VRF continues to push back efficiency boundaries
Three decades on VRV/F technology continues to evolve to improve the efficiency with which cooling and heating can be delivered. John Durbin traces the history of this technology
A major part of the energy efficiency of any climate-control system depends upon how efficiently energy can be transferred from one medium to another. This process always incurs some energy loss, and the more times energy is transferred between media, the greater that loss.
Moving energy from one location to another will also result in some loss — depending on the distances involved, the medium used to carry the energy and the mass flow. This loss will be compounded by the motive force and the pipe, duct, wire or however the energy source is contained.
It was this conundrum that spawned the invention of variable-refrigerant-volume technology (VRV) by Daikin nearly 30 years ago. This technology is now generically known as variable-refrigerant flow (VRF) air conditioning and has grown to be a worldwide air-conditioning solution. It has challenged and changed the thinking and practice of the global market for indoor climate-control.
The original thinking behind VRV/F [and we will now use the generic term VRF — Ed.] involved the realisation that, if the refrigerant that carried the heat to and from the compressor also passed through the evaporator and condenser, a significant energy saving would result. What is more, the change of phase between gas and liquid means the volume flow rate is tiny when compared to air or water, so the transportation loss is far less.
In the early days, restrictions to the length of pipework and control of the system posed problems but, by using the compressor as the ‘pump’ and incorporating clever oil-return technology, actual pipe lengths between indoor and outdoor units can now reach up to 150 m. A system comprises flow and return pipes with terminal units installed in parallel, and the exact quantity of refrigerant needed for cooling or heating function is supplied by the outdoor unit. Energy use is kept down by modulating the refrigerant volume to meet the exact needs of all the indoor units combined.
The easiest way match demand is to vary the flow of refrigerant by controlling the speed of the compressor. The most efficient speed control is by an inverter — thus the VRF air conditioning system was developed.
The system’s renowned benefit, of course, is that the outdoor unit, which can be a heat pump, varies the refrigerant volume within the system to match precisely the building’s requirements at any moment. Each area can thus continually maintain its desired temperature, avoiding fluctuations in the internal climate while minimising energy consumption.
The original systems showed a 25% to 30% saving in energy compared to a chiller system. The heating was by a reverse-cycle heat pump and showed even greater savings over a direct electric heater battery and similar savings when compared to a 4-pipe water-based heating and cooling system. But technology rarely stands still, and the addition of heat recovery to these systems resulted in a further 5 to 10% energy savings. Today, heat recovery incorporated within a well designed and fully integrated climate-control system can dramatically reduce energy consumption.
However, despite the worldwide success of VRF over the past three decades, there is a tacit understanding that its systems are only appropriate for small to medium buildings and that it does not offer the same level of design flexibility as a chilled-water system. This is no longer the case.
In fact, VRF systems offer much greater flexibility than some engineers realise and can be specified successfully for buildings of up to 10 000 m2, offering precise control of multiple zones. They also take up as little as a third of the central plant space required by traditional air-conditioning systems, so they are an attractive solution for major refurbishment projects in which the aim is to maximise the rentable floor space of a building.
Another great advantage is the modular nature of the system, making it ideal for phased refurbishment projects. A system can be retro-fitted floor by floor, avoiding the wholesale relocation of a building’s occupants, and it is perfect for improving the energy efficiency of older buildings — whether they be hospitals, schools, leisure centres or offices.
The evolution of VRF technology now means that it can be designed to operate in balanced mode, resulting in previously unheard of COPs — even reaching up to 10 and more (based on a Daikin REYQ10P in mixed mode at nominal conditions). There are water-cooled systems and even geothermal heating-only options. Heat-recovery options are evolving all the time, so that hot water can be generated energy efficiently and hot air supplied to air curtains.
VRF systems can also be combined with ventilation. Heat extracted can be transferred to the fresh-air supply being delivered to that room. The net result is a further reduction in the cooling/heating load on the air-conditioning system.
VRF systems now offer a highly energy-efficient and flexible solution to fit the requirements of modern energy-efficiency needs as well as providing end user comfort and affordability.
John Durbin is engineering-department manager with Daikin UK.