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 wetbulb temperatures (a metric that incorporates both the temperature and the humidity level 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 does not. A two-stage, or indirect-direct, evaporative cooler uses an indirect stage first 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 is exhausted from the building must be equal to the amount being supplied. If the amounts are not equal, the building will become pressurized, which leads to insufficient airflow plus 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 percent relative humidity in an evaporative process emerges at the wetbulb temperature, the theoretical limit for direct evaporative cooling. The effectiveness of such a (rare) cooler would be 100 percent. Achieving more than 90 percent effectiveness in a direct cooler is difficult, requiring a very thorough mixing of water and air. Most evaporative air coolers operate with 70 to 90 percent effectiveness using wetted fibrous or corrugated pads or media.
Indirect cooling. Indirect evaporative coolers use the evaporative cooling process without adding moisture to the building supply air (see Figure 1). This makes them suitable for a wider range of applications, including offices, 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, as the name implies, and thus can produce air cooler than is possible with either stage 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 is not possible with direct evaporative cooling alone. Because the two-stage approach introduces less moisture to the air than direct evaporative cooling alone, it can be used in more building types, but because IDECs still rely on the evaporative cooling process, they work best in dry climates. To ensure that peak cooling needs are met, especially on humid days, enlist the help of an HVAC designer to properly specify system components. One packaged IDEC is the OASys from Speakman CRS, which can be used in small commercial and residential applications.
Condenser air precoolers. This type of evaporative cooler has been available for many years for both 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. These types of systems can theoretically provide much of the same savings as evaporative condenser air conditioners, although there’s no independent research to quantify their savings potential.
The DualCool. The DualCool employs two approaches in one design intended for packaged rooftop units of 15 tons or larger. 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 better in drier climates. Originally designed by the Davis Energy Group, an HVAC consulting firm, it’s now offered by Integrated Comfort Inc.
A 2003 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, respectively, annual energy savings of 24 and 16 percent and demand savings of 0.43 and 0.19 kilowatts per ton.
The EER+. The EER+ is a heat-exchange module that can be attached to both 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.
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 if the exhaust air from the building is used; outdoor humidity will not significantly affect the heat exchangers. However, when using outdoor air in humid climates, the efficiency increase will not be as great as it is in dry climates.
The EER+ system can boost energy savings by as much as 40 to 50 percent, but 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 larger capacities can be accommodated by connecting multiple units. Paybacks vary based on the cooling load of the building.