Combined Heat And Power Units Research Articles

Performance of micro CHP unit based on SI engine with quantitative-qualitative load control

The paper presents data resulting by the preliminary experimental tests performed on a micro CHP (combined heat and power) 7 kWel unit. The engine load has been controlled by throttle position (quantitatively) or/and the value of air excess ratio (qualitatively) QQLC. By this way the engine efficiency can be improved in the range of partial loads by reducing the exergy losses during the inlet stroke. During the tests engine has been powered with LPG fuel. The engine performance together with environmental impact has been studied in this paper. Used method shows that despite the reduction of the load from 5.6 kW to 4.7 kW while burning the lean mixture, the efficiency of electricity generation increased slightly. The efficiency grew by approx. 1.41 percentage point comparing with the re-sults obtained for almost constant load but obtained by burning the lean mixture (λ = 1.3), followed by increased throttling and combus-tion of the stoichiometric mixture.

A New Wind Power Accommodation Strategy for Combined Heat and Power System Based on Bi-Directional Conversion

The extensive use of wind power can not only reduce dependence on fossil fuels, but also reduce emissions of polluted gases. However, large-scale wind power curtailments often occur in northeast China during the heat supply season, due to the fact that most of electrical demand is covered by the electrical power of the combined heat and power (CHP) during the off-peak hours. At present, for northeast China with heating demand, most of the research only focuses on how to accommodate more wind power on the spot by using one-directional conversion of the electric and thermal energy. But it is still difficult to realize the bi-directional conversion between the electro-gas or electro-thermal energy. In this paper, a combined electro-gas bi-directional conversion system (CEGBCS) is established by adding the power to gas (P2G), fuel cell and heat storage device in CHP system. This CEGBCS can not only realize bi-directional conversion of the electricity and gas, but also decouple the two operation modes of CHP unit, which greatly improve the ability of system to accommodate additional wind power. Finally, the effectiveness of the proposed CEGBCS is verified by comparing with two traditional methods.

Environmental impacts concerning flexible power generation in a biogas production

Combined heat-and-power units (CHPUs) of biogas plants can be operated at partial load or in intervals to respond to market conditions or interventions by the grid operators. In this study, we present calculations for the specific greenhouse gas (GHG) emissions of electricity from biogas driven CHPUs in dependence of engine load. Following the methodology of life cycle assessment and using measured data from five real-world biogas plants in Bavaria, we investigated the following scenarios for the operation of the CHPU: (1) full load, (2) 80% load response and (3) 60% load response. Our system boundary included the whole biogas production chain, starting from the production/supply of the input materials and ending at the supply of electricity (functional unit: 1 kWh of electrical energy fed into the grid). As electrical efficiency and engine load are positively correlated, partial-load operation results in higher specific GHG emissions of electricity from biogas. The impact of decreasing efficiency of the CHPU under partial load on overall specific GHG emissions turned out to be higher than the influence of methane slip from the engine. Furthermore, the increase of further efficiencies such as higher yields, efficient use of synthetic and organic fertilizers, as well as minimization of fuel consumption and the use of regenerative fuels also lead to further decrease of GHG emissions.

A water mass method and its application to integrated heat and electricity dispatch considering thermal inertias

Currently, most district heating networks in China are running in the heat-setting mode, which limits the adjustments of combined heat and power (CHP) units. By considering the electrical power system and district heating system together, the peak regulatory capabilities of CHP units can be improved, and renewable energy accommodation can be promoted. In this paper, a tractable integrated heat and electricity dispatch (IHED) model is described that addresses the thermal inertia characteristics of pipelines and buildings to increase flexibility. A water mass method (WMM) for pipeline thermal inertias is proposed to simplify the IHED model by removing the integer variables and differential equations. Benefiting from the WMM, the proposed IHED model is a single-level nonlinear model that is tractable. An efficient iterative algorithm based on the generalized Benders decomposition and a sequential approach combined with the iterative algorithm and Interior Point OPTimizer (IPOPT) are proposed to solve the IHED model. The WMM is tested in thermal inertia simulations compared to an existing node method (NM) and commercial simulation software. The proposed solution strategy is verified in a small-scale system and a practical system. The simulation results are discussed to demonstrate the feasibility and economy of the IHED model proposed here.

Optimal configuration of distributed heat source to promote the wind power penetration in combined heat and power system

With increasing heating demand and installed capacity of wind farms, combined heat and power (CHP) systems with inflexible generation are confronted with a major challenge for the efficient utilization of the wind power. Distributed heat sources (DHSs), such as electric boilers and heat storage, offer an effective way to enhance wind power penetration in a combined heat and power system by reducing the electric-heat coupling of the CHP unit. In this paper, an assessment approach for the wind power penetration of a CHP system with DHSs is proposed, and a two-stage multiple types of DHSs optimal configuration strategy to realize the minimum construction cost at a certain permissible wind power curtailment rate is established. After that, the proposed strategy is applied to a case study of a practical CHP system with in-depth analysis and detailed discussion, which reveals the effectiveness of the proposed strategy and indicates its promising applications for DHS configuration.

Integrated Heat and Electricity Coordinated Dispatch Considering Multiple Time-Scale Flexibility of CHP Based on Thermal Energy Storage of DHS

With the large-scale installation of combined heat and power (CHP) units and wind farms in China, the amount wind curtailment and system frequency stability problems have arisen. In this paper, from the perspective of coordinated operation of electric heating system, the multiple time-scale operation mode of CHP unit is studied, and the flexibility of CHP unit is released, thus alleviating the pressure of power system dispatching and operation control. Based on the analysis of thermal energy storage of district heating system (DHS), coordination model of heating network system and CHP unit are proposed, which effectively unlock the multi-time scale flexibility of the CHP unit (MTF-CHP). Based on the framework of the combined heat and power coordinated dispatch, an adjustable robust heat dispatch model in DHS and a power system scheduling model based on the allowable heat range of CHP (AHR-CHP) are proposed to realize the coordinated configuration of CHP unit scheduling flexibility and regulating flexibility in power system output and reserve.

Stochastic Multi-Timescale Energy Management of Greenhouses With Renewable Energy Sources

In this paper, we propose a stochastic multi-timescale energy management scheme of greenhouses with renewable energy sources (RES), including a photovoltaic (PV) system, combined heat and power (CHP) unit, and energy storage systems. The optimal energy management problem is formulated as a multi-timescale Markov decision process to address the randomness of RES and the outside weather conditions. In particular, a fast-timescale process is used to model the rapidly changing electrical process with fine granularity, while a slow-timescale process is used to model the gradually varying thermal process to reduce the computational complexity. Exact solution of the optimal energy management problem is derived to minimize the cost of greenhouse operation. To further reduce the computational complexity of finding the optimal energy management policy, an approximation solution is also derived. The proposed energy management scheme is evaluated based on a commercial greenhouse structure from Bonnyville Forest Nursery, Inc. for spruce, as well as real data of weather conditions, PV generation, CHP unit, and energy storage systems.

Thermodynamic and Economic Analyses of Reformative Design for High Back-Pressure Heating in Coal-Fueled Cogeneration Units.

High back-pressure (HBP) heating technology has been identified as an effective approach to improve the efficiency of combined heat and power (CHP). In this study, the novel concept of a HBP heating system with energy cascade utilization is developed and its probability examined. In the reformative design, the extracted heating steam from the intermediate-pressure turbine (IPT) is first drawn to an additional turbine where its excess pressure can be converted into electricity, then steam with a lower pressure can be employed to heat the supply water. As a consequence, the exergy destruction in the supply water heating process can be reduced and the efficiency of the cogeneration unit raised. A detailed thermodynamic investigation was performed based on a typical coal-fired HBP–CHP unit incorporating the proposed configuration. The results show that the artificial thermal efficiency (ATE) promotion was as much as 2.01 percentage points, with an additional net power output of 8.4 MW compared to the reference unit. This was attributed to a 14.65 percentage-point increment in the exergy efficiency of the supply water heating process caused by the suggested retrofitting. The influences of the unit power output, unit heat output, supply water and return water temperatures and turbine back pressure on the thermal performance of the modified system are discussed as well. In addition, the economic performance of the new design is assessed, indicating that the proposed concept is financially feasible.

An affine arithmetic-model predictive control approach for optimal economic dispatch of combined heat and power microgrids

This paper presents a novel approach for the optimal economic dispatch of Combined Heat and Power (CHP) Microgrids (MGs), which incorporates an Affine Arithmetic-based Economic Dispatch (AAED) problem into a Model Predictive Control (MPC) framework. The proposed algorithm solves an AAED problem each Δt minutes (e.g. 15 min) with time steps of Δt minutes over a time horizon T (e.g. 24 h). It uses the available forecast and the current state of the system to provide the schedule and the affine forms that represent the operation intervals of the generators and Energy Storage Systems (ESS) for the next time interval [t,t+Δt]. Online set points for generators and ESS are then obtained by computing the noise symbols values of the affine forms, based on the most updated information of electricity and heat demands and available renewable energy power. A theoretical CHP-based MG, comprising PVs, a gas boiler, a CHP unit, a battery, and a thermal tank, is used to assess the performance of the AA-MPC approach in both connected and isolated operation modes. The method is also compared with a deterministic MPC approach. Results show the ability of the method to better address forecasting errors, resulting in more cost-effective solutions without considerably affecting the computation performance.

Thermodynamic and economic analyses of a hybrid waste-driven CHP–ORC plant with exhaust heat recovery

A smart hybrid power plant comprising a waste-fired CHP (combined heat and power) plant accompanied by a small-scale organic Rankine cycle was recently designed and analyzed thermodynamically. The objective of this hybridization is to maximize the share of electricity production of waste-CHP plants rather than a higher heat production rate in a cost-effective manner. In this work, utilization of the exergy of the hot flue gas of the waste-fired CHP unit in order to increase the potential of the organic Rankine cycle for maximizing the net power output of the hybrid cycle is proposed and techno-economically analyzed. In addition, the effect of using alternative environmentally-friendly organic working fluids on the performance of the system was investigated. The results of the study show that by the flue gas potential utilization of the same CHP unit, the size of the organic Rankine cycle may increase significantly, leading to a larger power output of the plant. As such, the net exergy and energy efficiency values of the combined plant in various operational conditions are improved compared to its primary configuration by, respectively, about 10% and 20%, when both of the main CHP cycle and the organic Rankine cycle are working at nominal load. In addition, this exergy utilization is beneficial economically as well, decreasing the payback period of the parallelization project by about 10% (from 7.4 years to 6.7 years). The alternative organic working fluid does not make a significant change in the technical and economic performance indices of the system.

Large-scale combined heat and power economic dispatch using a novel multi-player harmony search method

Combined heat and power (CHP) plants have the capability of supplying power and heat energy altogether. The major aim of CHP economic dispatch (CHPED) is determining optimal generation management of cogeneration plants with a minimum operational cost of providing power and heat load demands. Several electrical and operational constraints should be studied in solving the CHPED problem, which includes valve-point influences of traditional thermal production units, the production capacity of cogeneration units and load balance. In addition, heat and power generated of CHP plants have a bidirectional association, which is known as a feasible operating zone, that makes the CHPED problem more complex. In this study, a novel multi-player harmony search (MPHS) method is introduced for solving the non-convex non-linear large-scale CHPED challenge in the power system. The presented MPHS algorithm in this study is based on utilizing experiences of multi-players for harmony improvisation during the optimization process of the problem. The presented MPHS algorithm is employed on a 24-unit test system and an 84-unit case study as a large-scale system to evaluate the efficiency of the presented MPHS method. The large-scale 84-unit system includes 40 thermal plants, 24 CHP units, and 20 boilers. The determined optimal solutions are compared with the literature to confirm the performance of the presented MPHS algorithm.

Network constrained economic dispatch of renewable energy and CHP based microgrids

Micro-grids (MGs) are appropriate solutions to various problems in power systems. Different types of energy sources such as renewable energy sources, combined heat and power (CHP) plants and distributed energy resources can be integrated in MGs. Local generation of mentioned energy systems can help system operators to satisfy energy demands with no need to the purchased power from grid. In this paper, a new optimization model based on mixed-integer non-linear programming (MINLP) is proposed to solve economic dispatch of MG containing CHP units, conventional power generators, heat-only units, wind turbines (WTs), photovoltaic (PV) system and battery storage system under uncertainties. The main goal of proposed paper is to find the optimal solution of economic dispatch of MGs in power systems in which technical constraints like AC power flow limitations are taken into account. In the proposed paper, the effects of dual dependency of produced heat and power by CHP units as well as the impact of electrical energy storage and heat buffer tank are studied in the optimal solution of economic dispatch of MGs. Uncertainties of renewable generation units, price of grid power and load are taken into account using scenario-based method, robust optimization approach and information gap decision theory (IGDT), respectively. The proposed model is studied regarding a 14-bus test system in various cases and the results are presented to evaluate the performance of employed methods.

Optimal economic dispatch for multi heat-electric energy source power system

An optimal economic dispatch model to balancing power and heat demand in a power system incorporating combined heat and power (CHP) units, intermittent energy sources such as wind and solar photovoltaic (PV), and battery energy storage system are proposed. The model is structured to include heat storage tanks and district heating network (DHN) with a goal of increasing the flexibility of CHP units, and satisfying better integration of the energy sources. A bi-level optimization model and its representative mathematical relations are formulated with taking into account the stochastic nature of wind and PV outputs and the storage constraints as well. Furthermore, the formulated relations are optimally solved. So, hybrid particle swarm optimisation (PSO) technique and sequential quadratic programming (SQP) method are applied to provide the optimal power delivered from each energy source. The objective function is formulated in a manner that the social welfare (SW) is maximized. The proposed model is applied to both of modified IEEE 24-bus and IEEE 118-bus test systems and simulated to show to a large extent that the model is real and robust as well as it provides a powerful solution to obtain the global optimal power dispatch. A comparative study is implemented for validation.

Optimal Economic and Emission Dispatch of a Microgrid with a Combined Heat and Power System

With the rapid development of the new concept of energy internet, electric power systems often need to be investigated together with thermal energy systems. Additionally, to reduce pollution from gas emissions, it is very important to study the economic and emission dispatch of integrated electrical and heating systems. Hence, this paper proposes a multi-objective optimization dispatch model for a microgrid (MG) with a combined heat and power (CHP) system. This CHP-based MG system consists of a CHP unit, a wind turbine, a PV system, a fuel cell, an electric boiler, an electric storage, and a heat storage. It can exchange electricity with the distribution network and exchange heat with the district heating network. Minimum economic cost and minimum environmental cost are considered as the two objectives for the operation of this CHP-based MG system. To solve the two objective optimization problem, the multi-objective bacterial colony chemotaxis algorithm is utilized to obtain the Pareto optimal solution set, and the optimal solution is chosen by the Technique for Order of Preference by Similarity to Ideal Solution method. Finally, numerical case studies demonstrate the effectiveness of proposed model and method for the optimal economic and emission dispatch of the CHP-based MG system.

Multi-objective combined heat and power unit commitment using particle swarm optimization

The unit commitment (UC) problem is formulated by undertaking dual operating mode combined heat and power (CHP) units along with thermal and heat units. The extraction and backpressure mode of the CHP unit provides the facility to adopt immediate change in the generation, which depends upon the ratio of the heat and power generation. The impact of dual-mode CHP (DM-CHP) unit has been examined on multi-objective (MO) economic and profit based models, in order to deal with pollutant emission along with cost/profit. The model of CHP-UC problem is a mixed integer model, hence to deal with binary and real variables, binary particle swarm optimization (BPSO) and PSO technique have been employed, respectively. To demonstrate the effectiveness of the optimization technique, two test systems of thermal UC problem have been undertaken and the results are compared. In order to verify the benefits of DM-CHP unit, three test systems of CHP-UC problem have been undertaken and the impact of backpressure and extraction mode of CHP unit has been investigated in terms of cost/profit and emission. It has been verified from the results that backpressure and extraction mode of the CHP unit provide diverse UC status and generation schedule.

A Framework for Incorporating Demand Response of Smart Buildings Into the Integrated Heat and Electricity Energy System

The electricity output of combined heat and power (CHP) units is constrained by their heat output corresponding to customers’ heat demand, which makes it difficult for the CHP units to frequently adjust their electricity output. Therefore, additional balancing power is required to integrate the variable wind power in the CHP-based heat and electricity integrated energy system (HE-IES). This paper expands the demand response (DR) concept to the HE-IES. A comprehensive DR strategy combining energy substitution and load shifting is first developed to exploit the demand flexibility of smart buildings. Besides electric balancing power, heat balancing power is also provided to relax the production constrains of CHP units. Moreover, a real-time DR exchange (DRX) market is developed where the building aggregators are stimulated to adjust buildings’ energy consumption behaviors and provide the required balancing power. Compared with the existing day-ahead DRX market, the real-time DRX market can balance the very short-term wind power fluctuation and reduce price spikes. Additionally, a novel optimum feasible region method is proposed to achieve the fast clearing of the DRX market to meet the higher requirement for clearing speed in the real-time market. Simulation results verify the advantages of the proposed technique.

Combined heat and power control considering thermal inertia of district heating network for flexible electric power regulation

The capability to perform rapid load changes is an important issue due to its considerable support on power system stability, and its improvement is increasingly becoming urgent due to large-scale integration of fluctuant wind energies. An optimized control strategy for improving the load-following capability of combined heat and power (CHP) units is therefore developed. The strategy aims to borrow the heat extraction output to perform fast load changes given that its influence in tens of minutes on heat consumers is slight owing to the large inertia of district heating networks (DHNs). The static and dynamic models of heat–power conversion are set up. Dual control strategy is adopted to combine heat source regulation (HSR) and traditional boiler–turbine coordinated control strategy, among which HSR is taken as the primary control of power load for rapid load response and fuel control as secondary control for the final load accuracy. Finally, field tests on a 330 MW CHP unit reveal that the maximum allowable ramp rate can be raised to 4% of rated power per minute and meanwhile it almost takes no impact on heat consumers.

Techno-economic assessment of providing control energy reserves with a biogas plant

Grid stability is being challenged by the increasing integration of power plants with volatile power generation into the energy system. Power supply fluctuations must be compensated by energy system flexibility. The storability of the energy carrier enables biogas plants to generate power flexibly. In this study, the technical and economic effects of providing positive secondary control energy reserves with an Austrian biogas plant were assessed. The plant’s main focus lies in biomethane production with the option of heat and power generation through combined heat and power (CHP) units. A detailed simulation model of the investigated biogas plant was developed, which is presented in this work. Ex-post simulations of one year of flexible plant operation were conducted with this model. The findings show that the installed biogas storage capacity is sufficient to provide control energy reserves while simultaneously producing biomethane. Profitability of providing control energy reserves largely depends on the prices at the control energy market and on CHP unit start-up costs. A cost efficiency analysis demonstrated that investing in a hot water tank with a volume of 5 m3 for short-term heat storage turned out to be economically viable.

Mixed-integer nonlinear programming framework for combined heat and power units with nonconvex feasible operating region: Feasibility, optimality, and flexibility evaluation

This paper presents an optimization framework to determine the optimal operating points of combined heat and power (CHP) units with nonlinear, nonconvex feasible operating region (FOR). The mentioned problem is the economic dispatch (ED) of heat-only units, thermal units, and CHP units. Also, the electric units are of thermal technology while their valve-point impact is taken into consideration. Also, the heat power curve of heat-only units is nonlinear. It is noted that the FOR of CHP units has been defined both as convex and nonconvex regions. For those with nonconvex characteristic, a method is proposed to convert it into a convex characteristic. Accordingly, a separate binary variable must be defined to determine the optimal operating point in each convex region. Thus, the presented problem in this paper is of mixed-integer nonlinear programming (MINLP) type modeled in General Algebraic Modeling System (GAMS) software. In this respect, different case studies have been presented to assess the optimality, feasibility, and the flexibility of the model. It is noteworthy that the valve-point effect of thermal units and different FORs of CHP units as well as the electrical power losses have been considered.

Improved Boiler-Turbine Coordinated Control of CHP Units with Heat Accumulators by Introducing Heat Source Regulation

It is significant for power system stability to improve the operation flexibility of grid-connected units. Such improvement has always been a hot topic, especially for coal-fired units. In recent decades, it has become increasingly urgent and challenging as large-scale fluctuant renewable energy is connected to the power grid. Boiler-turbine coordinated control strategy (CCS), which is employed to perform unit load control according to automatic generation control (AGC), has a slow ramp rate in general on account of large delay and inertia of boiler, so to improve the unit operating flexibility, it is necessary to explore usable heat storage and optimize the control strategy. In combined heat and power (CHP) units with heat accumulators, their heat and power are decoupled. Therefore the extraction steam that goes to the heating station can be regulated flexibly even operating in “heat-led mode”. The change of extraction steam flow has a significant influence on the turbine power output, so we propose to improve the load-following capability of CHP units by regulating the heat source flow. In this paper, the influencing model is set up, and it is about heat source flow variations on the electric power output. The load control strategy is further optimized and designed through combinations of CCS and heat source regulation. Finally simulations and analysis are performed on a 330MW CHP unit, and the results reveal that the power ramp rate with our strategy is two times faster than that with traditional strategy.