Behind the Curtain Look into Turntide's 8760 Energy Modeling Tool 

The following sections walk through the inputs and methodology for the 8760 model that Turntide uses to quantify the energy savings of the Turntide motor relative to the existing motor its replacing. We start with the Site and HVAC inputs. We then talk through the 8760 model, particularly how we apply balance points to TMY3 weather data. And finally we summary how we use motor performance maps of Turntide and induction motors in the 8760 model to calculate motor energy usage. 

1 - Site Inputs

Start by entering the following inputs to define the site for the 8760 model on rows 1 – 40 on the ‘Input’ tab:

  • Climate Zone selects the built-in TMY3 weather data set. This establishes the dry-bulb temperature for each hour of the year. The 8760 model does need any other weather data from the TMY3 file. The ‘Climates’ tab lists the built-in climates, including the 16 ASHRAE climate zones.

ROI Tool 1.png
  • Utility rate sets the energy rate ($/kWh) and demand charge ($/kW). The demand charge can be turned off for certain months since some utilities only apply demand charges during the summer. The peak demand savings is based on the kW reduction from the baseline motor to the Turntide motor during second-stage cooling (cool-2) or heating (heat-2) operation, assuming the RTU is operating in second-stage during the building’s peak demand window. 

  • Five different speed modes for the existing and Turntide motors can be defined. For RTUs, this is: heat-2, heat-1, vent, cool-1 and cool-2. For other HVAC systems, such as pumps or AHUs, the user can define these modes using speed bins (i.e. ‘mode 1’ = 20-30% speed, ‘mode 2’ = 30-40% speed). 

  • The building type establishes the default heating and cooling balance points. The heating balance points are the outdoor temperature below which the HVAC system goes into heating mode. The cooling balance points are the outdoor temperature above which cooling modes are used. Default balance points are included for six building types based on our experience measuring HVAC operation with respect to outdoor temperature. A brief description of each buildings behavior:

ROI Tool 2.png

Data Center: Extremely large internal heat gains drive balance points to cooler temperatures.

Restaurant: Large internal heat gains drive cooler balance points, albeit not as low as data centers.

Office: Internal heat gains in between Restaurants & Retail 

Retail: Small internal heat gain density result in warmer balance points

Supermarket: Internal refrigerated cases drive balance points to be very high. This causes a significant increase in heating throughout the year. We have scene supermarkets have continuous heat in their refrigerated section when the outdoor temperature reaches the mid-80s.

Industrial/Warehouse: These buildings typically have wider heating & cooling setpoints which results in a wider gap between the balance points. 

If the site is in a humid location, we reduce all the balance points by 5°F to account for the trend of maintaining lower heating & cooling set points to drive more moisture removal. Occupants in more humid climates will desire colder space temperatures to drive more compressor operation which in turn removes more moisture from the conditioned space.

2 - HVAC Inputs

You can define up to 10 different HVAC systems. These include RTUs, AHUs, pumps and exhaust fans. The 8760 model uses the following inputs to define each system:

2.1 - Cooling Size

The cooling size in ‘tons’ input is only applicable for RTUs and AHUs. The 8760 model uses this, along with a typical kW/ton cooling efficiency, to calculate each hour’s cooling electric energy. We use 1.2 kW/ton as the default efficiency, translating to 10.0 EER. If your rated EER efficiency is known, you can override the default. For RTUs and AHUs with DX coils, the cooling energy includes the compressor and condenser fan. For RTUs and AHUs with chilled water coils, the cooling energy includes the chiller and pumps.

ROI Tool 3.png

Important note, this cooling size input is optional. You should leave it blank for pumps and exhaust fans. And you can leave it blank for RTUs and AHUs when you are running the existing and Turntide motors at identical speeds during cooling modes. But we recommend you use it for RTUs and AHUs where, during cooling modes, you will run the Turntide motor at different speeds from the baseline motor. The reason is fan speeds impact the cooling efficiency. To explain this from a thermodynamic perspective, compared to AHRI nominal rated airflow rates (typically 370 cfm/ton for RTUs), lower airflow rates will reduce the heat exchange effectiveness of the DX or water coil, thereby reducing the system’s overall cooling efficiency.

Applying the same HVAC empirical modeling methodology as the DOE whole building simulation program EnergyPlus, our 8760 model captures the impact of increased or decreased fan speeds during cooling operation. We use an empirical (regression) equation developed from RTU lab testing by NREL at their Thermal Test Facility. Table 1 shows how this empirical equation increases the kW/ton at lower fan speeds. 

Table 1.png

Fortunately, you will want to leverage Turntide’s variable speed capability to reduce the fan speed during cooling operation. The reason is that the additional Turntide fan energy savings outweigh the slight cooling energy increase because Turntide compounds its higher motor efficiencies with the Fan Affinity Law’s cubic relationship between fan speed and power. 

Table 2 provides an example of a 10-ton RTU with a NEMA premium 3-hp baseline motor for a Retail store in Phoenix AZ. Scenario 2 shows the increased fan energy savings outweigh the slight increased cooling energy.

Table 2.png

2.2 - Motor Specs

Enter the existing motor specs including the nameplate horsepower (HP), NEMA standard or premium efficiency, Loading (described in the following paragraph) and nameplate speed. We used this information to calculate the existing motor power draw and torque in each of the five speed modes. We then use motor performance maps to establish the existing motor efficiency. 

The motor Loading is the brake horsepower (BHP) divided by the nameplate horsepower. The Loading can easily be measured on-site by dividing the amp draw on one of the motor phases by the nameplate Full Load Amps (FLA). The typical Loading ranges we have scene for various HVAC systems are: 

  • 50% to 70% for RTU supply fans

  • 60% to 90% for AHU supply fans and exhaust fans

  • 40% to 90% for pumps

The last motor input is the Turntide motor size. This is typically equal to the existing motor size. Yet there are times the Turntide motor can be smaller than the existing motor while still meeting the necessary load and meeting the correct NEMA frame size.

2.3 - Motor Speeds

For each of the five modes, you can enter a different speed for the existing and Turntide motors on the ‘Input’ tab in rows 71 to 75. These speeds will apply for all 10 of the HVAC systems. For constant speed applications, set all modes to 100% speed. Reference speeds for RTUs are provided. If necessary, using rows 79 to 94, you can override each HVAC system to assign it its own unique speeds.   

For RTU and AHU fan systems, you must select whether the fan is set to ‘ON’ vs ‘AUTO’ mode. These selections determine the fan operation during ‘Vent’ mode. ‘ON’ means the supply fan runs during occupied times even when neither heating or cooling is operating to provide ventilation. RTUs without outdoor air dampers will typically be set to ‘Fan AUTO’ mode, meaning the fan only turns on during heating or cooling operation. 

While building codes mandate that commercial and industrial spaces use ‘ON’ mode during occupied times for RTUs with outdoor air dampers, per ASHRAE Standard 62.1, in practice it is common to find buildings that run their RTUs in ‘Fan AUTO’ during occupied times to reduce comfort complaints. This behavior is particularly prevalent in colder climates where employees and customers become uncomfortable from cold outdoor air falling on them while the RTU is not in heating mode. Moreover, a multitude of research studies evaluating RTU outdoor dampers found that the failure rate of outdoor dampers due to some issue (broken actuator, unplugged, excessive corrosion preventing actuation) ranged from 43% to 100% of the RTUs evaluated in each respective study (ACEEE 2014 publication). Consequently, the outdoor air damper should be investigated in addition to ensuring the fan control is ‘ON’ if the RTU provides ventilation to the space.

3 - 8760 Model

The 8760 model is configured for RTU systems. It uses the Site and HVAC inputs, detailed in the previous sections, to calculate the RTU supply fan, cooling and heating energy usage each hour of the year. These can be rolled-up to establishe the annualized energy.

3.1 - Balance Points

At the heart of the model, the fraction of each hour spent in each of the five operational modes is calculated based on the balance points and outdoor temperature. Figure 1 gives a visual example of the 8760 model for a Retail store in ASHRAE climate zone 5b, Denver CO. The green area plot shows the annual hours across the temperature range. The heat-1 and cool-1 balance points start at 60°F. The heat-2 balance point is 10°F colder while the cool-2 balance point is 10°F warmer. 

As the outdoor temperature continues to drop below the heating balance points, the fraction of each hour spent in heating linearly increases (see red lines). The same holds true for cooling, as the outdoor temperature increases (see blue lines). The remainder of each hour is in vent mode (see grey line) which means the fan is operating if it is set to ‘ON’. If the fan is set to ‘AUTO’, the fan is off. 

Figure 1.png
Figure 2.png

The table inside Figure 1 above summarizes the annual hours spent in each of the five RTU modes. For non-RTU systems, the annual hours and percent time spent in each mode can be overridden in rows 108 to 126 in the ‘Input’ tab. When overriding the 8760 model, we recommend acquiring BAS trend data from the building which will reveal hours spent in each mode with respect to the outdoor temperature. This trend data can be easily extrapolated to capture the annualized time in each mode.

3.3 - Heat Pump Operation

If the RTU is a Heat Pump, the 8760 model provides all heating above 40°F outdoor temperature with heat pump operation. Any heating below this temperature is provided by back-up electric resistance heat.

4 - Motor Performance Maps

The 8760 model uses performance maps of the existing motor to establish the motor efficiency based on the operating RPM and torque. This is done at 100% speed. By knowing the power input into the motor and the motor efficiency, we calculate the power leaving the existing motor shaft. This is also known as the fan (or pump) brake horsepower since it is the power entering the fan blade. Using the fan brake horsepower and the existing motor speed, we calculate the power into the existing motor at each of the five modes. 

Similarly, we use the existing fan brake horsepower at 100% speed to calculate the fan (or pump) brake horsepower for each of Turntide’s five speed modes. Then, we use the Turntide motor performance maps to calculate the motor efficiency based on the torque and RPM output.  Finally, we use these motor efficiencies to calculate the power into the Turntide motor at each of the five speed modes. 

The hourly fan energy in the 8760 model is simply the multiple of the power into the motor at each of the 5 modes times the fraction of the hour in that mode. If the hours in each mode were override in the ‘Input’ tab, then the calculator uses those overrides instead of the 8760 model hourly output. Figures 2 and 3 show the motor performance map for a 2 hp Turntide motor and a 2 hp NEMA standard efficiency induction motor. 

Figure 3.png
Figure 3.png