• Energy Research

Abstract The present work investigated the outdoor performance of a basin type multi-stage solar still as well as the effect of collector over basin area (CBA) ratio on the distillate production. The effect of CBA was studied in outdoor experiments for the first time. The system consisted of a still coupled with one, two or three flat-plate solar collectors. In order to study the effect of CBA more accurately, the experiments were conducted in both winter and summer. In these experiments, the distilled water production of the multi-stage still was measured during the whole 24 h of a day on an hourly basis. The results revealed that in the winter, the basin coupled to one solar collector (CBA = 3.45) produced 11.56 kg of distillate. Adding a second collector (CBA = 6.90) to the system increased the production by 96%, whereas adding a third one (CBA = 10.35) increased it by only 23%. However, in the summer, 48% and 23% more productivity were obtained by adding the second and third collector, respectively.

Abstract In the present work, effects of height, length, and width of a single-slope basin-type solar still on its distillate production was investigated. For this purpose, a radiation model was developed which accounts for the influences of all walls on quantities of received solar radiation by the base and saline water of the still. Moreover, in this radiation model, the side walls of the still were considered as the trapezoid and the diffuse as well as beam components of solar irradiance were separately taken into account for the first time. Abilities of the present model were compared with earlier ones. Furthermore, the predictions of the current model were compared with the present experimental results, and it was observed that the predicted and measured values are in a close agreement. The results of this study show that by increasing the height of still's walls, its efficiency decreases. The model suggests that the height of the front wall should be less than about 0.10 m. Moreover, it is illustrated that by extending the still length, the side walls shadow reduces and consequently, the system efficiency increases. In addition, the optimal width to length ratio was found to be about 0.4.

Abstract Solar collecting area is one of the most important operating parameters of active solar stills. No experimental work has been performed to investigate the effect of this parameter to date. Furthermore in all of previous theoretical studies the effect of solar collecting area was examined during only the first 24-hour period of the operation of stills with one specified brine depth. However the present work experimentally studies the long-term simultaneous effects of collector area and brine depth on the performance of active solar stills. For this purpose four parallel active solar stills with different collector areas were fabricated and experiments were conducted for 5 consecutive days during which different amounts of brine depth were considered for active solar stills. The present results indicated that the overall trend of distilled water production and efficiency lessened with decreased brine depth for all solar collecting areas. Moreover, as the solar collecting area increased, water production of stills increased but their efficiency decreased for all brine depths. Furthermore the current results showed that for active solar stills with low brine depth and high solar collecting area the brine may boil and this boiling decreases the efficiency of active solar stills.

Abstract Effects of the amount and mode of input energy to an active multi-stage solar still were investigated in this work. To control the input energy, an electrical heater controlled by a PLC was utilized to simulate the energy absorbed by solar collectors. The study of the amount of input energy indicated that the freshwater production was a quadratic function of the collector over basin area (CBA) ratio. It was also found that stages 1 to 4 produced about 36%, 26%, 20% and 18% of the overall yield, respectively. Moreover, the effect of employing a thermal energy storage (TES) on the system performance was studied by comparing the mode of feeding energy according to the daily solar radiation pattern with the impulsive pattern (feed all of the energy at the beginning of the experiment). The current results showed that using TES increased the still's production by merely 5–10%. Thus, TES is suggested to be employed only if the CBA ratio is high and the system is not capable of operating at high temperatures.

Abstract One of the most important operating parameters which affects the performance and efficiency of active solar stills is brine depth. In all of the previous experimental or theoretical studies, effects of water depth were investigated during only the first 24-hour period (or even shorter periods) of the operation of active solar stills. In other words, only the first day was taken into account. However, the production of an active solar still depends on several parameters such as brine temperature at sunrise (initial temperature), which are all affected by the depth variation after the first day of operation. However, the present research experimentally investigates the long-term effects of water depth on the performance of active solar stills for the first time. For this purpose, two parallel active solar stills coupled with solar collectors with different surface areas were fabricated and an experiment was conducted for 10 continuous days. Unlike previous studies, results indicated that the overall trend of distilled water production and efficiency increased with increased water depth. Hence, higher water depth is recommended for practical uses of solar stills (more than two days).

Abstract In this work, the effect of the number of stages on the productivity of a multi-effect active solar still was experimentally investigated for the first time. Moreover, system performances in continuous and non-continuous modes were compared. For this purpose, indoor experiments were conducted on 4 similar solar still devices with different stages (1–4 stages) in order to accurately control the environmental conditions. In addition, water production was hourly measured during the whole 24-h experiment. The results show that with increased number of stages, distillate production can be predicted with a quadratic function. Moreover, adding a maximum of 6 and 10 additional stages can significantly increase production in continuous and non-continuous modes, respectively. It was also concluded that with more stages, the production enhancement is more in the continuous mode compared to the non-continuous mode, in a way that there is no significant difference between the performance of the single-stage device in continuous and non-continuous modes.

The effects of water depth in solar stills were studied in many earlier works. It was revealed that in the previous experimental works, the water surface-cover distance (WCD) was altered with the change of the water depth. However, in this research, the effects of water depth and WCD were investigated separately, and effects of water depth on the performance of solar stills with the same WCD were examined for the first time. In this way at first, some experiments were conducted in the summer and winter seasons using the stills with the same water depths, but different basin depths (i.e. different WCDs). It was found that WCD can affect the amount of distillate yield up to 26%. Thus, it was concluded that to study the effect of water depth accurately, different stills should be employed at the same time (to keep WCD constant). In the second step, some experiments were conducted using four stills in the summer, fall and winter seasons to examine the effects of water depth, while the WCD was constant. In addition, the stills with different water depths were modeled analytically and their performance was investigated. Moreover, an empirical relationship was obtained between the distillate yield and the water depth. By comparing the results of this empirical relation with previous studies, it was revealed that the past researches reported a lower dependency (in the average 15%) of the distillate yield on the water depth, since in their experimental works, WCD was changed along with the water depth.

In this paper, a new radiation model for a single-slope solar still has been developed which for the first time takes into account the effect of all walls of the still on the amount of incident solar radiation on the water surface and each wall. In this model, the walls are projected on the cover to calculate the amount of beam radiation received by any components inside the still. This is in contrary to the previous models in which stills' walls were projected on a horizontal surface and their shadows on the water surface and the walls were either neglected or not computed properly. The predictions of the proposed model are compared with the experimental data obtained in the present work and data from the literature. It was found that there is a good agreement between the theoretical results and the experimental data. Moreover, time variations of the incident beam radiation on different parts of a single-slope solar still are investigated. The results show that the effect of the back and side walls is not inconsiderable and they should be taken into consideration to improve the accuracy of the thermal radiation analysis of single-slope solar stills' performance.

This study investigated the effect of an internal reflector (IR) on the productivity of a single-slope solar still (during the summer and winter) experimentally and theoretically. A mathematical model was presented which took into account the effect of all walls (north, south, west and east) of the still on the amount of received solar radiation to brine, and the model was validated with the experimental data. The model can calculate the yield of the still with and without IR on various walls. The results show that the simultaneous use of IR on front and side walls enhances the still’s efficiency by 18%. However, installation of an IR on the back wall can increase the annual efficiency by 22%. The installation of IRs on all walls in comparison to a still without IR can increase the distillate production at winter, summer and the entire year by 65%, 22% and 34%, respectively. Furthermore, the effect of cloud factor on the installation of IRs on all walls was examined, and the results indicate that the increasing the cloud factor decreases the influence of IRs significantly.