Development and Performance of a Vacuum-Based Seawater Desalination System Driven by a Solar Water Heater

Abstract

This work proposes the design, construction, and field test of a vacuum seawater desalination system (VSDS) driven by an evacuated tube solar collector (with a total absorption area of 1.86 m²) under tropical climatic condition (Thailand ambient at latitude 13°43′06.0″ N, longitude 100°32′25.4″ E). The VSDS prototype was designed and constructed to be driven by hot water, which is produced by two heat source conditions: (1) an electric heater for laboratory tests and (2) an evacuated tube solar collector for field tests under real climatic conditions. A comparative experimental study to assess the ability to produce fresh water between a conventional dripping/pipe feed column and spray falling film column is proposed in the first part of the discussion. This is to demonstrate the advantage of the spray falling film distillation column. The experimental method is implemented based on the batch system, in which the cycle time (distillation time) considered is 10–20 min so that heat loss via the concentrated seawater blow down is minimized. Later, the field test with solar irradiance under real climatic conditions is demonstrated to assess the freshwater yield and the system performance. The aim is to provide evidence of the proposed vacuum desalination system in real operation. It is found experimentally that the VSDS working with spray falling film provides better performance than the dripping/pipe feed column under the specified working conditions. The spray falling film column can increase the distillated freshwater volume from 1.33 to 2.16 L under identical cycle time and working conditions. The improvement potential is up to 62.4%. The overall thermal efficiency can be increased from 33.7 to 70.8% (improvement of 110.1%). Therefore, the VSDS working with spray falling film is selected for implementing field tests based on real solar irradiance powered by an evacuated tube solar collector. The ability to produce fresh water is assessed, and the overall performance via the average distillation rate and the thermal efficiency (or Gain Output Ratio) is discussed with the real solar irradiance. It is found from the field test with solar time (8.00–16.00) that the VSDS can produce a daily freshwater yield of up to 4.5 L with a thermal efficiency of up to 19%. The freshwater production meets the requirement for international standard drinking water criteria, indicating suitability for household/community use in tropical regions. This work demonstrates the feasibility of VSDS working under real solar irradiance as an alternative technology for sustainable fresh water.

1. Introduction

1.1. Rationale of Research

Freshwater scarcity caused by population growth, industrialization, and climate variability has increased the reliance on seawater and brackish-water desalination in addition to conventional water management. This is due to limitation of freshwater source which is available 3–5% of global water source. Although reverse osmosis (RO) is preferred for seawater desalination because of its low specific energy consumption (SEC), there are some limitations as follow: system pretreatment; fouling control; and brine management. The thermal-driven vacuum desalination system (TVDS) is a promising solution for sustainable fresh water. This is because it can produce fresh water by distillation via boiling seawater. Hence, heat is required to promote pooling or evaporation of water contained within seawater. Later, water vapor is condensed to produce the distilled water (fresh water). More interestingly, TVDS can also be driven by a relatively low-temperature heat source when the distillation column is working under vacuum pressure. As a result, the boiling point of seawater within the distillation column is lower in association with the working saturation pressure, and, hence, it is possible to use a low-grade heat source such as hot water produced by a solar collector, industrial waste heat, etc. Therefore, TVDS can be driven by a solar water heater to produce sustainable fresh water using a renewable source (solar energy). The solar-driven methods is a free source and requires a low electricity demand and low carbon emissions. This can support small-scale decentralized desalination systems for remote communities, especially in tropical countries where solar resources are abundant [1,2,3,4,5,6].

In thermal desalination, the vacuum-based desalination system can be operated at a relatively low boiling point of around 50 to 70 °C. These conditions align well with evacuated tube solar collectors (ETCs), which are widely used and provide high optical-thermal efficiency in practical implementation. Integrating ETCs with TVDS units has demonstrated significant freshwater yields and favorable energy, economic, and environmental performance, including from energy and exergy perspectives [7,8,9,10,11,12].

Enhancing heat and mass transfer at the liquid–vapor interface is key to improving efficiency in small- and medium-scale thermal desalination. Spray/flash and falling film configurations can increase the heat transferring area and can produce thin films or droplets, which improves the evaporation, reduces operating time, and limits internal conduction losses. Recent studies on spray flash desalination under a vacuum have identified feed temperature, nozzle configuration, and vacuum level as critical factors for evaporation rate, productivity, and efficiency [13,14,15]. Research on falling film hydrodynamics and heat transfer, including updated correlations, highlights the influence of film thickness, interfacial waves, and surface conditions on design coefficients for columns and condensers, with further insights from saline and brine experiments [16,17,18].

From the literature surveys shown in

Table 1. Summary of technology and process of desalination.

Configuration/Process	Heat Source	Temperature/Vacuum (as Reported)	Key Findings	Ref.
Solar thermal desalination (STD) pathways	Solar thermal (general)	~50–70 °C; low pressure (overview)	Seven design pathways to improve STD; reduced STEC via coupling and latent heat recovery	[1]
Solar-driven desalination (PV-RO and solar thermal)	Solar PV/solar thermal	-	State-of-the-art of solar-driven desalination; integration roadmap	[2]
Solar energy-driven desalination (broad)	Solar thermal/PV	-	Renewable desalination for SDGs; climate mitigation perspective	[3]
Spray flash vacuum distillation with internal heat recovery	Low-grade thermal (model-based)	Low-grade heat + vacuum (model space)	Internal heat recovery boosts yield and lowers thermal duty	[4]
Novel spray flash desalination + CSP (RSM optimization)	Concentrated solar power (thermal)	Vacuum; CSP-level hot side	RSM identifies the best operating window to improve productivity/energy metrics	[5]
Spray-enhanced flash desalination (vacuum)	Ocean thermal/low-grade heat	Vacuum; small ΔT driving	Spray intensifies flashing and freshwater yield under vacuum	[6]
Falling film hydrodynamics and heat transfer (horizontal tubes)	Thermal (general)	Covers correlations, including low-pressure cases	Updated correlations; role of film thickness/waves/surface	[7]
Falling film evaporation on horizontal tube	Thermal (general)	Parametric (not field-specific)	New heat transfer correlation via multi-precision expansion	[8]
Falling film heat exchangers in desalination	Thermal (general)	Covers MED/MVC/MEE, including vacuum operations	Comprehensive review linking hydrodynamics and heat transfer to desalination design	[9]
Evacuated tube solar collector (ETC/ETSC)—advances	Solar thermal (ETC)	Typical ETC outlet 50–70 °C (review)	Recent advances and performance of ETC for thermal supply	[10]
Modified ETC basin solar still for RO brine	Evacuated tube solar collector (ETC)	Elevated brine temperature; no vacuum (still)	ETC + condenser geometry significantly increases productivity	[11]
Integrated solar still + external condenser	ETC heat pipe solar collector	Lower condenser-side temperature; no vacuum (still)	External condenser and ETC coupling enhance condensation and yield	[12]

, the experimental work on the vacuum-based desalination system is still lacking, especially for operating with a heat source temperature of 50–70 °C, as the unwanted heat (heat rejection at condenser) is rejected at a relatively high ambient tropical climatic condition. Moreover, there are no available studies on TVDS driven by a solar water heater under solar time. This is a research gap that is addressed in this work. Table 1 provides some previous work on desalination systems powered by a solar water heater.

1.2. Research Gaps

From Table 1, there is a gap in spray falling film vacuum desalination driven by an evacuated tube solar water heater (ETC) under tropical applications; however, empirical ambient evidence under realistic tropical variability (fluctuating solar irradiance, high ambient temperature/humidity) remains scarce, despite calls to validate solar-thermal coupling in practical climates [16,17,18,19]. Direct head-to-head benchmarks of spray falling film columns versus conventional dripping/pipe feed columns under identical operating conditions is still lacking which hinders quantification of structural benefits [20,21,22,23,24,25]. The combined and potentially nonlinear effects of cooling water flow, hot water temperature, and flow rate, as well as condensation temperature (T_cond) on freshwater yield and thermal efficiency, have not been mapped systematically. Addressing these gaps, the present work conducts outdoor experiments under tropical conditions using an ETC heat source; establishes a controlled head-to-head benchmark against a dripping/pipe feed column; performs parametric sweeps of coolant flow and T_HE/T_cond; and reports yield, efficiency, and full energy metrics with uncertainty for reproducible engineering comparison [26,27,28,29].

1.3. Research Purpose

This work proposes the design, construction, and field testing of a vacuum seawater desalination system (VSDS) driven by an evacuated tube solar collector (with a total absorption area of 1.86 m²) under tropical climatic conditions (Thailand ambient; 13°43′06.0″ N, 100°32′25.4″ E). The prototype VSDS is designed and constructed for testing with hot water generated by an electric heater (laboratory test) and evacuated tube solar collector (field test under real climatic conditions). The experimental method is implemented based on the batch duration with a cycle time of 10–20 min so that heat loss via the concentrated seawater blow down is minimized. A comparative experimental study to investigate the ability to produce fresh water between a conventional dripping/pipe feed column and spray falling film column is proposed in the first part of the discussion. Later, the field test with solar irradiance under real climatic conditions is demonstrated to assess the performance. The aim is to provide evidence of the proposed vacuum desalination system in real operation.

2. Experimental Apparatus and Field Test Setup

2.1. System Description of a Vacuum Seawater Desalination System (VSDS)

The proposed vacuum seawater desalination system (VSDS) was motivated by the need to utilize low-grade waste heat or renewable sources to produce fresh water. The technical feasibility of such systems relies heavily on the proper design of the vacuum distillation column, which enables the evaporation process to occur under sub-atmospheric pressure (or vacuum pressure). Under these sub-atmospheric conditions, seawater can boil (or evaporate) at a lower temperature (approximately 50–90 °C), allowing low-grade thermal energy to be effectively utilized as the driving force for evaporation. In this case, the system is designed to be tested with both a simulated heat source (via a heater) and real heat source (via a solar water heater). Figure 1, Figure 2 and Figure 3 provide an overview of the system and its components. In this work, two distillation columns based on the dripping/pipe feed column (DPF) and on the spray falling film column (SFF) are initially studied in the laboratory to demonstrate the improvement potential of the SFF over PFF. The differences in design between them are depicted in Figure 3.

The operational scheme of a vacuum desalination system (VSDS) is illustrated in Figure 1. The major components include a vacuum distillation column, condenser, heat exchanger, circulation pump, hot water storage chamber, and necessary instrumentation. The process starts when a low-temperature heat source is supplied to the heat exchanger. Thermal energy is transferred to the circulating seawater, resulting in an increased temperature at the heat exchanger outlet. The heated seawater is then sprayed into the top section of the distillation column through a spray falling header, forming a thin film of falling liquid along the inner wall of the column. This enhances the liquid–vapor interface and promotes the evaporation process. During evaporation, water within the seawater is boiled into vapor and flows toward the upper part of the column before entering the condenser. After that, liquid carried over with the vapor is eliminated to prevent seawater contamination of the freshwater production. Inside the condenser, the vapor is cooled by ambient air or cooling water and condenses into liquid fresh water, which is subsequently collected in the product tank. Since the operating pressure throughout the system is lower than the atmospheric pressure, a pump is required to discharge the collected distilled water to ambient conditions.

The vacuum seawater desalination prototype in this study uses low-grade thermal energy from a solar water heater via an evacuated tube collector (ETC). The system combines a solar collector hot water loop, a vacuum distillation column, and a condensation unit to accelerate evaporation at 55–80 °C, making it suitable for tropical climatic condition. Its main benefits are its reliance on renewable energy, low pump power, minimal vacuum maintenance, and a compact design for coastal communities.

The evacuated tube collector provides higher and more stable hot-water temperatures under high-irradiance conditions, and its application in distillation systems has been widely reported [26,27,28,29]. Hot water is circulated by a hot water pump to an insulated storage tank to buffer energy and attenuate temperature fluctuations due to fluctuation of solar irradiance. The hot water from the tank is then supplied to the distillation column using spray falling film. A structural frame and piping manifold support equipment installation and provide a drain for the concentrated brine.

The core component is the vacuum distillation column, which is a sealed stainless-steel vessel aiming to produce sub-atmospheric pressure via a vacuum pump during operation. Seawater, supplied by a circulating pump, passes through the heat exchange and absorbs heat from the hot water loop, and, hence, the seawater is evaporated at a working pressure of 6–10 kPa. Because the boiling point decreases with pressure, the natural or solar-vacuum method for small-scale systems offers strong potential for both performance and structural simplicity. The resulting vapor is directed to a condenser, in which the coolant is provided by a cooling tower. The water vapor is converted into fresh water and collected in a receiver tank, while concentrated brine is periodically discharged through the base piping to control salt buildup and prevent scaling.

The heat exchanger is designed and evaluated using the logarithmic mean temperature difference (LMTD), which is around 15–20 °C as listed in

Table 2. The components of the vacuum seawater desalination system (thermal-driven).

Equipment	Description and Specification	Significant Parameters
Heat exchanger (hot side)	Plate heat exchanger	A = 0.65 m2 Designed LMTD = 15–20
Condenser (cold side)	Plate heat exchanger	A = 0.65 m2 Designed LMTD = 15–20
Pipe feed distillation column	-Cylindrical shape with a total volume of 4.7 L, 120 cm height -7 trays were installed along the column (shown in Figure 3) -Made of stainless steel (SUS-316) with a feeding pipe diameter of 12.5 mm	-Tray is installed to increase the heat transferring area during the evaporation
Spray falling film distillation column	-Cylindrical shape with a total volume of 4.7 L, 120 cm height -Spray header diameter of 2.5 inches with 100 holes, each hole diameter of 1 mm -Made of stainless steel (SUS-316) with sight glass to observe the liquid level	-The spray falling header is designed to work as the atomized nozzle to increase the evaporation and reduce the surface tension of seawater
Receiver tank	-Capsule shape with a total capacity of 6 L -Made of stainless steel (SUS-316) with sight glass to observe the liquid level	-Accumulating the fresh water before pumping out the system for usage
Seawater circulating pump	-Magnetic coupling centrifugal pump (power of 200 W)	-It is necessary for this application to avoid leakage
Evacuated tube solar collector	-20 Pcs tubes -Total aperture area of 1.86 m2 -Collector orientation of 15°, southwest direction	-It is used to implement the field test under real solar irradiance
Heater (simulating heat source in laboratory)	-An immersion heater with a rated power of 7 kW -Achieving the desired temperature via a digital thermostat with solid state relay	-It is only used for laboratory testing so that the hot side can be maintained at a constant desired temperature

. This method and a counter-current setup are used to maximize the temperature difference and area efficiency [20]. When using spray falling film surfaces or spray nozzles in the distillation chamber, the film flow regime improves heat and mass transfer under vacuum pressure, as supported by systematic reviews and empirical data [10,11,12,19].

A control cabinet and instrumentation manage the operation, integrating switches for the hot water and seawater pumps, temperature and pressure displays, and a vacuum gauge. This setup allows operators to maintain key setpoints within target ranges. Auxiliary equipment and pumps, including the vacuum pump and backup power supply, are mounted on a common base for compactness and easier maintenance. In this setup, the evacuated tube collector is designed to replace the heater. Recent studies have shown that well-designed solar-powered distillation supports sustainability goals and reduces greenhouse gas emissions when effective heat transfer and recovery are employed [30,31,32,33,34,35,36].

In summary, the energy cycle begins with the storage of solar heat, then delivers heat to the vacuum distillation chamber to accelerate seawater evaporation, and finally condenses the vapor in a plate heat exchanger to produce fresh water. The utility cycle manages the seawater feed, periodic brine discharge, and closed-loop control. The prototype utilizes little electrical power for pumping and vacuum maintenance, relying primarily on solar energy. This design is well-suited for deployment in coastal or remote communities with high solar potential, supporting a sustainable freshwater supply and reducing the reliance on fossil fuels and long-term energy costs.

2.2. Performance Parameter

During the investigations, the heat rate for heating the seawater at the plate heat exchanger can be determined by Equation (1), which indicates the mass flow rate of the hot water and the temperature difference at the inlet/outlet port of the exchanger.

$$\dot{Q_H}={\dot{m}}_H{c}_p(T_{H-in}-T_{H-out})$$

(1)

The useful energy for the distillation process is implied by the latent heat transfer rate, which can be calculated by Equation (2).

$${\dot{Q}}_u={\dot{m}}_f{h}_{fg}$$

(2)

The evaporation rate is estimated by measuring the volume of the fresh water under a certain time interval (cycle time operation), and, hence, it can be calculated by Equation (3).

$${\dot{V}}_f={\displaystyle \frac{V_f}{∆t}}$$

(3)

The thermal efficiency of the VSDS can be determined by Equation (4).

$$\eta ={\displaystyle \frac{{\dot{Q}}_u}{{\dot{Q}}_H}}\times 100\%$$

(4)

For the field test based on solar irradiance, the heat rate for heating the fresh water can be determined by measuring solar radiation falling on the solar collector, as calculated by Equation (5).

$${\dot{Q}}_s=I_s{A}_c$$

(5)

Also, the daily useful energy for heating can be calculated by Equation (6) (daytime period for testing the VSDS).

$$Q_s=I_{tot}{A}_s{∆t}_{dis}$$

(6)

Meanwhile, the daily useful energy for distillation is calculated by Equation (7).

$$Q_u=\rho V_f{h}_{fg}{∆t}_{dis}$$

(7)

Therefore, the thermal efficiency based on the field test can be determined by Equation (8).

$$\eta ={\displaystyle \frac{Q_u}{Q_s}}\times 100\%$$

(8)

2.3. Uncertainty Analysis

The measurement uncertainty listed in

Table 3. List of instruments and their uncertainties.

Parameter	Equipment/Instrument	Model	Accuracy
Solar irradiation	Pyrometer	Lutron, model SPM-116SD	±2.5%
Air velocity	Hot wire anemometer	Fluke 925	±2.0%
Relative humidity	Hygrometer	Climomaster, model 6501	±2.0%
Data collector	Data logger	Hioki-LR8431	±1.5 °C
Temperature	Thermocouples	K-Type	±0.5%

was evaluated using the root-mean-square (RMS) method assuming independent uncertainty sources, which are commonly adopted in experimental thermal system analysis. Based on this approach, the uncertainty of the heat transfer rate to the hot-side heat exchanger was estimated to be 1.75%. The uncertainties of the useful latent heat transfer rate and freshwater production were 2.2% and 1.7%, respectively. The uncertainty of the thermal efficiency was 2.7%, while that of the solar collector efficiency was 2.5%.

3. Results and Discussion

3.1. Experimental Study of the VSDS Working with Different Distillation Columns

This experiment is implemented to compare a conventional dripping/pipe feed (DPF) distillation column with a spray falling film distillation column (SFF) under identical operating conditions. The primary outcomes are the freshwater volume per batch cycle time, average distillation rate (L·min⁻¹), and overall thermal efficiency. To ensure a fair and consistent comparison, the experimental set points of both DPF and SFF distillation column are identical, as outlined in

Table 4. Experimental set points for head-to-head comparison.

Parameter	Symbol	Set Point	Unit	Note
Hot water temperature	TH	60	°C	Controlling the heat source temperature
Cycle time per batch	tbat	20	min	Duration per experimental round
Cooling water flow rate	${\dot{V}}_{cool}$	12.5	L min−1	Maintaining condensation process
Seawater feed flow rate	${\dot{V}}_{sw}$	10	L min−1	Promoting spray falling film/pipe feed
Hot water flow rate	${\dot{V}}_H$	20	L min−1	Heating loop
Seawater volume per batch	$V_{bat}$	3	L	Initial volume of seawater
Heater power	Ph	7000	W	Heat source simulation

.

The improvement potential of the VSDS working with the SFF distillation column over the DPF distillation column is shown in

Table 5. Values and relative improvements.

Metric	Dripping/ Pipe Feed	Spray Falling Film	Absolute Δ	Improvement
Distillate volume (L)	1.33	2.16	+0.83	+62.4%
Distillation rate (L min−1)	0.067	0.108	+0.041	+61.2%
Thermal efficiency (–)	0.337	0.708	+0.371	+110.1%

. It is clearly seen that the SFF distillation consistently outperforms the DPF distillation column (as previously shown in Figure 3). The distillated volume of fresh water per cycle time is increased from 1.33 L to 2.16 L (improvement potential of +62.4%), average distillation rate rises from 0.067 to 0.108 L min⁻¹ (improvement of +61.2%), and overall thermal efficiency improves from 0.337 to 0.708 (+110.1%). The results indicate gains of approximately 162% to 210% across all three performance indicators, which indicates a clear advantage for the spray falling film distillation column.

The reason to support the advantage of the spray falling film distillation column is that it can generate fine droplets and a thin film on the internal surface, increasing the interfacial area and enhancing surface renewal under a vacuum environment. This reduces film thickness, surface tension, and internal thermal resistance, which strengthens heat and mass transfer and accelerates evaporation, as supported by the recent literature [3,7,8,10,11,12]. Therefore, the spray falling film distillation column performs better than the dripping/pipe feed. Hence, this present work comprehensively discusses the performance of the VSDS working with SFF.

3.2. Working Characteristic of the VSDS Working with SFF

This section evaluates the spray falling film distillation column in the laboratory when hot water is simulated by an electric heater, and, hence, the inlet hot water temperature can be precisely controlled at the desired value. Specifically, the following working conditions (shown in

Table 6. Experimental variables and conditions of seawater desalination process.

Parameter	Symbol	Value/Level	Unit	Note
Hot water temperature	TH	60	°C	Fixed
Distillation time	tdis	10, 15, 20, 30	min	Varied
Hot water flow rate	${\dot{V}}_H$	20	L min−1	Fixed
Cooling water flow rate	${\dot{V}}_{cool}$	12.5	L min−1	Fixed
Seawater feed rate	${\dot{V}}_f$	10	L min−1	Fixed
Seawater volume per batch	$V_{bat}$	3	L	Per cycle time

) are used in this study:

• Temperatures are monitored at key points, including the hot water inlet/outlet of the heat exchanger, seawater inlet/outlet of the heat exchanger, coolant inlet/outlet of the condenser, and distillate (freshwater) temperature;
• Freshwater production and thermal efficiency are quantified to identify the operating condition that yields the highest efficiency.

Figure 4 shows the temperature at the point of interest against time (cycle time). It is seen that the system exhibits a start-up transient. At start up, the inlet hot water temperature slightly decreases from 60 °C to 57 °C, the outlet hot water temperature remains constant throughout, and the seawater temperature jumps from 30 °C to 45 °C. Later, all three streams reach quasi-steady operation, T_H-in = 59–60 °C, T_H-out = 55–56 °C, and T_sw = 47–48 °C, while the vacuum pressure is around 70 mbar (7 kPa).

At a quasi-steady state, the hot-side temperature drop is maintained at ΔT_hot = 3–4 °C, which implies an adequate hot water flow rate (20 L·min⁻¹). This may limit thermal pinch on the hot water outlet. The minimum approach (T_{H, out} − T_{sw, out}) is around 7–9 °C, which is consistent with the effectiveness of the compact plate heat exchanger. This implies that the hot side heat exchanger should be fed with seawater at a temperature close to the hot water temperature.

Feeding the seawater at approximately 47–48 °C into the distillation column under a vacuum environment enhances flashing and evaporation because the energy required to reach evaporation is reduced. These mechanisms are consistent with findings from falling film theory and spray or flash studies supported by [3,7,8]. To achieve higher feed temperatures, the most effective approaches are to increase the heat transfer area and convective heat transfer coefficient, such as by expanding the plate heat exchanger area or optimizing the chevron angle and fouling control, and to adjust flow-rate ratios to raise the logarithmic mean temperature difference (LMTD).

Figure 5 illustrates the variation in the condenser temperatures against time. The outlet coolant temperature (T_c-out) rises rapidly in the first minute due to heat transfer from the vapor condensation, then increases gradually to peak at around minute six before declining as the vapor load decreases. The inlet coolant temperature (T_c-in) and distillated water temperature are almost constant throughout the test. These results agree with the reported condensation behavior in compact plate heat exchangers [34,37,38,39,40], which demonstrates good performance.

The distilled volume (or freshwater yield) against the cycle time is depicted in Figure 6. Under the same working conditions, the distillated volume increases linearly with the cycle time, with a best-fit line (R² = 0.981). This indicates a near-constant average evaporation process (0.10–0.11 L min⁻¹). The average product yields are 1.17 at a cycle time of 10 min, 1.55 L at a cycle time of 15 min, and 2.17 L at a cycle time of 20 min. However, Table 5 shows that at a cycle time of 30 min, even though the freshwater production still increases, the distillation rate and thermal efficiency are decreased. This is because the water contained within the seawater is quite low (concentrated brine), and, hence, the surface tension is increased. This results in producing a poorer evaporation. Therefore, the cycle time of 20 min is an optimal time for distillation.

Figure 7 and

Table 7. The experimental results influenced by the cycle time.

Time (min)	Fresh Water (L)	Distillation Rate (L/min)	Efficiency (%)
10	1.17	0.117	67.5
15	1.78	0.103	68.4
20	2.35	0.118	67.8
30	2.88	0.096	55.4

also show that operating the VSDS with a longer cycle time from 10 to 30 min raises the cumulative distillated volume from 1.17 to 2.88 L. However, the thermal efficiency of the VSDS with cycle time below 20 min decreases slightly from 67 to 63% when increasing cycle time from 10 to 20 min. It is interesting to see that the thermal efficiency is found when operating the VSDS with a cycle of 30 min, which is significantly decreased compared to the cycle time of 20 min. This behavior is consistent with a progressive reduction in the heat and mass transfer driving force as the batch proceeds: a higher brine temperature and the limited heat transferring area of the heat exchanger results in a lower logarithmic temperature difference (LMTD), the concentrated brine (longer batch operating time) increases the effective boiling point and depresses vapor pressure, and cumulative boundary losses become more significant [9,10,11,12,20].

Figure 8 and

Table 8. The experimental results of the VSDS with SFF influenced by heat source temperature.

Temperature (°C)	Fresh Water (L)	Distillation Rate (L.min−1)	Efficiency (%)	UA	LMTD
50	1.44	0.072	45.0	1289.2	4.65
60	2.35	0.118	67.8	579.4	11.22
70	2.60	0.130	67.0	369.5	19.0

(indexed to 50 °C = 100%, baseline) show a clear trade-off when increasing the hot water temperature. The distillated volume and distillation rate increase with the hot water temperature, while the thermal efficiency increases from 45% to 67%. A higher hot water temperature strengthens the heat and mass transfer driving forces in the spray/falling film operation under a vacuum environment. This can enlarge the interfacial area and accelerate the evaporation rate, which can also reduce the surface tension for the brine, resulting in better evaporation performance as supported by refs. [3,7,8,10,11,12]. In this test, the hot water temperature of 70 °C provides the maximum freshwater yield; however, the hot water temperature of 60 °C offers the best performance for this test. It is also seen from Table 8 that the LMTD of the heat exchanger (hot side) increases when increasing the hot water temperature at a fixed flow rate. This causes a significant reduction in the overall heat transfer coefficient (U) of the heat exchanger, which results in a lower effectiveness. This behavior implies that the heat transfer area is key to the heating performance of the seawater. In this case, it is noticeable that a larger temperature difference between the hot water outlet and seawater temperature is found when increasing the hot water temperature. This mitigates the ability to induce evaporation. In such a case, a lager heat transfer area is required or a higher hot water flow rate can compensate.

Figure 9 and

Table 9. The experimental results of VSDS with SFF influenced by coolant flow rates.

Coolant Flow Rate (L.min−1)	Fresh Water (L)	Distillation Rate (L.min−1)	Efficiency (%)	LMTD	UA
11.5	1.82	0.091	52.5	7.64	851.0
12.0	2.19	0.110	63.2	7.42	875.8
12.5	2.35	0.118	67.8	7.07	920.0

show the effect of the coolant flow rate on the key performance indicators. As the coolant flow rate increases (11.5, 12.0, and 12.5 L.min⁻¹), the distillated volume is increased by 1.82, 2.19, and 2.35 L and the average distillation rate is increased by 0.091, 0.110, and 0.118 L min⁻¹. The thermal efficiency increases from 52.5 to 67.7% when increasing the coolant flow rate. The reason is that increasing coolant flow rate generally increases the convective transfer coefficient (due to operating with a higher Reynolds number). This results in the overall heat transfer coefficient (UA) to increase from 851 to 920 W/K (seen from Table 7). However, the logarithmic mean temperature difference (LMTD) is not much different when increasing the coolant flow rate. This implies the condensation process is not sensitive to the change in the coolant flow rate. Therefore, a higher amount of fresh water is achieved when the coolant flow rate increases because of the ability to perform better condensation processes (higher overall heat transfer coefficient, UA). It is shown that using a too low flow rate may cause incomplete condensation and vapor carry-over with fresh water, whereas using a too high flow rate wastes pumping power and limits heat transfer potential. It has been proven that using a coolant flow rate of 12.5 L min⁻¹ produces maximum distilled water volume; however, the flow rate of 12.0 L min⁻¹ shows better system efficiency.

3.3. Field Test of the VSDS Driven by an Evacuated Tube Solar Collector

As discussed earlier in Section 3.1, it has been shown that the VSDS working with the spray falling film distillation column performed better than working with the drip/pipe feed column. It was found experimentally that the performance of the VSDS is acceptable under specified working conditions in which the hot water temperature varies between 50 and 70 °C and the cooling water temperature is fixed at 30 °C. This implies that it is feasible to operate the VSDS with the solar water heater under real solar radiation and tropical climatic conditions. In this section, the field testing of the VDS driven by evacuated tube solar collector is proposed and discussed. The aim is to provide evidence for further development of a larger scale solar-driven VDS. This is to provide alternative sustainable desalination technology for sustainable fresh water.

The field testing evaluates how the hot water flow rate from an evacuated tube solar water heater (ETC) influences a spray falling film vacuum distillation unit. Specifically, the temperatures of hot water at the heat exchanger inlet/outlet, seawater at the heat exchanger inlet/outlet, coolant at the condenser inlet/outlet, and distillated water (freshwater) are observed for discussion. Additionally, the freshwater quality is assessed against reference criteria. The working condition of the field test is listed in

Table 10. The operating conditions for the VSDS driven by an evacuated tube solar collector.

Item	Set Point/Value	Unit	Notes
Heating element	Evacuated tube solar collector	–	Converting solar radiation to hot water
Number of ETC tubes	20	tubes	–
Solar aperture area	1.86	m2	Collector illuminated area
Collector orientation	Southwest, 15°	–	Its best angle for Thailand
Water storage capacity	20	L	For thermal energy storage
Coolant flow rate	12	L.min−1	Ensuring the complete condensation process
Seawater flow rate	10	L.min−1	Promoting evaporation under a vacuum
Seawater per batch	3	L	Suitable for the distillation column
Test duration	08:00–16:00	8 h	Standard solar time
Preheat period	08:00–09:00	1 h	Performing thermal energy storage
Distillation period	09:00–16:00	7 h	Performing distillation
Data logging interval	Every 5 min	min	Thermocouples and flow meters

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Figure 10 depicts the solar irradiance and the hot water temperature at the heat exchanger inlet. It is seen that, even though some fluctuations in solar irradiance are found in the afternoon, the hot water inlet temperature rises steadily. Between 08:00 and 11:30, solar irradiance increases from approximately 250 to 900 Wm⁻², while the hot-water temperature rises from about 35 to 47 °C, reaching nearly 55 °C around noon. In the afternoon, irradiance drops sharply due to intermittent clouds, with several periods below 300–400 Wm⁻². However, the hot water temperature continues to increase, stabilizing near 65 °C. This consistent temperature is due to the thermal inertia and buffering effects of the evacuated tube collector and storage loop, which filter out short-period solar fluctuations and provide a stable 50–70 °C heat supply suitable for a vacuum spray falling film desalination system [12,14,15].

To provide more details of the working characteristics of the ETC-VSDS, the hot water temperature and seawater temperature at the outlet port of the heat exchanger is depicted against the solar time in Figure 11. It is found that the hot water outlet and seawater temperatures increase together from approximately 35 and 33 °C in the morning to 65 °C in the afternoon. The curves converge with an approach temperature of about 1 to 3 °C. This indicates the high effectiveness of the plate exchanger. A lower temperature difference between hot water and seawater, the log-mean temperature difference (ΔT_lm), decreases toward a pinch condition, so additional hot-side heating provides diminishing returns [10,11,12,20]. A slight decrease in the seawater temperature near 15:00 likely results from transient flow or vacuum disturbances due to concentrated brine blow down. However, the system recovers quickly. This demonstrates the thermal inertia and buffering of the ETC with the storage tank, which stabilizes the hot water supply despite irradiance fluctuations and concentrated seawater blowing down. This performance agrees well with the vacuum distillation characteristics, as a hot-side feed accelerates evaporation.

Figure 12 indicates the coolant temperature at the inlet/outlet port of the condenser and seawater temperature against solar time. It shows stable heat rejection throughout the day, which is implied by the coolant temperature difference, which is not much different (0.5–1.2 °C). The freshwater (distillated water) temperature is found to be higher than the coolant water outlet temperature, which is around 0.5–2 °C and is consistent with effective condensation under a vacuum environment. A slight transient at around 12:00, fresh water peaking at 36–37 °C at 15:00, and a small bump in coolant out likely reflect temporary increases in vapor load or short flow/valve disturbances due to brine blow down; the rapid return to baseline shows an adequate capacity margin and good dynamic stability. Overall, the condenser appears to operate in a high-flow/low-temperature difference regime.

Effect of the Hot Water Flow Rate

To demonstrate this effect, a three-day field test is implemented. The cooling water flow rate varies at 15, 17.5, and 20 L min⁻¹. Each test begins at 8.00 and finishes at 16.00 under real solar irradiance. The cooling water flow rate is kept at 12 L min⁻¹. Three parameters of interest—the freshwater yield, average distillation rate, and the thermal efficiency—are considered for discussion. The test results under solar time are depicted in Figure 13 and

Table 11. The tested results at different hot water flow rates.

Test Conditions: Fixed Coolant Flow Rate at 12 L/min Solar Time: 8.00–16.00
Hot Water Flow Rate	Total Solar Irradiation	Useful Desalination Energy (Qu)	Energy Storage in Water (Qsto)	Fresh Water	Thermal Efficiency	Collector Efficiency
15.0 L.min−1 (day 1)	39.17 MJ	10.05 MJ	16.25 MJ	4.05 L	25.65%	67.14%
17.5 L.min−1 (day 2)	31.47 MJ	7.76 MJ	11.85 MJ	3.45 L	24.66%	62.31%
20.0 L.min−1 (day 3)	39.45 MJ	7.31 MJ	16.42 MJ	3.25 L	18.53%	60.15%
Hint: Solar collector efficiency can be determined: $\eta = {\displaystyle \frac{Q_u+Q_{sto}}{Q_s}}\times 100\%$

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From the tested results shown in Figure 13 and Table 11, the VSDS driven by ETC can be operated steadily even when there is fluctuation in solar irradiance due to real operation. The thermal inertia caused by using the storage tank helps the system to operate stably with variations in solar irradiance. This can be observed by the hot water temperature at the heat exchanger inlet, which increased continuously with the variation in the solar irradiance. To further investigate the influence of operating conditions, three different hot water flow rates (15, 17.5, and 20 L min⁻¹) were tested during three separate days under real solar irradiance. During these tests, the coolant water flow rate was maintained at 12 L min⁻¹. The corresponding experimental results are summarized in Figure 13 and Table 11, demonstrating the effect of hot water flow rate on freshwater yield, average distillation rate, and thermal efficiency.

Table 11 indicates that the freshwater volume and distillation rate decrease with an increase in the hot water flow, whereas the thermal efficiency reaches a maximum value (25%) at the hot water flow rate of 15 L min⁻¹ before dropping to around 18% at 20 L min⁻¹. An increase in the hot water flow rate through a plate heat exchanger tends to drive the heat exchanger operating regime into a high-flow/low-ΔT regime, which results in a higher convective heat transfer coefficient (higher mass flux): the heat exchanger is working near a pinch point in which the logarithmic mean temperature difference is lower. Therefore, the delivered hot water temperature and the film temperature on the spray falling side can collapse, reducing the evaporation driving force [10,11,12,20]. It can be summarized that if the main goal is the maximum daily freshwater yield, a hot water flow rate of 15 L min⁻¹ is preferred (4.05 L per run; 0.578 L h⁻¹). However, if the goal is focused on better energy usage at only a lower cost per unit volume, a hot water flow rate of 17.5 L min⁻¹ is also useful. Operation of a hot water flow rate of 20 L min⁻¹ seems to be unnecessary, as it demonstrates no advantage on the three parameters of interest. This implies that the heat exchanger used for this application must be operated within the optimal range of the hot-side flow rate.

3.4. Effect of the Cooling Water Flow Rate of the System Driven by Solar Water Heater

To demonstrate this effect, a three-day field test is implemented. The cooling water flow rate varies at 8, 10, and 12 L min⁻¹. Each test begins at 8.00 and finishes at 16.00 under real solar irradiance. The hot water flow rate is kept at 17.5 L min⁻¹, which is an optimal value based on the energy efficiency perspective as discussed earlier. Three parameters of interest—the freshwater yield, average distillation rate, and the thermal efficiency—are considered for discussion. The test results under solar time are depicted in Figure 14 and

Table 12. The tested results at different coolant flow rates.

Test Conditions: Fixed Coolant Flow Rate at 12 L/min Solar Time: 8.00–16.00
Coolant Flow Rate	Total Solar Irradiation	Useful Desalination Energy	Energy Storage Conversion	Fresh Water	Thermal Efficiency	Collector Efficiency
8.0 L.min−1 (day 1)	30.33 MJ	5.87 MJ	11.25 MJ	2.61 L	19.35%	56.45%
10.0 L.min−1 (day 2)	22.38 MJ	4.21 MJ	9.55 MJ	1.87 L	18.81%	61.48%
12.0 L.min−1 (day 3)	32.07 MJ	6.17 MJ	12.78 MJ	2.74 L	19.21%	59.10%
Hint: Solar collector efficiency can be determined: $\eta = {\displaystyle \frac{Q_u+Q_{sto}}{Q_s}}\times 100\%$

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From the test results shown in Figure 14, the VSDS driven by ETC can be operated steadily even when there is fluctuation in solar irradiance due to real operation. The thermal inertia as a result of using the storage tank helps the system to operate stably with variations in solar irradiance. This can be observed by the hot water temperature at the heat exchanger inlet, which increases continuously with the variation in the solar irradiance.

Table 12 indicate stable operation throughout the day for a test period of three days. The coolant temperature at the condenser inlet remains nearly constant at 32–33 °C, while that at the condenser outlet is maintained at 32.5–34 °C, resulting in a slight temperature difference of only 0.5–1.0 °C. This reflects the condenser operation under an appropriate flow rate in which the logarithmic mean temperature difference is a good value. This ensures a reliable condensation driving force. Referring to the freshwater (condensate) temperature, it is slightly higher than the coolant temperature outlet (0.5–2 °C), which is consistent with vacuum condensation conditions. A slight fluctuation is observed around noon to early afternoon, with temperature peaks of 36–37 °C likely caused by an increased vapor load or changes in valve position and flow rate.

Table 12 provides a comparative illustration of the desalination performance at three different coolant flow rates. As compared to the baseline of 10 L min⁻¹, it is evident that the condenser working with the coolant flow rate of 8 and 12 L min⁻¹ yields better outcomes in terms of freshwater production and distillation rate. At 12 L min⁻¹, the system achieves the highest freshwater yield (≈2.74 L) and distillation rate. This means using a higher coolant flow rate can enhance the driving force for condensation processes. Conversely, although the coolant flow rate of 8 L/min also demonstrates improved freshwater production relative to the baseline, its performance remains slightly below that of 12 L/min.

More Interestingly, the thermal efficiency remains nearly constant (19%) even if the coolant flow rate is varied. In other words, the thermodynamic efficiency of the system is relatively insensitive to changes in the coolant flow rate. This implies that optimization of the system’s operation should focus on balancing water yield and energy consumption.

Overall, the tested results have shown that higher coolant flow rates enhance water recovery without significantly affecting efficiency, but the marginal gains should be weighed against the potential increase in pumping power and operating costs [30,31,32,33,34].

3.5. Product Water Quality and Compliance with MWA and WHO Criteria

In this work, the quality of the distilled water or fresh water is questionable, and, hence, it must be validated using the water quality standards of both the Metropolitan Waterworks Authority (MWA, 2013) [41] and World Health Organization (WHO) for drinking water. The comparative results are shown in

Table 13. Distillate quality benchmarked against the Metropolitan Waterworks Authority (MWA, 2017) criteria and WHO guidelines for drinking water quality (operational ranges).

Study/System	Desalination Configuration	Distillate pH (–)	Comment on Water Quality
Present work	Thermally driven vacuum desalination system (TDVDS) driven by ultra-low-temperature heat source (50–70 °C)	7.47–7.86 (Seawater = 8.0)	Near-neutral pH; all samples fall within MWA/WHO guideline range 6.5–8.5 for drinking water.
Dayem [33]	Solar natural vacuum desalination system (SNVD); natural vacuum created by 9.8 m elevation of evaporator	≈7.00	Review of SNVD performance reports a “neutral fixed pH value of 7.00” with ~100% reduction in conductivity, indicating excellent potable-water quality.
Kabeel et al. [35]	Modified single-basin solar still with nanofluids and auxiliary vacuum fan	7.1 (feed 8.9 and distillate 7.1)	Measured TDS decreased from 932 to 82 mg/L and pH from 8.9 to 7.1 after desalination; authors concluded that the distillate quality satisfies WHO drinking-water guidelines.

Notes: TDS is approximated from conductivity by TDS ≈ 0.65 × EC (typical range 0.5–0.7; the parenthetical values show this range). The MWA limits used here are as follows: salinity ≤ 1 g/L, pH 6.5–8.5, EC ≤ 2000 µS/cm. WHO GDWQ provides operational ranges for pH (6.5–8.5) and taste/acceptability guidance based on TDS bands (e.g., <300 mg/L “very good”, 300–600 “good”).

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It is seen from Table 13 that the distillated water met the requirements of both MWA (2017) and WHO drinking water quality standards for all tests. Salinity ranged from 0 to 0.02 g/L, pH from 7.47 to 7.86 (within the 6.5–8.5 guideline), and conductivity from 74 to 516 µS/cm, corresponding to an estimated TDS of approximately 48 to 335 mg/L. This falls within the WHO’s ‘very good’ to ‘good’ taste and acceptability bands. The lowest electrical conductivity (74 µS/cm) was observed during ETC runs with hot water flow variation, while the highest (516 µS/cm) occurred in the heater-driven case, though this was still well below MWA limits. This difference may result from minor droplet entrainment at the spray–condenser interface, slight contact material leaching, or varying CO₂ absorption after condensation. The near-neutral pH aligns with expectations for vacuum distillation and carbonate equilibrium. To further reduce residual conductivity, consider adding a demister or mist eliminator, optimizing spray and vacuum/flow conditions, using food-grade stainless steel contact surfaces, performing routine system rinsing, and calibrating meters with replicate sampling. If desired for taste or buffering, light remineralization with calcium or magnesium can be applied. These results support the distillate’s suitability for potable use and demonstrate the potential of solar-driven spray falling film vacuum distillation for household or community applications in tropical regions (MWA 2017; WHO GDWQ).

4. Conclusions

Laboratory and field tests of the vacuum seawater desalination system were implemented in this work. Two different types of distillation column (dripping/pipe feed (DPF)) and spray falling film (SFF)) were tested with the test bench in the laboratory under identical working conditions and demonstrated the improvement potential of the SFF over DPF. Since the SFF provided better performance than the DPF, it was selected for implementing in a field test under real solar irradiance. The field test was implemented during a test period of 6 days, in which the controllable parameters (hot water flow rate and coolant flow rate) were investigated under solar time at 8.00–16.00 under Thailand’s ambient conditions (latitude: 13°43′06.0″ N, longitude: 100°32′25.4″ E). The total solar irradiation was also measured, and the performance parameters were observed for discussion. The significant findings of this work can be summarized as:

• The laboratory test indicated that the VSDS working with SFF performed better than that working with DPF in terms of both the freshwater production yield and thermal efficiency. The key to this improvement is the heat and mass transfer enhancement because of larger interfacial areas for heat/mass transfer and reduced surface tension. The improvement potential of the SFF over DPF is 61–110% depending on the working conditions.
• It was experimentally found that a longer cycle time for distillation yielded a higher freshwater production. However, using too long of a cycle time produced only a slightly higher amount of freshwater due to concentrated brine. An increase in the hot water flow rate results in an increase in the thermal efficiency.
• The VSDS driven by evacuated tube collectors can work steadily to produce fresh water even when there is a fluctuation in solar irradiance. The freshwater production depends significantly on the total solar irradiation falling on the evacuated tube arrays. The collector efficiency of 55–68% and thermal efficiency of up to 26% are achieved depending on the working flow rate of the hot water and coolant.

Overall, VSDS working with the spray falling film column powered by ETC is a promising desalination technology under a tropical climate. This is because of the design’s simplicity and renewable power. Finally, product water tests showed that the distillate met MWA (2017) and WHO drinking water criteria (pH 7.47–7.86; EC 74–516 µS cm⁻¹; salinity 0–0.02 g L⁻¹), underscoring the suitability of the system for potable household/community use in tropical settings and further supporting field deployment where reliable, low-grade solar heat is available [MWA 2017; WHO GDWQ].

However, there are some limitations for industrial application that must be addressed in future research. First, vacuum-based desalination systems such as the proposed VSDS need to be further investigated for integration with ultra-low-temperature heat sources (50–70 °C), including industrial waste heat and solar thermal collectors, where conventional desalination technologies are inefficient. Second, careful optimization of the heat exchanger sizing should be implemented, as it significantly affects both freshwater productivity and distillate quality; moderate flow rates were found to provide a favorable balance between efficiency improvement and system stability. Third, from an industrial perspective, future work should focus on system scaling, long-term operational stability, and integration with heat recovery units to improve overall energy utilization. From an extended research perspective, further investigations on dynamic operating conditions, fouling mitigation, and techno-economic optimization are recommended to enhance the feasibility of large-scale implementation.