Combined Heat and Power(CHP)
Technical
Combined heat and power (CHP, also known as cogeneration) can provide thermal energy for buildings or processes, while simultaneously generating part of the electricity needed at the site. CHP or cogeneration is the sequential production of two forms of useful energy from a single fuel source. It takes advantage of the heat rejected in the thermo-dynamic conversion process from primary fuel to power and heat supplied for useful purposes.
A CHP system recovers the heat from electricity generation for productive uses such as heating, cooling, dehumidification, and other processes—heat that is usually wasted at conventional power plants. And because the electricity is generated near the point of use, it is subject to fewer transmission losses than electricity supplied by distant central power plants. For these reasons, properly designed CHP systems can be more than twice as efficient as the average U.S. fossil fuel power plant. Growing numbers of federal facilities are turning to CHP technologies to gain greater control, reliability, supply quality, and flexibility in their power systems, as well as to cut costs and to meet federal energy efficiency and emissions reductions goals.
The conversion of fuels to electricity produces large quantities of waste heat as a by-product, which conventional power plants simply reject to the environment. There has been an upsurge in interest in fuel-efficient distributed energy resources such as CHP among project developers, facility managers, and policy makers, because these systems help to mitigate some key power sector constraints. For example, CHP systems decentralize power generation to locations near facilities having thermal requirements that can be met with waste heat. Also, CHP systems are potentially 70% - 85% efficient in utilizing fuels. And they can meet increased energy needs, reduce transmission congestion, increase power quality and reliability, and increase the energy security of a facility.
Micro CHP - there are a small number of examples of micro CHP serving small groups of dwellings and small commercial applications, typically providing around 5 kWe output and 10-15 kW heat. Smaller units of around 1 kWe based on Stirling engines are planned for the market.
Small scale CHP - is most commonly retrofitted to existing building installations although CHP can prove to be even more beneficial in new buildings. Small-scale plant has electrical outputs of up to about 2 MWe, and usually comes as packaged plant, often based on gas-fired reciprocating engines, with all components
assembled ready for connection to a building's central heating and electrical distribution systems. Small gas turbines and micro-turbines are also available within this size band.
Large scale CHP – generally above about 2 MWe is designed specifically for each application. Larger multi-building installations (e.g. hospitals, universities) and community heating use either gas turbines or large reciprocating engines, fuelled by gas or oil. Gas turbines are favoured when high grade heat is required for steam raising. However, large gas turbines are more complex to maintain, have lower electrical efficiencies (up to the size range covered by this guide) and have a poorer efficiency at part load than engine based CHP. Community heating with CHP is a particularly efficient means of supplying large portfolios of domestic and/or commercial properties;
When to consider using CHP
The best time to consider CHP in existing buildings is when the heating plant is being
replaced, as the CHP can be more easily integrated with the new heating system. To achieve the greatest potential from CHP, it should always be considered at an early stage when:
• Designing a new building
• Installing new boiler plant
• Replacing/refurbishing existing plant
• Reviewing electricity supply
• Reviewing standby electrical generation capacity or plant
• Considering energy efficiency in general CHP should be considered right at the start of
a project, when a full option appraisal should nbe carried out.
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