Tallahassee mulls number, timing of new gas engine additions

The City of Tallahassee in Florida plans to retire some existing power capacity in the near term and over time to add new capacity, including solar and gas-fired engines that can even out the output of highly-variable renewable energy facilities.

The sity’s Electric Utility presently serves approximately 117,825 customers located within a 221 square mile service territory, it said in a Ten Year Site Plan filed April 1 with the Florida Public Service Commission.

The Electric Utility operates three generating stations with a total summer season net generating capacity of 746 MW. It has two fossil-fueled generating stations, which contain combined cycle (CC), steam and combustion turbine (CT) facilities. The Sam O. Purdom Generating Station, located in the City of St. Marks, Florida, has been in operation since 1952; and the Arvah B. Hopkins Generating Station, located on Geddie Road west of the city, has been in commercial operation since 1970. The city has also been generating electricity at the C.H. Corn Hydroelectric Station, located on Lake Talquin west of Tallahassee, since 1985.

The site plan noted: “There are several generating unit retirements scheduled in the near term (2016-2020). A total of 56 MW (summer net rating) of generating capacity provided by four (4) small combustion turbines (Hopkins CTs 1 & 2 and Purdom CTs 1 & 2) are planned for retirement by the spring of 2018. Though the retirement dates of these units have been postponed several times in the past the City believes it would not be prudent to consider them as dependable capacity beyond their currently planned retirement dates. In addition, the City’s Hopkins Unit 1, which first went into service in 1971, is planned for retirement at the end of 2020. All of these generating units are in excess of 40 years old.”

The city said it continues to seek out suitable projects that utilize the renewable fuels available within the big bend and panhandle of Florida. In February 2015, the city issued a request for proposals (RFP) for a purchase power agreement (PPA) for a 10 MW (ac) utility scale solar PV project. During the negotiations of the Purchase Power Agreement, the project developer offered to expand the project from 10 MW (ac) to 20 MW (ac). On Feb. 24, 2016, the City Commission voted to authorize negogiations for a PPA for 20 MW (ac). It is expected that the project will be located within the city’s service territory or adjacent to a city-owned facility. Due to the intermittent nature of solar PV the PPA will be for energy only and will not be considered firm capacity. It is anticipated that this facility will be operational by the summer of 2017.

In 2011, the City of Tallahassee signed contracts with SunnyLand Solar and Solar Developers of America (SDA) for over 3 MW of solar PV. These demonstration projects are to be built within the city’s service area and will utilize new technology pioneered by Florida State University. As of Dec. 31, 2015, both of these projects continue to face delays due to manufacturing and development issues associated with the technology. While the project developers have not announced a revised commercial operations date (COD), the city remains optimistic that the technology will mature into a viable energy resource.

The city currently projects that additional power supply resources will be needed to maintain electric system adequacy and reliability through the 2025 horizon year. As a result of the lower demand and energy reductions expected from demand side management, the city has determined that additional capacity will be needed by the summer of 2018 in order to satisfy its 17% reserve margin criterion.

A generation project is being developed at the city’s Substation 12. The project is primarily intended as a solution to a transmission constraint. Standard industry practice is to have to have at least two power lines serving each substation to ensure reliability. However, Substation 12 is currently only served via a single transmission line. Substation 12 serves a number of critical loads within the city’s service territiory including Tallahassee Memorial Hospital (TMH), a number of community medical offices/facilities adjacent to TMH, and the city Police Department. Due to the density of businesses, residences and roadways in the area, it is not cost feasible to add another transmission line here. As an alternative, a generation project located at the substation will provide about 18 MW (in the form of natural gas-fueled reciprocating internal combustion engines (RICE or IC)) to back up the critical loads from this substation. This capacity addition would satisfy the load and reserve requirements from summer 2018 through the summer of 2020.

In addition to the generating capacity to be added at Substation 12 new generating capacity will be needed following the planned retirement of the city’s Hopkins Unit 1 (76 MW) at the end of 2020. Based on the city’s 17% reserve margin criterion an additional 64 MW of generating capacity would be needed by the summer of 2021. City staff has been exploringalternatives for addressing the need to replace Hopkins Unit 1 and has found that there may be  opportunities to achieve additional benefits. The option that currently appears most attractive is the installation of 50 MW-100 MW of small (10-20 MW each) RICE generators similar to those planned for Substation 12.

The RICE generators could provide additional benefits including but not necessarily limited to:

  • The RICE generators could all be installed at the same time or phased over a period of years. This flexibility would allow the city to add the resources as needed.
  • The RICE generators could be installed at either the city’s Hopkins plant or split between the Hopkins plant and Purdom plant.
  • The RICE generators are more efficient than the units that are being retired. As a result, preliminary analyses indicate significant potential fuel savings.
  • The RICE generators can be started and reach full load within 5-10 minutes. In addition, their output level can be changed very rapidly. This makes them excellent for responding to the changes in output from intermittent resources such as solar energy systems and may enable the addition of more solar resources in the future.
  • The CO2 emissions from the RICE generators are much lower than the units scheduled to be retired.
  • Hopkins Unit 1 currently has a minimum up time requirement of 100 hours. This may at times require the unit to remain on line during daily off-peak periods when the unit’s generation is not needed and/or may represent excess generation that must be sold, possibly at a loss. Replacing Hopkins Unit 1 with the smaller, “quick start” RICE generators would allow the City to avoid this uneconomic operating practice.
  • There may be merit to retiring Hopkins Unit 1 earlier and advancing the in-service dates of these RICE generators. Preliminary analyses indicate that some of the associated debt service could be offset by the fuel savings from the efficiency gains achieved by these units. There is also a possibility that better pricing could be obtained if the city purchased more RICE generators than just those needed for the Substation 12 project.
About Barry Cassell 20414 Articles
Barry Cassell is Chief Analyst for GenerationHub covering coal and emission controls issues, projects and policy. He has covered the coal and power generation industry for more than 24 years, beginning in November 2011 at GenerationHub and prior to that as editor of SNL Energy’s Coal Report. He was formerly with Coal Outlook for 15 years as the publication’s editor and contributing writer, and prior to that he was editor of Coal & Synfuels Technology and associate editor of The Energy Report. He has a bachelor’s degree from Central Michigan University.