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Article Excerpt
INTRODUCTION
The energy use of HVAC secondary systems (1) is often much higher than is necessary to condition the outdoor air and meet the zone sensible and latent loads. The extra energy use can be a result of the characteristics of HVAC processes, the performance limits of HVAC components, the choice of system configuration and the restrictions in system operation that this imposes, and the optimality of the HVAC system supervisory control strategy (Zhang et al. 2006). The potential for poor system performance increases with the number of zones served by the system and the degree to which temperature, humidity, and indoor air quality must be controlled. Perhaps the best known example of a system configuration that results in higher-than-necessary energy use is a variable air volume (VAV) system with central cooling and zonal reheat; such systems have an energy overhead due to the simultaneous cooling and reheating of the supply air.
The global need to reduce building energy use and associated carbon emissions has increased interest in the optimum design and operation of HVAC secondary systems (Wright et al. 2004; Zhang 2005). However, to date, there is no means of judging the optimality of a particular design solution. The optimality of a system and its operating strategy is commonly judged by comparing the system performance with that of similar systems (or by measuring the improvement in system performance resulting from a change in the design or operation of a system). However, this approach is unsatisfactory, since it is possible that all systems in the comparison have poor performance and, as such, all that is identified is the best among a set of poorly performing systems. Further, when the system performance is measured in terms of energy use, there is a loss of transparency, as energy use can only be used to indicate the average performance of the system over the operating period (since energy use is the time-integral of the rate of system energy transfer at each operating condition).
This paper describes an approach to calculating the minimum HVAC system capacity (2) required at a particular operating point; the minimum capacity forms a benchmark against which the system performance may be judged. By basing the analysis on the system capacity at a particular operating condition, the cause of poor performance can be determined in most cases as a choice of system components and topology and/or the system operating strategy. The effect of system configuration and operating strategy on system performance would be less transparent if the analysis were based on the system energy input, as the system configuration and operation effects may be masked by the inefficiency in energy transfer between input and output (although most HVAC secondary-system components are passive devices for which the useful output energy equals the input energy).
The minimum capacity of an HVAC secondary system at a particular operating condition is conventionally assumed equal to the outdoor air and zone thermal loads (the sensible and latent loads are treated separately). However, this assumption is only valid for a single-zone system, as the potential reduction in required system capacity due to the system transferring energy between zones must be considered when calculating the minimum capacity of a multi-zone system. The calculation of the reduction in system capacity achievable through the use of interzonal energy transfer is a complicated task for any system serving more than a few zones. This problem is addressed for the case in which the energy transfer between zones is facilitated by interzonal airflow. This paper also describes an extension to the approach in which the minimum system capacity is used to evaluate the thermal effectiveness of the system. Finally, an example minimum capacity calculation is given for a multizone system operating with zone conditions that have been optimized to promote the energy transfer by interzonal airflow. The concepts presented herein have also been applied in a related study of the performance analysis of both novel and conventional HVAC systems (Wright and Zhang 2008).
Minimum System Capacity
The minimum system capacity at a given operating point is a function of the ambient and zone boundary conditions together with the potential for energy transfer between zones. The zone boundary conditions include the zone temperature, humidity, and the minimum outdoor airflow rate required to maintain indoor air quality. Ultimately, the zone boundary conditions would include the radiant temperature and be expressed as temperature, humidity, and air pollutant distributions (fields). Specification of the zone boundary conditions in terms of a distribution within the room are necessary in order to evaluate the performance of displacement ventilation systems and radiant cooling and heating systems. However, for the purposes of this paper, the zone boundary conditions are considered to have point values and, as such, relate to fully mixed conditions in each zone.
The potential for energy transfer between zones exists when there is a difference in the air temperature and/or humidity ratio between one or more zones served by the system. When these conditions exist, it may be possible to reduce the HVAC system capacity by transferring energy between zones. An HVAC system can facilitate the energy transfer in two ways: (1) by promoting airflow between zones and/or (2) by use of a heat pump. To some extent, the two approaches are complimentary, as each can provide energy transfer under some conditions when the other can not. For instance, a heat pump is ineffective in transferring energy between two or more zones when the zones have the same thermal load; however, provided there is a difference in the zone boundary conditions, energy transfer using interzonal airflow could be effective in reducing the required system capacity. (3) For instance, when all zones require heating, but one or more zones is at a higher temperature than the others, the sensible load on the HVAC system can be reduced by exhausting some air from the high temperature zone(s) through the low temperature zone(s). Further, interzonal airflow has the ability to transfer both latent and sensible energy, whereas this generally is not possible using a heat pump. In this paper, the potential for energy transfer between zones is restricted to that achievable by interzonal airflow only, but includes the transfer of both sensible and latent heat energy.
The calculation of the minimum system capacity can be considered in two parts: (1) the calculation of the minimum capacity associated with moving air through the system (the fan capacity) and (2) the calculation of the minimum capacity associated with heating, cooling, dehumidifying, and the humidification processes. Calculation of the minimum fan capacity requires the minimum possible flow resistance for the given airflow rate to be defined. Since there is no logical thermodynamic basis for determining the minimum possible resistance, it is defined as zero, with the result that, in concept, the minimum possible fan capacity is also zero. Defining the minimum fan capacity as zero also allows the analysis to be applied to naturally ventilated (and mixed-mode) buildings, which have a zero fan capacity.
However, unlike the fan capacity, the minimum capacity associated with heating, cooling, dehumidifying, and the humidification processes can be calculated as a thermodynamic function of the energy transfer between the ambient environment and the zones, together with potential to transfer energy between zones.
Calculation of Minimum System Capacity with Outdoor and Interzonal Airflow
The calculation of the minimum system capacity with outdoor and interzonal airflow is described here as an optimization problem. The problem is formulated for the optimization of the outdoor and interzonal airflow rates, together with the zone air temperatures and humidity ratios. By including the air temperature and humidity ratios in the set of optimization problem variables, the approach can be used in the study of zone setpoint supervisory control strategies. Note that, in this respect, it is perceived that an HVAC secondary system operating with the minimum system capacity would not only have a system configuration that facilitated interzonal airflow, but would also operate with an optimum...
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