By Pete McGuigan
Gas Turbines (GTs) operate at a constant volume of (hopefully) very clean air. It’s the density of this air however (weight per unit volume) that holds the key if we want to keep our Gas Turbine Power Output levels high.
On a hot summer day, air expands and is less dense (occupies more volume for the same weight) than on a cooler day. For Gas Turbines operating at a constant volume of ingested air, this then results in less mass flow of air to the compressor which will significantly reduce performance and power output.
Gas Turbines are rated for performance at 15°C (59°F), 60% Relative Humidity (RH) and at sea level air pressure. If your GT is located at and primarily operates during such conditions, then the efficiency and power output you see advertised is what you should expect to generate. If the above conditions are not met however, then you will see a significant drop off from the rated values.
All GTs have reduced performance levels at higher temperatures (and also at higher altitudes). A few common rules of thumb can be employed to understand the impact.
Expect about a 0.4% reduction in power output plus 0.1% increase in heat rate for each 1°F (0.85°C) rise in ambient temperature above 59°F (15°C).
Expect about a 0.4% reduction in power output plus 0.1% increase in heat rate for each inch Water Gauge (250Pa) of pressure drop.
Altitude has a minimal effect on heat rate but for each 1000ft increase in site elevation above sea level, there is about a 3.5% loss in power output.
By way of an example, if the ambient air temperature rises to 27°C (80°F), power output can drop by up to 3% for older ‘frame’ engines with a compression ratio of ~10 and approximately 8% for aeroderivative engines with compression ratios of ~30. This rises to 7% and 17% respectively as temperatures reach 38°C (100°F). When you crunch the numbers, the financial impact of this can be enormous.
On hot days, the losses in turbine output are further compounded because the market demand for power increases and (typically) the price goes up. Consumers turn on air conditioning units during hot days, driving up energy prices to a premium. Indeed, at peak times, energy prices can double to ~ $100 per MWhr or higher, making this a time when power companies really want to maximize output and generate increased profits. In many applications, however, the opposite is happening and GT performance is being reduced because of the ambient air conditions and the power output from the machine is actually dropping.
The most common way of compensating for this drop in performance is to employ supplementary devices which function to cool the inlet air, counteracting the drop in density and recovering a proportion of the losses in power output.
A payback analysis should be performed with the assistance of the technology supplier in order to understand the financial benefits to be had by introducing air cooling technologies. The technology supplier can also help define the best cooling methods for any given GT application or/and operational timeframe based on historical local area ambient temperature and humidity data. For retrofits they can also provide details about extending foundations and adding support to any new cooler support structure.
Different technologies are available to achieve GT air intake cooling, most commonly based on either water evaporation within the airstream or by using tube and fin style heat exchangers.
Evaporation of water is one of the simplest and oldest methods of cooling air. Even with all the sophisticated technology available today, including mechanical chillers, absorption chillers, and thermal energy storage systems, the simple principles of evaporative cooling remain a cost efficient method for GT air intake temperature control.
The performance of an evaporative cooler is based on the ratio of the number of degrees it can cool the air compared to the wet bulb temperature depression. This terminology can appear confusing but really all it means is the difference between the dry bulb (which is just another term for the ambient air temperature) and the wet bulb temperature (which is the temperature this air would be if it were 100% saturated i.e. at 100% Relative Humidity (RH)).
As air passes through an evaporative cooler system, heat energy is transferred from the air to the water. This energy transfer causes the water to evaporate and the water vapour then mixes with the air, manifested as increased humidity. The total amount of energy in the air, however, remains constant so the process can be considered as adiabatic.
Water evaporation based designs use what is termed as latent heat transfer. This is when heat is transferred from one substance (the hot air) without a corresponding temperature increase in the other substance (the introduced water). In this application the other substance (water) instead changes physical state from liquid to gas as it evaporates, thus the terminology ‘evaporative cooling’.
The most commonly used evaporative cooling systems employ a ‘wetted’ media. In this type of system, the GT inlet air passes through a bank of water-soaked evaporative cooling media. Evaporation of a portion of the water contained in the media lowers the dry bulb temperature of the air. A moisture separator stage is located immediately downstream of the media bank, the function of which is to remove any liquid water droplets that may become re-entrained in the airstream. The cooling media through which the inlet air passes is typically located between the inlet filter compartment and the inlet plenum, upstream of the silencer. An additional skid is used to house the cooler feed water tank, pumps, controls, and water quality (blowdown) sampling system. For larger systems the feed water tank (sometimes called the sump) can be located directly underneath the sets of media banks.
Wetted media evaporation systems offer the greatest benefit in hot, dry climates and/or at high altitudes where the air is thin. They are the most widely used and proven solution to reduce gas turbine losses in high temperatures and can offer low initial investment cost and small auxiliary power load.
Evaporation efficiency is directly controlled by contact time between airflow and the moist media. Contact time is a function of airflow velocity and effective media area. The longer the air remains in contact with the media, the greater the cooling that can be achieved through evaporation. Maximum saturation efficiency can be obtained by maximizing the contact area while maintaining relatively low speeds for the airflow. Low speeds are typically a function of filterhouse size, so a balanced compromise is needed to decide on the most cost effective overall solution.
Large quantities of water are required for the system to operate and so this needs to be readily available as a local utility or through on-site storage tanks. Water also needs to be of a relatively clean quality to protect the gas turbine from corrosion and scale formation and to help reduce maintenance frequency of the evaporative cooler system and media.
Water always contains a certain amount of dissolved minerals, unless it is treated and termed demineralized (more on this later). The process of evaporative cooling removes liquid water from the re-circulating flow and leaves behind the solids that had been dissolved in the water when it was added as makeup. Accordingly, for recirculating systems, enough water must be blown down (removed) from the re-circulating flow to control the level of these solids and to avoid build-up of insoluble minerals on the media pad surface (also known as scaling), which results in an increase in pressure drop, and a loss of evaporation efficiency. Blow down is a function of evaporation rate and the cycles of concentration. The chemistry of the sump is established by determining the maximum cycles of concentration that the makeup water can go through before needing to be changed.
The other main water evaporation based cooling methodology used for GT cooling is when atomized water is sprayed directly into the air intake. This is termed ‘fogging’. Fogging is a method of cooling where demineralized water is converted into a ‘fog’ by means of arrays of atomizing nozzles operating at high pressures. The fog, which consists of billions of small droplets, mixes with the hot ambient air and evaporates. This evaporation is again a latent heat transfer process whereby the ambient air temperature decreases. Care needs to be taken with fogging because atomizing nozzles are prone to wear which increases droplet sizes and can increase the risk of droplet erosion on the GT compressor blades as well as decreasing the effectiveness of heat transfer. It is also important to ensure that no ‘overspray’ occurs. This is the term used when the fog generated does not have sufficient time to interact with the air and fully evaporate or when more water is injected than is actually required to raise the relative humidity to 100%.
Fouling of the gas turbine inlet air system and the compressor will occur with inadequate water quality for both methods of water evaporation based cooling. For fogging systems, demineralized water is necessary to limit spraying nozzle blockage and there is no recirculation of the water employed, however for wetted media designs, ‘blowdown’ or continuous sampling of the (normally) recirculated water is necessary to ensure the water quality remains sufficiently clean and is topped up as necessary.
If either of these technologies is used, air temperatures cannot be lowered below the wet-bulb temperature (which if you recall from earlier is the temperature when the air is completely saturated i.e. at 100% Relative Humidity). It is important to then also realize that the effectiveness of these systems are limited if local levels of ambient humidity are already high, as the air already contains increased moisture levels to start with. Improvement gains to be had are directly linked to the delta between the ambient humidity and this 100% humidity (wet bulb) condition. Using wetted media also increases differential pressure across the system noting that the way the technology is employed and maintained means it can (and should) be removed in cooler times of the year when not required. The differential pressure associated with fogging systems is minimal.
Chiller coil cooling systems work like a radiator in a car. Cool fluid flows through tubes and is radiated into the inlet using fins which cool the surrounding inlet air, removing water vapour from it. This technology is not dependant on local ambient humidity and can lower the air temperature below the wet-bulb temperature. However, this solution adds a very high parasitic load, which can typically be around a third of the output gain achieved (several thousands of kW for a 100MW turbine), and increases differential pressure across the installation all year round (which negatively impacts GT performance), without the ability to easily remove when not needed.
A comparison of technologies is provided below with the relative pros and cons of the existing technologies commonly employed for GT air intake cooling, today.
A cool payback (case study)
To demonstrate the sort of payback period inlet for air cooling, let’s look at an example of a gas turbine installation in inland North Africa. The site has two GE 9E turbines and wishes to use the proven wetted media evaporation technology. Temperature and humidity levels assume the evaporating system operates between the hours of 10 a.m. and 8 p.m. from June to September. At peak temperatures (maximum evaporation) the two units combined will require approximately 42 m3/hour (184 US GPM) of water.
The average decrease in inlet air temperature as a result of the cooling system is 12°C (21°F). This equates to approximately 8.5% of reduced loss in turbine output. The 9E turbines are ISO rated at 126 MW (15°C). If an average of output of 110 MW is assumed, the cooling system saves 9.4 MW. As this saving is during peak periods, taking a price of $90/MWhr means in one year (excluding water treatment) the cooling system will save:
9.4 MW x 10hrs x 122 days x $90 = $1,032,120
For these installation conditions, a typical payback period of just one year makes the wetted media evaporation system an appealing option. This assumes a scope of supply including evaporative cooling system and the required supporting structure.
Not a component in isolation
A complete inlet system needs to consider multiple aspects to ensure the turbine is best protected – both in terms of its performance and against costly damage. Components may include weatherhoods and moisture separators, pulse systems if dust levels are high, filters to handle wet and dry contaminants as well as inlet acoustic and cooling systems. The cooling system itself requires water pumping, distribution and quality monitoring systems to help ensure desired results are being obtained. All need to be designed to offer robust and reliable performance based on local, often harsh, environmental conditions.
Companies such as Parker (formerly CLARCOR Industrial Air) can offer fully designed and engineered solutions to cover all aspects of a gas turbine inlet system. Experience, knowledge and expertise across all areas means customers can have peace of mind that they are getting optimum, reliable and consistent performance from their gas turbine all year round.
The design and effect of a gas turbine inlet system is highly dependent on local environmental conditions. Seasonal variances, site location, different contaminants, operational procedures, criticality of turbine availability and value of turbine output all come into play. Whatever the turbine technology used, lower air density will reduce power output. Companies such as Parker can help customers ensure their system is optimized for their specific installation requirements, minimizing losses and optimizing profit levels.
Pete McGuigan is senior product manager of the Gas Turbine Filtration Division at Parker Hannifin.