1. Introduction
Water resource management in arid and semi-arid regions has become a crucial issue due to many factors such as population increase, climate change, and regional changes. Therefore, proper management of these resources saves a large amount of water and helps grow the economy and narrow the water shortage gaps. One of the main sources of water losses in these areas is the evaporation from lakes and large water bodies, which leads to billions of cubic meters of freshwater lost due to dry weather and high temperature. Evaporation loss varies from one reservoir to another and changes according to the change in hydro-meteorological data. Studying evaporation will allow a better understanding of the mechanisms and regularities that guide the water circulation in nature and also its associated processes [
1,
2,
3,
4].
Several methods were used to estimate evaporation from water bodies, including evaporation pans, water budget methods, the bulk aerodynamic method, and some empirical formulas such as Penman’s equation. Recently, the utilization of images provided by satellite sensor technology and remote sensing (RS) can help in determining evaporation and evapotranspiration with no previous knowledge of soil or crop conditions [
5]. Many studies estimated evaporation loss rates around the world using different methods. For example, the evaporation losses estimated from reservoirs in South East Queensland, Australia, were 40% of the total water storage capacity per year [
6], while they were 3.1 mm/day from Sparkling Lake, northern Wisconsin, USA [
7], 3.6 mm/day from Lake Okeechobee in South Florida [
8], and 4.87 mm/day from a tropical African lake in Ethiopia [
9]. In Turkey, the total evaporation loss was 6.8 × 10
9–4.1 × 10
9 m
3/year from reservoirs and 2.7 × 10
9 m
3/year from lakes [
10], while in Japan, the mean annual evaporation was 911 mm from Lake Kasumigaura [
11].
Evaporation losses in arid and semi-arid regions such as Egypt are generally high. For example, the evaporation from Lake Nasser in Egypt ranges from 12 × 10
9 to 16 × 10
9 m
3/year [
12]. Egypt has already been facing water scarcity, and the construction of the Grand Ethiopian Renaissance Dam will further reduce Egypt’s share of water from the Nile River significantly [
13]. Therefore, there is an urgent need to save every drop of water and manage the resources. One of these ways, which we suggest here, is to reduce the water evaporation from Lake Nasser, the second largest manmade lake in the world. The evaporated water loss from the lake ranges between 10 × 10
9 and 16 × 10
9 m
3 every year, which is equivalent to 20 to 30% of the Egyptian income from Nile water [
14].
Hassan [
15] estimated an evaporation rate of 6.6 mm/day using the Surface Energy Balance Algorithm for Land (SEBAL). Abdel Wahab et al. [
12] estimated the annual evaporation losses from Lake Nasser from 2001 to 2013 and they ranged from 12.3 × 10
9 to 12.9 × 10
9 m
3. Hamdan and Zaki [
16] presented a long-term estimation for the evaporation losses from Aswan High Dam Reservoir (AHDR) using local hydrological and meteorological data collected from nine stations. The bulk aerodynamic method was applied using monthly available hydro-meteorological data with a record of 20 years (1995/1996 to 2014/2015). Annual water losses by evaporation varied from 12 × 10
9 (in 1995/1996) to 15.53 × 10
9 m
3 (in 2007/2008) with an average of 13.62 × 10
9 m
3/year.
Some methods were used to reduce evaporation from Lake Nasser. For example, Hassan et al. (2007) estimated an average evaporation rate of 6.3 mm/day and an annual water loss by evaporation of 12.5 × 10
9 m
3. They used pontoon framework and circular foam sheets and found that 0.500 km
2 must be covered to save one million cubic meters annually. Ebaid and Ismail [
17] studied the reduction in evaporation from Lake Nasser by disconnecting some of its secondary channels (khors). The results showed that the evaporation rate ranged from 2.73 in the middle of the lake to 9.58 mm/day at the edge. The evaporated water loss throughout the entire lake was about 0.86 billion m
3/month in March. The study showed that disconnecting two khors could approximately save an evaporation loss of 2.4 × 10
9 m
3/year. Elba et al. [
18] investigated the impact of lowering the lakebed by removing sediments from the High Aswan Dam Reservoir (HADR), emphasizing evaporation losses. A digital elevation model for the HADR was developed to describe the hydrological characteristics and to assess the consequences of removing sediment deposits. The results showed that the removal of sediments would reduce evaporation losses by 1.1 km
3 of the lake projected for 2100, which represents 6.5% of the total projected evaporation losses.
One of the promising methods that may be used to reduce evaporation from open water surfaces is the floating photovoltaic system (FPVS). It has many advantages compared to overland installed solar panels, including fewer obstacles that block sunlight, convenience, energy efficiency, and higher power generation efficiency owing to its lower temperature underneath the panels. Additionally, the aquatic environment profits from the solar installation because the shading of the plant prevents excessive water evaporation, limits algae growth, and potentially improves water quality [
19]. A number of studies have been conducted on FPVSs, which assessed them from different points of view. Ranjbaran et al. [
20] presented an analytical analysis and updated review that studied different aspects of FPV systems as a power generation system.
There has been little work conducted on using FPVSs in reducing evaporation. Abid et al. [
21] presented a review study to assess the prospects and highlight the importance of floating solar panel technology. They discussed multiple opportunities of FPVSs in different regions of the world and highlighted the importance of such technologies in already water-scarce areas. The FPVS is a novel idea in renewable energy production without putting an additional burden on water and land resources. Since the FPVS is a relatively new concept, only a few demonstrator projects have been deployed worldwide [
22].
Figure 1 shows a 40 MW FPVS at Huainan, China [
23].
A typical overland photovoltaic (PV) module, depending on the type of solar cells and climatic conditions, converts 4–18% of the incident solar energy into electricity. For the rest of the incident, solar radiation is converted into heat, which significantly increases the temperature of the PV [
24,
25,
26,
27,
28]. Being installed on water, an FPVS has a significantly lower ambient temperature in virtue due to water’s cooling effect. Consequently, the efficiency of floating-type solar panels is 11% higher than ground-installed solar panels [
29,
30,
31,
32,
33]. The first pilot floating PV plant was built in California in 2008. By the end of 2014, a total of 22 photovoltaic power plants were built in different areas of the world with an installed capacity from 0.5 to 1157 kW [
34]. At present, the developments on marine floating PV systems are in the pipeline to examine the effects of corrosion of seawater on unit and PV configuration and the link with energy production efficacy [
35].
The main purpose of the present work is to investigate the use of the floating photovoltaic system to reduce the water evaporation loss from Lake Nasser in Egypt and produce clean energy. In this work, evaporation was estimated based on meteorological data for the period from 2009 to 2020. Different scenarios of covering the lake’s surface with an FPVS were studied and discussed. Further, the effect of covering shallow depths is studied. This study is significant, given the fact that Egypt is a water-stressed country and further shortage in water supply from the Nile River is expected due to the construction of the Grand Ethiopian Renaissance Dam.
4. Discussion
Evaporation from open water surfaces is a great challenge in arid and semi-arid regions. Therefore, there is an urgent need to reduce evaporation in these areas, which will help manage the reduction in evaporation and could be very useful in these areas to manage the water resources more efficiently. Lake Nasser is considered one of the largest lakes in the world exposed to a very high rate of evaporation.
This study suggested using an FPVS to cover parts of the lake to reduce water losses due to evaporation. Different ratios of the coverage area were suggested, ranging from 25% to 100%. Although these different scenarios of covering the water surface were analyzed for Lake Nasser, the large area of 5775 km2 of the lake makes it unlikely that the decision-makers in Egypt will go for covering 100% of the lake or even 50%. This has motivated the authors to look for more practical solutions that can be practically visible and may be implemented by decision-makers. One of the optimum covering scenarios that can be followed for Lake Nasser is to cover only the shallow areas of the lake. For example, covering the water depths of 1.0–3.0 m, which only represent 15% of the total area, will provide an annual water saving of about 2.66 million m3. This water saving represents about one third of the total water saving should an FPVS cover the entire water surface in the lake.
Not far from the lake, and specifically near Aswan, Egypt has recently built Benban, the largest solar park in Africa, which is constructed over an area of 37 square kilometers. According to the European Bank, this solar park will generate 1.5 GW, enough to provide renewable energy to more than 1,000,000 homes [
43]. Lake Nasser has similar weather to this site, and sunshine is there most of the year with very high temperatures normally above 40 °C in summer, which means the use of an FPVS is a proper choice to cover the lake, especially areas with water depths below 1.0 m [
31]. In addition to saving 1.9 billion m
3 of water, enough renewable energy can be generated to fill the consumption of millions of homes in Egypt.
5. Conclusions
Evaporation is a complex phenomenon with many factors and various methodologies to measure. Factors influencing evaporation include wind, temperature, vapor pressure, and exposed surface area. The reduction in evaporation in arid and semi-arid regions is a vital issue due to water shortages in such areas. In this study, we investigated different scenarios to reduce evaporation from Lake Nasser, one of the main sources of water in Egypt that store the excess water from the Nile. We used meteorological data including temperature, relative humidity, and wind speed from 2009 to 2020, and then the bulk aerodynamic method was adopted to determine the annual evaporation from the lake. The results showed that the average amount of water lost annually due to evaporation from Lake Nasser is about 12.00 BCM/year, representing approximately 22% of Egypt’s current share from the Nile River, which is 55.5 billion m3 of water per year. This share will very likely be reduced because of the construction of the Great Renaissance Dam in Ethiopia. This study suggests using an FPVS to cover parts of the lake to reduce water losses due to evaporation. Although covering 100% of the lake can save up to 8.4 billion m3 of water every year, this is not a practical solution due to the large area of the lake that reaches 5775 km2. Therefore, we investigated the water saved by only covering the shallow parts of the lake for water depths up to 3.0 m. We did that at different stages by first covering the water depths from 0.0 to 1.0 m (6.5% of the lake’s area), then from 1.0 to 2.0 m (5% of the lake’s area), and finally from 2.0 to 3.0 m (3.5% of the lake’s area). The results suggested that covering the very shallow parts of 0.0 to 1.0 m will provide the highest water saving of the lake, which reaches 1.9 billion m3 of water. Extension of the covered area to depths of 1.0–2.00 m added an annual water saving of 500 million m3. A further water area covering for depths of 2.0 to 3.0 m, which represent 3.5% of the lake’s area, can save an additional 260 million m3 of water. Another benefit of the use of an FPVS to cover parts of Lake Nasser is renewable energy generation. The estimated annual energy generated will be 740 × 106, 570 × 106, and 400 × 106 MWh. Since the average annual household energy consumption in Egypt is 3130 kWh (World Bank, 2014), the energy generated from covering the depths up to 1.0 m can fill the consumption of 238 homes. A further benefit coming from this is that it helps Egypt meet climate change targets by producing more renewable energy.