Gaunts House Aerobic Digester Project

Location: Gaunts House, Petersham Lane, Wimborne, Dorset, BH21 4JQ
Type of Low Carbon Technology: Agricultural Anaerobic Digester
Project Contact: Derek Moss

Overview of project: The Gaunts Estate installed a Biogas Plant on its grounds in Wimborne. The Biogas Plant is a mesophilic (relatively cool) agricultural anaerobic digester (AD) fed with slurry, manure, maize, and hybrid forage rye.

Biogas is used to fuel a combined heat and power (CHP) engine, with electrical capacity of 150kW, and a useful thermal capacity of 213kW.

Electricity is used in the 5 buildings of the Gaunts House campus, with any excess exported to the grid. The private-wire network connecting the buildings is capable of operating in island mode without the grid.

The heat is used to warm the biogas tank, and to provide heating and hot water to the two largest buildings of the Gaunts House campus via a district heating scheme.

The output digestate is stored in a lagoon and then spread on the fields surrounding the digester, providing fertiliser for the grass eaten by the slurry-producing cows, and for crops.

“This system meets our heat needs, provides us with electricity without fossil fuel guilt, and helps fertiliser the fields. Despite the problems, it’s awesome!” Derek Moss, Project Manager

Suppliers of technology installed:

  • Biogas Plant (agricultural anaerobic digester) supplied by Evergreen Gas
  • Combined Heat & Power (CHP) Engine supplied by Shentons
  • District Heating Scheme designed and supplied by Pipework Energy Solutions

How the project was developed: A detailed study of how to make the Gaunts Estate energy self-sufficient and zero carbon within planning constraints was conducted, and settled on an AD plant.

It took over 4 years to get from this decision to commissioning the system. Planning constraints discouraged a large AD plant at the Estate’s dairy, and so the possibility of several smaller plants was pursued. Planning permission, environmental permits and grid connection agreements were achieved for four small AD plants, the Gaunts House plant being the largest capacity.

Careful analysis went in to sizing the plant to best meet the local heat demands and local fuel supplies, rather than maximising electricity generation capacity.

How the technology was installed: Installation meant the clearing of a field, the construction of access track, and a year of fevered work to secure FiT subsidy at the level expected. As well as the main digester tank, a silage clamp and digestate lagoon, needed to be constructed. SSE installed a new transformer to allow grid access, and extensive trenches were dug to bury the pipes and cables which distribute the heat and electricity from the engine.

How the technology is run and maintained: AD is one of the more demanding of the renewable energy technologies in terms of ongoing attention. A member of staff is employed to operate the plant, with daily activities including feeding, cleaning, monitoring, and preventative maintenance.

The fact that an AD plant features many working parts means that time is inevitably spent fixing malfunctioning items (pumps, sensors, etc).

Issues experienced with the technology, its installation, and the ongoing maintenance: The main issue experienced was the liquidation of the technology supplier (Evergreen Gas) before the plant had reached full operation. This was largely due to flaws in the design of the mixing system. Fixing this flaw across the many systems they had sold incurred more costs than the company could absorb.

The system that was installed at Gaunts House suffered from this flaw. Commissioning was completed without help from the supplier, and numerous construction mistakes were fixed by the operators, who managed to make the plant work at a low capacity with the flaw. The operators, with help from the owners of other ex-Evergreen plants, designed a fix to the flaw, which will be installed shortly, allowing an increase to full production.

Benefits of the technology: The plant provides zero-carbon heat and electricity to the charity based at Gaunts House, zero-carbon electricity to the local grid, and non-fossil-fuel fertiliser to the farms of the Gaunts Estate.

Total annual carbon savings are around 750 tCO2e/yr plus the savings from reduced fertiliser use.

Cost of technology (including installation and ongoing maintenance): The entire site, including the biogas plant, the CHP engine, the silage clamp, the lagoon, the grid connection and the heat and power networks cost around £900k.

Payback is expected in 12 years.

How was the project funded? The project was funded by a private loan, which in turn is being paid back with help from the Feed-In Tariff (FIT) for the electricity generated and the Renewable Heat Incentive (RHI) for the heat.

Lessons learned from this project: Caution should be taken in the procurement process, companies can appear to be highly professional, with an excellent technical pedigree, but flaws in designs can lead to substantial project failures.

We, like many others, made the mistake of investing in a slightly experimental system, attracted by prices lower than the market’s main players.

However, potential AD investors should not be put off by our problems; AD can, in the right places, provide energy generation which fits in well with demands and fuel supplies.

Blue lines on the photos below show the electricity cables. Red lines show the heat pipes.

  • Date March 9, 2018
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