Open Water Source Heat Pump Development
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Article from the January 2014 edition of the CIBSE Journal written by Alex Smith.
A development that uses heat stored in the water of the Thames could be the prototype for low-cost renewable energy systems all over the world, according to project engineer Chris White. Alex Smith visits Kingston in Surrey to find out more.
A modest single-storey building on the riverbank in Kingston upon Thames doesn't look like a prototype for an innovative renewable system that, according to its engineer, could be the answer to the world's looming energy crisis.
Its brick walls house part of a heat pump'system that harvests naturally stored energy from the River Thames, and then delivers under-floor heating and hot water for 56 affordable homes, 81 private apartments and, later this year, a new 145-bedroom hotel.
According to White Associates managing director, Chris White MCIBSE, it could help provide cheap and plentiful energy, and potentially offer coefficiency of performance ratios approaching double figures.
"Every process in this system has the potential, using known and proven technology, to provide a 100% renewable, zero-carbon thermal energy source for the planet if powered by clean electricity," says White.
The open water source heat pump system at the 70m Kingston Heights development is the first of its kind in the UK, and has been a labour of love for both White and Mike Spenser-Morris, the owner of NHP Group, the developer.
It works by using pumps in the plant building to extract water through intakes positioned 2.5m below the river surface, where the water temperatureis relatively constant year-round. After a two-stage filtration process, heat is transferred from the river water to a secondary circuit that links to a plant room on the fifth floor in the apartment block. Water source heat pumps then increase the temperature of the low-grade heat before sending it to mini plant rooms, where the second part of the heat pump upgrades temperatures further. The system is capable of delivering 2.3MW of heat. (See How it works below).
The most innovative element of the scheme is the taking of such a high thermal load from an open body of water. This gives far higher load potential than a traditional open loop borehole system, where the abstraction rate on groundwater is limited to how much can be pulled physically without causing instability to aquifer temperatures.
While Spenser-Morris was the driving force in delivering the project and helped convince local MPs Ed Davey, Secretary of State for Energy and Climate Change, and Zac Goldsmith to back the project, it was White who had to deliver the technical solution. Not only was it the first of its kindin the country, but it was also on a very challenging site the apartments were being built on top of an existing four-storey electrical substation.
As the concept had not been tested in the UK at this scale, White and Spenser-Morris had to convince a diverse number of stakeholders of its viability. These included Kingston Council, affordable housing provider Affinity Sutton, Redrow Homes and the Environment Agency, which is responsible for the health of the river. Indeed, two separate applications - for abstraction and rejection - licences had to be submitted to the agency as there was no precedent for applying for a joint licence.
"We had to speak to a lot of people to convince them it could work," says White. "Itwas because no-one had done it before. To convince others of the technology's viability, Mitsubishi Electric Europe, the chosen heat pump manufacturer for the scheme, organised a trip to a project in Osaka in Japan, which had used the technology - although in a commercial rather than residential scheme."
White had to start from scratch. He couldn't find a contractor that had worked on open water source heat pumps, and so employed a ground source equivalent. He soon found that the knowledge required to extract and return thousands of tonnes of Thames water was embedded "not in a renewables firm but a company specialising in water infrastructure. B&V Group worked in water treatment, and was used to industrial precision, which we required," says White.
The firm carried out the works to the plant room, including the self-priming pumps, heat exchangers and filters - working closely with Industrial Purifi cation Systems, which developed a double filtration system that met the exacting requirements of the Environment Agency: to return to the Thames clean river water that varied by no more than 30C from the abstracted water. And it had to do so without harming any marine life.
"Working with the agency was a challenge",says White. "There was no available licencefrom the Environmental Agency for whatwe wanted to do, he says. There was one for extraction of river water and one for rejection but none for the combination of both," says White. The team also had to ensure that the intakes did not interfere with river traffic.
White had the added complication of having to route the pipes over the roof of the electrical substation. Taking two 315mm pipes up the side of a building was a challenge because of weight on the point loads at fi xing intervals, and the overall end load when pipes are full.
The pipework required for transporting "the low-grade heat is low cost," says White, who specified medium density polyethylene (MDPE) pipes off the shelf.
He says the next challenge will come with educating occupiers as they arrive in the months ahead. "We have to educate people about the differences between heat pumps and conventional gas boilers, and change their mindsets to ensure they keep their heating on. We will be giving them 5kW of heat, but we need to tell them how to look after it."
NHP is forming a company to oversee the system. It calculates that, for a couple living in a one-bed flat, heating bills will be 15% cheaper than if energy was supplied by gas boilers.
White is enthusiastic about the potential of open source water heat pumps. He contrasts transporting river water at low temperatures to the potential heat losses incurred by district heating systems carrying hot water or steam at high temperatures. "Whereas traditionaldistrict heating loses effi ciency due to lower ground temperatures, either river, sea or lake water will typically be within a few degrees of ground temperatures at a depth of 1m."
"Dependent on where in the world the system is employed, it is probable that the further we distribute the water, we will actually pick up a rise in temperature from geothermal energy, and traditional distribution losses would actually become gains."
White estimates that the temperature at Kingston Heights will increase by 1-2 degrees C as it travels through pipework. White says transporting low-grade heat avoids overheating 'in corridors, which occurs when high-grade heat can't be contained.
The biggest paybacks will occur when the hotel is built, says White. A heat recovery VRF system will allow heat rejected in the summer to be injected into the development's primary circulation route, which provides hot water to the flats and hotel. White says that, with balanced loads between cooling, heating and hot water, there could be COP ratios of six and upwards for this type of mixed-use scheme. The more solar energy we collect, the more we inject it into our primary circulation, and the more efficient the heat pumps.
White says the technology opens up the possibility of large-scale town planning on a river or seawater network. This will allow buildings to be heated and cooled with loadmatched sites and full heat recovery.
The Kingston Heights scheme costs 10-15% more than an equivalent biomass system. "The costs were expected, and it was in part because we were using industrial plant," says White. "It was the first time it had to be done, and there" were a lot of unknowns.
"A couple of times we were under a lot of pressure to go the tried and tested route of biomass, but we fought on. This is a large-scale change in thinking, which will take years, if not generations, to implement. But, from little acorns mighty oaks do grow and I hope that, through the system installed here, we have planted the seed in people's minds that this can be done."
How it works
The community heating at Kingston Heights is based on an open water heat pump system, whereby the temperature of low-grade heat from energy stored in river water is increased to provide heating and hot water for residents and hotel guests.
A new plant building on the site of an old coal hopper, houses the pumps that abstract water from the Thames. IPS designed the two intake filters, which connect with the pumping system designed by B&V. These sit above the river bed clear of silt and away from navigation channels. They are 600 mm in diameter, and 1.04 m long, and are barrel shaped to prevent marine life and debris being trapped against the intake openings.
A two-stage filtration system ensures that no river life gets sucked into the system. The 1.5 mm mesh on the intakes is small enough to block elvers (baby eels) and a second epoxy-coated carbon steel mesh of 100 microns prevents other debris from entering the pump house. Having two intake filters provides backup if one fails, and enables them to be cleaned separately.
The Environment Agency stipulated that the temperature must not vary by more than 3 degrees C when it was returned to the river. Chris White calculated that at a flow rate of 150 l/s the system could abstract around 2 MW of low grade energy, and rejected the water back into the river at a temperature less than 3 degrees C lower.
The heat from the river water passes through a high-efficiency heat exchanger, which transfers the lo-grade heat to an internal closed watersystem. This is taken 200m via MDPE pipes over the existing electrical substation to a plant room on the 5th floor of the development.
Here 41 Mitsubishi Electric Ecodan water source heat pumps link to this "closed" loop and increase temperature up to 45 degrees C, before sending it across the development. In mini plant rooms, the second part of the Ecodan system upgrades the temperature further to deliver underfloor heating and domestic hot water to apartments and hotel rooms.
The heat pumps work by using the vapour compression cycle to upgrade low temperature renewable heat and raise it to usable temperatures. Inside sealed copper piping refrigerant vapour is compressed to raise both pressure and temperature. This hot, pressurised vapour is passed through a condenser where it liquifies and gives off usable heat.
The liquid refrigerant is allowed to expand, which lowers its temperature and pressure. The liquid then absorbs naturally occurring heat from the renewable source - in this case the Thames. Once this has happenned the liquid changes back to vapour and the process starts again.
Developer: NHP Leisure Developments
Building services engineer: White Associates
Contractor: United House
Affordable housing: Affinity Sutton
Private housing: Redrow London: 56 homes
Heat pumps: Mitsubishi Electric Ecodan heat pumps
Specialist river water engineering installation: B&V Water Treatment
Specialist filtration of river water: Industrial Purifi cation Systems