Efficiency of a photovoltaic thermal stepped solar still: Experimental and numerical analysis

doi:10.1016/j.desal.2018.04.014

Desalination

Volume 441, 1 September 2018, Pages 87-95

https://doi.org/10.1016/j.desal.2018.04.014 Get rights and content

Highlights

•
Connection of PVT collector to stepped solar still is studied numerically and experimentally.
•
Freshwater productivity can be improved by PVT collector connection to stepped solar still.
•
A desired value for mass flow rate and PVT collector area is obtained to enhance energy efficiency.

Abstract

In this study, the performance of a stepped solar still connected to photovoltaic thermal (PVT) water collector was numerically and experimentally analyzed. By considering energy balance for the different components of solar still system, a transient thermal model was achieved (i.e. PVT water collector, saline water, glass cover, absorber plate). Moreover, an expression for the system energy efficiency was obtained. An experimental setup was designed and manufactured. The simulation results were validated with experimental data. Then, parametric studies were performed and the influence of various design and operating parameters on the freshwater productivity, energy efficiency and output electrical power were studied. It was shown that there would be a desired value for the area of PVT water collector and mass flow rate of saline water maximizing the energy efficiency. The mass flow rate and PVT water collector area desired value was 0.068 kg/min and 1.33 m², respectively. Moreover, connecting the PVT water collector to stepped solar still enhanced the energy efficiency and freshwater productivity and for other uses could provide additional electrical power.

Graphical abstract

In this paper, the connection influence of photovoltaic thermal collector to stepped solar still is investigated experimentally and numerically. Furthermore, the effect of design and operating parameters on energy efficiency, freshwater productivity and output electrical power is studied.

Introduction

In recent years, the excessive growth of the human population has caused extra demand for fresh water. Fresh water resources are limited and only 3% of Earth's water is included [1]. Direct solar desalination could be a suitable solution for the production of water in domestic scale especially in arid and remote areas. Because there is a high potential for solar radiation and abundance of saline water resources in these areas [2]. Two common and low-cost designs for direct solar desalination include stepped solar stills and basin solar stills. However, the fresh water production efficiency of stepped solar stills is higher than the basin solar stills. Because in stepped solar stills, the thickness of saline water layer is less and salt water as forced mode in the solar still flows and the evaporation rate is much higher [3]. However, electrical power is needed for pumping saline water in stepped solar still. By connecting the photovoltaic thermal (PVT) water collector, electricity power needed to pump saline water in the system can be supplied. Moreover, the PV/T collector operated as preheater of water and by raising the temperature of the saline water in the system will increase the production of fresh water. On the other hand, the electrical performance of photovoltaic cells in the PVT water collector due to cooling that with saline water would be improved. Performance analysis of stepped solar stills can provide criteria for the improvement of fresh water production. In recent years, numerous studies have been conducted on the performance analysis of stepped solar stills.

The beginning of research on stepped solar stills is attributed to Headley [4] in the early seventies. He designed and tested a stepped solar still. His study showed that the freshwater productivity of stepped solar stills is much higher than basin solar stills. Because in stepped solar still, the thickness of saline water layer is less and consequently the rate of evaporation is higher.

Khelif and Touati [5] carried out a theoretical and experimental study on the performance of a stepped solar still. They introduced three main parameters, namely optical efficiency, global losses factor, and time constant as the characterizations of solar still, and finally provided solutions to improve the performance of solar still by these three parameters.

Radhwan [6] studied the performance of a stepped solar still connected to the latent heat storage by a transient numerical model. He reported that the efficiency and the daily production of fresh water in solar still connected to the latent heat storage, is 57% and 4.6 l/m² respectively.

Abdel-Rehim and Lasheen [7] have investigated the performance of an active solar still connected to solar parabolic trough collector as a preheater, experimentally. Their results have shown that the freshwater productivity of their solar still 18% is more than the conventional basin solar stills due to the usage of parabolic trough collector.

Sadineni et al. [3] designed and manufactured a stepped solar still. The results showed that freshwater productivity of stepped solar stills is around 20% higher than the basin solar still.

Farshchi Tabrizi et al. [2] performed an experimental study on the impact of saline water flow rate on the heat transfer and mass transfer coefficients in a stepped solar still. They presented based on experimental data a corrected equation for evaporative heat transfer coefficient for stepped solar still.

Dashtban and Farshchi Tabrizi [8] investigated the thickness of the air gap between the glass cover and absorber plate in a stepped solar still. They introduced the optimal air gap thickness of about 3 cm.

Kabeel et al. [9] studied the effect of depth and width of trays on freshwater productivity of a stepped solar still. They stated that if the depth and width of trays could be 5 mm and 120 mm respectively, the maximum freshwater productivity would be achieved.

Sampathkumar and Senthilkumar [10] have investigated the performance of an active solar still connected to evacuated tube collector (ETC), experimentally. They have observed that the productivity of ETC solar still is twice the productivity of a conventional basin solar still due to preheating of saline water by ETC collector.

Alaudeen et al. [11] studied the impact of connecting a flat plate collector into stepped solar still. The results showed that because of the saline water preheating by flat plate collector, the amount of freshwater productivity increased to a good extent.

Hansen et al. [12] experimentally studied the effect of using different wick materials on the efficiency of an inclined solar still. The results of their study showed that the use of wick material of coral fleece type with weir mesh-stepped absorber plate can increase the freshwater productivity up to 4.28 l/day.

Sathyamurthy et al. [13] showed that the connection of baffle plates on the absorber plate in stepped solar stills could appropriately increase the freshwater productivity. This is due to the increase of surface exposed to solar radiation.

Nagarajan et al. [14] examined the impact of the use of baffles in an inclined solar still. Their results showed that the use of baffles increased the freshwater productivity up to 168%.

In previous studies [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]], the influence of connecting the PVT water collector on the stepped solar still has not been investigated. While connecting the PVT water collector to stepped solar still that has been investigated in this study could have some advantages as follows:

•
The PVT water collector provides the electrical power needed to pump saline water in stepped solar still.
•
The PVT water collector by preheating the water at the entrance to the stepped solar still would lead to the increase of the saline water temperature and improving the fresh water production efficiency.
•
The PVT water collector provides additional electrical power for other uses.
•
The electrical performance of photovoltaic cells in the PVT water collector due cooling by saline water improves.

In this study, an experimental setup for stepped solar still connected to the PVT water collector has been designed and fabricated. By performing some tests on the mentioned setup, some experimental data has been achieved. Due to the limitations of the experimental tests and to investigate other various conditions, the numerical modeling of stepped solar still connected to the PVT water collector will be carried out. After validating the results of numerical modeling, parametric studies are addressed and the influence of various design and operating parameters on energy efficiency, freshwater productivity and output electrical power is investigated.

Section snippets

Energy analysis

A schematic diagram of stepped solar still connected to PVT water collector and its components are shown in Fig. 1.

According to Fig. 1, the saline water is pumped into the PVT collector and then preheated by receiving heat from the PV cells. Preheated saline water evaporates by receiving solar radiation in the stepped solar still. Evaporated steam is condensed on glass cover and fresh water is produced. In addition to supplying pumping power, the PV cells provide extra power for other uses. To

Validation

An experimental setup for stepped solar still connected to PVT water collector in Research Laboratory of Renewable Energy and Electromagnetic Fluids, University of Sistan and Baluchestan has been designed and fabricated. The experimental setup of stepped solar still connected to PVT collector and its components are shown in Fig. 3.

The experimental setup includes stepped solar still, PVT water collector, pump, storage tank and other equipment. In the stepped solar still in the study, the

Results and discussion

In this section, parametric studies will be addressed. The aim of parametric studies is the investigation of design and operating parameters effect on energy efficiency, freshwater productivity and output electrical power.

In Fig. 7, the changes of energy efficiency versus PVT water collector area and the mass flow of saline water are drawn in three dimensions. To draw the corresponding figure, mean values of solar radiation intensity, ambient temperature and wind speed during the testing period

Conclusion

In the present study, the idea of connecting the PVT water collector to stepped solar still was experimentally and numerically investigated and the main results are as follows:

•
The numerical modeling of PVT stepped solar still is in good agreement with experimental data.
•
There is an optimal point for the mass flow of saline water and PVT water collector area that maximize the energy efficiency.
•
Connecting the PVT water collector to stepped solar still makes >2-time increase in energy efficiency

Nomenclature

area (m²)

specific heat capacity (J/kg·°C)

distance between glass cover and water surface (m)

energy rate (W)

relative error (%)

F_R

heat removal factor

\overset{´}{F}

fin efficiency factor

gravity acceleration (m/s²)

solar radiation intensity (W/m)

Gr′

modified Grashof number

overall heat transfer coefficient (W/m²·K)

h_fg

latent heat of water vaporization (J/kg)

h_p1

penalty factor due to the presence of solar cell material, glass and EVA

h_p2

penalty factor due to the presence of interface between absorber plate and

References (15)

V. Velmurugan et al.
Single basin solar still with fin for enhancing productivity
Energy Convers. Manag.
(2008)
F. Farshchi Tabrizi et al.
Effect of water flow rate on internal heat and mass transfer and daily productivity of a weir-type cascade solar still
Desalination
(2010)
S.B. Sadineni et al.
Theory and experimental investigation of a weir-type inclined solar still
Energy
(2008)
O.S.T.C. Headley
Cascade solar still for distilled water production
Sol. Energy
(1973)
C. Khelif et al.
Semi-empirical characterization of a greenhouse-effect cascades solar distiller
Renew. Energy
(2000)
A.R. Radhwan
Transient performance of a stepped solar still withbuilt-in latent heat thermal energy storage
Desalination
(2005)
Z.S. Abdel-Rehim et al.
Experimental and theoretical study of a solar desalination system located in Cairo, Egypt
Desalination
(2007)

There are more references available in the full text version of this article.

Cited by (67)

A novel numerical model for solar still combined with collector and reflector
2024, Applied Thermal Engineering
Despite numerous advantageous, Solar Stills have low efficiency. This novel 2D numerical study aims to enhance the purified water productivity of a domestic-scale solar still through integrating it with three advantageous solar components of a Photovoltaic-Thermal Water Collector, a Solar Evacuated Heat Pipe Tube Collector, and an external reflector. The governing equations encompass mass, momentum, and energy conservation, and species transport equations, along with equations for radiation and turbulence models. User-defined function codes are utilized to couple these equations and apply source terms. The findings pointed out that the following scenarios offer the highest potential for yield improvement: (1) 39.6 % increase by integrating with a Photovoltaic collector having circulation water flow rate of $0.1 k g / m i n$ and collector area of $2 m^{2}$ . (2) 59.5 % increase by simultaneous use of Photovoltaic collector having circulation water flow rate of $0.068 k g / m i n$ and collector area of $1.33 m^{2}$ , plus reusing output electrical energy. (3) 76.9 % increase by integrating with heat pipe collector having heat pipe and solar still circulation water flow rate of $0.11 k g / s$ and $0.51 k g / m i n$ , and number of evacuated tubes of $15$ . (4) 40.3 % increase by using an external reflector with an angle of $90^{°}$ between glass and reflector.
Recent advances in the applications of solar-driven co-generation systems for heat, freshwater and power
2024, Renewable Energy
Increasing the demand for water and energy resources as well as the clean energy policies and the need for pollution emission reduction has developed the research on the renewable energy resources. This paper reviews the recent advances in multi-generation units for heat, electricity, and desalinated water production. It tries to investigate the new papers in this arena which were not mentioned in the previous reviews. The main parts of this work introduce the co-generation systems for the production of “water and electricity”, “heat and water”, and “electricity, water, and heat". The results show that among the solar energy receiving devices, Photovoltaic/Thermal panels (PV/Ts) and parabolic trough solar collectors are the most used in co-generation systems. On the other hand, a combination of solar collectors and power cycles makes them a suitable choice for the establishment of a large-scale co-generation plant. Therefore, solar collector-based electricity and water co-generation systems have at least 10 times more output capacity than PV- or PV/T-based systems. On the other hand, most heat and water co-generation systems can achieve an output temperature of 60 °C or more and produce around 10000 kg/year of freshwater based on system size. Examining the co-generation systems of water, electricity and heat also shows that these systems have a high production capacity compared to other co-generation systems. Systems based on heliostat solar tower can produce maximum powers of more than 18 MW and fresh water of 90 kg/s.
Exploring stepped solar still developments with a case study for potable water provision in salt farming regions
2024, Sustainable Energy Technologies and Assessments
All living things' most critical need is access to potable water, especially humans. India is one of the countries where there is still a serious shortage of fresh, drinkable water. Technology that uses solar energy to desalinate water has proven to be noteworthy for these problems, allowing for the free provision of fresh water. This article reviews recent developments in solar still designs, notably stepped solar stills. Additionally, approximately 10,000 salt farmers situated in coastal areas and the Little Rann of Kutch, Gujarat, play a central role in India's salt farming activities. An in-depth examination of their work life and daily need for potable water suggests that implementing stepped-solar stills could be a viable and cost-effective solution in this area that yields an average of 5–7 L of fresh water each day. It is estimated that employing around 2857 stepped solar distillers, each with a 2 m² areas, could fulfil the entire potable water requirements of the salt farming communities. Furthermore,economic study suggests 0.0065$/L potable water cost to salt farmers by adopting stepped solar distillers. Introducing straightforward stepped solar distillers near salt farms can significantly enhance these communities' labour productivity and, more importantly, protect them from life-threatening diseases.
Hybrid thermal desalination systems for sustainable development – A critical review
2024, Solar Energy
Accessing economic, clean drinking water, and affordable clean energy should be achieved without compromising climate action, is a target of the United Nations agenda for 2030 sustainable development. Earth covers 70% of the water, of which fresh water is only 3%, and two-thirds of that amount is not readily usable; hence, it is exceedingly scarce for drinking, irrigation, and industrial use. Low yield and low efficiency are the key issues that standalone thermal desalination systems are facing, and numerous studies have been conducted to improve them same. Advanced technologies encourage low-cost clean water production along with any other effects like electricity, cooling, and heating with hybrid desalination systems using renewable energy or waste heat recovery. However, energy demand is escalating daily for human comfort, and around 300 crores of the population still use fossil fuels, causing a majority share of global greenhouse gas emissions. The objective of this article is to consolidate the scattered literature on various hybrid thermal desalination systems and contrast the desalination yield, efficiency, and production costs. The first section provides a brief introduction to different thermal desalination techniques and their working principles. The following section consolidates the reported literature on the potential integration of solar photovoltaics, cooling, and heating systems with various thermal desalination techniques, including solar stills, humidification-dehumidification desalination, greenhouse desalination, membrane distillation, multi-stage flash distillation, and multi-effect distillation and thermal vapor compression. The gained output ratio (GOR), recovery ratio (RR), freshwater yield, coefficient of performance (COP), thermal efficiencies, and energy utilization of integrated systems are some of the many performance parameters summarized. The key findings from the literature review are summarized and suggested permutations of the hybrid systems are highlighted. The recommendations for future research areas. The novelty of the present review is to consolidate the scattered literature on all thermal desalination techniques and suggest the future scope of research.
Investigating the effects of air inlet jet on the performance of solar still: A comprehensive numerical study
2024, Desalination
Solar stills are a simple method of producing potable water, but their low efficiency makes them insufficient for meeting daily water needs. This study aims to improve the efficiency of simple solar stills (simple SS) by investigating the effect of an air inlet jet. Factors such as air velocity, entrance angle, and inlet and outlet location were studied. The effect of inlet velocity was studied for air inlet velocities of 1, 2, 3, 4, 5, and 9 m/s, and the effect of inlet angle was studied under six different angles ranging from −45 to +45 degrees. Results showed at $U_{in} = 5 m / s$ for the case where the glass cover and basin water were in the highest temperature difference when the inlet was located on the left wall (left-inlet mode), water production rate ( $\dot{m}$ ) and heat transfer rate (Nu) improved by %125 and %126, respectively, and for the right-inlet mode, $\dot{m}$ and Nu improved by %209 and %205, respectively. For an angled inlet, at $U_{in} = 9 m / s$ with inlet angle of +15 degrees, for left-inlet mode, $\dot{m}$ and Nu improved by 211 % and 207 %, respectively, And for right-inlet mode, the $\dot{m}$ and Nu improved by up to 264 % and 257 %, respectively.
Experimental investigations on a sustainable cogeneration system for power and desalination with 4-E analysis
2023, Solar Energy
Several research investigations were carried out on the potential integration of solar stills with photovoltaic modules to improve freshwater production and electric output. The current work aims at integrating stepped solar still (SSS) with a bi-directional flow serpentine thermal absorber-based Photovoltaic-Thermal (PVT) module. Near the vernal equinox days, a laboratory-scale experimental facility is designed, developed, and tested in Vellore (12.9165° N, 79.1325° E). The novelty of the proposed system is that it effectively utilizes the waste heat recovered from PVT with a unique thermal absorber for preheating saline water supplied to the SSS. Experiments were carried out on a PVT-SSS system to estimate the electricity and freshwater generation by PVT and SSS for saline water flowrates of 0.3, 0.5, 0.75, and 1.00 LPM. The estimated average electrical efficiency of the PVT panel is 14.84 %, 15.46 %, 14.74 %, and 14.50 %, and the freshwater productivity from SSS is 3460 ml, 2970 ml, 2460 ml, and 1950 ml per square meter. The average thermal efficiency of the proposed PVT-SSS system is 34.5 %, 31.8 %, 20.1 %, and 16.3 %; 49.36 %, 47.30 %, 34.90 %, and 30.83 %, respectively, for the mentioned flow rates. Further, the average exergy efficiency of the system is observed as 18.5 %, 18.7 %, 17.2 %, and 16.6 %, respectively. In addition, the unit cost of freshwater production for various flow rates in USD is estimated as 0.07, 0.08, 0.09, and 0.10, respectively. An environmental analysis was also performed to determine the CO₂ emissions and carbon mitigation for the proposed hybrid desalination system. The results are validated with previous literature.

View all citing articles on Scopus

View full text

Efficiency of a photovoltaic thermal stepped solar still: Experimental and numerical analysis

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Energy analysis

Validation

Results and discussion

Conclusion

Nomenclature

Energy Convers. Manag.

Desalination

Energy

Sol. Energy

Renew. Energy

Desalination

Desalination