The two basic DualCool performance goals are to:

 

Compressors and Blower

The economic value of kW demand reduction accrues principally to the electric utility, although “demand,” “time-of-use” and “real-time” rate structures transfer some of the benefits to the customer. DualCool can be installed on most rooftop units at lower “cost per kW” than new generation, while eliminating the new generator’s fossil fuel consumption. DualCool peak demand reduction varies with climate, refrigerant, and ventilation setting. DualCool can also reduce the peak demand of blowers and condenser fans.

The following table shows typical DualCool kW demand reduction for refrigerant R410A (used in newer cooling units) at four peak outdoor weather conditions, with outdoor air quantity at 20%, 35%, and 50% of total air delivered to the indoors. This table is based on independent monitoring. kW reduction is about 5% lower for (older) units with R-22 refrigerant.

 

 

Table is based on the following assumptions:

1. 350 fpm DualCool media face velocity
2. 350 cfm/ton supply air
3. 2.5 sq. ft. per ton condenser face area
4. 60% vent air coil effectiveness (vs. DBT-WBT)
5. RTU efficiency 11 EER @ 95F
6. 1% blower speed reduction per 2% vent air
7. Condenser savings based on net load after reduced vent air and blower
8. Based on 60% direct evaporative effectiveness
9. Blower savings corrected for pump energy and 15% vent air coil penalty

 

DualCool Pump

From calculated demand reduction, deduct DualCool pump kW from the table below:

 

Overview

DualCool energy savings are more difficult to predict than demand reduction, since energy savings accrue over an entire cooling season. DualCool energy savings are possible in all three “motor sets” in the heating/ventilating/air conditioning (HVAC) unit: the compressor(s), the blower, and the condenser fan(s). The compressor is the driving force in the refrigerant system, the blower moves air across the cooling coil and into the building, and the condenser fan(s) move(s) air across the condenser coil that discharges heat to the outdoors. The DualCool pump slightly increases energy use, as is also covered in this section.

 

Compressors

The compressor(s) typically consume 35% to 55% of annual rooftop unit electricity. Of the three motor sets (compressors, blower, condenser fans), DualCool compressor energy savings are the most difficult to estimate. Compressor savings result from lower condensing temperatures caused by pre-cooled condenser air, and from lower cooling loads that result from pre-cooling ventilation air and reducing blower speed. They vary with equipment design, refrigerant type, outdoor and indoor temperatures, occupancy schedule, and ventilation air percentage.

At present the best strategies for predicting DualCool compressor energy savings are: 1. Apply the DualCool simulation developed by PG&E (contact ICI for details) 2. Use the approximate savings tables below; interpolate as necessary.

 

 

Blower

The blower typically consumes 40% to 60% of annual HVAC electricity. Most rooftop units use fixed-speed blowers that run during all occupancy hours to supply fresh air. Over a full year the blower often consumes more energy than the compressors. Design engineers specify a blower speed based on the peak expected cooling load. Since blower energy use increases with the cube of the blower speed, small reductions can save a lot of energy. For example, reducing blower speed by 15% reduces blower energy use by nearly 40%. Also, virtually all blower motor energy heats the building. So reducing blower speed also reduces the cooling load.

By pre-cooling ventilation air, DualCool reduces the required cooling capacity and allows the rooftop unit to satisfy peak loads at reduced blower speed. This results in cooler supply air and increased dehumidification, improving occupant comfort- allowing the thermostat to be set a little higher for equivalent comfort. In general, total flow velocity should not be reduced below about 275 cfm/ton of rated capacity (“cfm” is cubic feet per minute airflow rate).

DualCool’s annual blower energy savings can safely be estimated as: (kW reduction) X (annual occupancy hours) where: kW reduction = (blower base kW) x (1-(% cfm reduction))3 and: annual occupancy hours = (hrs/day) X (days used/week) X (50 weeks/yr)

Use this table to quickly estimate blower energy savings per “brake horsepower” (BHP) of specified blower power (available from the rooftop unit specification charts):

 

 

Condenser Fans

Condenser fans typically consume 4 to 9% of annual HVAC electricity. DualCool only saves condenser fan energy when a fan is disabled, or when condenser fan motors are replaced to improve efficiency. This typically occurs to limit airflow velocity across the DualCool evaporative media. Contact ICI engineers to assess the opportunity for condenser fan savings on a particular project.

 

DualCool Pump

DualCool pump size varies with tonnage. The table below estimates annual energy use.

 

Example Specs

20 ton rooftop unit, R-410A refrigerant, 20% outdoor air Retail application, 104F peak temperature Blower at 400 cfm/ton, bhp= 3.5, assume 15% speed reduction

 

Demand Reduction

From chart, 104F, 20% outdoor air, reduction = 0.39 kW/ton Peak Demand Reduction= 20 X .39 – .12 (pump) = 7.7 kW

 

Energy Savings

Compressor- from chart, 321 kWh X 20 tons= 6420 kWh Blower- from chart, 2371 X 3.5 bhp= 8299 kWh Condenser fans no savings DualCool Pump from chart, -288 kWh Total Annual Savings 14,431 kWh

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