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Organic Rankine Cycle Energy Efficiency Improvement for AD Plants

Energy efficiency improvement measures are a condition of Environmental Permits issued to Anaerobic Digestion plant operators. Organic Rankine Cycle generators recovering the high temperature exhaust gas from the Biogas electricity generators has proven a commercially viable energy efficiency improvement for several AD plants in the UK, until the recent changes in Ofgem permitting rules.

Since March 31st 2017 under the Renewable Obligation Certificates (ROCs) and March 31st 2016 under the Feed In Tariff (FIT) scheme, adding an ORC generator to an AD electricity generating plant is viewed by Ofgem rules as an increase in Total Installed Capacity (TIC) and no longer eligible for the incentives.

Typically recovering the otherwise exhausted lost energy from the Biogas generator, fitting an ORC generator can provide an additional 7 to 8% electricity to power the parasitic loads of a plant, improving the Net amount of electricity available for export to the grid.

Under the new Ofgem rules the extra amount of net electricity only qualifies for the low export tariff rate and not the sites agreed FIT or ROC rate, reducing a commercially attractive simple 3-year payback for an ORC energy efficiency measure to an unattractive investment of longer than a 7-year payback.

In France, a consortium of AD operators faced with a similar change in permitting rules demonstrated the integration of ORC technology onto the Biogas Generator as an “Energy Efficiency Improvement Measure” and set a precedent for projects to receive the AD tariff incentives. A similar approach by the AD market to the UK Government Regulators would lead to a continued roll out of ORC generators with good commercial returns for AD operators, whilst improving their environmental performance and satisfying the conditions of their Environmental Permits.

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