Xiaohua Xia

Xiaohua Xia
University of Pretoria | UP · Department of Electrical, Electronic and Computer Engineering

PhD

About

236
Publications
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10,903
Citations

Publications

Publications (236)
Article
Full-text available
Solar irradiance forecast is closely related with efficiency and reliability of renewable energy systems. Multi-timescale irradiance forecast is a new and efficient way to simultaneously predict solar energy generation on different timescales for hierarchical decision making. This paper newly adopts the multi-task learning mechanism to study the mu...
Preprint
Solar irradiance forecast is closely related with efficiency and reliability of renewable energy systems. Multi-timescale irradiance forecast is a new and efficient way to simultaneously predict solar energy generation on different timescales for hierarchical decision making. This paper newly adopts the multi-task learning mechanism to study the mu...
Conference Paper
Temperature has a significant impact on the performance and ageing of electrochemical storages, especially the battery. It is therefore of paramount importance to understand and control their thermal stress for increased longevity. This paper presents the preliminary findings on a thermo-electric management of a battery-supercapacitor hybrid system...
Article
This paper presents a two-layer control strategy designed for easy integration of supercapacitors in a grid-integrated solar photovoltaic-battery hybrid renewable system, initially controlled by a typical model predictive control method. To operate the upgraded energy system, either without or with little modifications of the pre-existing architect...
Article
Full-text available
This paper presents an optimal control strategy of a grid-tied microhydro-wind power system for a rural dairy farm in South Africa. The problem is formulated as a multi-objective optimisation programme in discrete time domain to minimise grid imported energy cost under Time of Use Tariff (TOU) while at the same time maximising revenue generated fro...
Article
Energy and water are two inseparable resources that are crucial for human survival, yet, most developing nations are struggling to reliably provide them to the population especially in rapidly growing urban areas. Increasing demand is forcing governments, organizations and private sectors to encourage end-users to increase efficiency and conservati...
Article
Full-text available
Low hydropower generation in dry season due to low water levels in hydropower dams is a common problem, particularly in drought-prone regions such as Southern Africa. In this paper, an optimal hydrokinetic-powered pumpback retrofit for recycling a part of the dam's downstream discharge to optimise the performance of the dam is proposed. The optimis...
Article
Full-text available
Developing nations such as South Africa are energy insecure despite their high potential for renewable energy sources such as solar. This has forced various governments to initiate energy efficiency and conservation programs. Water heating contributes to a significant percentage of the electricity consumption in domestic and office buildings. There...
Article
Full-text available
Downhill conveyors are important potential energy sources within conveyor belt systems (CBSs). Their energy can be captured using regenerative drives. This paper presents a generic optimisation model for the energy management of CBSs that have downhill conveyors. The optimisation model is able to optimally schedule three configurations of a case-st...
Article
Full-text available
Many remote areas in developing countries such as Africa and island developing nations rely on expensive diesel grid even though there is a high potential for renewable sources such as wind and solar. Further, in domestic houses, water heating contributes to a huge percentage of the overall electricity consumption. Therefore, integration of renewab...
Article
In this technical note, an adaptive model predictive control (MPC) is proposed for unconstrained discrete-time linear systems with parametric uncertainties. The control objective is reference tracking. The adaptive MPC is designed by combining an adaptive updating law for estimated parameters and a constrained MPC for an estimated system. It is pro...
Article
The management of energy-water nexus in buildings is increasingly gaining attention among domestic end-users. In developing countries, potable water supply is unreliable due to increasing demand, forcing end-users to seek alternative strategies such as pumping and storage in rooftop tanks to reliably meet their water demand. However, this is at an...
Article
Developing countries are grappling with energy and water insecurity, impelling governments to encourage end-users to conserve and efficiently consume these vital resources. With water heating being among the largest energy users in residential buildings, it is imperative that the efficiency and conservation of both energy and water are achieved. He...
Article
In this paper, we consider two kinds of sampled-data observer design for a class of nonlinear systems. The system output is sampled and transmitted under two kinds of truncations. Firstly, we present definitions of the truncations and the globally uniformly ultimately bounded observer, respectively. Then, two kinds of observers are proposed by usin...
Article
In this paper combined demand side management strategy for residential consumers is studied for five households in South Africa. This study is twofold; the first part proposes an energy management system that combines demand side management strategies with a view of minimizing the consumer’s cost and reducing the power consumption from the grid. Ap...
Article
In this paper, a new demand-side management problem of networked smart grid is formulated and solved based on evolutionary game theory. The objective is to minimize the overall cost of the smart grid, where individual communities can switch between grid power and local power according to strategies of their neighbors. The distinctive feature of the...
Article
M&V (Measurement and Verification) has become an indispensable process in various incentive EEDSM (energy efficiency and demand side management) programmes to accurately and reliably measure and verify the project performance in terms of energy and/or cost savings. Due to the uncertain nature of the unmeasurable savings, there is an inherent trade-...
Article
In this paper, a nonlinear control is proposed for trajectory tracking of heavy-haul trains. The distinctive feature of the proposed control strategy is that, no active control efforts are exerted on wagons; and braking forces of wagons are considered as resistance, such that constraints of negative values on controls for wagons are avoided. It is...
Article
Full-text available
The expected savings from energy efficiency projects are divided into two, direct savings and indirect savings. Direct savings refer to savings obtained through the personal effort of an individual implementing some energy efficiency measures. Indirect savings are achieved through information transmission of energy efficiency measures from an indiv...
Article
The management of energy and water is increasingly gaining attention among domestic end-users. In developing countries, potable water supply is unreliable leading to alternative strategies such as rooftop water harvesting (RWH), storage and pumping. Since urban garden irrigation is the highest outdoor water consumer, optimal scheduling is important...
Article
Full-text available
As electricity prices continue to rise, the increasing need for energy management requires better understanding of models for energy-consuming applications, such as conveyor belts. Conveyor belts are used in a wide range of industries, including power generation, mining and mineral processing. Conveyor technological advances are leading to increasi...
Article
This paper develops an optimal control (OC) model of a heat pump water heater (HPWH) supplied by a wind generator-photovoltaic-grid system. The objective function is energy cost minimization, taking into account the time-of-use electricity tariff, which is an important control parameter. The control variables are the supply switch to the HPWH and t...
Article
Full-text available
The truck dispatching problem in open-pit mines is formulated into an integer programming problem in order to optimally determine the trip numbers of trucks from a shovel (dump site) to a dump site (shovel). The dispatching result aims to answer the question of where should a truck go so that the production target is achieved with minimum operating...
Article
Full-text available
The building sector is a large consumer of the world's energy and is responsible for 40% of the European Union's (EU) total final energy consumption. The most effective and efficient method to reduce energy consumption of buildings is to retrofit existing buildings with more energy efficient facilities. This paper presents a multi-objective optimiz...
Article
Full-text available
This paper develops an optimal control strategy for power dispatch of the grid tied photovoltaic (PV)-battery-diesel system to power heat pump water heaters (HPWH). The system comprises of the PV modules, grid, battery, HPWH, diesel generator (DG) and other domestic appliances. The PV can simultaneously feed-in the excess power to the grid and supp...
Article
In the paper, a typical coal trade process is described and modelled, where one logistics enterprise with blending equipments lies in the core and two types of common contracts are elucidated to define constraints. A mixed-integer model is built and featured by addressing contract violation, blending operation, real-time price information and arbit...
Article
Full-text available
In this paper, we integrate a Demand Response (DR) program into the multi-objective dynamic economic emission dispatch (DEED) optimization problem. The resulting optimization problem is termed DR-DEED. The DR program is a time based program known as the Time of Use DR program. The DR program has been developed using the customers' Price Elasticity...
Article
Full-text available
In the electricity market, customers have many choices to reduce electricity cost if they can economically schedule their power consumption. Renewable hybrid system, which can explore solar or wind sources at low cost, is a popular choice for this purpose nowadays. In this paper optimal energy management for a grid-connected photovoltaic-battery hy...
Article
This paper discusses the maintenance plan optimization problem for the energy efficiency purpose in the building energy efficiency retrofitting context. A subproblem namely the Building Retrofitted Facilities Corrective Maintenance Planning (BRFCMP) problem is proposed, where the corrective maintenance for malfunctioning retrofitted items are invol...
Article
This paper develops an optimal control strategy for power dispatch of the grid-tied photovoltaic (PV)–battery–diesel system to power heat pump water heaters (HPWH). The system consists of the PV modules, grid, battery, HPWH, diesel generator (DG) and other domestic appliances. The PV can simultaneously feed in the excess power to the grid and suppl...
Conference Paper
This paper models residential demand response using a bottom-up approach for five households in South Africa. This study is twofold; the first part proposes an energy management system that combines demand side management strategies with a view of minimizing the consumer’s cost and reducing the power consumed from the grid. Appliance scheduling wit...
Conference Paper
This paper models residential demand response using a bottom-up approach for five households in South Africa. This study is twofold; the first part proposes an energy management system that combines demand side management strategies with a view of minimizing the consumer’s cost and reducing the power consumed from the grid. Appliance scheduling wit...
Article
In the current situation of the energy crisis, the mining industry has been identified as a promising area for application of demand side management (DSM) techniques. This paper investigates the potential for energy-cost savings and actual energy savings, by implementation of variable speed drives to ventilation fans in underground mines. In partic...
Article
A new adaptive switched model predictive control (MPC) strategy is designed in this brief for energy dispatching of a photovoltaic-diesel-battery hybrid power system, where the battery is unpermitted to charge and discharge simultaneously. The distinguishing feature of the proposed switched MPC is that, new switched constraints are constructed to d...
Article
Renewable hybrid systems with small capacities are widely installed for joining demand response in modern communities. Based on hybrid systems, renewable energy generation can be stored and used for power supply during peak load period. It is impractical to control power flow in such small-scale systems, because power flow dispatching asks for extr...
Article
The efficiency of any energy system can be charaterised by the relevant efficiency components in terms of performance, operation, equipment and technology (POET). The overall energy efficiency of the system can be optimised by studying the POET energy efficiency components. For an existing energy system, the improvement of operation efficiency will...
Article
The energy savings achieved by implementing energy efficiency (EE) lighting retrofit projects are sometimes not sustainable and vanish rapidly given that lamp population decays as time goes by if without proper maintenance activities. Scope of maintenance activities refers to replacements of failed lamps due to nonrepairable lamp burnouts. Full rep...
Article
The aim of this paper is to develop an optimal scheduling strategy model for a grid-tied photovoltaic (PV) system to power a heat pump water heater (HPWH). The system is composed of PV modules that are grid-tied and a backup battery. The PV is capable of supplying power simultaneously to the HPWH and domestic load, whilst the grid and the battery a...
Article
Full-text available
Model predictive control (MPC) has been successfully applied to many transportation systems. For the control of overhead cranes, existing MPC approaches mainly focus on improving the regulation performance, such as tracking error or steady-state error. In this paper, energy efficiency as well as safety is newly considered in our proposed MPC approa...
Article
This paper presents an optimal energy management model of a solar photovoltaic-diesel-battery hybrid power supply system for off-grid applications. The aim is to meet the load demand completely while satisfying the system constraints. The proposed model minimizes fuel and battery wear costs and finds the optimal power flow, taking into account phot...
Article
This paper addresses the problem of output feedback stabilization for nonlinear systems with sampled and delayed output measurements. Firstly, sufficient conditions are proposed to ensure that a class of hybrid systems are globally exponentially stable. Then, based on the sufficient conditions and a dedicated construction continuous observer, an ou...
Article
Full-text available
During a demand response program, cost saving in households is achieved by optimal scheduling of appliances. This paper introduces inconvenience to the general problem of appliance scheduling. It measures the disparity between the baseline and optimal schedule and the consumer seeks to minimize this objective in addition to cost minimization. A non...
Conference Paper
Full-text available
Human thermal comfort is an important concern in the energy management of commercial buildings. Human thermal comfort research focuses mostly on the temperature control or the humidity control while based on human thermal comfort index control is ignored. In this paper, an optimal human thermal comfort control model (OHTCM) for a desiccant wheel co...
Article
Full-text available
The aim of this paper is to develop an optimal scheduling strategy model for a grid-connected photovoltaic (PV) system to power heat pump water heaters (HPWH). The system is composed of PV modules which are grid-tied. The PV is capable of supplying power simultaneously to the HPWH and domestic loads whilst the grid is a complimentary source. The co...
Article
Full-text available
In this paper, an optimal energy management model of a solar photovoltaic-diesel-battery hybrid power supply system is presented. The aim is to meet the load demand completely while satisfying the system constraints. The model proposed minimizes fuel and battery wear costs, and finds the optimal power flow taking into account photovoltaic power ava...
Article
Full-text available
A coal washing plant is studied for the purpose of reducing energy consumption while ensuring coal quality. The pumping system that supplies dense medium to the dense medium cyclone separation process, one of the biggest energy consumers in the coal washing plant, is studied in particular. The medium flow rate and density are optimized to reduce en...
Article
Full-text available
This paper presents an optimal control model to improve the operation efficiency of a vertical shaft impact (VSI) crushing process. The optimal control model takes the energy cost as the performance index to be minimized by accounting for the time-of-use (TOU) tariff. The control variables in the developed model are the belt conveyor feed flow rate...
Article
Full-text available
In this article, the optimal sampling and maintenance plans are both designed for the EE lighting retrofit projects. By adopting the optimal solutions, the project sponsors receive additional and sustainable energy savings over a 10-years’ crediting period. The reported project performance is accurate enough to satisfy the project sampling accuracy...
Article
In this paper, a mathematical model is formulated to quantify the social impact an individual has on his/her community when he/she performs any energy efficiency project and transmits that information to his/her neighbours. This model is called the expected power savings model; it combines direct and indirect expected power savings of the energy ef...
Article
This paper proposes a metering cost minimisation model that minimises metering cost under the constraints of sampling accuracy requirement for clean development mechanism (CDM) energy efficiency (EE) lighting project. Usually small scale (SSC) CDM EE lighting projects expect a crediting period of 10 years given that the lighting population will dec...
Article
For some existing buildings with out-of-date facilities, energy efficiency retrofit is a promising method to reduce energy consumption of buildings with small amount of investment. Among many choices of alternative efficient interventions, different strategies to select them are closely related with retrofit cost, energy saving and net present valu...
Article
Full-text available
The current trend of electricity market deregulation ushers in increasingly dynamic electricity pricing schemes. The cost-optimal scheduling of industrial loads with accurate price forecasts is therefore important. However, results in the current literature suggest that mean absolute percentage error (MAPE) is poor at indicating the economic benefi...
Article
In this paper, finite time dual neural networks with a new activation function are presented to solve quadratic programming problems. The activation function has two tunable parameters, which give more flexibility to design the neural networks. By Lyapunov theorem, finite-time stability can be derived for the proposed neural networks, and the actua...
Article
In this paper, an energy dispatch model that satisfies the load demand, taking into account the intermittent nature of the solar and wind energy sources and variations in demand, is presented for a solar photovoltaic–wind–diesel–battery hybrid power supply system. Model predictive control techniques are applied in the management and control of such...
Article
A new adaptive switched model predictive control (MPC) strategy is designed in this brief for energy dispatching of a photovoltaic-diesel-battery hybrid power system, where the battery is unpermitted to charge and discharge simultaneously. The distinguishing feature of the proposed switched MPC is that, new switched constraints are constructed to d...
Article
Full-text available
The dense medium cyclone (DMC) process used in coal beneficiation plants is studied from a control system perspective. Employing the dynamic model of the DMC process derived from mass flow balance, a model-based control strategy is proposed. The controller adjusts the density of medium used to enhance separation in the DMC process according to meas...
Article
The dynamical behaviors of an Euler discretized sliding mode control (SMC) systems based on equivalent control strategy are studied. A periodic-2 orbit in steady state is found for the switching function of the Euler discretized SMC systems. The time steps for the switching function to converge toward the periodic-2 orbit are obtained. When the dis...
Conference Paper
We design observers for nonlinear systems with sampled and delayed output measurements. The observers are of continuous and hybrid in nature. Based on an auxiliary integral technique, the exponential stability of the estimation errors is achieved, and the sampling period and the maximum delay are also given. Finally, numerical examples are provided...
Article
An improved model for reducing the cost of long-term monitoring in Clean Development Mechanism (CDM) lighting retrofit projects is proposed. Cost-effective longitudinal sampling designs use the minimum numbers of meters required to report yearly savings at the 90% confidence and 10% relative precision level for duration of the project (up to 10 yea...
Article
Full-text available
Immunological failure is identified from the estimation of certain parameters of a mathematical model of the HIV infection dynamics. This identification is supported by clinical research results from an original clinical trial. Standard clinical data were collected from infected patients starting Highly Active Anti-Retroviral Therapy (HAART), just...
Article
Full-text available
Small- and medium-sized enterprises (SMEs) are the important driving forces for the growth of China's economy. However, financing difficulty has always been the important problem besetting the development of SMEs for a long time. In particular, in recent years, US subprime crisis in 2008 caused a heavy blow to the development of some externally ori...
Article
In this paper, dynamical behaviors of the discrete-time sliding-mode-control (DSMC) uncertain systems are studied. First, a discrete reaching law with a disturbance compensator is presented, and a sliding-mode controller is designed by using the proposed reaching law. Second, the time steps for the system trajectories to converge to the switching m...
Article
Full-text available
The building sector contributes a large proportion of the world's total final energy consumption. As a result, considerable attention has been paid to energy efficiency in the building sector. At the current stage, building retrofitting is the most feasible and cost-effective method to improve building energy efficiency. This paper presents a multi...
Article
In practice, design parameters of a belt conveyor likely drift away from their design values by maintenance, readjustment, retrofit, abrasion, and circumstance change. For the purpose of energy optimization, these parameters should be estimated through experiments. In this paper, a new energy model of a DC motor driven belt conveyor is presented. T...
Conference Paper
In this paper, the leader-following consensus problem of multi-agent is studied. A decentralized adaptive design method is presented for multi-agent systems with non-identical unknown nonlinear dynamics, and for a leader to be followed that is also nonlinear and unknown. By parameterizations of unknown nonlinear dynamics of all agents, a decentrali...
Article
Full-text available
Generalized method of moments (GMM) has been widely applied for estimation of nonlinear models in economics and finance. Although generalized method of moments has good asymptotic properties under fairly moderate regularity conditions, its finite sample performance is not very well. In order to improve the finite sample performance of generalized m...
Article
A pump storage system (PSS) is introduced to the coal preparation dense medium cyclone (DMC) plants to improve their energy efficiency while maintaining the required medium supply. The DMC processes are very energy intensive and inefficient because the medium supply pumps are constantly over-pumping. The PSS presented is to reduce energy consumptio...
Article
This paper develops two optimal control models for the energy management of a mining crushing process based on jaw crushers. The performance index for both models is defined as the energy cost to be minimized by accounting for the time-of-use electricity tariff. The first model is referred to as a variable load-based optimal control with the feeder...
Article
Full-text available
The design of a robust controller for the voltage source inverter is essential for reliable operation of distributed energy resources in future smart microgrids. The design problem is challenging in the case of autonomous operation subsequent to an islanding situation. In this article, a dual-loop controller is proposed for voltage source inverter...
Article
Traditional building retrofitting planning is a decision making process prior to the building retrofitting investment. Although the long term energy and financial issues of the retrofitting project are considered in some cases, the energy efficiency potentials due to the maintenance and operation of the retrofitted facilities are not sufficiently e...
Article
Thermal comfort is an important concern in the energy efficiency improvement of commercial buildings. Thermal comfort research focuses mostly on temperature control, but humidity control is an important aspect to maintain indoor comfort too. In this paper, an optimal humidity control model (OHCM) is presented. Model predictive control (MPC) strateg...
Article
Clean development mechanism (CDM) project developers are always interested in achieving required measurement accuracies with the least metering cost. In this paper, a metering cost minimisation model is proposed for the sampling plan of a specific CDM energy efficiency lighting project. The problem arises from the particular CDM sampling requiremen...
Article
Full-text available
Insufficient energy supply is a problem faced by many countries, and energy efficiency improvement is identified as the quickest and most effective solution to this problem. Many energy efficiency projects are therefore initiated to reach various energy saving targets. These energy saving targets need to be measured and verified, and in many countr...
Article
This paper is concerned with the finite-time consensus problem of distributed agents having non-identical unknown nonlinear dynamics, to a leader agent that also has unknown nonlinear control input signal. By parameterization of unknown nonlinear dynamics, a Lyapunov technique in conjunction with homogeneity technique is presented for designing a d...
Article
Hybrid systems present a new dimension to the time correlation of intermittent renewable energy sources. The paper considers the daily energy consumption variations for winter and summer weekdays and weekends in order to compare the corresponding fuel costs and evaluate the operational efficiency of the hybrid system for a 24-h period. Previous stu...
Conference Paper
Model Predictive Control (MPC) was applied in Economic Dispatching (ED) problem, but its computation burden was large due to high complexity and dimension of problems. In this paper, a decomposed MPC approach is proposed to reduce the computational time without decreasing the performance of convergence and robustness. The decomposed MPC uses the “d...
Conference Paper
The expected savings from energy efficiency projects are divided into two the technical or direct savings and the nontechnical or indirect savings. The human and social contributions also known as the non-technical savings are rarely acknowledged when quantifying the total expected energy savings although humans are the ones that use the energy. Th...
Conference Paper
A model derived from existing biological logistic population growth equations is proposed for modelling the decay of Compact Fluorescent Lamp populations over time. Its adequacy is analysed by implementing a least-squares system identification approach using accepted benchmark data. The model is found to be as accurate as the existing CFL decay mod...
Conference Paper
In this paper, a metering cost minimisation model is proposed to find out the optimal solutions to the meter commitment problem for the CDM energy efficiency lighting projects. Under the typical CDM sampling accuracy requirement, known as 90/10 criterion, the optimal confidence and precision levels for each lighting group have been characterised. T...
Article
Considering time-of-use electricity pricing, the optimal scheduling problem of a pumping station is reformulated into a control sequence (CS) optimal scheduling problem, for which a reduced dynamic programming algorithm (RDPA) is proposed to obtain the solution. It is shown that the RDPA allows a reduction of the operational cost by about 60% compa...
Conference Paper
For the multiobjective problems, some global search methods may fail to find the Pareto optima with both accuracy and diversity. To pursue the two goals at the same time, a new memetic multiobjective differential evolution algorithm (MMODE) is proposed to hybridize the local search with differential evolution (DE) algorithm. The local search is con...
Article
Full-text available
A multiple objective optimisation model is formulated to help decision makers to make an optimal decision when investing in energy-efficient building retrofitting. The objectives are to maximise the energy savings and minimise the payback period for a given fixed initial investment. The model is formulated as a multi-objective optimisation problem...
Conference Paper
In this paper, we consider the adaptive synchronization problem for multi-agents systems with unknown nonlinear dynamics. The topologies of the networks are switching. A novel adaptive synchronization algorithm is proposed by using linear parameterizations of unknown nonlinear dynamics of all agents. By stability analysis and parameter convergence...
Conference Paper
In this paper, the consensus problem of multiagent systems with non-identical unknown nonlinear dynamics following an unknown and nonlinear leader is studied. By parameterizations of unknown nonlinear dynamics of all agents, decentralized adaptive consensus algorithms are proposed in networks with fixed and switching topologies by incorporating loc...
Article
Full-text available
The optimal operation scheduling of a pumping station with multiple pumps is formulated as a dynamic programming problem. Based on the characteristics of the problem, an extended reduced dynamic programming algorithm (RDPA) is proposed to solve the problem. Both the energy cost and the maintenance cost are considered in the performance function of...