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Energy Storage Systems Suitable for Stand-Alone Maritime Site
In this studied, an islanded site is considered. In such study case, both the load of the island and the renewable energy production will be highly in-termittent. Therefore, balancing the production and the load on the gird presents a key feature. For these reasons, it is necessary to optimize the system behavior by associating an energy storage system to the renewable energy sources. In fact, energy storage systems can store electricity or trans-form electrical energy into other forms of energy and storing it in the process. Different energy storage systems can be applied to an islanded site. The more used ones will be presented in this part of the study.
Pumped hydroelectric storage
Pumped hydroelectric storage (PHS) (fig.1.6) presents the largest energy stor-age capacity form for grid applications (about 95% of the total energy stored) [41, 42]. Such systems use the gravity potential energy of the water for stor-age purposes. In fact, at times of low demand, excess generation capacity is used to pump water from a lower reservoir into a higher one. When power is needed, water is released back into the lower reservoir threw a turbine, generating electricity . The largest PHS station is the Bath country station in the U.S.A., with a production capacity of 3 GW [41, 42].
The energy stored in a PHS station is related to the volume V of water stored in the upper reservoir and the altitude difference h between the upper and lower reservoirs. In fact, the gravity potential energy respects the following equation: Ep = mgh (1.1).
where, Ep is the gravity potential energy, m is the mass of the water stored in the upper reservoir, and g is gravitational constant. The mass of water is considered proportional to its volume, m = ⇢V (1.2).
where ⇢ is the density of the water. Therefore, Ep = ⇢V gh (1.3) finally, by adding the efficiency of the PHS µphs EP HS = µphs⇢V gh (1.4).
The PSH is the most developed between the different storage systems in this study. It scores a 9 TRL level . PHS systems present also two key fea-tures: their long discharge time and their remarkable lifetime . However, PHS systems present a low efficiency per volume unit since it needs two reservoirs, and each reservoir volume must be higher or equal to the one presented in equation (1.4), which presents a problem when applied in small islanded sites.
Referring to IRENA , PHS presents a total capital cost between 1 $ and 4 $ per W of installation and a total production cost of 0.05 $ to 0.15 $ per kW h.
Offshore Energy transmission
After studying different renewable energy systems related to the considered islanded site, mostly based on ocean energies (tidal turbines, offshore wind turbines and wave energy). One of the biggest problems faced is the off-shore transmission of energy, which proved to be challenging and costly . Three major transmission methods exist: Medium Voltage Alternative Cur-rent (MVAC), High Voltage Alternative Current (HVAC) and High Voltage Direct Current (HVDC). Each method has its own strength and weaknesses. In fact, higher the voltage is lower the losses are; but at the same time, higher the voltage is more complex and expensive the system’s equipments are. Ta-ble 1.2 shows a comparison between the alternative current and the direct current systems [8, 49].
Tidal Characteristics in the Area Around the Island
Tidal energy presents a high potential in numerous countries, counting: Canada, Argentina, United Kingdom, France, Ireland, Russia, Australia and China . Furthermore, tidal energy presents high energy density, high level of Energy Storage System for the Studied Stand-Alone Site predictability, and low environmental impact [100, 101]. It presents an important energy source, the tidal energy potential worldwide is estimated to 450 TWh/year, where the European coast presents around 24 TWh per year.
The United Kingdom, France, and Ireland present 98 % of the European potential with 42 % on the French coast [102–104].
However, tidal turbine deployment faces a number of problems. These renewable energy systems are not as mature as wind turbines or PV systems [104, 105]. The biofouling can cause major problems on the system efficiency . Another important problem is the identification of a suitable tidal system location, which is limited. Indeed, waves can cause a major disturbance on tidal turbines, when placed in shallow waters . Moreover, deep water sites are unattractive since the current velocity decreases with depth and the cost of the project increases.
For theses reasons, and considering the critical context of such project, a specific tidal current energetic study for the positioning of tidal turbines is prerequisite for the project success. Such study will focus on the evaluation of the tidal energy variations with the position, and the implications of such tidal turbine design options (bidirectional fixed axis turbine or with a yaw [107, 108]).
All maps of this section respects the following characteristics:
• Geographical coordinates (latitude and longitude) are expressed in a RGF93 system (official French system).
• Geographical North is considered as a reference for the direction and counted from 0$ (North) till 359$.
• The maps are meshed in elementary spatial areas. The center of each of these areas is considered as a measurement node.
Marine current velocity
It is essential to present the characteristics and parameters of the tidal velocity, before presenting the different relations and mathematical equations that will help with its calculation. First, vsw and vnw are two essential parameters for the calculation of the marine current velocity (vt). The bathymetric features have a high effect on vsw and vnw. Therefore vt changes with the location, and all three current velocities (vsw, vnw, and vt) are dependent on the longitude and latitude (Figs. 2.8 and 2.7). Moreover, the tidal forcing is the main force affecting the tidal velocity. Such tidal forcing is a function of time, and presents a periodic variation. given that vsw and vnw are typical current velocities, they will have periodic variations too, and depend on a specific time (tn), which refers to the coincident tide cycle time interval. Second, the tidal coefficient C varies from one tidal cycle to another, while keeping a constant value during a given cycle, since it depends on H. As a result, vt depends on position (longitude (L) and latitude (l)) and time (t). Moreover, vt is defined as a vector, therefore it is composed from an amplitude and a direction.
Table of contents :
1 State of the Art of Renewable Energy and Energy Storage Systems Technologies in a Stand Alone Maritime Context
1.2 Stand-Alone Maritime Site Suitable Renewable Energy Systems
1.2.1 Onshore wind turbine
1.2.2 Photovoltaic panel
1.2.3 Offshore wind turbine
1.2.4 Tidal turbine
1.2.5 Wave power
1.3 Energy Storage Systems Suitable for Stand-Alone Maritime Site
1.3.1 Pumped hydroelectric storage
1.3.2 Compressed air
1.4 Offshore Energy transmission
1.5 Renewable Energy Systems Regulation
1.5.1 Vector Control
1.5.2 Observer-Based Control
2 Methodology of Analysis of the Energy Resource and the Energy Storage System for the Studied Stand-Alone Site
2.2 Ouessant Island Energy Consumption
2.3 Wind Characteristics on the Island
2.3.1 Resource Characteristics
2.3.2 Turbine Properties
2.4 Tidal Characteristics in the Area Around the Island
2.4.2 Existing measurements
2.4.3 Marine current velocity
2.4.4 Tidal energy and turbine properties
2.4.5 Turbine Properties
2.5 Solar Characteristics on the Island
2.6 Diesel Generators
2.7 Pumped Hydroelectric System
3 Sizing Method of a Hybrid Renewable-based System for a Stand- Alone Site
3.2 Hybrid Renewable-based Farm Control Strategy
3.3 Sizing and Optimization Objectives
3.4 Simulation Results and Discussion
3.4.1 Results using fixed ESS, wind turbine, and tidal turbine models sizes
3.4.2 ESS sizes variation
3.4.3 Reducing wind turbine sizes
4 Design and Analysis of Inverter Control Methods for Micro-grid Applications in a Stand-Alone Site
4.2 Design and Analysis of Single Inverter Regulation for Renewable Energy-based Systems
4.2.1 System Elements Description
4.2.2 P/Q Control Strategy
4.2.3 V/f Control Strategy
4.2.4 IVSG Control Strategy
4.2.5 Simulation Results
4.3 Design and Analysis of Inverter Control Methods in a Multi- Source Case
4.3.1 Traditional Droop Control Strategy
4.3.2 VSG Control Strategy
4.3.3 Simulation Results
4.3.4 Comparison and Discussion
4.4 Application to the Renewable Sources-based System for Ouessant Island
4.4.2 System Elements Description
4.4.3 Simulation Results and System Performances Analysis
Conclusion and Perspectives
A Power Scheme of the Full System
B IVSG Block Diagram