Research

Present Research Projects

• PV Differential Power Processing (DPP) Systems

  Photovoltaic (PV) cells and modules are typically connected in series to reach high voltages needed for grid-tied applications. However, because of the series connection, they experience characteristics mismatch that reduces output power. The source of mismatch can come from irradiance variation, temperature variation, manufacturing tolerance, and cell degradation. Differential power processing (DPP) converters are a solution that achieves high system efficiency by processing a fraction of the total power. DPP converter systems are able to maintaining distributed local maximum power point operation and minimize losses in the system, making the architecture a significant improvement over traditional architectures. There are still many open questions related to converter topology, system control schemes, and PV module integration.
  
  
  Related Publications:
  Pradeep S. Shenoy, Katherine A. Kim, Brian B. Johnson, and Philip T. Krein, “Differential power processing for increased energy production and reliability of photovoltaic systems,” IEEE Trans. Power Electron., vol. 28, no. 6. pp. 2968–2979, June 2013.
  Katherine A. Kim, Pradeep S. Shenoy, and Philip T. Krein, “Converter Rating Analysis for Photovoltaic Differential Power Processing Systems,” IEEE Trans. Power Electron., vol. 30, no. 4, pp. 1987-1997, April 2015.
  

• PV Hot Spot Detection and Protection

  Future PV systems require additional functionality, such as the ability to monitor their operation for problems or faults and intelligently adapt the system as needed. This research focuses on adding detection and protection to PV systems for a temporary fault called hot spotting that occurs in series-connected PV cells. Hot spotting can be detected by monitoring the ac small-signal characteristics of a PV string at various frequencies to measure the parallel capacitance and resistance of the string. An algorithm that dynamically measures the PV string impedance during normal operation is pursued as a low-cost hot spot detection approach. For hot spot prevention, there are a few potential approaches to mitigate hot spot damage, but open-circuit protection is the only one that completely stops hot spotting. A protection device is developed that replaces the bypass diodes to provide both bypass and hot spot prevention capability. This hot spot prevention device was validated in experimental tests to stop the hot spotting within one minute after open-circuit protection is activated.
  
  
  Related Publications:
  Katherine A. Kim and Philip T. Krein, “Reexamination of Photovoltaic Hot Spotting to Show Inadequacy of the Bypass Diode,” IEEE J. Photovoltaic., vol. 5, no. 5. pp. 1435-1441, Sept. 2015.
  Katherine A. Kim, Gab-Su Seo, Bo-Hyung Cho and Philip T. Krein, "Photovoltaic Hot-Spot Detection for Solar Panel Substrings Using AC Parameter Characterization," IEEE Trans. Power Electron., vol. 31, no. 2, pp. 1121-1130, Feb. 2016.
  

• Photovoltaic Electro-Thermal Modeling

  As PV systems become larger and more complex, an enhanced photovoltaic (PV) cell model is needed for more accurate simulations. The conventional single-diode PV model is commonly chosen for simulation due to its simplicity in calculating PV characteristics in the forward-biased voltage region. However, to more fully model a PV cell using real-time simulation, the single-diode model must be improved. This research focuses on incorporate dynamic characteristics, reverse-biased voltage region characteristics, and thermal characteristics. Parallel capacitive and series inductive components are included in the PV cell model to achieve dynamic characteristics. Reverse-biased and breakdown features are also integrated to capture the reverse-bias behavior that occurs during some PV faults. Thermal characteristics of the cell and heat transfer with the cell’s surroundings are used to determine the PV cell temperature, which affects the electrical characteristics and output power.
  
  Related Publications:
  Katherine A. Kim, Chenyang Xu, Lei Jin, and Philip T. Krein, Dynamic photovoltaic model incorporating capacitive and reverse-bias characteristics, IEEE J. Photovoltaic., vol. 3, no. 4. pp. 1334-1341, Oct. 2013.
  Jee-Hoon Jung and Shehab Ahmed, “Real-time simulation model development of single crystalline photovoltaic panels using fast computation methods”, Solar Energy, vol. 86, no. 6, pp. 1826-1837, June 2012.
  

• Energy Harvesting Shoes

  Wearable applications have recently become popular as consumer products, but changing or recharging batteries often can be cumbersome for user. Energy harvesting is a solution that can be integrated into wearable applications and harvest energy as we go about our daily life. There are various technologies in energy harvesting systems, like the solar panel, piezoelectric, thermoelectric, and magnet vibration transducers. In this project, the goal is to research that how to harvest and store the energy from a shoe when people walking.