Synergistic freshwater and electricity production using passive membrane distillation and waste heat recovered from camouflaged photovoltaic modules
Introduction
At the time of writing, more than 1.42 billion people live in areas facing severe water stress all over the world. Despite the significant technological advancements in the last decades, today still less than 3 per cent of the global water resources is safely drinkable. Moreover, given the expected population growth and increasing urbanization in the next years, the global freshwater demand is estimated to increase by 20 to 30 per cent by 2050 in a range of different sectors (UNICEF, 2021). The second major problem associated with the current projections for global development relates to the increasing demand of energy, and to the consequent potential effect on climate change (De Angelis et al., 2021). Mitigation policies foresee an increasing use of renewable energy sources, towards a global sustainable development and an equal access to energy and freshwater sources (UN General Assembly, 2015).
In this view, water treatment technologies driven by renewable energy sources have great potential towards the above mentioned goals and have been extensively investigated and continuously improved in the recent years (Giudici et al., 2019, Bologna et al., 2020, Nassrullah et al., 2020, Yusuf et al., 2020). In particular, several articles proposed the coupling of photovoltaic (PV) modules with traditional water treatment technologies such as reverse and forward osmosis (Koutroulis and Kolokotsa, 2010, Khayet et al., 2016, Lee et al., 2016), electrodialysis (Campione et al., 2018, Xu et al., 2020) and mechanical vapour compression hybrid systems (Helal and Al-Malek, 2006). However, most of the proposed integrated systems use PV modules to power conventional desalination devices, while only a few works have experimentally investigated the coupling of PV with desalination devices (Manokar et al., 2018, Abd Elbar et al., 2019, Elminshawy et al., 2020, Chen et al., 2021), proving the complexity of this objective. Recently, novel advanced materials paved the way for passive devices in the field of simultaneous freshwater and electricity production (Liu et al., 2020, Dao et al., 2021). Passive technologies have the particular advantage to: (i) include no moving mechanical parts, thus typically requiring low maintenance and associated costs; (ii) be especially suitable for operation in off-grid areas, where no connection to the electric power grid is available. Among the others, promising results were obtained by bio-inspired systems (Jiang et al., 2020, Zhou et al., 2020), advanced nano-structured materials (Xue et al., 2017, Zhang et al., 2020), traditional (Wang et al., 2019, Wang et al., 2021) and advanced (Yang et al., 2017) membrane technologies. However, the novelty of these materials and the current fabrication techniques relegate these technologies to lab-scale and centimeter-sized prototypes with a low power density output, thus still lacking cost-effectiveness and a comprehensive evaluation of their scalability on a device-scale perspective.
The coupling of PV and multi-stage passive devices, a technology based on membrane distillation recently reported by some of the authors of the present work (Chiavazzo et al., 2018), offers a good compromise between modularity, scalability, low cost and high productivity. The device operates at ambient pressure using a multi-stage rationale, where high efficiency is obtained thanks to a series of multiple evaporation and condensation stages where latent heat is recovered several times (Signorato et al., 2020). The water feeding relies on capillary action, which makes it free from moving mechanical parts and additional energy needs. Thus, the design results to be completely passive, compact, and the employed materials inexpensive. While in the original concept solar thermal energy was exploited to power the system, the required low-temperature thermal energy may be also recovered from other sources of waste-heat (Morciano et al., 2020a).
Based on this idea, in this work we propose a coupled solution for efficient and sustainable freshwater and electricity production. The above mentioned multi-stage distillation device is indeed coupled with a PV module. The desalination unit is integrated on the back side of the PV unit, which provides the waste-heat to empower the distillation stages. This latter energy recovery has the additional advantage to lower the working temperature of the PV module, thus increasing its conversion efficiency (Letcher and Fthenakis, 2018). While similar integrated solutions have already been explored (Manokar et al., 2018, Sultan and Efzan, 2018, Wang et al., 2019), here we address the additional problem related to the environmental and visual impact of PV installations (Sánchez-Pantoja et al., 2018, Al-Karaghouli et al., 2010). To this, we propose and investigate the performance of a PV module with an innovative aesthetic surface coating, which allows to reduce the impact with the surrounding landscape. The resulting compact unit results to be particularly suitable for floating installations (Sahu et al., 2016, Cazzaniga et al., 2018). In order to assess the proposed solution, here we experimentally tested the performance of a working prototype. A numerical model is also developed and compared with the experimental results, to provide deeper insight on the sensitivity to the operating conditions.
The article is organized as follows. The working principle of the multi-stage desalination unit and its integration with the PV module are outlined in Section 2. Section 3 reports on the experimental setup for testing the performance of the prototype. The modeling approach and the adopted numerical framework are discussed in Section 4; whereas, the results are reported in Section 5. Finally, the conclusions are drawn in Section 6, along with the perspective developments of the proposed work.
Section snippets
Passive membrane distillation unit
The adopted multi-stage membrane distillation unit powered by solar thermal energy was originally introduced by Chiavazzo et al. (2018). It consists of a series of identical stages, which allow to recover the latent heat of vaporization several times before it is evacuated to the environment. A schematic of the working principle is reported in Fig. 1a. Each stage consists of: a synthetic microfiber (hydrophilic) cloth, acting as evaporator; a microporous polytetrafluoroethylene
Setup for testing the distiller
A schematic of the experimental setup used to test and characterize a single distiller module is shown in Fig. 3. The module is fed with the waste heat from the back side of the PV panel. The specific mass flow rate () of distilled water, expressed in L m−2 h−1, is evaluated under the reference conditions of irradiance equal to one sun, namely W m−2; salinity and temperature of the sodium chloride solution are equal to 35 g L−1 and 20 °C, respectively. In these tests, the PV
Analytical model
The Maxwell–Stefan equations combined with the Dusty Gas Model are used to describe the mass transfer of multi-components mixture of gases through porous media, such as membranes or air gaps. The model equation can be written as (Chiavazzo et al., 2018, Alberghini et al., 2020, Morciano et al., 2020a): where and are the absolute pressure and mean temperature within the membrane; is the universal gas constant; ,
Distillation performance of a single distiller
The freshwater production of a single distiller under the camouflaged PV panel obtained in the experiments is shown in Fig. 7a, where the mean value 1 s.d. is reported from five different tests, repeated according to the same protocol presented in Section 3.1. When powered with the constant irradiance from sun simulator, the produced distillate increases linearly over time. To monitor the evolution of the performance during the experiments, the specific flow rate of distilled water [L m−2 h−1
Conclusions
In this work, we have analyzed the coupling between a camouflaged photovoltaic technology and a passive, thermal desalination device. The water treatment unit is mounted on the back side of the PV module, which allows to recover and exploit the waste heat for freshwater production. Overall, the main original contributions of the presented solution are: (i) the innovative aesthetic surface coating, which allows to reduce the visual impact on the surrounding landscape; (ii) a mosaic-like
CRediT authorship contribution statement
Giovanni Antonetto: Methodology, Software, Formal analysis, Investigation, Data curation, Writing – original draft, Visualization. Matteo Morciano: Methodology, Software, Formal analysis, Writing – original draft, Visualization. Matteo Alberghini: Methodology, Formal analysis, Resources. Gabriele Malgaroli: Methodology, Formal analysis, Investigation, Writing – original draft, Visualization. Alessandro Ciocia: Methodology, Formal analysis, Resources. Luca Bergamasco: Methodology, Formal
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
M.F. acknowledges financial support by Politecnico di Torino, Italy (starting grant no. 56RIL16FAM01). We thank Eliodoro Chiavazzo and Pietro Asinari for helpful discussions.
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