ORNL’s Solar Technologies program supports the U.S. Department of Energy (DOE) Solar Energy Technologies Office – SunShot Initiative goal to make solar energy cost-competitive with other forms of electricity by the end of the decade. To achieve this goal, ORNL performs innovative research and development activities in Photovoltaics (PV), Concentrating Solar Power (CSP) and Systems Integration (SI) that can make the abundant solar energy resources in the U.S. more affordable and accessible for all Americans.
Areas of research interest include novel earth-abundant and nano-engineered PV materials, understanding of degradation mechanisms using in-situ characterization and modeling, bandgap engineering through multi-layering approach, innovative and non-planar solar cell architectures, smart inverters, modeling of by-directional power flow and impact of high solar penetration on distribution networks, development and in-situ characterization of corrosion-resistant containment alloys, and innovative approaches to mitigate efficiency losses due to obstructions.
Modular 3-phase cascaded H-bridge inverter
One of the major issues associated with solar power efficiency is the reduction in incident radiation even before energy conversion begins. For solar reflectors and PV cells, surface obstruc¬tions such as dust and sand accumulation are main contributors to this reduction. Studies have shown that reflectivity can be reduced by as high as 50% in 14 days for the mirrors employed in CSP plants. Similar findings have also been reported for PV systems efficiency. Presently, scheduled routine manual cleaning and brushing using deionized water is the dominant solution.
In order to reduce cost and enhance efficiency, ORNL is developing a low-cost, transparent, anti-soiling (self-cleaning) coating that can be applied on most surfaces using conventional painting or spraying methods. Initial testing of mirror samples has commenced in September 2014 at the Ivanpah Solar Electric Generating System site in California. Preliminary results on non-optimized anti-soiling coating showed that soiling rate has been cut in half, and reflectivity of coated mirrors returns to the original values after each rain event.
Advanced inverters and controls are being developed to mitigate the issue of transient shading of PV
Similarly, Solar PV systems that supply electricity to com¬mer¬cial and residential buildings are frequently subjected to transient shadings from rooftop structures or nearby trees. The panel areas that are affected by shading change with time of day, and conventional central inverters are not designed to provide maximum output under these conditions. While micro-inverters can provide optimum power due to independent maximum power point tracking (MPPT) controls, the significant number of required units makes this solution costly and less reliable. ORNL and University of Tennessee researchers are working to develop advanced “Cascaded H-Bridge Inverter” that uses multiple levels of MPPT control that adjusts to transient shading conditions, providing maximum value of power output available to the systems. In addition, modulation compensation is used to balance 3-phase grid current.
Reliability and protection challenges are increasing in importance as wind plants become more prevalent
ORNL has supported the DOE Wind Energy Technologies Office’s mission in wind power development and deployment by engaging in research activities such as gearbox reliability, geo-spatial analysis, turbine blade improvement, etc. National resources such as the Carbon Fiber Technology Center can contribute to lowering technical and market barriers through advanced manufacturing of low-cost, high-strength carbon fibers. Most recently, ORNL has concentrated on advancing wind integration into the electric grid by leveraging its extensive expertise in power system engineering, protection of renewable resources and grid assets, advanced control and operation methodologies, as well as smart grid and microgrid technologies. ORNL is working extensively with utilities, manufacturers and developers to broadly disseminate the specific knowledge necessary to ensure reliability and instill confidence that are critical for high penetration of wind power.
Oak Ridge National Laboratory is managed by UT-Battelle for the Department of Energy