Northern Ireland's top auditor is to begin an investigation following allegations about another green energy scheme in NI.
It is understood the comptroller and auditor general, Kieran Donnelly, will launch the inquiry in February over payments to anaerobic digesters.
They use organic material like slurry and silage to create a biogas which is burned to produce electricity.
Scores of them have been built in Northern Ireland in recent years.
The subsidies paid in Northern Ireland are more lucrative than they are in Great Britain.
Earlier this year, a BBC Radio 4 documentary made allegations about the operation of some of them.
It claimed that in a number of cases, the generating facilities were "phantom plants" - an allegation denied by the companies named.
Mr Donnelly's investigation will focus on the wider system of support payments for renewable energy called Renewable Obligation Certificates.
But it is understood it will drill into the issue of AD plants in particular, given the issues raised.
What is anaerobic digestion?
- Process by which micro-organisms break down biodegradable material in the absence of oxygen
- Organic material such as manure, crops, grass or slurry is put into large containers
- Once this material breaks down it produces biogas such as methane
- Methane can be converted and fed into the National Grid
The payments to companies which generate electricity from renewables come from a subsidy on electricity bills rather than direct from the public purse.
But because both the Economy Department and the Utility Regulator have a role, any inquiry falls within Mr Donnelly's remit.
In some cases, the finances for the construction of AD plants in Northern Ireland has been provided by London-based venture capital companies.
Farmers on whose land they are built are then paid a fee to feed and run the facility, with the lucrative subsidies reverting to them after a decade for the remainder of the 20-year term.
In several cases, concerns have been raised that plants have been built and run in breach of environmental and planning guidelines.