Complementing renewable energy with gas turbines

Alongside renewable energy technologies, the role of gas turbines in the 21st century electricity mix is assured.

Renewable energy and natural gas are the big winners in the race to meet the demand for power over the next 25 years, according to the 2016 World Energy Outlook. The rapid growth in renewables—notably wind and solar power—is changing the electricity supply landscape and how gas turbines are being called on to generate to the grid. The modern power grid needs intelligent resources able to ramp up and down swiftly, efficiently, and repeatedly.

Increasingly, network operators are giving dispatch priority to renewable energy. Due to these industry changes, gas turbine operators are taking on an additional new role: switching from providing baseload to providing power at times of peak demand. Plant flexibility is critical for on-call gas turbine generation. Gas turbine plants that can be dispatched within minutes are important assets for balancing electric system loads and maintaining grid reliability.

Simple cycle gas turbines have traditionally served as peaking units because they can be started within minutes and ramped up and down quickly. They are less thermodynamically efficient (< 40 percent) than combined cycle turbine plants but can be profitably deployed during peak times when the price paid for electricity is higher.

Reserve turbines can be kept spinning—either without producing any energy or operating below optimum output—and brought rapidly online if required. However, additional fuel is required to ramp up and operate these turbines, even if they're not synched with the grid. This increases emissions and reduces the net efficiency of the power system. Operating at lower loads may mean decreased combustion temperature and less conversion of CO to CO2.

Depending on environmental permit requirements, plant configuration, and post-combustion emissions-control systems, the minimum environmental load for most gas turbines is generally about half of full outputs. However, through the use of modern control systems, managers have the ability to improve efficiency in this area.

OEMs are designing upgraded control technologies that improve emissions compliance in turndown and minimize efficiency impacts at part load. For example, the use of variable guide vanes decreases compressor mass flow, and refinements to combustion chamber design, such as sequential firing (reheat), produces higher combustion temperatures while operating at low loads. Similarly, software upgrades in the control system can fine tune combustion temperatures, enabling the gas turbine to maintain a higher level of output when the external temperature drops.

The same software can adjust operators' control of the water spray within heat recovery steam generators. The steam temperature can now be maintained using a model-based control approach, anticipating the attemperation requirement based on the gas turbine activity. This means operators do not have to suppress the temperature, which, in turn, stays more stable, increasing steam turbine efficiency and reducing thermal and mechanical wear.

Improving air compression, sealing technology, and optimizing the hot gas path lifetime and turbine inlet temperature levels may also achieve marginal gains. There are new, efficient combustion systems optimized for part-load performance, which, in some cases, allow the unit to operate at as low as 10 percent load while maintaining emissions compliance.