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Combined Heat and Power (CHP) or Cogeneration (Cogen) is a well-established technology that simultaneously generates electricity and heat from a fuel input.
Cogeneration can save up to 30% on primary energy costs when compared to the separate purchase of electricity from the electricity grid and gas for use in on-site boilers.
CHP has been used for many years all over the globe across different industries in several forms (steam boilers and steam turbines, gas turbines, steam boilers and steam turbines, reciprocating engines and heat recovery systems). The arrangements selected are generally detected by the available fuel source, market sector and scale.
In the UK following the privatisation of British Gas in 1986 and the dash for gas in the 1990s, CHP in industry grew rapidly with the installation of gas turbines and steam boiler plants, and within the commercial sector and hospitals with the installation of gas engine plant. Today, due to the reduction in plant size and the requirements for greater operational flexibility gas engine CHP has become the leading technology of choice.
With the drive towards a sustainability zero carbon economy all the major manufactures have biogas and biomethane green gas engine variants and are future proofing their technology to use hydrogen (mixed or pure).
CHP is a proven technology and can deliver generation efficiencies up to 95% utilising low carbon natural gas as a fuel source. As the gas grids become green, the carbon signature of CHP will reduce. However, CHP can use zero-carbon fuels directly to deliver zero or a negative carbon position.
When businesses are planning the building of new facilities or improvements to existing, they should always consider CHP or Trigeneration, known also as Combined Cooling Heat and Power (CCHP). CHP can play a valuable role to support business to deliver a sustainable, secure, and flexible energy solution.
A fuel source (fossil or renewable) is combusted, and the energy produced is used to drive a prime mover (reciprocating engine or gas turbine) which produces mechanical power and heat. The mechanical power is converted into electricity via an alternator and the heat is available to use directly or converted to meet the requirements of the site.
For some applications a CHP system can achieve energy utilisation of up to 95%, typically they meet between 70-80% which is a significant improvement over importing grid electricity and conventional heat production. The improvement in the efficiency of energy production lowers the cost of production and where carbon fuels are being used as fuel it produces a lower carbon signature per unit of energy produced.
Further power efficiencies are achieved because the cogenerated electricity and heat is produced and consumed on-site, avoiding the transmission losses of around 7.5% that occur in transporting electricity from remote generators to consumers.
The price differential between, and depending on a site particular requirement, CHP can deliver improvement in energy efficiency, reduction in energy cost and an improvement in the site carbon signature. Depending on the fuel sources used (e.g biogas, hydrogen) CHP can deliver a carbon neutral or negative position.
Where sites have little demand for heat but require cooling for process, users can introduce absorption cooling into their CHP plant which can operate at temperatures as low as -5 deg C. This is called Combined Cooling Heat, Power (CCHP), or Trigeneration which can deliver cooling for air conditioning and refrigeration.
Where an organisation has a requirement for CO2 in their processes (glasshouse growers, carbonated drinks industry, food processing industry, brewing industry) the production of CO2 from the combustion of carbon-based fuels can be an economical consideration.
The term Quadgeneration has been given to CHP with the inclusion of equipment that takes the exhaust gases and processes them to meet the user requirements of the particular use. For a grower, this may be simple cleaning contaminants from the exhaust gases and cooling the gases, whereas for the food and drink industry requiring 99.99% pure CO2 this will require a more sophisticated processing plant.
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There are several important considerations that are important to maximising the potential benefits of a CHP project. These include:
We’ll consider each of these issues in turn and explain how you can make the most of CHP at the site.
The energy efficiency and cost savings associated with CHP can reduce if a project is not sized correctly. Make your scheme too big, and it won’t run at maximum efficiency. Make it too small, and you could miss out on the financial benefits of maximising low-cost self-generated power.
CHP specifiers need to consider several variables before selecting an appropriate engine. It’s important they understand some of the underlying motivations behind the project as well as cost and carbon requirements.
If your motivation is to maximise the CHP system efficiency, there is no need to size a plant for the highest peak electricity or heat demand. If a CHP plant can cover 100% of peak annual energy demand, then the unit will be running at reduced capacity for most of the year.
CHP generators are less efficient when operating at a reduced load.
Running at maximum efficiency, a CHP unit will only generate enough heat and power to meet a site’s base load demand.
Power and heat from the CHP system can be topped up with additional electricity from the grid and heat from existing boilers to meet peak demands and optimise performance.
From a commercial (rather than a pure efficiency) perspective, the optimum engine size would provide slightly more than the base load. It would be sized to run at a full load for as long as possible, maximising the economic benefits of self-generation and resulting in only minor energy wastage.
Although wasting heat makes CHP less efficient, the financial benefits of self-generated power mean that it can be worthwhile to run CHP at a high electrical load, even if heat demand is relatively low.
This may only be true up to a certain point, however, because inefficient CHP systems risk missing out on certain tax incentives like an exemption from the Climate Change Levy and eligibility for the Annual Investment Allowance if they do not meet CHPQA efficiency standards.
For some sites, the amount of wasted heat can be reduced by:
Correctly sizing a CHP system means closely matching a unit to a site’s energy load. This requires careful analysis of a site’s energy demand over time.
Energy consumption data can come from several sources. To build a clear picture of energy consumption, it is preferable to use half-hourly metering data or information from a building energy management system.
Plotting electricity and heat consumption over time will produce a pattern showing how and when energy is used. It will also show up variations in demand at different times of day, during the week and across the year. Mapping these variations is crucial to finding the optimum level at which a CHP unit should be operating at maximum output.
If you are trying to size a CHP system for a new building, then demand profiles can be estimated using a range of sources including design data, occupancy patterns, simulation modelling and benchmark profiles from comparable projects.
Using inaccurate energy data is one of the biggest causes of under or mis-sizing a CHP project, so take care to make sure this data is a true representation of real life.
Natural gas is the main source of fuel used in CHP systems in the UK and Ireland. It has a high heat value, so is ideally suited to on-site power generation, but each CHP project will have a number of considerations based on a site’s available gas supply.
A reliable gas supply is critical to success. Mains gas pressure varies significantly across the UK and Ireland. In the project planning phase, specifiers must make sure the local gas supply network can meet the pressure demands of a CHP engine.
Gas compressors can provide a pressure boost if the local supply is too weak, but this will incur extra capital and running costs and could make on-site gas generation unsuitable.
Gas-fired CHP will significantly increase a site’s natural gas demand. If the installed network cannot cope with the additional volume demand, then the supply infrastructure may need to be upgraded.
If a site connection cannot meet the new peak requirement, it may need to be updated, or a separate gas supply may need to be opened up to serve the CHP plant. Any supply issues will need to be raised with the local gas distribution network.
The quality of natural gas is regulated, but composition can vary from location-to-location. Small variations in quality are not usually sufficient to affect on-site gas generation, but it could have a small impact on efficiency. Over time, this small discrepancy will need to be factored into output calculations.
Specifiers can judge the quality of gas at a site by measuring the composition of gas and using it to calculate quality parameters, like calorific value.
A new energy centre, containerised gas engine or existing building CHP installation may need to be approved by the local authority. Planning permission can be easier or more difficult to obtain based on the site’s local area.
If a site is in a residential area, or close to a major city, the building design may need to be altered to reduce the impact on the local community. In more industrial areas away from large population centres, planning permission is generally easier to attain.
The noise a CHP unit makes is a key consideration, particularly when there is a residential dwelling nearby.
The impact of noise can be reduced by situating a CHP unit as far away from residences as possible. Alternatively, noise abatement solutions can be added to an energy centre or containerised gas engine, which can make the units more expensive.
This is often a straightforward procedure that can also look at other aspects of the installation such as the height of the exhaust gas flue systems. This aspect is regulated through a D1 calculation and dictates the height of the flue requirement.
Local gas dispersion may also cause an issue with a local authority. In some cases, dispersion modelling must be carried out to discover how dispersion will affect the area.
If local pollution levels are particularly high, or prevailing winds will have a serious impact on a local community, adjustments may need to be made to the exhaust gas flue system.
The Medium Combustion Plant Directive (MCPD) places a limit on the amount of nitrogen oxide (NOx) released from combustion generators in the 1-50 MW range in England and Wales. This can cause an issue for some plants close to major cities – particularly those in or near London.
CHP providers will ensure that all installations meet the MCPD restrictions. In some cases, selective catalytic reduction technology can be used to reduce emissions, though this can increase CHP running costs.
A Distribution Network Operator (DNO), or a Distribution System Operator (DSO), is a company responsible for distributing electricity throughout the UK and Ireland. The DNO own and operate the system of cables and infrastructure that transport electricity from the national transmission network to homes and businesses.
To connect your CHP plant to the electricity distribution network, you must apply to the local Distributed Network Operator (DNO) by following the G99 connection procedure.
The Data Registration Code of the Distribution Code sets out the obligations on the generator and Distribution Network Operator (DNO) to exchange data as part of the design process and lists the data items that may need to be exchanged. The purpose of completing the application process is to simplify and clarify this data exchange process.
For guidance on connection of generation assets to the UK and Ireland electrical network you should visit the Energy Networks Association (ENA), who are the industry body representing energy network operators within these territories.
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