Fast Cycling Capability for New Plants and Upgrade Opportunities
H. Emberger, E. Schmid, E. Gobrecht, Siemens Power Generation (PG), Germany (Source: Siemens Energy)

1 Introduction
Increasing demand for electricity, replacement of ageing plants and ongoing liberalization characterize the Asian power market. The customer focus is the optimization of life-cycle-cost (LCC) driven by features like
High efficiency, high power output and operational flexibility.

The Siemens Reference Power Plant (RPP) development takes that into account and uses LCC models for the optimization of plant concepts. Additionally, customer feedback and experiences from units in construction and operation are considered in the RPP development and lead to a continuous improvement of the products. In current research and development projects for combined cycle plants, major improvements regarding operational flexibility could be reached. Focus was on start-up time after overnight shutdown because market analyzes showed that many customers operate their plants two shifts weekdays and perform up to 200 starts/ year after overnight shutdown (approx. eight hours standstill). Experts from steam turbine (ST), gas turbine (GT), balance of plant (BoP) including I&C systems and heat recovery steam generator (HRSG) identified bottlenecks and potentials for improvement.

These analyzes were basis for a modified start-up procedure which allows a significant reduction of start-up time after overnight shutdown compared to a state of the art start-up procedure. The steam turbine start-up is modified to enable an early roll-off and fast loading of the turbine. An improved GT control ensures usage of maximal possible load ramps over a wide operating range. To minimize HRSG lifetime consumption, a BENSON® once through system developed by Siemens Power Generation (PG) is implemented in the HP pressure stage.

 This boiler design does not limit the GT load ramp. Optimized unit coordination logic brings together all improvements on component side and ensures a fast start-up with a “onepush-button” control. Siemens PG as an OEM and turnkey supplier has a detailed knowledge about components’ performances and plant operation. Only with this knowledge, the improvement in operational behavior described in this paper can be achieved. A fast start-up plant gives plant owner business opportunities like utilizing hourly and seasonal market arbitrage, participation in ancillary energy markets or peak shaving. These factors can be implemented in an economic evaluation model and improve the economic benefit of a project.

This paper gives an overview about recent improvements with respect to operational flexibility, the technical features of a cycling plant and it shows an approach for economic evaluation of flexibility.

2 Market environment
Looking at the development of the power sector in Asia, two effects can be identified: A growing demand for electricity and an ongoing liberalization of the market (Figure 1).

Deregulated markets

New capacities are added as well as ageing plants have to be replaced. These new power plants must cope with the challenges of liberalized and deregulated markets. Further on, the compliance with increasing environmental requirements is a challenge for new projects. In this business environment combined cycle power plants are favored over steam power plants because of their shorter pay-back periods, advantageous emission levels and excellent operational flexibility.

Looking closer to the requirements of a power plant in modern power markets, operational flexibility becomes a major topic. Drivers for this demand are risks like fluctuating fuel and electricity prices. Additionally a flexible plant opens up new business opportunities like utilizing hourly and seasonal market arbitrage, participation in ancillary energy markets or peak shaving (Figure 2).

Risk and opportunities in liberalized markets

An operating profile optimized for this market circumstances increases the economic value of the plant. Depending on seasonal load and the dispatch rank of the plant, driven by ompetition and fuel  prices, it is likely that the plant will be partly operated as baseload unit and partly as cycling unit over its lifetime. Therefore a plant needs advanced cycling capability and highest efficiency at baseload.

Siemens RPPs are developed to meet these market requirements. They show excellent operationalflexibility and highest efficiency.

3 Current Cycling Plant Design
Combined cycle power plants dominate the world wide power markets since years. Most of these plants were initially designed for baseload operation due to low fuel prices in the 90ties resulting inlow electricity costs. Nowadays, many operating combined cycle plants are shifted to intermediateload and new plants are specified for cycling load regimes because of today’s high gas prices. Therefore  features for high operational flexibility like short start-up and shutdown times are emphasized bycustomers. Also the focus of Siemens Reference Power Plant development changed according to thesemarket requirements. In the past, the RPPs were designed for baseload operation with a low number ofstarts per year. Start-up time for a 400 MW single shaft plant after overnight shutdown (approx. eight hours) was 90 minutes. As an answer to the changed market requirements, Siemens developed a fast start-up concept and implemented it into the RPPs. With this design, a reduction of the single shaft start-up time after overnight shutdown of approx. 50% is achieved.

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