2.11 Variable Refrigerant Flow (VRF)
Commonly referred to as VRF systems, Variable Refrigerant Flow systems are produced by a number of manufacturers and consist of both heat pump and heat recovery units. Similar in nature to a Water Source Heat Pump system, the VRF system consists of an outdoor condenser unit tied to a group of indoor heating and cooling units using a piping loop. Unlike a wate source heat pump system, however, the VRF system uses a refrigerant loop connection between the indoor and outdoor units. In the case of the heat recovery version of the VRF system, indoor units are able to exchange heating and cooling energy via the refrigerant loop.
Given the nature of partial loading on the outdoor unit from the various indoor units, using a simple full load EER to approximate unit performance will result in a gross underestimation of system performance. Accurate hourly loading of each indoor unit is necessary to derive the maximum accuracy. Each indoor unit must be modeled as a zone or thermostatic control element in the software. In situations where a single zone contains a number of indoor units, it is acceptable to group those units as a single zone, provided each area served by the indoor units is thermally similar. Once the thermal zones have been determined, each indoor unit will be modeled based upon actual heating and cooling capacity and fan characteristics. Some VRF systems include optional components that provide outdoor air heat recovery and/or economizer capability, so these features will need to be specified at the zone level inputs of the software.
Once all the indoor units have been defined, each indoor unit will need to be associated with a loop that connects it to an outdoor unit. As part of the specification of the loop, heat recovery may also be included if the unit has this capability. Each outdoor unit will be specified with inputs for capacity and full load efficiency. In addition, operating curves will be input to adjust the full load efficiency for part load conditions and varying outdoor conditions.
Additionally, some VRF systems include the ability to connect all of the outdoor condensing units to a water or glycol based condenser water loop that includes a cooling tower and possibly a boiler. If this is the case, additional parameters will need to be input to define this water loop including the description of the pumps, cooling tower and boiler. This description will be similar to the parameters that are described for a conventional water source heat pump condenser water loop.
A final variation on the water cooled condensers is the inclusion of a ground coupled system. In this case, the condenser water will be pumped through a series of wells that couple the condenser water to the ground temperature. This portion of the modeling should generally follow the procedures specified in the section on ground source heat pumps elsewhere in this chapter.
The baseline building does not have a VRF system.