*2.1. Energy and Water Needs of San Andrés Island*

San Andrés Island is located to the west of the Caribbean Sea in the Atlantic Ocean. The island has a warm climate, between 26 ºC and 29 ºC all year round. Throughout the year, there are two seasons (i) the dry season, usually between January and April, but it can last a maximum of five months, (ii) the other months are part of the rainy season with strong winds, usually between May and December [38].

The economy of the department is based mainly on tourism and commerce. Its main export product is coconut, but it also produces avocado, sugar cane, mango, orange and yucca [39].

It is the least extensive department in the country and has the highest population density, which places the islands in a delicate resource management situation. It is estimated that around 75,000 people live on San Andrés Island, and every year, one million tourists arrive [40]. A considerable figure for a territory of 26 square kilometres with no rivers.

The particular geography of the San Andrés Island, its condition of insularity, having only two aquifers, the amount of population that inhabits the islands, as well as the floating population that arrives throughout the year and depends on imported food are just some of the characteristics that make this territory highly vulnerable to climate change and shortage of potable water.

Currently, the San Andrés Island electricity demand is approximately 160– 187 GWh/year [41], which is supplied by diesel-powered thermoelectric plants, consuming around 40 million L of diesel each year, which are brought by boat from Cartagena. In addition, there is also a thermoelectric power plant that works with the burning of garbage, which has an installed capacity of 1.6 MW and a helpful power of 1 MW [42]. This situation, combined with the variable costs of fuel and the transport high prices, serves as an incentive for companies and communities to seek new energy alternatives.

On 17 July 2018, the Colombian government delivered the new desalination system and treated water line for the neighbourhoods of La Loma, El Cove and San Luis, at San Andrés Island, which facilitates the supply of treated water in the communities of this sector and avoids that emergencies occur due to the shortage of water in times of drought. The work had a total initial investment of 4.2 million dollars, and the plant can treat 25 L of water per second, enough to benefit more than 23,000 inhabitants [43].

It is convenient to study more thoroughly the possibilities offered by an OTEC plant for the San Andrés Island since not only can the natural resources available be used to generate electricity, but in addition, OTEC plants can convert seawater into potable water at a cost similar to that of a conventional desalination plant [44]. This last one would be quite beneficial for the island population because it would help to supply this indispensable resource.

#### *2.2. Ocean Thermal Energy*

OTEC is a type of renewable energy that uses the Δ*T* between the surface and deep layers of the sea to move a thermal machine and produce valuable work, usually in the form of electricity [12]. On the other hand, the oceans cover more than 70% of the earth's surface, and by absorbing heat, it can store a large part of the energy emitted by the sun. In this sense, using a small portion of this stored energy could meet the energy needs of a country [45]. The water column temperature in the Colombian Caribbean depends on the origin of the water masses from different latitudes, such as the North or Central Atlantic, and the contribution of the great rivers of the southern Orinoco and Amazon. Each mass of water has a characteristic temperature, salinity, and density [46]. In the Colombian Caribbean, three thermal layers can be differentiated in the water:


In an OTEC system, the cycle efficiency is directly related by the Δ*T* [47]. The larger the Δ*T* is, the higher efficiency obtained. OTEC systems should be located close to shore to reduce the transmission costs of the electricity generated [14].

#### 2.2.1. OTEC Cycles

Surface water heats a liquid using a heat exchanger, transforming it into steam, which drives a turbine that generates electricity. The cycle cools the steam with another heat exchanger in deep water, restarting the generation cycle. Currently, the primary cycles are open, closed or hybrid. In an open cycle OTEC system, the hot water found on the surface is taken to a vacuum chamber using a vacuum pump, which operates at a maximum of 3% of atmospheric pressure [48]. The water evaporates rapidly through this pressure drop, and the expanding steam drives a low-pressure turbine connected to an electric turbine. An advantage of this cycle is that the steam leaves the minerals in the vacuum chamber, producing desalinated water, which can be used depending on its physical-chemical characteristics for water for human consumption or irrigation [49]. Closed cycle OTEC systems, on the other hand, use refrigerant, which is heated directly by heat from surface water; the evaporated refrigerant drives an electric turbine and is cooled in deep water. The main advantage of this cycle is lower construction and operation costs; however, it requires more outstanding care in the handling of the refrigerant; it should be noted that the closed cycle does not produce desalinated water [50]. The hybrid cycle contains characteristics of both the open-loop and the closed-loop. In this case, the water is taken through a pump to a vacuum chamber to be evaporated. The water vapour heats a refrigerant that activates an electric turbine; the water vapour is condensed in a heat exchanger located in deep water. The hybrid cycle has desalinated water as an indirect product [51].

#### 2.2.2. Desalinated Water as a Derived Product

An advantage of OTEC systems is the possibility of generating desalinated water. For example, a 1 MW hybrid OTEC system can produce around 4500 m3/day of desalinated water [52]. However, it should be noted that the cost of producing desalinated water by this method is comparable to standard desalination plants [48].

OTEC systems can be a solution to both the water and energy needs of San Andrés Island. However, the implementation costs are high. Considering that the Island has limited financial resources, it is necessary to carry out a financial assessment that serves as a starting point when implementing a size project to ensure its long-term viability and profitability. This work is aimed at contributing to this point.

#### **3. Methodology to Perform the Economic Analysis**

The proposed methodology is divided into three parts: first, selecting a location for the system; second, a power plant's technological description and third, an economic analysis, which are described below.

#### *3.1. Methodology Used in the System Location*

The OTEC system's location selection aims to determine the place to settle the plant and select if an onshore or offshore installation is carried out. Bathymetry and temperature differences at San Andrés Island, along with the best location selection are presented. To this end, environmentally data collected from NASA are used. Then, the differential temperature between the sea surface and the 1000 m depth around San Andrés Island is obtained. Next, bathymetry is performed at 7 points on the island, and the point with the most appropriate profile is selected according to [14].

### *3.2. Power Plant's Technological Description*

The power plant's technological description is introduced to perform its economic analysis. To this end, various software will be used to simulate the operation of an OTEC power plant at San Andrés Island, understand its benefits and obtain its technical characteristics. Google Earth for the positioning of the plant, Autodesk Inventor for the 3D sketch and Cadesimu for the electrical circuit.

The technical conditions of the OTEC system are defined, and it should be clarified that simplifications are made to obtain an overview of the implementation. The energy model, the sketch of the power plant, the electrical transfer scheme, and the emissions analysis are provided in this step.
