As gas turbines continue to become key allies for power generation, different inlet air cooling methods for gas turbines are emerging to improve turbines’ performance.
This variety of inlet air cooling alternatives is particularly valuable when it comes to designing the right inlet cooling strategy, where factors such as ambient temperatures and humidity have been proven to greatly determine an installation’s performance and efficiency.
As seen below in this article, for locations with hot and humid summer weather, the choice of inlet air cooling methods for gas turbines can make a substantial difference and cascade into accessing important economic advantages.
Drawing on ARANER’s thermal engineering expertise in designing cutting-edge Turbine Inlet Air Cooling solutions, this article explains the main working principle behind each method, and reviews key scientific literature and engineering intelligence around each strategy’s suitability.
Importance of Inlet Air Temperature in gas turbine performance
Maximizing turbine performance stands at the heart of achieving a power plant’s goals, as it has an impact on its overall power capacities but also on its economic results and its sustainability.
At a basic level, the main principle behind all inlet air cooling methods for gas turbines is to reduce air temperatures at the compression entry with the aim of improving the turbine’s performance.
Here, it’s key to understand turbine performance is highly dependent on ambient conditions, including humidity and ambient temperatures.
As a rule, a turbine’s power output is directly proportional to the air mass flow rate that enters its engine. However, as humidity and ambient air temperature rise, they cause a reduction in air density, which in turn lowers the mass flow rate into the compressor. In other words: increased temperatures and humidity cause less air to pass through the turbine, thus provoking a noticeable decline in turbine performance. In fact, a rise of 1 ºC in compressor air inlet temperature has been documented to lead to a decrease of turbine power output of 1% (Rahman and Abdalla, 2012).
Conversely, thermal engineering experts have designed a number of solutions that rely on cooling inlet air: by reducing its temperature, its density increases, so that more air mass enters the turbine’s compressor.
Here, inlet air cooling methods for gas turbines represent a valid strategy for power generation plants in locations where ambient temperatures and humidity are high, or where large differences exist between the winter and summer months.
In cases like these, drops in turbine power output are particularly concerning as they take place in seasons where power demand is at its highest caused by extended air conditioning use. This contrast between production capacities and demand causes a myriad of troubles for operators: from higher operational costs, to a risk of power outages and grid instability.
This is where the different inlet air cooling methods for gas turbines come in as a valuable strategy to boost performance and maintain reliable power generation.
A solution that can boost turbine power output between 10% to 30%, thus offering a viable alternative to expanding turbine installation capacity through new installations (which represents a more costly choice).
Objectives of Turbine Inlet Air Cooling
As seen above, TIAC emerges as a key ally to overcome some of the main challenges in the power generation industry and, more specifically, turbine performance.
As such, the specific goals of inlet air cooling methods for gas turbines include:
- Power output increase:TIAC allows for a significant increase in a turbine’s power output. This technology’s effect is particularly important in difficult scenarios, such as in hot seasons, where demand is high but turbine power output decreases.
- Avoid turbine’s output fluctuation due to ambient conditions: TIAC manages to reduce the impact of changing temperature and humidity values, generating stable conditions for turbines to produce more consistent and reliable power output. This, in turn, is key for improving grid stability and ensuring energy supply meets the demand reliably.
- Lower production costs: the main goal of TIAC is to optimize existing equipment, rather than requiring higher capital expenditures. As such, TIAC helps reduce the cost per unit of electricity generated, as it increases power output without the need to add new turbines or plants.
- Sustainability: TIAC is recognized as a green technology with carbon credits. It improves efficiency and reduces fuel consumption and greenhouse emissions compared to running turbines less efficiently or expanding power plants.
An overview of the existing Inlet Air Cooling Techniques
Evaporative cooling
This technique relies on exposing warm air to water in order to achieve a reduction in temperatures. In order to do so, air is passed through a wetted medium, where the principle of latent heat of vaporization operates: as water evaporates, it absorbs heat from the air.
The relatively easy installation and operation of evaporative cooling have meant this method has been a preferred option for many installations. However, the fact that it relies on water supplies to perform cooling limits its applicability, especially in projects where water conservation is crucial. At the same time, techniques based on evaporation are limited by wet-bulb temperature, so that the presence of great humidity in air can limit this system’s efficiency, as evaporation becomes less effective.
Fogging
Fogging techniques are categorized as a subset of evaporative cooling. This approach relies on mixing inlet air with fine mist to reduce inlet air temperatures.
A mist made of tiny water droplets, usually between 20 microns and 40 microns, although size can change depending on ambient conditions and expected evaporation time.
In order to produce fog, high-pressure demineralized water passes through atomizing nozzles. Then, water hits impaction pins, breaking the stream into billions of tiny droplets.
The pros and cons of fogging among inlet air cooling methods for gas turbines are largely similar to those of evaporative cooling: it represents a relatively affordable method, but its effectiveness is greatly limited to the presence of dry air, as the capacities of cooling water are limited up to wet-bulb temperature. When humid air enters the picture, the effectiveness of fogging decreases greatly. Blade erosion and a high water consumption are also issues that must be considered when opting for fogging techniques.
Keep learning: Basics of fogging system for inlet air-cooling
Chiller-based cooling systems
Under this category different types of systems are reunited that base their cooling capacities on the incorporation of a chiller. These include:
- Compression chillers: mechanical refrigeration systems that provide cooling based on a vapor-compression cycle where water is cooled and pumped to a cooling coil, installed within the turbine’s filter house. In order to do so, compression chillers typically employ electric motors.
- Absorption chillers: this piece of equipment uses thermal energy (heat, rather than electricity) to drive the cooling cycle via a chemical absorption process. Absorption chillers can employ waste heat as an energy source (such as turbine exhaust gasses), thus maximizing the system’s energy efficiency.
- Engine-driven chillers: here, internal combustion engines (typically powered by natural gas, or other fossil fuels) drive the compressor mechanically.
The chiller approach facilitates cooling the air below wet-bulb temperatures, thus surpassing some of the key limitations in other inlet air cooling methods for gas turbines when it comes to air humidity and climatic variations. Additionally, some chiller-based options operate without water consumption.
Thermal Energy Storage (TES)
TES technologies involve a range of systems that are capable of storing thermal energy (heat or cold) for later use. When applied to inlet air cooling, TES involves storing chilled water or another cooling medium during off-peak hours to use it to cool the turbine inlet air during peak demand periods.
An example of this approach are ARANER’s TESTIAC systems, which combine thermal energy storage tanks and chiller-based TIAC systems.
A comparative analysis of cooling methods
As seen above, the different inlet air cooling methods for gas turbines offer various approaches to increase turbine performance, each with their own specific benefits and limitations.
The choice between inlet air cooling methods for gas turbines should be guided by thorough research as well as attention to the project’s specific needs. As an aid, here’s a review of some of the key factors to compare the different strategies, according to noteworthy research in the field, as well as ARANER’s extensive expertise in designing TIAC solutions.
Power output enhancement
All inlet air cooling methods for gas turbines offer important improvements in turbine power output. Some key findings in this respect include:
- For evaporative cooling, Hosseini et al. (2007) analyzed a system installed in gas turbines of a combined cycle power plant in Fars (Iran). The paper shows the gas turbine at 38 ºC ambient temperature and 8% relative humidity achieved an increase in output of 11 MW, with “the temperature drop of the inlet air (...) about 19 °C with media evaporative cooling installations.”
- For chiller-based systems, Dos Santos et al. (2012) used thermodynamic models to compare the effects of different inlet air temperatures on heat rate, power output, and efficiency. Results found “evaporative cooling brought an increment of 8.4 % and the absorption chiller represented a power output gain of 12.7 %”. More specifically, solutions based on absorption chillers managed “a larger temperature drop at different ambient conditions.” The study made some important comments on how each of these systems align with different climatic conditions, as mentioned below in this article.
Capital and operational costs
Hosseini et al. (2007) mentions a payback period of about 4 years for their evaporative cooling study. However, when it comes to other sources in scientific literature, there’s limited data to perform a comparative analysis of the different inlet air cooling methods for gas turbines.
ARANER’s extensive expertise and experience in implementing TIAC solutions confirms these technologies are capable of reducing the cost per kW to 30%, down from typical investment values of 600 up to 1,000 USD per kW.
As such, the typical EPC cost for a Turbine Inlet Air Cooling system ranges from approximately 150 to 400 USD for each additional kilowatt of power generated.
Drawing on ARANER’s experience, the typical payback period for TIAC installations is between 3 to 5 years, and a Net Present Value of between 6 to 8 times of investment value.
In any case, working with TIAC experts such as ARANER involves having access to a simulation of the system’s hourly performance for a complete year, so that the project can be understood from a return on investment perspective.
These calculations will necessarily take into account the project’s specifics, including expected ambient temperatures and humidity which, as specified below, should determine the choice of inlet air cooling methods for gas turbines.
Climate suitability and limitations
To a great extent, climate suitability should be a main guiding principle in the choice of inlet air cooling methods for gas turbines.
Mentioned above in this article are the limitations concerning fogging and evaporative cooling, which are bound by the air humidity conditions present on site.
The Dos Santos et al. (2012) paper mentioned above makes the case for chiller-based TIAC solutions “when large temperature reductions are needed under extremely high, dry ambient conditions”.
The availability of water or lack of thereof should also guide the choice between the different inlet air cooling methods for gas turbines. As projects increasingly consider sustainability and resource conservation as key parameters, chiller-based solutions are also poised to become crucial.
TES technologies in combination with TIAC solutions are also emerging as key allies for pushing turbine efficiency in warm climates. For instance, ARANER’s TESTIAC system at Jebel Ali Power Plant has managed to achieve a turbine inlet temperature of 20° degrees in an environment with ambient temperatures around 50° and extreme humidity.
A project that is a testament of how advanced thermal engineering expertise can make a difference when paired with a commitment to design ad hoc systems for each project’s needs.
With more than two decades of experience, at ARANER we develop tailor-made TIAC technologies to achieve cost-efficient, reliable and sustainable TIAC and TESTIAC solutions.
Get in touch with us and speak to our team about how we can help you achieve this.
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