energies Article Development of Economizer Control Method with Variable Mixed Air Temperature Jin-Hyun Lee 1, Yong-Shik Kim 2, Jae-Hun Jo 3, Hyun Cho 4 and Young-Hum Cho 5,* 1 Department of Architectural Engineering, Graduate School of Yeungnam University, Gyeongsan 38541, Korea; [email protected] 2 Division of Architecture and Urban Planning, Incheon National University, Incheon 22012, Korea; [email protected] 3 Division of Architecture, Inha University, Incheon 22212, Korea; [email protected] 4 R&D Center, POSCO E&C, Incheon 22009, Korea; [email protected] 5 School of Architecture, Yeungnam University, Gyeongsan 38541, Korea * Correspondence: [email protected]; Tel.: +82-53-810-3081 Received: 14 August 2018; Accepted: 4 September 2018; Published: 14 September 2018 Abstract: Achieving energy efficiency by improving the operating method of the system used in existing buildings is attracting considerable attention. The Building Design Criteria for Energy Saving was established to induce energy saving design in the domestic construction field, and the introduction of a free-cooling system, such as an economizer system, as an item of the mechanical sector, was evaluated. The economizer is an energy saving method that reduces the building load by introducing outdoor air through damper control according to the indoor and outdoor conditions. The system consists of dry-bulb temperature control and enthalpy control and the mixed air temperature is kept constant in the conventional economizer controls. On the other hand, in dry-bulb temperature control, when the set value of the mixed air temperature is changed according to the load, additional energy savings are expected compared to the conventional control method. Therefore, this paper proposes an economizer control that makes the mixed air temperature variable according to the load in a Constant Air Volume single duct system. For this, a load prediction is needed and an Artificial Neural Network is used for the load prediction. In addition, the relationship between the mixed air temperature and energy were analyzed using the BIN method and TRNSYS 17. Based on the results of previous analysis, a control method which predicting the load using Artificial Neural Network and controlling the mixed air temperature according to the predicted load in the dry-bulb temperature control of a Constant Air Volume single duct system is proposed and the proposed control was applied to the dynamic simulation program and compared with the conventional control. The results show that the temperature of each room was 21–23 ◦C in summer and 22.5–26 ◦C in winter when the economizer was controlled using the proposed control method and the energy consumption analysis showed that 19% of the energy was reduced compared to the conventional method when the proposed method was controlled. Keywords: economizer; mixed air temperature; Constant Air Volume single duct system; Artificial Neural Network 1. Introduction Reducing greenhouse gas emissions and energy use is a global issue [1,2]. In Korea, various policies are being implemented to reduce greenhouse gas emissions. The demand for technology development for energy conservation is increasing and various laws and regulations have been enacted. Moreover, various studies are underway in the construction sector, which comprises a large share of domestic energy consumption [3]. Energies 2018, 11, 2445; doi:10.3390/en11092445 www.mdpi.com/journal/energies Energies 2018, 11, 2445 2 of 18 Energy efficiency by improving the operation method of the system used in existing buildings is attracting considerable attention. In the domestic construction sector, the Building Design Criteria for Energy Saving was established to induce energy savings design, and it suggests an energy saving design for the passive, active, and electric sector. As one of the active items in the building design criteria, it evaluates free-cooling systems, such as the economizer, which is one of the energy efficiency methods by improving the operating method. This evaluation criterion shall be rated when the outdoor cooling system such as the economizer system is applied to reduce the cooling load by introducing outdoor air during intermediate season [4]. According to ASHRAE standard 90.1, “The economizer means that the outdoor air temperature is low or the air is introduced by adjusting the damper to reduce mechanical cooling” [5]. According to the equipment engineering manual, “The outdoor air cooling cycle can reduce the cooling cost when the outdoor air is low enough to be used as a cooling medium. The return, exhaust and outdoor side damper are adjusted for the cooling setting value when the outdoor air temperature is below the upper limit value, when the outdoor air temperature is above the set temperature value, the return and exhaust damper are opened and closed separately, and the outdoor air side damper is controlled to be opened to the minimum” [6]. In this study, reducing the Heating, Ventilating, and Air Conditioning (HVAC) load by introducing the outdoor air through damper control according to the indoor and outdoor air conditions was defined as the economizer. Typical control methods of the economizer include dry-bulb temperature control and enthalpy control. In conventional economizer control, the mixed air temperature is constant. Dry-bulb temperature control is a method for determining the amount of outdoor air intake by comparing the indoor air and outdoor air according to the dry-bulb temperature reference. In other words, when the outdoor air temperature is lower than the return air temperature, the outdoor air side damper is opened to increase the outdoor air damper opening rate and reduce the opening rate of the return damper to increase the outdoor air intake amount. In the HVAC system, the airflow rate of the outdoor air and the return air can be expressed using Equation (1) according to the law of conservation of mass, and the relationship between temperature and airflow of outdoor air, return and mixed air can be expressed as Equation (2). In addition, the outdoor air intake amount can be expressed using Equation (3). That is, in dry-bulb temperature control, the outdoor air intake ratio can be calculated from the outdoor air temperature, airflow of outdoor air, return air temperature, and airflow of the return air. It is a simple control method considering only sensible heat. In the case of the enthalpy control, the latent heat, which is not considered in the dry-bulb temperature control, is taken into consideration, and the temperature control values are controlled in place of the enthalpy in a manner complementing the limit that cannot consider the humidity of the dry-bulb temperature control. That is, the outdoor air is introduced by considering the enthalpy of the outdoor air and return air simultaneously. When the outdoor air enthalpy is smaller than the return air enthalpy, the air is introduced for cooling. In the case of enthalpy control, however, it is difficult to calibrate the humidity sensor, and it is difficult to install and operate it economically. Figure1 shows the intake ratio of the outdoor air temperature to the outdoor air temperature during operation of the economizer system. moa + mr = mmix (1) moaToa + mrTr = mmixTmix (2) T − T α = min r mix , 1 (3) Tr − Toa To analyze the operation status of the domestic economizer, a high-rise building with economizer control was selected as the target building. The selected building is a business building and hotel located in Songdo, Incheon. The building has three floors underground and 68 floors aboveground. Floors 1–35 are used as a business building and the Floors 36–65 are used as a hotel. In addition, Floors 66–68 are composed of a machine room and electric room. Table1 provides an overview of target Energies 2018, 11, x FOR PEER REVIEW 3 of 19 Energies 2018, 11, x FOR PEER REVIEW 3 of 19 Energies 2018, 11, 2445 3 of 18 overview of target building and Figure 2 is the picture related to the target building. Air Handling overview of target building and Figure 2 is the picture related to the target building. Air Handling Unit (AHU), Fan Coil Unit (FCU), Variable Airflow Volume (VAV) system, and Constant Airflow buildingUnit (AHU) and, FigureFan Coil2 is Unit the picture (FCU), related Variable to theAirflow target Volume building. (VAV) Air Handling system, and Unit Constant (AHU), Fan Airflow Coil Volume (CAV) system units are installed in the target building for air conditioning, and a medium UnitVolume (FCU), (CAV) Variable system Airflow units Volumeare installed (VAV) in system, the target and building Constant for Airflow air conditioning, Volume (CAV) and system a medium units temperature water absorption type refrigerator, an air cooling type inverter refrigerator, a cooling aretemperature installed inwater the targetabsorption building type for refrigerator, air conditioning, an air and cooling a medium type inverter temperature refrigerator, water absorption a cooling tower, and a heat exchanger are used as the heat source. Two HVAC systems are installed in each of typetower, refrigerator, and a heat anexchanger air cooling are typeused inverter as the heat refrigerator, source. Two a cooling HVAC tower, systems and are a heatinstalled exchanger in each are of floors that are used for business, and the two units are in charge of one floor. Six outboards are usedfloors as that the are heat used source. for Twobusiness, HVAC and systems the two are unit installeds are inin eachcharge of floorsof one that floor. are Six used outboards for business, are responsible for introducing the outdoor air of the entire building to supply outdoor air to each floor andresponsible the two for units introducing are in charge the of outdoor one floor. air Sixof the outboards entire building are responsible to supply for outdoor introducing air to the each outdoor floor air conditioner.