Experimental Study of a Tubular Solar Distillation System with Heat Exchanger Using a Parabolic Trough Collector

Abstract

One way to overcome the scarcity of clean water through sustainable approach is by utilizing a solar distillation system. This easy-to-use technology is adopting tubular solar distillation. The three main components, which are the most essential for producing the amount of permeate, are the solar collector, tubular and heat exchanger (HE). This study aims to determine the performance of a tubular solar distillation device equipped with HE using a parabolic trough collector (PTC). The PTC has an area of 5.1 m² covered with a solar reflective chrome film. Aluminum tubular acts as the feedwater heater. The HE is placed inside the tubular, which acts as a coolant to convert the steam phase into freshwater/permeate and as a feedwater heater to flow into the tubular. In the present study, several parameters were tested: comprise temperature, solar radiation, pressure, humidity, mass flow rate, permeate productivity and efficiency. This study demonstrated the production of a sufficient amount of permeate, which was 5.32 L for 6 h. The efficiency of this device yielded a peak of 48.2% during solar radiation of 813 W/m² in an average ambient temperature of 32 °C, with an overall average of 44.59%.

1. Introduction

The scarcity of clean water has become a big problem for humanity that endangers food security, human health and natural ecosystems [1]. Existing water sources, such as underground and river water, are observed to continuously decrease in time. The scarcity of clean water issue is predicted to be more critical in the upcoming years driven by socioeconomic and climate changes [2]. The problem of water scarcity can be overcome by several methods, such as rainwater harvesting and mist collection [3]. Another solution to increase the world’s need for clean water is through the desalination process, starting from seawater, brackish water and liquid waste [4]. As we generally understand, around 71% of the earth’s surface is covered with water, roughly 97% of the water in the world is salt water, which cannot be used directly for human, animal and plant needs [5]. In regard to this problem, the desalination process can be utilized as it is able to transform salt water into freshwater.

Desalination is a process in which the feedwater (sea water) is separated into two substances, namely water that has a low concentration of dissolved solids (permeate) and water that has a high concentration of dissolved solids (concentrate) [6]. Water with a low concentration of dissolved solids is considered the clean water. Clean water contains Total Dissolved Solids (TDS) of less than 1000 mg/L [7]. Generally, the desalination process can be carried out by either a thermal process or a membrane process. The thermal process comprises Multi-stage flash distillation (MSF), Multi-effects distillation (MED) and Vapor compression (VC). Membrane processes comprise reverse osmosis (RO), Nano-filtration (NF), Electro dialysis (ED) and Forward osmosis (FO) [8]. The difference between these methods lies in the source of energy, in which the thermal method uses thermal energy while the membrane process uses electrical energy [9]. To protect the environment from pollution and to fully utilize the potential of renewable energy, the thermal process is considered to be promising for research related to desalination applications due to the abundance of natural heat sources [10]. One of the natural heat sources in this matter is solar energy.

Solar energy is a fairly stable source of sustainable energy and is consistently available, particularly in tropical countries [11]. The amount of solar energy that reaches the earth is four million exajoules (1 EJ = 1018 J) annually [12]. In Indonesia, the annual average of energy received from the sun is 4.8 kWh/m²/day [13]. The Aceh region has a relatively good level of solar radiation even though it is slightly lower than the ideal solar radiation of 5.1 kWh/m²/day, and its sky clearness index is very sufficient. Such a large amount of energy should be able to be utilized for various applications, especially for solar water heaters (SWH) [14,15], solar drying [16], dryers, water heaters, power plants [17] and solar desalination [18]. One of the most optimal and efficient ways to utilize solar energy is to use system concentrated solar power (CSP) technology. This CSP technology consists of solar towers (ST) [19], parabolic dish collectors (PDC) [20], parabolic trough collectors (PTC) [21] and solar Fresnel reflectors (SFR) [22]. Among the four CSP technologies, many researchers used the PTC system for solar desalination applications because of its maturity, cost-effectiveness and ease of manufacturing process [23,24].

In the PTC system, the energy received by the mirror reflector is linear by concentrating sunlight into the receiving tube and heating the transferring fluid, which is then transformed into superheated steam [24]. The working temperature of PTC ranges from 20 to 400 °C, a relatively wide range which enables the usage for a variety of applications [25]. Jebasingh and Herbert conducted a literature study on the desalination system using CSP technology, which showed that the MSF system supported by PTC is more economical than the RO method [21]. Several studies using PTC technology of the MSF system have been conducted in recent years. Al-Othman et al. conducted research on seawater desalination with a capacity of 40,000 m³/day, producing freshwater of 1880 m³/day [26]. Separate research conducted by Luqman et al. on desalination yielded up to 1140 m³/day of freshwater, which is sufficient to meet the needs of about 2280 people with an efficiency of 34.5% at 550 W/m² solar radiation [27]. According to Alsehli et al., the average daily freshwater production obtained is 53 kg/m²/day. As for the solar collection area of 45,552 m², it can produce fresh water of 2230 m³/day [28].

Two of the most common types of solar distillation are active solar distillation and passive solar distillation [29]. Passive solar distillation works by simply utilizing the heat received by the feedwater reservoir. Active solar distillation uses additional heat other than the heat received by the water reservoir [30]. Active solar distillation is assisted by the addition of heat to accelerate the change of the feedwater phase. Parsa et al. conducted passive solar distillation system experiments to determine the effect of altitude on the production performance of passive solar distillation systems. In the study, experimental tests were carried out at an altitude of 3964 and 1171 m above sea level. Testing was carried out over four days with the same start and end times. From the tests carried out, the results of maximum production at an altitude of 3964 and 1171 m were 720 and 500 mL/m²/h, respectively [31]. The use of passive solar distillation can also be found in tubular solar distillation systems as in the research of Talib et al. In the study, experimental tests were carried out using three pieces of test equipment: the Conventional Tubular Solar Still (CTSS) system, the Tubular Solar Still with Phase Change Material (TSS-PCM) system and the Tubular Solar Still with Nano Phase Change Material (TSS-NPCM) system. The test was carried out on three systems with the same water level of 1 cm. Based on the testing process, the following results were obtained: 4.3, 6.0 and 7.9 kg/m² for CTSS, TSS-PCM and TSS-NPCM, respectively [32]. Hassan conducted a study to see a comparison of the performance of single still active and passive solar distillation. Single still (SS) and double still (DS) using PTC and without PTC were recorded. The study was conducted in the summer and winter. The results showed that double still without or with PTC has a greater freshwater yield in summer than in cold mucin. The daily yield of DS with PTC is 8.53 kg/m² in summer and 4.03 kg/m² in winter. In addition, the energy efficiency of DS + PTC is 23.28% in summer and 15.61% in winter, which is better than that of SS + PTC [33]. Dawood et al. conducted a conventional solar still research integrated with two PTCs. A pipe, which is fed by the working fluid consisting of oil and nano oil, is installed on each PTC receiver. One of those PTCs, under the pipe, had a tube containing paraffin wax added. The maximum daily yield of freshwater products reached 9.7 L/m²/day, and the maximum efficiency obtained is 22% [34]. Elashmawy conducted research on tubular solar still (TSS) by comparing the performance of solar tubes using rectangular tubs with Black cotton clothing and half-cylinder tubs without Black cotton clothing. The result obtained was a solar tube with a half-cylinder tub without Black cotton clothing producing the highest amount of freshwater production of 1.66 L/day [35].

Building upon the above explanation, it can be said that solar distillation is sufficient to produce permeate in the amount of L/day. It is necessary to design a solar distillation system capable of producing large amounts of permeate. As several studies suggest, CSP technology using a PTC system is very helpful in improving the performance of a solar distillation in solar desalination applications. The tubular model is also the solar distillation system, which is considered suitable to be integrated with PTC. To accelerate the condensation process, the installation of HE in the tubular is needed [36]. Therefore, this research focuses on designing and testing a tubular solar distillation apparatus with HE using a PTC system. The purpose of this study is to determine the performance of a tubular solar distillation apparatus equipped with a heat exchanger using PTC in solar desalination applications. The contribution of this research is to investigate a solar distillation device that is able to produce and increase the amount of permeate production by the integration of heat exchanger device. The application of this research is subjected to help the use of clean energy to overcome the scarcity of clean water for people in rural areas with sufficient heat from sunlight. This paper consists of 4 parts: (1) Introduction: discusses the background and literature review of related studies; (2) Materials and Methods: elaborates the experimental setup and data acquisition technique; (3) Result and Discussion: provides analysis of the obtained data corresponding to relevant research in regard to tubular solar active solar distillation device with PTC and HE integration; and (4) Conclusion: remarks on all the result and the contribution of this study.

2. Materials and Methods

2.1. The General Setup of Research

This research was carried out experimentally, starting from designing the entire solar distillation system test equipment; followed by the manufacturing process of frames, PTC and reflectors, feedwater receivers/reservoirs, permeate reservoirs and heat exchanger (HE); and then equipment setup, the process of installing measurement instruments and the process of testing and data collection.

2.2. Solar Distillation Prototype Design and Manufacturing

Figure 1 is the seawater desalination test kit for a tubular solar distillation system designed specifically for this study. The system consists of a feedwater reservoir, permeate reservoir, HE, sight glass, reflector solar collector (parabolic trough), water valve, water pump, feedwater drum, permeate drum and concentrate drum. All components, materials and functions of each of these components are shown in

Table 1. Material of the components—modification components of the tubular active solar distillation system.

No	Name of Component	Material	Function
1	Tubular	Aluminum	Feedwater heating place
2	Permeate container	Aluminum	Early permeate shelters
3	HE	Copper	Steam cooling and feedwater inflow
4	Sight glass	Glass	Steam cooling and feedwater inflow
5	Reflector (parabolic trough)	Iron plate	Place for laying solar reflective film
6	Solar reflective film	Chrome	For the reflector of the sun’s rays
7	Feedwater valve	-	feedwater flow rate regulator
8	concentrated valve	-	feedwater flow rate regulator
9	permeate valve	-	Permeate flow rate regulator
10	Water pump	-	Flowing feedwater
11	feedwater tank	HDPE	feedwater storage
12	Measuring cup	Glass	Final permeate storage
13	Concentrated tank	HDPE	Concentrate storage

.

The solar collector, which functions as a solar energy collector and as a light reflector to the receiver (permeate reservoir), has an area of 5.1 m² as shown in Figure 2. On the solar collector, a sheet of the solar reflective film made of chrome is installed or glued. The receiver serves as a heat receiver from the collector. This receiver is made of aluminum with a high heat conductivity. Inside the receiver, a copper heat exchanger—which is used as a steam heat exchanger from the feedwater and also preheated for the feedwater—is installed. The dimensions and properties of the heat exchanger are shown in Figure 3. The material of the heat exchanger is copper with thermal conductivity of 418 W/m²K, thermal diffusivity of 1.15 × 10⁻⁴ m²/s, density of 8960 kg/m³ and specific heat of 406 J/kgK [37].

2.3. Working Principle of Solar Distillation System

Figure 4 depicts the working principle of the solar distillation system designed in this study. At the initial stage, the feedwater (salt water) entering the tubular tube passes through the HE. Inside the tubular, the volume of water is set to as much as 12.7 L. The volume is measured from the sight glass that has been installed on the tubular. After it is confirmed that the feedwater was filled at a volume of 12.7 L, the PTC system starts the process. PTC heats the tubular tube from the reflection of light waves concentrated from the collector to the receiver/tubular. This particular process is similar to the principle of heating water using a traditional stove. The feedwater that is inside the tubular will change its phase into steam towards the tubular surface. A container of freshwater reservoir/permeate is placed inside the tubular. Then, the feedwater in the tank is re-pumped to flow into the HE. At this stage, HE acts as a coolant to convert the steam phase into freshwater/permeate and a feedwater (salt water) heater, which later are subjected to flow into the tubular. The steam that has changed into dripping water falls into the permeate holding container and then flows into the measuring cup. The hot water in the HE is collected back in the tubular and is ready to be reheated according to the working principle of PTC. The event occurs repeatedly until the amount of heat has begun to decrease due to the reducing intensity of solar radiation. The position of each component, such as HE, permeate holding container and feedwater reservoir inside the tubular, can be seen in Figure 4.

2.4. Experimental Setup Equipment and Measurement Parameters

Figure 5 is a schematic of testing a tubular solar distillation system. The test equipment and measuring instruments have been compiled according to the research procedure. Measuring instruments are marked with numbers for ease of data acquisition and processing. The parameters measured are the temperature of the incoming feedwater (T_whi), the temperature of the outgoing feedwater (T_who), the temperature of the feedwater (T_w), the temperature of steam (T_v), ambient temperature (T_a), solar radiation (I_r), pressure (P_v), humidity (H_v), mass flow rate ($\dot{m}$) and permeate productivity. Figure 5 depicts the parameters’ location with detailed information in

Table 2. Description of measuring instrument placement, measurement parameters and name of measuring instrument.

Point	Parameters	Unit	Measuring Instruments	Type	Accuracy Level	Range
Twhi	Inlet Feedwater Temperature	°C	Thermocouple	Type K	±0.1%	−270 to 1260 °C
Twho	Outlet Feedwater Temperature	°C	Thermocouple	Type K	±0.1%	−270 to 1260 °C
Tw	Feedwater temperature	°C	Thermocouple	Type K	±0.1%	−270 to 1260 °C
Tv	Steam Temperature	°C	Thermocouple	Type K	±0.1%	−270 to 1260 °C
Ta	Ambient temperature	°C	Thermocouple	Type K	±0.1%	−270 to 1260 °C
Pv	Vapor Pressure	Bar	Pressure Transducer	SEN0257–RK5–2,1B	±1.5%	0 to 12 Bar
Ir	Solar Radiation	W/m2	Pyranometer	Sentec (SEM228A)	±2%	0 to 1800 W/m2
$\dot{m}$	Mass flow rate	L/s	Flow Meter	YF–S201	±1.5%	1 to 30 L/min
Hv	Vapor Humidity	%	Humidity sensor	FS400–SHT31	±0.5%	0 to 100%RH
AG	-	-	Data Acquisition (DAQ) System	Agilent Type A 97410
PC	-	-	PC	Free
MC	Amount of permeate	-	Measuring Cylinder	-

. The accuracy for the use of different measuring instruments is summarized in Table 2.

2.5. Testing Procedure

After the experimental setup equipment is well organized, the experiment for tubular solar system was started at 9:00 a.m. until 5:00 p.m. during daylight with clear sky and high heat condition. The place where the data were taken was in the environment of the Faculty of Engineering, Samudra University, Langsa City, and Aceh, Indonesia at a location of 4°27.5′ N, 97°58.3′ E. At first, T_whi, T_who, T_w, T_v, T_a, I_r, P_v, H_v and $\dot{m}$ were measured simultaneously using measuring instruments as in Table 2. All measuring instruments are connected with a TYPE a 97410 Agilent DAQ. Data from DAQ are read and processed using a computer device as shown in Figure 5. The amount of permeate production is obtained from the results of distillation and measured using measuring cups. All obtained data are then processed in the form of tables and graphs.

3. Results and Discussion

3.1. Initial Measurement

Figure 6 depicts the distribution of temperature, solar radiation intensity, pressure, humidity and the water mass flow rate according to time in the tubular distillation system. The temperature distribution consists of the feedwater temperature (T_w), the feedwater vapor temperature (T_v) and the ambient temperature (T_a). The temperature of the feedwater (T_w) is meant for the feedwater contained in the tubular. Then the parameters are connected between the distribution of temperature and solar radiation to time (Figure 6a), the distribution of temperature and pressure variation according to time (Figure 6b), the distribution of temperature and humidity to time (Figure 6c) and the distribution of temperature and mass flow rate to time (Figure 6d).

The research was carried out from 9:00 a.m. to 5:00 p.m. The magnitude of T_w and T_v follows the amount of solar radiation. At 09.00 a.m., T_w and T_v were at 36.7 °C and 38.9 °C, respectively, while I_r and T_a were at 662 W/m² and 34.5 °C, respectively. T_w and T_v continue to increase according to I_r. The value of I_r reached a maximum of 813 W/m² at midday (12:00 p.m.) while T_a was stable at 32 °C. The increase in T_w stopped when it reached 75.9 °C due to the heat received by T_w being used to change its feedwater phase. This phenomenon is shown in the graph in Figure 6b. The atmosphere was protected from the steam inside the tubular evaporating. The entire valve on the copper pipe and the tubular tube was then tightly closed. At the same time that the steam inside the tubular was increasing, the pressure inside the tubular was also rising. Because of the rise in pressure and the quantity of steam, the amount of permeate also increased. It can be seen that from 09.30 WIB to 11.00 WIB, the pressure increased from 0.19 bar to 0.3 bar, respectively. When there is an increase in pressure, the boiling point will increase as well, therefore it will affect the amount of steam content.

Using a humidity sensor, the amount of vapor content in the tube can be determined. In Figure 6c, the percentage of moisture caused by steam can be seen. At a temperature of 36.7 °C, the heat received by the feedwater is still utilized to increase T_w. At this temperature, the humidity measured about 55%. At a temperature of 63.7 °C, humidity began to increase to about 78%. The highest amount of humidity is obtained at a temperature of 74.5 °C, where T_w has begun to be constant. In this condition, the faucet contained in HE began to open to produce permeate.

The average mass flow rate ($\dot{m}$) of feedwater in the HE is shown in Figure 6d to be around 2.61 g/s. For 4 h, there is a temperature drop of 10 °C at this mass flow rate. There is a temperature difference between T_who (50 °C) and T_whi (32 °C) as a result of the heat exchanging process between the feedwater vapor in the tank and the feedwater in the HE. T_who and T_whi are separated by 18 °C. As can be observed from the graph, due to the different values of T_w (70 °C) and T_who (50 °C), T_w decreases over time.

From the overall results obtained, the amount of I_r (665 W/m²) for 30 min can increase T_w by 25.3 °C and start to produce steam with a pressure of 0.19 bar. In accordance with the water vapor table, at a steam temperature of 62 °C, the vapor pressure is 0.19 bar. Research conducted by Kumar et al. [38] for heating water with PTC also showed results in accordance with the increase in T_w. The effect of $\dot{m}$ on T_who also shows results that are in accordance with research conducted by Alsadaie et al. [39].

3.2. Permeate Production Rate

Permeate is obtained from the phase change of feed moisture to liquid. To change the moisture of this feed, an HE is used with feedwater from the tank as a coolant. The performance of this HE is determined based on the convection properties of the feedwater that are affected by the speed of its flow. Figure 7 depicts the influence of the speed of the feedwater on other parameters such as the temperature of the feedwater in the tubular tube (T_w). Figure 7a shows the influence of the speed of mass flow rate ($\dot{m}$) on productivity. It is known that the mass flow rate in HE is 15 g/s; this value caused the difference of temperature between T_who (50 °C) and T_whi (32 °C) in HE. Under these conditions, the amount of productivity is 0.9 g/s. However, at this flow rate, there has been a decrease in T_w which is very rapid. T_w dropped from a temperature of 74.8 °C to 57.7 °C which had a difference of 17.1 °C within 65 s. Because the decrease is too large, the tubular’s power will be employed once more to increase the temperature of its feedwater. In Figure 7b, the mass flow rate in HE is 2.63 g/s. This flow rate triggered a difference of temperature between T_who (51 °C) and T_whi (32 °C) in HE. At this condition, the productivity is 0.3 g/s with no decrease in T_w for 65 s. The amount of productivity at a mass flow rate of 2.63 g/s is lower than the mass flow rate of 15 g/s. However, at a mass flow rate of 2.63/s, T_w becomes more stable which is beneficial for the stability of productivity.

Because the permeate production at a flow rate of 2.65 mL/s is more stable, the distillation process continues without increasing the feedwater flow rate in the HE. From Figure 8, it can be seen that the water productivity rate reaches a maximum of about 0.32 g/s at 2:30 p.m. At 03:00 p.m., the productivity starts to decrease which reduces the addition of total permeate (Yield in Hour). The amount of permeate that can be achieved in the tests carried out is about 5483.45 g or an equivalent of 5323.74 L in a period of 6 h. Figure 7 shows that the intensity of the sun has an effect on the initial heating of the feedwater which will further increase the rate of permeate production. The effect of $\dot{m}$ on the productivity rate is in accordance with the research conducted by Alsadaie et al. who investigated the effect of fouling on the performance of HE [39].

3.3. Efficiency of Tubular Solar Distillation

The tubular efficiency of solar distillation (${\eta }_d$) is obtained using Equation (1), where $\dot{m}$ is the mass flow rate of HE feedwater (kg/s); ${\lambda }_{fg}$ is the enthalpy of feedwater (kJ/kg); $I_r\left(t\right)$ is solar radiation (W/m²); and $A_{bs}$ is the area of the PTC reflector (m²). The enthalpy of the feedwater can be determined using Equation (2), which is a function of T_w (°C) [40].

Table 3. Measurement data for calculating the efficiency of tubular solar distillation.

$\dot{\mathit{m}}$ (kg/s)	Tw (°C)	${\mathit{\lambda }}_{\mathit{f}\mathit{g}}$ (kJ/kg)	Ir (W/m2)	Efficiency (%)
2.68	75.90	2326.07	788	43.08
2.68	75.00	2328.08	809	42.00
2.66	74.90	2328.30	813	41.49
2.65	74.91	2328.28	783	42.91
2.64	74.50	2329.19	774	43.27
2.61	74.00	2330.31	765	43.30
2.59	74.00	2330.31	727	45.21
2.58	72.00	2334.77	701	46.80
2.57	71.00	2337.01	678	48.24
2.57	70.4	2338.35	600	47.43
2.58	68.2	2343.28	590	46.97
2.57	62	2357.24	250	45.38
2.58	54	2375.39	137	43.63

shows the results of calculating the efficiency of tubular solar distillation in Equations (1) and (2).

$${\eta }_d=\frac{\sum \dot{m} {\lambda }_{fg}}{\sum I_r\left(t\right)A_{bs}3600}$$

(1)

where

$${\lambda }_{fg}={10}^3\times \left[2501.9-2.40706\times T_w+1.192217\times {10}^{-3}\times T_w{}^2-1.5863\times {10}^{-5}\right]$$

(2)

Figure 9 depicts the tubular solar distillation efficiency based on the calculation results of Equations (1) and (2). The efficiency of tubular solar distillation increased from 11:00 a.m. to 3:00 p.m. The efficiency at 11:00 a.m. was 43.1%. The average efficiency obtained in this study was around 44.6%. The highest efficiency was obtained at 48.2% at 3:00 p.m. A comparison of this study with several studies related to tubular solar distillation is shown in

Table 4. Comparison of several studies related to tubular solar distillation with the present study.

S. No	Reference and Author	Design	Area (m2)	Yield (L)	Efficiency (%)
1.	Arunkumar et al. [41]	parabolic concentrator-concentric tubular solar still (CPC-CTSS)	2	3.23	76.78
2.	Elashmawy [35]	parabolic concentrator-solar tracking system (PCST-TSS)	0.059	3.53	28.5
3.	Ahmed et al. [42]	parabolic concentrator solar tracking-wicked-tubular solar still (PCST-W-TSS)	0.87	5.1	39.7
4.	Essa et al. [43]	tubular drum solar still (TDSS) with parabolic solar concentrator (PSC)	0.5	5.75	63.8
5.	Kabeel et al. [44]	tubular solar still with a cylindrical parabolic concentrator (CPC)	1.2	7.4	60.4
6.	Present Study	tubular solar distillation device equipped with HE using a parabolic trough the collector	5.1	5.32	44.59

. The production and efficiency obtained in this study, 5.32 L and 44.59%, respectively, are not much different from those obtained by some researchers who integrate the tubular solar distillation system with other kinds of technology/devices in Table 4. This indicates that the implementation of HE is also sufficient for tubular solar distillation.

4. Conclusions

An analysis of the tubular solar distillation equipped with heat exchangers using PTC has been successfully carried out. PTC has been able to work by providing heat to the tubular tube filled with feedwater so that the feedwater can change its phase from liquid to vapor. The performance of HE is also sufficient, where HE can work as a good heat collector. HE can cool saturated vapors until saturated vapors turn into permeate. HE is also able to work as a feedwater heater, which eases the work of PTC on heating the feedwater in the tubular. With a pressure of 0.19 bar and a feedwater temperature of 62 °C, this device can convert the feedwater phase into steam. HE-integrated tubular solar distillation research using PTC can produce a permeate production amount of 5.32 L for 6 h. This tubular solar distillation achieved a maximum efficiency of 44.59%, indicating the effectiveness of HE integration. The PTC system, which is used as a light trap for solar waves in this solar distillation system, has proven that renewable energy is very promising for the future. With relatively simple design, the device developed in this study has been able to produce clean water and can be potentially implemented in rural areas, particularly in tropical regions with sufficient sunlight and heat conditions throughout the year.