The role of small scale CHP in energy-efficient buildings

A 30% reduction in CO2 emissions and attractive payback on investment are just two attractions of small-scale CHP. Chris Marsland of Ener-G Combined Power Ltd takes up the story.

Small-scale combined heat and power (CHP) is widely regarded as one of the lowest-cost carbon-abatement technologies due to the significant primary energy savings it can achieve. In buildings with a large heating or cooling demand over extended periods, its energy saving potential is often unmatched.

Since CHP is suitable for new or refurbished buildings, where it can replace ageing boilers or reinforce existing heating supplies, it is also versatile.

CHP engines are most commonly fuelled by natural gas, which is converted into electricity at around 33% efficiency and heat at around 52% efficiency. A high efficiency gas CHP system will deliver an efficiency increase of up to 25% on the separate energy systems it replaces. Other fuel options include biogases, bio-liquids and bio-fuels.

CHP should be considered when:

• designing a new building or considering energy efficiency in general;

• installing or replacing a new boiler plant;

• reviewing electricity supply or standby electricity generation or plant;

• exploring options for building regulations compliance;

• reducing CO2 emissions and environmental impact.

CHP has many applications, but is particularly suited to leisure centres, hotels, hospitals, universities, military bases, prisons, manufacturing (particularly pharmaceuticals), commercial/municipal premises, horticulture, airports, data centres and district heating. Bio-gas CHP is used for wastewater treatment works and anaerobic digestion facilities, such as farms and dairies.

Benefits of CHP

When a building's heating/cooling and power demand are suited to CHP, payback on investment is typically three to five years, delivering energy cost stability and savings over an average asset lifespan in excess of 15 years. CHP systems can reduce CO2 emissions by up to 30%.

There are also tax benefits, including reduction or exemption from the Climate Change Levy and access to Enhanced Capital Allowances — for 'good-quality' schemes accredited by the CHPQA.

Third-party or supplier funding options, such as the Ener-G discount energy purchase scheme, also provide the option of outsourcing the CHP system and gaining energy saving benefits without any capital outlay.

CHP contributes significantly towards legislative compliance with part L2 of Building Regulations, while providing BREEAM assessment points. When configured in island mode, it can also reduce grid dependency to improve security of supply.

Ener-G Combined Power, CHP, Energy efficient building systems, energy efficiency
This CHP installation at Manchester’s historic town hall and library comprises two units, each with an electrical output of 230 kW. Heat is rejected to an existing absorption chiller to provide 24-hour cooling to the ICT communications rooms.

CHP feasibility

There are three main stages in considering the feasibility of a CHP system.

1. Data collection — using utility consumption data for the site to gain an accurate picture of the baseload. It might be beneficial to use temporary metering to better understand heat and power demands. In new buildings, it is possible to use building design data, simulation modelling of building, benchmark profiles from comparable buildings, occupancy patterns and data from energy models.

2. Initial desktop feasibility study — evaluating validated data, taking into consideration the 'spark spread' (the difference between the input natural-gas price and electricity tariff). It's generally accepted that a minimum spark-spread of 2.5 is required to make a scheme economically viable. Future changes in consumption patterns that could affect baseload should also be factored in — such as energy-efficiency measures, expansion or contraction of facilities, changes in processes, operation or occupancy.

High electrical and thermal utilisation (particularly the latter) is essential to maximise efficiency, so it is necessary to understand the minimum energy demands during the running period, as well as the maximum demands. The large differences between night and day and winter and summer demands must also be understood and considered.

3. On-site review — this should further assess suitability by considering location, gas availability, space allocation, planning implications, noise issues; local regulations, maintenance restrictions, electrical connections (i.e. LV, HV and network restrictions) and thermal integration. This is the start of a detailed feasibility review to ensure suitability and compliance.

Plant optimisation

Sizing is critical to efficiency and to ensure the system is certified as 'good quality' by the CHPQA. If the selected CHP unit is too small then the maximum savings won't be delivered. If it's too large, it will be operating inefficiently at part-load, have fewer run hours and lower utilisation figures.

It is rare to achieve a continuous match in heat and power demands, so the planned operating strategy may require additional heat from conventional boilers, or a heat-rejection facility, scope to import or export power and modulation of CHP output. Various operating strategies should be considered to achieve perfect optimisation — e.g. is part-load operation or heat rejection preferable to exporting power? Is night time operation worthwhile? Is thermal storage beneficial?

Cooling demand is unlikely to determine CHP capacity, so the general approach would be to assess how much surplus heat is available in summer months and whether this matches. It is also necessary to estimate the baseload cooling demand profiles for optimum efficiency of the absorption chiller.

Chris Marsland is technical director for Ener-G Combined Power Ltd. Ener-G has published a detailed new 'Essential guide to small scale combined heat and power', which is available free to download from the link below.

About the Author:

Chris Marsland, BEng (Hons) CEng MIET, is the Technical Director for ENER-G Combined Power Ltd, which is Europe’s leading provider of small-scale CHP systems.

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