Scenario Five - Variable Refrigerant Flow

The final scenario to examine, again completely changes the HVAC equipment while leaving the building envelope, and internal loads the same. Variable refrigerant flow (VRF) incorporates multiple heat pumps or fan coils on a common refrigerant line, tied to a condenser unit and boiler, or to air source heat pump for the heating and cooling source. Again, outdoor air will be provided with a DOAS and ducted separately to the space. It will also be controlled via CO2 control for even further outdoor air reduction. The key building components are summarized with a prescriptive target in the table below.

Table 11: Variable Refrigerant Flow Prescriptive Summary

This type of system, while the most efficient of the options, also has challenges to face in a retail scenario. Again, many of them are related to communication between the design team and the tenant as previously recounted, but additional challenges include:

  • The air source heat pump that provides the heating and cooling for some variations of this system, performs very poorly in cold conditions. As the
    outdoor air temperature drops, the COP drops until it is acting as nothing more than an electric resistance heater. Subsequently, if the outdoor air
    temperature continues to drop below -20°C, the heating will no longer function. To rectify this, a backup boiler is often used.
  • Similar to the hydronic heating option, options that include a boiler, either as the primary heating source, or as a backup will require the use of previously leasable floor space.

Assuming an average utility rate of $0.10/kWh for electricity, $0.25/m³ for natural gas, and $0.004/ekWh for purchased offset, the estimated utility costs for the ‘VRF’ scenario are summarized along with the economics of including PV in the following table.

Table 12: Variable Refrigerant Flow Cost Summary

To ensure that 51% of the site energy use is covered by onsite generation, the ‘VRF’ option will require 19 kW of roof mounted PV with no parking lot mounted PV. An estimate of the layout required is presented below.

Figure 11: Variable Refrigerant Flow PV Array – System now fits entirely on roof

Directly comparing the ‘VRF’ option to the ‘Business as Usual’ scenario, it easily becomes the most efficient of the five scenarios. The effects of the upgraded building on the energy end(uses can be explored in more detail with the aid of the following graphic.

Figure 12: Energy End Use 4 Business as Usual vs. Variable Refrigerant Flow

Domestic Hot Water – No change from previous scenario.

Fans - Fan power is showing as slight drop in energy consumption. This is due to the variable speed fans in both the ERV and the zone units.

Pumps – Although pumps are showing up as minimal in this analysis, it is due to a work around required to correctly model the system as a whole. Since the pumps again are variable, the pumping power should be similar to the hydronic heating scenario. The energy required to run the pumps isn’t lost however, it is included in the fan energy.

Space Cooling – A small decrease in cooling is apparent from an increase in the cooling efficiency of the system.

Space Heating - Space heating has been reduced again. In this case, the heating reduction is due to the increase in heating efficiency, as a heat pump with a seasonal COP of 2.6 is providing most of the heating. Another benefit for conditioning in shoulder seasons is the common refrigerant line. Since each zone could independently be in heating or cooling, the heat rejection and addition to the line in each zone can bypass the need to use the heating or cooling source.

Miscellaneous Equipment – No change from previous scenario.

Lighting – No change from previous scenario.