INVESTIGATIONS ON SOLAR AND OCEAN WAVE BASED HYBRID ENERGY SYSTEM

Abstract

The thesis pertains to the investigations on a hybrid system integrating solar photovoltaic (PV) power and ocean wave energy. The integration of DC sources is easier than of AC sources as former is independent of problems related to frequency, harmonics, reactive power, phase and frequency synchronization as well as have less control complexity and easy deployment. Further, energy storage system, i.e., battery bank, is essential to maintain the power balancing in renewable energy sources. Therefore, power from solar PV, ocean wave energy and battery bank are fed at DC-link to supply continuous power to an off-grid load. To regulate the energy supply to the load from solar PV, ocean wave and/or battery bank and charging/discharging of the battery bank, an energy management scheme and control strategy plays an important role. Further, it ensures extraction of maximum power from solar PV and provides a constant amplitude sinusoidal AC voltage at the load end. For development of a hybrid system, three distinct off-grid systems, namely, off-grid solar PV system, off-grid ocean wave energy converter (OWEC) system and off-grid hybrid system, are considered to study and analyse the system behaviour and contribution of energy management schemes and control strategies. Although solar PV systems are being used commercially, its efficiency should be improved for adequate utilization of available solar power. The efficiency of off-grid solar PV – battery backup system can be improved by change in maximum power point tracking (MPPT) topology and/or its control system. A multi-cell resonant converter with classical perturbation and observation (P&O) algorithm is selected as MPPT device as it offers high gain, reduction in power rating of power switch, zero current switching, ease of control, modular structure and lower current stress on power switch. Another multi-cell resonant converter is used for battery discharging. The primary power source (solar PV) and secondary power source (battery bank) through multi-cell resonant converters are connected at DC-link to supply a common inverter connected AC load. An energy management system is developed to generate power by solar PV at MPP, supply to load and/or battery bank from solar PV and/or battery bank according to available power from solar PV. The voltage and power balancing approach is used to control battery discharging converter. Further, a closed loop PI controller is used to provide the inverter control pulses by energy management system. The simulation and experimental prototype results are presented to validate the theoretical concepts. In contrast to the solar PV system, the OWEC system has been in its nascent stages of research and development. Firstly, a suitable OWEC device is selected for the Indian coastal region by a comprehensive literature survey of all the existing OWEC devices. Among the OWECs tested at open sea, 13 OWECs are shortlisted on the basis of ocean wave power flux density in Indian coastal region. A systematic approach is used to extrapolate unknown quantities, such as, total annual cost per MW, system overall efficiency and relative impact on the surrounding. Finally, a suitable OWEC is selected based on total annual cost per MW, efficiency and effect on surrounding environment. It was concluded that direct drive permanent magnet linear generator (PMLG) based OWEC (L1-L3) (Sweden) and sea-wave slot-cone generator (SSG) (Norway) are cost-effective, efficient and eco-friendly and the most suitable for Indian coastal region. As direct drive permanent magnet linear generator (PMLG) based OWEC (L1-L3) model are suitable at ocean wave flux density similar to Indian cost, similar model is selected for further experimental and simulation study. As the PMLG is not available in market, a laboratory-scale prototype of 0.5 m height with 18 windings is developed using a linear actuator, motor, gear, chain and piston. The main drawback of the developed system is that it can only generate continuous wave pattern, i.e., for the wave pattern of fixed time period and wave-height. As the physical system in the ocean will encounter variable height and discontinuous wave pattern at the coastal line, a single-phase power electronic PMLG emulator is also developed to study the performance of the proposed OWEC system with discontinuous wave pattern. The power generated by direct-drive PMLG based OWEC (L1-L3), with both continuous and discontinuous wave pattern, is a high-frequency variable amplitude discontinuous AC power. The amplitude and frequency of the generated power depend on the motion of the mover, i.e., ocean wave height and time period. Such a discontinuous surge power cannot be utilized directly by the load. Therefore, to generate continuous AC power with constant amplitude and frequency, an energy buffer system is developed for both continuous and discontinuous wave pattern systems. Further, a voltage control strategy is used to control the inverter power switches and consequently, to stabilize the amplitude of AC voltage at load end. For continuous wave pattern, two different inverter topologies, i.e., topology-1 and topology-2, are utilized. The topology-1 operates in buck and boost modes and generates a complete AC cycle. In contrast, topology-2 operates in three different modes, namely, buck, boost and buck-boost modes, and generates positive half cycle by buck and boost modes and negative half cycle by buck-boost mode. For topology-2, the current in the inverter inductor for the negative half cycle is found to be larger than that for the positive half cycle. Consequently, system losses increase and higher power rating of the inductor and switches are required. Therefore, in comparison to topology-2, topology-1 has lesser current across inductor and in turn, lower power loss and required power rating of the components. From the small signal analysis of the inverter, it was observed that the inverter topology-1 is most stable during buck-mode of operation. Further, the inverter operation in buck mode reduces current in inverter inductor as well as ensures high power transfer capability and high efficiency. Therefore, inverter topology-1 in buck mode (or buck-inverter) is used for PMLG emulator for discontinuous wave pattern. Further, a voltage and power control algorithm is developed to limit the variation of DC-link voltage using multi-cell resonant converter and a battery charging buck-converter. A buck-inverter with voltage control strategy is connected at DC-link to provide AC voltage with constant amplitude and frequency at load end. The experimental results from the laboratory-scale prototype and the simulation results are presented which validate the proposed concept. Finally, two time varying renewable energy sources i.e. direct-drive PMLG based OWEC and solar PV are integrated with battery backup for off-grid fixed AC load. The DC-link voltage is controlled by battery energy storage system (BESS) and AC load side voltage is controlled by step-down inverter. Resonant boost converters are implemented for battery discharging and as MPPT device for solar PV. A control logic is developed to regulate DC-link voltage by controlling battery charging and discharging current. The input power varies due to change in solar irradiance and discontinuous movement of PMLG mover (surge power) whereas output power varies due to change in inverter load. However, the inverter output voltage at load end is successfully established to be constant with the proposed energy management system. A laboratory prototype model with dSPACE controller is developed to validate simulation results.