**1. Introduction**

Solar energy is a renewable energy source capable of addressing environmental concerns and concerns about energy sustainability. The best known and most widely used technology in renewable energy generation is the use of photovoltaic (PV) modules that convert solar energy into electrical energy through photovoltaic cells. Admittedly, much of the solar energy absorbed by PV cells causes an increase in the temperature of the PV module, which causes a reduction in power output and energy efficiency, and negatively affects its performance and lifetime [1]. In addition, the most common climatic factors that influence the conversion efficiency of PV modules are solar radiation [2], ambient temperature and module surface temperature [1], relative humidity, wind speed and accumulated dust or shading problems on the module surface [3].

In recent years, researchers have conducted many reviews and comparative analyses on various proposed cooling techniques [4,5], of which water cooling systems stand out as a good solution to improve electrical efficiency and decrease the degradation rate of PV cells. On the other hand, Vivar et al., 2010 [6], proposed a novel technology called SolWat, which was later proposed [7,8] to improve environmental sustainability (scarcity, water stress and deterioration of water quality) and reduce the energy consumption of a wastewater treatment plant (WWTP) by implementing this technology as a tertiary treatment. This proposal would be very beneficial since energy consumption in the WWTP is a critical

**Citation:** Torres López, J.; Vivar García, M.; Fuentes Conde, M.; Palacios Villa, A.M. Open SolWat System with Cooling of the Secondary Wastewater Effluent from a WWTP on the Front Surface of the Photovoltaic Module for efficient Energy Generation and Reclaimed Water Production. *Environ. Sci. Proc.* **2023**, *25*, 81. https://doi.org/ 10.3390/ECWS-7-14321

Academic Editor: Athanasios Loukas

Published: 3 April 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

part of its operation and maintenance costs, especially for tertiary treatments that require high energy consumption, although as they are not required by law, they are not usually included in the wastewater treatment line. In view of these problems, possible solutions can be found in the use of renewable treatment plants [9,10] (to obtain energy) that also allow water to be reused.

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The results obtained by Vivar et al., 2021 [7], and Torres et al., 2022 [8], managed to obtain reclaimed water from the secondary effluent of wastewater from a WWTP, after SODIS disinfection treatment [11] under real sunlight for 4 h, and to produce energy simultaneously. Now, this study proposes a new design in SolWat technology, the Open SolWat system. Following the objectives already proposed in previous research, this work aims to: (a) improve the final quality of reclaimed water for reuse and for final discharge to the environment; and (b) obtain energy, but this time the new prototype will significantly increase the energy efficiency in the SolWat PV module by means of a cooling system based on a thin film of water on the front surface of the module.

### **2. Materials and Methods**

The SolWat technology is based on a solar–photovoltaic hybrid system (water disinfection reactor coupled to a photovoltaic module) that uses only solar energy to purify water and generate electricity. This technology receives solar radiation and uses its broad electromagnetic spectrum for (1) water disinfection thanks to the germicidal effect of ultraviolet (UV) radiation and the thermal effect of far infrared radiation (FIR) that drastically reduce the amount of pathogenic microorganisms present in the water, and (2) electricity production thanks to the visible (VIS) and near infrared (NIR) radiation reaching the photovoltaic module.

The new prototype is called the "Open SolWat system" (Figure 1) and has a dynamic operation mode (with water recirculation). The system consists of a PV module through which a thin water film (1 mm) circulates, allowing cooling of the PV module and solar disinfection of the water, from the top of the module until it is collected in an open water tank, which is exposed to solar radiation and therefore also acts as a solar water disinfection reactor. A reference PV module was also used experimentally as a control system.

**Figure 1.** (**a**) Open SolWat system and reference photovoltaic module. (**b**) Use of the electromagnetic spectrum in Open SolWat.

The PV modules were polycrystalline silicon cells (PV LOGIC, Bredon, UK) consisting of 36 solar cells connected in series with dimensions of 634 × 535 × 25 mm (0.251 m<sup>2</sup> cell area) and a nominal power of 45 Wp. This prototype has two water reactors: (a) a reactor formed only by an L-shaped aluminium profile at the top of the photovoltaic module, where the perforated pipe with 53 water outlet microtubes (3 mm water inlet and 2 mm

water outlet) with a separation of 1 cm between them is attached; and (b) the open and translucent water tank (62 × 45 × 18 cm). Water recirculation in the SolWat was performed by a pumping system, with a Xylem Flojet magnetic coupling water pump (NDP14/2, supply voltage: 230 V, input power: 10 W) through a set of pipes that propelled the water sample from the open water tank to the SolWat, with an average flow rate of 8.9 L/min. The water pump was primed with 200 mL of the experimental sample. Experimental tests with 4.2 and 6.2 L volumes were carried out with wastewater and purified and ultrapure (Milli-Q) water samples to study the influence of water flow and turbidity on PV performance. The homogenisation of the samples was achieved thanks to the drop of the water sheet into the open tank and the water pipe with 7 orifices distributed inside the tank.
