Experimental Investigation of a Direct Evaporative Cooling System for Year-Round Thermal Management with Solar- Assisted Dryer

Building cooling is achieved by the extensive use of air conditioners. These mechanically driven devices provide thermal comfort by deteriorating the environment with increased energy consumption. To alleviate environmental degradation, the need for energyefficient and eco-friendly systems for building cooling becomes essential. Evaporative cooling, a typical passive cooling technique, could meet the energy demand and global climatic issues. In conventional direct evaporative cooling, the sensible cooling of air is achieved by continuous water circulation over the cooling pad. Despite its simple operation, the problem of the pad material and water stagnation in the sump limits its usage. Moreover, the continuous pump operation increases the electrical energy consumption. In the present work, a porous material is used as the water storage medium eliminating the pump and sump. An experimental investigation is performed on the developed setup, and experiments are conducted for three different RH conditions (low, medium, and high) to assess the porous material’s ability as a cooling medium. Cooling capacity, effectiveness, and water evaporation rate are determined to evaluate the direct evaporative cooling system’s performance. The material that replaces the pump and sump is vermicompost due to its excellent water retention characteristics. There is no necessity to change material each time. However, the vermicompost is regenerated at the end of the experiment using a solar dryer. The passing of hot air over the vermicompost also avoids mould spores’ transmission, if any, present through the air. The results show that vermicompost produces an average temperature drop of 9.5°C during low RH conditions. Besides, vermicompost helps with the energy savings of 21.7% by eliminating the pump. Hence, vermicompost could be an alternate energy-efficient material to replace the pad-pump-sump of the conventional evaporative cooling system. Further, if this direct evaporative cooling system is integrated with solar-assisted drying of vermicompost, it is possible to provide a clean and sustainable indoor environment. This system could pave the way for year-round thermal management of building cooling applications with environmental safety.


Introduction
The rapid population growth has attributed to the drastic rise in energy consumption. The critical factor driving energy consumption is the weather effect, which leads to an increase in cooling and heating requirements to provide thermal comfort. Air conditioners play a vital role in providing thermal comfort to the occupants of the building. The continuous operation of these devices increased greenhouse gas emissions. The rise in emissions directly related to energy demand poses a severe threat to the environment. In the current scenario, sustainable and energy-efficient technologies would be convincing to provide a clean and safe ecosystem. Since ancient times, passive cooling techniques have provided comfortable living space without any emissions. Moreover, the use of renewable energy sources could reduce energy demand. Hence, passive cooling integrated with renewable energy sources could be the best alternative to meet conventional mechanically driven equipment's challenges.
Evaporative cooling, a passive cooling method, had been in use since 2500 B.C. An evaporative cooler makes use of water to cool the hot and dry air. The water takes up the air's  Costelloe and Finn [11] Indirect contact evaporative cooling system Office buildings

Dublin and Milan Temperate
As a consequence of 16°C cooling water from October to May, the office buildings in Dublin can be cooled throughout the year without the use of conventional cooling. In Milan, a similar performance can be achieved from November to March Heidarinejad et al. [12] Two-stage indirect-direct evaporative cooling system Public buildings Tehran Multi Two-stage indirect-direct evaporative cooling system can be preferred to mechanical vapour compression systems in regions with higher wetbulb temperatures to minimise energy consumption.
However, water consumption of the two-stage system is 55% greater than that of traditional DEC Ibrahim et al. [13] Direct evaporative cooling system equipped with porous ceramic evaporators Residential buildings Nottingham Multi Direct evaporative cooling system supported with porous ceramic evaporators can provide 6-8°C in dry-bulb temperature with a 30% rise in relative humidity. The maximum cooling achieved from the system is 224 W/m 2 Hajidavalloo [14] Indirect contact evaporative cooling system

Residential buildings
Khoozestan, Iran Very hot Through this system, power consumption can be decreased by about 16% in very hot climates, and the COP can be enhanced by 55% compared to conventional air conditioners He and Hoyano [15] Passive evaporative cooling wall (PECW) Residential courtyards Japan Hot and dry PECW constructed as ceramic pipes absorb water using capillary action. It can reduce the surface temperature of the pipe by 3-5°C than the ambient, and the ambient air temperature is dropped to 3-4°C Katsuki et al. [16] Porous ceramic plates -Korea Hot and dry The study reports that porous properties and relative humidity of air are the key factors influencing the self-cooling effect of porous ceramic plates 2 International Journal of Photoenergy sensible heat and gets evaporated to produce cold and humid air. It takes shape in several forms, and the modern evaporative coolers consist of a pad-pump system with a reservoir to recirculate the water. The working principle of a direct evaporative cooling system is shown in Figure 1. Evaporative cooling pads are generally made of cellulose materials. These materials are used in desert air coolers due to their excellent cooling performance with high durability. However, its high cost limits its utility to higher-end coolers. Cellulose impregnated cooling pads were employed for different studies. Rong et al. [2] studied the dynamic performance of a cross-fluted cellulose pad by controlling the pump's on and off time (control time cycle), water flow rate, etc. A relationship between cooling efficiency and the waterair ratio was one of the useful parameters for the controller. This prediction model for transient conditions provides the solution for poultry houses' control systems in a hot-arid climate. Xu et al. [3] developed an evaporative cooling setup with corrugated cellulose pads to provide thermal comfort for greenhouses in a humid subtropical climate. Evaporative coolers made of cellulose pads were combined with air conditioners and tested by Harby and Al-Amri [4] and Dhamneya et al. [5] to study their performance in achieving energy savings of a split air conditioner and window air conditioner, respectively. Aspen cooling pads were preferred in some air coolers due to their lower cost and effectiveness. Bishoyi and Sudhakar [6] compared the performance of aspen and honeycomb cooling pads. The results had shown that the honeycomb cooling pad achieved the highest effectiveness compared with the aspen cooling pad.
Many natural and synthetic fibres and other materials were also tested for their suitability to be used as a cooling pad material. Al-Sulaiman [7] proposed an experimental setup to evaluate the cooling pads' performance made from the fibres of date palm (stem), luffa, and jute. The results were compared with a commercially available cooling pad. The results showed that the highest effectiveness of 62.1% was obtained for jute material, while the conventional cooling pad material gave 49.9% effectiveness. Doğramacı et al. [8] used eucalyptus fibres as cooling pad material and showed that these fibres perform better in cooling efficiency at lower velocities. Jain and Hindoliya [9] tested the effectiveness of a cooling pad with coconut and palash fibres and compared them with a conventional pad made of aspen and khus fibres. The palash fibres offered a low pressure drop than the aspen pad, and its effectiveness was comparatively higher.
Many researchers tested different evaporative cooling systems for their local climatic conditions. Cuce and Riffat [10] presented the types of evaporative cooling systems, thermodynamic analyses, thermal performance assessment, and their applications in buildings. The different types of direct and indirect evaporative cooling are employed in the cooling 3 International Journal of Photoenergy system based on humidity conditions. Based on the building's cooling load requirement, novel evaporative cooling systems were tested for different climatic conditions and are presented in Table 1.
From the extensive literature, it is understood that evaporative cooling, which is quite simple and inexpensive, is suitable for hot and dry climates. However, the pad material is subjected to problems like sagging, clogging, and scaling. Watt [17] had addressed these problems, which led to the deterioration of pad material and reduced its life. The continuous operation of the pump increases the electrical energy consumption. The reservoir, which stores the water for recirculation, provides a favourable environment for breeding disease-spreading insects. In the present work, a porous material-vermicompost is used to replace the pad, pump, and reservoir. Vermicompost is highly porous with high water storing capability [18]. The specific objective of the system is to study the performance of vermicompost based     International Journal of Photoenergy direct evaporative cooling system with three different velocities (0.9 ms -1 , 1.8 ms -1 , and 2.7 ms -1 ) under different relative humidity conditions of ambient air (high, medium, and low). The water retention ability of vermicompost is evaluated in terms of evaporative cooling capacity, effectiveness, and water evaporation rate. Thus, vermicompost's cooling potential for use in direct evaporative cooling to provide a sustainable and clean indoor environment for year-round thermal management is explored in this work.

Materials and Methodology
2.1. Vermicompost: An Alternative Material for Direct Evaporative Cooling. Vermicompost, a porous material with high water storage capacity, is extensively used in agriculture to promote plant growth during water stress conditions. Many scientists had studied the performance of evaporative cooling systems with porous materials. However, vermicompost, which possesses high porosity and high water retaining ability, is not explored yet, as a cooling medium in the evaporative cooling system. The problems of continuous pump operation and water stagnation in the sump are to be eliminated using vermicompost as a water storage medium. Vermicompost loaded in the system can be used again, and there is no necessity to change material each time. However, the vermicompost should be regenerated at the end of each cycle to sterilize the material from the mould growth. Vermicompost drying is achieved using an indirect solar dryer. The hot air from the solar collector dries the vermicompost.

Site Selection.
The outstanding performance of evaporative coolers in arid regions facilitated to test the vermicompost as a cooling medium in a direct evaporative cooling system at Vellore, an Indian city with hot and dry weather. The yearly average temperature history of Vellore from 2009 to 2019 is shown in Figure 2. The average temperature throughout the year is 30°C ± 5°C. The maximum temperature of 42°C and minimum temperature of 19°C are almost reached every year. This temperature history indicates that Vellore weather conditions have great potential for evaporative cooling throughout the year.

Experimental Investigation.
This section describes the experimental setup and the operational procedures adopted to evaluate the performance of the vermicompost. Figure 3 shows a vermicompost-based evaporative cooling system integrated with an indirect solar dryer. The experimental setup consists of a cooling chamber tray with a baffle arrangement, air blower, buffer tank of 0.5-liter capacity to supply water, the monitoring sensors, and instruments with a data acquisition system. The cooling chamber tray is made of a rectangular box of 400 × 300 × 100 mm for charging the vermicompost. The tray consists of five baffle plates with perforations at the end to create turbulence in the flowing air. The photographic view of the cooling chamber loaded with vermicompost is shown in Figure 4. A centrifugal blower of 180 W capacity forces the air into the cooling chamber through a galvanized iron pipe of 25.4 mm diameter. A flow control valve is provided after the blower for varying the velocity of air. Water is supplied to the vermicompost through small tubes of 4 mm diameter. An insulation material (polyurethane foam) of 5 mm thickness is provided over the entire setup to avoid heat entry from the ambient to the cooling system. The vermicompost in the cooling chamber is dried with a solar dryer. The solar dryer consists of a solar collector to receive the radiation. It has an aluminium absorber plate, glass cover, plywood, and insulation material. Figure 5 shows the photographic view of the solar collector. Asbestos is used as the insulation material at the bottom of the absorber. The absorber plate is coated with black paint to enhance solar absorption. The absorber area is 0.4 m 2 . A chimney of 300 mm height is used for discharging the air after drying the vermicompost in the chamber. Temperature and relative humidity sensors (DHT 22) are placed at three locations of the setup. DHT 22 uses a capacitive-type sensor for humidity measurement and a thermistor for temperature measurement. The sensor (TRHS1) used for measuring the ambient conditions is kept exposed to the atmosphere on the outer surface of the setup. TRHS2 placed at 30 mm before the entry of the chamber and TRHS3 located 30 mm after the exit of the chamber measure the temperature and RH of air entering and leaving the chamber, respectively. The measurements are made continuously at a scanning rate of 1 minute using a data acquisition system (Agilent Keysight, Model no. 34972A). The details of the instruments used for measurement are presented in Table 2.  The parameters measured directly using the instruments are used to obtain the derived parameters of interest. Table 3 presents the uncertainties of the parameters which were measured directly using instruments and the evaluated parameters using the rules of error propagation suggested by Taylor [20].

Experimental
Procedure. Cattle manure-based vermicompost was dried before its use in the cooling system. A known quantity of vermicompost (1000 grams) was measured using the load cell. The material was loaded in the cooling chamber. Initially, trial experiments were conducted for various water quantities to find the vermicompost's maxi-mum water holding capacity. It was found that with the vermicompost : water mass ratio of 100 : 75, the material attained fully saturated condition. Then, water was supplied to the vermicompost in the above ratio until its saturation. No additional water was provided during the operation of the system. The blower was switched on to allow the air to pass over the vermicompost, and the flow control valve was adjusted to set the desired velocity of 0.9 ms -1 . Usually, direct evaporative coolers are not operated with air velocity exceeding 3 ms -1 to avoid aerosols' formation [21]. The temperature and relative humidity were measured by the sensors placed at different locations and were recorded continuously. The same procedure was repeated for other velocities of 1.8 ms -1 and 2.7 ms -1 . The same material can be used again, and there is no necessity to change material each time. However, the vermicompost is regenerated at the end of the experiment using a solar dryer. The passing of hot air over the vermicompost also avoids mould spores' transmission, if any, present through the air. The experiments were conducted during October 2019, February 2020, and May 2020 for high RH, medium RH, and low RH conditions. Direct evaporative coolers are employed in places with hot and dry weather to provide cooling during peak sunshine hours (11 a.m. to 3 p.m.). In Vellore, where the experiments were conducted, three distinct seasons could be observed in a year (peak summer, winter, and monsoon season). Hence, three months were selected that represent the above three seasons. Accordingly, it is found that in September, the RH value is high in the range of 60% to 75%; in February, the RH is in the midrange of 45% to 60%; and in May, the RH value is low in the range of 30% to 45%. The duration of the experiments was 180 minutes, 360 minutes, and 480 minutes for high, medium, and low RH conditions, respectively. Though all the experiments were started at 10 a.m., the experiments were terminated when there was an appreciable change in ambient RH during certain months. However, it was continued till 6 p.m. in May (peak summer month) when the low ambient RH existed till late evening. Since the end of the experiment varied in different months, the experiments' duration also varied. Figure 6 shows the detailed methodology of the experimental analysis in a flowchart form.

Data Analysis
This section explains the formulae used for calculating the following derived parameters: evaporative cooling capacity, effectiveness, and evaporation rate of water.
3.1. Evaporative Cooling Capacity. The evaporative cooling capacity is calculated using the following equation: 3.2. Effectiveness. Effectiveness is one of the key factors in evaluating the evaporative cooler performance. The ratio of temperature difference of inlet air and outlet air to the temperature difference of inlet air and its wet-bulb temperature is called effectiveness.
3.3. Evaporation Rate of Water. The amount of water required to produce the cooling effect is calculated using the formula given by

Results and Discussion
In this section, the variations of temperature and relative humidity of air are initially presented, followed by the discussion of derived parameters such as evaporative cooling capacity, effectiveness, and evaporation rate of water. Subsequently, the temperature drop and effectiveness of the previous studies are compared with the present system.

Variations in Temperature and
Relative Humidity of the Air. Figure 7 shows the temperature and relative humidity of ambient air and outlet air from the cooling chamber when the air was supplied at three different velocities for high RH conditions. It is seen from the figures that the ambient conditions play a significant role in deciding the performance of the cooling system. In Figure 7(a), the fluctuations of the temperature and relative humidity of ambient air were quite small except for a short duration. A sudden change in ambient conditions was observed after 150 minutes, as shown in Figure 7(b), and 100 minutes, as shown in Figure 7(c). However, the average drop in temperature was 4°C to 5°C for all velocities up to 150 minutes. The outlet relative humidity is 99%, as the ambient relative humidity varied from 60% to 75%. It is also found from the figures that when the system is operated at lower velocities, maximum temperature drop was achieved with slight fluctuations. Moreover, the time taken to attain this condition is large. When the system was operated at higher velocities, the maximum temperature drop was achieved within a few minutes, and the same drop was sustained for a more extended period. Figure 8 shows the temperature and relative humidity of ambient air and outlet air from the cooling chamber when the air was supplied at three different air velocities for medium RH conditions. It is seen in Figure 8(a) that the system produced an average temperature drop of 5°C for 60 minutes for a velocity of 0.9 ms -1 . After this, there was a continuous decrease in the temperature drop until the end of the experiment. For the operational velocity of 1.8 ms -1 , it is observed in Figure 8(b) that an average temperature drop of 7°C was produced until 150 minutes, followed by a gradual decrease in the temperature drop. Figure 8(c) shows that an average temperature drop of 8°C was produced by the system for 300 minutes when the system was operated at a velocity of 2.7 ms -1 . The outlet relative humidity produced by the system is 99% for all velocities.
It is inferred from the above results that an average temperature drop of 5°C to 8°Cand a relative humidity of 99%   International Journal of Photoenergy could be achieved with this system based on the operating velocity if the ambient air has a temperature of 30°C to 34°C and relative humidity of 45% to 60%. Figure 9 shows the temperature and relative humidity of ambient air and outlet air from the cooling chamber when the air was supplied at three different air velocities for low RH conditions. It is illustrated from the figures that the temperature drop produced by the vermicompost is maintained for a longer duration at all velocities during low RH conditions compared to other RH conditions. An average temperature drop of 6.5°C, 9.4°C, and 8.8°C was produced by the system up to 400 minutes when the system was operated at velocities of 0.9 ms -1 , 1.8 ms -1 , and 2.7 ms -1 , respectively. The relative humidity of air leaving the system is 99% for all velocities. It is inferred from the above results that an average temperature drop of 6.5°C to 9.5°C and relative humidity of 99% could be achieved with this system based on the operating velocity. The ambient air has a temperature of 38°C to 42°C and relative humidity of 30% to 45%. Figure 10 shows the average temperature drop produced by the cooling system operated at three different velocities during three different RH conditions.
The above results elucidate that the vermicompost-based evaporative cooling system performs better at velocities of 1.8 ms -1 and 2.7 ms -1 during low and medium RH conditions. However, for high RH conditions, though the temperature drop produced by vermicompost at 0.9 ms -1 velocity is high, the time taken to achieve a higher reduction is more. At the same time, the system operated at higher velocities produces a maximum drop within a few minutes. Hence, it is clear that ambient relative humidity and operating velocity are the key factors that influence the performance of a vermicompostbased evaporative cooling system. Figure 11 shows the instantaneous cooling capacity calculated using Equation (1) for three different velocities under three different RH conditions. It is clear from the figures that the duration of sustained cooling effect varies based on the temperature drop for all velocities during different RH conditions. The period of sustainability is 120 minutes for high RH conditions, 300 minutes for medium RH conditions, and 400 minutes for low RH conditions. Figure 12 shows the average instantaneous cooling capacity produced by the cooling system at three different velocities under three different RH conditions. It is seen from the figure that the system produces a higher cooling capacity for a velocity of 2.7 ms -1 during all RH conditions. It is also observed that there is a dependency between the mass flow rate and evaporative cooling capacity. The higher cooling rate could be achieved by increasing the airflow velocity and area of flow. Since the maximum velocity should be limited to 3 ms −1 to avoid health issues, the area of flow can be increased, and hence, mass flow rate can be increased. However, 1000 grams of vermicompost with the same mass flow rate is capable of producing a higher cooling effect during low RH conditions. This increases the amount of water to be evaporated for providing the required cooling effect. Hence, a large sump is needed for

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International Journal of Photoenergy water storage. This vermicompost-based cooling system is producing the cooling effect by utilizing only the initial water supply and thus eliminating the water sump system and related health issues. Hence, the vermicompost-based cooling system can produce sustained cooling at higher velocities during all RH conditions.   Figure 13 shows the effectiveness calculated using Equation (2) for three different velocities under three different RH conditions. It is noticed from Figure 8(a) that for all velocities, the system is very effective for 60 minutes. After that, there are fluctuations which are mainly due to the change in ambient conditions. In Figures 8(b) and 8(c), it is  15 International Journal of Photoenergy found that higher average effectiveness was achieved when the velocities of air were 2.7 ms -1 and 1.8 ms -1 . The system's effectiveness was less at 0.9 ms -1 compared to other velocities. It is further observed that the effectiveness was maintained until the end of the experiment, which is mainly due to the water stored in the vermicompost to attain sustainable cooling. Figure 14 shows the average effectiveness of the vermicompost-based evaporative cooling system tested during three different RH conditions operated at three different velocities. It is clear from the figure that effectiveness is above 75% when the system is operated at high velocity during all RH conditions. It is also understood that the ambient temperature and relative humidity influences the effectiveness of the vermicompost-based cooling system. Figure 15 shows the instantaneous evaporation rate calculated using Equation (3), cumulative evaporation rate, and ambient relative humidity for three different velocities under high RH condition.

Water Evaporation Rate.
It is observed from the figures that due to the relative humidity, which varies from 60% to 90%, the instantaneous evaporation rate is in the range of 0.2 g min -1 to 0.7 g min -1 at all velocities. However, the average instantaneous evaporation rate is more at 1.8 ms -1 velocity compared to other velocities. The average cumulative evaporation rate is in the range of 60 grams to 80 grams.
It is clear from Figure 16 that the instantaneous evaporation rate increases with velocity, and the average instantaneous evaporation rate is highest for 2.7 ms -1 . Its value is in the range of 0.5 g min -1 to 1.0 g min -1 . The average cumulative evaporation rate is increased from 200 grams to 400 grams for medium RH conditions. It is seen from Figure 17 that the instantaneous evaporation rate is more when the air enters dry, i.e., with low relative humidity in the range of 30% to 45%. The cumulative 17 International Journal of Photoenergy evaporation rate is also increased, and it is in the range of 250 grams to 550 grams. Figures 18 and 19 show the average values of instantaneous and cumulative evaporation rates at all velocities for three different RH conditions. It is construed from the figures that instantaneous evaporation and cumulative evaporation rates are inversely proportional to ambient relative humidity and directly proportional to velocity. The highest values are obtained by 1000 grams of vermicompost for low RH condition operated at 2.7 ms -1 .

Economic
Analysis. If a pump had been used for supplying the water at the rate of 1 liter per hour, a 0.05 kW pump would have been employed for producing the same cooling effect. The energy consumption for the pump is calculated as follows: It is understood from the analysis that vermicompost as an energy-efficient material provides thermal comfort by eliminating the pump and issues related to water storage in the sump with energy savings of 21.7% and an annual electricity cost savings of Rs. 1551.24.

Comparison with Previous Studies.
Many researchers had done extensive work in evaporative cooling systems with different pad materials. The results of their investigations are given in Table 4.
It is observed from the table that the conventional padpump system using various pad materials had been used for a hot and dry climate. The effectiveness of the systems varies from 60% to 80%. The highest temperature drop produced by eucalyptus fibre has an effectiveness of 71%. The present vermicompost-based evaporative cooling system produces an average temperature drop of 9.5°C with an average effectiveness of 80% for low RH conditions. Moreover, this system could produce an average temperature drop of 5°C, even during high RH conditions. It is construed that vermicompost has the potential to replace the pad-pump system of the conventional direct evaporative cooling system. Hence, this sys-tem could be used for cooling applications throughout the year.

Conclusion
The following conclusions are arrived from the present investigation performed on a vermicompost-based evaporative cooling system.

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International Journal of Photoenergy (viii) Instantaneous evaporation and cumulative evaporation rates are inversely proportional to relative humidity and directly proportional to velocity.
The highest values are obtained by 1000 grams of vermicompost for low RH condition operated at 2.7 ms -1 (ix) It could be used under all operating conditions; however, excellent performance characteristics are observed when the system is operated under low RH conditions. Hence, the vermicompost-based evaporative cooling system could be used in places with hot and dry weather conditions (x) The use of a solar dryer for regenerating the vermicompost ensures the prevention of mould growth in the cooling chamber This energy-efficient cooling technology has the potential to provide a clean and sustainable indoor environment throughout the year in places with hot-dry weather conditions. This system acts as a water storage medium and thus eradicates the problems of water stagnation. The elimination of the pump increases the energy savings of the system by 21.7% and an annual electricity cost savings of Rs. 1551.24. This system could pave the way for year-round thermal management of building cooling applications in an environmentally friendly manner. The present setup technically demonstrates the lab-scale feasibility of a vermicompostbased evaporative cooling system. When it is proposed to make it a user-friendly commercial setup, new limitations and challenges may arise. The future scope of the work is to test the life cycle of the vermicompost in the repeated charging and discharging cycles for long-term utilization.
Abbreviations DEC: Direct evaporative cooling DHT: Digital hygro thermometer TRHS: Temperature relative humidity sensor RH: Relative humidity.

Symbols
T amb : Ambient temperature (°C) T al : Temperature of air leaving the cooling chamber (°C) T wbt : Thermodynamic wet-bulb temperature of entering air (°C) RH amb : Ambient relative humidity (%) RH al : Relative humidity of air leaving the cooling chamber (%) _ m a : Mass flow rate of air (kgs -1 ) c p : Specific heat capacity of moist air (J kg -1 K -1 ) Q ec : Evaporative cooling capacity (W) _ m wi : Instantaneous evaporation rate of water (g min -1 ) _ m wc : Cumulative evaporation rate of water (g).

Data Availability
The data used to support the findings of this study are included within the article.