Energy and Buildings 49 (2012) 310–316 Contents lists available at SciVerse ScienceDirect Energy and Buildings j our na l ho me p age: www.elsevier.com/locate/enbuild Air handling unit supply air temperature optimal control during economizer cycles Gang Wang a,∗ , Li Song b a University of Miami, 1251 Memorial Drive, Rm. 319, Coral Gables, FL 33146-0630, United States b University of Oklahoma, 865 Asp Avenue, Room 217, Norman, OK 73019-0601, United States a r t i c l e i n f o Article history: Received 19 January 2012 Accepted 17 February 2012 Keywords: Energy efficiency Free cooling Air handling unit Optimization Control a b s t r a c t Most air handling units (AHUs) in commercial buildings have an air economizer cycle for free cooling under certain outside air conditions. During the economizer cycle, the outside air and return air dampers are modulated to seek supply air temperature at its setpoint. The supply air temperature is typically set at 55 F (13 ◦ C) to control humidity in the space. However, dehumidification is not necessary when the outside air is dry. Meanwhile, the space may have less cooling load due to envelope heat loss and/or occupant schedule changes. These facts provide an opportunity to use higher supply air temperature to reduce or eliminate mechanical cooling and terminal box reheat. On the contrary, a higher supply air temperature requires increased air flow as well as fan power. Therefore, an optimization question is formed, through which an optimal supply air temperature is identified to minimize total energy consumption. In this paper a steady-state energy consumption model is established for AHU systems under the economizer, and then an analytical optimization method is used to seek an optimal supply air temperature setpoint to minimize the energy cost. This paper presents AHU system energy modeling, supply air temperature optimization, simulated energy savings, and control sequence development. © 2012 Elsevier B.V. All rights reserved. 1. Introduction With the technology advancements of variable frequency drives (VFDs) and building automation systems (BAS), variable air vol- ume (VAV) air handling unit (AHU) systems conserve a significant amount of energy while maintaining indoor comfort. In contrast to constant air volume systems where only supply air tempera- ture is a controlled variable due to a fixed constant airflow rate, VAV systems have both supply air temperature and supply airflow rate as controlled variables. Thus, there is a need to identify opti- mal supply airflow rate and supply air temperature for minimizing energy consumption with the constraints of meeting zone comfort conditions. The systematic use of optimization techniques has been introduced in the heating, ventilation and air conditioning (HVAC) industry in recent years [1], with a substantial focus on VAV sys- tem optimal control. Lumped dynamic AHU models were used to compute the optimal supply air flow rate, zone air tempera- ture and predicted mean vote (PMV) index for minimizing the cost function through the sequential quadratic programming (SQP) method [2,3]. To solve the problems caused by physical models, such as computation power requirements and inaccurately defined ∗ Corresponding author. Tel.: +1 305 284 5555; fax: +1 305 284 3492. E-mail address: g.wang2@miami.edu (G. Wang). input parameters, Wang and Jin [4] developed an adaptive phys- ical model using online-identification and a self-tuning method for solving AHU optimization problems through a genetic algo- rithm method. Sun and Reddy [5] integrated detailed dynamic models with the SQP algorithm to determine the optimal controls. However, even though simulation-based SQP has greatly reduced computational requirements, it is still not practical for real-time control implementation. Parameshwaran et al. [6] studied the use of two fuzzy-based objective functions to identify optimal setpoints of supply air temperature, supply duct static pressure, chilled water temperature, and zone temperature. Overall, optimization on a VAV system is a complicated issue if transient system behavior is con- sidered. Assuming that the AHU has the fastest dynamic in HVAC systems at a time constant of a minute, compared with the building thermal load dynamics at a time constant of 1 h [7], the energy consumption of AHUs during the transient time is insignificant in an entire oper- ation cycle of AHUs. In this paper, analytical optimization method is explored using a steady-state energy consumption model based on the first principle to obtain optimal supply air temperature reset schemes for minimizing the total energy consumption of a VAV system during economizer seasons. Typically AHUs have an economizer cycle to reduce or eliminate mechanical cooling under certain outside air conditions. During the economizer cycle, return air and outside air dampers are modu- lated to seek the supply air temperature setpoint [8]. A high-limit 0378-7788/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2012.02.024