Nexus Water & Energy: A Case Study of Wave Energy Converters (WECs) to Desalination Applications in Sicily

The studies about interactions among energy and water are considered as a new and recent field of research.

The procedure applied in the present article are the result of previous application made in different technical fields that show a good replicability [1]–[4].

In general, electrical energy processes require one huge amounts of water, anyway water needs energy for treatment and transport, so these sources are inextricably joined. This relation is called “energy-water nexus”.

It emerged from an understanding that natural resources are beginning to limit, to a substantial degree, economic growth and human well-being goals. As population and economies grow many regions of the world experience water and energy security challenges that must be addressed now.

In the next 20 years, towns in developing countries will increase their population as the demand of 70 million more people each year. Recently FAO’s studies describe that by 2050, nine billion people will want a 60 percent increase of water in agricultural production and a 6 percent increase in already-strained water withdrawals [5]

In addition to this, over 1.3 billion people worldwide still lack access to electricity; most of them live in sub-Saharan region or East-Asia [6]. About 2.8 billion people live in areas of high water scarcity and 1.2 billion live in areas of water deficiency [7].

Other interesting aspect regard primary energy production and electricity production in Asia where new analysis describe an incredible increase. For example, Latin American countries, increased production will come from non-conventional sources and the amount of electrical energy produced will be fivefold so the direct consequences will be the increase of fresh water [7].

Water is an essential element for energy. Energy depends on water – for power generation, the extraction, transport and processing of fossil fuels, and the irrigation of biofuels feedstock crops – and is vulnerable to physical constraints on its availability and regulations that might limit access to it.

Water is necessary to produce all forms of energy. For primary fuels, water is used in resource extraction, irrigation of biofuels feedstock crops, fuel refining and processing, and transport. In power generation, water provides cooling and other process-related needs at thermal power plants; hydropower facilities harness its movement for electricity production.

These uses can, in some cases, entail significant volumes of water. Additionally, they can have adverse effects on water quality via contamination by fluids that contain pollutants or physical alteration of the natural environment.

The global demand for water is assumed to grow for all major water use sectors, with the largest proportion of this growth occurring in countries with developing or emerging economies as India or China.

Another interesting aspect is energy requirement for supplying and treating water. In particular, it is necessary for the water provision: pumping and treatment. In literature, desalination is defined as the most energy all-out water process.

Desalination is a process purifies saline water so in general means to remove salt [8] from seawater or generally saline water.

Their disadvantage is their high salinity of ocean. The energy cost of treating low salinity seawater is about ten times greater than a typical freshwater source. Desalination is therefore an appropriate option only when there are no other sources or the cost of energy for transporting water is very high.

However, the desalination industry is working on reducing energy costs. The International Desalination Association has a goal of achieving a 20% energy reduction by 2015, and some companies have started to experiment with using renewable energy for desalination. Anyway, recent economic and technical problems could effect a delay to reach the aim.

So desalination represents one the most case of water and energy interconnections and in particular, the principal aim of this work is its application using renewable energy.

As above mentioned, desalination is the most energy intensive water treatment technology but it could be solution for many problems in water supply for areas with chronic debt of water.

The case study for our work is Sicily, in fact in the end of the 20th century, the island had the problem of aridity, so to respond of water demand, there were the planning of various desalination plants for different uses as civil, agricultural and industrial uses.

As for Sicilian climate, northern part has often been helped by the presence of mountain chains which have significantly contributed to the water needs, in the southern part of Sicily the availability of water sources has been continuously worsening [9] due to a decrease in rainfall.

In Sicily, the biggest desalination plants were installed in Gela and Porto Empedocle from 1974 onwards. Today six other plants are operating in Sicily: one in Trapani and the others on the five islands of Lampedusa, Linosa, Pantelleria, Ustica and Lipari. So the principal innovation of this work is using wave potential in Mediterranean sea (near islands as Pantelleria or Ustica) to nurture desalination plants [10].

The paper will be divided into three section: the first will be a presentation of state of the art of seawater desalination using renewable energy source, in the second we will describe a possible integration of wave energy to desalinate (a presentation of prototype realized by DEIM) and in the final section a real application near Pantelleria.

T Recent studies defined exploitation from sea wave as one of the most talent among the renewable energy. It is approximated that in the imminent period it will have one huge increase because these electrical devices used become more complete. In general, these generators are called Wave Energy Converters (WECs).

Wave energy devices need large investment of money throughout the process of producing a valid device: overcoming successive steps that can lead them to validate their concepts, build and test prototypes and optimize parameters with the final outcome of a successful product that can be deployed in arrays and sold.

The current panorama with many concepts being patented in every country can be confusing; some technologies have the potential of being successful, whereas many others will not pass the concept phase. An interesting research on wave energy conversion is excellent in countries near oceans, where the wave energy potential is a big resource. In Europe, most of the pilot plants are located along the Atlantic coast in countries such as Ireland, Portugal, Spain, Norway and the UK [17].

Energy accessibility is one of the most important aspect to determine a wave energy production, but high energy potential usually includes uncommon wave during extreme events. In flat opposition, other characteristics or physical aspects regard semi closed sea as Mediterranean where is possible a minor availability, but interesting researches are addressed to optimize wave energy extraction. For example, some studies demonstrated a wave energy production along Italian coasts. As mentioned before, sea wave information, about fundamental features as its temporal and spatial variability and of its distribution or different sea states, is necessary to WECs’ realization

Therefore, thanks to the large amount of information taken from these wave buoys, wave atlases along the Italian coastline were realized, in which the values of wave power and the average wave energy offshore are exposed. These reference books of the Italian coasts are elaborated with local wave parameters measured by buoys sat in Mediterranean [18] [19]. Wave buoys represent the best and direct measure of wave parameters.

On the other hand, there are some drawbacks. In fact, time series obtained from buoys definite local wave climate and sometimes present large data gaps caused by momentary collapse of the buoy or by habitual keeping operations. Wave energy resource is strongly connected to the bathymetry: a greater water depth increases wave potential due to the lower friction losses. On the other side, a less water depth decreases wave potential. According to this, all wave buoys are located offshore, several kilometers from the coastline. Moreover, fetch represents an important characteristic too. It is the distance where the wind blows: a large fetch should be preferred, because it defines a more intense and regular wave climate. Focusing on the Sicilian coastline, all these studies have allowed the creation of this wave power map [20], in which the red flags represents the measuring buoys.

The minimum wave power potential is identified along the coasts of Catania, in which values vary from 2.1 kW/m to 3 kW/m. Along Palermo’s coasts the values are higher, included between 3.1 kW/m and 4 kW/m, while the higher ones are referred to Mazara del Vallo, where average wave power is bigger than 5.1 kW/m.

The use of third generation wave model allowed the creation of further wave power maps, which are characterized by a higher resolution. Therefore, the presence of these maps represents an important breakthrough not only in the description of wave power along the Sicilian coastline, but also in the assessment of the production of electrical energy from sea wave through the use of WECs. Moreover, the implementation of the GIS technology (Geographic Information System) will increase the feasibility of the installation of Wave farm. This instrument is able to include different data in a specific geographic map, such as the placement of protected areas with the distribution of wave power. In this way, we are able to know exactly the usable areas for exploitation of wave energy and the ones where this exploitation should be avoided.

3.1 Methodology and Sicilian wave power evaluation

P In this work a study of the wave energy potential along the Sicilian coast was performed using data acquired by sea wave buoy such as DEIM Buoy [21] and a third generation ocean wave model. Particularly, the potential wave map was obtained thanks to the large amount of data that came from the wave buoys of the Rete Ondametrica Nazionale (RON). In 1989, it was originally composed by 8 pitch-roll buoys, but in 2007 the total number of wave buoys was of 15. These buoys are able to record different parameters, such as Significant wave height Hs [m], Peak period T_p [s], Average period T_m [s], and Average wave direction Φ_m. At first, recording period was about 3 hours, then (with the replacement of all the wave buoys with more technological ones) was about every 30 minutes. In this way, RON was able to describe wave climate with a higher time resolution (but the spatial resolution remained still low). In deep waters, Wave power P can be obtained through H_s and T_e data [22]:

$P=\frac{\rho g^{2}}{64 \pi} H_{S}^{2} T_{e}$ (1)

in which ρ is seawater density (equals to 1025 kg/m^3)  and Te represents the Energetic period [s], equals to the Peak period multiplied by a constant, which depends on the exact collocation of the buoy. Finally, average wave energy can be obtained summarizing every sea state wave energy (fig. 3).

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Figure 3. Evaluation of sea wave energy in Sicily for year 2013

3.2 Wave Energy Converter by DEIM

F In this paragraph we expose the electricity production scenario from marine waves along the Sicilian coastline, due to the use of an innovative conversion device nowadays in the design phase by DEIM department of University of Palermo [23]. This is a new device called Point Absorber (shown in Fig. 4) in which will be able to transform wave energy in electrical energy directly, without the use of intermediate devices or polluting fluids. Every Point Absorber presents a nominal power of 80 kW, consisting of 8 linear generators of 10 kW each one, and had an external diameter of 5 meters. The working stroke of the translators is about 4 meters: so, the WEC will be able to exploit the most energetic sea state too. Moreover, thanks to the high performance of the linear generators, the overall efficiency of this WEC will be about 50 %.

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Figure 4. Model design of wave energy converters

The Permanent Magnet Generator (PMG) is a bilateral generator with two stators. The main material is “laminated” iron that is it’s made up by overlapping of a lot of sheets (126 as exact number), with a thickness of 0.5 mm. This choice is justified to cut down the eddy current and other losses.. As for its design, the first and last three slots have a width of 8 mm while internal slots have a width of 12 mm because they must contain two winding, 13.5 mm teeth are built, except the first and last tooth that measure 7 mm. We use two tables made up by bakelite, used for coming together the stator block.. There are 32 holes to 10 mm.  The Fig. 5 does an internal view of this new device.

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Figure 5. Internal view of DEIM Point Absorber

As for the translator is realized by an alternating assembly of 40 permanent magnets that interspersed with soft iron pole pieces mounted on a bakelites sheet. The material used is Neodymium– Iron–Boron (Nd–Fe–B), the arrangement is in pairs such that opposing magnetomotive forces (mmfs) drive the flux through the soft iron. In the end, the coil material used is copper and in particular is copper wire of diameter 0.5 mm. The coils have 375 turns of rectangular shape with an average size of 85x135 mm. Each coil has a weight of 278 g. We identify 72 coils that are arranged in 36 in each side of the armature (coils A and coils B). Tab.1 describes all technical data.

Table 1. PMG Technical sizes

3.3 Water resource and desalination plant in Sicily

In the end of the 20th century Sicily had high problem of dryness. Sicilian government/ answer was the construction of a number of desalination plants to satisfy water needs for municipal, rural and industrial demand. While the northern part of Sicily has always been helped by the presence of mountain chains which have significantly contributed to the water needs, in the southern part of Sicily the availability of water sources has been continuously worsening due to a decrease in rainfall [10].

Desalination could be a solution for some of these areas. One summary of the desalination system was carried by different authors [24]. Table 1 and figure 6 describe geographical sites, main technical data and performances of all the desalination plants in Sicily [10].

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Figure 6. Geographical distribution of desalination plants [24]

Table 2. Desalination plant :typologies, capacities and energy consumption. Multistage flash distillation (MSF), multieffect distillation (MED) and thermal vapour compression (TVC), reverse osmosis (RE) and electrodialysis (ED) [24]

The table 2 shows a state of the art of desalination plant in Sicily. It is important underline how these plants (MSF, RO, MED, MVC) are now working in the island producing fresh water for a population living in the south and western coast of Sicily. The plants, owned by the Regional Government, were built in different years to face the problem of water supply which has affected the population since the early 1970s [24].