Figure 1 - Courtesy of Venmar CES
This is my favorite engineering topic, bar none. It fascinates me how much energy is required to heat and cool our buildings. Even more interesting is how to reuse that energy to cut waste heat.
Design engineers often ask me, “Which method of energy recovery do you prefer?” My answer: “It depends on the application” (shocking, right?).
This post covers energy recovery in airside HVAC systems and what I look for in helping my engineers determine the right fit.
The Difference Between Homes and Commercial Buildings
Here in Upstate New York the most economical way to heat our homes is with a furnace. Our furnace is an air handler that recirculates air using the vents throughout the house. It typically uses gas to heat the air to make up for what was lost through the walls, windows and roof. No outside air is introduced to the air handler.
Commercial building code does not allow a closed system for most applications. Building codes require higher indoor air quality, which drives a requirement for at least some minimum ventilation of outside air into the system. To maintain building pressure, every part of air introduced from the outside requires equal amounts to be exhausted. So, on the coldest day of the year in Buffalo we will be heating 0 degree outside air to about 70 degrees. This requires tremendous amounts of energy. And since our space is already tempered, that warm exhaust is dumped right outside.
Enter: Energy Recovery
Many years ago engineers developed a way to claim some of this exhausted energy without compromising the quality of the ventilation air. Using various forms of heat exchangers placed in the outside air and exhaust air streams, we can use the exhausted heat to pre-condition the inbound ventilation air. This can substantially reduce the load on the heating element.
Here are the 2 most common forms of energy recovery:
- Heat wheels:
Heat wheels are placed within a central air handler. They rotate to transfer the heat within a space. Typically heat wheels are most popular in a non-critical or contaminated space because they can transfer heat and moisture.
- Efficient energy transfer
- Requires larger air handlers to accommodate the wheel
- Moving parts require maintenance, energy input
- Most potential for cross contamination between air streams
- Heat plates:
Heat plate type exchangers are fixed between airstreams. They transfer energy through touching components in the plate. This type of exchanger is slightly less efficient than a wheel, but still very popular.
- Requires no moving parts, no external energy input to operate
- Inexpensive form of energy recovery
- Less potential for cross contamination of air streams
- Most plates don't transfer moisture
- Require larger air handlers to accommodate
Engineers, It's Your Turn
I recommend to my engineers that they consider the application when determining the best form of energy recovery. Are they exhausting contaminated air (think laboratories, bathrooms, etc…) where the airstreams can't cross? Do they have adequate space to locate the air handler? Are the latent loads (moisture) high? Usually from here we can select an energy recovery form and begin selection of the air handler.
Of these two types of technologies, what have been your experiences – good/bad/indifferent?
I look forward to hearing from you!
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