Humid Air Injection Boosts CT Output

Additional megawatt-hours (MWH) can be obtained at low cost during peak demand periods from gas turbines and combined cycle power plants by injecting externally compressed, humidified, and heated air into a combustion turbine (CT) up-stream of combustors. This novel approach is denoted as CT-HAI, (HAI is an acronym for Humidified Air Injection) for simple cycles and CC-HAI for combined cycles. It results in a significant power augmentation over the whole range of ambient temperatures, but it is the most effective at high ambient temperature conditions when reduction in power output is most severe.

The simplified explanation for reduced power production by CT and CC plants is that lower inlet air density, a result of the high ambient temperature, reduces mass flow through a CT with a corresponding reduction in power.

With HAI, power output can be maintained essentially constant over the range of 0 F to 95 F at about 20 % above the nominal 59 F rating. The overall heat rate for the total output of the power augmented CT also drops by about 8%-12% over that temperature range, saving fuel as the temperature rises. The heat rate for the incremental power is approximately 6000-6400 Btu/kWh, i.e. in the range of CC plants. Engineering and mechanical aspects of the air injection for CT-HAI concept are similar to the steam injection for the power augmentation, which has accumulated significant commercial operating experience.

This system can be operated to produce additional MW for sale whenever market conditions are attractive. The value to individual utilities will vary according to the number of hours that the additional megawatts can be sold at attractive prices. Specific capital costs of additional kWs (i.e. for installing HAI) are less than $200/kW. With lower net heat rates, the cost of electricity obtained with this technology can provide power at lower production costs in peak power markets.

The process is an interesting coming together of two separate ideas for getting more out of CTs: (1) adding humidity, and (2) (externally) compressing the air:

Just Add Water —
The output of a CT can be increased by adding water in various ways, like evaporative cooling, wet compression, and inlet chilling. Unfortunately, these technologies that may have low initial capital costs introduce the water into compression process and can create significant operational problems. For example, GE has told users to cease inlet fogging and evaporative cooler operation until compressor blade erosion inspections can be performed. Technologies that introduce condensation or carryover of water into the compressor section can cause blade erosion and ductwork corrosion, pitting and thermal stress.

While steam injection technology also bypasses the compressor, with HAI, humidity is introduced in the form of humidified air that, as compared with the steam injection, provides for a safer and more stable combustion process, and allows for higher injection rates with associated greater power augmentation. Steam injection flow is limited by a number of combustion related and other considerations.

Compressed Air —
The other development behind HAI is compressed air energy storage (CAES), a diurnal peak shifting method where air is compressed off-peak and stored in underground formations or piping systems. On-peak, the compressed air is fed to the CT, relieving it of the need to do its own compression and thus increasing output. From there it was a short step to realizing that an external compressor could be beneficial under certain operating conditions. Adding humidity to this external air supply enhances the performance even more.

Dr. Michael Nakhamkin, President, Energy Storage and Power Consultants (ESPC), has fourteen patents; including five on CAES technology and another five on the power augmentation technologies with humid and dry air injection into CT.

– Combustion Turbine with Humid Air Injection (CTHAI) -pat. 6038849
– Combustion Turbine with [Dry]Air Injection (CTDAI) -pat. pending

Both methods can increase power output by 15%-25% or more; use proven equipment; and are simple to implement and operate. The humid version also reduces NOx by 15%. Developers have also come up with a clever means to avoid entraining impurities in the water, simplifying water treatment. A once-through boiler with partial steam generation requires only demineralized water.

Several HAI/DAI concepts as applied to simple-cycle (CT) and combined-cycle (CC) plants are available for commercial implementation. Successful validations have been done at Calpine on GE 7241 FA. HAI can be practical for any CT 5 MW and larger.

Hill Energy System, a subsidiary of Hill International, is a licensee of the HAI technology, and is actively marketing systems. The website has contact information and a number of helpful documents.

Also see a full discussion in the July 2003 issue of Power Engineering Magazine:
“Humid Air Injection Turns to Out-Of-Shelf Equipment to Enhance Viability for Combustion Turbine Power Augmentation”

“Air Injected Power Augmentation Validated by Fr7FA Peaker Tests”, Gas Turbine World, March/April 2002.


Ron Wolk, prominent power technology expert, has been involved in this program for years, and can provide additional insights. Contact him at:

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