AHRI 920 Performance Rating and Comparisons of DX-DOAS Unit Efficiency

Introduction

 

In 2015, the Air-Conditioning, Heating, and Refrigeration Institute (AHRI) introduced a performance rating for dedicated outside air systems. All references in this document refer to the AHRI Standard 920 (I-P) document. This was a very timely announcement as it corresponded with the increasing demands for measurable energy efficiency solutions and methods to reduce global warming. At the same time the ASHRAE SPC 90.1- 2016 committee was updating their standard and wished to include minimum rating values for this type of product.

 

DX-DOAS Definition

 

DX-Dedicated Outdoor Air Systems (DX-DOAS) condition outdoor ventilation air independently from the building HVAC system. This approach to handling ventilation air results in superior humidity control by limiting the primary source of humidity in most buildings – ambient humidity carried in by the ventilation air – directly at its source before it enters the building. The decoupling of the ventilation air's requirements from the building's internal sensible requirements allows both systems to be designed for maximum energy efficiency. The energy savings can be obtained by running the separate, sensible cooling only, interior cooling system less often or at a higher evaporating temperature, thus improving the overall energy efficiency.

Further energy savings may be realized by providing only the amount of ventilation air necessary and/or by using energy recovery from the building exhaust air to pre-condition the ventilation air.

While the concept of introducing ventilation air is simple enough, in actual practice there are many different configurations that can be utilized to solve this goal. These different configurations have a significant impact on the energy efficiency of the DX-DOAS units, so the first step in the evaluation is to determine what configuration will be used at the specific jobsite. The category choices are:

• Energy recovery pre-conditioning
  If exhaust air can be brought back to the DOAS unit, then pre-conditioning the outside aire with an energy recovery wheel or plate heat exchanger will improve the energy efficiency of the system.
• Condenser
  There are several condenser types that impact the minimum efficiency of ASHRAE 90.1-2016
  • Air cooled (either split or packaged)
  • Water cooled (water temperature will impact system efficiency)
• Cooling Tower
  The dehumidification only system utilizes a cooling tower water loop for its condensing water source. The temperature of the fluid usually varies from 55°F to 90°F.
• Chilled Water
  The dehumidification only system utilizes a chilled water loop for its condensing water source. The temperature of the fluid usually varies from 40°F to 60°F.
• Ground source, closed loop
  A heat pump that uses fluid circulated through a subsurface piping loop as a heat source/heat sink. The temperature of the fluid is related to climate and operating history conditions and usually varies from 25°F to 100°F.
• Ground-water source
  A heat pump that uses water pumped from a well, lake or stream as a heat source/heat sink. The temperature of the water is related to climate conditions and usually ranges from 45°F to 75°F for deep wells.
• Water Source
  A heat pump that uses fluid circulated in a common piping loop as a heat source/heat sink. The temperature of the piping loop fluid is usually mechanically controlled within a moderate temperature range of 61°F to 89°F.

This rating standard has been developed to provide design engineers with a relative guide to quantify the energy a DX-DOAS system will consume while dehumidifying and reheating ventilation air to a neutral 70°F condition at several different outside air conditions. Unlike air conditioning systems, the dehumidification process requires the system to condition the air to the design dewpoint (this is established at a 55°F dewpoint in the case of this standard) in order to remove the moisture before it enters the space. The system then must use 75% to 90% site recovered energy, no new energy, to reheat to the neutral condition as stated in ASHRAE 90.1. Use of the hot gas reheat coil is a common solution utilized to achieve the site recovered requirement. The ambient conditions that the unit must be designed for vary throughout the year, so the AHRI standard utilizes four different dehumidification entering air conditions or rating points as a method to demonstrate the moisture removal efficiency (MRE) at part load conditions. This is expressed as the amount of moisture removed per kilowatt-hour of energy used.

Refer to Figure 1 for a psychrometric plot of the calculation of the total cooling required to meet the 55°F dewpoint and the amount of moisture that must be removed. In order to meet this demand, DOAS units must have a very low sensible heat ratio and have unique refrigeration circuits to handle this wide range of entering air conditions. A detail psychrometric review can be found in Desert Aire's Technical Bulletin #3, Dehumidification and the Psychrometric Chart.

 

 

If the system is a heat pump, the standard also defines two rating points for winter heating. The heating performance is stated as coefficient of performance (COP), which is units of watt output divided by the watt input. Refer to Tables 1 and 2 for the input values based on the configuration selected for the specific application.

To model real world conditions, the standard also has established an external static pressure for various supply air volumes and has added a pumping penalty to account for the energy consumption adder of water-cooled systems.

 

 

 

Integrated Seasonal Ratings

 

The AHRI committee used a composite bin hour data set of multiple cities to provide a weighted value moisture removal efficiency and coefficient of performance value. This weighted value puts emphasis on the part load values in calculating the integrated seasonal ratings. Table 3 lists the weighting percentage values.

 

Comparison Dehumidification

 

The AHRI 920 standard provides a method to compare a specific manufacturer’s design to the minimum standard as published by the AHRAE 90.1-2016. This allows the design engineer the ability to evaluate the different configurations of manufacturers of DX-DOAS equipment in order to meet the energy goals of the building owner.

For an air cooled DOAS that has an ISMRE value equal to 8.0, the comparison to the ASHRAE 90.1-2016 minimum value of 4.0 would provide a conclusion that the system was two times as efficient (8.0 / 4.0 = 2.0). To estimate the true cost of energy, you will need to convert t he ISMRE value into a kilowatt value and annual energy cost. One way to roughly estimate the cost for dehumidifying the outdoor air could be as follows:

 

1) Using the weighted average moisture removed from the integrated values, calculate the average capacity (see sidebar for details of the constant derivation).

2) Using the average moisture capacity, convert the efficiency to average power.

3) Multiply the average power use by the local utility rate and the bin hours about the design dew point.

For example, a 5,000 cfm DOAS unit would remove a weighted moisture content of roughly 100 lb/hr. A 4.0 ISMRE system would have an average power of 25 kW during dehumidification. Using a bin hour total in the dehumidification mode of 4,800 hours and a utility rate of $0.10/kWh would provide an annual cost of $12,000. The selected unit at an ISMRE value of 8.0 would cost only $6,000, thus the selected unit would save the owner $6,000 per year of operation.

Although there are variances in loads for any particular area and the weightings of the 920 standard are a composite of many different area of the US, experience has shown that the formula above gives a very good estimate of the energy costs.

Results can be used to make appropriate decisions regarding the initial system cost verses continued energy use.

Comparison Heating

 

A heat pump system can also have its heating energy cost compared for its annual comparison. For a ground source, closed loop DOAS that has an ISCOP value equal to 4.0, the comparison to the ASHRAE 90.1-2016 minimum value of 2.0 would provide a conclusion that the system was two times as efficient (4.0 / 2.0 = 2.0). To estimate the true cost of energy, you will need to convert the ISCOP value into a kilowatt value and annual energy cost. One way to roughly estimate the cost for heating the outdoor air could be as follows:

1) Using the weighted average heating requirement removed from the integrated values, calculate the average heating capacity (see sidebar for details of the constant derivation).

2) Using the average moisture capacity, convert the efficiency to average power.

3) Multiply the average power use by the local utility rate and the bin hours above the design dew point.

For example a 5,000 cfm OAS heat pump unit would require a weighted heating requirement of 62.5 kW (213 MBH). A 2.0 ISMRE system would have an average power of 31.25 kW during heating. Using a bin hour total in the heating mode of 3,950 hours and a utility rate of $0.10/kWh would provide an annual cost of $12,344. The selected unit at an ISMRE value of 4.0 would cost only $6,172, thus the selected unit would save the owner $6,172 per year of operation.

 

Conclusion

 

DOAS units operate in many different climates and can be selected with a wide variety of designs and condensing options. This makes it difficult to compare the energy efficiency of treating outside air from one design to another or from one manufacturer to another manufacturer. However, this is extremely important as the process of treating outside air to improve indoor air quality is energy intensive. The AHRI 920 standard is a tool that can be used to compare the energy efficiency of different manufacturers' offerings to optimize the energy consumption for the end user.

In addition, ASHRAE has established minimum values for DOAS units. This provides the designer the base level to establish the specification criteria. By using the calculations in this application note, the design engineer can also compare the payback time required to specify the higher energy efficient designs.