Improving the energy efficiency of a refrigerated warehouse through the use of palm tree pruning was

Abstract

Keywords: Palm Tree Pruning Waste; Thermal Insulation; Refrigerated Warehouse; Energy Efficiency; Dynamic Thermal Simulation

Introduction

Thermal insulation is an important technology for reducing energy consumption in buildings by preventing heat gain/loss through the building envelope. There are many different insulation materials such as, polystyrene,

polyurethane, fiberglass, mineral wool, foam, expanded pure agglomerated cork, wood-wool, cellulose, etc. [1–5]. Table 1 summarizes some advantages and disadvantages of the common types of thermal insulators. Because of

*Corresponding author. *E-mail address: h.lakrafli@usms.ma This paper was recommended for publication in revised form by Regional Editor Erdal Çetkin Published by Yıldız Technical University Press, İstanbul, Turkey Copyright 2021, Yıldız Technical University. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).

their advantages such as low cost, abundance, recyclability, availability, no toxicity, renewable and eco-friendly features, natural fibers can replace the synthetic materials and its related products for the less mass and energy conservation applications [2–9]. Several research works have been developed to highlight the thermal insulation capacity of various materials: La Gennusa et al. [10] have studied the insulating properties of some materials composited from waste or vegetal products. Gounni et al. [11] have developed new thermal insulation material made from textile waste based on acrylic and wool using needle punching method. Philip and Rakendu [12] have investigated the use of water hyacinth-based panels as a raw material for the production of thermal insulation materials. Lertsutthiwong et al. [13] have developed new composite panels, with low thermal conductivity, produced from a mixture of solid waste from tissue paper manufacturing and corn peel. Manohar [14] has investigated the potential of biodegradable coconut and sugarcane fiber for use as thermal insulators in building. The thermodynamic properties of tropical wood were studied by Ngohe-Ekam et al. [15] and correlated with the basic density of material. In another works, the thermal properties of leather and carpentry wastes were investigated and the applicability of these materials as insulators was highlighted [16–18]. Other studies were conducted using straw [19], matter material derived from recycled bottle [20], textile sub waste [21], rubber waste particles [22], recycled cellulose fibers [23], duvet waste notably polyester and duck feathers [24], composite material based on date palm leaflets and expanded polystyrene wastes [25], etc. With the increasing concerns with the quality and the safety of foods, several standards and guidelines have been developed to improve the design, construction, and operation of storage warehouses [26]. Historically, the use of cold through freezing or refrigeration has been one of the most common methods for food preservation over long periods. In this context, refrigerated warehouses are used to store fruit, vegetables, and food because they keep their freshness and flavor. They help in eliminating sprouting, rotting and insect damage. Cold storage also helps stabilize market prices and evenly distribute goods both on demand basis and time basis. Cold storage facilities have one of the highest electrical energy consumption rates in the commercial building sector. One of the factors causing variation in the temperature of foods and other products is the heat transmitted in or out through the walls of the refrigerated warehouse by the process of conduction and radiation. This problem can be avoided using thermal insulation which creates a barrier that prevents the transmission of energy and maintains the interior at a suitable temperature. The most insulating materials used in cold storage installations are cork, expanded polystyrene sheeting, sandwich panels with a polyurethane or polyisocyanurate insulation core, etc.

The main objective of this research work is to analyze the potential of using a natural agro-waste material, namely palm tree pruning waste (PTPW) as alternative thermal insulation material for refrigerated warehouses. Thermal properties of PTPW were first highlighted and a dynamic thermal simulation (DTS) tool was then used to predict the thermal behavior of a model cold installation.

Materials And Methods

Materials The agro-waste material used in this study is palm tree pruning waste (PTPW) which is a renewable resource. The abundance of this material presents an opportunity to develop green building materials at low cost. Palm trees become heavier as they grow, this can cause them to break or crack. This is why it is essential to prune cut them periodically. This operation generates a large amount of fibrous wastes. PTPW tested in this work are from gardening wastes of the city of El Jadida (Morocco). The material was first dried in an oven at 60°C for 24 h and then grounded to obtain particle size less than 5 mm. The density of palm bark fibers is around 0.064 g/cm3. Their chemical composition is as follow: Carbon 38.79%, Hydrogen 5.12%, Nitrogen 0.28%, and Sulphur 0.22%. The contents of cellulose, hemicellulose, and lignin in PTPW are 47.62%, 18.00% and 16.38%, respectively (Table 2) [27]. Scanning electron microscopy (SEM) micrograph (Fig. 1) of crushed PTPW shows a porous structure with an irregular shape. Thermal Conductivity Measurement Thermal conductivity is a property of the material. It expresses the extent to which a material conducts heat in stable conditions. In this work, thermal conductivity measurements were conducted by the box method (Fig. 2) [17,18] based on the use of an isothermal enclose with a heat exchanger at its base, which contains water-glycol fluids maintained at low temperature by a cryostat. The device has two boxes provided with a heater and there are two environments above and below the sample. Each box has an open face and contains on the interior of its superior face an electrical resistance heating R. The interior of boxes plays the role of the hot atmosphere. The material (Length 27 cm × Width 27 cm × Height 4 cm) to be tested is placed (Fig. 2) in such a way that side flows are negligible. When the permanent regime is established, the expression of thermal conductivity is deduced as follows: l=

Table 1. Some advantages and disadvantages of thermal insulating materials [2–5] Material

– Non-flammable, resistant to moisture damage, easy to install, and effectively blocks heat flow.

Results And Discussion

Thermal Conductivity of PTPW Because of its divided form, palm tree pruning waste was placed between two plaster panels (Length 27 cm × Width 27 cm × Height 1.2 cm) to give a multilayer material P/PTPW/P (Length 27 cm × Width 27 cm × Height 4 cm).

Zones 1→6: Cold rooms (Area of each room: 180 m2; Height: 7.8 m); Zone 7: Corridor (Area: 540 m2; Height: 5 m); Zone 8: Loading dock for trucks (Area: 36 m2; Height: 5 m) Figure 4. Different areas of the refrigerated warehouse and their dimensions.

Table 3. Refrigerated warehouse envelope characteristics. Component

Separate panels were used so that heat cannot be transferred easily because there is no connection between their parallel surfaces. In order to obtain layers of different densities, the available volume between plaster panels (1166.4 cm3) was filled by various amounts of PTPW ranging from 0 to 0.064 g/cm3. It should be noted that if the mass of PTPW between the two plasterboards is less than 0.04 g/cm3, the interior volume which is (27 × 27 × 1.6 cm3) will not be completely occupied by PTPW. This can cause heat transfer in a heterogeneous medium. The thermal conductivity and the thermal resistance of the composite specimens P/PTPW/P were first measured using the box method and then the values of the parameters corresponding to PTPW were theoretically deduced. The effect of PTPW material is well revealed from the results shown in Fig. 5 and Fig. 6. The first thing we can see (Fig. 5) is that, in absence of PTPW (ratio: 0 g/cm3), the thermal conductivity of specimen is relatively higher

than that of plaster panels separated by PTPW because the mobile air when it is used alone and subjected to convection, it is not a good thermal insulator. For this reason, insulator materials are used to stabilize air and improve consequently the effect of insulation. Obtained results highlight that heat transfer decreases considerably when PTPW are used to separate plaster panels. As can be seen (Fig. 5), the thermal conductivity of the composite specimen P/PTPW/P decreases from 0.127 W/m.°C to 0.084 W/m.°C when increasing the ratio “PTPW mass/available volume between plasterboards” from 0 to 0.064 g/cm3. However, the thermal resistance which is inversely proportional to the thermal conductivity increases from 0.315 to 0.476 m2.°C/W. The thermal properties of the system “plasterboard/ PTPW/plasterboard”, considered as multilayer composite material, were used to deduce those of PTPW material. The thermal conductivity of plasterboard was first measured, it is about 0.320 W/m.°C. Obtained results (Fig. 6) highlight

Figure 5. Thermal conductivity and thermal resistance of multilayer composite material as a function of “PTPW mass / volume” ratio.

Figure 6. Thermal conductivity and thermal resistance of PTPW placed between panels.

Table 4. Comparison of the thermal conductivity of PTPW with that of some unconventional and conventional insulation materials. Materials

that the thermal properties of PTPW depend strongly of its compactness, i.e. how much it weighs divided by how much space it takes up. For example, λ of the volume limited by plasterboards decreases from 0.220 to 0.069 W/m.°C when increasing the mass of PTPW from 0 to 0.064 per cm3. However, the thermal resistance increases from 0.073 to 0.232 m2.°C/W. A comparison of the thermal conductivity of PTPW with that of some materials discussed in the literature [28?30] is shown in Table 4. As it can be seen, λPTPW is comparable or lower than that of other natural materials. Generally,

we notice that PTPW can compete with several insulation materials. Thermal Simulation Study In the refrigerated warehouse, the insulation of the cold rooms, the corridor and the loading dock for trucks is ensured by means of polyurethane foam sandwich panels which are composed of two sheets of metal and a rigid core in between. The rigid core has a polyurethane material known for its thermal insulation properties. Polyurethane foam sandwich panels are widely used in industrial

Figure 7. Variation of outside temperature and that of the zones 4 and 7 during the (a) coldest week and the (b) hottest week.

Figure 8. Variation of temperatures of zones 4 (a) and 7 (b), during the very hottest days of summer, considering polyurethane and different thicknesses of PTPW as insulators.

buildings, cold storages, warehouses, office buildings and other commercial units. The objective of this part is the simulation of the thermal behavior of the building in the case where polyurethane or PTPW is employed. The energy efficiency of the considered refrigerated warehouse will be evaluated in both cases. A preliminary test was performed by studying, at first, the effect of an insulation layer of polyurethane (λ = 0.028 W/m.°C, ρ = 40 kg/m3, e = 10 cm). It was found that variations in temperature are generally dependent of the defined area. This trend is especially more pronounced in cold rooms and corridor during the cold and hottest periods. Fig. 7 shows the measured air temperatures in these areas (zone 4 as example and zone 7) compared with the external air temperatures recorded from January 12 to 19 (the coldest week of winter season) and from July 24 to 31 (the hottest week of summer season). During the coldest (2.5–18.5°C) and hottest (24.5–44.6°C) weeks, all internal

temperature values recorded in zone 7 are lower than those in zone 4. The difference is attributed to the orientation of the two zones. Fig. 8 shows the variations in temperature of zones 4 and 7 of the refrigerated warehouse during the typical 48 h of summer (29 and 30 July) when considering various thicknesses of PTPW (10, 30, 40 and 75 cm) instead of polyurethane. Results show that there is an obvious effect on the temperature which is strongly dependent on the thickness of the PTPW applied layer. For example, by increasing the thickness of PTPW from 10 to 75 cm the highest temperatures recorded in zones 4 and 7 decrease from 35.3°C to 16.3°C (Fig. 8a) and from 33.9°C to 13.6°C (Fig. 8b), respectively. In order to highlight the optimal thickness of PTPW layer recommended to minimize heat transfer as much as possible, the effect of this biomaterial was compared with that of a 10 cm thick polyurethane layer with which the

Figure 9. The monthly energy loads of the refrigerated warehouse when PTPW (30 cm) or polyurethane (10 cm) is considered.

0.069 W/m.°C when it is compacted with a density of 0.064 g/ cm3. The simulation results, obtained by the use of TRNSYS program, show that PTPW material can acts as a thermal barrier for refrigerated warehouse (RW) envelopes. It can reduce energy consumption and therefore the expenditures associated with it. The effect on the temperature of RW areas is strongly dependent on the thickness of the PTPW applied layer. Compared to 10 cm thick polyurethane, a decrease in temperature during the very hottest days was highlighted essentially by the use of 30 cm thick PTPW. Likewise, a significant reduction in the annual unit energy consumption has been achieved when considering this thickness. So, PTPW material can compete with conventional insulating materials and can provide multiple applications in particular to decrease energy consumption and costs in space cooling.

Nomeclature

thermal insulation of the refrigerated warehouse is normally ensured. The comparison between simulation results (in zones 4 and 7) during the very hottest days of summer is presented in Fig. 8. Compared to the control state (insulation with 10 cm of polyurethane), a considerable improvement in the temperature of zones 4 and 7 was revealed essentially with the thickness 30 cm. The temperature values recorded in this case are 1 to 2°C lower than those reached with the use of polyurethane. Energy Consumption Reducing energy consumption is one of the main challenges in most countries. It is one of the economic necessities and of the main sustainability trends among many sectors. Because of its location in a hot semi-arid climate, the refrigerated warehouse studied must be kept at a temperature suitable for storing food. The thermal conditions which must be imposed are as follows: 0 to 15°C for cold rooms and 15 to 25°C for corridor and loading dock. Fig. 9 shows the monthly energy loads of the refrigerated warehouse for PTPW (30 cm) and polyurethane (10 cm). From the results obtained, it turned out that PTPW (30 cm) compared to polyurethane (10 cm) can reduce energy requirements whether during all seasons of the year mainly in the hottest period. For example, the energy loads in July can decrease by 25.7% when PTPW (30 cm) is used. The annual unit energy loads are 22.5 kWh/m² and 18.1 kWh/m² when polyurethane (10 cm) and PTPW (30 cm) were considered, respectively. This corresponds to a reduction of 19.6% in the annual unit energy consumption.

thermal conductivity (W/m.°C), thermal resistance (m2.°C/W), coefficient of thermal loss from the box (0.16 W/ºC), heat flux penetrating through the composite specimen (W), e thickness of the sample (m), h1 individual heat transfer coefficient for the hot fluid (W/m2.°C), h2 individual heat transfer coefficient for the cold fluid (W/m2.°C), q heat flux emitted by joule effect (W), U voltage (V) R Resistance of the heater element (Ω), S sample area (cm2), Ta room temperature experiments (°C), Tb inside temperature of box (°C), Tc temperature of cold surface of sample (°C), Text surface temperature of air in contact with external surface of the material (°C), Th temperature of hot surface of sample (°C), Tint surface temperature of air in contact with internal surface of the material (°C), ρ density (kg/m3), DTS dynamic thermal simulation, P plaster panels, PTPW palm tree pruning waste, RW refrigerated warehouse, SEM Scanning electron microscopy.

Conclusion

Palm tree pruning waste (PTPW), which is a natural material known by its abundance (low price), has good thermophysical properties. It has a low thermal conductivity of about

Data Availability Statement

The authors confirm that the data that supports the findings of this study are available within the article. Raw

data that support the finding of this study are available from the corresponding author, upon reasonable request.

Conflict Of Interest

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Ethics

There are no ethical issues with the publication of this manuscript.

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