In December 2017, a 20-MW solar farm located at the Tallahassee International Airport was connected to the city’s power grid, marking a milestone in achieving its commitment to safe, reliable, cost effective and environmentally responsible power generation and delivery.
Interim City Manager Reese Goad told the media, “This is another great step in Tallahassee’s efforts to be a leader in implementing sustainable energy practices.”
The addition of solar power into the system that has a peak load of 633 MW was made possible through a power purchase agreement with Origis Energy. This project is a unique arrangement between the city and Origis, which financed, built and operates the 234,000-panel solar farm on 120 acres of city property. As part of the agreement, the city pays for all energy delivered from the farm.
Tallahassee’s power system has been based on fossil fuel generation that provides power on demand with the output controlled by the operators. In considering the addition of the solar farm, the city understood that this new resource would generate power on its own schedule and not one dictated by the system operators.
Power generated from this farm is available to residents for a small premium and this option has proven popular. The success of this first farm has, in part, led to plans for a second, larger solar farm that will begin delivering power to the city’s grid in early 2020. The first solar farm produces 37 million kWh of clean energy, enough to light 3,400 homes. This farm reduces the city’s carbon dioxide (CO2) emissions by approximately 18,000 tons per year. The second solar farm will power an additional 7,000 homes and further reduce the CO2 emissions by approximately 36,000 tons per year.
The Beginning of the Story – Preparing for Solar
In February 2015, the City Commission approved the procurement of the first solar farm. Immediately the utility staff began preparations for solar power by conducting and RFP process, followed by a series of integration studies, including feasibility, system impact and facilities studies.
The studies analyzed the solar farm’s generation characteristics and the existing grid. Stanley Consultants was engaged to perform these studies for varying farm sizes and locations. The airport location was determined to be the best location and size for the city.
The utility sought to minimize adding new grid infrastructure and cost, so it chose to deliver the power through a new 12.47kV express feeder to an existing distribution substation, and then to residents. Further studies then determined the necessary substation upgrades that were required to integrate this power into the system.
Stanley Consultants engineered the substation’s minor modifications and assisted with the express feeder, and all were completed before solar power was connected. The city’s early start on these grid upgrades allowed these projects to be completed at a reasonable pace to avoid acceleration costs, and kept the solar farm on the project critical path.
The Current Situation – Performance and Operational Impact
Florida is flat, hot, humid and largely bordered by ocean. Easterly trade winds routinely cause low cumulus cloud cover that forms quickly. Therefore, solar power generation is difficult to predict even a brief time into the future. Despite this uncertainty, solar power development in the state is continuing unabated.
Each utility’s situation is unique, which has led to different approaches to solar power. The different situations have led to various approaches to solar, from small to large utility-scale projects; from utility-owned to third party-owned and operated. Despite this variation, one thing is constant: Solar projects are gaining in popularity and have proved to be a successful source of environmentally friendly power.
The solar farm almost immediately began contributing a full 20 MW into the system right from the time of the system hookup. Keep in mind, this is December. Fluctuations occurred during the day for brief periods. In short durations, the power delivered was varying from virtually 0 to the max 20 MWs. Operators had to closely monitor the load, the weather, and have the right mix of other generation online and ready for fast-response operation. Operating a power system has always been challenging, and just as expected by the utility, it became even more challenging by adding solar power. However, through May 2018, the city has met the requirements each day.
The most challenging period is expected to occur during the fall and spring shoulder months, when minimum generation is running, photovoltaic power is fluctuating, and industrial load is spiking on and off. These periods have been identified and are being studied further, including identification of best generation mixes. In January 2017, the City Commission voted to increase its power purchase agreement from 20 to 60 MW total, scheduled to come on line in early 2020. Such a significant increase will require further modifications to the city’s power grid and adjustments to the operational approach.
The Future – Near and Long-term
The addition of renewable energy changes how a utility thinks about its future generation fleet, and Tallahassee is no different. But in the utility market today, no aspect of the business is static. Major equipment and infrastructure reaches the end of its useful life. Many utilities today are experiencing that with coal-fired generators. The city experienced it with simple cycle combustion turbines. Several of these peaking generators have been decommissioned, and other will in the future. Much of that generation capacity is being replaced by natural gas-fired reciprocating internal combustion engines (RICE).
Two projects are underway that consist of six Wartsila reciprocating natural gas engines—two 9 MW units at one location, and four 18 MW units at a second location. RICE technology has the advantages of not only an impressive efficiency and low operation cost, but also a quick start reaction time. With a ramp-up time of less than 10 minutes, the Wartsila engines should give the city more flexibility. As technology continues to advance, other power options are being studied, including utility scale energy storage.
Considering the new 40 MW solar farm planned for early 2020, Mayor Andrew Gillum said, “There are few better ways to celebrate the new year than our solar farm powering up. When the farm is fully operational, it will make Tallahassee a solar leader in the Sunshine State.”
About the authors: Ben A. Cowart, P.E., is Manager of Alternative Energy for the City of Tallahassee’s Electric System Integrated Planning Division and is a Certified Public Manager by Florida State University.
Terry L. Blalock, P.E., is a business development manager with Stanley Consultants.