In my previous life at ENGIE I specialized in the technical modeling of combined heat & power (CHP) plants.
I developed models to support sales projects and to optimize operation of existing sites – such as the district heating scheme at the Olympic Park in London.
CHP is an attractive technology for maximizing the recovery of heat & electricity from fuel. The technologies are mature and will deliver carbon benefits in most electricity grids.
This post aims to give concise practical details about the two most commons forms of gas based CHP. Together the gas engine and gas turbine are around 90 % of installed capacity of CHP in the UK.
Table 1 – CHP in the UK (from DECC)
This post focuses on the facts that matter in the day to day world of energy management.
common for both gas engines & gas turbines
- Both have total efficiencies (electric + thermal) roughly around 80 % HHV.
- Both operate with a maximum electric efficiency at full load.
- In part load operation total efficiency remains around 80 % HHV – reductions in electric efficiency are counteracted by increases in thermal efficiency.
Figure 1 – a simple gas engine schematic
- High electric efficiency.
- Cheap maintenance cost.
- Cheap capital cost.
- Half of the recoverable heat is generated as low quality (<100 °C).
- Usually only economic at sizes below 5 MWe.
A gas engine has a high electric efficiency (30-38 % HHV).
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.
Gas engine CHP is low maintenance (0.6 – 1.2 p/kWh @ 8,000 hours/yr) and captial (500 – 1,500 /kWe total project) cost.
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, boiler feedwater heating on sites with low condensate return rates and low-temperature process heating. This makes district heating and hospitals good applications of gas engine CHP.
Figure 2 – a simple gas turbine schematic
- All of the heat generated is high quality.
- Supplementary firing can be used to generate more heat at high efficiency.
- Potential to combine with steam turbines to generate more power.
- Lower electric efficiency.
- Complex emissions control systems.
- Usually limited to sizes above 5 MWe.
A gas turbine operates with a lower electric efficiency (25-35% HHV) than a gas engine.
A gas turbine generates roughly twice as much heat as power – ie the heat to power ratio is around 2:1.
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.
This also allows gas turbines to be used in combined cycle mode (steam is generated off the exhaust and used to drive a steam turbine). More gas can be fired into the exhaust to further increase steam generation (known as supplementary firing).
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.