Last year’s start of the commercial operation of South Africa’s first new open cycle gas turbine plants since democracy was introduced in1994 was a significant milestone in Eskom’s plan to increase generation capacity and ensure a reliable supply of electricity, but the story is far from over.

Heather Johnstone, Senior Editor

Since the end of Apartheid and the introduction of democracy in the mid-1990s South Africa has experienced phenomenal economic growth, with GDP increasing at an average rate of five per cent a year. It now has the most advanced economy in the African continent. Coupled with this economic growth is a soaring demand for power. South Africa’s national utility Eskom generates around 95 per cent of the country’s electricity, but this growing demand means that its excess electricity generation capacity is being rapidly depleted, especially when it comes to supplying electricity during peak demand periods.

In 2004, in an effort to address the growing demand for electricity, the South African government approved a R93 billion ($11 billion) electricity infrastructure investment programme, which encompassed the entire electricity value chain – from generation to transmission to distribution. Most importantly it gave Eskom the green light to begin building new power stations, after several years of underspending on generation capacity. One of the first initiatives the utility embarked upon was to determine what type of peaking plant could be built in a relatively short time, to address the growing peak demand deficit.

Ankerlig peaking power plant with four OCGT units
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After due consideration Eskom opted to build two open cycle gas turbine (OCGT) plants. According to the utility, it chose OCGT technology primarily because it is well-proven and has a good track record (high availability and reliability), there are many OCGT suppliers across the world, and probably most importantly, OCGT plants can be built in a relatively short period of time and can be easily fast tracked to produce even shorter project lead times. Eskom aim was to have the two OCGT plants, which would add around 1000 MW of power to the national grid, operational to meet the peak demand during the winter months of 2007.

Initially, seven potential locations for the OCGT plants were identified in the Western Cape province. The two locations eventually chosen were the Atlantis Industrial Area and Mossel Bay adjacent to the PetroSA fuel manufacturing facility. There were a number of reasons why those two sites were chosen, but the main ones were the presence of existing infrastructure meaning less time and money was required for site development, and relatively easy access to liquid fuel. Furthermore, both sites are strategically placed to make use of the opportunity to use gas as the primary energy source should natural gas become available in the future.

Meeting peak demand

In July 2005, after receiving three submissions from multi-national companies, Eskom awarded the R1.86 billion contract to Siemens of Germany. The scope of the contract was for the design, manufacture, supply, delivery to site, installation, commissioning and testing of seven OCGT units, split between the two power islands.

The Ankerlig peaking plant at Atlantis was to comprise four 150 MW OCGT units, initially fuelled by kerosene, with the Gourikwa power plant in Mossel Bay, consisting of three 150 MW OCGT unit also fuelled by kerosene. An option to use natural gas at a later date was also part of the contract.

As mentioned above both plants had to be operational in time to meet the peak demand of the winter period of 2007, so the deadline for completing the project was tight. Construction work began on the Ankerlig power plant on 17 January 2006, while the Gourikwa station started on 24 January 2006. The fourth and final unit of Ankerlig station was released for commercial operation on 25 June 2007, with the final unit of Gourikwa power station signed off three days prior to that.

Commenting at the official opening of the two new OCGT power stations, which took place in October last year, Eskom’s chief executive Jacob Maraga said: “Their commissioning in time for this year’s winter was an important achievement for Eskom’s capacity expansion programme”.

In September 2006, the Eskom board approved a further investment of to expand the installed capacity of both plants, increasing their combined capacity to 2100 MW. This project involves the addition of five 150 MW turbines to the Ankerlig plant and two 150 MW turbines to the Gourikwa site.

The new contract was once again placed with Siemens after a bidding process. The additional units are currently under construction, with the project expected to be completed before winter 2009.

Workhorse of the Power industry

Both OCGT plants had to be designed to meet the daily electricity peaks in the morning and in the evening. This represents approximately 520 starts per year, which is a challenging mode of operation because each start and shutdown causes detrimental effects on the operating lifetime of the gas turbine’s hot gas parts, primarily through repeated thermal stress.

The SGT5-2000E’s rotor with blading
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To address these concerns Siemens decided to install its proven workhorse, the SGT5-2000E (formerly the V94.2) gas turbine, which although developed back in the 1970s has been updated over the years and remains a backbone technology for power generation. Because the SGT5-2000E was originally developed as a peaking unit, it is perfectly designed for rapid starts and frequent start and shut down – a major consideration in this project.

The SGT5-2000E is well suited for peak demand applications because of its lower turbine inlet temperatures and the investment made in ensuring the optimum performance of its hot gas parts and rotor components under these conditions. Its robust design means it is able to burn a variety of fuels, including both low and high calorific gas and liquids, while still achieveing the lowest possible emissions. Furthermore, its advanced burner technology enables it to burn liquid fuels in a dry premix operation, which means that the OCGT plants can run without water injection, while still achieving the required emission levels.

One of SCT5-2000E’s major design features is its light and robust disc type rotor, with a central tie bolt. This effectively reduces material accumulation by exhibiting greater rigidity, and enables lower thermal and mechanical stress. Moreover the rotor discs have a Hirth serration that not only takes over the mechanical torque in the rotor, but also enables a self-centering of the discs once the rotor is stacked. This self-centering design feature is also beneficial during turbine service because after the restacking of the rotor it is automatically centered, and thus the rotor becomes immediately operational.

ESKOM’s Future plans

South Africa still has some way to go before it is able to effectively meet its growing power needs, so power restrictions and rolling blackouts look set to continue for the foreseeable future. According to recent projections by Eskom, the country will require the addition of around 3000 MW of power in the next two to three years, consisting of both baseload and peaking electricity generating capacity. Furthermore, the utility also announced it is to spend R1 trillion to double capacity through to 2025.

Because of this medium-term forecast in the demand for electricity and constraints associated with meeting this projected demand, the conversion of these OCGT to CCGT units is an obvious and relatively easy option to help Eskom meet this projected demand increase.

This conversion will increase the generating capacity of the plants through a more efficient process. Overall thermal efficiency is expected to be increased from approximately 34 per cent for the current OCGT to approximately 50-55 per cent for the proposed CCGT. As with the OCGT units the CCGT units will be fuelled by liquid fuel and at a later stage natural gas.

Although the SGT5-2000E gas turbine was originally designed for open cycle its versatile and robust design makes it equally suitable for operating in combined-cycle, and in fact the first example of its commercial combined-cycle operation was back in 1983. The heat recovery steam generator is the major component to be added to the existing gas turbines as part of an conversion. Furthermore, unlike the current OCGT operation, water will be required for the CCGT power generation process. Eskom is currently investigating various water resource options, including the use of groundwater from the underground aquifer in the Atlantis area, the use of wastewater from a nearby wastewater treatment plant, and the use of desalinated seawater piped to the power station from the coast.

Eskom has ambitious plans for the South African electricity sector and Ankerlig and Gourikwa OCGT plants look set to continue to help in satisfying the country’s hunger for power.