Gas Turbine Efficiency Calculation: Avoid Higher Cost in Fuel Consumption

Power generation is largely still based on natural gas. The gas turbine technology has lately been rushing towards harnessing natural gas power more efficiently.  As a result, many users are now talking about efficiency of the gas turbine.

On the other side, major turbine manufacturers are competing to build the “most efficient” product. As the fight for bragging rights continues, the need to dig deeper into what efficiency has never been more appealing. In this article, we seek to define gas turbine efficiency calculation and review how to reduce fuel consumption.

Expression of Gas Turbine Efficiency

Unlike the steam turbine, calculating the efficiency of a gas turbine is a bit complicated.  A GT presents vapor and vapor conditions that are very dynamic. These conditions are largely dependent on the atmospheric conditions and type of fuel. The conditions in the GT being variable, they must be expressed as thermal efficiency, heat rate, kilowatt-hour and BTUs per horsepower.

We could compare the mpg rating for a car with the Btu/kWh for a gas turbine power plant. The only difference is that lower heat rate depicts higher efficiency, unlike in a car.

Gas turbines are devices for converting fuel energy into electric power (via electric generators) or mechanical power. They normally use the Brayton Cycle, which is a thermodynamic cycle that involves compression and expansion of a gaseous medium. An ideal cycle may offer 100% performance, but a real gas turbine will always have a certain level of losses and friction.

Turbine Inlet Air Cooling Case Study
 
The overall balance equation looks like this in its simplified form:

Shaft Power =Fuel Energy–Power Required for Compression–Exhaust Energy- Mechanical Losses

From the equation, it means that profitability is based on the turbine’s output and the cost of fuel. The operator can only control the turbine’s output, as the cost of fuel is beyond his/her control. That is why the song about enhancing turbine efficiency is not about to stop any time soon.

For a better understanding of the equation above, a review of the sections of the gas turbine is worthwhile. A natural gas fired power plant may be complex in the eyes, but it features three major sections:

  1. Compressor: draws, pressurizes and directs the air to the combustion chamber. Air is at extremely high speed here.
  2. Combustion Chamber:  comprises of a fuel injector set that creates a stream of fuel for mixing with air. After being burnt at 2000 degrees F, this mixture forms a high-pressure high-temperature gas stream that is ready to enter the turbine.
  3. Turbine Section: it is made of a complex combination of rotating and fixed aero-foil-section blades. The rotating blades respond to the expansion of the hot combustion gas. These blades have two essential functions- spin the generator for electricity generation and drive the compressor for more compressed air into the combustion chamber.

Performance Deterioration – Costly Effects

Every gas turbine suffers reduced performance during operation, with one of the causes of this deterioration being high ambient temperatures. Performance deterioration results in power output loss and higher fuel consumption. Both of these impacts can affect an organization negatively because they affect operation costs and revenue.

Overall, these factors can make the lifecycle costs of the gas turbine unbearable. When it comes to power generation, operating costs could exceed $1m for power loss of 3% and fuel consumption increase of 1%.

The gas turbine is composed of several components- turbine, combustor and compressor. When the characteristics of any of these components are altered, there might be increased heat rate and power output loss. This will increase fuel costs incurred by the power plant.

How can you avoid the higher costs?

There are several ways of achieving this, but we will outline one- Enhancing Power Plant Output.

GT power augmentation is ARANER’s specialty. Alongside that, we raise productivity to ensure that power plants sustain reliable performance. The company is renowned for inlet conditioning, which refers to reducing the inlet air temperature of GT. There are several approaches to this including inlet chilling, evaporative cooling and fogging.

ARANER offers inlet chilling in a technology it calls Turbine Inlet Air Cooling (TIAC). This ensures that the temperature entering the turbine does not exceed a preset temperature. TIAC provides optimal output through consistent cooling.  Moreover, the technology lowers maintenance costs and extends turbine life.

Other Improvements for Higher Efficiency

A look into the market reveals that GT manufacturers are using various methods to increase gas turbine efficiency for lower costs. These methods are based on enhancing compressor pressure ratio, increasing turbine inlet air and increasing mass flow.

Efficiency Boost through Thermal Energy Storage

Araner provides turbine inlet air cooling (TIAC) solutions that can blend with thermal energy storage (TES). Having such a setup eliminates the need for a million dollar peaking natural gas power plant. It provides the opportunity to reap from power generated during the night, using the same to chill water stored in a TES tank. The stored water is used the following day when demand is at peak.

Although the chiller operates on a 24/7 basis, it is split between generating chilled water and serving the gas turbines with cold water. Cold water from the tank augments the chiller during the day when demand increases.

This is crucial because the natural gas turbine can lose significant capacity during hot summers. Storage using a TES tank helps offsets the lost efficiency. An added benefit of this efficiency enhancing method is that operators and owners do not have to endure the lengthy permitting processes associated with installation of new power plant or gas turbine.

Compressor Pressure Ratio

The pressure ratio is one of the most important parameters related to performance and efficiency of the gas turbine. You can optimize the efficiency of the engine by increasing the difference or ratio of compressor discharge pressure to inlet air temperature. Manufacturer design determines this compressor pressure ratio.

In the analysis of this ratio, two gas turbine designs are prominent- aero-derivative and industrial (heavy frame) designs. Heavy frame GTs are designed to operate with low a low ratio of about 18:1, compared to aero-derivative GTs that have a ratio of about 30:1. The latter are lightweight, more fuel-efficient and produce less emission, so they are ideal for jet engines. Unfortunately, they tend to be more responsive to compressor inlet temperature.

Heat Recovery Steam Generator (HRSG)

An HRSG is fixed at the exhaust of the turbine to boost efficiency of the system. This component captures the waste heat produced by the turbine and uses it to preheat the discharge air of the compressor before it goes to the combustion chamber.

The HRSG is the heart of the Combined Cycle Power Plant (CCPP) set-up, which also includes steam turbines. The HRSG simply transfers energy from the waste heat to a water system, the result being steam. The steam operates a steam turbine, which in turn supplements the energy produced by the simple cycle natural gas power plant.

The steam turbine and the gas turbine run simultaneously, with the gas turbine running as it would alone, except that in the CCPP set-up, waste heat is diverted to the HRSG. The HRSG a lot of thermal energy that would otherwise have gone to waste is used to heat water.

Waste heat emanates from process thermodynamic limitations and system/equipment inefficiencies. Combating industrial efficiency is through either reduction of energy consumed or adding newer equipment that is more efficient. Capturing and reusing waste heat is more a more appealing solution, as a lot of energy is lost during operations.

Reusing waste heat increases efficiency significantly, but the effect emissions has been notable too. Combined cycles use less fuel, thereby reducing the amount of pollutants and greenhouse gases in the atmosphere. Modern combined plants are also using Selective Catalytic Reduction systems (SCR), reducing up to 90% present in waste heat.

How Does Temperature Affect Turbine Efficiency?

The power output of a gas turbine is inversely proportional to temperature i.e. when the temperature decreases, the output increases. To explain this relationship, here are some insights:

  • Air density increases with decreasing temperature
  • The GT is a fixed volume device
  • Higher mass of air through the GT increases power output
Fig 1:  Gas Turbine Performance vs. Ambient Temperature

Fig 1:  Gas Turbine Performance vs. Ambient Temperature. Source

In hot climates, there is a specific problem whereby poor GT performance coincides with peak electricity demand. The engineer designing a system for such climates must make this important consideration for reliable gas turbine efficiency calculation.

Engineers at ARANER model the gas turbine system for a year, collecting ambient temperature data at regular intervals throughout this period. You can contact the team for more details about this process.

Conclusion

To reduce high fuel consumption in a gas turbine, one must first look at the different forms of gas turbine efficiency calculation.  Peter Drucker once said, “If you can’t measure it, you can’t improve it”. You need an experienced partner for thorough gas turbine efficiency calculations.

Team up with ARANER for the most comprehensive gas turbine service solutions. Services include diagnostics, monitoring and upgrades. The TIAC technology is a popular solution for reduced fuel consumption, but it does not have to stop there. Following a review of your system and the existing possibilities, other approaches could apply as well.

If you enjoyed this post, you may be want to read Ice Storage Tanks: cost effective solution for smaller footprints.
 

 

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