The Physical Factors to Consider When Specifying Your Combined Heat and Power (CHP) Project

Posted by Ian Hopkins on 25-Apr-2017 10:00:00

Develop an understanding of client drivers and project demands, as well as the physical constraints of the project site to deliver successful Combined Heat and Power (CHP) projects.

The essential principles of physical parameters surrounding your Combined Heat and Power (CHP) project.jpgThe development of a good specification requires a thorough understanding of the client’s requirements, a detailed analysis of their heat and electricity use profiles and careful consideration of the physical constraints of their site.

A generic ‘copy and paste’ specification developed without an in-depth understanding of client drivers and project demands will result in an inadequate CHP plant. Here, we detail what is required for the development of an optimal system specification.

Understanding client drivers

The client’s reasons for adopting Combined Heat and Power (CHP) must guide development of the specification. In some cases the priority will be electricity generation, particularly if there is a potential for exporting surplus electricity to the grid for sale. In other cases CHP systems will be installed to reduce CO₂ emissions and comply with building regulations or voluntary standards, such as the Building Research Establishment Environmental Assessment Method (BREEAM).

The sizing of the CHP unit will require more detailed analysis of heat and electrical demand than can be provided by SBEM and other compliance software, and will also be affected by the client’s priorities. For example, where electricity generation is the priority, it may be desirable to include a heat rejection unit in the system, to enable generation to continue even when there is no or little demand for heat.

Physical parameters

As well as defining the capacity and configuration of the CHP unit, the specification must also address the physical parameters of the CHP on site, most notably: connections to the grid and gas networks, the impact on the building and its immediate environment, and provision for maintenance.

Network connections

A CHP unit can only be connected to the grid with the permission of the Distribution Network Operator (DNO), irrespective of whether electricity will be exported. A G59 application to connect should be made well before the CHP unit is to be commissioned. It is not so rare for an application to be rejected outright, but approval may require an improvement of the existing grid connection which can be costly and a lengthy process.

The gas supply must have a high enough pressure and capacity for the CHP engine. Where the existing site pressure is too low, a pressure booster can be used to raise it. Alternatively,  arrange to have the gas supplied at a higher pressure, which would be the more cost effective option.

The impact on the building and its environment

NOx emissions from on-site generation must be managed to prevent problems with local air quality. Where analysis indicates dispersal will be insufficient, advanced catalytic treatment can be used to keep emissions within permitted levels.

CHP units generate more noise and vibration than gas boilers of a similar capacity. The level of airborne sound transmission can be reduced by acoustic insulation to the CHP unit and to its intake and exhaust ductwork. Transmission of sound and vibration through the fabric should be controlled by anti-vibration mounts and flexible connectors for ductwork.

Access for maintenance

CHP units typically require five or six maintenance visits per year. To facilitate maintenance there should be a minimum of 1m free space on all sides of the unit and above it.

Measuring success

A successful specification will result in a CHP installation that runs at full capacity for the maximum number of hours (while still allowing for maintenance time) and provides the expected rate of return on investment, with a suitably short payback period.


  1. Developing the best system specification possible requires a thorough understanding of client drivers and project demands, as well as knowledge regarding their energy usage and the physical constraints of the project site.

  2. The client’s priorities (electricity generation, reduced emissions, connections to gas network etc) must inform the sizing and specification of the CHP.

  3. The location and physical parameters the CHP must be chosen correctly to ensure optimal safety, easy maintenance access and accessibility to the local grid or gas networks.

  4. Ensure you take the appropriate and timely steps to connect to your local energy network, such as ensuring gas is supplied at the right pressure and capacity, and making a G59 application to the DNO to permit grid connection.

  5. A successful specification will allow you to run at full capacity for the maximum number of hours while still allowing time for maintenance, and will also provide the expected returns in a suitable payback period.

After fully considering the physical factors affecting your CHP project, incorporate this information into writing the highest standard of CHP specification possible. Read The ENER-G Quality CHP Plan.

Download: The Ener-g high Quality CHP Plan

Topics: CHP / Cogeneration

Ian Hopkins

Ian Hopkins is a technical sales professional and business leader with more than 15 years’ experience in delivering energy efficiency projects and strategy in Europe and the United States. Ian currently heads up the Sales and Marketing function as one of the board directors at ENER-G Combined Power Ltd.