SHARC Energy Systems invited Infinitas to design the UK’s first sewage heat recovery heat pump system for Scottish Borders College in Galashiels. The college is a higher education establishment with five separate campus buildings of various ages and uses. Each building had its boiler plant of various types, ages and conditions, and different heat emitters, including radiators, air handling units, fan coils and hot water cylinders.

Our task was to link the five buildings to a new central water source heat pump energy centre via a buried low-temperature heat network. While working on the scope of work, our responsibilities covered all stages of the process, starting with an initial consultation, site survey and feasibility study.

The primary energy source for the heat pump system was the nearby combined sewer. In general, sewers tend to run at temperatures exceeding those found in the ground or natural water sources due to the discharge of showers, washing machines and other domestic and commercial waste. Scottish Water provided us with the sewer flow rates during the feasibility study. We calculated the energy capacity and availability for the SHARC system from the data.

To establish the annual heat demand profile for the college, we used a combination of half-hourly metered gas data and monthly gas bills. Our team also considered non-heat uses of gas, such as cooking. Then, we established the building’s peak thermal demand. When combined with the available heat capacity within the sewer, we found an 800kW heat pump system to supply almost 100% of the demand in all but the very coldest of years.

However, our cooperation didn’t end there. Following the contract closeout, the client appointed Infinitas to conduct a detailed design on the energy centre, the 1km heat network and the interfaces to each of the five gas-fired plant rooms. We initially retained the gas boilers to provide individual backup to buildings. Such a solution would enable each unit to be isolated from the network under different circumstances. Our team used 2 Carrier 30XWH water source machines, rated at 400kW each. Throughout the project lifecycle, Infinitas completed:

• The end-to-end system schematic
• General arrangement drawing and specifications for the heat pumps and supporting equipment, pre-insulated buried pipework and direct interface to each plant room

The system, installed in 2015, has been operating with a seasonal coefficient of performance (COP) of 4.2 in the period since.

Infinitas completed the successful renewable heating incentive (RHI) application for the system. But our work continues. We carry on working with SHARC Energy Systems to improve system efficiency. In 2018, Infinitas included additional solar PV, battery storage and consolidation of all remote boiler plants for discharge into the shared network either as a top-up for or replacement of heat from the heat pumps.

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