Experimental Study on Extremum Seeking Control for Efficient Operation of Air-side Economizer

The air-side economizers are a major class of energy-saving devices for ventilation and air conditioning systems by taking advantage of outdoor air during cool or cold weather. Typical rule based control cannot justify energy optimal operation, while model based optimization of air-side economizer operation depends on the accurate knowledge of system model and enthalpy sensing of the ambient and return-air. Such optimal operation is hard to achieve in practice due to inaccurate model and degradation/failure of temperature and relative humidity (RH) sensors. As pointed out by Seem and House (2010), under certain indoor/outdoor air conditions, there exists a convex map between damper position and energy consumption of an air handling unit (AHU), which implies an optimal damper opening minimizing the cooling-coil load. Such convexity guarantees the use of gradient-search type of real-time optimization methods. An Extremum Seeking Control (ESC) was proposed by Li et al. (2009), where the chilled water flow rate of the cooling coil (equivalently the energy consumption) is minimized by tuning the damper opening. The proposed framework was validated with a Modelica based dynamic simulation model of an air-side economizer. This study is conducted to perform experimental evaluation of the ESC control of air-side economizer. The experimental setup is anchored on a Lennox XC25 variable-speed air conditioner. The Lennox, CBX40UHV indoor air handler unit is equipped with duct work to form an air-side economizer, connected to a foam based 16\u27X8\u27X8\u27 test chamber. The Lasko 751320 electrical heaters are used as heat source. The Honeywell HCM-890 humidifiers and Soleus Air SG-DEH-70EIP-6 dehumidifiers are used to regulate the indoor air humidity. A National Instruments CompactRIO-9024 platform is used for data acquisition and control. Major measurements include temperature, relative humidity (RH) and power consumption. A Watt Node Pulse WNB-3D-240-P electric power meter is used for power measurement. The Omega P-L-1/10-1/8-6-0-T-3 temperature sensors and Veris Industries HN3XVSX RH sensors are installed to monitor the indoor and outdoor air conditions. The Omega HHT13 speed sensors are used to measure fan speeds, while Fluke 80i-110s current sensors are used to measure the compressor motor current. The ESC controller is implemented with the damper opening as input and the total power consumption as feedback. Two experiments have been performed under different indoor/outdoor air conditions. The first experiment was performed under outdoor air temperature 23°C and RH 65%, a heat load of 6000 W and indoor temperature setpoint 28°C. The ESC turned on the outdoor damper 100% automatically to allow maximal outdoor air resulting in indoor RH 50%. The total power consumption was reduced from 540 W to 450 W with an energy saving of 16.67%. The second experiment was performed under same conditions with indoor RH regulated to 40%.The ESC turned off the outdoor damper to allow minimal outdoor air. The power consumption was reduced from 620 W to 600 W with an energy saving of 3.33%. More experiments will be performed in warmer weather in February and March to further validate the performance of the ESC controller

Extremum Seeking Control of Hybrid Ground Source Heat Pump System

The ground source heat pump (GSHP) technology is a renewable alternative for space conditioning by rejecting/absorbing heat to/from the ground, which has demonstrated higher energy efficiency for residential and commercial buildings. As the system capacity is limited by the initial cost of construction of ground-loop heat exchanger (GHE), developing the so-called Hybrid GSHP system by utilizing supplemental heat rejecters such as cooling towers has emerged as a cost-effective alternative. In practice, operational efficiency of Hybrid GSHP system mainly depends on 1) the actual characteristics of heat pump, cooling tower, GHE and other equipment; 2) ambient air and ground conditions. In particular, the GHE heat transfer is heavily affected by the ground thermal characteristics which, however, is difficult and expensive in practice to determine due to the complexity of soil type and distribution. In addition, the actual cooling tower characteristics can vary significantly. Such uncertainties bring forth dramatic difficulty for successful application of model based control or optimization methods. In this study, an extremum seeking control (ESC) strategy is proposed for efficient operation of a hybrid GSHP system with cooling tower, which minimizes the total power (i.e. GHE loop water pump, cooling tower fan and pump, and the heat-pump compressor) consumption by tuning the air-flow rate of the cooling tower fan and the GHE loop water flow rate. To evaluate the proposed control method, a Modelica based model of the Hybrid GSHP system is developed by utilizing the Buildings Library developed by the Lawrence Berkeley National Laboratory, which consists of a 20-borehole GHE, a water-to-water heat pump, a counter-flow cooling tower and a plate heat exchanger. The transient conduction model of vertical GHE in the Buildings Library is adopted, which is based on a finite-volume method inside the borehole and cylindrical source model outside the borehole. A variable-flow water pump model is constructed for the GHE water loop, which gives power consumption under different operating scenarios. A cooling tower model in the Buildings Library is adopted, which is a static polynomial model based on a York cooling tower correlation. The relative air flow rate can be regulated to maintain the leaving water temperature at the setpoint, and then the corresponding fan power consumption is obtained. The heat pump model is based on the evaporator temperature, condenser temperature and Carnot efficiency. An inner-loop proportional-integral (PI) controller is implemented to regulate the evaporator leaving water temperature at 7 deg-C. Under the air wet-bulb temperature of 35 deg-C and dry-bulb temperature 23 deg-C, steady-state simulation of the plant model yields the static map of the total power with respect to the cooling tower relative air flow rate and the GHE water flow rate, which indicates about 25% power variation across the adjustable range of inputs. Simulation was conducted in two conditions: change in evaporator inlet water temperature and change in ambient air condition. The simulation study under way is to validate the effectiveness of the proposed ESC strategy, and the potential for energy saving will also be evaluated

Experimental Study on Extremum Seeking Control for Efficient Operation of Air-side Economizer

The air-side economizers are a major class of energy-saving devices for ventilation and air conditioning systems by taking advantage of outdoor air during cool or cold weather. Typical rule based control cannot justify energy optimal operation, while model based optimization of air-side economizer operation depends on the accurate knowledge of system model and enthalpy sensing of the ambient and return-air. Such optimal operation is hard to achieve in practice due to inaccurate model and degradation/failure of temperature and relative humidity (RH) sensors. As pointed out by Seem and House (2010), under certain indoor/outdoor air conditions, there exists a convex map between damper position and energy consumption of an air handling unit (AHU), which implies an optimal damper opening minimizing the cooling-coil load. Such convexity guarantees the use of gradient-search type of real-time optimization methods. An Extremum Seeking Control (ESC) was proposed by Li et al. (2009), where the chilled water flow rate of the cooling coil (equivalently the energy consumption) is minimized by tuning the damper opening. The proposed framework was validated with a Modelica based dynamic simulation model of an air-side economizer. This study is conducted to perform experimental evaluation of the ESC control of air-side economizer. The experimental setup is anchored on a Lennox XC25 variable-speed air conditioner. The Lennox, CBX40UHV indoor air handler unit is equipped with duct work to form an air-side economizer, connected to a foam based 16\u27X8\u27X8\u27 test chamber. The Lasko 751320 electrical heaters are used as heat source. The Honeywell HCM-890 humidifiers and Soleus Air SG-DEH-70EIP-6 dehumidifiers are used to regulate the indoor air humidity. A National Instruments CompactRIO-9024 platform is used for data acquisition and control. Major measurements include temperature, relative humidity (RH) and power consumption. A Watt Node Pulse WNB-3D-240-P electric power meter is used for power measurement. The Omega P-L-1/10-1/8-6-0-T-3 temperature sensors and Veris Industries HN3XVSX RH sensors are installed to monitor the indoor and outdoor air conditions. The Omega HHT13 speed sensors are used to measure fan speeds, while Fluke 80i-110s current sensors are used to measure the compressor motor current. The ESC controller is implemented with the damper opening as input and the total power consumption as feedback. Two experiments have been performed under different indoor/outdoor air conditions. The first experiment was performed under outdoor air temperature 23°C and RH 65%, a heat load of 6000 W and indoor temperature setpoint 28°C. The ESC turned on the outdoor damper 100% automatically to allow maximal outdoor air resulting in indoor RH 50%. The total power consumption was reduced from 540 W to 450 W with an energy saving of 16.67%. The second experiment was performed under same conditions with indoor RH regulated to 40%.The ESC turned off the outdoor damper to allow minimal outdoor air. The power consumption was reduced from 620 W to 600 W with an energy saving of 3.33%. More experiments will be performed in warmer weather in February and March to further validate the performance of the ESC controller

Evaluation of an Extremum Seeking Control Based Optimization and Sequencing Strategy for a Chilled-water Plant

Chilled-water plants with multiple chillers account for a significant fraction of energy use in large commercial buildings. Real-time optimization and sequencing of such plants is thus critical for building energy efficiency. Due to the cost and complexity associated with calibrating a chiller plant model to field operation, model-free control has become an attractive solution. Recently, Mu et al. (2015) proposed a model-free real-time optimization and sequencing strategy based on extremum seeking control (ESC) for chilled-water plants with multiple parallel chillers. In this ESC scheme, the variable to be optimized is the total power from the chiller compressors, cooling tower fans, condenser and evaporator loop water pumps, while the manipulated inputs include the tower fan airflow, condenser water flows and evaporator leaving chilled-water temperature setpoint. Two schemes are proposed for chiller sequencing: A) A chiller is turned on based on the measurement of chilled water valve position and is turned off when a chiller compressor is running at its nominal minimum speed. B) A chiller is turned on and off based on the measurement of operating cooling load. For Scheme A, Mu (2015) performed a comprehensive case study, and the simulation results demonstrated that the proposed framework performed well under various ambient, load and equipment conditions. However, for Scheme B, only one case was simulated. Although the two chiller sequencing schemes have a shared physical process in terms of chiller plant operation, it is necessary to evaluate the load-based Scheme B in terms of energy efficiency performance. This paper aims to provide a comprehensive evaluation for the Scheme B-based optimization and sequencing strategy for a multi-chiller chilled-water plant. Three ambient conditions are considered: i) 27 °C and 60%RH (Mild), ii) 37°C and 30%RH (Dry Hot), and iii) 37 °C and 80%RH (Humid Hot). For each of these ambient conditions, simulations are performed for the scenarios listed below, Scenario #3 is simulated with dynamic ambient and load profiles. Scenario #1. Two-chiller ESC with no sequencing under fixed ambient conditions Scenario #2. Chiller sequencing under variable load and fixed ambient conditions Scenario #3. Chiller sequencing with realistic ambient and load profile Scenario #4. Penalty Function based ESC Chiller Sequencing Scenario #5. ESC for Efficiency Recovery: Chiller A properly charged and Chiller B with a low refrigerant charge Scenario #6. ESC for Efficiency Recovery: Chiller A with nominal operation and Chiller B with heat exchanger fouling References B. Mu, Y. Li, T.I. Salsbury, J.M. House, Extremum Seeking Based Control Strategy for a Chilled-Water Plant with Parallel Chillers, ASME Dynamic Systems and Control Conference, Columbus, OH, paper no. 9949, 10 pages, 2015 B. Mu, Self-optimizing Control for Building Ventilation and Air Conditioning Systems,Ph.D.Dissertation, Department of Electrical Engineering, University of Texas at Dallas, December 2015