Final Report: HybridAir: An Integrated Ventilation, Vapor Compression, and Indirect Evaporative Cooling System

EPA Contract Number: EPD06039
Title: HybridAir: An Integrated Ventilation, Vapor Compression, and Indirect Evaporative Cooling System
Investigators: Bourne, Dick
Small Business: Davis Energy Group, Inc.
EPA Contact: Richards, April
Phase: I
Project Period: March 1, 2006 through August 31, 2006
Project Amount: $69,988
RFA: Small Business Innovation Research (SBIR) - Phase I (2006) RFA Text |  Recipients Lists
Research Category: Pollution Prevention/Sustainable Development , SBIR - Green Buildings , Small Business Innovation Research (SBIR)


The purpose of this project was to complete “proof-of-concept” development of a novel, high-efficiency single-package residential cooling system called “HybridAir.” The technology combines advanced indirect evaporative cooling and mechanical ventilation components with vapor compression cooling components to reduce energy use significantly when compared to conventional cooling systems. Specific project goals were to:

  • Demonstrate 3-ton cooling capacity at 95°F/67°F dry bulb/wet bulb.
  • Achieve 100 percent indirect evaporative effectiveness.
  • Reach steady-state energy efficiency ratios (including all blower and parasitic energy), with both evaporative and vapor-compression stages operating, as indicated below:
    • 20 at 104/70 outdoor air and 80/67 indoor air
    • 17 at 95/75 outdoor air and 80/67 indoor air
    • 22 at 95/67 outdoor air 80/67 indoor air


Conventional air-cooled vapor compression cooling systems are well evolved and thus are unlikely to experience significant future efficiency improvements. These systems cause high electrical demands and, because of their split-system residential configuration, do not deliver the ventilation air that is necessary to improve indoor air quality.

The HybridAir concept combines advanced indirect evaporative and vapor compression cooling processes to resolve problems associated with conventional cooling systems. Unlike two-stage evaporative cooling systems, whose applications are limited essentially to dry climates, HybridAir offers advantages in a broad climate range. The HybridAir blower delivers a mixture of outside air and building return air into a quasi-counterflow, indirect evaporative cooling stage. Supply air then is further conditioned by the evaporator coil of a downsized vapor compression cooling stage.

Project Work

Because of Phase I cost constraints, Davis Energy Group’s (DEG) proposed effort for this proof-of-concept project involved modifying an available indirect evaporative cooling module (ICM), which was developed in a previous DEG project; this module was combined with a small vapor compression stage customized for this project. The proposal specified the following tasks:

  • Identify indirect stage and condenser and evaporator coil design options.
  • Build, and laboratory test, a proof of concept prototype and measure performance.
  • Develop description of operating modes and preliminary control strategy.

In addition to these specified tasks, the project also evaluated HybridAir benefits and economics based on project results.

Task 1 Summary: Optimize Indirect and Vapor Compression Stages. Task 1 pursued two possible strategies for achieving the ambitious performance goal of 100 percent indirect effectiveness. Neither strategy proved successful under the constraints imposed by the basic platform of the available ICM. The project team then considered demonstrated results from other plate-type indirect evaporative heat exchangers that were designed for higher effectiveness, and use longer airflow paths. Figure 1 shows the results of these analyses, which indicate that with proper design, extending the current 14-inch flow path to approximately 22 or 27 inches should increase the effectiveness to 80 or 100 percent.

Figure 1. Comparative Performance of Indirect Evaporative Plates
Figure 1. Comparative Performance of Indirect Evaporative Plates

The project team also designed an appropriate vapor compression stage. The goal of this design effort was to select a relatively small compressor (rated at approximately 16,000 BTU/hr) that would deliver approximately 2 tons of cooling at typical operating conditions.

Task 2 Summary: Build and Laboratory Test Prototype; Measure Performance. The project team fabricated a HybridAir proof-of-concept prototype and conducted tests in the simulated outdoor chamber of the DEG test laboratory. This facility allows relatively precise control of the outdoor dry and wet bulb conditions. The laboratory is equipped with high-quality airflow, humidity, and energy use sensors and data logging devices for accurate monitoring of HVAC components.

Figure 2 shows tested and projected performance results with both HybridAir cooling stages operating. In this full capacity mode, the prototype HybridAir unit, with low (55%) indirect effectiveness and an air-cooled condenser coil, delivered cooling roughly comparable to a 13 seasonal energy efficiency ratio (SEER) conventional unit. HybridAir efficiency increased markedly when the condenser was cooled evaporatively. The third and fourth bar sets indicate that with 80 percent indirect effectiveness and a water-cooled condenser, performance can be substantially above conventional systems, and very near project targets.

Figure 2. System Performance
Figure 2. System “Full Capacity” Performance

Task 3 Summary: Describe Operating Modes and Preliminary Control Strategy. As with hybrid cars, an effective control strategy will be vital to maximizing HybridAir efficiency. In this task, the project team focused on simplification by reducing the number of HybridAir operating modes from six possible modes to no more than three—a two-stage cooling set and an optional heating mode.

Added Task Summary: Evaluate HybridAir Benefits and Economics. In this task, the team simulated annual cooling performance and developed installed cost estimates to assess HybridAir marketability in a range of U.S. locations compared with conventional systems. Figure 3 shows estimated cooling energy savings percentages based on measured effectiveness and predicted performance at targets of 80 and 100 percent indirect effectiveness. Substantial energy savings result primarily from very efficient Stage 1 (noncompressor) operation during many cooling load hours.

Figure 3. Annual Percentage Energy Savings
Figure 3. Annual Percentage Energy Savings

The team also developed a detailed cost comparison and determined that by eliminating the condensing unit shell, fan, and associated components and labor, the HybridAir technology may achieve a near-term lower installed cost (instant payback) compared with a conventional split system.

Summary/Accomplishments (Outputs/Outcomes):


There is a large potential HybridAir market, with the strongest market consisting of new apartments and single-family homes. Table 1 illustrates that air conditioning has become standard equipment in most new U.S. homes over the last 30 years. These data show that homebuyers are increasingly demanding the comfort afforded by air conditioning, even in locations where it is seldom needed. HybridAir can help reverse the peak demand consequences of this trend on electric utilities, while providing improved indoor ventilation, which may be mandated in the next decade.

Table 1. Percentage of New Single Family Homes With Air Conditioning






All of the U.S.

























Production Prototype Design

For optimal performance, HybridAir needs its own platform, which incorporates a longer airflow path through the indirect heat exchanger. Under another project, DEG is developing a high-speed, low-cost indirect heat exchanger with a 24-inch by 30-inch plate size. Although the airflow patterns of the plate design are not appropriate for HybridAir, the size and production processes are suitable. A custom-designed 24-inch by 30-inch HybridAir plate system should easily attain 80 percent indirect effectiveness and may approach 100 percent effectiveness.

Other Benefits

HybridAir will provide efficient cooling and fresh air ventilation in all climates and will reduce energy costs by minimizing compressor operation and applying an efficient variable speed blower. HybridAir also can deliver other homeowner benefits, including peak load reduction, improved ventilation and indoor air quality, and elimination of the condensing unit. The flush wall-mount HybridAir configuration also promises to enhance aesthetics, available outdoor space, and public safety.

Building standards and regulations are moving toward requiring mechanical ventilation. HybridAir provides ventilation without requiring additional equipment and offers societal benefits, including reduced electricity use and greenhouse gas emissions. Because HybridAir will reduce power plant NOx emissions during peak periods, it also will help reduce smog on hot sunny days when air quality is poor.


Results from this proof-of-concept project suggest a significant potential for HybridAir. The laboratory prototype demonstrated the benefits of integrating a small vapor-compression system with indirect evaporative cooling; economic studies based on actual project results indicate significant HybridAir energy-savings benefits, even at the 55 percent indirect effectiveness measured for the prototype. At 80 percent indirect effectiveness, which is clearly achievable with an improved design, annual cooling energy savings should range from approximately 30 percent in eastern U.S. cities to about 70 percent in mountain cities such as Denver. HybridAir also should deliver 20 to 35 percent peak demand savings, depending on climate.

Based on the proof-of-concept work completed in this Phase I SBIR project, HybridAir offers significant advantages over conventional residential cooling systems in a wide range of applications.

Supplemental Keywords:

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