VRF air conditioning adopts water-cooled technology
Water-cooled VRF air-conditioning in the Zetter Hotel in London saves space and increases energy efficiency. The hotel is served by seven Mitsubishi Electric WR2 heat-recovery air-conditioning units. The system was designed by Buro Happold and installed by JCW Air Conditioning. Energy is rejected to and extracted from water drawn from an aquifer
The scope of VRF air conditioning has been opened up enormously with the development of water-cooled condensing units.Since VRF air-conditioning was introduced to Europe by Daikin over 20 years ago in 1982, this approach to air conditioning has escalated. Several manufacturers now offer VRF systems in the UK, and their capability has expanded enormously over the years. At first just a handful of indoor units could be served by one outdoor unit. Permissible pipe runs were also much shorter than the 120 m real length and 40 m level difference between indoor and outdoor units that can be expected with modern systems. Outdoor units can also be installed side by side to work as a single entity — greatly increasing the cooling capacity. The development of inverter drives for compressors has vastly improved control capability, and a range of approaches is available to use energy recovered from an area being cooled to heat another. However, VRF systems have traditionally been served by packaged air-cooled units, which have to be installed outdoors so that the restrictions on refrigerant pipework impose design limitations.
Water cooled Those limitations have now been overcome with the introduction of water-cooled condensing units by two major manufacturers — Daikin and Mitsubishi Electric. Condensing units can be installed indoors, close to the VRF circuits they serve, and there is quite simply no restriction on the length of water pipework. Plate heat exchangers provide the link between refrigerant circuits and the water loop. Both systems can be used with a wide range of indoor units. Daikin and Mitsubishi have very similar views on the potential of water-cooled VRF (or the VRV trademark used by Daikin). Philippe Vandesteene of Daikin explains that such water-cooled systems are eminently suitable for tall multistorey or large buildings because there are no limitations on water piping length. He also perceives the potential for existing water pipework to be used to enable a VRF system to replace existing equipment.
The compact condensing units of Daikin’s water-cooled VRV system come as 10 hp modules, and three can be connected together.
Mitsubishi Electric’s Philip Ord highlights the potential for phased installation in a new building or major refurbishment where future occupancy is unknown. The first phase of the installation is to install a 2-pipe riser with valved tap-offs to each floor and area that will subsequently be served by a VRF system. This pipework should be sized at the outset for the maximum load of the building and water flow rate. By this means, initial capital expenditure is minimised. Only when a floor is let, explains Philip Ord, need fit-out work comprising the installation of a water-cooled VRF unit and indoor units be carried out. It is also at this stage that a boiler and heat-rejection method such as dry-air cooler is added. As more air-conditioning circuits are installed, boiler and heat-rejection capacity can be added. With much VRF equipment offering the potential for energy recovery, Philippe Vandesteene highlights a significant energy-saving opportunity. Not only can energy removed by cooling units be used for heating on the same VRF circuit, but the water circuit also enables energy to be transferred between VRF circuits. ‘We don’t just cool a room,’ says Philippe Vandesteene, ‘we remove calories and use them to heat other spaces.’ Such energy balancing reduces the need for heat from boilers and heat rejection.
Less refrigerant Both companies use environmentally friendly refrigerants, but the introduction of a water loop considerably reduces the quantity required. Philippe Vandesteene puts the reduction in refrigerant charge at 46%. All the capabilities of VRV systems are retained, including sophisticated control. For the enterprising engineer, however, the water loop provides potential for imaginative engineering. To summarise ideas from Philip Ord and Philippe Vandesteene, why use dry air coolers to reject heat? Cooling towers can be used instead, with the potential to increase COP and smaller space requirement — which could be crucial in high-rise buildings. Indeed, why not reject heat into the ground or a lake? Going one stage further, why not dispense with boiler plant and use a ground-source heat pump to provide thermal energy? Solar energy could also provide heat. Ground sources provide relatively stable temperatures compared with the atmosphere. At a depth of 1.7 m, the temperature is 10°C in winter and 17°C in summer. At a depth of 50 m, the temperature is 12°C all year round. Water-cooled VRF units are small, compact and quiet — so they can be installed virtually anywhere in a building. Daikin’s condensing unit, for example, measures 1000 x 780 x 550 mm and weighs just 150 kg. To save more space, one unit can be stacked on top of another. They use R410A and are available in both heat-pump and heat-recovery versions. Nominal cooling capacities range from 27 to 82 kW, with heating from 31 to 95 kW. Cooling COPs of 5.21 and EERs of 4.53 are achieved at full load. Operating sound pressure is 51 dB(A).
Project Mitsubishi has designed water-cooled VRV equipment into bespoke installations for some time, and one in London demonstrates its capabilities. The conversion of a 19th-centruy warehouse in Clerkenwell into a stylish hotel uses the space-saving capabilities of the WR2 system to enable valuable roof space to be used for penthouse suites. The water loop for this project is linked to a 130 m-deep aquifer at a steady 13 to 14°C. The water is pumped up a borehole into the plantroom, passed through a plate heat exchanger and metered before being piped to the sewer. The installation is designed to transfer heat within circuits and between circuits using the water loop. Monitoring of this system at the Zetter Hotel shows an average COP of 3.48 at full load with a water flow of 5 m3/h and temperature of 32°C. There is potential to increase the COP to 5.1 by reducing the water temperature to 20°C and increasing the flow to 7.5 m3/h. Combining the benefits of VRF air conditioning with water cooling is an important development for the VRF sector and the air-conditioning market in general.
Related links:
