**1. Introduction**

Small off-grid islands have costly, limited, and intermittent supply of electricity and water due to dependence on importation. In the case of the Philippines, they are often electrified by diesel generators that are supplied with diesel imported from the mainland, resulting in high electricity prices (i.e., six times that of the mainland) because of high transportation costs [1]. In addition, freshwater for general purposes on these islands is sourced from an unreliable rainwater supply or a limited and vulnerable groundwater source [2]. Drinking water may be imported from the mainland at a cost of around 1 USD/L [3]. Economic activity on these islands are adversely affected as a result of the high costs of electricity and water. In particular, the International Labor Organization identified that freshwater scarcity hinders economic growth, as businesses would compete with residents for the resource [4]. The scarcity of clean freshwater can be solved through seawater desalination, but it is energy intensive [5]. Putting additional conventional power generation capacity must be considered alongside desalination capacity, but this may be an unsustainable and more costly option [1]. In contrast, putting up renewable energy (RE) generation capacity to augment desalination may be more feasible because of the decreasing costs of solar photovoltaic (PV) [6] and battery energy storage systems (BESS) [7]. Hybrid energy systems consisting of solar PV, diesel generators, and energy storage were shown to reduce electricity costs in off-grid areas [8]. In the Philippines, Ocon and Bertheau estimated a 20% energy generation cost reduction for Philippine off-grid islands upon transition from a diesel-only system to a solar PV-based hybrid system [1], while Bertheau and Blechinger considered a 10-year load growth in their analysis and found the same trend [9]. Note that the works mentioned were focused on finding the lowest levelized cost of electricity only.

Table 1 summarizes the literatures that conducted feasibility studies for both energy and desalination systems. Some works have focus on the optimal combination of components. For example, Fernández-Gil and Petrakopoulou showed that implementing RE-desalination systems on Skyros Island, Greece yields freshwater at a cost of 2.75 USD/m3, which is 52% lower than imported freshwater [10]. Tafech et al. show that reverse osmosis (RO) powered by a PV-wind system can yield freshwater at a cost of 1.87 USD/m<sup>3</sup> on King Island, Australia [11]. For comparison, the freshwater cost in those literatures reviewed are more expensive than that of mainland Philippine water cost which amounts to 0.70 USD/m<sup>3</sup> [12]. Other works focused on dispatch algorithms. For instance, Gökçek simulated solar PV, wind, and diesel-based energy systems coupled with RO as an additional electrical load [13]. Corsini and Tortora demonstrated the load leveling capabilities of RO by absorbing excess RE generation in a PV-diesel hybrid system [14]. Lastly, Bognar et al. compared different RO dispatch strategies which shows both energy and water generation costs may be minimized by treating RO as a deferrable load [15]. Note that excessive desalination turndown could arise, which will damage the desalination plant [16].


**Table 1.** Summary of energy-desalination feasibility studies. RO: reverse osmosis; PV: photovoltaic.

Table 2 compares different desalination technologies from different works in the literature, which generally shows insufficient technical detail. In contrast, research listed in Table 1 involve rigorous model-based simulations, but are mostly limited to RO. This is not surprising considering that the technology has a low energy consumption [19] and decreasing investment costs [22], making it practical for installation. The technology constitutes the largest share of installed global desalination capacity with 69% of the global desalinated water production [23].


**Table 2.** Review of comparative desalination studies. MED: multi-effect distillation; MSF: multi-stage flash; MVC: mechanical vapor compression.

<sup>1</sup> Learning rate in these studies refers to the percentage by which the cost of desalination decreases when the cumulative installed capacity doubles. Other definitions, such as those based on the doubling of cumulative generation, also exist [29].

RO is a membrane-based desalination technology in which saltwater passes through a semipermeable membrane that blocks ions and other contaminants. Freshwater is collected at the other end of the membrane [5]. In a continuous process, the feedwater channel, membrane, and freshwater permeate collectors are pressed and wound in a cylindrical package (Figure 1). At the center of the cylinder is the permeate tube with freshwater [30]. Despite its advantages, the technology is still hindered by frequent membrane degradation [31]. Many works have tackled the technical issue by using different membrane types [32] and managing membrane fouling [33]. This has led some studies to consider other desalination methods. Other membrane-based technologies include electrodialysis (ED), wherein saltwater is passed between alternating layers of anodic and cathodic exchange membranes. These membranes allow only anions and cations to pass, respectively. Ions are separated and trapped between every other layer when an electric field is applied, forming alternating layers of freshwater and brine [34].

*Energies* **2020**, *13*, 2261

**Figure 1.** Reverse Osmosis (RO).

Thermal desalination systems generally produce freshwater from the vapors of heated saltwater [24]. An example is multi-effect distillation (MED, Figure 2). Saltwater is placed in a series of heat exchange vessels. The first stage receives external heating and some steam is produced. This steam serves as the heat source of the next stage and is collected as freshwater upon condensing. This is repeated across several stages. At the final stage, the steam preheats the feed saltwater before condensing [35]. Another thermal desalination system is multi-stage flash (MSF, Figure 3) [36]. The saltwater is heated using recovered heat and external heating sources. It is then flashed at the first stage to produce steam and concentrated saltwater. The steam condenses after heating the feed saltwater. The condensate is collected as freshwater. The concentrated saltwater is flashed at a lower pressure in the next stage. In mechanical vapor compression (MVC, Figure 4) [37], saltwater partially vaporizes in a closed vessel. The vapors are passed into a compressor, which raises the temperature of the vapor stream. Heat is then exchanged with the saltwater in the feed and the vessel to produce more vapor. Although this technology is classified as a thermal process, it does not use external heating sources [38]. MSF and MED constitute 18% and 7% of the global desalinated water generation, respectively [23].

**Figure 4.** Mechanical vapor compression (MVC).

Performing techno-economic studies to integrate water desalination to the off-grid energy systems requires accurate models [39], while different economic dispatch algorithms must be tested as they generate varying results. In addition, results that use deterministic generation and demand profiles such as the literatures reviewed may not be accurate due to uncertainties in demand and/or RE generation [40]. Lastly, results that use constant cost over time may also not be accurate as it is expected that desalination costs will decrease as the technology matures [26]. For the Philippine context, Castro et al. have reviewed different RO dispatch algorithms. The results suggest that treating RO as a sink while demanding additional generation when water storage is low can minimize water generation costs [41]. However, testing the techno-economic feasibility of different desalination technologies under different economic dispatch algorithms while accounting stochastic behavior of generation and demand profiles and projected decreasing technology has not been done yet.

In this work, a techno-economic comparison of MED, MSF, MVC, and RO desalination coupled to a solar PV-diesel hybrid system with lithium ion (Li-ion) BESS is performed. The desalination technologies were selected based on their commercial maturity. The analysis of the desalination technologies was divided into four parts. First, the optimum sizes of energy and desalination components and their uncertainties were determined using stochastic generation and demand profiles. Second, power flows were generated to investigate the interaction between energy and desalination components. Third, energy and water generation costs were calculated at different starting periods to analyze the risks and tradeoffs of delaying plant construction. Lastly, a sensitivity analysis of optimum sizes to the overnight diesel and coal cost was conducted to account for uncertainties in diesel prices.
