Exergy-Optimum Environmental Design and Control of Morphing Air-to-Air Heat Recovery Ventilation in Sustainable Buildings

This paper introduces an axially morphing heat recovery ventilation unit, which addresses the thermodynamic gaps in the current theory and practice due to the lack of recognition of the Second Law of thermodynamics, and the fixed design geometry and control parameters. Three major gaps are identified: 1-All standards, guides, and design practices and controls are based only on the First Law of Thermodynamics. 2-Exergy of fan power consisting of electric power and the exergy of thermal power obtained by the heat recovery process are different and additional carbon dioxide emission responsibility occurs in proportion to the difference. This recognition needs the second law. 3-The thermal effectiveness changes with instantaneous air-change demand and operating conditions, like the temperature of the outdoor air. But the design geometry and operating regime is constant except variable fan speed, if included in the design. To avoid these gaps a novel, axially morphing heat recovering exchanger has been modeled and analyzed using both the First and the Second laws. The model consists of an axially expanding or retracting heat exchanging chamber such that the decarbonization effect of the recovery is maximized. Sample analyses show that such a design may benefit up to 150% increase of carbon-dioxide reduction potential when electric power from a grid-connected natural-gas power plant is supplied, compared with a conventional design with fixed geometry, based on the First Law only. The impact of the new findings on several standards and green building metrics are discussed, and correction factors are introduced.