Variable Refrigerant Flow: An Emerging Air Conditioner and Heat Pump Technology

 


Ammi Amarnath, Electric Power Research Institute
Morton Blatt, Energy Utilization Consultant


ABSTRACT
This paper reviews the attributes of  an emerging space conditioning technology;
variable refrigerant flow (VRF) systems. Material presented in this paper was synthesized from
the open literature, private interviews with industry experts and data (sometimes proprietary
data) obtained from manufacturers. VRF systems are enhanced versions of ductless multi-split
systems, permitting more indoor  units to be connected to each outdoor unit and providing
additional features such as simultaneous heating and cooling and heat  recovery. VRF
technology uses smart integrated controls, variable speed drives, refrigerant piping, and heat
recovery to provide products with attributes that include high energy efficiency, flexible
operation, ease of installation, low noise, zone control, and comfort using all-electric
technology. VRF systems are very popular in Asia and Europe and, with an increasing support
available from major U.S. and Asian manufacturers are worth considering for multi-zone
commercial building applications in the U.S.

 This paper provides an overview of variable refrigerant flow system technology,
including the market situation, advantages and disadvantages for the customer, possible impact
on the electric utility, applications recommendations, and technology attributes. Also addressed
are what is holding back the technology, including lack of verified third party field data; codes
and standards issues; technology improvements needed; and market actions needed to increase
penetration of these systems.


Evolution of the Technology

Ductless space conditioning products, the forerunner of multi-split and VRF systems,
were first introduced in Japan and elsewhere in the 1950s as split systems with single indoor
units and outdoor units. These ductless products  were designed as quieter, more efficient
alternatives to window units (Smith, 2007).

Products have evolved from a few indoor units operating off each outdoor unit, to multisplit products with 4 units to 8 units in the late 1980s, to 16 units in the early 1990s, to 32 units
by 1999. Today’s advanced systems permit as many as 60 or more indoor units to operate off one
outdoor unit, enabling application in large commercial buildings. Electronically commutated
motors, inverter-driven/capacity modulated scroll compressors, multiple compressors, versatile
configurations and complex refrigerant and oil circuitry, returns, and controls have enabled this
addition of up to 60 indoor units. Refrigerants have also changed. The early “mini-split” systems
used R-22 refrigerant, then R-407C, and today’s systems rely on R-410A. Figure 1 shows the
piping and refrigerant flow for conventional split systems (one indoor and one outdoor unit)
multi-split systems, and variable refrigerant flow systems (Dyer, 2006).

Multi-splits offer some of the major advantages of VRF systems, such as zoning, capacity
control, ease of retrofit, low installation costs, and minimizing ducting and use of secondary
2008 ACEEE Summer Study on Energy Efficiency in Buildings 3-1fluids and associated costs and losses. On the other hand, their simpler piping results in longer total length of piping compared to VRF systems. Similarly, multi-split heat pumps do not have the opportunity for heat recovery between units that are cooling and units that are heating. As such, multi-split systems should be considered  for smaller, simpler buildings where heat recovery is not a possibility and fewer numbers of zones need to be conditioned.


How Does VRF Work?
 
Multi-splits include multiple indoor units connected to a single outdoor unit. Ductless
products are fundamentally different from ducted systems in that heat is transferred to or from
the space directly by circulating refrigerant to indoor units (evaporators or condensers) located
near or within the conditioned space. (When the indoor units are in the cooling mode they act as
evaporators and when they are in the heating  mode they act as condensers.) In contrast,
conventional ducted systems transfer heat from the space to the refrigerant by circulating air (in
ducted systems) or water (in chillers) throughout the building.

VRF systems are enhanced versions of ductless multi-split systems, permitting more indoor
units to be connected to each outdoor unit and providing additional features such as simultaneous
heating and cooling and heat recovery. VRF heat pump systems permit heating in all of the indoor
units, or cooling of the all the units, not simultaneous heating and cooling. Heat recovery systems
provide simultaneous heating and cooling as well as heat recovery to reduce energy use during the
heating season.

Over the past 15 years the technology has advanced in a number of areas:
• Standard compressors to variable speed and capacity modulated scroll compressors
• Direct driven outdoor fans to variable frequency drive, inverter-driven fans
• Direct driven indoor coil motors to direct current or ECM-type motors
• Variable capacity indoor units
• Better heat exchanger surfaces with multi-segmented coils
• Improved controls and diagnostics
• R-22 to R-410A
• Better refrigerant charge and oil management
 
 Other features include the addition of  concealed ducted units and ceiling cassette
configurations to the traditional wall-mounted units. Refrigerant piping runs of more than 200
feet are possible and outdoor units are available in sizes up to 240,000 Btu/ hr.

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