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Retrofitting refers to the addition of new technology or features to older systems.
- power plant retrofit, improving power plant efficiency / increasing output / reducing emissions
- home energy retrofit, the improving of existing buildings with energy efficiency equipment
- seismic retrofit, the process of strengthening older buildings in order to make them earthquake resistant
Power plant steam turbine retrofits 
Retrofit projects replace or add equipment to existing power plants to improve their energy efficiency, increase their output and extend their lifespan, while decreasing emissions.
Benefits of a retrofit 
- Saving on capex while benefiting from new technologies
- Optimization of existing plant components
- Adaptation of the plant for new or changed products
- Increase in piece number and cycle time
- Guaranteed spare parts availability
Principally retrofitting describes the measures taken in the manufacturing industry to allow new or updated parts to be fitted to old or outdated assemblies (like blades to wind turbines).
The production of retrofit parts is necessary in manufacture when the design of a large assembly is changed or revised. If, after the changes have been implemented, a customer (with an old version of the product) wishes to purchase a replacement part then retrofit parts and assembling techniques will have to be used so that the revised parts will fit suitably onto the older assembly.
Environmental management 
The term is also used in the field of environmental engineering, particularly to describe construction or renovation projects on previously built sites, to improve water quality in nearby streams, rivers or lakes. The concept has also been applied to changing the output mix of energy from power plants to cogeneration in urban areas with a potential for district heating.
Sites with extensive impervious surfaces (such as parking lots and rooftops) can generate high levels of stormwater runoff during rainstorms, and this can damage nearby water bodies. These problems can often be addressed by installing new stormwater management features on the site, a process that practitioners refer to as stormwater retrofitting. Stormwater management practices used in retrofit projects include rain gardens, permeable paving and green roofs. (See also stream restoration.)
See also 
- Green retrofit
- Aftermarket kit
- Diesel particulate filter (DPF)
- Electric vehicle conversion
- Scrappage program
- Software patching