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POWER FLUCTUATION PHENOMENA
The attraction of tidal current renewable energy lies in high energy density and high predictability of the tidal current resources. Seawater is more than 830 times denser than air and it enables a MCT to be 2~3 times smaller than a wind turbine for a same power rating. Moreover, the astronomic nature of the tides makes tidal current a very predictable energy resource on long-period scale for decades. However, the MCT is subjected to two kinds of power fluctuation phenomena.
The mechanical power harnessed by a horizontal marine current turbine can be calculated by the following equation, P = 1 ρCp AV 3 (I.1). In this equation, sea water density ρ and turbine blade swept area A are considered as constants; V represents the marine current velocity; Cp is the power capture coefficient and is related to the tip top speed ratio and the marine current speed when the blade pitch angle is fixed. For typical MCTs, Cp is estimated to be in the range of 0.35-0.5 [3]. When a Maximal Power Point Tracking (MPPT) strategy is used, the turbine rotor speed is able to be controlled to keep Cp at its optimal value. That means in first order, the power produced by the MCT is proportional to the cubic of current speed in the turbine cross section. It can be seen from (I.1) that the power produced by a MCT would change greatly when there are variations in the marine current speed.
Two main kinds of power fluctuations can be identified. On a daily time scale, the MCT generated power varies with a period of about 6 or 12 hours which is related to tidal astronomical phenomenon. In Europe, the semi-diurnal tide is dominant and the tidal current direction changes about every 6 hours [1]. On a much smaller time scale, the MCT power may fluctuate with a period of a few seconds caused by swell disturbances. These short-time fluctuations are mainly related to long wavelength swells which are considered as the main disturbance for the marine current turbine system. Figure I.7 gives the measured swell waves data at Les Pierres Noires in the Fromveur passage near Brest. This correlogram of swell typical period Tp and significant height Hs is from CANDHIS (French National Archiving Center for In-situ Swell Measurements) [22]. The data are recorded during winters in the last few years (2005-2013). There are totally 46889 records in this table. If we consider that the swells corresponding to sea states of 10 s < Tp and 2 m < Hs can not be neglected for MCTs, it still remains about 33206 records.
BATTERY STORAGE TECHNOLOGIES
Battery is a classical solution for storing electricity in the form of chemical energy. Battery storage technologies presented here refer to rechargeable batteries which can be used as energy storage sources. A battery system usually consists of one cell or multiple cells connected in series or in parallel depending on the desired output voltage and capacity. Each battery cell comprises the cathode (positive electrode), the anode (negative electrode) and the electrolyte which provides the medium for transfer of electrons between the two electrodes. During discharge, electrochemical reactions at the two electrodes generate a flow of electrons through an external circuit with the cathode accepting electrons and the anode providing electrons. During charging process, the electrochemical reactions are reversed and the battery absorbs electricity energy from the external circuit.
Lead-acid Batteries
Lead-acid batteries are the oldest type of rechargeable batteries. They are considered as very mature technologies. They are easy to install and have a low cost. Valve regulated lead-acid batteries require negligible maintenance. The self-discharge rates for this type of batteries are very low, around 2-5% of rated capacity per month, which make them ideal for long-term storage applications. However, disadvantages of lead-acid batteries are low energy density and short service life. The typical energy density is around 30 Wh/kg and the typical lifetime is between 1200 and 1800 cycles [26]. The cycle life would be affected by depth of discharge and they are not suitable for discharges over 20% of their rated capacity [27]. The performance of lead-acid would also be affected by temperature: higher temperature (with the upper limit of 45°C) will reduce battery lifetime and lower temperature (with the lower limit of -5°C) will reduce the efficiency.
Nickel-based Batteries
In a nickel-based battery, nickel hydroxide is used on the positive electrode but for the negative electrode different materials can be used. This fact explains the existence of various technologies. There are three kinds of nickel-based batteries namely the nickel-cadmium (NiCd) battery, the nickel-metal hydride (NiMH) battery and the nickel-zinc (NiZn) battery. The NiCd technology uses cadmium hydroxide, the NiMH uses a metal alloy and the NiZn uses zinc hydroxide. Nickel-based batteries have larger energy densities than lead-acid batteries, 50 Wh/kg for the NiCd, 80 Wh/kg for the NiMH and 60 Wh/kg for the NiZn.
NiCd batteries are now reaching the level of maturity as lead-acid batteries. NiCd batteries have a longer lifetime about 3000 cycles and can be fully discharged without damage [28]. As an example, this technology is used in the energy storage system of the Alaska Golden Valley project which provides a backup to an isolated electrical power system. This project is claimed to be the world’s most powerful battery system which can produce up to 52 MW of emergency backup power for about 15 minutes [29]. However, two drawbacks limit future large-scale deployment of this technology. One is the high price, for the NiCd battery may cost up to 10 times more than the lead-acid battery. Another is the environment concerns about cadmium toxicity and associated recycling issues [30-31].
NiMH batteries have high energy density witch is over twice than lead-acid batteries. This type of batteries can be recycled and their components are harmless to the environment. They also can be used in large temperature ranges and high voltage operation. However, repeatedly discharged at high load currents would shorten the life of NiMH batteries to about 200-300 cycles and the memory effect reduces the useful SoC (State of Charge) of the battery. NiZn batteries have the same advantages of NiMH batteries and have deep cycle capability as NiCd batteries, but they suffer from poor life cycle due to the fast growth of dendrites.
Lithium-ion Batteries
Lithium-ion batteries achieve excellent performances in portable electronics and medical devices. This technology is now typically driven by the consumer electronics market (smartphone, tablet, digital camera, etc.) and is very attractive for electric vehicle applications, because lithium-ion batteries are lighter, smaller and more powerful than other batteries. They have the highest energy density (100-250 Wh/kg) and the highest power density (800-2000 W/kg) among all the batteries [32]. Other advantages of lithium batteries include high efficiency, low memory effect and low self-discharge rate. This is the reason why lithium-ion batteries are very promising to be used in the next-generation electrical vehicles or hybrid vehicles [27, 32-33].
Some drawbacks exist in this battery technology. Lithium-ion batteries are theoretically characterized by a lifetime about 3000 cycles at 80% depth of discharge. However in actual, lithium-ion batteries are not robust and sometimes very fragile. Life cycles are affected by temperature and would be severely shortened by deep discharges [28]. Usually, lithium-ion batteries require special protection circuit to avoid overload and need sophisticated management systems to maintain safe operational conditions. Another drawback is that the cost of lithium-ion batteries, from $900/kWh to $1300/kWh. These facts would limit the use of lithium-ion batteries in large-capacity cases and applications where low SoC would be reached.
In 2011, a 32-MW lithium-ion battery system was installed adjacent to a wind farm near Elkins in West Virginia (USA) for smoothing out some of the wind’s variability. This battery system is capable of discharging the rated power for about 15 minutes [29]. A recent research shows that lithium-ion batteries could be the most cost-effective solution for integrating renewable sources only when the required depth of discharge is limited around 10% [34].
Battery Technologies Comparison
Table I.2 summarizes the main merits and demerits for the battery technologies discussed above. For marine energy application, these batteries are reasonable supposed to be installed underwater or on an offshore platform and they may be discharged deeply in order to achieve a required smooth effect. In the first place, low maintenance and robust long service life (deep discharge ability) should be considered as important criteria, and in that term the lead-acid and lithium-ion batteries are not favorable due to their short cycle life for deep discharge. Low cost should also be emphasized, which make lithium-ion and nickel-based batteries not attractive for megawatt-scale applications.
Table of contents :
INTRODUCTION
CHAPTER I: State of the Art Survey of Energy Storage Technologies for Marine Current Turbines
I.1 INTRODUCTION
I.2 TIDAL CURRENT TURBINE BACKGROUND
I.3 POWER FLUCTUATION PHENOMENA
I.4 BATTERY STORAGE TECHNOLOGIES
4.1 LEAD-ACID BATTERIES
4.2 NICKEL-BASED BATTERIES
4.3 LITHIUM-ION BATTERIES
4.4 SODIUM-SULPHUR BATTERIES
4.5 FLOW BATTERIES
4.6 BATTERY TECHNOLOGIES COMPARISON
I.5 FLYWHEEL TECHNOLOGIES
I.6 SUPERCAPACITOR TECHNOLOGIES
I.7 PHS AND CAES TECHNOLOGIES
7.1 PUMPED HYDRO STORAGE
7.2 COMPRESSED AIR ENERGY STORAGE
I.8 COMPARISIONS OF ENERGY STORAGE TECHNOLOGIES
I.9 CONCLUSION
CHAPTER II: Power Smoothing Control with Supercapacitors for Compensating Swell Effect on a Grid-Connected MCT System
II.1 INTRODUCTION
II.2 SWELL EFFECT MODELING
II.3 MODELS FOR A GRID-CONNECTED MCT SYSTEM
3.1 MARINE CURRENT TURBINE MODEL
3.2 MARINE CURRENT GENERATOR MODEL
3.3 POWER CONVERTER AVERAGE-VALUE MODEL
II.4 GENERATOR–SIDE POWER SMOOTHING CONTROL
4.1 PI CONTROLLERS TUNING
4.2 PROPOSED MPPT FOR REDUCING POWER FLUCTUATION
4.3 COMPARISON OF THE PROPOSED MPPT WITH TORQUE-BASED MPPT
II.5 GRID-SIDE POWER SMOOTHING CONTROL
5.1 GRID-SIDE CONVERTER CONTROL
5.2 SUPERCAPACITOR ESS FOR GRID POWER SMOOTHING
II.6 COST AND LAYOUT ISSUES OF THE SC ESS
II.7 CONCLUSION
CHAPTER III: Daily Power Management with Flow Battery ESS
III.1 INTRODUCTION
III.2 MCT-BASED HYBRID SYSTEM
III.3 FLOW BATTERY MODELING
3.1 BATTERY EQUIVALENT CIRCUIT MODEL
3.2 BATTERY PARAMETER CALCULATION PROCESS
3.3 BATTERY SIZE AND BASIC CHARGE-DISCHARGE CHARACTERISTIC
III.4 HYBRID SYSTEM DAILY POWER MANAGEMENT
4.1 SYSTEM CONFIGURATION AND BESS CONTROL SCHEME
4.2 SIMULATION WITHOUT THE DG
4.3 SIMULATION WITH THE DG
4.4 SIMULATION WITH A SMALLER DG
III.5 ISLAND POWER MANAGEMENT CASE
5.1 ISLAND LOAD ESTIMATION
5.2 POWER MANAGEMENT FOR THE ISLAND CASE
5.3 CONCLUSION OF THE ISLAND CASE
CHAPTER IV: Control of the Non-Pitchable PMSG-Based Marine Current Turbine at Over-rated Current Speed
IV.1 INTRODUCTION
IV.2 GENERATOR OPERATING CHARACTERISTICS…
2.1 OVER-RATED SPEED OPERATION
2.2 STEADY-STATE ANALYSIS
IV.3 ROBUST FLUX-WEAKENING CONTROL
3.1 THE SYSTEM CONTROL SCHEME
3.2 SPEED CONTROL AND TORQUE CONTROL
3.3 COMPARISON OF THE CAP MODE AND THE MAP MODE
IV.4 DISCUSSIONS ON GENERATOR PARAMETERS
IV.6 CONCLUSION
CONCLUSIONS & PERSPECTIVES
I. INTRODUCTION
II. ETAT DE L’ART SUR LES SYSTEMES DE STOCKAGES D’ENERGIES POUVANT ETRE ASSOCIES AUX HYDROLIENNES
II.1 PROBLEMATIQUE DE L’HYDROLIEN
II.2 ETAT DE L’ART SUR LES SYSTEMES DE STOCKAGES D’ENERGIES
II.3 COMPARAISONS ET CONCLUSIONS SUR LES SYSTEMES DE STOCKAGE D’ENERGIE
III. LISSAGE DE LA PUISSANCE AVEC DES SUPER-CONDENSATEURS
III.1 MODELISATION DE LA HOULE
III.2 LISSAGE DE PUISSANCE PAR ACTION SUR LA GENERATRICE
III.3 LISSAGE DE LA PUISSANCE INJECTEE COTE RESEAU
III.4 DISCUSSION
IV. INTEGRATION D’UNE FLOW BATTERIE POUR LA GESTION
QUOTIDIENNE DE LA PUISSANCE
IV.1 MODELISATION DE LA BATTERIE
IV.2 SYSTÈME HYBRIDE HYDROLIEN/BATTERIE/DIESEL
IV.3 CAS D’UNE ALIMENTATION INSULAIRE ISOLEE
V. STRATEGIE DE LIMITATION DE PUISSANCE AUX VITESSES DE COURANTS MARINS ELEVEES
V.1 CARACTERISTIQUES DE FONCTIONNEMENT DE LA GENERATRICE
V.2 CONTROLE ROBUSTE PAR DEFLUXAGE
V.3 DISCUSSION SUR LES PARAMETRES DE LA GENERATRICE
VI. CONCLUSION ET PERSPECTIVES
REFERENCES
