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    316 IEEE Transactions on Consumer Electronics, Vol. 59, No. 2, May 2013

    Contributed Paper

    Manuscript received 03/30/13

    Current version published 06/12/13

    Electronic version published 06/12/13. 0098 3063/13/$20.00 2013 IEEE

    Smart Heating and Air Conditioning Scheduling

    Method Incorporating Customer Convenience

    for Home Energy Management System

    Hyung-Chul Jo, Sangwon Kim, and Sung-Kwan Joo,Member, IEEE

    Abstract AHome Energy Management System (HEMS)

    is expected to be vital for saving energy costs considering the

    time-varying price of electric power in a smart home

    environment. Studies on various energy resources such as

    energy storage systems and fuel cells in a smart home

    environment are required for HEMS development. In the area

    of the HEMSs, however, there exists very limited research on

    heating and air conditioning scheduling incorporating

    customer convenience. This paper presents a smart heating

    and air conditioning scheduling method for HEMS thatconsiders customer convenience as well as characteristics of

    thermal appliances in a smart home environment. The

    prototype software based on the proposed method for HEMS

    is also implemented1.

    Index Terms Home energy management system, smart

    heating and air conditioning scheduling, customer convenience,

    energy resource scheduling

    I. INTRODUCTIONTime-varying retail pricing schemes are being designed by

    utility companies for reducing the increasing energy demand.

    A Home Energy Management System (HEMS) [1]-[5] canplay an important role in saving the energy costs considering

    the time-varying price of electric power in a smart home.

    Wireless communication networks that control and monitor

    appliances in a smart home have been studied. HEMSs that

    incorporate communication technologies for reducing energy

    costs have been developed. In [6] and [7], a model and an

    algorithm for a HEMS for coordinating the operating schedule

    of a few devices in a building and a micro-grid were

    proposed; however, electric vehicle (EV), fuel cell (FC), and

    energy storage system (ESS) lifetime were not considered.

    In addition, very limited research has been conducted in the

    area of scheduling algorithms that control thermal appliances

    1This work was supported by the Human Resources Development of the

    Korean Institute of Energy Technology Evaluation and Planning (KETEP)

    grant funded by the Korea government Ministry of Knowledge Economy

    (No.20114010203010)

    H. -C. Jo is with the School of Electrical Engineering, Korea University,

    Seoul, 136-701, Republic of Korea (e-mail: [email protected]).

    S. Kim is with Future IT R&D Lab., LG electronics, Seoul, Republic of

    Korea (e-mail: [email protected]).

    S. -K. Joo is with the School of Electrical Engineering, Korea University,

    Seoul, 136-701, Republic of Korea (e-mail: [email protected])

    in a smart home environment, such as heating, ventilating, and

    air-conditioning (HVAC) systems, which consume more

    energy than others appliances in a home. HVAC models, such

    as a model considering outdoor environment information [8]

    and a resistance-capacitance equivalent model considering

    building structure [9], for thermal energy management in a

    building have been proposed. However, these HVAC models

    are highly complicated for use in a HEMS and it is difficult to

    obtain the information necessary for the model from the smart

    home.This paper presents a reduced HVAC model that can be

    applied to the scheduling problem in a HEMS. The proposed

    model is based on an ideal model that considers the customer

    convenience and characteristics of the HVAC system. For

    improving the practicality of the model, parameters related to

    indoor heat capacity and heat energy dissipation are estimated

    statistically from historical data.

    Scheduling problem with energy resource models in a smart

    home can be formulated as a mixed integer non-linear

    programming (MINLP) problem. The formulation is non-

    convex owing to bilinear constraints. The nonlinear, non-

    convex nature of the problem affects a significant reduction inthe convergence ratio of the problem and makes it difficult to

    arrive at the global optimal solution. In general,

    transformation techniques [10]-[12] have been applied to the

    original problem for overcoming bilinearity and obtaining a

    global optimal solution to the problem. In [13], a

    parameterization method was used to the scheduling problem.

    Because the method is based on the search tree, the

    optimization time of the problem may be longer. Therefore, in

    this paper, a linear transformation [12] and optimization

    technique is proposed for determining the optimal operating

    schedule of the energy resources.

    Furthermore, the prototype software based on the proposed

    method for HEMs has been developed. In order to determine

    the least-cost energy schedules for a smart home, the

    implemented software has four modules such as a data input

    module, a parameter estimation module, a scheduling module,

    and a database management module.

    This paper is organized as follows. First, in Section II, a

    model considering the HVAC system characteristics and

    customer convenience is proposed, and the HEMS scheduling

    problem is formulated. In Section III, a method using linear

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    H.-C. Jo et al.: Smart Heating and Air Conditioning Scheduling Method Incorporating Customer Convenience for Home Energy Management System 317

    transformation and optimization is proposed for solving the

    scheduling problem for the HEMS. Section IV describes the

    implementation of the prototype software. The numerical

    results are analyzed to show the effectiveness of the proposed

    method in Section V, and the conclusions of this study are

    presented in Section VI.

    II. HEATING AND AIR CONDITIONING SCHEDULINGPROBLEM WITH CUSTOMER CONVENIENCE FOR SMART

    HOME

    A method for the integrated scheduling of all energy

    resources in the home is required so that customers can reduce

    the overall energy cost through the introduction of a smart home

    and time-varying price. Fig. 1 shows the overview of HEMS in

    a smart home environment. The energy resources are restricted

    to a power grid, photovoltaic (PV) system, ESS, EV, FC, and

    HVAC because it is impossible to control all the equipment in a

    smart home environment without causing inconvenience to the

    customer. In this study, several profiles and parameters are

    assumed to be known from forecasts based on the historical data

    and information obtained from utility companies.

    Fig. 1. Overview of HEMS in a smart home environment.

    Models of the equipment such as ESS, FC, and EV that

    have been studied with regard to the large-scale power system

    or building can be applied to the HEMS. HVAC models

    almost similar to a real HVAC system were proposed in [8]

    and [9]. However, these HVAC models require additional

    information that is difficult to obtain within the home

    environment. A reduced and practical HVAC model that can

    be applied to a HEMS is required. Therefore, a model that

    considers the characteristics of the HVAC system and

    customer convenience is used in this study.

    A. HVAC ModelIt is difficult for a customer to frequently modify the HVAC

    setting for reducing energy cost considering the time-varying

    price. Therefore, the HVAC system is assumed to have an

    automatic control system that uses the control signal obtained

    from the HEMS scheduling module. An HVAC model for

    scheduling the operation mode and output by considering the

    temperature and time-varying price is proposed.

    The utility companies provide the information about the

    coefficient of performance (COP) which represents the ratio of

    change in heat to supplied electrical demand. Therefore, a

    HVAC model is typically expressed as follows:

    )()( tpCOPtq HVACHVAC (1)

    where qHVAC(t) is the cooling or heating demand generated by

    the HVAC system at time t andpHVAC(t)is the electrical demand

    of the HVAC system at time t.For estimating the HVAC systems power demand, the

    cooling and heating demands should be calculated considering

    the COP and temperature in the discrete time domain.

    The indoor cooling or heating demand, according to the first

    law of thermodynamics, is expressed as follows:

    )}()(}{)(){(

    )}()1({)(

    tTtTtZ

    tTtTtq

    inoutoffHVACoffon

    ininload

    (2)

    where qload(t)is the cooling or heating demand at time t; Tin(t)isthe indoor temperature at time t; Tout(t) is the outdoortemperature at time t;ZHVAC(t)is a variable denoting the HVAC

    system operation at time t; is the indoor heat capacity; and on

    andoffare parameters related to the outdoor temperature.The proposed model is considered as ideal if it is applied to

    the scheduling problem without correcting the value. For

    improving the practicality of the model, the estimated value

    must be close to the actual value. For that reason, a method to

    update thevalue with the historical data is adopted.

    B. Customer ConvenienceFor minimizing overall energy cost, the HVAC system

    should not be operated when the home is unoccupied. An

    estimate of house occupancy is required for determining the

    HVAC operating time. To this end, algorithms based on the

    customers manual selection or using sensors have been

    commercialized. The nonintrusive load monitoring (NILM)[14]can be used as an alternative method for estimating the time for

    which a house remains unoccupied. The NILM is a process for

    analyzing changes in the power or voltage supplied to a house,

    as shown in Fig. 2, and for detecting the appliances that are used

    in the house as well as the total energy consumption.

    Fig. 2. Overview of the prototype NILM system.

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    318 IEEE Transactions on Consumer Electronics, Vol. 59, No. 2, May 2013

    If the method based on the manual selection is not used, the

    customer occupancy can be statistically estimated by analyzing the

    information for the energy consumption of each appliance provided

    by the sensors or the algorithm based on the NILM.

    Because the temperature in the home decided by the scheduling

    may be so high or low that the customer feels inconvenience, the

    indoor temperature should be limited to the preferred temperature

    established by the customer within the HVAC operating time S.

    This requirement can be expressed as follows:

    StTtTT in ,)( maxmin (3)

    where Tmin and Tmax are the minimum and maximum values,

    respectively, of the indoor temperature.

    C. Mathematical Model for Scheduling ProblemIn this section, the objective function and constraints for the

    home appliances based on the aforementioned model and the

    formulation described in [6]-[7], are formulated as follows:

    1) Objective functionThe objective of the scheduling problem is to minimize the

    overall energy cost incorporating the costs of electricity andnatural gas. The objective function partially considers the

    ESS lifetime, and can be expressed as follows:

    24

    1

    )()()()()(

    t

    bobgasgrid tZBPtpNPtVtEPtpMin (4)

    wherepgrid(t)is the power bought from the power grid or soldto the grid at time t; EP(t) is the electricity price at time t;

    Vgas(t) is the volume of natural gas used by the FC at time t;

    NP is the natural gas price; pb(t) is the power of the ESScharging or discharging at time t;BP is the penalty value forESS discharging based on the Ah-throughput model [15]; and

    Zbo(t)is a binary variable denoting the discharging state.

    2) Constraints for ESS and EVThe EV model is based on the ESS model, and the

    automotive characteristics of the EV are considered.

    - SOC dynamicsb

    b

    Cap

    tptSOCtSOC

    )()()1( (5)

    where SOC(t) is the state of charge (SOC) in the ESS or theEV at time t; and Capbdenotes the capacity of the ESS or theEV.

    -ESS and EV capacity limitsmaxmin )( SOCtSOCSOC (6)

    where SOCminandSOCmaxare the minimum and the maximumSOC, respectively, of the ESS or the EV.

    - Target SOC (only EV)tarfinal SOCtSOC )( (7)

    where tfinaldenotes the expected departure time of the EV; and

    SOCtaris the target SOC.

    -ESS and EV charger limits0],,[],,[)( maxminmaxmin bibibobob PPPPtp (8)

    where minbiP andmax

    biP are the minimum and the maximum

    charging power, respectively, of the ESS or EV charger; and

    minboP and

    maxboP denote the minimum and the maximum

    discharging power of the ESS charger only.

    3) FC ConstraintsGiven its role as a small cogeneration unit, the FC generates

    heat as a by-product. FC characteristics can be formulated asthe combined heat and power system model proposed in [7].

    - Volume of natural gas used by FCnorelec

    FCgas

    qEftptV

    )()( (9)

    where pFC(t) is the power generated by the FC; Efelec is the

    electrical efficiency of the FC; and qnoris the energy per unitvolume of natural gas.

    - Quantity of heat energy generated by FC)(

    )1()( tp

    Ef

    EfEftq FC

    elec

    elecheatFC

    (10)

    where qFC(t)is the heat energy generated by the FC at time t;andEfheatis the FCs thermal efficiency.

    -FC generation limits0],,[)(

    maxmin

    FCFCFC PPtp (11)where minFCP and

    maxFCP are the minimum and the maximum

    power generated by the FC, respectively.

    4) HVAC system constraints-Heat balance

    )()()( tpCOPtqtq HVACFCload (12)

    },{)},()1({

    })(){()}()({)( ,,,

    FCHVACEtTtT

    tZtTtTtq

    inin

    Ei

    offiioffioniinoutload

    (13)

    where i,onand i,offare the values in the operating state and

    non-operating state of the thermal device i, respectively.

    - Customer-preferred temperatureStTtTT in ,)( maxmin (14)-HVAC power consumption limits

    0],,[)( maxmin HVACHVACHVAC PPtp (15)

    where minHVACP andmax

    HVACP are the minimum and the maximum

    electrical demand of the HVAC system, respectively.

    5) Power balance constraints)()()()()()( tptptptptptp loadHVACbFCsolargrid (16)

    wherepsolar(t)is the power supplied by the PV at time t; andpload(t)is the total demand from the uncontrolled appliances at time t.

    III. SMART HEATING AND AIR CONDITIONINGSCHEDULING METHOD FOR HEMSThe solution to the problem formulated as an MINLP using

    the above-described model could be global or local optimum

    depending on problem convexity. Constraint (13) is non-

    convex owing to the sum of the bilinear terms. Therefore, it is

    necessary to verify that the obtained solution is globally

    optimal. Owing to the complexity of this process, the problem

    must be converted into a convex form through convexification.

    A number of methods to transform a non-convex constraint

    into a convex one have been discussed. In this study, an easy-

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    H.-C. Jo et al.: Smart Heating and Air Conditioning Scheduling Method Incorporating Customer Convenience for Home Energy Management System 319

    to-implement linear transformation technique is adopted [11].

    Constraint (13) can be rewritten in a linear form using a set

    of slack variablesX(t)for the Tin(t)variables as follows:

    },{)},()1({)](

    )())}(1()(){([)(

    2,,

    1,,,,

    FCHVACEtTtTtX

    tXtZtZtTtq

    ininioffi

    ioni

    Ei

    ioffiionioutload

    (17)

    The bilinear term in constraint (13) can be substituted by

    the slack variables. Constraints (18)(23) are added to definethe new variables required for the transformation.

    ))(1()()())(1()( max1,min1, tZKtXtTtZKtX iiinii (18)

    )()()()()( max2,min2, tZKtXtTtZKtX iiinii (20)

    )()()( max1,min tZKtXtZK iii (21)

    ))(1()())(1( max2,min tZKtXtZK iii (22)

    Ei (23)

    whereKminandKmaxare, respectively, the lower and the upper

    bounds of Tin(t).

    The difference betweenKminandKmaxshould be as small as

    possible. For satisfying constraint (14), Kmin should be lower

    than Tmin and Kmax should be greater than Tmax. For selectingappropriateKminandKmaxvalues, historical Tin(t)data are used.

    Fig. 3 presents the proposed scheduling method for HEMS.

    The entire problem is written in a linear form by replacing the

    nonconvex constraint (13) with a linear one (17) and by

    adding constraints (18)(23). For solving the problem, first,

    the profiles and parameters obtained from the forecast

    algorithm are entered into the scheduling problem. Kmin,Kmax,

    , and HVAC operating time S are estimated from the

    historical data. Additionally, the preferred temperature is

    selected manually by the customer or from the estimation

    results using past selections. The optimal scheduling result can

    be obtained by solving the modified formulation.

    Fig. 3. The proposed scheduling method for HEMS.

    IV. IMPLEMENTATION OF PROTOTYPE SOFTWARE USINGSMART HEATING AND AIR CONDITIONING SCHEDULING

    METHOD WITH CUSTOMER CONVENIENCE FOR HEMS

    The prototype software using the proposed method has

    been developed using a commercially available, cross-

    platform optimization suite. It performs optimization for

    scheduling of energy resources and manages the scheduling

    results. Fig. 4 shows a configuration of the proposed prototype

    software for smart heating and air conditioning scheduling

    system. The software is composed of four modules that

    include a data input module, a parameter estimation module, a

    scheduling module, and a database management module.

    Fig. 4. Middleware Architecture of the proposed HEMS.

    A. Data Input ModuleAs shown in Fig. 5, the module collects the data that is

    directly input to the software by the customer or transmitted

    from the prediction modules. The customer can enter

    information related to the customer convenience for HVAC

    and EV into the software through the interface manager. The

    rest of the essential information is obtained from theprediction modules. The collected information is stored in the

    database.

    Fig. 5. Screenshot of data input in the implemented prototype software.

    B. Parameter Estimation ModuleThis module is designed to estimate the parameters from the

    historical data in the database. As mentioned in the previous

    sections, and K values are necessary to solve the proposed

    problem. Given the value estimated from the volume and mass

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    320 IEEE Transactions on Consumer Electronics, Vol. 59, No. 2, May 2013

    of indoor air, can be statistically estimated using equation (2)

    and historical data, as shown in Fig. 6. The output power of the

    device for smart heating and air conditioning, the indoor

    temperature, and the outdoor temperature constitute the

    historical data. If necessary, can also be estimated using a

    constantvalue via a process similar to that shown in Fig. 6.

    Fig. 6. estimation procedure for the proposed method for HEMS.

    For selecting the appropriate value ofK, the candidates ofKare determined by analyzing the historical data. Furthermore,

    the preferred temperature and the candidates are compared to

    determine the value of K. The parameters selected by the

    estimation engine are transmitted to the scheduling module.

    C. Scheduling ModuleThis module is designed to obtain the schedule of the

    energy resources using the proposed method and input data. It

    is operated when press the solve button. Given that the values

    of , , and K are estimated by the parameter estimation

    module, optimization based on the branch and bound

    algorithm can be performed to obtain the optimal schedule.

    D. Database Management ModuleThe input data and the optimal solution from the scheduling

    method is stored in the database through database

    management module and displayed, as shown in Fig. 7. Data

    that can be displayed in the software include the electrical

    demand for the uncontrolled appliances, PV output, prices of

    various energy resources, HVAC operation, EV charging, FC

    operation, and ESS charging/discharging.

    Fig. 7. Screenshot for displaying total energy consumption and electric

    energy of energy device.

    V. NUMERICAL RESULTSFor testing the proposed method, a scenario based on the

    data collected from several institutions during winter is

    created. A smart home with a volume of 1,145 m 3 is

    assumed for the simulation. It is also assumed that two

    adult occupants have a full-time job and the unoccupied

    period to be statistically estimated is from 08:30 am to

    19:00 pm. The energy resources in the smart homeenvironment include the PVs, FCs, ESS, EV, HVAC, and

    the uncontrolled appliances. Several parameters of each

    device to be assumed are listed in Table I. The preferred

    operating time of HVAC and EV charging time to be

    assumed in Table I is based on the unoccupied period. The

    value of is estimated from the volume and mass of the

    indoor air. The historical data for the estimation of are

    obtained from the library building.

    TABLEI

    SPECIFICATION OF ENERGY RESOURCES IN SMART HOME

    Energy Resource Specification

    Energy storage system

    Capacity 10 kWh

    Maximum battery

    charge/discharge6 kWh

    Electric vehicle

    Capacity 16.4 kWh

    Maximum batterycharge/discharge

    3.3 kWh

    Charging time 19 pm7 am

    Fuel cell

    Capacity 1 kW

    0.03 (not operated)0.04 (operated)

    Efficiency40% (electricity)

    45% (heat)

    Heating, ventilating, and

    air conditioning

    COP 2.73

    0.03 (not operated)0.55 (operated)

    Preferred operating

    time of HVAC19 pm 9 am

    Preferredtemperature

    19C

    The several profiles used in this study are shown in Fig. 8.

    Fig. 8 (a) shows the electrical demand profile forecasted from

    the electrical energy consumption data of the uncontrolled

    appliances and the occupied period. Fig. 8 (b) shows the 3-

    kWh PV generation profile. For developing the PV generation

    profile, the PV output power data to be obtained from the PVcell in the school building is used. The prices of natural gas,

    power bought from the utility, power sold to the utility, and

    BPare shown in Fig. 8 (c). The electricity price and natural

    gas price are, respectively, based on the time-of-use (TOU)

    and natural gas rate for the cogeneration of the utility in

    America. The BP is determined by using the ESS price and

    the amount of discharge during the lifetime. For the

    calculation of the amount of ESS discharge during the lifetime,

    it is assumed that the ESS lifecycle at 80% depth of discharge

    (DOD) is 3,000 cycle.

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    H.-C. Jo et al.: Smart Heating and Air Conditioning Scheduling Method Incorporating Customer Convenience for Home Energy Management System 321

    Fig. 8. (a) Electrical demand profile, (b) PV generation profile, (c) Prices

    profile of various resources.

    Based on several parameters and profiles, the smart heating

    and air conditioning scheduling method for HEMS provides a

    smart home with an optimal schedule for each energy resource.

    The total electrical demand incorporating the demands of

    HVAC and ESS charging and electrical energy resource

    composition are shown in Fig. 9. As can be seen in the figure,

    only the FC is operated when natural gas price is lower than

    electricity price. For generating profit using a BP lower than

    the TOU, the ESS is discharged at 20:00. In fact, during peak

    hours, all energy supplying resources are used for decreasing

    the amount of energy purchased from the grid.

    Fig. 9. Total electrical demand and composition of electrical energy

    supplies in smart home.

    As shown in Fig. 10, the proposed HVAC model is

    successfully applied to the scheduling problem. There is no

    preheating in the unoccupied period, during which the

    electricity price is at its peak. In contrast, the HVAC system is

    operated within the preferred temperature during the occupied

    period. Furthermore, owing to constant indoor temperature, as

    shown in Fig. 10, it can be observed that the part representing

    the indoor temperature dynamics in constraint (13) is ignored,

    and the HVAC output power is proportional to the difference

    between the outdoor and indoor temperatures.

    Fig. 10. Scheduling result for the HVAC operation and the indoor

    temperature change at constant preferred temperature in smart home.

    Setting different values of the preferred temperature in the

    method could have an effect on the overall energy cost. The

    cases with several different Tminand Tmaxvalues are compared

    to demonstrate the influence of preferred temperature on the

    overall energy cost in the smart home environment. The

    preferred temperatures in all these cases are listed in Table II.

    In cases I, II, and III, the preferred temperature of each case is

    respectively 19C, 20C, and 21C. In contrast, in case IV, thepreferred temperature is determined for the indoor temperature

    to be in a range of 19C to 21C for preventing customer

    inconvenience.

    TABLEII

    PREFERRED TEMPERATURE

    Case Temperature (C)

    Case I 19

    Case II 20

    Case III 21

    Case IV 1921

    It can be observed from the results shown in Fig. 11 that the

    overall energy cost can be reduced by setting the preferred

    indoor temperature within a specific range rather than as a

    constant value while avoiding inconvenience to the customer.

    The overall energy cost in the case IV is respectively 28%,

    32%, and 36% lower than other cases. Furthermore, in case of

    the constant preferred temperature, the overall energy cost

    increases by approximately 3% when the constant preferred

    temperature increases by 1C.

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    Fig. 11. Comparison of overall energy cost for various preferred

    temperatures.

    VI. CONCLUSIONThis paper proposes a smart heating and air conditioning

    method with customer convenience for HEMS. Optimalscheduling various energy resources in a smart home is

    needed to save energy costs. This paper describes a reduced

    HVAC model that considers customer convenience and a

    method for solving the scheduling problem of the HEMS with

    the reduced HVAC model. The HEMS based on the proposed

    method can be used to determine the least-cost schedules of

    the available energy resources while minimizing

    inconvenience to the customer in a smart home environment.

    The numerical results show that the HEMS based on the

    proposed scheduling method, which can be installed at various

    smart homes, has great potential to reduce customers overall

    energy costs.

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    DE-AC36-83CH10093, Jan. 1997.

    BIOGRAPHIES

    Hyung-Chul Jo received his B.S. degree in the school of ElectricalEngineering from Korea University, Seoul, in 2010. He is currently pursuinghis Ph.D. degree at Korea University. His research interests include smart

    home and building energy management systems and large- and small-scale

    system optimization.

    Sang-Won Kim received his B.S. degree from Seoul National University,

    Seoul, Korea in 2002 and M.S. degree from Korea University in 2013. Since2002, he has been with LG Electronics, Seoul, Korea, where he is

    currently a Senior Research Engineer in the Future IT R&D Lab.His main research interests include data mining and optimization algorithms

    for smart homes and buildings.

    Sung-Kwan Joo (M05) received his M.S. and Ph.D. degrees from theUniversity of Washington, Seattle, in 1997 and 2004, respectively. From 2004

    to 2006, he was an Assistant Professor in the Department of Electrical andComputer Engineering at North Dakota State University, Fargo, U.S. He is

    currently an Associate Professor in the School of Electrical Engineering atKorea University, Seoul, Korea. His research interests include

    multidisciplinary research related to energy systems, involving information

    technologies, optimization, and intelligent systems.