270.

Phase Change Material ID	TRNSYS Parameter Value
901.23QFGM27	901.2327
901.23QFGM51	901.2351
901.23QFGM91	901.2391
901.25QFGM27	901.2527
901.25QFGM51	901.2551
901.25QFGM91	901.2591
901.27QFGM27	901.2727
901.27QFGM51	901.2751
901.27QFGM91	901.2791
901.29QFGM27	901.2927
901.29QFGM51	901.2951
901.29QFGM91	901.2991

.

The PCM properties for the selected 12 sustainable Bio-PCM with melting points of 23, 25, 27, and 29 °C are listed in

Table 5. The properties of sustainable phase change materials in Type1270 [50].

Melting Point (°C)	Latent Heat (J/g)	Energy Storage Capacity (kJ/m2)	Specific Heat (J/gK)	Thermal Conductivity (W/mK)	Relative Density (g/mL)	Phase State
23, 25, 27, 29	210–250	400–1250	2.2–4.5	0.15–2.5	0.85–1.4	Liquid, viscous gel, solid-solid gel

.

As depicted in Figure 6a, the sustainable Bio-PCM layer is examined at three different locations on the facing sandwich panel wall inside the room, in the middle of the wall, and on the facing wall outside the portable cabins.

In Type1270 PCM model, a pure sustainable PCM is assumed, which means the melting/solidification process of the PCM layer occurs at constant temperature, and the solid/liquid-specific heats of the PCM are constant, as shown in Figure 6b.

Therefore, as shown in Figure 6b, the Type1270 Bio-PCM model uses the following assumptions:

• The specific heats of the Bio-PCM are constants but with different values for liquid and solid phases.
• The contact resistance between the PCM layer and the adjacent wall layers is neglected.
• The melting and solidification are constant temperature processes.

The temperature of PCM in solid state (${\mathrm{T}}_S$) is calculated from initial state ($T_I$) using the following [42]:

$${\mathrm{T}}_S=T_I+\frac{{\dot{q}}_1+{\dot{q}}_2}{m_{BioPCM}{Cp}_S}$$

(1)

where ${\dot{q}}_1$ and ${\dot{q}}_2$ are energy rates to/from the PCM layers by adjacent wall layers, $m_{BioPCM}$ is the mass of Bio-PCM and ${Cp}_S$ is the specific heat capacity of the Bio-PCM in the solid state.

The temperature of PCM in the liquid state (${\mathrm{T}}_L$) is calculated from the initial state ($T_I$) using the following:

$${\mathrm{T}}_L=T_I+\frac{{\dot{q}}_1+{\dot{q}}_2}{m_{BioPCM}{Cp}_L}$$

(2)

where ${Cp}_L$ is the specific heat capacity of the PCM in liquid state.

The energy performance index (EPI) is a sustainable suitable index for evaluating the overall performance of the portable cabin for heating, cooling, and total energy consumption. EPI determines annual heating and cooling consumption and is usually expressed by:

$$\mathrm{E}\mathrm{P}\mathrm{I}=\frac{\mathrm{A}\mathrm{n}\mathrm{n}\mathrm{u}\mathrm{a}\mathrm{l}\mathrm{E}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{y}\mathrm{d}\mathrm{e}\mathrm{m}\mathrm{a}\mathrm{n}\mathrm{d}}{\mathrm{F}\mathrm{l}\mathrm{o}\mathrm{o}\mathrm{r}\mathrm{A}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{o}\mathrm{f}\mathrm{R}\mathrm{o}\mathrm{o}\mathrm{m}}(\mathrm{k}\mathrm{W}\mathrm{h}/{\mathrm{m}}^2/\mathrm{y}\mathrm{e}\mathrm{a}\mathrm{r})$$

(3)

The EPIs for cooling and heating demands are separately calculated, and the total EPI is simply the sum of the two.

In TRNSYS, it is useful to determine the air infiltration which has an impact on the heating/cooling of the studied room. The air infiltration can be approximated based on temporal wind speed available from weather data using the following:

$$Infiltration(\mathrm{L}/\mathrm{h})=0.07V+0.4$$

(4)

The value of air infiltration varied from 0.5 to 3 L/h, the lower limit corresponds to more airtight buildings, and the higher limit corresponds to older buildings with less air tightness.

3. Experimental Results and Discussions

In order to investigate the sensitivity of the two portable cabins under some changes and also to create a data bank for validating software simulations, a similarity check was first conducted to ensure that both cabins behave in a thermally identical manner. Then, a common and a new sustainable cool roof methods are investigated that can be easily integrated in the architecture of the buildings in Kuwait. The main goal is however to validate the TRNSYS software for intensive study of sustainable Bio-PCMs in the Kuwaitis’ buildings.

3.1. Similarity Checks

A similarity check was conducted from 30 September to 9 October 2022 to assess the similarity of the thermal characteristics and AC power consumption of both rooms under similar conditions. The average difference between the two rooms was only 0.48% for temperature and 1.44% for humidity on 30 September 2022. The AC system was turned on from 4 to 9 October 2022 at the pre-set temperature of 22 °C for both cabins. The average energy consumption for the left room was 0.323 kWh, and for the right room, it was 0.318 kWh on 8 October 2022, indicating a difference of only 1.7% [51]. To analyze energy saving of various sustainable Bio-PCMs in different climates for the portable cabins, the TRNSYS version 18 models are developed here. For evaluating the accuracy of these sustainable models, TRNSYS simulation results must be validated against experimental data. TRNSYS uses the available weather data (the year 2020) for the site at Kuwait International Airport. The measurements of the energy consumption of the AC systems in the portable cabins were discontinuously conducted at the Australian University of Kuwait in October 2022. As shown in Figure 7, measurements were carried out during 4–15 October 2022 and during 23–31 October 2022. The AC cooling power measurement with an average of 314.6 W was 4.8% higher than the simulation results, which had a value of 299.5 W (see Figure 7a). In late October, as shown in Figure 7b, the AC cooling measurement with an average of 163 W was −8.2% lower than the simulation prediction of 176 W. Considering the cumulative readings, a total of 475.7 W was simulated from the two measured intervals, while a total of 477.4 W was measured. This gives merely a 0.37% cumulative error. Hence, the overall accuracy of TRNSYS simulation (as a sustainability tool) is exceptionally well for the cooling power measured here.

3.2. Application of Shingles as a Cool Roof

Shingled Roofs are sustainable roofs made from overlapping flat or curved pieces of various materials. Shingles are not recommended for low-slope or horizontal roofs and are usually recommended for sloped roofs. However, no report on the application of shingles on insulation walls in Kuwait is found in the literature, and most houses in Kuwait have horizontal roofs. Therefore, as shown in Figure 8a, the curved shingles ceramic roof tiles are mounted on the roof of the Right cabin from 6 to 20 November 2022. From 6 to 13 November 2022, the AC power was on, and the energy consumption of the cabin with shingles was examined. From 13 to 20 November 2022, the thermal characteristics of the two cabins were compared when the AC power was off.

From 6–13 November 2022 when air-conditioning was on, the power and energy consumption are compared between the left room (without shingles) and the right room (with shingles). For the studied period, the left cabin consumed 14.45 kWh (daily average of 1.81 kWh), while the right cabin consumed 14.86 kWh (daily average of 1.86 kWh), indicating a 2.8% increase in AC energy consumption with the use of shingles (see Figure 9). In all the days from 6 to 13 November 2022, the right cabin consumed more energy than the left cabin. Therefore, shingles are not suitable for a low-slopped roof for sustainable energy saving.

3.3. Application of a Finned Metal Cool Roof

A finned metal cool roof is manufactured from 3 mm aluminum sheets. A 45-degree fin angle is made from a 130 mm wide sheet bent at 30 mm and riveted to a base sheet (see Figure 8b). Hence, the width of the fins is 100 mm that were mounted at 90 mm distance from each other. Ten fins with the length of 1200 mm were riveted to the base sheet. The base was made from 4 square aluminum sheets of 1200 mm length and 3 mm thickness that nearly covered the 3000 mm × 3000 mm roof of the right cabin with a gap of 300 mm from each side (see Figure 8b). The measurement was performed from 21 to 28 November 2022 with AC turned off. Then, the AC was turned on from 28 November to 21 December 2022.

As shown in Figure 10, the energy consumption of the finned metal roof was higher in all days. The total energy consumption for the studied period (from 28 November to 21 December 2022) was 24.7 kWh for the left cabin and 26.3 kWh for the right cabin. This corresponds to the average value of 1.03 kWh and 1.1 kWh for the left and right cabins, respectively. Hence, the energy saving of −6.4% was observed. This means the finned metal roof consume 6.4% more energy than the normal sustainable roof.

4. Simulation Results and Discussions

TRNSYS models are developed as a sustainable tool to analyze the energy savings of various Bio-PCM in different climates for portable cabins. To validate the accuracy of these models, TRNSYS simulation results are compared against experimental data obtained from discontinuous measurements of AC energy consumption in portable cabins conducted at the Australian University of Kuwait in October 2022 [51,59]. The AC cooling power measurement showed a 4.8% higher average value than the simulation result in the first interval and an 8.2% lower average value in the second interval. However, considering the cumulative readings, the simulation result and the measured value were quite close, with a mere 0.37% cumulative error. Therefore, the overall accuracy of the TRNSYS simulation is considered exceptionally well for the cooling power measured here (see more details in [51,59]).

4.1. Energy Saving of Bio-PCMs in Kuwait

In Figure 11a, the heating savings of 12 sustainable Bio-PCMs in three different locations within the wall layer in Kuwait are shown. The Bio-PCM models were evaluated using TRNSYS for the weather of Kuwait International Airport. The best heating savings were obtained when the Bio-PCM layer was located in the middle of the wall. The Bio-PCM type 901.2391 provided the highest sustainable energy saving of 54.43%. On average, all 12 Bio-PCMs provided 48.25% energy savings when placed in the middle, reducing to 40.97% and 40.45% when used inside and outside cabin walls, respectively.

Figure 11b shows the annual cooling savings for 12 sustainable Bio-PCM types in portable cabins. The results indicate that the highest cooling energy savings can be achieved when the Bio-PCM layer is located outside the cabin walls. Bio-PCM type 901.2591 provides the highest saving of 28.95% when located outside the wall, compared to 24.09% and 15.92% when located in the middle or inside the cabin walls, respectively. On average, all Bio-PCMs perform better when located outside the cabin wall, with an average saving of 28.76%. Cooling demands are the main building sustainable energy consumption in Kuwait, and no heating system is practically used.

Figure 11c shows the annual total energy saving (heating and cooling) of sustainable Bio-PCMs for the portable cabins in Kuwait. The Bio-PCM type 901.2391 performs slightly better compared with the rest of the Bio-PCMs when it is installed on the outer cabin walls and roof, with a total saving of 30.22%. This reduces to 26.46% and 18.85% if the Bio-PCM is installed in the middle or inside the of the cabin walls and roof, respectively. On average, all the Bio-PCMs provide total energy savings of 29.72%, 25.83%, and 17.67% if located on the outside, middle, and inside of the cabin, respectively.

Figure 12 shows the performance of the optimal Bio-PCM type 901.2391 at the optimal location, i.e., outside the cabin walls and roof. It can save 46.6% annually on heating, 28.7% on cooling, and 30.2% on total energy consumption in Kuwait. With Bio-PCMs, the sustainable energy performance index (EPI) decreases from 38.3 to 20.5 kWh/m²/year for heating, from 424.6 to 302.6 kWh/m²/year for cooling, and from 462.9 to 323 kWh/m²/year for total energy consumption. Compared to conventional materials like AAC, the sandwich panel with Bio-PCMs saves 70% energy in Kuwait.

4.2. Energy Saving of Bio-PCMs in Australia

In TRNSYS simulations for Australia, the west wall exhibited rapid and high temperature variations in the Bio-PCM layer, which exceeded the limits predefined by the manufacturer. To remedy this, the Bio-PCM type 901.2391 was identified as the most stable material and used for the west wall. For all selected sustainable cities in different climates of Australia, the Bio-PCM attached to the outside wall performed better; hence, the results presented below are for Bio-PCMs installed outside.

Figure 13 shows the overall performance of the optimal Bio-PCMs for the portable cabin in the equatorial climate in Australia (Darwin). Results for Darwin in Figure 13 indicate that no saving in heating was obtained. In contrast, the best saving of 25.4% was achieved for the cooling and the total energy consumption when Bio-PCM type 901.2391 was installed at the optimal location (outside the walls and roof) of the portable cabin. As shown in Figure 13 using Bio-PCM, the energy performance index (EPI) in sustainable Darwin is not changed for heating but reduced to 309.1 kWh/m²/year for cooling and to 309.2 kWh/m²/year for total energy consumption.

In the tropical city of Normanton in Australia, as shown in Figure 14, the overall performance of the optimal Bio-PCMs is presented by an energy saving of 27% and 18% for cooling/total and heating, respectively. Bio-PCM type 901.2991 performed slightly better than the other Bio-PCMs. The energy performance index (EPI) in Normanton was found to be 0.05, 284.1, and 284.2 kWh/m²/year for heating, cooling, and total energy, respectively. For all studied types of Bio-PCMs, the heating saving is identical (18%). The average total sustainable energy saving is 14.53, 21.41, and 26.83% for Bio-PCMs fixed on the inside, middle, and outside of the walls, respectively, with the optimal location being on the outside surfaces.

Rockhampton is located in Queensland in eastern Australia in a subtropical climate. Application of Bio-PCM type 901.2991 on the outside walls has led to nearly 100% saving on heating, 30.7% on cooling, and 30.9% on total energy consumption (see Figure 15). The sustainable energy performance index (EPI) using Bio-PCM in Rockhampton is reduced to 0.02, 186.12, and 186.14 kWh/m²/year for heating, cooling, and total energy, respectively.

Meekatharra is a desert in the west of Australia. The Bio-PCM type 901.2591 performs better here as the optimal PCM. Heating, cooling, and total savings of 95.7, 38.3, and 39.5% are obtained, as shown in Figure 16. The energy performance index (EPI) in the sustainable city of Meekatharra is reduced by 0.28, 178.27, and 178.55 kWh/m²/year for heating, cooling, and total energy, respectively.

In the grassland area of Alic Springs in central Australia, the Bio-PCMs have performed better compared with previous climates. A total energy saving of 44.1% is observed, as depicted in Figure 17. The optimal Bio-PCM type 901.2391 performed slightly better than other PCMs. The sustainable energy performance index (EPI) is reduced from 298.4 to 166.9 kWh/m²/year for the total energy consumption.

Sydney is located in the temperate climate of the southeast of Australia. So far, Bio-PCMs perform best in this climate, with a total energy saving of 46.5% (see Figure 18). The optimal Bio-PCM type 901.2991 performed better than the other Bio-PCMs. The sustainable energy performance index (EPI) reduced from 157.18 to 84.11 kWh/m²/year for the total energy consumption in Sydney.

5. Conclusions

This research focused on the sustainable energy efficiency of buildings in different climates of Kuwait and Australia using two portable cabins constructed with insulated sandwich panels. The present research investigated the thermal characteristics and energy consumption of the AC systems in the cabins, with one cabin featuring two sustainable cool roof techniques, shingles, and a finned metal roof, which were tested separately in November and December 2022. A TRNSYS model was developed to evaluate the portable cabins with and without sustainable Bio-PCMs, with twelve built-in Bio-PCMs assessed at three test locations. The importance of using insulation materials is highlighted for the development of low-energy buildings in Kuwait, despite current government regulations prohibiting their use for fire safety reasons.

• The experimental results show that the use of sustainable cool roofs has altered the thermal characteristics of the cabin, with the cabin fitted with these cool roofs being cooler than the base cabin, although shingles increased AC energy consumption by 2.8%, and the finned metal roof raised it by 6.4%.
• The TRNSYS model was validated against AC power measurements in the portable cabin with a cumulative error value of only 0.37%.
• The study assesses the desert climate of Kuwait and six different climate regions in Australia for utilizing sustainable Bio-PCMs. By placing the Bio-PCM layer outside the cabin walls, 30% of the total (heating/cooling) energy can be saved in Kuwait.
• In Australia, the sustainable city of Sydney, with a temperate climate, can save 46.5% as the highest total energy saving, while the sustainable city of Darwin, with an equatorial climate, can save 25.4% as the lowest total energy saving.

It is usually recommended to use metal roofs with reflective paints. This and other sustainable remedies may be tried for this cool roof in summertime. The future work includes using solar reflective paints on the roof and walls and applying cool roof paints for the finned metal roof. Long-term experimental work on sustainable PCM performance studies and techno-economic analysis will be conducted in a continuation of the present work. Additionally, TRNSYS models may be extended to simulate sustainable portable cabins with cool roofs and various PCM models.