Photovoltaic power for Alcatraz

July 14th, 2016, Published in Articles: Energize

 

A commercial-scale micro-grid system was designed and built on Alcatraz Island, San Francisco, US, as a solution to high diesel fuel costs, pollution in the San Francisco bay area, and high carbon emissions.

When a ship’s anchor ruptured the underwater power lines which linked the island to San Francisco in 1950, Alcatraz turned to diesel generators as its primary source of power. In 2010, Princeton Power Systems was engaged to take on the challenge of enabling the historic landmark and tourist attraction, known as “The Rock”, to provide its own clean and efficient independent power source.

This project reflects the US’s National Park Service’s initiative to find an alternative way of powering the island in order to reduce fuel costs and pollution. Alexandra Picavet, the US National Parks’ public affairs officer, says the switch to solar power on Alcatraz was one of the most challenging and rewarding goals to meet in striving toward a more sustainable operation in the Golden Gate National Recreation Area.

A micro-grid system, comprising inverters, a solar array, advanced batteries, a system site controller, and back-up generators, was selected to independently power the island (Figs. 1 and 2). Designing and building the system on one of the US’ most well-known historic landmarks with over 1-million visitors per year, created many challenges.

Fig. 1: Equipment set up diagram.

Fig. 1: Equipment set up diagram.

 

Fig. 2: Rooftop PV panels.

Fig. 2: Rooftop PV panels.

System highlights

The system comprises 350 kW of PV panels, a 400 kW storage battery offering 1900 kW/h, eight 100 kW grid-tied inverters, a site controller, two diesel-powered generators and a lead-acid battery pack.

The inverters are said to have an efficiency factor of 96,5% with built-in maximum power-point tracking (MPPT) for the charge controller.

The site controller controls the generators, determining when to start and stop them; manages the charging of the batteries; oversees PV smoothing; and logs all data which is aggregated and displayed on a remote information kiosk (Figs. 3 and 4).

Fig .3: Information kiosk display 1.

Fig .3: Information kiosk display 1.

 

Fig. 4: Information kiosk display 2.

Fig. 4: Information kiosk display 2.

The system has to supply a load of between 50 and 80 kW per day. The use of solar PV panels is said to have reduced the island’s carbon emissions by as much as 80%.

Component placement

Preserving the island in pristine condition while completing the installation was the greatest challenge. Given that a system of this size requires a large construction effort, component placement was key. To prevent the solar array from being visible from San Francisco, it was placed on the roof of the prison in a flat configuration rather than a traditional angled configuration. The inverters, battery rack and generators were placed in the old generator room, as this space is isolated and not accessible for tourists. The room is also protected from the harsh salt water environment.

Commissioning

The fragile natural environment and wildlife, particularly the birds (Alcatraz is an old Spanish word meaning pelican) added to the challenge. Extra attention was given to the solar panels after being damaged by rocks and shells which were dropped by birds as they flew overhead. Despite the coarse condition of the generator room, engineers were able to insulate the room to prevent future problems and ensure reliable long-term operation.

Information kiosk

The information kiosk on Alcatraz Island allows tourists to see the daily, weekly, and yearly performance of the micro-grid system. Figs. 3 and 4 show how much of the island’s power is produced by solar, battery storage, and generator.

While the majority of the island’s power is produced by solar, batteries provide additional help when the sun is not present, as does the generator when both solar and battery storage are not available. The generator is automatically turned on and runs at maximum efficiency to recharge the battery, and is then turned off for three to four days at a time by the site controller.

Alcatraz Island’s power data

Fig. 5, which shows the island’s power data, is based on a pre-commissioning test conducted the month before the system became fully operational. The total island load varies typically between 50 and 80 kW. The PV array produces a peak power each day of roughly 175 kW. The batteries absorb all of the excess electricity from the PV panels when their output exceeds the load requirement, and the batteries deliver whatever load the PV cannot provide at all other times. In Fig. 5, for illustrative purposes, the backup generator was turned on, allowing the system to charge all of the batteries to 100% state of charge.

Fig. 5: Power data graph.

Fig. 5: Power data graph.

The chart shows the batteries’ power absorption diminishing to zero as the batteries become fully charged. The excess PV power in this case was fed back into the generator, resulting in the generator output becoming negative. When the generator is shut down, as a result of the batteries being fully charged, and there is no capacity to absorb any further PV power which exceeds the load, the system automatically limits the PV panels’ electricity production at the time to be equal only to what the load and the batteries can absorb.

This is managed using a frequency-shifting control strategy that requires zero communications among the micro-grid inverters. Once the PV production is less than the island loads, the batteries again resume supplying the balance of required power.

Contact Monica Hannah, Princeton Power Systems, mhannah@princetonpower.com

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