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WR2 Application Study – Zetter is Better


The installation of seven Mitsubishi Electric WR2 PQRY-P250YMF-C heat recovery air conditioning units into the Zetter Hotel in Clerkenwell is now helping the stylish conversion provide exactly the right environment for its guests, whilst using the natural underground lake deep below the city streets as a cooling and heating source to save on energy consumption.

The Challenge

The Hotel, which is within walking distance of Sadlers Wells, the Barbican, Smithfields, St Pauls and the City, has been refurbished to an exceptionally high standard.

The refurbishment was designed by architects Chetwood Associates, using sustainable materials and environmentally responsible principles with the main atrium providing natural ventilation and the air conditioning cooled by water pumped from a bore-hole which sinks 130m below the building.

The air conditioning system, which was supplied by Mitsubishi Electric’s Direct Bristol office, was put together by a design team from engineering consultancy, Buro Happold and Gratte Manly Mechanical Services and installed by J C W Air Conditioning of Bristol.

The architects were looking for a system that not only allowed for a sustainable and energy efficient design, but also avoided losing valuable roof space that has now been used for penthouse suites

“If we had installed standard air conditioning units we would have lost at least one penthouse suite which earns valuable revenue for the hotel,” explained Todd Billo, Maintenance Manager at the 59-bedroomed hotel, which includes seven rooftop studios with patios providing panoramic views over London. All rooms have giant sash windows and are reached via the dramatic five-storey atrium.

The 18-month conversion project has retained many of the building’s original features and scars while blending the traditional and the modern, to create an unpretentious urban space in which to sleep, eat and meet. The building’s large sash windows for example, are a dominant feature of every room and particularly on the south side of the building, this can leave rooms hot as the guests walk in, so it is vital that the air conditioning is able to cool quickly to a comfortable level.

The solution

One of the unique features of the WR2 system is the fact that the condensing units can be sited indoors, and in the case of Zetters, the seven units are installed in small service rooms on each floor. Each unit can provide up to 31.5 kW of heating and 28 kW of cooling and link in this occasion to PFFY-P VLRM-E floor standing units concealed in each of the bedrooms.

Along with this capability to free up vital roof space, the WR2 is also unique in its ability to link to a building’s water loop and transfer heating or cooling energy between them, allowing the units to offer incredibly efficient double heat recovery, producing heat recovery from indoor units on the same refrigerant circuit in addition to using the water circuit to transfer energy between different WR2 circuits.

In the case of the Zetter Hotel, ground sourced water from an aquifer approximately 130m below ground provides water at a constant 13 – 14°C. The water is pumped up the borehole and passed through a plate heat exchanger and then finally metered before being piped to the sewer.

The refurbished building, which opened in March 2004, is utilising the WR2’s 2 pipe simultaneous heating and cooling ability to precisely control the various temperature requirements within the Hotels rooms.

By using the open loop bore hole the “WR2” systems efficiency benefits from the constant year round water temperature of between 13 ~14°C. Unlike conventional air cooled systems where the heating efficiency drops at low ambient temperature and with defrost cycles, the WR2 system efficiency remains high regardless of ambient conditions.

The air conditioning is controlled by Mitsubishi Electric’s G50 system which links to the overall BMS, although each room has an individual controller. When a guest enters the room and places their VIN card (room key) in the slot which controls all lights and services, the air conditioning will come on and will automatically switch off when the room is vacated. The system is also connected to sensors on the sash windows so that if they are opened, the air conditioning will switch off to stop energy being used when it is not needed.

“The system has been so well set up that it practically runs itself and we are now fine tuning it so that we can get maximum efficiency,” says Todd adding that the user-friendly G50 is easy to pick up and understand.

Water has figured largely in the history of Clerkenwell, whose name derives from a well used to bring water up to medieval London from the underground reservoir.

In addition to using this water to take away excess heat from the air conditioning systems, the Zetter Hotel also filters it and offers it to guests in the form of still or sparkling bottled spring water.

The hotel and its restaurant have already proved very popular and the installation of the WR2 system will ensure that the building has a reliable, energy efficient environmental control for years to come.

Ongoing monitoring

The principle of the WR2 system in use at the Zetter Hotel is double heat recovery, firstly extraction of heat from the underground water source and then heat recovery between condensing units and then the indoor units so that the system provides maximum efficiency at all times.

Continual monitoring of the system shows that the WR2 system at Zetters has already been providing an average COP of 3.48 at full load and at current water volume flow rates 5m3/h and temperature of 32°C.

The ability to monitor on an ongoing basis will allow the Hotel’s owners to see how efficiency varies depending on the water volume flow rate and temperatures and any seasonal changes within the hotel and in clientele numbers.

The owners can then examine ways of matching any changes required or can look to increase energy efficiency – For example, reducing the water temperature to 20°C and increasing the water flow rate to 7.5m3/h, will increase the COP to 5.1.

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