A fuel source (fossil or renewable) is combusted, and the energy produced drives a prime mover (reciprocating engine or gas turbine) that produces mechanical power and heat. The mechanical power is converted into electricity via an alternator. CHP systems generate electricity and heat simultaneously at the point of use, with the heat becoming usable to meet the site's requirements as illustrated below:
For some applications, a CHP system can achieve energy utilisation of up to 95%, supplying both electricity and heat more efficiently; typically, they meet between 70-80%, which is a significant improvement over importing grid electricity and conventional heat production. Improvements in energy production efficiency lower production costs, and when renewable fuels are used, they produce a lower carbon signature and lower carbon dioxide emissions per unit of energy produced.
Further power efficiencies are achieved because the cogenerated electricity and heat are produced and consumed on-site, avoiding the grid transmission losses of around 7.5% that occur in transporting electricity from remote generators to consumers.
Depending on a site-specific requirement, CHP can deliver improvements in energy efficiency, reductions in energy costs, and an improvement in the site's carbon signature. Cogeneration can save up to 30% on energy costs and offer a return on investment between 2 and 3 years. 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 degree Celsius, redirecting recovered energy to cooling rather than rejecting heat that is not otherwise needed. This is called Combined Cooling, Heat, and Power (CCHP), or Trigeneration, which can deliver cooling for air conditioning and refrigeration.
Where an organisation requires CO2 in its 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 option.
Quadgeneration refers to an advanced form of CHP that integrates additional equipment to process exhaust gases according to specific user needs. For example, in agricultural applications such as glasshouse growing, this may involve simply removing contaminants and cooling the exhaust gases. In contrast, industries like food and beverage manufacturing require more complex processing plants to produce ultra-pure CO2 (up to 99.99%) for use in their operations.
Several important considerations 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 be reduced 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. They must understand some of the project's underlying motivations, as well as the 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, unlike conventional power generation, which does not match base heat and power loads as efficiently. If a CHP plant can cover 100% of peak annual energy demand, the unit will run 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 generate only enough heat and power to meet a site’s base-load demand and produce electricity at the level the site consistently needs.
Power and heat from the CHP system can be supplemented with additional electricity from the grid and heat from existing boilers to meet peak demand 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 full load for as long as possible, maximising the economic benefits of self-generation and resulting in only minor energy waste. However, oversizing can also leave more unrecovered heat and some additional waste.
Although wasting heat makes CHP less efficient, the financial benefits of self-generated power can make it worthwhile to run CHP at a high electrical load, even if heat demand or process heat is relatively low, because it still generates heat that may be recovered and used.
This may only be true up to a certain point, however, because inefficient CHP systems risk missing out on tax incentives, such as 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 or unused thermal energy can be reduced by:
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 variations in demand at different times of day, during the week and across the year. Mapping these variations is crucial to finding the optimal level at which a CHP unit should operate at maximum output.
If you are sizing a CHP system for a new building, demand profiles can be estimated from 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 ensure this data accurately reflects real-world conditions.
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 ensure that 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 additional capital and operating costs and could render 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, 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 they 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 its composition and using the results 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 CHP plant more expensive.
This is often a straightforward procedure that can also examine other aspects of the installation, such as the height of the exhaust gas flue systems and the wider exhaust system, as part of the flue design and compliance review. A D1 calculation regulates this aspect and determines the flue height 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 are likely to have a serious impact on a local community, adjustments may be needed 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 issues for some plants near major cities – particularly those in or near London.
CHP installers 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, including the wider power systems and connection infrastructure affected by network review. The DNOs 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 Distribution Network Operator (DNO) by following the G99 connection procedure.
The Data Registration Code for the Distribution Code sets out the obligations of the generator and the Distribution Network Operator (DNO) to exchange data during the design process. It 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 the connection of generation assets to the UK and Ireland electrical networks, you should visit the Energy Networks Association (ENA), which is the industry body representing energy network operators in these territories.
Contact us about our CHP solutions
Interested in on-site CHP? Discover our approach to CHP installation, optimisation, and asset-care.
