Improvements to cooling systems are one of the most effective ways to increase your bottom line and meet sustainability goals, especially if your building requires a lot of cooling. Schools, data centers, offices, clinics, warehouse/distribution centers and others can make great candidates for capturing the benefits of indirect-direct evaporative cooling (IDEC), aka “2-stage evaporative cooling”. IDEC systems can use 60% – 75% less electricity than conventional refrigerant-based mechanical AC systems[i].
Due to their design, these systems are best suited for arid or semi-arid climates. Recently we worked on a 130,000 ft2 medical office building located in Salt Lake City, UT where an IDEC system can provide low-cost cooling up to 3,460 hours per year. This amounts to about 80% of the cooling load for the building in a typical year, saving tens of thousands of dollars per year in operating costs.
How do IDEC systems work?
IDEC systems provide cooler supply air at a lower relative humidity than direct evaporative coolers.
Figure 1: IDEC System [Adapted from Al-Juwayhel, et al. (2004)]
Stage 1 – Indirect Cooling
Shown in the diagram above, the first indirect stage cools the outdoor air without adding moisture. This cooling is typically accomplished using one of two methods. In one method a cooling tower may be used to provide naturally cooled water to a cooling coil in the air handler. More commonly, the inlet air is cooled using a cross-flow heat exchanger where outside air that is cooled using direct evaporation. First-stage cooling follows a line of constant humidity ratio as no moisture is added to the primary airstream. This method improves energy efficiency by lowering the dry-bulb and wet-bulb temperatures without the use of compressors.
Stage 2 – Direct Cooling
The same air stream passes through a water-soaked pad (evaporative media) where additional cooling takes place and the air picks up additional humidity required to condition the space. This evaporative media is designed to be 90 percent efficient. The second stage follows the wet bulb temperature (WBT) line at the condition of the air leaving the first stage. Because the air stream has already been cooled without adding moisture, final, near-saturated state has a lower humidity than it would using only a direct evaporative process.
Figure 2: IDEC process (ASHRAE, 2008)
Note: Functional Outside Air (OA), Return Air (RA) dampers, and enthalpy sensors are required to control this process
IDEC Advantages
IDEC systems offer two big advantages for customers. They:
- satisfy residential/commercial building loads at lower cost, and
- reduce environmental impact by providing CFC free cooling.
Drawbacks of IDEC systems
There are a few disadvantages and considerations of IDEC systems that can discourage installation. They are:
- IDECs are best suited for hot and dry climate areas – not every building will be an ideal candidate for an IDEC system.
- IDECs increase water consumption at the site. However, studies have shown that they have a net water savings relative to typical packaged rooftop units if one includes the source water usage due to typical power generation.
- Maintenance is key – poorly maintained IDECs, in particular in dusty environments, lose their effectiveness. Evaporative media need to be replaced, and circulating pumps require maintenance and attention.
- In areas with higher wet bulb temperatures, a third-cooling stage (e.g. direct expansion refrigeration unit or a chilled water coil) may be used in combination with IDEC to provide additional cooling if needed.
- Increased complexity of the system.
Moving Forward with and IDEC System
In the right location, IDEC systems can offer big rewards from reduced energy consumption and CFC-free cooling. It’s important to consider your building’s location and needs before moving forward with this type of project. In fact, we developed our excel add-in for psychrometrics to analyze the performance of these systems. If you need any assistance planning your next cooling retrofit, feel free to reach out anytime.
[i] According to American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE, 2008), HVAC System and Equipment, Chapter 40
