Answer by Dale Blundell (Atkins)
Apart from electrical power and heat, cogeneration systems may also be combined with other equipment to optimise the benefits for a particular application.
Steam can only be raised from a high grade heat source, and for spark ignition systems it is therefore only the exhaust heat that can be used for this purpose. The low grade heat from the engine cooling water system must be used elsewhere, for example to preheat cold boiler feedwater.
To produce steam, the exhaust gases must be passed through steam raising equipment. In small scale systems, this would normally be by a steam generator. In medium to large scale sizes, the gases would be passed through a waste heat boiler or a heat recovery steam generator (HRSG). Such systems have to be carefully designed to ensure that the steam is raised safely and the steam conditions (temperature, pressure, dryness fraction) meet the requirements of the existing system.
By integrating an absorption chiller, chilled water can be generated in addition to heat and power. Absorption chillers can utilise both steam and LPHW – the higher the input temperature, the greater the Coefficient of Performance of the chillers. Where a spark ignition engine is used, the LPHW from the engine cooling circuit can be used to operate the chiller, leaving the high grade heat of the exhaust to be used elsewhere.
Absorption chillers are a good application where there is a cooling demand at times when not all of the heat from the CHP can be utilised. Where systems have a COP < 1, then it is economically better to utilise heat from the CHP directly, rather than convert this into cooling. Absorption chillers are generally more expensive than their equivalent vapour compression system, however the cost of maintenance is lower.
Organic Rankine Cycle – ORC
The operation of ORC is similar to that of the Rankine cycle in a steam turbine. In an ORC, instead of the mass flow of the steam turning the turbine, waste heat evaporates an organic refrigerant, the expansion of which drives a turbine. The operation of the turbine generates electrical power through an alternator coupled to the turbine drive shaft. In an ORC application, high grade waste heat can be used to heat the organic medium which is the input media to an ORC turbine. The exhaust gases from a CHP system (Reciprocating or Gas Turbine) can also be used as the thermal input to an ORC. Here, the heat is converted into power, which may be more economically beneficial to a site than the heat from the CHP.
This is an extension of the application of Tri-generation to generate a fourth utility – food grade carbon-dioxide. In this application, some or all of the exhaust gases from a CHP system are passed through a system which selectively extracts the carbon-dioxide from the gas stream. An amine based chemical is employed which has a strong affinity for CO2,that captures the gas, which is then purified and the amine regenerated. As bulk gases are becoming increasingly expensive, it can be an attractive consideration for manufacturers of products containing CO2 e.g. fizzy soft drinks, beer etc.