Recycling thermal energy is next level of efficiency

Energy efficiency has come a long way. Modern energy codes have cut nearly 50 percent of the energy use from the code minimum of the 1980s. Taking energy efficiency to the next level will require us to fundamentally change the way we think about energy.

There was a time when all of our garbage went to landfills, but recycling is now commonplace. We need to think about energy the same way. We throw away a tremendous amount of heat (energy) and then pay for new energy to make domestic hot water and heat our buildings. The thermal energy we throw away during the cooling process can often be recycled.

Some unique applications, such as immersion cooling for high-performance computing, can create low-temperature heating water directly. For most applications though, a heat pump is needed to convert waste thermal energy to a usable temperature. When heating and cooling are simultaneously needed, a heat pump can efficiently satisfy both with a single refrigeration cycle. Instead of using a condenser or cooling tower to throw the heat away, the energy rejected from cooling can be recycled to the heating system.

Heat recovery chillers can be used to simultaneously produce chilled water and heating water (or domestic hot water). For large buildings and campuses, hydronic heating and cooling systems allow thermal energy to be moved large distances to where it is needed. Short-term thermal energy storage, in the form of ice or stratified water tanks, can enhance the overlap of heating and cooling loads. When short-term storage is not enough, geothermal heat exchangers can be added to store heat from the summer for reuse in the winter.

One commonly overlooked opportunity is that energy recycling can be enhanced by shutting off economizers when the heat is valuable somewhere else. Economizers provide “free cooling” by throwing away heat. If you can economically use the heat somewhere else in the building, or on the campus, it is wasted heat. If you disable a waterside economizer, an airside economizer or the economizer mode of an air-to-air energy recovery system, the heat can be captured by the cooling coil and recycled through a heat recovery chiller into the heating water system.

So, is your economizer “free cooling” or “wasted heat”?

For more information, contact Matt Slager at

Posted on November 18, 2016