**2. Overview of FPV System**

Photovoltaics are now the fastest growing source of generating electricity worldwide. There are different methods to install PV systems in different infrastructures. The most famous mounting methods are:


In 2007, the first FPV system was installed with 20 kW capacity in Japan [24]. At present, they are installed in many locations worldwide, with a total installed capacity of 1.10 GW according to the World Bank report in 2019 [25]. Compared to ground or roof-mounted PV systems, water-mounted PV systems are proved to have very good performance among different countries and conditions. The main advantages of FPV systems are the absence of land acquisition, increased efficiency, less soiling and shading losses, the maintenance of water quality, the prevention of algal blooms, automatic water evaporative cooling and saving water from evaporation [26–31]. The most famous FPV stations worldwide are:


There are remarkable benefits from floating PV technologies, which are:


The structural components involved in the FPV system are floating platforms, cables, mooring and an anchoring mechanism, as shown in Figure 1. Floating platforms are the supporting structure with enough buoyancy to float itself with the installed PV array. Pontoon-based floating structures made up of high density polyethylene (HDPE), is the most commonly used floating structure [27]. Apart from that, metal-based structures with steel pipes are also used to support the PV array on water [28,29]. In order to ensure the stability of the floating structure from the action of waves and wind currents, a proper anchoring mechanism should be implemented. Based on the soil type and water level of the reservoir, the floating structure is anchored. Anchors made up of concrete blocks are placed on the bottom of the water body and connected to the edges of the PV array through mooring lines. [27,29]. Based on the position of the FPV array, anchoring on the embankment of the reservoir or on the nearby land area is possible [13,20,21,32]. Mooring lines ensure the flexibility and stability of the FPV system during severe wind and waves. Elastic mooring lines are used to make the FPV structure more flexible during a drift in water level during monsoon and empty reservoir conditions [30]. The power generated from the PV array installed on the floating structure is connected to the substation through underwater cables. Based on the distance of the substation from the FPV array, the inverter

station is either placed on the ground or on a separate floating platform near the PV array to reduce the resistive losses [20–22,29,33]. Consequently, to increase the overall efficiency of the system, a cleaning and tracking mechanism can be implemented. In the present study, a pontoon-based floating PV, and a bottom anchoring system with elastic mooring lines is considered.

**Figure 1.** Key design elements for power generation through FPV system.

#### **3. Need for Reducing Evaporation Rate**

Egypt is situated in the north-eastern part of Africa that experiences hot desert climatic conditions all year round. The daytime temperatures in the geographical location are extremely hot, with temperature levels greater than 45 ◦C, and the annual average sunshine hours is more than 3500 h. With very good irradiation levels, Egypt experiences a high number of patent evolutions in the energy generation, transmission and distribution sector, in which solar PV technology from the renewable energy sources play a major role [7,8]. Additionally, these high radiation levels increase the rate of evaporation, which leads to water scarcity conditions.

The entire population of Egypt depends on the Nile River as the major water source to satisfy the irrigation, domestic and industrial water needs. A significant number of dams and canals were constructed in the early 1950s to preserve and consume the water during dry summer seasons. As mentioned above, this clear, bright and sunny yearround season elevates the evaporation rate of open water storage surfaces. Thus, an increase in evaporation rate increases the water loss, and the purpose of the water storage system is affected. Additionally, the drift in the water head level of the reservoir affects the continuous power generation through hydroelectric power plants. Various covering systems are practiced, reducing the impacts of radiation and temperature on a reservoir with a large surface area. Floating PV panels are an effective covering system in mitigating the evaporation rate while generating power. Additionally, with high radiation and sunlight hours of the region, the conventional PV system experiences issues of overheating of the panels by the formation of hotspots which experience the over proportional heating of solar cells in comparison with other cells of the module. These issues are eliminated by the water evaporative cooling of panels by placing PV panels on the water surface; thus, the generated power is more highly efficient than the rooftop and ground-mounted PV systems. Further, the FPV system with an equivalent capacity of the hydroelectric power plant has the potential to provide intermittent operation, ensuring a continuous power supply.

#### *Potential Evapotranspiration Estimation*

Initially, in order to calculate the amount of water saved by the FPV as it covers surface, it is necessary to estimate the annual water loss (liters/year) through potential evapotranspiration (PET) from the reservoir [30]. Many conventional methodologies are in practice to estimate the PET at different times of year based on the geographical conditions of the location [30]. The calculation needs the data of radiation with duration, temperature (surface, air, wet-bulb and dew point), wind velocity, latitude, and latent heat of vaporization, humidity, pressure and albedo over a period of time. Evaporation estimation techniques are mainly categorized into pan evaporation, the water budget method, the mass transfer method, and the water and energy balance method [30,33].

The Penman–Monteith method of estimating the evaporation rate is the most commonly used methodology, as it is highly recommended for its accuracy. The calculation needs the daily meteorological data of temperature, relative humidity, wind speed and irradiation incident on the horizontal surface with the geographical information including latitude and altitude above the sea level of the particular location. This meteorological data for Aswan High Dam and Aswan Reservoir for the period of 10 years (2010–2019) were obtained from the NASA website to calculate the annual water loss through PET [18].

These two dams also have a specific installed capacity and currently operate as the source of hydroelectric power projects. Since water is the direct source of power generation in HEPP, the rate of evaporation directly affects the power production, which is the major reason for selecting these dams for the location of the FPV system to balance the water–energy nexus. The evaporation rate is calculated using the Penman method by Equation (1) [33].

$$\text{ET}\_{\text{o}} = \frac{0.408 \,\Delta \,(\text{R}\_{\text{n}} - \text{G}) + \,\gamma \,\frac{900}{\text{T} + 2\text{T}\%} \,\text{u}\_{2} \,(\text{e}\_{\text{s}} - \text{e}\_{\text{a}})}{\Delta + \,\gamma (1 + 0.34) \text{u}\_{2}} \Big( \text{mm } \text{day}^{-1} \Big) \tag{1}$$

where ETo is the reference evapotranspiration mm day−<sup>1</sup> , Δ is the slope of the vapor pressure curve - kPa ◦C−<sup>1</sup> , u2 is the wind speed at 2-m height ms−<sup>1</sup> , Rn is the net radiation - MJ m<sup>−</sup>2day−<sup>1</sup> . <sup>G</sup> is the soil heat flux density - MJ m<sup>−</sup>2day−<sup>1</sup> , es is the saturation vapor pressure (kPa), ea is the actual pressure (kPa), γ is the psychometric constant - kPa◦C−<sup>1</sup> and T is the mean daily air temperature at 2 m height (◦C).

From the calculated results, Figure 2 shows the sum of the monthly potential evaporation rate (mm), which is calculated from 2000 to 2019, that has a higher level of PET half of the year, with July being the peak [34]. Additionally, from the trend line of Figure 3a, it is observed that the cumulative rate of PET is increasing every year and Figure 3b shows the annual water loss of the reservoir through evaporation. This gives a major concern in regard to mitigating the loss of available freshwater resources through evaporation. Over the selected period, Figure 4 shows the month-wise sum of PET values. In the months of June and July, the water loss through evaporation is higher than others, with values reaching more than 200 mm.

**Figure 2.** Evaporation rate in Egyptian dams among years in mm/day.

(**b**)

**Figure 3.** (**a**) Annual average of potential evaporation rate per day (**b**) annual water loss through evaporation (mm).

**Figure 4.** Sum of monthly potential evaporation rate over the period of (2000–2019).
