The Key Design Considerations You Need to Make When Specifying Combined Heat and Power (CHP)

Posted by Chris Marsland on 04-Apr-2017 11:00:00

Making the right decision in the specification of a Combined Heat and Power (CHP) system will determine success. Here are the points you'll need consider at design stage.

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While the final specification for every CHP system should be tailored to the specific requirements of the project, there are a number of universal design considerations that  are key, and must always be addressed. We’ve laid them out here.

Sizing the CHP unit

Matching the thermal and electrical capacity of a Combined Heat and Power (CHP) plant to the demands of the building is paramount. The aim should be to have the CHP unit running for at least 4,000-5,000 hours per year, ensuring maximum financial benefit.

Sizing the unit requires detailed analysis of the thermal and electrical loads on the project, looking at patterns on a half-hourly, daily, weekly and monthly basis across the year’s seasons to gauge the fluctuations of heat and electricity demand.

There may be times when demand for electricity outstrips the demand for heat. If you are considering an electrically lead design i.e. electricity is the priority then it may be acceptable to continue generating it while dissipating or “trimming”some unwanted heat through something akin to an air blast radiator. However, heat trim may only be financially viable when grid electricity costs are high compared to gas costs, and it will of course increase the CO₂ emissions associated with the project.

An alternative strategy for taking up spare thermal capacity when electrical demand is high is to use thermal stores, which is considered as part of a thermally lead design i.e. satisfying the thermal load on site is prioritized over the electricity. These large water storage cylinders a.k.a thermal stores (holding tens of thousands of litres) can be heated during periods of low site thermal demand then used at times when heat demand is higher. This enables the CHP unit to continue its operation and therefore maximise it’s run hours.

Also, the heat trim and and thermal store strategies can be used in harmony to maximise efficient CHP operation.

CHP and other systems

The relationship between the CHP unit, other heat generators and the building management system is critical. The system must be configured so the CHP unit acts as the lead heat source and so the boiler acts as a backup heat source. The boiler would then only fire if the demand can’t be satisfied by the CHP alone.

The heating and hot water distribution systems must be designed to ensure the return temperatures will be low enough to keep the CHP firing. In new buildings this should not be problematic, but when retrofitting CHP into existing buildings a careful analysis of the heating load will be required in order to provide suitable return temperatures.

Grid connections

To ensure safe operation of the grid, any CHP unit needs a G59 approval from the District Network Operator (DNO) before it can be connected to the mains supply, irrespective of whether or not electricity will be exported. G59 approval is rarely refused, but may require an improved grid connection for the site.

Spatial requirements

CHP units are bigger than gas boilers of the same thermal capacity and need more space for maintenance, typically 1m around and above. There must also be provision for removing the engine block from the unit for off-site overhaul, which is likely to be required at least once during its service life, however CHP suppliers should be able to advise of options when space is tight. For all the CHP engines, care must be taken to ensure sufficient ventilation air is allowed within the plant room. A nominal volume of air of will also be required for combustion within the CHP engine.

Acoustic impact

Generally, a CHP unit comprises of a natural gas fired reciprocating engine.These engines tend to create some noise. Therefore, it is essential to surround the CHP in an  acoustic enclosure. This enclosure will reduce any noise emitted to levels well below the acoustic regulations of the site. For a packaged unit, an acoustic enclosure can be used to reduce airborne sound transmission down to as low as 65dBA@1m. However, CHP units installed within a building like a residential development or a hospital may require more stringent acoustic treatment of the enclosure; that is, any ventilation or exhaust ductwork may also need to be insulated.

The transmission of sound and vibration through the building fabric can be attenuated by isolating the unit with resilient materials or anti-vibration mountings, and using flexible connectors at key points so there is no transmission to the building.

A specification that properly addresses all the above issues will provide a good basis for a successful CHP installation.

Takeaways:

  1. Analysing the thermal and electrical loads of your project and the patterns of half-hourly, daily, weekly and annual energy demands is crucial to the optimal sizing of your CHP system.

  2. Configuration must work in conjunction with other systems in the building.

  3. CHP unit needs G59 approval from the District Network Operator (DNO) before it can be connected to the mains supply, irrespective of whether or not electricity will be exported.

  4. Take spatial requirements into account - CHP systems can be bigger than gas boilers, requiring more space for maintenance and to be accessible for off-site overhauls.

Depending on the model and sizing of your CHP and the building it is being installed in, specific acoustic and vibrational treatment may be necessary.

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Topics: CHP / Cogeneration

Chris Marsland

Chris has worked in the CHP industry since 1998, before that he spent 12 years heading up the Product Development Team in an industrial process control company. He is a Chartered Engineer and a member of The IET. His role at ENER-G covers CHP New Product Design, Development and Production as well as supervising ENER-G’s R&D team.