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].
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.
Figure 1 shows the intake ratio of the outdoor air temperature to the outdoor air temperature during operation of the economizer system.
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.
Table 1 provides an 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 Volume (CAV) system units are installed in the target building for air conditioning, and a medium temperature water absorption type refrigerator, an air cooling type inverter refrigerator, a cooling tower, and a heat exchanger are used as the heat source. Two HVAC systems are installed in each of floors that are used for business, and the two units are in charge of one floor. Six outboards are responsible for introducing the outdoor air of the entire building to supply outdoor air to each floor air conditioner.
The Building Automation System (BAS) is monitored to determine the status of the economizer system operation of the target building. The target building is controlled by a dry-bulb temperature economizer and the minimum air volume is 50% of the maximum air volume. On the other hand, the outdoor air damper, return air damper, and mixed air damper are not controlled by the economizer control algorithm. The outdoor air damper is closed when the mixed air damper is opened 100%. The mixed air damper and the outdoor air damper are opened at 100%.
Figure 3 shows the rate of damper opening per hour of the target building. Therefore, the current building is equipped with an economizer that is not operated by a control algorithm but operated by the experience of the system operator. In addition, the current mixed air temperature is operating at a fixed value. That is, the use of an economizer is low in Korea, and it is not used properly when it is applied at present.
Khalaj developed a set of nine dry-bulb temperature control cycles to use the economizer dry-bulb temperature control to control the cooling system’s energy requirements in the design and operation of a data center. Prior to this, Australia was divided into 23 regions according to climatic conditions and the data center load was measured. The proposed control method was applied to evaluate the energy performance based on the annual and monthly energy savings with the existing method [
7]. Nassif proposed a new economizer control called a split signal damper control strategy that reduces the static pressure of the damper and minimizes the energy use of the ventilation and supply fans, while the control signal controls only the damper of the outdoor air and the remaining two dampers are opened at the 100%. This control method can save energy in the supply and ventilation damper when compared with the conventional method, which controls all three dampers [
8]. Wang developed a steady-state energy consumption model to determine the optimal supply air to minimize the energy cost of an air conditioner with the economizer system. The optimal supply air according to the outdoor and indoor conditions was presented, and the energy savings were estimated through simulation [
9]. To compare the energy performance according to the economizer control method, Son selected the case of dry-bulb temperature control, enthalpy control, and the case where the economizer did not operate as a simulation case, and evaluated it using Energy Plus. According to the control method, the temperature of the mixed air and the introduction ratio of the outdoor air were analyzed further. As a result, enthalpy control during the cooling period has the greatest energy savings [
10]. Kim compared the dry-bulb temperature control and enthalpy control using the TRNSYS simulation program to evaluate the performance according to the economizer control system. According to the economizer control method, the indoor air quality, heat environment, outdoor air introduction rate, and energy consumption according to the outdoor temperature were analyzed. As a result, both control methods satisfied the indoor thermal environment and air quality, but enthalpy control saved more energy than dry-bulb temperature control [
11]. Choi evaluated the simulation cases based on the economizer control methods applicable to the Energy Plus for office buildings. The results were compared and analyzed in terms of the indoor air quality, indoor heat environment, and energy consumption. As a result, the differential enthalpy control method was optimal to apply to the target building in terms of energy [
12]. Lee evaluated the energy conservation measures used for cooling by directly introducing outdoor air, which is one of the energy management methods by improving the driving method. In other words, various variables applied to outdoor air cooling control for office buildings were analyzed using the dynamic energy analysis program, and the energy efficiency of each control method was compared and analyzed based on this analysis [
13]. Kim proposed an outdoor air-cooling method that uses outdoor air information and cooling load information that were predicted beforehand and introduced outdoor air to maintain the room temperature within a comfortable range at the same time as removing the load. Unlike previous studies, the effect of outdoor air-cooling was compared and analyzed in various regions [
14]. As such, active research is being conducted to apply the economizer to the system at home and abroad. On the other hand, previous studies focused mainly on economizer control, and there has been little research on controlling the mixed air temperature in an economizer system.
Therefore, the purpose for checking the novelty of this paper is to propose an economizer control method that makes the mixed air temperature variable according to the load in a CAV single duct system with a constant flow rate. For this, a load prediction is required and an Artificial Neural Network (ANN) is used to predict the load. The business building was selected as the target building and the load prediction model was developed through the simulation data of the target building. Through the BIN method and TRNSYS simulation, the relationship between the mixed air temperature and energy was analyzed. Based on these results, an economizer control was proposed to predict the load using an ANN and to make the mixed air temperature variable by the economizer dry-bulb temperature control method of a single duct system with a constant airflow according to the predicted load. The results were compared with the conventional dry-bulb temperature control in terms of the room temperature and energy.
3. Evaluation of Economizer Control Method
The TRNSYS program was modeled in the flow of
Figure 12 to evaluate and apply the proposed economizer control. To predict the load through the ANN, Type 16 was applied to transfer the state of the input variable to the HVAC system. The predicted load was applied to the HVAC system through Type 16. When the fluid moves from the HVAC to each zone of the building, the terminal is controlled by the on/off controller to operate according to the indoor environment when necessary.
Room temperature and energy were analyzed for the evaluation, and the energy was evaluated by comparing the energy consumption of the proposed control with that of the conventional control.
Figure 13a,b shows the room temperature at peak load in summer and winter. According to the proposed control method, the indoor environment of the target building is 22.5–26 °C in summer and 22–23.5 °C in winter, which is 24 ± 2 °C.
To analyze the energy savings of the conventional dry-bulb temperature control and proposed control, each was selected as a simulation case, as listed in
Table 7, and evaluated. The result are shown in
Figure 14 and
Figure 15. The energy from the cooling coil was reduced by 28% in Case 2 compared to Case 1, and the energy saving rate through reheat coil was 20%. The total energy was 626,499MJ in Case 1, the energy requirement of Case 2 was 509,585 MJ, and 116,914 MJ of energy was reduced, giving a saving rate of 19%. All HVAC energy was saved in Case 2 compared to Case 1 over all cooling periods. The HVAC energy showed the largest energy saving rate of 37.67% in May and the lowest energy saving rate of 17.09% in August.
4. Conclusions
The economizer system is one of the energy efficiency measures through the operation and management of buildings. In the Building Design Criteria for Energy Savings of domestic buildings, it has proposed to introduce the economizer system, which is one of the free-cooling systems. On the other hand, in Korea, there are few places where the economizer system is installed, and, when it is installed, it is not operating properly. The existing economizer dry-bulb temperature control is controlled by maintaining a mixed air temperature constantly. However, energy saving is expected if the mixed air temperature is made variable according to the load. Variables affecting buildings vary widely and it is very difficult to predict the loads on buildings. The ANN makes a prediction based on existing data, and a load prediction through an ANN is a very effective method. Therefore, this paper proposes economizer control, which estimated the load using ANN and made the mixed air temperature variable according to the predicted load, and evaluated the proposed control on the room temperature and energy side.
The results of this study are as follows.
- (1)
The factors affecting the building load vary widely, and the load varies according to the prediction method. The factors affecting the prediction of the building design are different. Most studies on load prediction use existing data to predict the load. The ANN learns and predicts the existing data, and it can analyze all the variables. Therefore, the ANN model was used for the load prediction in this study.
- (2)
In the case of a CAV system, the relationship between the mixed air temperature and energy was evaluated using the BIN method and TRNSYS simulation while the mixed air temperature was increased from 10 to 16 °C in 1 °C increments. As a result, the mixed air temperature was not constant when the energy demand was smallest depending on the load. Therefore, economizer control was proposed based on the analysis results and developed ANN model.
- (3)
The proposed control method was applied through TRNSYS and evaluated in terms of room temperature and energy. 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. A comparison of the energy requirement with the conventional control system showed 19% of the energy was reduced.
In this study, we propose a control method to control the mixed air temperature according to the load during economizer dry-bulb temperature control in a CAV system for the purpose of energy saving. The proposed control satisfies the room temperature and it is confirmed that it is possible to save energy compared to the existing control. However, in selecting the system, the study was limited to the CAV system, and it was limited to the dry-bulb temperature control of the economizer. Therefore, in the future, it is necessary to study various HVAC systems other than a CAV system and to apply and demonstrate the proposed control in real building.