Tallahassee to retire several gas units later this decade; may add new one

The City of Tallahassee currently expects that additional power supply resources will be required to meet future system needs out toward the end of the period covered by a Ten-Year Site Plan it filed March 31 at the Florida Public Service Commission.

For the purposes of this plan, the city has identified the addition of a combustion turbine (CT) generator (similar to the city’s existing Hopkins CT3 and CT4) at its existing Hopkins Plant site. The timing, site, type and size of this new power supply resource may vary as the nature of the need becomes better defined. Alternatively, this proposed addition could be a generator(s) of a different type/size at the same or different location or a peak season purchase or the planned retirement of Hopkins Unit 1 could be postponed.

The city’s 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.

In total, 222 MW (net summer rating) of CC generation and 20 MW (net summer rating) of CT generation facilities are located at Purdom. Hopkins includes 300 MW (net summer rating) of CC generation, 76 MW (net summer rating) of steam generation and 128 MW (net summer rating) of CT facilities.

The Hopkins 1 steam generating unit can be fired with natural gas. The CC and CT units can be fired on either natural gas or diesel oil but cannot burn these fuels concurrently.

The total capacity of the three units at the C.H. Corn Hydroelectric Station is 11 MW. However, because the hydroelectric generating units are effectively run-of-river (dependent upon rainfall, reservoir and downstream conditions), the city considers these units as “energy only” and not as dependable capacity for planning purposes.

In 2005, the City Commission gave the Electric Utility approval to proceed with the repowering of Hopkins Unit 2 to combined cycle operation. The repowering was completed and the unit began commercial operation in 2008. The former Hopkins Unit 2 boiler was retired and replaced with a combustion turbine generator (CTG) and a heat recovery steam generator (HRSG). The Hopkins 2 steam turbine and generator are now powered by the steam generated in the HRSG. Duct burners have been installed in the HRSG to provide additional peak generating capability. The repowering project provides additional capacity as well as increased efficiency versus the unit’s capabilities prior to the repowering project. The repowered unit has achieved official seasonal net capacities of 300 MW in the summer and 330 MW in the winter.

City working on several solar additions, with two of them stalled

The city said it continues to seek out suitable projects that utilize the renewable fuels available within the big bend and panhandle of Florida. Most recently the city has issued a request for proposals (RFP) for a purchase power agreement (PPA) for a 10 MW (ac) utility scale solar PV project. 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.

Although there are ongoing concerns regarding the potential impact on service reliability associated with reliance on a significant amount of intermittent resources like PV on the city’s relatively small electric system, the city will continue to monitor the proliferation of PV and other intermittent resources and work to integrate them so that service reliability is not jeopardized. As of the end of calendar year 2014 the city has a portfolio of 232 kW of solar PV operated and maintained by the Electric Utility and a cumulative total of 1,550 kW of solar PV has been installed by customers.

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 the end of 2014 both of these projects continue to face delays due to manufacturing and development issues associated with the technology. Such delays are to be expected with projects involving the demonstration of emerging technologies, the plan said. 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. Until a new COD is announced, this will be the last reporting of these projects.

Retirement timing for Hopkins 1 figures into later capacity need

No new resource additions are expected to be needed in the near term (2015-2019). Resource additions are expected in the longer term (2020-2024). The city has identified the need for additional capacity in the summer of 2021 following the retirement of Hopkins 1 (which has been delayed by one year) in order to satisfy its 17% reserve margin criterion. The timing, site, type and size of any new power supply resource may vary dependent upon the metric(s) used to determine resource adequacy and as the nature of the need becomes better defined. Any proposed addition could be a generator or a peak season purchase.

Alternatively, the planned retirement of Hopkins 1 could be further postponed. The suitability of this resource plan is dependent on the performance of the city’s demand side management (DSM) portfolio and the city’s projected transmission import capability. If only 50% of the projected annual DSM peak demand reductions are achieved, the city would require about 3 MW of additional power supply resources to meet its planning reserve requirements in the summer of 2019.

The possible new capacity would be the gas-fired 46-MW (summer) Hopkins Unit 5, with a possible May 2018 construction start. For the purposes of this report, the city has identified this unit as a General Electric LM 6000 combustion turbine generator (similar to the city’s existing Hopkins CT3 and CT4).

The currently planned retirements and their timing are:

  • Hopkins CT 1 (12 MW summer, gas), March 2016;
  • Purdom CTs 1 and 2 (10 MW apiece summer, gas), October 2016;
  • Hopkins CT 2 (24 MW summer, gas), March 2017; and
  • Hopkins ST 1 (76 MW summer, gas), January 2021.
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.