Designing Energy Recovery Systems for High Summer (Cooling) Efficiency

Rudolf Zaengerle, Konvekta USA Inc.

High-Performance Runaround Energy Recovery Fundamentals:

High-performance runaround energy recovery systems with advanced control software are operating at efficiencies of net 70-90 percent (based on annual energy consumption for heating and cooling). Advanced control software provides continuous recovery and efficiency reporting and verification.

It's critical that high-performance systems operate at optimum performance under varying operating parameters. With several variable input parameters, controlling and optimizing a system requires a numerical-simulation-based controller that allows variable amounts of heat transfer fluid to be circulated throughout the system. In multi-functional systems, additional heat and/or cold is introduced into the glycol circuit, either to boost the heating/cooling capability of the energy recovery system from waste heat/cold sources, or to control the supply air temperature to the building to eliminate the need of separate heating/cooling coils. These features add yet another level of complexity to the controller function.

Designing for high Summer (cooling) Efficiency:

Any energy recovery system can only be operational in summer (cooling) mode while the air temperature (or enthalpy) entering the exhaust [heat exchanger] is lower than the outside air temperature (or enthalpy). The air entering the exhaust is typically in the range of 72-76F DB, 50-65% relative humidity. In almost any climate zone, there are 2000-3000 hours per year where cooling/dehumidification of the outside air is required, but outside air temperature is below 75F DB, so energy recovery is not possible. If evaporative cooling is added in the exhaust, reducing the DB temperature of the air entering the exhaust heat exchanger to 60-65F, during most of these 2000-3000 hours per summer, cold energy recovery will be possible.

Another approach to extend summer operating hours is to combine run-around and a ‘wrap-around' into one energy recovery system: a ‘wrap-around' consists of two coils, one before the cooling coil (precooling outside air) and one after the cooling coil (reheating supply air), and combining it with a run-around system means to add a third coil in the exhaust air. Even if the entering exhaust air temperature is too high to recover cold energy, in bypassing the exhaust coil, the ‘wrap-around' portion of the system can still be in operation, extending the summer operating window to all hours when the cooling coil is in operation.

We'll show the impact of these designs on peak cooling reduction and annual cold energy recovery for Climate Zones 2, 3, 4 and 5.

Learning Objectives

  • Understanding the thermodynamics and necessary controls of a run-around energy recovery system.
  • Understanding the limits of an energy recovery system in summer (cooling) operation.
  • Explore design options to extend operating hours and recovered energy for summer (cooling) operation.
  • Understand Psychrometrics of outside air, supply air and entering exhaust air in summer (cooling) operation.


Rudolf Zaengerle is the President of Konvekta's North American operation, a subsidiary of the Swiss based Konvekta AG, a manufacturer of high performance energy recovery systems. He holds a Master of Mechanical Engineering degree and a PhD in Business Administration, both from the Swiss Federal Institute of Technology, Zurich. He has also studied at Harvard Business School. He was an Assistant Professor at the Swiss Federal Institute of Technology's Energy Sustainability & Urban Planning Institute.


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