Turbine inlet air cooling is a group of technologies and techniques consisting of cooling down the intake air of the gas turbine. The direct consequence of cooling the turbine inlet air is power output augmentation. It may also improve the energy efficiency of the system. [1] This technology is widely used in hot climates with high ambient temperatures that usually coincides with on-peak demand period. [2]
Gas turbines take in filtered, fresh ambient air and compress it in the compressor stage. The compressed air is mixed with fuel in the combustion chamber and ignited. This produces a high-temperature and high-pressure flow of exhaust gases that enter in a turbine and produce the shaft work output that is generally used to turn an electric generator as well as powering the compressor stage.
As the gas turbine is a constant volume machine, the air volume introduced in the combustion chamber after the compression stage is fixed for a given shaft speed (rpm). Thus the air mass flow in is directly related to the density of air, and the introduced volume.
where is the mass, is the density and is the volume of the gas. As the volume is fixed, only density of the air can be modified to vary air mass. The density of the air depends on the relative humidity, altitude, pressure drop and temperature.
where:
The performance of a gas turbine, its efficiency (heat rate) and the generated power output strongly depend on the climate conditions, which may decrease the output power ratings by up to 40%. [4] To operate the turbine at ISO conditions [5] and recover performance, several inlet air cooling systems have been promoted.
Different technologies are available in the market. Each particular technology has its advantages and inconveniences according to different factors such as ambient conditions, investment cost and payback time, power output increase and cooling capacity.
Inlet air fogging consists of spraying finely atomized water (fog) into the inlet airflow of a gas turbine engine. The water droplets evaporate quickly, which cools the air and increases the power output of the turbine.
Demineralized water is typically pressurized to 2000 psi (138 bar) then injected into the inlet air duct through an array of stainless steel fog nozzles. Demineralized water is used in order to prevent fouling of the compressor blades that would occur if water with mineral content were evaporated in the airflow. Fog systems typically produce a water spray, with about 90% of the water flow being in droplets that are 20 microns in diameter or smaller. [6]
Inlet fogging has been in commercial use since the late 1980s and is a popular retrofit technology. As of 2015, there were more than 1000 inlet fog systems installed around the world. [7] Inlet fog systems are, “simple, easy to install and operate” and less expensive than other power augmentation systems such as evaporative coolers and chillers. [8]
Inlet fogging is the least expensive gas turbine inlet air cooling option and has low operating costs, particularly when one accounts for the fact that fog systems impose only a negligible pressure drop on the inlet airflow when compared to media-type evaporative coolers. [9] [10]
Fog nozzle manifolds are typically located in the inlet air duct just downstream of the final air filters but other locations can be desirable depending on the design of the inlet duct and the intended use of the fog system. [11]
On a hot afternoon in a desert climate, it is possible to cool by as much as 40 °F (22.2 °C), while in a humid climate hot-afternoon cooling potential can be just 10 °F (5.6 °C) or less. Nevertheless, there are many successful inlet-fogging installations in humid climates such as Thailand, Malaysia and the American Gulf States. [12]
Inlet fogging reduces emissions of Oxides of nitrogen (NOx) because the additional water vapor quenches hot spots in the combustors of the gas turbine. [13]
Fog systems can be used to produce more power than can be obtained by evaporative cooling alone. This is accomplished by spraying more fog than is required to fully saturate the inlet air. The excess fog droplets are carried into the gas turbine compressor where they evaporate and produce an intercooling effect, which results in a further power boost. This technique was first employed on an experimental gas turbine in Norway in 1903. There are many successful systems in operation today. [14]
Several gas turbine manufactures offer both fogging and wet compression systems. Systems are also available from third-party manufacturers.
The evaporative cooler is a wetted rigid media where water is distributed throughout the header and where air passes through the wet porous surface. Part of the water is evaporated, absorbing the sensible heat from the air and increasing its relative humidity. The air dry-bulb temperature is decreased but the wet-bulb temperature is not affected. [15] Similar to the fogging system, the theoretical limit is the wet bulb temperature, but performance of the evaporative cooler is usually around 80%. Water consumption is less than that of fogging cooling.
In a mechanical compression chiller technology, the coolant is circulated through a chilling coil heat exchanger that is inserted in the filter house, downstream from the filtering stage. Downstream from the coil, a droplet catcher is installed to collect moisture and water drops. The mechanical chiller can increase the turbine output and performance better than wetted technologies due to the fact that inlet air can be chilled below the wet bulb temperature, indifferent to the weather conditions. [16] Compression chiller equipment has higher electricity consumption than evaporative systems. Initial capital cost is also higher, however turbine power augmentation and efficiency is maximized, and the extra-cost is amortized due to increased output power.
The majority of such systems involve more than one chiller unit and the configuration of the chillers can have a great bearing on the system parasitic power consumption. The series counterflow configuration can reduce the compressor work needed on each chiller, improving the overall chiller system by as much as 8%. [17]
Other options such a steam driven compression are also used in industry. [18]
In vapor-absorption chillers technology, thermal energy is used to produce cooling instead of mechanical energy. The heat source is usually leftover steam coming from combined cycle, and it is bypassed to drive the cooling system. Compared to mechanical chillers, absorption chillers have a low coefficient of performance, however, it should be taken into consideration that this chiller usually uses waste heat, which decreases the operational cost. [19]
A thermal energy storage tank is a naturally stratified thermal accumulator that allows the storage of chilled water produced during off-peak time, to use this energy later during on-peak time to chill the turbine inlet air and increment its power output. A thermal energy storage tank reduces operational cost and refrigerant plant capacity. [20] One advantage is the production of chilled water when demand is low, using the excess of power generation, which usually coincides with the night, when ambient temperature is low and chillers have better performance. Another advantage is the reduction of the chilling plant capacity and operational cost in comparison with an on-line chilling system, which produce delays during periods of low demand.
In areas where there is demand cooling, daily summer on-peak periods coincide with the highest atmospheric temperatures, which may reduce the efficiency and power gas turbines. With the vapor mechanical compression technologies, cooling can be used during these periods so that the performance and the power output of the turbine may be less affected by ambient conditions
Another benefit is the lower cost per extra inlet-cooling kilowatt compared to newly installed gas turbine kilowatt[ citation needed ]. Moreover, the extra inlet-cooling kilowatt uses less fuel than the new turbine kilowatt due to the lower heat-rate (higher efficiency) of the chilled turbine. Other benefits may include the incrementation of steam mass flow in a combined cycle, the reduction of turbine emissions (SOx, NOx, CO2), [21] and increase in power-to-installed volume ratio.
Calculating the benefits of turbine air cooling requires a study to determine payback periods, taking into consideration several aspects like ambient conditions, cost of water, hourly electric demand values, cost of fuel. [22]
A heat engine is a system that converts heat to usable energy, particularly mechanical energy, which can then be used to do mechanical work. While originally conceived in the context of mechanical energy, the concept of the heat engine has been applied to various other kinds of energy, particularly electrical, since at least the late 19th century. The heat engine does this by bringing a working substance from a higher state temperature to a lower state temperature. A heat source generates thermal energy that brings the working substance to the higher temperature state. The working substance generates work in the working body of the engine while transferring heat to the colder sink until it reaches a lower temperature state. During this process some of the thermal energy is converted into work by exploiting the properties of the working substance. The working substance can be any system with a non-zero heat capacity, but it usually is a gas or liquid. During this process, some heat is normally lost to the surroundings and is not converted to work. Also, some energy is unusable because of friction and drag.
A heat pump is a device that uses work to transfer heat from a cool space to a warm space by transferring thermal energy using a refrigeration cycle, cooling the cool space and warming the warm space. In cold weather, a heat pump can move heat from the cool outdoors to warm a house; the pump may also be designed to move heat from the house to the warmer outdoors in warm weather. As they transfer heat rather than generating heat, they are more energy-efficient than other ways of heating or cooling a home.
The Brayton cycle, also known as the Joule cycle, is a thermodynamic cycle that describes the operation of certain heat engines that have air or some other gas as their working fluid. It is characterized by isentropic compression and expansion, and isobaric heat addition and rejection, though practical engines have adiabatic rather than isentropic steps.
A compressor is a mechanical device that increases the pressure of a gas by reducing its volume. An air compressor is a specific type of gas compressor.
An evaporative cooler is a device that cools air through the evaporation of water. Evaporative cooling differs from other air conditioning systems, which use vapor-compression or absorption refrigeration cycles. Evaporative cooling exploits the fact that water will absorb a relatively large amount of heat in order to evaporate. The temperature of dry air can be dropped significantly through the phase transition of liquid water to water vapor (evaporation). This can cool air using much less energy than refrigeration. In extremely dry climates, evaporative cooling of air has the added benefit of conditioning the air with more moisture for the comfort of building occupants.
The Rankine cycle is an idealized thermodynamic cycle describing the process by which certain heat engines, such as steam turbines or reciprocating steam engines, allow mechanical work to be extracted from a fluid as it moves between a heat source and heat sink. The Rankine cycle is named after William John Macquorn Rankine, a Scottish polymath professor at Glasgow University.
A cooling tower is a device that rejects waste heat to the atmosphere through the cooling of a coolant stream, usually a water stream, to a lower temperature. Cooling towers may either use the evaporation of water to remove heat and cool the working fluid to near the wet-bulb air temperature or, in the case of dry cooling towers, rely solely on air to cool the working fluid to near the dry-bulb air temperature using radiators.
A chiller is a machine that removes heat from a liquid coolant via a vapor-compression, absorption refrigeration, or absorption refrigeration cycles. This liquid can then be circulated through a heat exchanger to cool equipment, or another process stream. As a necessary by-product, refrigeration creates waste heat that must be exhausted to ambience, or for greater efficiency, recovered for heating purposes. Vapor compression chillers may use any of a number of different types of compressors. Most common today are the hermetic scroll, semi-hermetic screw, or centrifugal compressors. The condensing side of the chiller can be either air or water cooled. Even when liquid cooled, the chiller is often cooled by an induced or forced draft cooling tower. Absorption and adsorption chillers require a heat source to function.
The General Electric LM6000 is a turboshaft aeroderivative gas turbine engine. The LM6000 is derived from the CF6-80C2 aircraft turbofan. It has additions and modifications designed to make it more suitable for marine propulsion, industrial power generation, and marine power generation use. These include an expanded turbine section to convert thrust into shaft power, supports and struts for mounting on a steel or concrete deck, and reworked controls packages for power generation. It has found wide use including peaking power plants, fast ferries and high speed cargo ship applications.
An air cycle machine (ACM) is the refrigeration unit of the environmental control system (ECS) used in pressurized gas turbine-powered aircraft. Normally an aircraft has two or three of these ACM. Each ACM and its components are often referred as an air conditioning pack. The air cycle cooling process uses air instead of a phase changing material such as Freon in the gas cycle. No condensation or evaporation of a refrigerant is involved, and the cooled air output from the process is used directly for cabin ventilation or for cooling electronic equipment.
An absorption refrigerator is a refrigerator that uses a heat source to provide the energy needed to drive the cooling process. Solar energy, burning a fossil fuel, waste heat from factories, and district heating systems are examples of convenient heat sources that can be used. An absorption refrigerator uses two coolants: the first coolant performs evaporative cooling and then is absorbed into the second coolant; heat is needed to reset the two coolants to their initial states. Absorption refrigerators are commonly used in recreational vehicles (RVs), campers, and caravans because the heat required to power them can be provided by a propane fuel burner, by a low-voltage DC electric heater or by a mains-powered electric heater. Absorption refrigerators can also be used to air-condition buildings using the waste heat from a gas turbine or water heater in the building. Using waste heat from a gas turbine makes the turbine very efficient because it first produces electricity, then hot water, and finally, air-conditioning—trigeneration.
In thermodynamics, the thermal efficiency is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, steam turbine, steam engine, boiler, furnace, refrigerator, ACs etc.
Economizers, or economisers (UK), are mechanical devices intended to reduce energy consumption, or to perform useful function such as preheating a fluid. The term economizer is used for other purposes as well. Boiler, power plant, heating, refrigeration, ventilating, and air conditioning (HVAC) uses are discussed in this article. In simple terms, an economizer is a heat exchanger.
Vapour-compression refrigeration or vapor-compression refrigeration system (VCRS), in which the refrigerant undergoes phase changes, is one of the many refrigeration cycles and is the most widely used method for air conditioning of buildings and automobiles. It is also used in domestic and commercial refrigerators, large-scale warehouses for chilled or frozen storage of foods and meats, refrigerated trucks and railroad cars, and a host of other commercial and industrial services. Oil refineries, petrochemical and chemical processing plants, and natural gas processing plants are among the many types of industrial plants that often utilize large vapor-compression refrigeration systems. Cascade refrigeration systems may also be implemented using two compressors.
Vapor-compression evaporation is the evaporation method by which a blower, compressor or jet ejector is used to compress, and thus, increase the pressure of the vapor produced. Since the pressure increase of the vapor also generates an increase in the condensation temperature, the same vapor can serve as the heating medium for its "mother" liquid or solution being concentrated, from which the vapor was generated to begin with. If no compression was provided, the vapor would be at the same temperature as the boiling liquid/solution, and no heat transfer could take place.
A turboexpander, also referred to as a turbo-expander or an expansion turbine, is a centrifugal or axial-flow turbine, through which a high-pressure gas is expanded to produce work that is often used to drive a compressor or generator.
Thermodynamic heat pump cycles or refrigeration cycles are the conceptual and mathematical models for heat pump, air conditioning and refrigeration systems. A heat pump is a mechanical system that allows for the transmission of heat from one location at a lower temperature to another location at a higher temperature. Thus a heat pump may be thought of as a "heater" if the objective is to warm the heat sink, or a "refrigerator" or “cooler” if the objective is to cool the heat source. In either case, the operating principles are similar. Heat is moved from a cold place to a warm place.
Free cooling is an economical method of using low external air temperatures to assist in chilling water, which can then be used for industrial processes, or air conditioning systems. The chilled water can either be used immediately or be stored for the short- or long-term. When outdoor temperatures are lower relative to indoor temperatures, this system utilizes the cool outdoor air as a free cooling source. In this manner, the system replaces the chiller in traditional air conditioning systems while achieving the same cooling result. Such systems can be made for single buildings or district cooling networks.
Ice storage air conditioning is the process of using ice for thermal energy storage. The process can reduce energy used for cooling during times of peak electrical demand. Alternative power sources such as solar can also use the technology to store energy for later use. This is practical because of water's large heat of fusion: one metric ton of water can store 334 megajoules (MJ) of energy, equivalent to 93 kWh.
Compressed air dryers are special types of filter systems that are specifically designed to remove the water that is inherent in compressed air. The compression of air raises its temperature and concentrates atmospheric contaminants, primarily water vapor, as resulting in air with elevated temperature and 100% relative humidity. As the compressed air cools down, water vapor condenses into the tank(s), pipes, hoses and tools connected downstream from the compressor which may be damaging. Therefore water vapor is removed from compressed air to prevent condensation from occurring and to prevent moisture from interfering in sensitive industrial processes.
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