COSTS | MARINE SOURCE HEAT PUMPS Cost model Seawater source heat pumps have the advantage of a thermally stable water source, but the equipment needs protection from corrosion. Aecom weighs up what the cost would be for a 12.4MW energy centre in the Thames Estuary Marine source heat pumps T he UK government is pushing the construction industry to decarbonise heating systems as part of the legal commitment to meet its 2050 net zero carbon target. This has led to a reduction in use of fossil fuels and an increase in the specification of centralised systems, with common-plant district heating systems and off-grid heat sources being specified far more widely. Water source heat pumps are increasingly being considered as a reliable low carbon alternative to traditional heating systems. Ground source heat pumps (GSHPs) are well established and the groundwater heat source is far more stable than air in an air source heat pump. There is risk, however, that the temperature of the water and the adjacent strata could change over time. Another option is marine source heat pumps, which use seawater as a heat source. With an annual temperature range of 11C, seawater has thermal stability throughout the year. Tides also move the water around continuously, thereby diluting any thermal impact because of the sheer scale. Using saltwater as the heat source has its own challenges, though, including corrosion, geography and environment. Saltwater is an aggressive substance, so all of the primary pipework, pump sets, filters and plate heat exchangers need to be specified appropriately; this increases the cost over non-saltwater solutions. From an environmental perspective, the primary loop is frequently an open-source loop, with risk of contamination of the source water from the system. The Environment Agency would be involved at all stages of the project to ensure that natural resources are safeguarded, pollution risks are minimised, and heat load does not impact the ecosystem adversely. The design solution for a saltwater-based system is similar, typically, to a GSHP one. The primary difference 30 July 2020 www.cibsejournal.com CIBSE July 2020 p30-31 Cost model.indd 30 is the need to minimise ingress of marine life into the system and to ensure mechanical separation of the open-loop saltwater system from the closed-loop glycol (or equivalent) system. This is achieved with grilles/screens, filtration and a plate heat exchanger. Incoming water is additionally filtered to protect the plate heat exchanger from unnecessary corrosion. The cost model The cost model looks at a large-scale urban development incorporating residential units, commercial office space, hotels and retail units including extensive food and beverage offerings using water-based heat pumps. The scheme is located in the South East of England, along the Thames Estuary and within 50m of the water. It has 8MW of heat pumps and 4.4MW of water-cooled chillers. Costs are for energy source only and exclude inflation beyond first quarter 2020, main contractors costs, design and other fees, and VAT. The cost model makes due allowance for an aggressive environment with highly saline water. So, the pipework is primarily stainless steel, and there are upgraded extraction pumps and plate heat exchangers to accommodate a harsher operating environment. Only the services elements for the scheme are looked at in the cost model, which does not include any works associated with building or civil engineering. These elements are highly specialised and cover items such as coffer dams, dewatering and guided auger bores. Additionally, the cost model excludes any works associated with the river wall, environmental protection measures such as anti-scouring devices to protect the sea/river bed fish bypass systems, and so on. It makes no allowance for extraction licences, river and environmental agency fees, and the like. ABOUT THE AUTHOR The engineering services cost management group of Aecom specialises in the cost estimating, procurement and cost management of building services installations. It is producing a series of cost models for CIBSE Journal in 2020 on areas such as data centres and Londons commercial buildings. 19/06/2020 17:04