Combined Heat And Power System Research Articles

Comprehensive analysis and optimization for a novel combined heating and power system based on self-condensing transcritical CO2 Rankine cycle driven by geothermal energy from thermodynamic, exergoeconomic and exergoenvironmental aspects

In this paper, a novel combined heating and power (CHP) system is proposed to realize full-scale utilization of geothermal energy and efficient multi-generation, which not only performs preferable overall performance than previous homogeneous system, but also offers an effective energy cascade utilization approach for self-condensing transcritical CO2 (TCO2) Rankine cycle. Based on the established mathematical models, the performance comparison is conducted for proving the superiority of the novel CHP system. Then, an overall performance analysis is implemented to reveal the combined effects for six key parameters on system thermodynamic, exergoeconomic and exergoenvironmental performances. Furthermore, multi-objective optimization considering system overall performance is conducted. The results show that for the novel CHP system, the largest relative improvement rate of system exergy efficiency (ηexg) and declining rate of total unit product exergy cost (cP,total) versus the previous CHP system are 15.03 % and 18.89 %, respectively. The final optimization results of ηexg, cP,total and total unit product exergy environmental impact (bP,total) are determined as 51.10 %, 14.12 $/GJ and 9.00 mPts/GJ, respectively. This paper fulfills an elaborate performance analysis and optimization for the novel CHP system, which fills the research gap of efficient and promising CHP system based on self-condensing TCO2 Rankine cycle.

Impact of heat recovery and thermal load control on combined heat and power (CHP) performance

This research assesses the energy efficiency and techno-economic viability of a Combined Heat and Power (CHP) system designed for a typical building that meets both its electrical (97 kWh/d) and thermal (92 kWh/d) loads. The CHP system comprises wind turbines (WT), photovoltaic panels (PV), batteries, micro gas turbines (MGT), and boilers, which are evaluated for their techno-economic performance. To enhance the system's efficiency and minimize energy wastage in CHP, two strategies, namely heat recovery (HR) from MGT and Thermal Load Control (TLC) for converting surplus renewable power generation into thermal energy, are implemented. Four case studies are conducted to analyze the impact of each strategy on the CHP system's performance. The optimization of hybrid renewable energy systems, considering the overall system's economic performance, is achieved through the integration of MATLAB and HOMER PRO. The study reveals that the incorporation of TLC and HR results in significant reductions in the Cost of Energy (COE), Net Present Cost (NPC), CO2 emissions, loss of Power Supply (LPS), and energy sizing while increasing the renewable fraction in the CHP system. Sensitivity analysis is performed on the capital cost of components, TLC, and HR variations, demonstrating their substantial influence on the economic performance of CHP. The cash flow results show that the integration of these two components reduces the system cost by 25 %. It is also shown that almost 70 % of the CC of the system is related to PV and batteries. The proposed strategies and findings provide valuable insights for enhancing the reliability and techno-economic evaluations of hybrid renewable.

Impact Evaluation of Wastewater Treatment Based on the Anaerobic Digestion of Sewage Sludge Using the Life Cycle Assessment Method

All the inputs and outputs of a technical system can be interpreted from an environmental point of view. Using the life cycle assessment (LCA) approach, some changes that are less harmful to the environment can be included in the system. This research aims to evaluate the environmental effects of the wastewater treatment plant (WWTP) in South Tehran, and the LCA method was used in this study. Based on the data of qualitative parameters obtained from the measurement of Tehran province’s water and sewage company, the environmental emissions were calculated and analyzed using SimaPro software (9.0.0) and the standards defined under the ReCiPe 2016-midpoint method. In the ReCiPe 2016 method, the results were expressed in two intermediate levels (including three classes of influence) and final (including 18). The results showed that the treated wastewater and chlorine factors had the most adverse environmental effects. Among the 18 effect classes, the treated wastewater in the class of marine environmental toxicity with the amount of 101.1531 kg 1,4-DCB had the most environmental impacts among other classes. The power consumed by the biogas-burning combined heat and power (CHP) unit in the wastewater treatment (WWT) process reduced the environmental effects in most impact classes. The most adverse environmental effects of the WWT process are related to damage to human health and the ecosystem. According to the findings, the use of CHP systems is suggested for energy saving and also for reducing harmful effects on the environment.

Thermal and environmental analysis of an infectious medical waste-to-energy

This work presents an infectious medical waste-to-energy (IMWtE) thermal and environmental analysis using combined heat and power (CHP) technology. Steam sterilization can be operated with an infectious medical waste (IMW) of 375 kg/h⋅unit, a maximum per day of 12,000 kg/day for double sterilization units, and a running time of 16 h/day. The CHP system uses a dried IMW of 797 kg/h, generating a power output of 128.98 kWe, providing a drying heat of 382.91 kW, and achieving an overall system efficiency of 8.45 %. Results are obtained for a life cycle assessment (LCA) of the IMWtE by CHP system technology. The endpoint effectiveness comprises considerations of human health: 2.83E+01 DALY, ecosystem quality is represented value of: 9.32E+00 Species⋅y, and natural resource value of: 1.08E+06 USD, all of these are fundamentally linked to the utilization of steel, copper, paint, and gypsum. The LCA impacts are primarily due to the operation phase (93 %), with the smaller contribution of the decommissioning phase (4 %), and the construction phase (3 %), respectively.

Optimal bidding strategy for integrated CHP system by modified heuristic algorithm considering CO2 emission in energy and reserve markets

Purpose This study aims to solve optimal pricing and power bidding strategy problem for integrated combined heat and power (CHP) system by using a modified heuristic optimization algorithm. Design/methodology/approach In electricity markets, generation companies compete according to their bidding parameters; therefore, optimal pricing and bidding strategy are solved. Recently, CHP units are significantly operated by generation companies to meet power and heat, simultaneously. Findings For validation, it is shown that profit is improved by 0.04%–48.02% for single and 0.02%–31.30% for double-sided auctions. As heat price curve is extracted, the simulation results show that when CHP system is integrated with other units results in profit increase and emission decrease by 3.04%–3.18% and 2.23%–4.13%, respectively. Also, CHP units significantly affect bidding parameters. Originality/value The novelties are pricing and bidding strategy of integrated CHP system is solved; local heat selling is considered in pricing and bidding strategy problem and heat price curve is extracted; the effects of CHP utilization on bidding parameters are investigated; a modified heuristic and deterministic optimization algorithm is presented.

Design and optimization of a combined heat and power system with a fluidized-bed combustor and stirling engine

In this study, a fluidized-bed biofuel system was integrated with a Stirling engine (SE) to create a combined heat and power (CHP) system with high energy efficiency. Methods for ensuring the stability of the SE and fluidized-bed combustion were investigated and implemented. The proposed design was tested experimentally, and the system successfully produced electricity from the heat of the flue gas generated during biomass combustion. The Taguchi method was employed to maximize the temperature of the fluidized bed by modifying three parameters: the sand height, air flow rate, and biomass feed rate. The biomass used in this study was discarded mushroom sawdust waste. The biomass feed rate was varied from 9.5 to 17 g/min, and the air flow rate was varied from 50 to 60 L/min. The integrated CHP system was found to yield 90−100 W of electric power and 1077.3 W of heat energy for producing hot water. The oxygen, carbon dioxide, carbon monoxide, and nitric oxide concentrations of the flue gas produced under the optimized system parameters were analyzed, and minimal emissions of carbon monoxide and nitric oxides, which are harmful gases, were discovered. In most experiments on SEs in the literature, liquid or gaseous fuels have been used to power the engine. However, this study successfully used solid biomass fuel to power a practical SE, thus widening the knowledge on the utilization of this renewable energy source for electricity generation. The results of this study indicate that the proposed CHP system is stable, clean, and efficient. This system potentially provides a solution for two problems, namely the disposal of mushroom sawdust waste and the eco-friendly generation of heat and electricity. Therefore, the proposed system is promising for green and sustainable power generation in the future.

Real-time modeling and optimization of molten salt storage with supercritical steam cycle for sustainable power generation and grid support

This research article presents an innovative approach to enhance sustainable power generation and grid support by integrating real-time modeling and optimization with Molten Salt Energy Storage (MSES) and a Supercritical Steam Cycle (s-SC). As renewable energy usage grows, intermittent resource availability challenges grid stability and reliable power supply. To address this, we develop a system that merges real-time modeling and optimization for precise control of MSES and s-SC components. This integration ensures uninterrupted energy generation, storage, and distribution, optimizing renewable energy use during high-demand periods. Mathematical models and simulations assess the system's dynamic behavior, performance, and economic viability. Rigorous techno-economic analysis highlights cost-effectiveness and environmental benefits. Findings reveal exceptional energy efficiency and grid support, making it a promising solution for sustainable power generation and grid stability amid renewable energy growth. Real-time modeling and optimization emerge as crucial components in modern energy systems. The Combined Heat and Power (CHP) system achieves 56% energy efficiency with off-design impacts considered and 63.61% without. Also, the overall system exergy efficiency decreased from 73.36% at design to approximately 63.55% under off-design scenarios. Regarding the economic aspect, the levelized cost of storage (LCOS) for the CHP system is estimated at 114.4 €/MWh with off-design conditions and 106.8 €/MWh without.

Thermal economy simulation study for a carbon capture power plant with combined heat and power based on absorption heat pump technology

The Combined Heat and Power (CHP) system represents an energy-efficient and environmentally-friendly approach to energy utilization. However, in the context of the “carbon peaking and carbon neutrality” strategy, ensuring low-carbon operation of CHP systems becomes imperative. In this study, based on absorption heat pump (AHP), an extraction condensing carbon capture (EC&CC) CHP coupled system and a low-pressure cylinder zero-output carbon capture (LCZ&CC) CHP coupled system are proposed, respectively. Furthermore, under the 100 % turbine heat acceptance (THA) operating condition, a variable operating condition study of heating load and carbon capture capacity is conducted. The results demonstrate that under identical heating load, the EC&CC CHP coupled system has superior thermal economy, with an average reduction of 1011.03 kJ/kWh in heat consumption of power generation and 34.54 g/kWh in coal consumption of power generation compared with the LCZ&CC CHP coupled system; For equivalent carbon capture capacity, the LCZ&CC CHP coupled system has superior thermal economy, with an average reduction of 771.28 kJ/kWh in heat consumption and 26.35 g/kWh in coal consumption of power generation compared with the EC&CC CHP coupled system. The EC&CC CHP coupled system has lower CO2 capture heat consumption, resulting in an average reduction of 135.67 t/h in carbon capture extraction compared with the LCZ&CC CHP coupled system. This study can provide guidance for optimizing the design of carbon capture CHP coupled systems.

Using response surface methodology for multi-objective optimization of an efficient/clean combined heating/power system based on sugarcane bagasse gasification for environmental sustainability

Developing CHP (combined heat and power) systems-based biomass gasification is crucial for achieving environmental sustainability. These systems offer numerous profits, counting abridged greenhouse gas emissions, efficient energy utilization, and sustainable waste management. By investing in and promoting the adoption of biomass gasification-based CHP systems, it can be moved towards a more sustainable and greener energy future. In this regard, a clean CHP system is integrated by comprising a sugarcane bagasse gasifier, a proton conducting solid oxide fuel cell, a supercritical carbon dioxide Brayton cycle, and a heating recovery unit. The system performance is evaluated with respect to input variables from energy and environmental viewpoints. The integrated system performs more efficiently at lower equivalence ratios, higher temperatures, lower utilization factors, and higher pressure ratios. The efficiency initially improves with an increase in current density from 1400 A/m2 to 3000 A/m2, but starts to decline when the current density is pushed beyond this threshold to 5400 A/m2. According to the results, it has been concluded that the most favorable conditions for attaining the optimum goals are an equivalence ratio of 0.4, current density of 5400 A/m2, temperature of 1200 K, utilization factor of 0.7, and pressure ratio of 3.7. The best performance yields a power generation of 344.6 kW, heating generation of 3422.9 g/s, efficiency of 28.60%, and emissions of 1281 g/kWh.

Corn ethanol production with thin stillage anaerobic digestion for bioenergy recovery: A technical and economic evaluation

The generation of bioenergy from thin stillage in a corn ethanol mill provides a new perspective for the use of by-products in a circular economy concept. Alternative technological routes for the corn ethanol by-products energy and nutrients recovery must be investigated to assess present and future decision-making toward economic and environmental indicators. The theoretical biogas production from an up-flow anaerobic sludge blanket (UASB) reactor in a corn-ethanol mill and the potential thermal and electrical power generation along with the avoided greenhouse gas (GHG) emissions were estimated for a large-scale industrial facility. This work aimed to assess the anaerobic digestion (AD) contribution to techno-economic indicators of a corn ethanol mill in three scenarios: 1) Standard corn-to-ethanol mill; 2) AD-CHP - thin stillage AD to produce biogas and its burning in a combined heat and power (CHP) system; 3) AD-BU - biogas purification to obtain and commercialize biomethane. The existent Brazilian market and fuels and biofuel policies place the system without AD in a slightly better position when analyzing financial indicators related to investment return. In contrast, the global energy and mass balances considering thin stillage AD showed that in the AD-CHP scenario, processing 1 ton of thin stillage would produce 10.37 m³ of biogas, which could generate 119.9 MJ of heat and 24.7 kWh of electricity in a CHP system, favoring a greener energy scenario. The energy recovered from biogas returns to the industry, supplying 7.15% of the heat required for steam production and 52.74% of the electricity requirement for industrial processing. As a renewable energy source, biogas would replace fossil sources for energy generation avoiding GHG emissions. Extrapolating the results of the present work to the Brazilian national scale, the biogas yield from the corn-to-ethanol industry would be 179.68 million m³ , the thermal energy generation from biogas could mitigate 125,574.42 ton CO2eq, and the electricity replacement could mitigate 58,193.02 ton CO2eq. Besides the mentioned energy generation and environmental benefits, the financial indicators for the examined scenarios showed that the Brazilian market and public policies related to biogas/biomethane are constraints to overcoming to larger adoption of AD by the corn-to-ethanol industry.

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%.

Thermodynamic analysis of a novel combined heating and power system based on low temperature solid oxide fuel cell (LT-SOFC) and high temperature proton exchange membrane fuel cell (HT-PEMFC)

Low-temperature solid oxide fuel cell (LT-SOFC) and high temperature proton exchange membrane fuel cell (HT-PEMFC) have attracted significant attention due to outstanding advantages over traditional fuel cells. To increase the energy utilization ratio of LT-SOFC and solve the fuel supply problem of HT-PEMFC, a new combined heating and power (CHP) system mainly composed of a LT-SOFC unit, gas processing unit, HT-PEMFC unit and waste heat recovery (WHR) unit is proposed. For the CHP system, LT-SOFC can not only be used as power generation system, but also as fuel source of HT-PEMFC and heat source of WHR unit. Moreover, the influences of the recirculation percentage, fuel utilization and operating temperature of LT-SOFC on the system performance are revealed. Under typical conditions, the net electric power and total output power of the CHP system are 1002.7 kW and 1388.6 kW, and the net electric efficiency and the total energy efficiency are 62.5% and 86.5%, respectively. The superior performance of CHP system compared with other relevant systems proves its feasibility, and the analysis results also provide theoretic foundation for LT-SOFC based cogeneration systems.

Comparison of the carbon footprints of combined heat and power (CHP) systems and boiler/grid for supplying process plant steam and electricity

This paper focusses on comparing carbon footprints of supplying process plant steam and electricity using boiler/grid, gas turbine and steam turbine CHP systems, and grid heat & power (grid-HP), and investigating the effect of key parameters on the carbon footprints. These parameters include fuel type, grid carbon intensity, plant heat to electric power ratio, plant steam pressure, and operational parameters particular to the CHP systems. The analysis highlights the critical importance of the grid carbon intensity. There exist grid carbon intensity transition values, whereby below a certain value the grid-HP provides the lowest footprint, and above a certain value one of the CHP systems provides the lowest footprint. These transition values are independent of the plant heat to electricity ratio. However, they are influenced by other factors outlined in the paper. Consequently, many fossil fuel boiler/grid and CHP systems may become obsolete due to decreasing grid carbon intensity.

A constraint equivalent model of heat network for combined heat and power systems cooperation considering uncertainties

The cooperation of combined heat and power (CHP) systems can improve efficiency in energy utilization by fully utilizing the complementarity between heat and power. However, heat and power systems are operated by different companies where data sharing is restricted for privacy reasons, thus posing significant obstacles to collaboration. This paper proposes a coordinated operation method for CHP systems based on the constraint equivalent model. To preserve the operational constraints of heat networks and protect data privacy, a boundary point search method is firstly developed, where the operational constraints of heat networks are mapped to the feasible region of CHP units. Then, a multi-period cooperation model considering uncertainties of both networks is established based on the proposed constraint equivalent model and the rolling horizon strategy. Moreover, the impact of heat storage tanks and uncertainties of heat loads on the boundaries of the feasible region for heat networks is further analyzed. Simulation results demonstrate the effectiveness of the proposed constraint equivalent model and the cooperation method.

Regional impacts on air quality and health of changing a manufacturing facility’s grid-boiler to a combined heat and power system

ABSTRACT Poor air quality is linked to numerous adverse health effects including strokes, heart attacks, and premature death. Improving energy efficiency in the industrial sector reduces air emissions and yields health benefits. One of these strategies, replacing an existing grid boiler (GB) with a combined heat and power (CHP) system, can improve a facility’s energy efficiency but can also increase local air emissions, which in turn can affect health outcomes. Previous studies have considered air-emissions and health outcomes of CHP system installation at a single location, but few studies have investigated the regional air quality and health impacts of replacing an existing GB with new CHP system. This study estimates the emission changes and associated health impacts of this shift in 14 regions in the US, representing different electricity generation profiles. It assumes that one manufacturing facility in each region switches from an existing GB to a CHP system. The monetized annual US health benefits of shifting a single GB to a CHP in each of the 14 regions range from $-5.3 to 0.55 million (2022 USD), while including CHP emission control increases the benefits by 100–170% ($9,000 to 1.15 million (2022 USD)). This study also includes a sensitivity analysis, which suggests that the facility location (region, state, and county), boiler efficiency, and emission control of the CHP are key factors that would determine whether shifting from a GB to CHP system would result in health benefits or burdens. Implications: Combined heat and power (CHP) systems offer industrial facilities the opportunity to improve their energy efficiency and reduce greenhouse gas emissions. However, CHP systems also combust more fuel on site and can also increase local air emissions. This study evaluates how converting an existing grid boiler (GB) system to a CHP system (with or without emission control) affects local (from combustion) and regional emissions (from electricity consumption) and the associated health burdens in different US regions. A facility can use this study’s analysis as an example for estimating the tradeoffs between local emission changes, regional emission changes, and health effects. It also provides a comparison between the incremental cost of adding SCR (compared to uncontrolled CHPs) and the NPV of the monetized health benefits associated with adding the SCR.

Optimization of a CHP system using a forecasting dispatch and teaching-learning-based optimization algorithm

Using optimization algorithms and developing dispatch strategies are essential in sizing renewable energy systems to ensure optimal performance, cost-effectiveness, and sustainability. This study employs the Teaching-Learning-based Optimization (TLBO) algorithm to determine the optimal size of a Combined Heat and Power (CHP) system. The optimization results are validated using the Genetic Algorithm (GA) and Particle Swarm Optimization (PSO). Furthermore, a novel dispatch strategy is developed to make an informed decision when using different energy sources. The strategy considers a 24-h foresight of upcoming electrical demand, solar irradiation, temperature, and wind speed. The developed dispatch strategy has led to a reduction in cost and excess electricity compared to the pre-prepared strategies. The energy sources employed include Photovoltaic panels (PV), Wind Turbines (WT), Diesel Generators (DG) with heat recovery capability, battery banks, and boilers to supply electrical and thermal demand. A Levelized cost of energy (LCOE) of 0.142 $/kWh is obtained for the PV/WT/DG/Battery/Boiler system. Although the three algorithms find almost similar optimal solutions, TLBO exhibits better convergence speed than PSO and GA. A comparison with HOMER software control strategies shows the developed dispatch strategy is 3.4% and 15.5% more efficient than Cycle Charging and Load Following strategies, respectively. Lastly, a comprehensive economic sensitivity analysis is performed to investigate the effect of inflation and discount rates on the size of components and final objective functions.

A sustainability assessment of utilising energy crops for heat and electricity generation in Turkey

The share of bioenergy is expected to increase in the future, necessitating the use of various bio-based feedstock, including energy crops. This study sets out to determine life cycle environmental impacts and costs of heat and electricity generation from two potential energy crops in Turkey – poplar (Populus spp.) and Miscanthus (Mischantus x giganteus). Different bioenergy conversion pathways, comprising direct combustion (heat-only and combined heat and power (CHP)), gasification (CHP) and co-firing with lignite, are evaluated via life cycle assessment (LCA) and life cycle costing (LCC). LCA results show that CHP-combustion using poplar is a better option than the other conversion pathways considered. All bioenergy plants provide great reductions (>80 %) in global warming potential (GWP), fossil fuel depletion and human toxicity (non-cancer) in comparison with heat from natural gas and electricity from the grid. Furthermore, for both feedstocks, CHP systems have lower impacts in seven of 17 categories than the heat from natural gas and electricity from the grid. Overall, the environmental and economic profiles of poplar-based energy plants are better than those of Miscanthus. However, the latter is better for GWP if land use change is considered. The LCC range between $2.2–$11.2/MWhth and $10.6–$28.3/MWhe and all bioenergy plants (except the Miscanthus boiler) would pay back within their lifetime. Compared to the costs of current heat provision in Turkey bioheat is 64 % cheaper, while only direct combustion plants outperform the grid with a 14 % lower cost. If 20 % of fallow and degraded lands are used for Miscanthus, up to 73 Mt./yr (or 14 %) of GHG emissions could be avoided, in addition to saving $4.7 bn (or 37 %) in the fossil-fuel import costs. These findings can be used by energy companies and policy makers to enable a more sustainable transition to a bioeconomy in Turkey.

Investigations into the performance and emissions of a small-scale CHP system using producer gas obtained from gasification of forest residues

The cost of wood, which form traditional feedstocks for biomass gasification, has been sharply increasing in recent years. Hence, low cost alternative feedstocks, such as forest residues, need to be explored to retain the economical edge. Forest residues (FR) are the materials left behind on the forest floor after forest harvesting and management activities. The present study explores the effect of forest residues as feedstock in a downdraft gasifier coupled with a small-scale combined heat and power (CHP) system, consisting of a diesel engine-alternator setup. The findings reveal that the FR mixtures change the flow behaviors of both feedstock and air in the reactor. While the average composition of the producer gas is similar to that of wood chips, a stratified bed with uneven flow leads to an unpredictable nature in the equivalence ratio which is a process controlling parameter. This is also reflected in gas parameters such as the heating value and cold gas efficiency, which in turn leads to unpredictability in the overall performance of the CHP system, evident in its electrical and thermal efficiencies, and exhaust gas components like CO, CO2, and NOx. Diesel substitution rates (DSR) of up to 59% and 46% for partial and full electrical loads respectively were realized. While emissions of CO and CO2 increased with increasing DSR, supplementing diesel with producer gas has benefits in terms of the pollutants NOx and PM, which were found to be generally lower for the FR mixtures. In the case of NOx, reductions of over 65% was achieved.

Thermodynamic and dynamic analysis of a hybrid PEMFC-ORC combined heat and power (CHP) system

To improve the electrical efficiency of proton exchange membrane fuel cell (PEMFC) combined heat and power (CHP) systems, a novel hybrid CHP system combining PEMFC with organic Rankine cycle (ORC) has been proposed in this study. The proposed system recovers waste heat from the PEMFC stack, electrochemical reaction products, and air compressor, and then further generates electricity through the ORC subsystem. The ORC adopts seawater as the coolant to absorb the low-grade heat generated by the novel system. Based on a 60-kW typical PEMFC CHP system in a hydrogen-electric coupling demonstration project, a thermodynamic model of the novel system is established, and the steady-state, dynamic and economic analysis are performed. At rated stack current, the novel system shows an energy efficiency of 75% and an exergy efficiency of 51%. The electrical efficiency and the electricity exergy efficiency are improved by 4.3% and 5.1% respectively compared to the typical system. As the stack current increases, the electrical efficiency of the novel system declines more slowly. Moreover, the temperature sensitivity analysis shows that the lower the seawater temperature, the higher the system exergy efficiency. During 24 h dynamic operation, the novel system has higher electrical efficiency and exergy efficiency, the average response time of the cycle pump is reduced by 27.5%, and 4.56 kg hydrogen is saved compared to the typical system. In the economic analysis, the levelized cost of energy is 0.34 $/kWh for the novel system at rated operating condition, which is slightly better than the typical system. If the cost of PEMFC stack and hydrogen price decrease by 50%, the levelized cost of energy for the novel system will drop by 47.4% and reach 0.179 $/kWh.

An Evaluation Method of Comprehensive Performance of Retrofitted CHP District Heating Systems

There is a big wave in China of retrofitting single-purpose coal-fired systems for district heating into heat-oriented combined heat and power (CHP) systems to save energy. Back-pressure steam turbines (BPSTs) and extraction steam turbines (ESTs) are both common in retrofitted systems, but contrastive analyses of their effects on the systems’ operation performance are lacking. Moreover, comprehensive evaluation methods of the retrofitted systems remain unknown. In this paper, exergy, exergoeconomic, and exergoenvironmental analyses were conducted to evaluate the thermodynamic, economic, and environmental performances of two real CHP systems: system A using a BPST and system B using an EST. Additionally, a new multi-criteria evaluation method based on rank correlation analysis was proposed for the retrofitted system. The results show that system A is better than system B in thermodynamic and environmental aspects but poorer in the economic aspect. Overall, the multi-criteria evaluation result indicates that system A has a better comprehensive performance than system B. Therefore, the BPST has a better effect than the EST on the retrofitted CHP system for district heating in this study. The findings could provide a reference point for retrofitting work in the future.