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Answer by Dale Blundell (Atkins)
Cogeneration systems come in a wide range of sizes, and use different technologies depending on the application. The common elements of all CHP systems are the prime mover and electrical generator.
- Micro CHP is defined by the EU Energy Efficiency Directive as having electrical capacity of up to 50kW, which can be split into <5kW, and 5 - 50kW electrical capacity size ranges. The prime mover can be based on different technologies such as Stirling engines or small spark ignition engines. The heat to power ratio for these systems could be as high as 5:1, lending themselves to projects where heat demand, maybe hot water, forms a large proportion of overall energy demand. Typical applications for these smaller units include hotels, leisure centres, nursing homes, sheltered accommodation. Very small CHP equipment designed for use in domestic properties is now becoming available, and fuel cell based equipment has recently been launched featuring heat to power ratios of between 20:1 and 2:1, opening up new market opportunities.
- Small scale CHP means a unit of electrical capacity up to 1 MW. The prime mover in this application is typically the spark ignition (reciprocating) engine. Historically these would have been modified engines intended to run on liquid fuels, but more modern sets are purpose built for gas fired CHP applications. Alternative prime movers include micro-turbines (30kW to 100kW), small scale aero-derivative gas turbines (~500kW) and ORC (Organic Rankine Cycle) turbines. However the number of installations and total run-time in the field is significantly lower than spark ignition engines and they are more expensive. The ratio of heat production to electricity generation (heat to power ratio) ranges between 1:1 and 2:1, depending on the type of prime mover.
- Large scale CHP – these systems can range from 1 MW to as high as 1,250MW. These are generally bespoke systems where the application could comprise any one of a variety of technologies, including:
- Boilers (fossil fuel or biomass) raising steam for use in a steam turbine.
- Large reciprocating engines.
- Large gas turbines using heat recovery steam generators (HRSG) that recover high grade heat from exhaust gases.
- Combined cycle systems involving a mix of plant that typically uses a gas turbine with HRSG, boilers and steam turbine.