Energy/Heat recovery plays a pivotal role in HVAC systems. The primary goal is to reduce energy consumption and enhance overall system performance. By capturing and repurposing energy that would otherwise go to waste, these devices contribute to cost savings, environmental sustainability, and increased efficiency.
At the heart of any energy recovery system is the heat exchange device, which is what facilitates the transfer of sensible, and in some cases latent energy. In HVAC applications, air-to-air heat exchangers are most common and are devices that recover heat from the exhaust air stream, which is then used to pre-heating or pre-cooling the incoming fresh air. This process mitigates the load on the system, reducing heating and cooling requirements.
Here is a look at the main heat exchange devices found in air side HVAC systems:
Plate Heat Exchangers:
Designed for efficient heat transfer through closely spaced metal plates. This design allows for significant surface area exposure, ensuring effective heat exchange between the airstreams. Plate heat exchangers sit in the middle for overall effectiveness (60% or so, with lower latent performance), however they have advantages in that they have no moving parts and almost no cross contamination (typically 0% at the beginning of their life). Air pressure drop in plate type systems tends to be higher than other approaches which can increase brake horsepower in a system. Enthalpic cores are available, but they typically have even higher air pressure drop and lower overall effectiveness (50%). Plate exchangers are subject to defrost cycles when ambient temperatures drop, either by stopping the supply are blower allowing the return air to defrost the core, or employing a separate regeneration damper to recycle exhaust air through the supply air section of the heat exchanger.
Rotary Heat Exchangers:
Rotary Heat Exchangers – Swegon
Often referred to as a heat wheel, these devices rotate between the exhaust and supply air streams, transferring heat in the process. While offering generally better overall effectiveness than a plate type heat exchanger (generally in the 70% range) their continuous rotation does have a higher incidence of cross-contamination between the two air streams (typically 4% or so). Heat wheel technology is also more effective at latent transfer than other strategies and typically is a great choice when a bit of cross contamination is acceptable, and the wheel and its components to move it can be inspected on a regular basis. Heat wheels will be subject to a defrost strategy when ambient temperature drop, usually by slowing the wheel rotation and allowing the exhaust air stream to defrost the wheel.
Run-Around Coils:
Run-around coil systems utilize coils placed in the exhaust and supply air streams, with a fluid circulating between them. As the fluid absorbs or releases heat, it facilitates the transfer between the two air streams. Glycol run around loops tend to be the least effective solution and do not have the capability to provide latent energy transfer. Typically, this method is employed when you have an exhaust source that is not within the same unit as the supply air. These systems are not particularly effective and can require an external pump to transfer the fluid between both air streams. One upside is glycol coils are not subject to a defrost cycle.
Regenerative Cores:
Nagas SBE
The ultimate in effectiveness in heat recovery, regenerative cores can offer up to 95% effectiveness but typically run slightly lower. Regenerative cores utilize 2 plate type heat exchangers in parallel and charge one core while discharging from the other, utilizing a damper system rapidly flip the airstream between cores (typically between 20-60 seconds per cycle). Regenerative cores are the heaviest and largest of the devices mentioned here, but can be a game changer in systems that run constantly. The other benefit of regenerative cores is they typically have no defrost cycle – which results in constant energy recovery.
Energy recovery is ubiquitous in systems today and dependent on arrangement there is a device to suit any application. O’Dell specializes in providing the right heat recovery solution for each application and have several vendor options to meet project budget requirements and timelines.
