Air-to-Air Energy Recovery System
Understand how the chilled water loop works. Explore component overviews, measurements, and additional resources.
Overview
An air-to-air energy recovery (AAER) system consists of a heat exchanger component, air ducts and any associated device for its operation. AAER systems extract sensible (and sometimes latent) energy from a previously conditioned “outgoing” airstream and use that energy to pre-condition a separate “incoming” air stream. Typically, the outgoing airstream is drawn from occupied spaces in a building, and the incoming air is from the outdoors. The two airstreams must be at different temperatures for sensible heat transfer and different moisture contents for latent transfer.
Figure 1 shows the main components associated with a fixed plate heat exchanger including the heat exchanger, air ducts and fan and motors. In a AAER with a rotary wheel heat exchanger there is also a motor used to power the rotary wheel.
Components
Consists of a belt driven wheel that is built up by a matrix of foils. The wheel rotates and moves through both the incoming and outgoing air streams, effectively transferring heat between them. The pressure drop and efficiency of the exchanger depends on the wheel diameter and density of corrugation. Most rotary wheel exchangers transfer both sensible heat and latent energy.
Fixed plate heat exchangers
A fixed plate heat exchanger functions by providing neighboring channels, separated by plates, for airstreams to pass through and transfer energy. The channel separation determines both the pressure drop and efficiency of the heat exchanger. Many fixed plate heat exchangers only transfer sensible heat, but they can be designed to transfer latent energy (i.e., moisture) as well, depending on the material used.
Coil Energy Recovery (Runaround) Loops
Coil energy recovery loops, also known as runaround loops, make use of a heat exchanging coil that connects the incoming and outgoing exhaust air streams. The loop contains a heat transfer fluid (i.e., water, glycol) and a pump to move the liquid between both streams. Coil energy recovery loops transfer sensible heat only.
Heat Pipe Heat Exchangers
Heat pipe heat exchangers operate by using a working fluid that changes phases to transfer heat. One side of the heat pipe is placed in the exhaust stream and the other is placed in the supply stream, causing one side of the pipe to undergo evaporation and the other side to undergo condensation, effectively transferring heat. Heat pipe heat exchangers transfer sensible heat only.
Heat Exchanger Motor
Most heat exchangers are passive devices and rely on the fan and motor components in other systems to move air through them. In the case of a rotary air-to-air energy exchanger, the heat exchange medium is rotated through the airstreams, where the exchanger medium picks up or releases heat and possibly moisture. A small constant or variable speed motor is needed to rotate the heat exchanger.
Air Duct
Ducts transport air to and from served zones, heat recovery units and air handling units. Duct systems often include dampers and turning vanes to control flow volume and reduce pressure losses which can directly impact system efficiency.
How do I…
Assess System Energy Usage
A heat exchanger will increase the static pressure in the air streams which requires additional fan motor energy. This energy increase is quantified using the methods described in the VSVV AHU System. In a rotary wheel configuration, the motor that drives the wheel should be considered when estimating the energy impact. In a runaround loop (coil energy recovery loop) configuration, the motor that drives the pump should be considered.
Quantify Electricity Usage (kWh)
Determine motor true power and runtime.
Equipment to Measure
Heat Exchanger Motor
What to Measure:
- Hourly true RMS power draw (kW)
- Total operating time (hours)
Measurement Locations
Assess System Performance
The primary effect of air-to-air energy recovery is to reduce fuel and/or electricity usage by capturing waste heat. The impact of the energy recovery system on both temperature and humidity must be considered to gain a full picture of its energy impact.
Quantify Heat Recovered (Btu)
Measure supply, return, exhaust and outdoor air temperatures, supply, return, exhaust and outdoor air relative humidities, supply and return air flow rate, and hourly true RMS power draw of supply and exhaust fan motors.
How to Measure
Click any measurement below for detailed setup, data collection, and troubleshooting instructions.
Use this technique to measure the supply and exhaust fan motor hourly true RMS power draw.
Use this technique to measure average hourly outdoor air temperature.
Use this technique to measure the average hourly relative humidity of:
- Outdoor air
- Return air
- Supply air
- Exhaust air
Use this technique to measure the average hourly temperature of:
- Return air
- Supply air
- Exhaust air
Air Flow Rate (CFM)
Measure hourly supply and return air flow.
Borrow Equipment and Download Calculator
Reserve loggers, download the calculator, and enter your measured data.
Air-to-Air Heat Transfer Calculator
Provides estimated annual sensible and latent heat transfer during the heating and cooling season of an energy recovery ventilator with a rotary wheel heat exchanger.
For more details about the methodology behind this calculator, see Air-to-Air Heat Transfer.
Download CalculatordownloadMeasurement Locations
Further Reading
ASHRAE (2020). “ASHRAE Handbook: HVAC Systems and Equipment,” Chapter 1. HVAC SYSTEM ANALYSIS AND SELECTION. I-P Edition.
ASHRAE (2020). “ASHRAE Handbook: HVAC Systems and Equipment,” Chapter 26. AIR-TO-AIR ENERGY RECOVERY EQUIPMENT. I-P Edition.
CenterPoint Energy (2022). “Runaround Loops.” Energy Recovery Systems. https://www.centerpointenergy.com/en-us/Services/Pages/Runaround-loops-MN.aspx?sa=MN&au=bus
Heatex (2022). “Heat Exchanger Basics.” https://www.heatex.com/knowledge/heat-exchanger-basics. National Renewable Energy Laboratory (2003). “Laboratories for the 21st Century: Best Practices” https://www.nrel.gov/docs/fy04osti/34349.pdf
Paulina Kanaś (2019).” The influence of geometrical parameters on heat and mass transfer processes in rotary heat exchangers.” SN Applied Sciences.