Optimal Combined Heat And Power Research Articles

Modeling and optimization of a residential PEMFC-based CHP system under different operating modes

Proton exchange membrane fuel cell (PEMFC)-based combined heat and power (CHP) system can improve the system overall efficiency. In this study, with the electrical and thermal demand curves from a survey of 500 households as the input, a residential PEMFC-based CHP system is numerically studied to explore the system operating characteristics under different operating modes, including constant-power output, electrical-led output, and stepped electrical-led output. Effects of structural and operating parameters, such as tank volume, tank temperature, PEMFC temperature, and environmental temperature, are studied. The system performance is evaluated based on CHP efficiency, matching degree, and hydrogen consumption. The results show that while the optimal CHP efficiency is higher under the constant-power output mode, the corresponding matching degree is lower and the hydrogen consumption rate is higher. A new hybrid mode is proposed to combine the advantages of different modes, achieving both higher efficiency and matching degree. Additionally, the CHP system is compared with a conventional energy supply system in terms of carbon emission and operating costs. The critical hydrogen price for a more cost-effective CHP system than the conventional system is calculated. Furthermore, a dataset is utilized to establish the relationship between CHP efficiency and key input parameters, and an artificial neural network (ANN) model is developed to predict CHP efficiency. The ANN model demonstrates a high accuracy of 98.5% and is coupled with the genetic algorithm to optimize the system, leading to the final CHP efficiency of 95.62%.

Technoeconomic and environmental optimization of combined heat and power systems with renewable integration for chemical plants

Chemical industry mainly uses fossil fuels as energy resources and thus causes substantial amount of carbon emissions. Combined heat and power (CHP) systems have become an effective solution for reducing energy consumption and carbon emissions. This paper presents a bi-objective mixed-integer linear programming framework for technoeconomic and environmental optimization of CHP systems for chemical plants, accounting for steam turbine network design and renewable integration. The bi-objective optimization problem is solved to achieve a trade-off between total annual cost (TAC) and carbon emissions. Two typical quad-pressure and dual-pressure chemical plants are employed for demonstration. The results show that the optimal CHP system has a TAC of 21.1 and 18.9 million USD as well as carbon emissions of 50 and 35 kton for quad-pressure and dual-pressure plants. Both systems feature a sharp reduction of 33.3% and 42.5% in emissions and a mild increase of 26.7% and 30.2% in TAC when comparing with TAC minimization. Moreover, we find that gas boiler is mainly responsible for carbon emissions, and replacing it with electric boilers is an promising alternative for further emission mitigation. We observe that photovoltaics capital price has a significant impact on system TAC and emissions, while employing only carbon tax is unable to achieve deep decarbonization. Hence, we recommend combining system design, renewable technology advances, and carbon tax synergistically for chemical industry decarbonization. Our methodology provides an effective and flexible framework for further investigation of decarbonization. • A modeling framework for CHP system design for chemical plants is presented. • Optimal designs of CHP systems for two typical chemical plants are obtained. • A sensitivity analyses are performed to showcases the impacts of system parameters. • Some policy implications are provided for both designers and policymakers.

Impact of heat price regulation on the optimal district heating production mix and its policy implications

The use of district heating systems consisting of a combined heat and power (CHP) and a heat only boiler (HOB) is on the rise in South Korea. In this context, monopolistic district heating providers optimise cogeneration capacity composition to maximise their profits under price regulation. Building upon Joskow and Jones (1983), this study develops a theoretical framework to investigate the role of price regulation on the optimal capacity composition of a district heating system for a regional monopolist. Assuming varying heat demand, wherein the maximum heat load may exceed CHP capacity, the effects of price-cap and rate of return regulations on the optimal mix of the district heating system are examined and compared. The results show that the heat production mix is optimal for price-cap regulation. In contrast, the rate of return regulation hinders the monopolist's incentive to reduce costs. Finally, in case of internalising the environmental benefits of cogeneration, the optimal CHP size in the heat production mix should be increased; consequently, the energy efficiency for producing heat and electricity is likely to improve. This study, therefore, has far-reaching implications for policymakers and practitioners alike in terms of addressing the dual goals of energy efficiency and economic efficiency.

Optimal CHP Planning in Integrated Energy Systems Considering Network Charges

This paper proposes a novel optimal planning model for combined heat and power (CHP) in multiple energy systems of natural gas and electricity to benefit both networks by deferring investment for network owners and reducing use-of-system (UoS) charge for network users. The new planning model considers the technical constraints of both electricity and natural gas systems. A two-stage planning approach is proposed to determine the optimal site and size of CHPs. In the first stage, a long-run incremental cost matrix is designed to reflect CHP locational impact on both natural gas and electricity network investment, used as a criterion to choose the optimal location. In the second stage, CHP size is determined by solving an integrated optimal model with the objective to minimize total incremental network investment costs. The proposed method is resolved by the interior-point method and implemented on a practically integrated electricity and natural gas systems. Two case studies are conducted to test the performance for single and multiple CHPs cases. This paper enables cost-efficient CHP planning to benefit integrated natural gas and electricity networks and network users in terms of reduced network investment cost and consequently reduced UoS charges.

Optimal decision-making for combined heat and power operations: A fuzzy optimisation approach considering system flexibility, environmental emissions, start-up and shutdown costs

Variations in energy demands require combined heat and power (CHP) systems to be more flexible in adjusting its operations. However, high flexibility is often achieved at the expense of other aspects. In other words, increasing flexibility of a CHP system would require large number of equipment to start-up or shutdown. This would lead to potentially high emissions and high start-up/shutdown costs due to sub-optimal fuel choices and poor selection of technologies respectively. In this respect, it would be challenging to determine the optimal CHP operation with high flexibility, low emissions and low start-up/shutdown costs. To address this challenge, operational optimisation must be considered. In this paper, a multi-objective fuzzy optimisation model was developed to optimise system flexibility, fuel emissions, equipment start-up and shutdown costs for a CHP system operation to meet varying energy production requirements. Essentially, fuzzy optimisation is used to maximise the degree of satisfaction among the three mentioned objectives. To demonstrate the proposed approach, a multi-fuel CHP case study consisting of several scenarios was solved. Results from the case study show that a trade-off CHP operation was determined for increased production. The trade-off operation had the degree of satisfaction of 35 % for each objective. The obtained degree of satisfaction indicates that a balance between flexibility, emissions and costs was achieved. In addition, chosen technologies and fuels were analysed and classified under categories such as “must have”, “optional” and “must avoid”.

Economic output-maximizing share of combined heat and power generation: The case of South Korea

The combined heat and power (CHP) generation, which can produce both heat and electricity at the same time, is so efficient that it can reduce energy use and even mitigate CO2 and air pollutants emissions. For this reason, the share of CHP generation worldwide is increasing and the optimal share of CHP generation in electric power generation is a serious concern in South Korea. This article attempts to estimate the optimal share by modeling the plausible relationship between CHP generation and national output in the Cobb-Douglas production function setting and then deriving the output-maximizing share of CHP generation. The production function is estimated for annual data from 1995 to 2017 using several quantile regressions. The optimal share is computed to be 13.8%. Therefore, it is recommended that South Korea should increase the share of CHP generation slightly, given that the share was 8.6% in 2017. If the optimal CHP share instead of the actual CHP share had been achieved in 2017, GDP in 2017 could have increased by 0.82%.

Optimal configuration for a low-temperature geothermal CHP plant based on thermoeconomic optimization

In this paper, we propose a thermoeconomic optimization procedure for four configurations of low-temperature geothermal combined heat-and-power (CHP) plants. The series, parallel, preheat-parallel and HB4 configurations are investigated. Electricity is produced via an organic Rankine cycle (ORC) and two types of district heating (DH) systems are considered for the heat delivery: a 90/60 and a 65/40 DH system. The objective is to maximize the net present value (NPV) of the CHP plant for given DH system requirements. We conclude that, under the assumptions considered, the NPV can be increased from −2.81MEUR (unprofitable) for a stand-alone electrical power plant to 12.5, 28 and 58MEUR (economically feasible) for the optimal CHP, and for a heat demand of 5, 10 and 20 MW th, respectively. Also, the exergetic plant efficiency is higher for the CHP plants, which means that the geothermal energy source can be utilized in a better way. Furthermore, we have found that the series CHP is generally the optimal CHP configuration. Only for the 90/60 DH system and low heat demands, the HB4 is more suitable. Compared to the simple series and parallel CHPs, the HB4 configuration has a 16% and 5.5% higher NPV, for heat demands of 5 and 10 MW th, respectively.

Optimal combined heat-and-power plant for a low-temperature geothermal source

This work compares the performance of four combined heat-and-power (CHP) configurations for application in a binary geothermal plant connected to a low-temperature 65/40 and a high-temperature 90/60 district heating system. The investigated configurations are the series, the parallel, the preheat-parallel and the HB4 configurations. The geothermal source conditions have been defined based on existing geothermal plants in the northwest of Europe. Production temperatures in the range of 110–150 °C and mass flow rates in the range of 100–200 kg/s are considered. The goal is to identify the best-performing CHP configuration for every set of geothermal source conditions (temperature and flow rate) and for multiple values of the heat demand. The electrical power output is used as the optimization objective and the different CHP plants are compared based on the exergetic plant efficiency. The optimal CHP plant has always a higher exergetic plant efficiency than the pure electrical power plant; up to 22.8%-pts higher for the connection to a 65/40 DH system and up to 20.9%-pts higher for the connection to a 90/60 DH system. The highest increase of the exergetic plant efficiency over the pure electrical power plant is obtained for low values of the geothermal source temperature and flow rate.

Design of Biomass Combined Heat and Power (CHP) Systems based on Economic Risk using Minimax Regret Criterion

It is a great challenge to identify optimum technologies for CHP systems that utilise biomass and convert it into heat and power. In this respect, industry decision makers are lacking in confidence to invest in biomass CHP due to economic risk from varying energy demand. This research work presents a linear programming systematic framework to design biomass CHP system based on potential loss of profit due to varying energy demand. Minimax Regret Criterion (MRC) approach was used to assess maximum regret between selections of the given biomass CHP design based on energy demand. Based on this, the model determined an optimal biomass CHP design with minimum regret in economic opportunity. As Feed-in Tariff (FiT) rates affects the revenue of the CHP plant, sensitivity analysis was then performed on FiT rates on the selection of biomass CHP design. Besides, design analysis on the trend of the optimum design selected by model was conducted. To demonstrate the proposed framework in this research, a case study was solved using the proposed approach. The case study focused on designing a biomass CHP system for a palm oil mill (POM) due to large energy potential of oil palm biomass in Malaysia.

Performance of working-fluid mixtures in ORC-CHP systems for different heat-demand segments and heat-recovery temperature levels

In this paper, we investigate the adoption of working-fluid mixtures in ORC systems operating in combined heat and power (CHP) mode, with a power output provided by the expanding working fluid in the ORC turbine and a thermal energy output provided by the cooling water exiting (as a hot-water supply) the ORC condenser. We present a methodology for selecting optimal working-fluids in ORC systems with optimal CHP heat-to-electricity ratio and heat-supply temperature settings to match the seasonal variation in heat demand (temperature and intermittency of the load) of different end-users. A number of representative industrial waste-heat sources are considered by varying the ORC heat-source temperature over the range 150–330°C. It is found that, a higher hot-water outlet temperature increases the exergy of the heat-sink stream but decreases the power output of the expander. Conversely, a low outlet temperature (~30°C) allows for a high power-output, but a low cooling-stream exergy and hence a low potential to heat buildings or to cover other industrial thermal-energy demands. The results demonstrate that the optimal ORC shaft-power outputs vary considerably, from 9MW up to 26MW, while up to 10MW of heating exergy is provided, with fuel savings in excess of 10%. It also emerges that single-component working fluids such as n-pentane appear to be optimal for fulfilling low-temperature heat demands, while working-fluid mixtures become optimal at higher heat-demand temperatures. In particular, the working-fluid mixture of 70% n-octane+30% n-pentane results in an ORC-CHP system with the highest ORC exergy efficiency of 63% when utilizing 330°C waste heat and delivering 90°C hot water. The results of this research indicate that, when optimizing the global performance of ORC-CHP systems fed by industrial waste-heat sources, the temperature and load pattern of the cogenerated heat demand are crucial factors affecting the selection of the working fluid.

Heuristic approach for the economic optimisation of combined heat and power (CHP) plants: Operating strategy, heat storage and power

CHP plants produce power and heat in parallel and operate between power and heat consumers as well as prices for their supply. The profitability of a plant with given power depends on operating strategy and heat storage size. In this paper, we present a fast heuristic algorithm to determine the most profitable setting for given boundary conditions, as a substitute for the state-of-the-art approach based on mixed-integer linear programming (MILP). The presented heuristic algorithm is appropriate for thousands of plant calculations or real-time operation planning of single as well as interconnected plants. We used hourly resolved full-year data reflecting electricity market price and heat demand, which was simulated using a typical multi-family house configuration. The heuristic algorithm found the optimal operating strategy and heat storage size 34 times faster than the MILP solver. Additional optimisation runs of the operating pattern at various fixed heat storage sizes, resulted in equal solutions of both approaches. The heuristic algorithm found in this case the operating patterns around 200 times faster. Furthermore, the heuristic algorithm can be enhanced to find optimal CHP power, heat storage size and operating strategy in one process using characteristic curves for plant efficiencies and costs.

Examination of the optimal operation of building scale combined heat and power systems under disparate climate and GHG emissions rates

This work aims to elucidate notions concerning the ideal operation and greenhouse gas (GHG) emissions benefits of combined heat and power (CHP) systems by investigating how various metrics change as a function of the GHG emissions from the underlying electricity source, building use type and climate. Additionally, a new term entitled “CHP Attributable” reductions is introduced to quantify the benefits from the simultaneous use of thermal and electric energy, removing benefits achieved solely from fuel switching and generating electricity more efficiently.The GHG emission benefits from implementing internal combustion engine, microturbines, and phosphoric acid (PA) fuel cell based CHP systems were evaluated through an optimization approach considering energy demands of prototypical hospital, office, and residential buildings in varied climates. To explore the effect of electric GHG emissions rates, the ideal operation of the CHP systems was evaluated under three scenarios: “High” GHG emissions rates, “Low” GHG emissions rates, and “Current” GHG emissions rate for a specific location.The analysis finds that PA fuel cells achieve the highest GHG emission reductions in most cases considered, though there are exceptions. Common heuristics, such as electric load following and thermal load following, are the optimal operating strategy under specific conditions. The optimal CHP capacity and operating hours both vary as a function of building type, climate and GHG emissions rates from grid electricity. GHG emissions reductions can be as high as 49% considering a PA fuel cell for a prototypical hospital in Boulder, Colorado however, the “CHP attributable reductions are less than 10%.

Defining a technical criterion for economic justification of employing CHP technology in city gate stations

In order to prevent hydrate forming in city gate stations (CGS), natural gas is preheated before pressure reduction. In this work, employing combined heat and power (CHP) technology in CGSs, by which the required heat is provided and electricity is freely produced, is comprehensively studied to answer this main question that which CGS is an appropriate host for CHP technology. In this respects, a comprehensive thermo-economic analysis was accomplished on several CGSs of Iran to firstly, find the most optimal CHP system size and design for each station and secondly, define a general criterion by which one could realize that if a CGS can efficiently employ CHP technology. Net present value (NPV) approach is used to analyze and compare the performance of these stations economically. The results showed that only one of the case studies can hire CHP system efficiently. It was conceived that utilizing CHP system is more efficient if it works at higher power ranges. Therefore, considering the effective parameters on heating demand of CGSs, the novel criterion of RHD was introduced as the target index of this work. Finally, the results demonstrated that CGSs with RHD values greater than 0.23 are suitable for using CHP systems.

Small scale biomass CHP: Techno-economic performance of steam vs gas turbines with bottoming ORC

Small scale Combined Heat and Power (CHP) plants present lower electric efficiency in comparison to large scale ones, and this is particularly valid when biomass fuels are used. In most cases, the use of both heat and electricity to serve on site energy demand is a key to achieve acceptable global energy efficiency and investment profitability. However, the heat demand follows a typical daily and seasonal pattern and it is influenced by climatic conditions. During low heat demand periods, a lot of heat produced by the CHP plant is discharged. In order to increase the electric conversion efficiency of small scale biomass CHP plants, a bottoming ORC system can be coupled to the cycle, however this decreases the temperature and quantity of cogenerated heat available to the load. In this perspective, the paper proposes a thermo-economic analysis of small scale CHP plants based on steam turbine (ST) or externally fired micro gas turbine (EFGT) coupled to different typologies of bottoming Organic Rankine Cycles (ORC). The research assesses the influence of the thermal energy demand and CHP plant operational strategies on the global energy efficiency and profitability of the proposed cogeneration options, taking into account the part load efficiency and the heat to electricity ratio flexibility that could be achieved through a switch on-off of the bottoming ORC. The thermodynamic cycles and their part load efficiency are modeled by Gate-Cycle (Brayton cycles) and Cycle-Tempo (Rankine cycles). The research explores the profitability of bottoming ORC in view of the higher efficiency and electricity generation revenues but higher costs and reduced heat available for cogeneration in the case of bottoming ORC. The results indicate the optimal CHP technology and configuration for each energy demand segment and the relative key technical and economic factors in the Italian legislative framework.

Life Cycle Assessment of Biomass-based Combined Heat and Power Plants

Norway, like many countries, has realized the need to extensively plan its renewable energy future sooner rather than later. Combined heat and power (CHP) through gasification of forest residues is one technology that is expected to aid Norway in achieving a desired doubling of bioenergy production by 2020. To assess the environmental impacts to determine the most suitable CHP size, we performed a unit process-based attributional life cycle assessment (LCA), in which we compared three scales of CHP over ten environmental impact categories—micro (0.1 megawatts electricity [MWe]), small (1 MWe), and medium (50 MWe) scale. The functional units used were 1 megajoule (MJ) of electricity and 1 MJ of district heating delivered to the end user (two functional units), and therefore, the environmental impacts from distribution of electricity and hot water to the consumer were also considered. This study focuses on a regional perspective situated in middle-Norway's Nord- and Sør-Trøndelag counties. Overall, the unit-based environmental impacts between the scales of CHP were quite mixed and within the same magnitude. The results indicated that energy distribution from CHP plant to end user creates from less than 1% to nearly 90% of the total system impacts, depending on impact category and energy product. Also, an optimal small-scale CHP plant may be the best environmental option. The CHP systems had a global warming potential ranging from 2.4 to 2.8 grams of carbon dioxide equivalent per megajoule of thermal (g CO2-eq/MJth) district heating and from 8.8 to 10.5 grams carbon dioxide equivalent per megajoule of electricity (g CO2-eq/MJel) to the end user. Supporting Information S1: This supplement contains further details on life cycle assessment (LCA) methodology and the assumptions used for the case system, along with tables of the foreground life cycle inventories, contribution analysis for each product (1 megajoule [MJ] electricity and 1 MJ district heating to end user) for each impact category, and total environmental impacts at the combined heat and power plant (1 MJ useful energy at the plant). Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Gas turbines in district heating combined heat and power systems: influence of performance on heating costs and emissions

Much work is currently focussed on identifying economically and environmentally optimal strategies for increasing gas turbine based combined heat and power (CHP). In many such studies, only a few fixed parameters are used to describe the CHP plant. These are typically total and electrical efficiencies, investment and running costs, minimum and maximum acceptable size, and minimum acceptable part-load. However, for gas turbine based systems these characteristics are clearly functions of the operating conditions, especially for part-load operation. This study examines the effects of varying performance of the gas turbine on the overall heat production costs and CO 2 emissions of a medium sized community district heating plant. Both single and double-shaft engines are considered in the study. The results show that the assumption of constant efficiencies for all operating conditions leads to an overestimation of the optimal CHP plant size, thereby underestimating the heat production costs and overestimating the CO 2 emissions of the plant. The results also show marked differences according to the type of gas turbine used and part-load operating strategy adopted. In particular, the paper discusses the part-load operating difficulties for CHP plants running gas turbines equipped with low emissions burners.

Combined heat and power (CHP) vs separate heat and power (SHP) generation for primary energy conservation

The weighted net efficiency to be expected in the nineties from base-load systems generating exergy locally and electricity centrally from large coal plants, ranges from 36 to 42% if heat is produced with dispersed heat pumps. In CHP generation from coal, such efficiencies will only be achieved by large plants (several hundred MW) serving nearby heat demand. In all other locations, dispersed generation of base-load low-grade heat by heat pumps using electricity from the grid, or oil, or gas, may thus be the most efficient method for conserving primary energy, leaving optimal electricity or CHP generation to very large power plants using either coal or nuclear fuel.