Fig. 1 illustrates the schematic configurations of the DSSD-AH-ACC. The distiller consists of a 1 m × 0.5 m basin fabricated from galvanized steel sheet and coated with matte black paint to enhance solar energy absorption. The basin is insulated with 50 mm of polyurethane foam to minimize thermal losses. Transparent glass channels, positioned at a 30° tilt from the horizontal on both sides of the distiller, serve as the air heater (AH) and air-cooled condenser (ACC). Distillate collection channels are installed along the lower edges of these glass covers. The air-cooled condenser is located on the northern side, while the air heater is placed on the southern side. Each unit comprises a 0.5 m × 0.05 m glass channel. Ambient air is circulated through them either by natural thermosyphon action or by a DC fan, recovering the unavoidable thermal losses from the distiller covers.

Fig. 2 compares the energy analysis of the DSSD-AH-ACC with that of the CDSSD. Adding the air heater (AH) and the air-cooled condenser (ACC) interact with the solar irradiance, lowering solar transmittance to the distiller while simultaneously reducing convective and radiative losses to the ambient.

The following assumptions were adopted in the analysis: •

Constant thermophysical properties.

Air flows through the air heater (AH) and air-cooled condenser (ACC) of the DSSD-AH-ACC play a pivotal role in governing distillate condensation on the distiller covers, condensation losses recovery, minimizing thermal losses to the ambient surroundings, and, ultimately, determining the overall thermal efficiency of the DSSD-AH-ACC. Fig. 3 illustrates the impact of varying air flow rates through the AH and ACC, from 0.001 to 0.2 kg/s, on key performance metrics of the DSSD-AH-ACC. Notably, increasing air flow through the AH had a minimal impact on enhancing distillate production (Fig. 3a), with this effect vanishing gradually at higher flows. Conversely, increasing air flow through the ACC had an insignificant impact on distillate production. Overall, the higher the air flow rates through the AH and ACC, the higher the distillate production, and vice versa. The thermal efficiency (Fig. 3b) was significantly affected by increasing both air flows. Maximum efficiency was achieved at the highest air flows through the AH and ACC, while minimum efficiency corresponded to the lowest air flows. This highlights the contribution of recovering the distiller thermal losses on the overall performance of the DSSD-AH-ACC. The temperature difference of the exiting air from the AH (Fig. 3c) was primarily affected by AH flow. It significantly decreased with increasing AH flow and remained unaffected by ACC flow, particularly at higher AH flows. In contrast, the temperature difference of the exiting air from the ACC (Fig. 3d) declined sharply with increasing ACC flow, while increasing the AH flow slightly contributed to decreasing it.

Fig. 4 further explores the effect of uniform air flows through both the AH and ACC on the performance of the DSSD-AH-ACC. Increasing both flows equally, from 0.001 to 0.2 kg/s, had a minimal effect on improving distillate production. However, the g₁ cover distillate yield increased by nearly 34% while the g₂ cover distillate yield decreased by nearly 31%. This shows only a redistribution of the distillate across the distiller covers. The temperature difference of the exiting air from the AH and ACC declined by approximately 96%. Nevertheless, the thermal efficiency rose by about 94%. These findings are discussed in greater detail later in this section.

Fig. 5 illustrates the temporal temperature profiles within the DSSD-AH-ACC and CDSSD with the ambient temperature (T_amb) serving as a baseline across the three parts of the figure. Fig. 5a,b correspond to air flows of 0.01 and 0.1 kg/s, respectively, while Fig. 5c represents the temperature distribution within the CDSSD. At low air flows of 0.01 kg/s (Fig. 5a), the DSSD-AH-ACC exhibits temperature profiles with higher peaks compared to those at high air flows of 0.1 kg/s (Fig. 5b). The reduced convective heat transfer inside both the AH and ACC at low air flows limited thermal dissipation from the components of the DSSD-AH-ACC, resulting in elevated internal temperatures. Conversely, high air flows of 0.1 kg/s (Fig. 5b) noticeably reduced the air exit temperatures from the AH and ACC as a result of the increased heat capacity of the flowing air through them. On the other hand, the CDSSD (Fig. 5c) showed lower overall temperatures compared to the DSSD-AH-ACC. The absence of the auxiliary AH and ACC allowed for greater thermal dissipation to the ambient environment, leading to lower internal temperatures.

Fig. 6 illustrates the temporal evolution of distillate yield and thermal efficiency for the DSSD-AH-ACC under air flow rates of 0.01 and 0.1 kg/s, in comparison with the CDSSD. As shown in Fig. 6a, the DSSD-AH-ACC achieved thermal efficiency improvements of about 14% and 60% at air flows of 0.01 and 0.1 kg/s, respectively, relative to the CDSSD. These gains are primarily attributed to enhanced recovery of thermal losses within the AH and ACC, facilitated by increased air flow and improved convective heat transfer. Despite the higher thermal efficiency, the distillate yield of the DSSD-AH-ACC showed a reduction of about 19% under both air flows when compared to the CDSSD. This decline is attributed to elevated glass cover temperatures (g₁ and g₂) in the DSSD-AH-ACC, which surpassed condensation rates on these surfaces relative to the cooler covers of the CDSSD.

Fig. 6b offers detailed insight into the distillate contributions from the g₁ and g₂ covers. In the CDSSD, the g₂ cover is typically cooler than the g₁ cover because of its position on the shaded side of the distiller, while g₁ is directly exposed to solar radiation. This temperature differential favors greater condensation on the g₂ cover compared to g₁. In contrast, in the DSSD-AH-ACC system, the presence of the AH significantly reduces the solar irradiation reaching the g₁ cover, resulting in a lower temperature than g₂. Consequently, distillate condensation on g₁ exceeds that on g₂ under all air flows. The greater the airflow through the AH, the greater the distillate condensation on the g₁ cover. Additionally, the rise in distillate yield from g₁ is associated with a corresponding reduction from g₂, since the total distillate output is primarily governed by the thermal content of the saline water. Nevertheless, the presence of the AH and ACC elevated the temperatures of both g₁ and g₂ covers relative to those of the CDSSD. This temperature increase, in turn, led to lower total distillate yields on both covers of the DSSD-AH-ACC system compared to the CDSSD.

Fig. 7a presents a comprehensive energy analysis of the convective heat transfer from the glass covers to the circulating air within the AH and the ACC under varying air flow rates. In general, the heat transfer from the g₁ and g₂ covers to the air, under various air flows, is greater than that from the g₁₁ and g₂₂ covers, primarily due to their higher surface temperatures. In addition, the heat transfer from the g₂ and g₁₁ covers is greater than that from the g₁ and g₂₂, respectively, under various air flows, again because of their temperatures. Notably, the convective heat transfer from all covers at 0.1 kg/s air flow is approximately twice that at 0.01 kg/s air flow. This enhancement is attributed to the increased thermal capacity and velocity of the circulating air with increased air flow.

Fig. 7b illustrates the combined convective and radiative thermal losses from the g₁₁ and g₂₂ covers in the DSSD-AH-ACC in comparison to those from the g₁ and g₂ covers in the CDSSD. The losses from the g₁₁ and g₂₂ covers, at various air flows, are notably less than those from the g₁ and g₂ covers. This contributes to the superior thermal efficiency of the DSSD-AH-ACC relative to the CDSSD. Additionally, the losses from the g₁₁ and g₂₂ covers decreased with increasing air flow rates as a result of enhanced energy recovery by the circulating air, as stated earlier. Lastly, the losses from the g₁ and g₂ covers are consistently greater than those from the g₂ and g₂₂ covers owing to their higher surface temperatures.

This section examines the influence of climate conditions, including received solar intensity, ambient air temperature, and ambient wind speed, on key performance metrics of the DSSD-AH-ACC, namely water productivity, thermal efficiency, and the temperature difference of air exiting the AH and ACC, under varying air flow rates.

Fig. 8 illustrates the influence of incident solar intensity on key performance metrics of the DSSD-AH-ACC under air flows ranging from 0.01 to 0.1 kg/s. As expected, the increase in the incident solar intensity led to an enhancement in the evaporation within the DSSD-AH-ACC. This, in turn, improved the distillate production and elevated the temperature difference of the air exiting the AH and ACC, and consequently increased the overall thermal efficiency of the DSSD-AH-ACC. Specifically, the increase in solar intensity from 950 to 1150 W/m² boosted distillate production by approximately 40%, raised the air temperature difference by about 23%–25%, and thereby improved thermal efficiency by nearly 2.5%. Furthermore, increasing the air flow from 0.01 to 0.1 kg/s enhanced the thermal efficiency by about 40%, even though the air exiting temperature differences dropped to nearly one-quarter and the distillate production remained unchanged. This improvement is attributed to the enhanced convective heat transfer and greater energy recovery by the circulating air, whose thermal capacity increased tenfold as the flow rate increased from 0.01 to 0.1 kg/s.

Ambient air temperature governs thermal losses from the DSSD-AH-ACC to the surrounding environment, thereby determining its overall performance. As the ambient temperature increases, the temperature difference between the DSSD-AH-ACC components and their surroundings decreases, resulting in reduced heat losses and, consequently, improved thermal performance of the DSSD-AH-ACC. Fig. 9 illustrates the impact of ambient air temperature on key performance metrics of the DSSD-AH-ACC across air flows ranging from 0.01 to 0.1 kg/s. Under all air flow conditions, distillate production, thermal efficiency, and the temperature difference of air exiting from the AH and ACC increased with an increase in ambient temperature. Specifically, the efficiency increased by about 7%–8%, the air temperature difference elevated by about 0.5%–3%, and distillate production rose by nearly 9%–10%. These findings reveal that the DSSD-AH-ACC performs more efficiently in hot climates than in cold climates. In addition, the increase in the ambient temperature preserved the 40% enhancement in thermal efficiency and the 75% reduction in air temperature differences as air flow increased from 0.01 to 0.1 kg/s, while the distillate production remained unchanged.

Ambient wind speed has a direct impact on convective thermal losses from the DSSD-AH-ACC to the surrounding environment. As wind speed increases, losses from both the AH and ACC rise, reducing thermal losses recoveries by the circulating air through them and thereby worsening the overall thermal performance of the DSSD-AH-ACC. Fig. 10 illustrates this effect across air flows ranging from 0.01 to 0.1 kg/s. When wind velocity increased from 2 to 6 m/s, the temperature difference of the air exiting from the AH and ACC decreased by about 11%–12% under 0.01 kg/s air flow and by about 6%–8% under 0.1 kg/s air flow. Consequently, thermal efficiency declined by nearly 2% although distillate production remained unaffected. Remarkably, the rise in wind velocity maintained the 40% improvement in thermal efficiency and the 75% reduction in air temperature differences achieved by increasing air flow from 0.01 to 0.1 kg/s, while the distillate production remained unchanged.

The initial saline water mass directly regulates the temperature difference between the saline water and the distiller covers by governing the saline water temperature, and thereby influencing distillate production. Fig. 11 shows the impact of varying the initial saline water height, from 6 to 14 mm, on the performance metrics of the DSSD-AH-ACC under air flows ranging from 0.01 to 0.1 kg/s. Increasing the initial saline water height in the distiller basin reduced distillate production by about 4.5%–6%, the air temperature difference at the exit from the AH and ACC by about 11%–15%, and thermal efficiency by about 3%–4%. Consistent with earlier findings, increasing air flow from 0.01 to 0.1 kg/s sustained the 40% improvement in thermal efficiency and the 75% reduction in air temperature differences, for various water heights, while the distillate production remained unaffected.

Minimizing thermal losses from the DSSD-AH-ACC glass covers to the ambient surroundings during off-sun hours can improve the DSSD-AH-ACC performance. This can be achieved by covering the g₁₁ and g₂₂ glass covers with insulation boards after sunset to preserve residual heat. Fig. 12 compares the performance of the DSSD-AH-ACC with and without insulation covering during off-sun hours under air flows of 0.01 and 0.1 kg/s. With insulation covering, distillate production (Fig. 12a) decreased by nearly 1% under 0.01 kg/s air flow, while it remained unchanged at 0.1 kg/s. The average temperature of the air exiting the AH and ACC (Fig. 12b) increased by about 9% under 0.01 kg/s and 1% under 0.1 kg/s air flow. As a result, thermal efficiency (Fig. 12c) improved by approximately 5% under 0.01 kg/s and 4% under 0.1 kg/s air flow.

Table 1 compares the distillate production and thermal efficiency of the DSSD-AH-ACC with those of the most recent passive DSSDs reported in the literature. The distillate production of the DSSD-AH-ACC lies within the range documented for other DSSDs, despite its relatively simple design compared to most DSSDs included in the table. Notably, the DSSD-AH-ACC demonstrated the highest thermal efficiency among all listed DSSDs, as well as among related DSSDs [27–29] that employed the concept of recovering the inevitable heat losses from the distiller in auxiliary air or water heating.