Reducing Ventilation Energy Demand In Multifamily High Rise Buildings Through Preconditioning: Two Modeling Studies

Concerns over climate change and rising costs and demand of energy have fueled interest in various renewable and high efficiency energy technologies in recent years. The energy demand associated with ventilation in high rise multifamily buildings can be quite significant. A number of technologies have been developed with the aim of reducing the demand associated with the maintenance of acceptable levels of indoor air quality and comfort in residential buildings. Two such technologies were studied here. The unglazed transpired solar collector (UTSC) represents a radical shift from earlier collector designs with its perforated metal cladding and the absence of glazings and covers. It acts as a ventilation air preheater by delivering warmed air to a conventional makeup air heater which supplies any necessary supplemental heating prior to building delivery. The perforated design allows the boundary layer to be sucked into the collector, resulting in very high thermal efficiencies. A model consisting of a UTSC system and the building on which it is installed was developed. The model computes hourly performance based on TMY2 weather data. Results indicated the importance of the control system in maximizing energy capture and validated the realistic option of a nighttime bypass mode. Results of simulations in seven locations across the state of New York indicated that, while substantial energy capture was possible, economic performance appeared to be marginal. Recovery ventilation (including both heat and energy recovery ventilation) offers another potentially attractive method of offsetting ventilation energy demand by preconditioning ventilation air. Heat and/or moisture are transferred between the exhaust and supply streams of the ventilation system, allowing the capture of “waste” heat and moisture (or the lack of these two, depending on the season) from the exhaust stream. Recovery ventilation, which can also achieve very high efficiencies, has the advantage over solar technologies of operating continuously and unimpeded during both day and night and throughout the entire year since the heat/moisture exchange proceeds in either direction and is useful in both. Unfortunately, serious problems hampering the field performance of recovery ventilation systems have included low system flows and pressures and even flow reversal at exhaust terminals. A model of a typical high rise residential building was developed based on an existing Albany building involved in a recovery ventilation case study and exhibiting these problems. The model successfully predicted the key field observations of low pressures and flows along with flow reversal under certain conditions. Factors found to significantly affect ventilation system performance included stack effect, opening of windows, and duct leakage. Possible solution strategies were investigated including increasing system pressure, compartmentalization, and leakage reduction. An important finding was that typical operating pressures are most likely too low to resist external disturbances such as stack effect. However, significant duct leakage can pose a formiddable obstacle to achieving higher pressure systems. These results indicate the importance of considering both the necessary pressures for immunization against uncontrollable disturbances and the duct leakage present (in retrofit applications) in order to maximize the performance of recovery ventilation systems in the field.