The market share of renewable generation has dramatically increased and will continue to make up an ever-increasing proportion of total global capacity. Coal and nuclear plants will be retired and are unlikely to be replaced due to high capital costs and/or environmental issues. With a greater proportion of power generation coming from unpredictable sources there is a need for energy storage and more flexible sources of generation. Gas Turbines are the industry choice for flexible generation.

A Combined Cycle Gas Turbine (CCGT) plant can typically reduce its output from 100% to 53% of nominal power. Isentropic Ltd is currently focused on providing integrated energy storage systems for these machines, which enables this range to be increased to between 115% to 20%. Being able to operate in this way adds commercial value to both new and existing gas plants as it opens access to markets for selling fast-acting additional power and reduces the number of start-stop cycles, which have very high associated maintenance costs.


GTI-Storage Rapid Response

The Isentropic GTI-Storage Rapid Response system offers a low cost solution for Combined Cycle Gas Turbines (CCGT) to provide frequency regulation without losing capacity or efficiency compared to current CCGT operation. The system can boost output power by over 10% in under 5 seconds.


Speed of Response

The Rapid Response system is designed to react quickly to fluctuations in power demand, which enables operators to deliver grid services such as frequency response. The UK Grid Code specifies the technical requirements for frequency response for all units. In order to comply with the code, current CCGTs must typically run below their peak output at around 90% capacity. This is so that they can increase their output to meet a sudden increase in demand.

With Rapid Response CCGTs can run at full capacity and rely on energy storage to increase or decrease output at will. Output can be increased to 110% within 5 seconds.

Rapid Response provides 2x the rate of response from 100% load


Pipes instead of Caverns

Underground salt caverns are often proposed as containers for compressed air in energy storage systems. The main disadvantage with salt cavern storage is that it requires very specific geology and hence projects are geographically constrained. Furthermore, salt caverns take many years to develop and often face environmental concerns. Isentropic Ltd makes use of pipework and cylinders which are commercially available to produce a Compressed Air Energy Storage (CAES) system which can be installed almost anywhere.


Operational Overview

Rapid Response System Diagram

During the charging process, gas is bled from the exit of the compressor in a conventional gas turbine. The compressed air is then passed through an Isentropic Thermal Store transferring its heat to the crushed rock. After passing through the thermal store, the compressed air is pumped to pressure using a conventional reciprocating compressor into steel cylinders at up to 250 bar. On discharge, the gas returns from the pipe store, via a throttle valve, picking up heat through the thermal store before being injected back into the gas turbine.


Key Benefits

  • The system has a low capital cost compared to other storage systems due to utilisation of the existing gas turbine machinery to produce the majority of the system work. Parsons Brinkerhoff estimate the capital cost of the Rapid Response system to start at $400/kW for a First of a Kind (FOAK) system and drop to $200-$300/kW for a Nth of a Kind (NOAK) system.
  • The system can boost CCGT output by over 10% within 5 seconds, while the CCGT is generating at full load. The faster reaction time is advantageous for frequency response. The ability to provide this service while generating at full load is a benefit to the power plant owner (improved capacity and efficiency/heat rate).
  • All system components except the Isentropic Thermal Store are commercially available or based on existing technology. This reduces technical risk.
  • 20 year life. As the system utilises turbo-machinery and mechanical components, the design life for the system is expected to be at least 20 years, with proper maintenance and operation.
  • Potentially short turbine downtime during installation. The storage system can be installed before connection is made to the gas turbine. This reduces the GT down time.

GTI-Storage Enhanced Turndown

The Isentropic GTI-Storage Enhanced Turndown system offers the same advantages as Rapid Response but with an increased rate of energy storage, larger storage capacity and improved thermodynamic efficiency. This system is designed to improve the economics of plants that turn down to low load during their duty cycle as a result of power market economics (i.e. negative spark spreads). It also permits continuous operation of the plant at low part load, thus avoiding shutdowns and startups that otherwise would impact on maintenance and incur significant associated costs.

With Enhanced Turndown the operating range of a CCGT may be increased from 50-100% to 20-115% whilst offering up to 6 hours of energy storage.

Arbitrage on the Spark Price

With the additional capacity of an Enhanced Turndown system, the operational philosophy is that during periods of low electricity demand, the GTI-Storage system is operated in charge mode in preparation for periods of high demands when the output of the plant can increase to 115% of normal capacity. In this way it is possible to shift power output from periods of negative spark spreads to periods of positive spark spreads.


Lower Maintenance Costs and Reduced Turbine Degradation

Rather than switching off the turbine during periods of low demand, the storage system enables the CCGT to turn down to below normal minimum load. This provides operational benefits because gas turbines incur significant "equivalent operating hours" (EOH) penalties each time the engine is switched off and started or operated in a load following duty that involves fast load changes accompanied by rapid temperature changes. Degradation also affects the efficiency of energy conversion (i.e. increases heat rate).

Estimates of monetary savings, where they have been made, vary widely from $15,000 USD per day at the low end to ten times that value at the upper end.


The effect of Start-Stop operation on Turbine Life and Heat Rate (KEMA Ltd, 2008)


Operational Overview