The energy conversion process plays a vital role for energy transition toward renewables and de-carbonization. In this context, considerable research works are carried out at Khalifa University’s research centers to enhance the efficiency, reliability and stability of the energy conversion systems in various form.
Wind Energy Systems
In this topic, the research team at APEC developed several advanced technologies with focus on developing new configuration and control strategies for Wind Turbine Generators that include FSIG-WT, DFIG-WT and PMSG-WT. The following developed technologies and US Patents are given as examples:
- APEC’s team introduced novel Fault-Ride through (FRT) schemes that employ shunt and series system reconfiguration comprising a transient management scheme for doubly-fed induction generator (DFIG) based wind. The combination of shunt and series grid interface schemes demonstrated a low component count, simple fault tolerant and protection structure, and improved performance of fault ride through with effective compensation to the electric grid.
- The concept is further modified and implemented with the self-excited Squirrel Cage Induction Generator (SCIG) based wind turbine that comprises shunt capacitor banks and a combined shunt and series utility grid interface configuration named as unified compensation system (UCS) incorporating a transient management scheme to improve FRT capability for SEIG wind turbines. The system shows the flexibility utilizing one converter to perform different types of line voltage regulation and provide both series and shunt compensation.
- Then, a new Fault Ride-Through (FRT) configuration for Fixed Speed Induction Generator based Wind Turbine (FSIG-WT) is developed. It utilizes a nine switch converter (NSC) to enable both shunt and series compensation in response to the system dynamics and grid faults and augmented with advanced synchronization scheme.
- In another work, APEC team introduced a novel DC link scheme for enhancing the Fault Ride-Through (FRT) capability of Doubly Fed Induction Generator based Wind Turbine (DFIG-WT). The proposed system consists of parallel capacitors with a dedicated control strategy designed to provide means for power evacuation during grid fault conditions. This technically simple and cost-effective scheme was developed considering transmission line auto-reclosing which may cause multiple fault inceptions.
- Also, other schemes such as dynamic voltage restorer and high temperature superconducting fault current limiters, dynamic modulated braking Resistor as well as new protection and advanced control schemes have been examined with FSIG-WT, DFIG-WT, PMSG-WT at the level of wind turbine and wind park to enhance the Fault-Ride Through capability and overall all performance with efficient ancillary serveries.
- Several novel wind park controllers were developed by APEC team with focus on voltage regulation, active and reactive power regulation, frequency support, power oscillation damping and power quality enhancement in adherence to grid codes requirement.
US patent: M. S. El Moursi, W. Xiao, and P. Huang “Fault Handling System for Doubly Fed Induction Generator“, US 9,461,573.
https://patents.google.com/patent/US20140138949A1/en
US Patent: Vinod Khadkikar, Bharah Ambatii and Parag Kanjiya “Fault Tolerant Wind Energy Conversion System” U.S. 9,425,726
Solar Energy
Advanced research works are carried out at Khalifa University by Prof. Ammar Nayfeh and his research team to improve the efficiency of the solar cells efficiency using novel materials as follows:
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Metal and Semiconductor Nanoparticles to Improve Solar Cells
In this work, the research team studied both metal and semiconductor nanoparticles. They did both semiconductor and metal nano particles. First, plasmonic solar cells were fabricated and the effect of Au nanoparticles on the performance of a-Si:H solar cells was investigated experimentally. In addition, the performance of thin-film amorphous silicon-based n+-i-p+ solar cells in the presence of an exterior top thin film of mono-dispersed ultra-small 2.85 nm diameter silicon nanoparticles has been examined. -
2.2D materials: Investigation of interfacial layers for graphene/silicon schottky junction solar cells
Dr. Aaesha Alnuaimi finished her PhD in 2017. Her thesis was on the graphene-Si junction and interface which should behave like a metal/Si junction. To study Gr/Si junction experimentally, graphene was synthesized by chemical vapor deposition (CVD) method and PN junction solar cells fabricated. The effect of metal oxide interfacial layers has also been studied using ALD. The first study has been performed with chemically doped graphene and Al2O3 interfacial layer. Engineering the interface with Al2O3 resulted in improving the efficiency of Gr/Si solar cell from 3.77% to 8.75%. Subsequent experiments using HfO2 and ZnO have been carried out with chemical-doping-free graphene layer. The efficiency of the cells improved from 1.77% to 6.91% with HfO2 and to 6.28% with ZnO interfacial layers. This work was seminal in Graphene/Si junctions. -
3.III-V on Si Multi-Junction Step-Cell (break it up)
Dr. Sabina Abdul Hadi during her PhD worked on the development of III-V on Si multi-junction solar cells. Multi-junction (MJ) solar cells have the highest reported efficiency to date as they utilize solar spectrum more efficiently compared to single junction cells by having vertical stack of solar cells absorbing wider range of optical wavelengths. However, one drawback of MJ solar cells is their high costs, since the materials used in their fabrication involve expensive Ge and/or III-V group materials, most commonly GaAs. In her research, Dr. Abdul Hadi explored 2-terminal III-V / Si MJ solar cells, where top cell is made of GaAs1-xPx layers grown on inexpensive Si substrates. In such 2-terminal setup, sub-cells are connected in series where the sub-cell with lowest current limits overall MJ cell performance. Thus, optimization problem focuses on achieving maximum matching sub-cells currents. Another challenge is use od III-V on Si materials is their lattice mismatch, which can cause significant material defects in III-V layers if they are grown/deposited directly on Si substrate. One way to grow GaAs1-xPx layers on Si with minimum defects, is via graded SiGe buffer layers (~ 6-7 μm thick), allowing for fabrication of high quality III-V top cell. In her work, Dr. Abdul Hadi in our collaboration with MIT Professor Eugene Fitzgerald and his team of researchers have shown by experiment and simulations that monolithic III-V/SiGe/Si dual-junction cell performance would be limited by Si sub-cell due to undesired optical absorption in SiGe layers due to their low bandgap and high absorption. In order to reduce the optical losses in SiGe layer, a MJ step-cell design was proposed, where bottom cell is partially exposed to direct sunlight in order to boost photo-generated current in it. Step-cell design was analyzed numerically and by using detailed balance method. Theoretical efficiency limit analysis of step-cell, ignoring buffer layer optical losses, showed that step-cell provides added degree of freedom in MJ cell optimization, allowing use of conventionally non-optimum and potentially low-cost materials without significant efficiency losses. Furthermore, numerical modeling of GaAs1-xPx/Si bonded and GaAsP/SiGe/Si monolithic step-cells was carried out using TCAD Synopsys simulation tools, showing benefits of step-cell. Moreover, Dr. Abdul Hadi has demonstrated proof-of-concept step-cell device experimentally, where GaAs0.75P0.25 cell grown on SiGe/Si substrates served as the top cell. Finally, in her PhD work, Dr. Abdul Hadi showed that cost estimates indicate that III-V/Si MJ step-cell has potential to be low-cost high efficiency source of PV energy.
Patent: “Method and device for low cost, high efficiency step photovoltaic cells”, Application PCT/US2017/056335
https://patents.google.com/patent/US20200052141A1/en?oq=US20200052141A1
Book: Ammar Nayfeh and Sabina Abdul Hadi “Silicon-Germanium Alloys for Photovoltaic Applications” Elsevier, 2023.