What is Combined heat and power (CHP)?
Combined heat and power (CHP) generates electricity by burning fuel, and captures the usable heat that is produced in the process. Essentially, it is a system that is able to produce both electricity, and heating. Conventional electricity generation creates large amounts of heat that are wasted by being released into the atmosphere (usually seen as steam rising from cooling towers). CHP reduces carbon emissions considerable when it is designed effectively.
The CHP system harnesses the heat, reducing wastage and making a more efficient use of the electricity generation process. CHP is a highly efficient process, achieving efficiency rating of around 80%. Traditional gas power stations achieve around 50% efficiency, while coal fired plants only achieve around 38%.
How does CHP work?
CHP can use different fuels in the energy generation process, including fossil fuels and renewable fuels such as biofuels.
CHP is a technology that is used on a local scale. The energy produced is distributed to adjacent communities or buildings. Well insulated pipes underground, distribute the energy to local houses, and commercial buildings.
CHP can be employed on a micro or macro level. It can be used in large community settings (macro CHP), or on individual buildings (micro CHP). A key factor of CHP is that it must be producing energy near to the point of use.
CHP is only suitable where there is a year round demand. Buildings such as schools, hospitals, leisure centres, university halls, care homes and similar settings benefit from CHP.
CHP can offer carbon savings of between 5% and 20% depending on size and application. The system is still developing and not widely used yet. As the grid becomes more efficient using renewable fuels, the benefits of the CHP system will begin to fall.
Design considerations for Combined Heat and Power (CHP)
CHPs are best used where there is a high demand for space heating and hot water, usually at least 12 hours per day, throughout the year.
CHP can be unsuitable for new housing developments and buildings, as well insulated constructions have a lower heat demand, which means the CHP system is not as efficient. High demand buildings are preferable.
Storage must be considered, as CHP requires a thermal store for the heat that is generated. This needs to be well insulated, and can be situated below ground.
CHP tends to use gas as a fuel, although greener fuels are available, they are still in the early stages of development. Using biofuels will require further storage for the fuel, which must be considered.
CHP is noisy, so it needs to be installed in an acoustically separated room to avoid noise pollution and disturbance.
Micro CHP can be used for single buildings that have a large heating demand throughout the year. This is usually buildings such as leisure centres, care homes, nurseries, and other similar buildings. When used on single dwellings, the system must be running for a minimum of 4 hours a day. CHP is not suited to dwelling that have been designed with good insulation (zero carbon standards), as these buildings will have very low heat demand.
It is said that local generation of electricity is cheaper than buying off grid, and any excess can be sold back to the grid.
The larger the CHP the more efficient. Macro CHP is used for large non domestic buildings, or mixed use developments. The heating demand must be at least 12 hours a day, so it is suited to larger care homes, hospitals, leisure centres, swimming pools, university halls, hotels and other similar buildings. They are often used as part of a community or district heating strategy.
The system requires a back up boiler in case the CHP fails. As with the micro CHP, renewable fuels are also possible, but remain in the early stages of development. There are also issues with biomass supply, and fuel storage needs to be considered also.
Hot water storage must be included in the design of the CHP system, which is usually around 2-4 litres per m2of development.
Who is using CHP?
CHP is being used at Centre Paris in Woburn Forest, and has saved £200,000 and 900 tonnes of carbon a year.
CHP is being used by 40 London fire brigade stations, at one station is has reduced 13 tonnes of carbon and saved the fire service £2,500 in energy costs per year.
Rotherham Hospital has a CHP system which has saved them £273,000 and 1,300 tonnes of carbon.
Over 50 care homes are now using CHP and are seeing savings of around £15,000 per year and 6 tonnes of carbon.
Combined cooling heat and power (CCHP)
Trigeneration is the use of the standard CHP system, but it also has the capacity to provide cooling. This is considered to be a cost effective and low carbon way of providing a buildings heating and cooling needs from one system.
Environmental Design Pocketbook – Sofie Pelsmakers
Rough Guide to Sustainability – Brian Edwards