Updated: Dec 6, 2021
Written by Thomas Tsaros, PE | Senior Energy & Infrastructure Service Leader
I diligently read the paper daily, and it amazes me the regularity of articles I see lately regarding the topic of building electrification. Amazing. Front page articles, no less. Air-to-air heat pumps, neighborhood geothermal networks (“hey neighbor, can you spare some glycol?”), and injecting hydrogen into the natural gas pipeline—it’s all out there. Some articles paint perhaps too rosy a picture of the possibilities, while others strike the right balance of sound advice and caution.
It’s one thing to electrify a home (PV, heat pumps, even battery storage), but another to electrify energy-intensive facilities such as hospitals and labs. How to cost-effectively electrify thermal loads? Unless some disruptive technology is introduced, I think going back to basics is a way to approach this.
Old school. The classics. Or like my kids refer to me, “old timey.” Let’s go back to the original playbook. Thinking about the production of chilled water in such facilities got me thinking about all that available waste heat. So what about refrigerant heat recovery? It’s been a while I must admit that I have thought about desuperheaters. Many years ago, they were a more common sight in chiller plants. Though, unfortunately, sometimes these were bypassed due to service issues or concern of energy impacts on the chiller plant. But in the right applications, such as hospitals, this classic energy conservation technique offers a practical means to electrify a portion of a facility’s year-round thermal energy load.
Desuperheaters are available for both water and air-cooled chillers. Year-round thermal loads present in hospitals such as low temperature reheat or domestic water heating where a 140 degrees F supply temperature is not uncommon in modern systems, present ideal heat sink applications for desuperheaters.
Let’s take for example, a 150-ton air cooled chiller application. A desuperheater will recover around 30% of the condenser waste heat available. That translates to about 650 MBH of heating capacity at full load—not a trivial amount of heat. Assuming a building reheat load of around 4 Btuh/SF—I’ve seen this as a minimum reheat load in hospital settings—two, 150-ton air cooled chillers could meet the base reheat load for a hospital of about 325,000 SF or about the average size community hospital in the U.S.
How might this translate in enhancing a chiller’s coefficient of performance (COP)? Coefficient of Performance is defined in this case as the ratio of useful cooling energy produced to compressor energy input. Assuming the subject chiller above has a COP of 3.9, which corresponds to rating of 0.9 kW per ton, adding the heat recovery output to the cooling energy produced—both forms of useful energy—increases the chiller COP from 3.9 to 5.37 or a 38% increase above the cooling-only COP. Compared to a straight electric resistance water heater with a COP that can only approach 1 or the best gas fired water heater with a COP approaching 0.96, a chiller with a desuperheater makes far more sense.
So instead of shooting for the stars, stick with the planets. Consider that the same 300 tons of cooling with a geothermal heat pump would require a well field the size of an NFL football field (go Pats!), a packaged desuperheater would offset fossil fuel use for a fraction of the cost, substantially increase the efficiency of the cooling plant, and deliver real savings and not just “green,” good citizen points.
Thomas Tsaros, PE
Senior Energy & Infrastructure Service Leader