Alcatraz, a 22-acre island in San Francisco which until 1963 was home to a supposedly inescapable prison, boasts a unique solar microgrid which took almost two decades to develop.
A highly redundant microgrid
The tourist attraction is powered by a commercial scale microgrid system, reducing much of the diesel fuel costs required to power systems on the National Park Service property.
The microgrid system consists of a 350kW photovoltaic (PV) array, Princeton Power Systems inverters, a Princeton Power Systems site controller, two diesel generators, and a lead-acid battery rack.
The total island load varies typically between 50 and 80kW. The photovoltaic array generates a peak power each day of approximately 175kW. The batteries absorb all of the excess PV when it exceeds the load requirement, and they deliver whatever load the PV does not cover at all other times.
While solar produces the majority of the island’s power, batteries provide a back-up when the sun is not shining, as does the generator when both solar and battery storage are not available. The generator is automatically turned on and run at maximum efficiency to recharge the battery bank. It then switches off three to four days at a time.
Because of its autonomy, the microgrid must be highly redundant. A master controller monitors the system to create maximize solar use and reduce the use of diesel generators. If the master control system becomes damaged, each of the eight inverters in the system can operate autonomously.
Even if multiple components get compromised for whatever reason, each individual inverter will continue to power the island’s load. It therefore provides a lot of redundancy if anything should go wrong.
Becoming energy independent
The island became energy independent in the 1960s when a storm broke the electric cable to the mainland grid. After that, Alcatraz relied on diesel generators. However, the generators created an unpleasant air quality for tourists and fines for the park service.
Finally, in the 1990s, the park service started installing solar panels with the help of the Sacramento Municipal Utility District and the National Renewable Energy Laboratory.
The project was shelved due to protests from historical preservation groups but was revived several years later when US$3.6 million became available to solarize Alcatraz through the 2009 American Recovery and Reinvestment Act. Princeton Power Systems. Plans were put into place to design and provide equipment for a microgrid.
More discreet rooftop solar panels ( a total of 1,360) were installed at this time. Princeton Power Systems was able to install a flat system, invisible from the shore that didn't damage "the look and feel “of the landmark.
Other than pressure from historic preservationists, harsh conditions — wind, salt and fog — meant that equipment needed protective encasement. Since no new structures could be built, modern electronics had to be tucked unobtrusively into historic enclosures.
Microgrid uses existing infrastructure
The Alcatraz microgrid powers the main prison building, now a museum, as well as dozens of smaller structures, a media room, and a charging station for golf carts used for transportation around the island. The microgrid equipment was integrated into the existing diesel system and uses existing wiring.
Instead of using the solar power to charge the batteries and then batteries to supply the load, the solar directly supplies the AC microgrid. The solar power is generated directly into the grid, and the batteries are directly attached to the AC grid.
This makes the system highly efficient because the solar energy doesn't have to go into the batteries. This gives the island more control over the batteries’ charging and discharging which can improve the life span of the batteries.
Princeton Power Systems designed the microgrid so that it can interconnect with the main grid, should another undersea cable be built. But at this time, it is more cost-effective for the island to rely solely on the solar microgrid.
Because of the various challenges, the cost for the Alcatraz microgrid was higher than it would be under normal circumstances.Read more