According to the World Water Assessment Programme [1
] global water demand has increased approximately by 1% per year from the 1980s and is expected to continue to rise with a similar rate until 2050, ending up in a 20%–30% increase above the current global water demand. This is due to many factors, such as population growth, changes in consumption patterns, increased water needs of municipal, agricultural and industrial sectors, and socio–economic and technological development [1
]. Moreover, fresh water availability is expected to decrease because of climate change (CC) and water quality degradation. According to Sustainable Development Goal (SDG) No. 6 [3
], the availability and sustainable management of water and sanitation must be ensured for all people now and for the future. As a result, all the above projections are in direct conflict with this target of the United Nations [4
Greece is a typical example of challenging water resources management (WRM) for many reasons. First of all, its geomorphology in combination with its geographical position in the eastern part of the Mediterranean Sea leads to the inhomogeneous distribution of annual precipitation depth, an attribute that affects the hydrological and hydrogeological regime of the mainland. Secondly, around 100 islands in Greece are inhabited, most of which have insufficient water resources and need to be supplied via water importation (Table 1
). In general, islands face water scarcity at a higher intensity compared to the mainland, and, at the same time, they have a higher degree of autonomy, which can speed up the implementation of innovative processes [5
]. Thus, current practices to ensure water supply in small dry islands in Greece are not fully sustainable and effective, especially during summer months, as needs increase significantly due to tourism. Various WRM practices are used for the islands’ water supply, including rainwater harvesting (RWH), water transportation and desalination.
RWH, a water management practice followed for over 4000 years, has gained more and more ground as a modern and simple water saving technology [6
]. RWH can augment a water supply to meet urban and rural needs by adding flexibility and robustness to a system while avoiding more costly expansions in infrastructure [7
]. Untreated harvested rainwater can be used for non-potable uses, such as toilet flushing, cloth washing, other household uses and garden irrigation, while potable uses are also common in several countries (e.g., Australia and Spain), but the appropriate treatment of harvested rainwater might be required, depending on its quality [8
]. Several studies (e.g., [9
]) have shown that RWH can be a viable alternative source for domestic water, with quantities saved ranging from 12% to 100% of the overall demand [13
], depending on the specific environmental conditions and the technical characteristics of the system selected (e.g., tank size). The potential use of RWH combined with greywater recycling—which is defined as the urban wastewater that includes water from baths, showers, hand basins, washing machines, dishwashers and kitchen sinks but excludes steams from toilets [14
]—is another option which could lead to an even further reduction of publicly-supplied water [16
]. Regarding design parameters, the rainfall depth is the factor that directly affects the operational efficiency of the system, with other factors including the rooftop area, tank volume, water demand and the efficiency of runoff collection and the filter [19
]. Despite the benefits obtained, RWH is still limited in many countries, including Greece, as, among other reasons, the cost of installation and maintenance can lead to long payback periods [20
An evolving technology for water supply is sea and brackish water desalination. As reported by the International Desalination Association (2019), the global installed capacity of the desalination plants is about 100 million m3
/d, with about 59% of the installed global desalination capacity using seawater as the feed-water type. Technological improvements regarding membrane processes, energy recovery systems and desalination plants integrated with renewable energy sources (RES) can provide opportunities for cost reduction (e.g., [4
]). Consequently, desalination is an increasingly attractive technology that can be considered an economical and efficient solution to the water scarcity problem of remote areas and islands, especially for freshwater production (e.g., [3
]). However, the main disadvantage of desalination technologies is the high energy consumption requirements [2
]. Indicatively, the energy usage for seawater desalination lies between 0.5–16 kWh/m3
, depending on the technology used (e.g., [2
]). According to Gude [27
], desalination technologies are classified in two main categories: Thermal and membrane processes. About 65% of the total global fresh water produced is by utilizing membrane processes and, particularly, by reverse osmosis (RO) technology [30
]. RO is expected to remain the dominant technology in the new era due to the lower energy consumption it requires, its lower construction, operation and maintenance (O and M) costs, and its technological improvements [4
During the few last decades, there has been an increasing interest regarding the use of RES as power supplies for desalination plants (e.g., [2
]). As García Latorre et al. [35
] reported, this is mainly attributed to the limited availability of fossil fuels and to the unpredictability of their cost. The environmental issues of fossil fuels are also factors that encourages the change to RES [36
]. Solar, wind and geothermal energy are the main sources of renewable energy integrated with desalination technology, accounting for approximately 90% of renewable energy desalination worldwide [26
]. Among them, wind energy is the most frequently used in coastal areas, given the high wind-potential that is usually available [25
], with wave energy being also suitable in some cases. Regarding the latter, however, although it is a source with significant potential [38
], its application still entails many uncertainties [39
] because it is a rather new technology. Besides the technical aspects, the estimation of cost is a major one [40
Finally, various studies [41
] have evaluated the role of seawater pumping as power supply to desalination plants; this particular practice is called seawater pumped hydro storage (SPHS). SPHS methods are considered already mature and widely applied technologies [46
]. It must be stated that the operation of a system with a single reservoir is proposed for minimizing the costs [45
]. Therefore, SPHS can be proved to be an attractive option for the remote islands of Greece. Caldera et al. [2
] tried to estimate whether it is possible for a seawater RO desalination plant, powered solely by RES, to cover the global water demand by 2030. Katsaprakakis et al. [43
] present, in detail, a wind powered pumped storage system (WP-PSS) at which electricity surplus could be used in RO desalination plants for producing potable water. Segurado et al. [47
] optimized sizing regarding an operational strategy of an integrated power and water supply system that consisted of a wind-powered desalination and pumped hydro storage system.
The objective of this research paper is to evaluate five different WRM scenarios for meeting the increasing water demand in small remote islands of the Aegean Sea. These scenarios include: (i) Domestic rainwater harvesting combined with water transportation, (ii) wind-powered seawater desalination, (iii) wind-powered seawater desalination and domestic rainwater harvesting, (iv) wind-powered seawater desalination combined with seawater pumping, and (v) wind-powered seawater desalination combined with seawater pumping and domestic rainwater harvesting. Each alternative was examined for a 30-year time period, taking into account an increase in population and tourism, and evaluated in respect to cost, using the life cycle cost (LCC) methodology. As the performance of each alternative is directly related to the variability of meteorological conditions (e.g., wind speed for wind turbines operation and precipitation for rainwater harvesting), the best solution, as determined, was also assessed under six different climate change scenarios. While most of the above alternatives have either been proposed or implemented for water supply in various arid Greek islands, this is the first time that these different measures and combinations have been assessed and compared on the regional scale in seek of an optimal solution. This paper focused on eight small islands of the Aegean Sea which face significant water scarcity problems; however, the presented methodological framework and findings are applicable to other arid or semi-arid insular or isolated coastal regions of the mainland.
Water demand has been steadily increasing in the last decades in small Greek islands [5
], mainly as a result of tourism increases. Due to the limited local water resources, current water management practices depend on water importation by ship, a solution that entails significant costs, as validated by the implementation of the Directive 2000/60/EC for the Aegean Islands, Greece [56
]. In this frame, five different alternatives for meeting water needs were examined and compared for eight small islands located in the Aegean Sea based on their economic performance.
Domestic RWH can contribute to the reduction of the overall amount needed, but it can only be considered a supplementary source of water. In particular, water savings ranged between 11.6%–28.7% of the daily domestic water demand for the eight islands examined and for the tank capacities selected. In other studies conducted at the regional scale, the potential potable water saving was found to range between 12% and 79% in southeastern Brazil [9
], 29.9%–32.3% for high-rise buildings in Australia [57
] and 0.27%–19.7% in Jordan [11
]. Based on the findings of Londra et al. [6
] for the island of Naxos that is located in the Aegean Sea of Greece and is characterized by similar meteorological characteristics with the islands examined in the present study, only 30% of total water needs for a three-resident household could be fully satisfied. However, a tank capacity larger than 50 m3
and a rainfall collection area larger than 150 m2
would be required for this purpose.
On the other hand, desalinated water is a solution which can lead to the water autonomy of the islands, eliminating their dependency from imported water. Many studies have addressed the technical and economic issues of different technologies for powering desalination units [32
]. The investment cost of seawater reverse osmosis desalination plants lies in the range of 900–2500 €/m3
]. In the present study, a cost of 2500 €/m3
/d was used, given the relatively small capacity of the plants selected in all cases (140–780 m3
). The unit production cost of water (including all capital and maintenance costs) [2
], reflects the cost (€) required to produce the total quantity of desalted water (m3
). According to other researchers (e.g., [2
]), unit production cost lies between 0.6 and 12 €/m3
. Here, it was found to range from 1 to 5 €/m3
in relation to the examined scenario and the characteristics of each island. Among the alternatives, wind-powered desalination was found to be the most cost efficient in seven out of the eight islands. The only exception was the island of Donousa, at which RES desalination combined with sea water pumping for energy storage outperformed.
Overall, water desalination can become a key in meeting the increasing water needs, especially in water-stressed isolated areas with easy access to seawater, such as the small dry islands located in the Aegean Sea. Further, the gradual phasing-in of renewable energy sources to power desalination plants, such as the WPROD, WPROD-RWH, WPROD-SWP and WPROD-SWP-RWH scenarios examined, will help to ensure the long-term sustainability of such projects. At the same time, against the current unsustainable practices, the integration of RES can reduce impact on the environment, as freshwater can be provided without significantly increasing greenhouse gas emissions. In terms of cost, despite the fact that a high initial investment is typically required, funds expended were recouped in all cases examined, with payback periods ranging from five to fifteen years, depending on the technological solution chosen and the characteristics of each island. However, it should be addressed that as the best scenario’s performance under CC revealed a systematic decrease in the reliability of an RES-based supply, a fact that should be taken into account in water resources management in isolated islands.
The main objective of this paper was to investigate various water resource management scenarios regarding their performance in meeting fresh water demand. The analysis was carried out in eight small islands of the Aegean Sea, Greece, which face significant water scarcity problems. Most of the examined measures have been proposed or implemented in other similar cases in Greece; however, no previous study has focused on comparing this set of different options and combinations with the aim of determining the optimal one. The examined scenarios refer to five combinations that incorporate current water supply practices, domestic rainwater harvesting systems and desalinated water provided through the operation of wind-powered desalination plants and/or seawater pumping. All scenarios were evaluated for a 30-year lifespan, and an optimal solution was determined for each island, based on a life cycle cost analysis. Finally, the performance of the best solution regarding energy supply reliability was assessed under six climate change scenarios.
Results highlighted that the current water supply practices (mainly water hauling via ships) is neither sustainable nor economical. Among the different alternatives, wind-powered desalination was found to be the most efficient in most of the cases. The LCC analysis revealed that, despite the high initial investment that is typically required, capital return can be achieved after short periods, depending on the design parameters of the system and the characteristics of each island. Additionally, a decrease in the systems’ performance under all CC scenarios and for all islands was found. For the period 2021–2050, a systematic decrease in the scenarios performance, expressed in terms of decrease in renewable energy supply, was observed. The highest percentage reduction was found about 44%. Despite the inherent uncertainty of climate change models, the systematic reduction in RES reliability highlights the necessity of incorporating climate change in the design process of such projects.
The present work reveals that an integrated approach that includes the combination of different alternatives is essential in determining the best course of action for dealing with water resources management problems. In this respect, other potential technological solutions not examined here, such as greywater recycling and wave energy coupled with seawater desalination, need to be included and evaluated in a framework similar to the one presented. The latter, in particular, is a method with significant potential for coastal areas. Furthermore, issues related to environmental impacts and costs need to be assessed and incorporated into the future research.