3 Forms of Flexibility for Evolving Gas Turbine Power Plants

The US electricity industry has undergone rapid change in recent years. Demand for lower-carbon electricity options—both at the government and consumer level—is a primary driver of this change. The desire for cleaner electricity has resulted in a surge of retirements of coal-fired power plants, according to the Department of Energy, which have been largely replaced by gas turbine power plants and renewables like wind and solar power.

However, changes in public policy, the emergence of intermittent renewables, demands for higher efficiency, and changes in relative fuel prices are just a few factors that have created a need for greater flexibility in the traditional gas turbine model.

The Evolving Gas Turbine Plant

Historically, a baseload power plant started up and ran at a constant rate for most of the year to meet continuous electricity demands. Most gas turbine power plants were designed for relatively static operation. They were ramped up and down maybe once a month and tended to run at a steady state like nuclear power plants.

Gas power plants now have to be able to ramp up and down quickly in response to variations in wind and solar output. Some of these variations are predictable; others aren't. As a result, gas plants need to be flexible in a number of ways.

Commercial Flexibility

Plant owners need to meet a specific level of financial performance. That means they must make power available when it is needed, but also balance availability against the higher maintenance costs of frequent cycling. The National Renewable Energy Laboratory published a study of these cycling costs to help plant owners better estimate the economics and consequences of cycling.

One potential option to smooth out the cycling is to offer incentives if customers are willing to sign up for a particular load profile, such as peaking in the morning and afternoon, or if customers are willing to be on the load profile that the power plant specifies.

There are also different types of demand response programs (DRP), in which electricity prices are dynamically varied and customers can respond accordingly. Such programs can include critical or variable peak price and critical peak rebates, and even enable power companies to control high-demand applications, such as air conditioners and water heaters. According to an Advanced Energy Economy Institute report, using DRPs in Michigan's Lower Peninsula could not only offset significant demand in the summer season, but it would also lessen the need for new power plants, saving ratepayers up to $1.2 billion.

The plant manager must be sure that the plant is operating safely and in compliance with key performance indicators (KPIs). For example, they may need to make decisions regarding whether to idle the plant at a low level during times of low demand, while at the same time meeting the required emissions profile. They will need to determine whether sending some electricity down the line without being able to bill for it is cheaper than short-duration cycling. They may also decide that the increased cycling requires a change to the maintenance intervals based on the multiyear agreements.

Management must also determine which technologies and upgrades to the gas plant are an economical way to improve flexibility. These may include advanced control solutions, gas turbine axial fuel staging, emissions catalysts that can reduce carbon monoxide during turndown, and gas turbine upgrades that allow for improved heat conservation during intermittent shutdowns.

Operational Flexibility

A gas turbine that will be utilized to complement intermittent renewables must be capable of three types of operational flexibility, according to Utility Dive.

First, the plant has to be able to do system ramps multiple times per day that may vary according to the season, such as in response to solar output predictably rising and falling. Second, the plant must be able to respond rapidly to hourly system variability, such as in response to wind output suddenly falling. Third, the system has to be able to respond on a second-by-second basis to maintain system frequency.

Operations will need to include enough people on their staff to handle the startups and shutdowns. A gas turbine can come up and down quickly by opening and closing fuel control valves. However, those thermal changes have a detrimental impact on the heat recovery steam generator (HRSG). Tubes and fins can crack prematurely with frequent thermal cycling. Operations may find it advantageous to bypass sections of the HRSG entirely. Strategically placed heating elements and insulation may also help even out the thermal stresses.

Service Flexibility

As plants cycle more often, service intervals may need to be adjusted. This can impact the frequency of routine maintenance as well as scheduled plant outages.

Some historical inspection results that were performed under constant load may no longer be as informative when a plant shifts to a load-following operation. Components that were not critical in the past may now fatigue more rapidly and should assume greater importance in the maintenance schedule. This may include more intensive monitoring or online monitoring of these components.

Plant operators must decide how to best balance their manpower around the operational model they're fulfilling. Then they can make data-driven decisions to postpone maintenance tasks (or potentially perform them early) according to the forecasted operational model.

According to Power Engineering International, the importance of employee experience and deployment should not be underestimated. Work schedules may need to become more flexible to accommodate periods of high and low work intensity within the plant.

In some cases, it may be worthwhile to cross-train personnel to be able to accomplish multiple tasks. Mechanical technicians could learn to do basic electrical work or vice-versa.

Plant managers and operators should also determine which tasks and decisions to automate, rather than handling manually. There are often opportunities to remotely monitor conditions and provide advisories.

If a plant is becoming more of a peaker, which is often necessary to remain in service, it must change its maintenance strategy to revolve around starts-based operation and willingly sacrifice hours-based operation. For example, the plant could overfire the gas turbine to peak or enable a fast start by injecting ambient air into the gas turbine exhaust to avoid overheating the HRSG.

The evolution of the gas turbine power plant demands flexibility in multiple plant roles. Future plants will need to respond quickly to changing power demands while maintaining emissions compliance, limiting wear and tear on equipment, and earning an acceptable rate of return. Plant owners, managers, and operators can achieve this in a number of ways. These include improving operational procedures, restructuring multiyear agreements that modify maintenance intervals, changing commercial terms based on a customer's acceptance of a specific load profile, and upgrading equipment and technology.