Heat Recovery in Air Handling Units: Enhancing Energy Efficiency and Cost Savings

Introduction

Integrating heat recovery techniques in air handling units (AHUs) has become a crucial aspect of HVAC systems in the quest for energy efficiency and sustainability. By capturing and utilising heat from exhaust air, these systems can significantly reduce energy consumption and lower operating costs while maintaining comfortable indoor environments. Let's explore the various heat recovery methods used in AHUs.

The Importance of Heat Recovery in AHUs

AHUs are essential components of supply and extract systems in HVAC, comprising separate fans, air filters, and heating coils. Traditionally, heating coils provide the primary source of heat for buildings. However, by implementing heat recovery systems, a portion of warm exhaust air can be recirculated into the supply, reducing the need to heat fresh air. This approach conserves energy and enhances indoor air quality by minimising the influx of outdoor pollutants.

Heat Recovery Technologies

1. Plate Heat Exchanger (Recuperator): A plate heat exchanger enables efficient heat transfer from exhaust air to incoming fresh air without mixing the two streams. Studies show that plate heat exchangers can achieve heat recovery efficiencies ranging from 60% to 80%.

2. Run-Around Coil System: Run-around coil systems utilise separate coils in the exhaust and supply air streams connected by a heat transfer fluid. This fluid transfers heat from the exhaust to the supply air. Studies indicate that run-around coil systems can recover up to 70% of the heat from the exhaust air.

3. Thermal Wheel (Rotary Heat Exchanger): Thermal wheels have a rotating wheel with heat-absorbing material. As the wheel alternates between the supply and exhaust air streams, efficient heat exchange occurs without air mixing. Thermal wheels have demonstrated heat recovery efficiencies ranging from 60% to 90%.

4. Heat Pipes: Heat pipes use a sealed tube with an inner wick structure and a working fluid to transfer heat. Studies show that heat pipes can recover up to 70% of the heat from the exhaust air.

Cost Savings

The implementation of heat recovery in AHUs can result in significant cost savings. On average, studies indicate that heat recovery systems can reduce heating energy consumption by 20% to 40% in commercial buildings. Considering the average annual heating costs in the UK, a large office building with an annual heating cost of £100,000 could save £20,000 to £40,000 per year through heat recovery.

Conclusion

Heat recovery in air handling units is vital in enhancing energy efficiency and cost savings in HVAC systems. Technologies like plate heat exchangers, run-around coil systems, thermal wheels, and heat pipes allow efficient heat transfer from exhaust air to incoming fresh air without mixing the two streams. Building owners can achieve notable energy savings and reduce operating costs by incorporating these heat recovery systems.

Examples of Out of the Box Heat Recovery

This Mitsubishi Lossnay heat recovery system is often used with a VRF system.

Courtesy of Mitsubishi Electric