Evaluation of a cost-responsive supply air temperature reset strategy in an office building

Abstract

This paper describes a new supply air temperature control strategy for multi-zone variable air volume systems. We developed the strategy with the intent that it is simple enough to implement within existing building management systems. At 5-minute intervals, the strategy estimates the cost of fan, heating and cooling energy at three different supply air temperatures (current, higher, lower), and chooses the one with the lowest cost as the setpoint. We then implemented this strategy in a seven floor, 13,000 m 2 office building and compared the energy costs to the industry best practice control strategy in a randomized (daily) controlled trial over a 6-month period. We showed that the new control strategy reduced total HVAC energy costs by approximately 29%, when normalized to the typical annual climate data for this location and operating only during typical office hours. These findings indicate that the current industry best practice control strategy does not find the optimal energy cost point under most conditions. This new control strategy is a valuable opportunity to reduce energy costs, at little initial expense, while avoiding more complex approaches, such as model predictive control, that the industry has been hesitant to adopt. We describe the new control strategy in language common to the industry (see sequence of operations included as supplemental material) so that readers may easily specify and implement this immediately, in new construction or controls retrofit projects.

Optimal Control Strategy for Variable Air Volume Air-Conditioning Systems Using Genetic Algorithms

“…The HVAC system modeled in this study has an air handling unit that provides a VAV system to each room and a freezer that can be used also as a heat source in cold weather. Numerous controllable settings are required to manage a VAV system, from heating and cooling temperature settings and minimum airflow rate settings at the zone level to the minimum outside airflow, supply air temperature, and duct static pressure settings at the system level [25]. Among the various controllable variables in an HVAC system, the settings for supply air temperature and duct static pressure are selected specially for the study as the control variables in the VAV air-conditioning system.…”

Quantifying energy losses in hot water reheat systems

“…Some control strategies require a dynamic estimation of hot water reheat energy consumption using the instrumentation that is typically installed in a building management system. One example is a cost-responsive supply air temperature reset strategy [27], which dynamically selects the optimal AHU supply air temperature setpoint to minimize the combined cost of fan, cooling, and reheat energy consumption. The work presented in this document is in part a validation and refinement of the earlier method used to estimate reheat energy consumption by that control strategy.…”

“…We re-analyzed the reheat data from a previous study by the authors [27]in a different 10 building using an earlier version of the reheat estimation method 11 . In that building, extrapolating from the 6-month study period (September to February), the intentional reheat result was 9.7 kWh/m 2 per annum, or an average of 1.1 W/m 2 during operating hours.…”

“…a passing reheat coil), this wastes energy at the zone, but also often wastes energy at the whole building level, as the AHU must supply cooler air than would otherwise be necessary to all of the other zones served by that AHU. See [16,27] for more detailed discussion.…”

Comments to Paper Entitled: Development of a Data-Driven Predictive Model of Supply Air Temperature in an Air-Handling Unit for Conserving Energy. Energies 2018, 11, 407

“…However, the authors neglected that there is a supply air temperature (SAT) set point, which is conventionally based on outside air temperature [2], which is also the current best practice as proposed by ASHRAE GPC 36. The optimized SAT set point could also be a function of outside air temperature, outside air relative humidity, outdoor air flow rate and supply air flow rate [3], outside air temperature, minimum reheat valve position or return air temperature [4], outside air temperature, load ratio and power exponents [5], outside air temperature and total number of people [2], energy cost signal, and cooling demand [6]. If properly tuned, the SAT should be able to meet its set point.…”