Countering the growing pressures on HFCs
Ken Sharpe looks at the growing pressure on HFC refrigerants and how one manufacturer has developed a concept that addresses the issues for large systems.
Concerns about global warming are putting a lot of pressure on refrigerants, in the EU at least, for applications such as air conditioning. The phasing down of the use of HFCs, for example, is based on the combined effect of their GWP (global warming potential) and the amount of refrigerant used.
The chart at the bottom of the page shows just how dramatic the effect will be, with an 80% reduction by 2030 compared with 2014 for such refrigerants produced and imported into Europe. The phase down starts quickly, with over a third by 2017.
Reducing leaks from existing equipment to decrease the amount of refrigerant needed for topping up will help. But the main burden for meeting the programme will fall on manufacturers.
The main approaches are summarised below.
•Use refrigerants with a lower GWP. The use of R32 instead of R410A is already being adopted, especially for smaller systems with less than 12.5 kW of cooling. Not only is its GWP two-thirds less than that of R410A but it is also a more efficient refrigerant, so that a smaller charge is needed. The overall effect is a 71% reduction in total CO2 equivalence — very close to the 80% reduction by 2030.
•Use less refrigerant. This is the key approach for larger systems, and it also addresses the requirement of BS EN378, which limits the amount of refrigerant in occupied spaces — such as might occur if a large system were to leak. The effect is that the maximum refrigerant charge for a hotel system is about 20 kg. One way to address this issue is to install leak detection, which can be expensive, both in terms of capital and maintenance costs, especially in a hotel where leak detection would be needed in every air-conditioned guest room.
We are now in the territory of large VRF and chilled-water systems, with the value of the VRF market being about 50% greater than chiller-based air conditioning.
VRF has many desirable characteristics, which has led to this market prominence. That said, chilled-water systems have their desirable characteristics — not least of which in the current discussion is the complete absence of refrigerant in occupied areas.
Mitsubishi Electric has developed a system that combines the desirable characteristics of chilled-water and VRF systems.
Called hybrid VRF (HVRF), it resembles a VRF system from the City Multi outdoor units to the hybrid branch controllers (HBCs) that are the definitive feature of the concept. Between an HBC and the fan-coil units (FCUs) in the conditioned spaces heating and cooling, the system is water based.
An HBC can have eight or 16 ports with just a 2-pipe connection to the fan-coil units, each of which can provide heating or cooling; the HVRF system does not need 4-pipe FCUs to achieve this capability
Extensive systems are possible. Not only can each pair of ports on an HBC have up to three FCUs connected, but two main HBCs and two sub-units can be connected to the same outdoor units to deliver simultaneous heating and cooling to up to 50 rooms or areas.
There is a lot of kit in an HBU, so much in fact that only refrigerant and water connections are needed — along, of course, with an electrical supply. Inverter-driven DC pumps are built in, to supply chilled and heated water, as are refrigerant-to-water plate heat exchangers (PHEs) for heating and cooling. Valve blocks on the flow and return ports to each FCU select heated or chilled water and control the flow rate to achieve a ∆T of 5K; as the heating or cooling demand changes so does the flow rate of water delivered to the indoor units.
The heat-recovery effect is achieved by the outdoor unit supplying hot refrigerant gas or a mixture of hot gaseous refrigerant and liquid refrigerant to the heating PHE. When the needs of the heating circuit have been met, the by-then liquid refrigerant passes to the cooling PHE and then back to the outdoor unit.
Using less refrigerant addresses the EU phase-down timetable, and keeping refrigerant out of occupied spaces avoids the need for leak-detection equipment.
Mitsubishi Electric’s figures suggest that for a hotel the capital costs of an HVRF system would be about 10% less than a VRF system with leak detection. In the longer term, say 15 years, the need for maintenance of the leak-detection system and replacement of sensors, will lead to significant costs being incurred.
Sebastien Desmottes, product marketing manager for Mitsubishi Electric, summarises, ‘Hybrid VRF is designed specifically to deliver the function of a 4-pipe fan-coil chiller system with the efficiency and flexibility of modern VRF in one fully packaged system.
‘We expect to see BS EN378 legislation impact more on the built environment and have developed this range to answer that need.’
There were already a few installations in the field or being installed as we went to press.
One installation in progress was for a 17-storey 250-bed hotel. Hybrid VRF was chosen to avoid the need for leak detection.
An actual installation is in the new offices of consulting engineers Hoare Lea in Cardiff. Managing partner for the 500 m2 office Wynne Harris says that VRF would have been a perfectly good approach but that HVRF reduced the amount of refrigerant needed by 30 to 40%.
One desirable feature highlighted by Wynne Harris is the better temperature regulation that is achieved in occupied areas by water-based fan-coil units compared with refrigerant units.
From an engineering viewpoint, HVRF is a very interesting concept, but it is its ability to address concerns with refrigerant leaking into occupied spaces that is proving to be its most attractive feature to users.