Large-Scale Solar Integration: Some Assembly Required

By Scott Lee

The world’s power grid is going through a remarkable transformation. This shift towards distributed renewable energy is changing the energy landscape as the need to maintain pace with the ever increasing demand for electricity, coupled with recent concerns regarding global warming due to greenhouse gas emissions, has resulted in a tremendous increase in solar energy installations. Since 2002, installed photovoltaic generation capacity has doubled every 3.6 years while the median installed price of residential and commercial PV systems has fallen approximately 14% per year. While solar power is clean, renewable, and increasingly affordable, as with all new technologies the large-scale grid integration of it is not without its challenges.

One major issue is the intermittency of solar generation. Power systems operate on a precarious balance of demand and supply. The lion’s share of electrical generation is produced with heavy rotating machinery, throttled to match demand exactly. If a mismatch such as a drop in demand occurs, the massive generators are unable to reduce power output quickly and begin speeding up as excess energy accumulates within them. This leads to a steady increase in line frequency until supply and demand once again equilibrate. While small, slow-moving changes are normal in day-to-day operations, solar adds a complication into the mix. Solar systems are based on lightning fast silicon instead of lumbering tons of steel, and unlike turbines that require tens of minutes to throttle up or down, a solar system can do so in a thousandth of second.

Germany, a country with over 38 GW of solar systems accounting for over 31% of the country’s electricity generation, is no stranger to the potential hazards of using such highly dynamic systems. In 2008, it was discovered that the seemingly innocuous combination of bright skies and light load could result in a devastating blackout. During that time, all solar inverters were hard-coded to shut down should the line frequency exceed 50.2 Hz as a safety mechanism, as high line frequencies may damage sensitive electronic devices. As solar is capable of ramping up much more rapidly than generators are able to ramp down, line frequency could uncontrollably increase beyond the 50.2 Hz threshold. If this occurs, every grid-connected inverter would disconnect in unison. In this scenario, tens of gigawatts of generation are simultaneously lost and the resulting electrical shockwave propagates through the entire system. In severe cases the generation loss is too great to sustain, and a large number of rotating generators slow down or shut off entirely. The survivors stay online long enough to experience gigawatts of solar suddenly kick back on again as the line frequency decreases, restarting the cycle. In extreme cases this “yo-yo” effect could result in a national power system crippled until nightfall.

So far, Germany has avoided major catastrophe by being proactive. In 2012, the country enacted an initiative to retrofit every solar inverter with advanced controllers to address the “50.2 Hz problem” by curtailing solar generation as line frequency increases. As solar integration continues to increase, issues such as these become more of a concern, highlighting the need for advanced integration technologies. Organizations such as the IEEE and VDE are currently proposing new standards and technologies to address the challenges of high levels of solar integration.

In today’s renewable energy market, a flurry of research and innovation are taking place as companies search for solutions to solar’s large-scale integration problems. Companies such as Fulcrum3D and the University of California, San Diego are developing advanced cloud tracking technologies to better forecast solar generation while other such as Tesla and Aquion are developing economical large-scale energy storage systems to buffer the variability of solar energy. Smart microgrids, originally intended to improve grid resiliency after the Hurricane Sandy disaster, could also be used to allow communications between grid energy devices, enabling real-time response to changing conditions and deliver better grid management and control.

When the Pearl Street Station, the world’s first central power plant, fired up its steam engines in 1882, no one could have dreamed that a paltry 175 horsepower electrical grid would multiply in capacity over one hundred million times to become the terawatt engine powering the world today. In a similar sense, only time will tell how the solar transition will play out. The enormous scale and complexity of the grid makes it impossible to predict the impact of solar as it becomes a major player in the energy landscape. While the future solar powered grid is as unpredictable as the weather upon which it depends, one thing is certain: a successful transition into a future powered by clean, renewable energy will require a mix of innovation, new technologies, and some assembly required.

 

Scott Lee is a Ph.D. researcher at the Advanced Power and Energy Program of the University of California, Irvine. His research focus involves high penetration photovoltaic grid integration and advanced inverter and microgrid controls. He can be reached at sfl@apep.uci.edu.