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‘Reverse fridge’ is harnessing river water to heat historic mills and cut carbon emissions

Published: Thursday, 27 January, 2022

River water is once again powering historic mills – but this time cutting-edge green technology is using it to heat Stroud District Council's offices.

As part of the council’s commitment to net carbon zero emissions, gas boilers have been ditched and new water source heat pumps have been installed at its Ebley Mill headquarters and the council’s Brimscombe Port Mill.

The council is well-known for its green credentials but is not resting on its laurels and wants to showcase this new technology to help set an example of how to emit less carbon.

“One of our three main priorities in the new Council Plan is environment and climate change, with an objective of protecting our environment, and leading the district to carbon neutrality in 2030,” said Council Leader, Doina Cornell.

“This new way of heating these buildings is an important step towards meeting that target.”

The council instructed a detailed feasibility study to establish the most effective methods to reduce carbon footprint. Blessed with abundant natural resource in the nearby River Frome, the decision was made to commission water source heat pumps at both Ebley and Brimscombe Port Mills to provide heating and hot water using heat energy from the River Frome.

Both mills date from the early 19th Century and are Grade II* Listed.

Cotswold Energy Group was awarded the contract to design and install a suitable system, working in conjunction with another Stroud company (Renewables First) to obtain consents from the Environment Agency.

Heat pumps contain a refrigerant fluid which boils at an extremely low temperature – when compressed and cycled, extracting even 2 degrees centigrade from the river water is enough to create 21C of ambient temperature inside the buildings, delivered through the radiators.

The “closed loop” system installed at Brimscombe uses sealed pipes submerged below the river water level and filled with a sealed circuit of brine (a mix of environmentally friendly anti-freeze fluid and water). As the brine is pumped in a circuit between the river and the heat pumps it gathers heat energy from the river water. Using refrigerant fluid and a compression cycle this heat is transferred to the building’s internal heating system which is made up of internal pipes and radiators.

At Ebley Mill an “open loop” system was preferred due to the size and heat demand of the building. This technology instead relies on large volumes of water (3300 litres per minute) being pumped from the river, extracting heat energy via a heat exchanger and returning the slightly cooled water to the river.

The existing gas boilers have been replaced by a series of heat pumps that are powered only by electricity, saving more than 5,125 tonnes of carbon.

The council was successful in applying for funding under the government’s renewable heat incentive scheme and will recover the capital costs of both installations over a period of years. This means the investment will eventually pay for itself, save on bills, and save Council Tax payers money.

Watch Stroud District Council Leader, Doina Cornell talk about the new water source heat pumps.

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