CHP Cheat Sheet - Gas Engines & Gas Turbines

Combined heat and power (CHP) maximizes the recovery of heat and electricity from fuel. CHP is a mature technology, well suited for serving energy demands of industrial sites that consume both electricity and various grades of heat.

This post gives concise practical details about the two most common forms of gas based CHP - gas engines and gas turbines. Together the gas engine and gas turbine are around 90% of installed capacity of CHP in the UK.

Technical modeling of combined heat and power plants was my first area of professional specialization.

At ENGIE I developed technical models to support new projects. I also developed models to optimize the dispatch of existing plants, such as the Olympic Park district heating scheme in London.

This post focuses on the technical facts important to the design and operation of gas turbines and gas engines.

Common Characteristics

Both gas engines and gas turbines share several key characteristics:

• Total efficiency: Roughly around 80% HHV (electric + thermal)
• Maximum electric efficiency: Achieved at full load
• Part load performance: Total efficiency remains around 80% HHV - reductions in electric efficiency are counteracted by increases in thermal efficiency

Gas Engines

Key Practical Advantages

• High electric efficiency: 30-38% HHV
• Cheap maintenance cost: £0.6-1.2 p/kWh @ 8,000 hours/yr
• Cheap capital cost: £500-1,500 /kWe total project

Key Practical Disadvantages

• Half of the recoverable heat is generated as low quality: <100°C
• Usually only economic at sizes below 5 MWe

Operating Characteristics

A gas engine generates roughly the same amount of electricity and heat - i.e. the heat to power ratio is around 1:1.

The recoverable heat generated in a gas engine is split roughly half high grade (>500°C), half low grade (<100°C).

Gas engines are typically economic up until 5-6 MWe. Beyond that size gas turbines become competitive.

Heat Sink Requirements

Having half of the heat generated as low quality means that a low-grade heat sink is required.

Many industrial processes only require high-temperature heat (typically served using steam). Without a low-grade heat sink for the low-grade heat the economics of a gas engine will suffer.

Typical low-grade heat sinks include:

• Space heating: District heating networks and hospitals
• Boiler feedwater heating: On sites with low condensate return rates
• Low-temperature process heating: Various industrial applications

This makes district heating and hospitals good applications of gas engine CHP.

Gas Turbines

Key Practical Advantages

• All of the heat generated is high quality: >500°C
• Supplementary firing: Can be used to generate more heat at high efficiency
• Combined cycle potential: Can combine with steam turbines to generate more power

Key Practical Disadvantages

• Lower electric efficiency: 25-35% HHV
• Complex emissions control systems
• Usually limited to sizes above 5 MWe

Operating Characteristics

A gas turbine generates roughly twice as much heat as power - i.e. the heat to power ratio is around 2:1.

A gas turbine operates with a lower electric efficiency (25-35% HHV) than a gas engine.

Unlike a gas engine, all of the heat generated by a gas turbine is high grade (>500°C). This makes gas turbines ideal for industrial sites that need high-temperature steam to run their processes.

Advanced Configurations

This also allows gas turbines to be used in combined cycle mode:

• Combined cycle: Steam is generated off the exhaust and used to drive a steam turbine
• Supplementary firing: More gas can be fired into the exhaust to further increase steam generation

This can be a key advantage of gas turbines, as the marginal efficiency of supplementary firing is higher than generating heat in a shell or water tube boiler.

Summary

Gas engines and gas turbines are the two most common forms of gas-based CHP, representing around 90% of UK installed capacity.

Both achieve total efficiencies around 80% HHV, with trade-offs between electric efficiency and heat quality:

• Gas engines: Higher electric efficiency (30-38% HHV), lower capital and maintenance costs, but half the heat is low grade (<100°C). Best for applications with low-grade heat sinks like district heating
• Gas turbines: Lower electric efficiency (25-35% HHV), but all heat is high grade (>500°C). Ideal for industrial steam applications and can be configured in combined cycle with supplementary firing

Thanks for reading!