Several evaporative-cooling options are available, including direct, indirect, and two-stage evaporative coolers; condenser air precoolers; the DualCool; and the EER+.
Direct cooling. Direct evaporative coolers blow air over a wet surface. Heat in the air evaporates moisture from the surface, thereby lowering the air temperature (figure 1). Although these systems typically use less than a quarter of the energy that vapor-compression air conditioners do, they’re often restricted to industrial or warehouse applications in drier climates because they add moisture to the building air supply. Their suitability for a particular application depends on the cooling load and the range of outdoor wet-bulb temperatures, a metric that incorporates both the temperature and the humidity of the air.
Figure 1: Direct and indirect evaporative coolers
A direct evaporative cooler adds moisture to the building supply air, whereas an indirect evaporative cooler doesn’t. A two-stage or indirect-direct evaporative cooler first uses an indirect stage before passing the building supply air through a direct stage. The darker-colored arrows indicate that moisture has been added to the air stream.
Because evaporative cooling requires a moving air stream, the amount of indoor air that’s exhausted from the building must be equal to the amount being supplied. If the amounts aren’t equal, the building will become pressurized, which leads to insufficient airflow in addition to difficulty closing doors and air whistling through stairwells and elevator shafts. When comparing direct evaporative coolers, the most relevant metric to use is the effectiveness of the unit. Effectiveness is a term that quantifies, as a percentage, how close to the wetbulb temperature the unit can reach. Air that reaches 100% relative humidity in an evaporative process emerges at the wet-bulb temperature, the theoretical limit for direct evaporative cooling; the effectiveness of such a (rare) cooler would be 100%. Although older coolers typically achieved 50% to 80% effectiveness, properly functioning and well-designed systems with thicker media—at least 10 inches—can achieve 93% effectiveness.
Indirect cooling. Indirect evaporative coolers use the evaporative-cooling process without adding moisture to the building supply air (figure 1). This makes them suitable for a wider range of applications, and they can be combined with traditional compressor-based systems. Indirect evaporative coolers can take a couple of forms:
- Self-contained. The building supply air (or primary airflow) flows through a heat exchanger. The building exhaust air (or secondary airflow) is evaporatively cooled and passed through the other side of the heat exchanger, thereby removing heat from the supply air. This approach can be used in many climates because the outdoor humidity levels don’t significantly affect the evaporative-cooling process.
- Tower/coil approach. Often called a water-side economizer, this approach uses a cooling tower to produce cool water that’s fed to a separate finned cooling coil in the supply air stream. The cooling tower could be part of an existing water-cooled chiller plant.
Two-stage cooling. Two-stage evaporative coolers—also called indirect-direct evaporative coolers (IDEC)—employ both indirect and direct stages, and thus can produce cooler air than either stage can produce alone. The first stage uses an indirect section to cool the air without adding moisture. The air is then directly evaporatively cooled in the second stage. This produces air at a temperature lower than the outdoor wetbulb temperature, which isn’t possible with direct evaporative cooling alone. Because the two-stage approach introduces less moisture to the air than direct evaporative cooling does, it can be used in more building types; however, because IDECs still rely on the evaporative-cooling process, they work best in dry climates. To meet peak-cooling needs, especially on humid days, enlist the help of an HVAC designer to properly specify system components.
Condenser air precoolers. This type of evaporative cooler has been available for many years for large and small air-cooled systems. Large units typically use flat, rectangular rigid-media blocks with a sump and pump placed over the intake side of the condenser coil. For example, a precooler with 70% effectiveness can deliver 10% total energy savings and 20% peak demand savings; a precooler with 50% effectiveness can deliver 8% total energy savings and 15% peak demand savings.
The DualCool. The DualCool, which is intended for packaged rooftop units of 15 tons or larger, employs both direct and indirect cooling approaches. It uses a direct evaporative cooler to precool the condenser air and an indirect evaporative cooler to precool the building supply air. As with other evaporative coolers, the DualCool works best in drier climates. Originally designed by the Davis Energy Group, an HVAC consulting firm, it’s now offered by Integrated Comfort Inc.
A study by the Heschong Mahone Group consulting firm provides some savings estimates for the DualCool. The study estimates that units in Fresno, California (a hot, dry climate), and Santa Rosa, California (a milder, more humid climate), delivered annual energy savings of 24% and 16% and demand savings of 0.43 and 0.19 kilowatts per ton, respectively.
The EER+. The EER+ is a heat-exchange module that can be attached to existing air-cooled air conditioners and heat pumps to increase their efficiency. Manufactured by Global Energy Group, the module works by capturing waste condensate water from the rooftop unit and routing it over evaporative-cooling pads; exhaust air or outdoor air is blown across the pads (figure 2).
Figure 2: How to evaporatively cool an air conditioner
In the EER+ module, an evaporative-cooling pad uses condensate water to subcool and desuperheat the refrigerant.
As demonstrated in figure 2, the resulting evaporative cooling removes heat from the air-conditioner refrigerant after the compressor and subcools it after the condenser—thereby increasing the efficiency and capacity of the system. The EER+ works in most climates using the exhaust air from the building, as outdoor humidity won’t significantly affect the heat exchangers. However, when using outdoor air in humid climates, the efficiency increase won’t be as great as it is in dry climates.
The efficiency gains depend on the efficiency of the existing system: The lower the efficiency of the existing system, the more benefit the EER+ can offer. The EER+ costs from $400 to $1,100 per ton installed, depending on the size of the unit (smaller units are more expensive per ton). It’s available in capacities from 6 to 100 tons, and even larger capacities are possible by connecting multiple units. Payback periods vary based on the cooling load of the building.