Genetic algorithms and non-intrusive energy management system based economic dispatch for cogeneration units

By integrating neural networks (NNs) with turn-on transient energy analysis, this work attempts to recognize demand load, including the buyers’ load on the power systems and the internal load on the cogeneration systems, thereby increasing the recognition accuracy in a non-intrusive energy management (NIEM) system. Analysis results reveal that an NIEM system and a new method that is based on genetic algorithms (GA) can effectively manage energy demand in an optimal economic dispatch for cogeneration systems with multiple cogenerators, which generate power for buyers. Furthermore, the global optimum of economic dispatch under typical environmental and operating constraints of cogeneration systems is found using the proposed approach, which is based on genetic algorithms. Moreover, the use of the proposed GA-based method for economic dispatch can substantially reduce computational time, fuel cost, power cost and air pollution.     

A district heating system based on absorption heat exchange with CHP systems

In order to decrease the energy consumption of large-scale district heating systems with cogeneration, a district heating system is presented in this paper based on absorption heat exchange in the cogeneration system named Co-ah cycle, which means that the cogeneration system is based on absorption heat exchange. In substations of the heating system, the temperature of return water of primary heat network is reduced to about 25°C through the absorption heat-exchange units. In the thermal station of the cogeneration plant, return water is heated orderly by the exhaust steam in the condenser, the absorption heat pumps, and the peak load heater. Compared with traditional heating systems, this system runs with a greater circuit temperature drop so that the delivery capacity of the heat network increases dramatically. Moreover, by recovering the exhausted heat from the condensers, the capacity of the district heating system and the energy efficiency of the combined heat and power system (CHP system) are highly developed. Therefore, high energy and economic efficiency can be obtained.     

A district heating system based on absorption heat exchange with CHP systems

In order to decrease the energy consumption of large-scale district heating systems with cogeneration, a district heating system is presented in this paper based on absorption heat exchange in the cogeneration system named Co-ah cycle, which means that the cogeneration system is based on absorption heat exchange. In substations of the heating system, the temperature of return water of primary heat network is reduced to about 25°C through the absorption heat-exchange units. In the thermal station of the cogeneration plant, return water is heated orderly by the exhaust steam in the condenser, the absorption heat pumps, and the peak load heater. Compared with traditional heating systems, this system runs with a greater circuit temperature drop so that the delivery capacity of the heat network increases dramatically. Moreover, by recovering the exhausted heat from the condensers, the capacity of the district heating system and the energy efficiency of the combined heat and power system (CHP system) are highly developed. Therefore, high energy and economic efficiency can be obtained.     

Suitable operational strategy for power interchange operation using multiple residential SOFC (solid oxide fuel cell) cogeneration systems

A suitable operational strategy for a power interchange operation using multiple residential solid oxide fuel cell (SOFC) cogeneration systems for saving energy is investigated by an optimization approach based on mixed-integer linear programming. In this power interchange operation, electricity generated by residential SOFC cogeneration systems is shared among households in a housing complex without allowing a reverse power flow to a commercial electric power system in order to increase electric load factors of the system. For an SOFC cogeneration system operated continuously with the minimum output, two types of operational strategies for the power interchange operation are adopted: an operation to meet the total demand for electricity in intended households by the electricity output of SOFC cogeneration systems and an operation to meet the demand for hot water in each household by the hot water output of the SOFC cogeneration system. To clarify a theoretical limit of the energy-saving effects of the two strategies, this study numerically analyzes optimal operation patterns for 20 households on three representative days. The results show that the former operational strategy, which takes advantage of the high electricity generating efficiency of the SOFC, is more suitable for saving energy as compared to the latter strategy.     

新颖的核能燃气轮机总能系统的开拓性研究

基于系统综合和能的梯级利用的思路对核能燃气轮机总能系统开拓研究的进展,包括三方面：（1）提出若干高温气冷堆核能联合循环发电新系统;（2）开拓出两种核能联合循环多联产新系统;（3）探索出几种核电站联合循环技术更新改造新途径等;建立了完整的系统特性模型进行了模拟分析,揭示了它们的热力学特性,概括了各种用途的核电燃气轮机总能系统新系统,新方案的性能与特点。     

Performance improvement of a 70 kWe natural gas combined heat and power (CHP) system

Combined heat and power is the simultaneous production of electricity and heat. CHP plants produce energy in an efficient way. A natural gas CHP system based on an internal combustion engine (ICE) is described, which has been set up at the Building Energy Research Center in Beijing, China. The system is composed of an ICE, a flue gas heat exchanger, a jacket water heat exchanger and other assistant facilities. The ICE generates power on-site, and the exhaust of the ICE is recovered by the flue gas heat exchanger, and the heat of the engine jacket is recovered by the jacket water heat exchanger to district heating system. In order to improve the performance of the system, an absorption heat pump (AHP) is adopted. The exhaust of the ICE drives the AHP to recover the sensible and latent heat step by step, and the temperature of the exhaust could be lowered to below 30 °C. In this paper, the performance of the new system were tested and compared with conventional cogeneration systems. The results show that the new CHP system could increase the heat utilization efficiency 10% compared to conventional systems in winter. All the results could be valuable references for the improvement of the CHP system.     

The evaluation of small cogeneration for residential heating

The decision whether or not to install small cogeneration for residential purposes mainly depends on individual economic considerations, combined with ecological awareness. Since in most cases, the economic balance is still unfavourable, government grants are considered in order to bridge this economic barrier. It is however still unclear how these grants are best spent to obtain an optimal environmental benefit. In the case of cogeneration, mainly static and simplified methods are used, completely neglecting the dynamic interaction between the cogeneration systems and the central power system and the dynamic response of the cogeneration units themselves. In this paper, these issues are discussed in two parts. The first part clarifies how an actual cogeneration unit, if necessary in combination with a back‐up boiler and heat storage, will respond to a certain demand. For this purpose, experiments were performed to establish the transient and stationary behaviour of the system. It is shown that the transient heating of the cogeneration engine is rather slow (e.g. half an hour after cold start, the engine only produced 65% of the heat it would have in stationary regime) where the electric transient behaviour is negligible. In the second part of the paper, dynamic simulations are performed to quantify the impact (primary energy saving and reduction in greenhouse‐gas emissions) of the massive installation of cogeneration for residential heating. Two important parameters are isolated. First, the interaction with the expansion of the central power system is very important. If the installation of cogeneration prevents the commissioning of new power plants, the potential energy saving and (especially) emission reduction are reduced. The second parameter is the annual use of the cogeneration units. Here, the potential energy saving and emission reduction increase with increasing annual use. Copyright © 2002 John Wiley & Sons, Ltd.     

Performance comparison of three solid oxide fuel cell power systems

An energy analysis of three typical solid oxide fuel cell (SOFC) power systems fed by methane is carried out with detailed thermodynamic model. Simple SOFC system, hybrid SOFC‐gas turbine (GT) power system, and SOFC‐GT‐steam turbine (ST) power system are compared. The influences of air ratio and operative pressure on the performance of SOFC power systems are investigated. The net system electric efficiency and cogeneration efficiency of these power systems are given by the calculation model. The results show that internal reforming SOFC power system can achieve an electrical efficiency of more than 49% and a system cogeneration efficiency including waste heat recovery of 77%. For SOFC‐GT system, the electrical efficiency and cogeneration efficiency are 61% and 80%, respectively. Although SOFC‐GT‐ST system is more complicated and has high investment costs, the electrical efficiency of it is close to that of SOFC‐GT system. Copyright © 2013 John Wiley & Sons, Ltd.     

Effects of gas recycle on performance of solid oxide fuel cell power systems

An energy analysis of solid oxide fuel cell (SOFC) power systems with gas recycles fed by natural gas is carried out. Simple SOFC system, SOFC power systems with anode and cathode gas recycle respectively and SOFC power system with both anode and cathode gas recycle are compared. Influences of reforming rate, air ratio and recycle ratio of electrode exhaust gas on performance of SOFC power systems are investigated. Net system electric efficiency and cogeneration efficiency of these power systems are given by a calculation model. Results show that internal reforming SOFC power system can achieve an electrical efficiency of more than 44% and a system cogeneration efficiency including waste heat recovery of 68%. For SOFC power system with anode gas recycle, an electrical efficiency is above 46% and a cogeneration efficiency of 88% is obtained. In the case of cathode gas recycle, an electrical efficiency and a cogeneration efficiency is more than 51% and 78% respectively. Although SOFC system with both anode and cathode gas is more complicated, the electrical efficiency of it is close to 52%.     

Decentralised optimisation of cogeneration in virtual power plants

Within several projects we investigated grid structures and management strategies for active grids with high penetration of renewable energy resources and distributed generation (RES & DG). Those ”smart grids” should be designed and managed by model based methods, which are elaborated within these projects. Cogeneration plants (CHP) can reduce the greenhouse gas emissions by locally producing heat and electricity. The integration of thermal storage devices is suitable to get more flexibility for the cogeneration operation. If several power plants are bound to centrally managed clusters, it is called “virtual power plant”. To operate smart grids optimally, new optimisation and model reduction techniques are necessary to get rid with the complexity.There is a great potential for the optimised management of CHPs, which is not yet used. Due to the fact that electrical and thermal demands do not occur simultaneously, a thermally driven CHP cannot supply electrical peak loads when needed. With the usage of thermal storage systems it is possible to decouple electric and thermal production. We developed an optimisation method based on mixed integer linear programming (MILP) for the management of local heat supply systems with CHPs, heating boilers and thermal storages. The algorithm allows the production of thermal and electric energy with a maximal benefit. In addition to fuel and maintenance costs it is assumed that the produced electricity of the CHP is sold at dynamic prices. This developed optimisation algorithm was used for an existing local heat system with 5 CHP units of the same type. An analysis of the potential showed that about 10% increase in benefit is possible compared to a typical thermally driven CHP system under current German boundary conditions. The quality of the optimisation result depends on an accurate prognosis of the thermal load which is realised with an empiric formula fitted with measured data by a multiple regression method.The key functionality of a virtual power plant is to increase the value of the produced power by clustering different plants. The first step of the optimisation concerns the local operation of the individual power generator, the second step is to calculate the contribution to the virtual power plant. With small extensions the suggested MILP algorithm can be used for an overall EEX (European Energy Exchange) optimised management of clustered CHP systems in form of the virtual power plant. This algorithm has been used to control cogeneration plants within a distribution grid.     

燃气—蒸汽联合循环机组轴系振动问题综述

本文对燃气—蒸汽联合循环机组轴系布置方案进行了详细研究,将其分为普通单轴布置轴系、带有SSS离合器(同步自动离合器)的单轴布置轴系、多轴布置中的燃气轮机轴系和汽轮机轴系4种类型。并且分析了4种类型与常规机组汽轮机轴系的区别,并通过对某联合循环电厂轴系的典型振动故障分析,讲述了此方面研究的重要性。本文从现场振动故障诊断和理论研究两个方面总结了近些年联合循环机组轴系振动的研究进展,提出带有SSS离合器的单轴布置轴系和多轴布置中带有SSS离合器的汽轮机轴系是今后的研究方向。     

双碳目标下燃煤热电联产机组储能技术应用分析

  目的  “碳达峰、碳中和”目标要求现有能源结构进行深刻变革。提高原有燃煤热电联产机组灵活调节能力是保障新能源电力安全并网的重要内容之一。  方法  分别从燃煤热电联产系统灵活调节需求、潜在储能应用现状以及耦合储能技术的发展方向等方面进行了评述。  结果  分析认为深度“热电解耦”仍是提高燃煤热电联产系统的关键内容。其次,为满足“源–荷”匹配性,储能技术将在燃煤热电联产系统中发挥重要作用,其中具有应用潜力的储能技术主要包括储热、蓄电以及飞轮储能。  结论  最后根据燃煤热电联产机组耦合储能技术的应用特点,提出了储能性能老化、新能源消纳、扩容区域热电负荷中长期变更、初投资与回收期的经济性分析四个方面的建议以及需要注意的相关问题。     

Matching economical, energetic and environmental benefits: An analysis for hybrid CHCP-heat pump systems

The optimization of design and operation of combined heat, cooling and power systems usually leads to select different plant lay-outs and size of components, depending on the adopted optimization criterion (maximum profit or energy saving or minimum environmental impact). This occurs when the current energy prices and the normative provisions supporting cogeneration are not able to coincide with the specific customer’s interest and the overall “social interest” for a reduction in energy consumption and in pollutants’ emissions. At EU level, polygeneration is considered to have a large potential for residential and commercial buildings district network, for the tertiary sector and for industrial applications. In such applications, it is often convenient to integrate the trigeneration system with a reversible heat pump, because of a low ratio between electric demand and that for heating and cooling. In this paper, the design and operation of such hybrid systems is discussed. The results achievable through different operation modes are compared and, with reference to a 600-rooms hotel and a 300-beds hospital in Italy, the effects on plant design from an hour-by-hour optimization of plant operation are assessed. Finally, the need for a flexible support system for cogeneration plants is put into evidence and some criteria are listed for an effective regulation.     

Ecological efficiency and thermoeconomic analysis of a cogeneration system at a hospital

This work aims with an approach for cogeneration plants evaluation based on thermoeconomic functional diagram analysis. The second law of thermodynamics is used to develop a methodology to analyse cogeneration systems, based on exergoeconomics evaluation. The thermoeconomic optimisation method developed is applied to allow a better configuration of the cogeneration plant associated to a university hospital. Also ecological efficiency is evaluated. The method was efficient and contributes for thermoeconomics modelling and analysis and can be applied to any sort of thermal system, especially those with combined heat and power in thermal parity.     

Energy optimization in a pulp and paper mill cogeneration facility

Alarmingly low pulp prices in early 2009 left pulp and paper mills across North America desperate for any way to improve thin profit margins. One solution that continues to gain popularity among the industry is improved energy management systems for cogeneration systems, which use steam for two purposes – to provide heat for the pulping process and to generate electricity for sale to regional providers. This paper presents an energy optimization algorithm for use in a pulp and paper mill cogeneration system. The algorithm is applicable to a number of popular mill configurations, power sale contracts, and fuel purchasing scenarios. The method is also extended to address weather-dependent cooling limitations encountered by a mill cogeneration facility, in which case an iterative solution is proposed in order to maintain convexity of the optimization problem. Results are presented in the form of three case studies.     

Novel cogeneration power system with liquefied natural gas (LNG) cryogenic exergy utilization

We propose a new cogeneration power system with two energy sources of fuel chemical energy and liquefied natural gas (LNG) cryogenic energy, and two outputs of electrical power and cooling power. Due to the advanced integration of system and cascade utilization of LNG cryogenic energy, the system has excellent energy saving: chemical energy of fuel and LNG cryogenic energy are saved by 7.5–12.2% and 13.2–14.3%, respectively. As CO₂ is selected as working fluid and oxygen as fuel oxidizer, CO₂ is easily recovered as a liquid with LNG vaporization. In this paper, the typical recuperative Rankine cycle and the corresponding cogeneration system are described and a detailed thermodynamic analysis is carried out to reveal the principle of the cycle and system. Furthermore, the influence of key parameters on performance is discussed. Considering the engineering application, the technical advantages and concerns are pointed out.     

Comments on derivation of an index for evaluating economics of cogeneration systems and its applications

Industrial cogeneration systems usually must satisfy a power load and heat loads at different temperatures. The limitations of the economic index proposed by Pak and Suzuki for such cogeneration systems is discussed in this paper. The importance of a rational exergetic basis for evaluation of different grades of energy is emphasised. Thermodynamic criteria, e.g. the exergetic efficiency, relative fuel savings and fuel chargeable to power, are shown to provide useful information regarding cogeneration options. Any assessment scheme for cogeneration schemes must incorporate thermodynamic criteria in addition to economic criteria.     

串联型焦电联产系统的概念性设计与特性规律

根据能量的综合梯级利用原理,设计了串联型炼焦-发电联产系统.新系统缩小了燃料化学能释放侧与热能接受侧的品位差;尽量减少炼焦过程耗气量,把更多富氢的焦炉煤气提供给联合循环,以实现高效洁净发电;还变革传统的焦炉工艺流程,优化和整合燃气轮机和焦炉的排气系统,使得系统总排烟量和损失都降低.研究表明,新系统具有优良的热力特性,系统相对节能率高达23%.     

Thermodynamic evaluation of hydrogen and electricity cogeneration coupled with very high temperature gas-cooled reactors

Hydrogen production using thermal energy, derived from nuclear reactor, can achieve large-scale hydrogen production and solve various energy problems. The concept of hydrogen and electricity cogeneration can realize the cascade and efficient utilization of high-temperature heat derive for very high temperature gas-cooled reactors (VHTRs). High-quality heat is used for the high-temperature processes of hydrogen production, and low-quality heat is used for the low-temperature processes of hydrogen production and power generation. In this study, two hydrogen and electricity cogeneration schemes (S1 and S2), based on the iodine-sulfur process, were proposed for a VHTR with the reactor outlet temperature of 950 °C. The thermodynamic analysis model was established for the hydrogen and electricity cogeneration. The energy and exergy analysis were conducted on two cogeneration systems. The energy analysis can reflect the overall performance of the systems, and the exergy analysis can reveal the weak parts of the systems. The analysis results show that the overall hydrogen and electricity efficiency of S1 is higher than that of S2, which are 43.6% and 39.2% at the hydrogen production rate of 100 mol/s, respectively. The steam generators is the components with the highest exergy loss coefficient, which are the key components for improving the system performance. This study presents a theoretical foundation for the subsequent optimization of hydrogen and electricity cogeneration coupled with VHTRs.     

Applying a non-intrusive energy-management system to economic dispatch for a cogeneration system and power utility

Non-intrusive energy-management (NIEM) techniques are based on energy signatures. While such approaches lack transient energy signatures, the reliability and accuracy of recognition results cannot be determined. By using neural networks (NNs) in combination with turn-on transient energy analysis, this study attempts to identify load demands and improve recognition accuracy of NIEM results. Case studies are presented that apply various methods to compare training algorithms and classifiers in terms of artificial neural networks (ANN) due to various factors that determine whether a network is being used for pattern recognition. Additionally, in combination with electromagnetic transient program (EMTP) simulations, calculating the turn-on transient energy facilitate load can lead to identification and a significant improvement in the accuracy of NIEM results. Analysis results indicate that an NIEM system can effectively manage energy demands within economic dispatch for a cogeneration system and power utility. Additionally, a new method based on genetic algorithms (GAs) is used to develop a novel operational strategy of economic dispatch for a cogeneration system in a regulated market and approach the global optimum with typical environmental constraints for a cogeneration plant. Economic dispatch results indicate that the NIEM system based on energy demands can estimate accurately the energy contribution from the cogeneration system and power utility, and further reduce air pollution. Moreover, applying the NIEM system for economic dispatch can markedly reduce computational time and power costs.