Cogeneration and trigeneration
What are they?
Cogeneration is typically based on an on-site electricity generator that is fuelled by gas. The heat produced by the generator is captured for the building’s hot water system. This means both heat and electricity are being generated. In systems that also provide cooling, the hot water is fed to an absorption chiller which produces chilled water from the hot water resource.
‘Tri-generation’ refers to the production of electricity, heat and chilled water. Chilled water is produced through the incorporation of an absorption chiller into a co-generation system. Absorption chillers use the waste heat from the cogeneration plant to create chilled water for cooling a building.
Typically the cogeneration system will not provide all of the site heating needs and a trigeneration system will not provide all of the site’s chilled water needs. As a result the building will still have boiler and chiller plant.
Advantages / Disadvantages
Cogeneration and trigeneration make better use of the input fuel than the power stations on the grid, which throw away the waste heat. Also, gas is a lower impact fuel than the coal that comprises the input fuel to the majority of grid electricity.
Optimum applications require continuous electrical and thermal loads which are present in some retail and industrial applications (especially where the industrial process has process heating or cooling requirements) but are uncommon in offices.
Specialist maintenance is required to operate cogeneration and trigeneration systems with adequate reliability
It is not always possible to operate cogeneration or trigeneration within a local electricity distribution network. Permission must be sought from the owner of the electricity distribution network.
Less electricity is imported from the national grid as it is being generated on-site. CO2 emissions are reduced because the gas used provides both electricity and heat rather than just heat from a boiler, and grid electricity uses coal which has higher emissions.
Cogeneration and trigeneration are best suited to sites with a simultaneous year-round demand for heating / cooling and power, such as data centres, industrial buildings with high process loads, shopping centres and office buildings with large IT suites and server rooms (for example, trading floors).
Running costs vary dependent on the nature of site energy demands. Some installations cause a net increase in cost, while others can reduce costs.
Retrofit / improvement opportunities
Cogeneration and trigeneration systems require more plant room space than conventional gas boiler and chiller plant. If there is adequate plant room space then a retrofit is feasible. The plant room is likely to require an acoustic upgrade, as the generators can be noisy.
The building should have a high demand for heat and / or cooling if the cogeneration or trigeneration is to be cost-effective. For example, modern offices have a low demand for heat and a highly variable electrical demand, with the result that CHP may not be economically viable.
Floor plate implications
Noise levels can be a problem, so it is important to ensure that there is sufficient acoustic treatment to the plant room housing the cogeneration or trigeneration equipment.
Maintenance requirements are more frequent and complicated than for conventional plant. This extra cost can be an important factor when considering affordability.
The most common generator type is a large and very noisy gas engine. Turbine generators are also available which are much quieter and look like boxes, and are typically installed in banks of two or more.
Questions to ask
- Who maintains the cogeneration/trigeneration unit and at what are the costs of operation?
- How much, if any, of the energy generated is exported to the grid?
More references required