Exergy-optimum coupling of heat recovery ventilation units with heat pumps in sustainable buildings

Abstract

This study shows that as a result of exergy destructions in heat recovery ventilation units, additional but avoidable carbon dioxide emissions take place due to the imbalance between the unit exergy of thermal power recovered and the unit exergy of fan power required to overcome the additional pressure drop. Therefore, special attention needs to be paid in the design and control of heat recovery ventilation units to minimize such carbon dioxide emissions responsibility by a proper exergy-rational balance between the heat recovered and power required. The potential improvements about the exergy rationality of the heat recovery ventilation units were investigated for several alternatives. These alternatives were: heat recovery ventilation-only (base case), coupling with an air-to-air heat pump in tandem or parallel to the heat recovery ventilation unit, and a heat pump-only case. To carry out such an investigation, a new exergy-optimum design and dynamic control model was developed. Under typical design conditions, this model showed that a heat pump in parallel configuration does not improve the exergy rationality unless its coefficient of performance is over 11, which is not practical with today’s technology. Instead, passive solar and wind energy systems have been discussed and recommended. Results were also compared with condensing boiler, micro-cogeneration unit, fuel cell, and electric resistance heating cases. It has been shown that heat recovery ventilation with an air-to-air heat pump in tandem is the best in terms of the exergy-based coefficient of performance. Additional comparisons were made concerning avoidable and direct carbon dioxide emission responsibilities, climate warming-potential and ozone-depleting potential, embodied energy, embodied exergy, and carbon dioxide recovery periods. A new composite index, which recognizes the direct relationship between the ozone layer depletion and the greenhouse gas emissions has also been introduced for comparing system alternatives in terms of their atmospheric footprint.