Generation for future networks: Are flexible gas turbines the answer?



 

Gas turbines are seen as the bulk power generation choice of the future, for reasons of economy, lower emissions, and operational issues. Future demands place gas turbines in operating situations which differ from the classical usage, and will require new designs and new operating constraints.

Lower gas prices, increased availability of gas, lower CO2 emissions and the ability of gas turbines to load-follow rapidly have all led to the increasing use and proposed use of gas turbines in existing and future networks. One of the prominent applications for gas turbines is as a “dancing partner” for variable generation resources, particularly wind.

One of the more frequent proposed applications for gas turbines is to cater for the residual load in a network based primarily on wind and solar power. Wind power is a variable resource, and the pattern of variation is not constant, meaning that the residual load can vary both in magnitude and duration. Gas turbines are chosen for their rapid ramping capabilities and relatively low capital costs, as well as short on-site construction times.

Fig. 1: Residual demand in a network using renewable energy (Agora Energiewende).

Several studies have shown that it may be possible to combine wind farms and gas turbines to provide a reliable power source. Gas turbines (GTs) form part of the generation mix in a number of networks that include wind as a component, but at the time of writing there is no known network which applies the wind/gas turbine combination as a major component, and the principle remains a strictly theoretical one.

The choice of turbine type size and configuration will depend on the operating cycle regime imposed on the turbine by the renewable resource. Turbines have a limited lifetime, which is affected both by the time in service and the number of operating cycles. Different turbine types are used for continuous and peaking operation, and the choice of turbine can affect the success of a system.

The concept of peaking power has a different meaning in the wind/GT network to that in a traditional network. Traditionally peak generation coincides with peak demand, but in the
wind/GT network, the requirement for peak generation can also coincide with low wind, independent of the demand (Fig. 1).

Residual load or residual demand

Residual demand is the difference between a varying demand and a varying generation source, in a generation driven system based on renewable energy plant.

As can be seen the residual load varies both in magnitude and duration. The amount of residual load will depend on the configuration of the network, and the balance between gas turbine generation and the amount and characteristics of wind generation. Too much variable generation will result in curtailment and little use of the gas generation. Too little wind generation might result in over-usage of the gas component.  Optimising the balance will require a detailed study of the characteristics of the variable resource. The balance of generation will need to cater for worst case conditions as well as average conditions.

Many projected systems are based on average values and do not take worst case conditions into account. Countries such as Germany, which already has an extensive wind component, experience long periods (several days) of low wind and solar output over the whole country [1], but are able to import power from neighbouring countries during these periods. South Africa does not have this luxury, and any future network needs to take this into account.

Fig. 2: Typical residual demand duration curve [2].

Residual load duration curves

A common method for assessing the needs for additional generation capacity is the residual load duration curve (RLDC). These curves are useful for determining the loading and capacity factor of the turbine, and can be used to decide what turbine is best suited to the application, and also determine the expected lifetime of the turbine. Fig. 2 shows a typical residual load duration curve [1].

Turbine lifetime determination

Gas turbines wear differently in continuous duty application and cyclic duty application, as shown in
Fig. 5. Thermal mechanical fatigue is the dominant life limiter for peaking machines, while creep, oxidation, and corrosion are the dominant life limiters for continuous duty machines [1].

The working life of a gas turbine will depend on two factors:

  • Effective operating hours or duty cycle
  • Start and stop cycles

Generation patterns and gas turbine operation

Generation is traditionally classified into baseload, midterm and peak areas  Fig. 4

  • Baseload: operates 24/7 throughout the year, usually at a fixed output.
  • Midterm: operates for an extended period but not for the full 24 hrs of a load cycle.
  • Peak: operates for a short period during the peak load portions of the daily load cycle.

In a residual load system the gas turbine will have to operate in all three areas. The definition of baseload needs to be modified for this type of operation. There will arise circumstances where the GT will need to operate continuously for more than 24 hours, even over several days if necessary. For the purpose of this article baseload operation is considered to be operation in excess of the 24 hour load cycle.

Baseload operation-gas turbines

The most common baseload operation configuration for a GT is the combined cycle GT (CCGT). In this mode, the gas turbine is combined with a steam turbine and the output gas is used to generate steam to power the steam turbine. GTs used for combined cycle operation are of the large frame type, designed to operate at a fixed speed.

Fig. 3: Gas turbine lifetime dependence [1].

Large-frame GTs (LFGTs) are heavier than aero-derivative, and operate at lower pressures. LFGTs can be operated in a continuous mode, and some models are capable of load following when operated in open-cycle mode. CCGT are primarily used at fixed output and run at a rating below the maximum output rating of the turbine.

Peaking operation

GT used for peak operation is a different species altogether than that used for baseload operation. Aeroderivative GTs are usually used for peaking operation because of their rapid start-up and load following capability. Aeroderivative GTs cannot generally be used for continuous operation and are best suited for cyclic operation in cycles of less than 24 hours.

Efficiency and heat rate

GTs operate at maximum efficiency and heat rate (MJ/kWh) when running at close to the maximum load rating. When operated at part load the efficiency decreases and can be as low as 50% of the efficiency at full power. This means that the heat rate increases and the amount of fuel consumed per kWh at low loading will increase. Depending on the configuration of gas turbines used, following a residual load will result in gas turbines running at partial load for a considerable part of the operational cycle, and thus overall efficiency will be lower than at full load operation.

Cyclic operation and efficiency

Cyclic operation of CCGT has proved to be problematic in the past, and is generally confined to fixed cycles such as two shift operation [3]. Cyclic operation of conventional turbines requires open cycle operation. Cyclic operation also affects efficiency, depending on the number of ramps during an operational cycle and the ramp rate.

Gas turbines for residual load operation

It is clear from the preceding sections that conventional baseload (CCGT) and peaking turbines would not be suitable individually for operation in a load-following  residual load type of operation and that either a combination of the two types would be required or that new types of GT need to be developed.

Fig. 4: Typical demand curve (Source: Energy storage matters).

New developments

Taking future requirements into account, the industry has come up with CCGT units that can operate combined-cycle mode with an efficiency still typical of peak load power plants and can be used efficiently throughout the base, intermediate and peak load ranges [3]. In support of fluctuations in renewables and energy demand, fewer plants will be operating in baseload mode, and new GT technologies are engineered to deliver enhanced cyclic capabilities that allow utilities to ramp faster and more often, cycle on/off faster and more often, and provide more short-term reserves.

Development of CCGT for the future residual demand market has been undertaken by major manufactures of gas turbines, such as Alstom, GE Energy, MHI and Siemens. Flexibility of operation seems to be the current main performance criteria. Due to the increasing installation of intermittent renewables, and the use of GT as a partner to renewables, one manufacturer sees efficiency under part load operation as even more important than efficiency in baseload. A feature of the machines is the use of integrated control systems which link all of the technologies.

Another feature regarded as important is low load operation capability which allows a CCGT power plant to be “parked” at a much reduced minimum load point – about 20% load – to provide fast responding stand-by and significantly reduced fuel consumption during such low load periods. The plant is claimed to have a ramp up rate of 350 MW in
15 min from low load [3]. Rapid response and ramp rate is achieved by decoupling the gas and steam portions of the plants, allowing the gas turbine to dominate.

Combined cycle gas turbines that can operate efficiently and flexibly in baseload, mid-term and peaking mode are being developed and tested by most major manufacturers and promise to transform the gas generation market by providing plant that can be used effectively with variable generating sources.

References

[1]  J Janawitz, et al: “Heavy-Duty Gas Turbine Operating and Maintenance Considerations”, GE Power and Water.
[2]  P Vithayasrichareon: “Using renewables to hedge against future electricity industry uncertainties – an Australian Case Study”,  Energy Policy, November 2014.
[3]  ETD: “Damage to CCGTs Due to Cyclic Operation”, ETD Proposal No. 1097-gsp-prop07.
[4]  T Probert: “Fast starts and flexibility: Let the gas turbine battle commence”, Power engineering international Vol. 19 Issue 6.

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