Cascade Heat Research Articles

Modeling and operational characteristics analysis of a dual-source synergetic heat pump based on air energy and biomass waste heat

Air source heat pumps have gradually become a widely used heating solution worldwide due to their advantages of high efficiency, energy conservation, and environmental protection. However, air source heat pumps also face some challenges in practical applications, such as unstable heating and performance degradation in low-temperature environments. In order to improve these problems, based on the cascade heat technology, a new type of dual-source synergetic heat pump system is designed using air energy and biomass waste heat dual heat sources. In order to study its performance, a simulation model of the heat pump system was first constructed using distributed-parameter method. In order to verify the effectiveness of the model, an 18-kW heat pump experimental unit was constructed. By comparing the three key parameters of user side outlet water temperature, high-temperature compressor power, and system coefficient of performance (COP), the results show that the maximum error does not exceed 11 %; Secondly, in order to improve the stability and operational efficiency of the system, a three-level control strategy was designed based on considering the coordination of different grade heat sources; Finally, based on the above, the operating characteristics of the heat pump system were investigated through testing under different external input conditions. The research results indicate that by utilizing cascade heating and parallel heat exchange technology, the heat pump system can achieve stable heating even under large fluctuations in external working conditions. At the same time, compared with air source heat pumps, the COP can be improved by 69.7 % in low-temperature environments. The system results not only provide reference for the design and operation of new heat pump systems, but also provide practical and feasible solutions for the problem of decreased heat pump performance in low-temperature environments.

Thermodynamic analysis of new configurations of auto-cascade refrigeration cycles integrating the vortex tube

Auto-cascade refrigeration cycles have extensive application prospects for low-temperature refrigeration. To achieve low evaporating temperature, the cascade heat exchanger uses a significant portion of the cooling capacity, resulting in less cooling capacity entering the evaporator. A vortex tube with energy separation capability is integrated into the cycle. Then the refrigerant is further cooled, thereby reducing the cooling capacity of the cascade heat exchanger. Thus, more cooling capacity generated by the cycle is transferred to the evaporator. This paper proposes two novel auto-cascade refrigeration cycles integrating vortex tubes. The performance of novel configurations is evaluated through thermodynamic analysis. The results demonstrate that novel configurations outperform the baseline cycle in terms of energy and exergy efficiency. The coefficient of performance and exergy efficiency of cycles achieved maximum increases of 23.84 % and 11.62 %. The modified exergy analysis method distinguishes the destruction and indicates the direction of optimization. Furthermore, the economic and environmental analysis indicates that the novel cycles achieved a maximum reduction of 6.74 % in cost rate and 12.72 % in environmental impact under given conditions. Generally, the thermodynamic analysis reveal the potential of vortex tubes to enhance the performance of auto-cascade cycles.1

Optimizing working fluids for advancing industrial heating performance of compression-absorption cascade heat pump

Independent cascade hybrid heat pump (ICHHP) can bridge the large temperature gap between low-grade air sources and high-temperature industrial demands. However, the working fluids used in previous studies are just considered sufficient as long as they perform their functional role. They may not always be the most suitable choices in different situations, and the maximum efficiency of ICHHP has not been achieved. Ionic liquids (ILs) as alternative absorbents have shown the potential to enhance ICHHP performance with their unique ability to tailor thermal properties by freely combining anions and cations. However, the scarcity of IL thermodynamic properties data, underscored by limited vapor-liquid equilibrium experiments, has impeded the full exploitation of this inherent advantage. Obviously, the lack of dedicated research on screening optimal working fluids limits ICHHP performance. To address the identified limitations, the optimal fluid is recommended in this paper by comprehensively evaluating the performance among 100 fluid combinations under different working conditions. The results indicate that R161 is the best choice for the compression subloop. For the absorption subloop, the performance improvement is more sensitive to the anionic species, with the order of influence generally being [OAC]− > [Br]− > [OMS]− > [TFA]−. Specifically, H2O/[EMIM][OAC]—R161 stands out, with maximum improvements in coefficient of performance (COP) and exergy coefficient of performance of 9.4% and 5.6%, respectively, compared to other candidates. Furthermore, it doubles the COP relative to the reference fluid H2O/LiBr—R134a. Consequently, H2O/[EMIM][OAC]—R161 is the superior working fluid, significantly advancing industrial heating efficiency for ICHHP in large temperature lift conditions.

Effect of plate-fin heat exchanger structural parameters on the performance of a cascade refrigeration system

This study investigates the effect of various structural parameters in plate-fin heat exchanger on the performance of a cascade refrigeration system (CRS). Focusing on five key factors—fin thickness, fin type, fin height, porosity, and element axial length—we analyze their contributions to COP, exergy destruction, and entropy production within the CRS. The results reveal that the fin type barely influences the CRS’ performance, the element axial length has the greatest impact, followed by fin height, porosity, and fin thickness, respectively. The COP of the CRS drops by 26.25 % as the element axial length increases from 124 mm to 136 mm, 23.67 % (23.08 %) as the fin height of perforated (straight) fin increases from 6.1 mm to 6.7 mm, and 10.27 % (10.38 %) as the fin thickness of perforated (straight) fin increases from 0.1 mm to 1.3 mm, and the exergy destruction increases by 42.42 %, 37 % (37 %), and 13 % (10.38 %), respectively. The effect of porosity on the CRS’ performance is opposite to that of other parameters. When the element axial length is 124 mm, the fin height is 6.1 mm, and the fin thickness is 0.1 mm, the COP of the CRS reaches the maximum value (0.689). The conclusion can provide data support for the performance optimization of CRS from the structure of the cascade heat exchanger.

Optimal intermediate temperature of two-stage cascade heat pump with non-azeotropic refrigerants for simultaneous heating and cooling

This paper presents a model for finding the optimal intermediate temperature to achieve the highest coefficient of performance of the standard two-stage cascade heat pump with non-azeotropic refrigerants for simultaneous heating and cooling. The optimal intermediate temperature remains unaffected by the pinch point temperature at the cascade heat exchanger, though this temperature does impact heat pump performance. The optimal intermediate temperature rises with increasing high-side condensing and low-side evaporating temperatures and the value is deviating from the average temperature between the condensing and evaporating temperatures of the high and low-temperature sides of the cascade heat pump, respectively by a correction factor. The factor is found to be dependent on the critical temperature ratio of the working fluids in the low-temperature side and high-temperature side including the condenser temperature of the high-temperature side of the cascade heat pump cycle. A developed model consisting of the average temperature with the correction factor for estimating the optimal intermediate temperatures were examined with a set of non-azeotropic refrigerants with different compositions at different high-side condensing (65–95 °C) and low-side evaporating (5–15 °C) temperatures and pinch point (2–11 °C) temperatures. The results on the optimal intermediate temperatures and the maximum COPs agreed very well with those calculated from the enthalpy method within ±7 % and ±3 % deviations, respectively.

HAPPENING: an efficient cascade heat pump system for multifamily buildings

An innovative hybrid heat pump system is presented as a novel solution for multifamily building heating system retrofitting. This system is very efficient on the one hand thanks to its innovative configuration in cascade, minimum thermal losses and on the other hand, the maximization of local solar energy self-consumption due to decoupling generation and consumption and applying smart control strategies. It is aimed at facilitating retrofitting of existing heating systems in multifamily buildings, enabling an efficient decarbonization. Three variations of the system have been designed, installed, and tested in real buildings across Europe, within the H2020 project “HAPPENING”. The project is now under monitoring phase and the preliminary results are positive, demonstrating the high efficiency of the concept

Vapor absorption, compression and cascade heat pumps for carbon capture plants: Multi-criteria analysis and techno-economic assessment with different working fluids

Post-combustion CO2 capture plants include high costs that are associated with intense energy consumption. Heat pumps can recover heat from the flue gas and other sources in the CO2 emitting plant and upgrade it to reduce the use of fossil-based steam. The few studies regarding heat pumps in CO2 capture systems include arbitrary selection among limited working fluids, without optimizing the underlying cycle. This work implements a multi-criteria investigation of the performance of 20 working fluids for vapor compression heat pumps (VCHP) and 21 refrigerant/absorber pairs for absorption heat transformers (AHT), considering the operating optimization of each cycle for each fluid. A cascade cycle combining the two configurations (AHT-VCHP) is proposed, using the optimum fluids cyclopentane and water/potassium nitrate identified for the VCHP and the AHT. The annual operating costs are higher than the annualized capital expenditures. The proposed fluids outperform the conventional options. The VCHP with cyclopentane covers 58 % (2.68 GJ/t CO2) of the reboiler duty with recovered heat and exhibits the lowest cost of $33.8/tCO2. This is $18.7/tCO2 lower than the cost of the conventional direct contact cooling unit that is used in capture plants. The VCHP remains competitive for up to 57 % increase in the electricity price.

Techno-economic analysis and fuel applicability study of a novel gasifier-SOFC distributed energy system with CO2 capture and freshwater production

This study employs the water-gas-shift membrane reactor and multi-effect distillation with thermal vapor compression technologies to facilitate low-energy carbon capture and large-scale freshwater production in gasifier-solid oxide fuel cell (SOFC) systems. The supercritical CO2 Brayton cycle and double-effect absorption refrigeration cycle are introduced to achieve waste heat cascade recovery. This work expands the fuel selection to 12 biomasses to explore the fuel adaptability and application scenarios of the system, and selects four representative cases to reveal their thermodynamic and economic attributes. The base scheme produces 21,635 ton/year freshwater and 1333 ton/year CO2, and achieves energy and exergy efficiencies of 70.68 % and 30.60 % with total cost rate of 23.21 $/h and levelized cost of products of 43.55 $/GJ. The gasifier and SOFC effort contributes to over 60 % of total irreversibility. Larch wood is preferred in resource-rich regions while wood residue, switch grass and wood chip are also prioritized; rice straw, rice husk, cotton stem and algal biomass are suitable for water-scarce areas. As key parameters change, Case-II consistently excels in power output, efficiencies and economic aspects, but lags in cooling capacity compared with other cases. This work provides a reference for low-carbon transition in the energy sector and large-scale freshwater production.

Submesoscale Processes in the Kuroshio Loop Current: Roles in Energy Cascade and Salt and Heat Transports

AbstractThe Kuroshio Loop Current (KLC) is an important form of Kuroshio intrusion into the northeastern South China Sea (NESCS), which has significant influences on dynamical and biogeochemical processes in the NESCS. Recent studies suggested that the KLC is a hot spot of submesoscale processes (submesoscales) with spatiotemporal scales of O(1–10) km and O(1–10) days, but submesoscales' roles in energy cascade and salt and heat transports remain obscure. Here, we investigate this issue through analyzing outputs from a 1/48° simulation. The kinetic energy exchange rate between submesoscale and larger‐scale processes (KER) is overall positive in the KLC region, which suggests the dominance of forward cascade. The magnitude of KER is comparable with the temporal change rate of larger‐scale kinetic energy in the upper 200 m. We also find that magnitude and direction of KER are closely associated with strain rate and horizontal divergence of background flows, respectively. In addition, the KLC region shows elevated submesoscale salinity and heat diffusivities with magnitudes reaching O(102) m2 s−1. During the KLC period, horizontal mixing by submesoscales can transport 0.90 × 1013 kg salt and 0.71 × 1020 J heat westward into the NESCS interior, which are an order of magnitude larger than those caused by the mesoscale eddy shedding from the KLC. These results suggest that submesoscales play important roles not only in energy cascade but also in salt and heat transports in the KLC region. Therefore, the roles of submesoscales should be taken into account when studying energy, salt, and heat budgets in the NESCS.

An intelligent data-driven investigation and optimization integrated with an eco-friendly thermal design approach for a marine diesel engine to study its waste-to-liquefied hydrogen generation potential

The waste heat generated by vehicles, especially heavy-duty engines, has motivated the current research to propose a novel thermal design for a ship's engine. Hence, the suggested system comprises an organic-flash power plant, a bi-evaporator cooling subsystem, a thermal desalination cycle, a water electrolysis-based hydrogen generation unit, and a Claude unit for liquefying hydrogen. The innovative aspects associated with the proposed design include utilizing a novel cascade heat recovery process aimed at reduced irreversibility for the engine's waste and the integration of bi-evaporator technology and Claude unit to facilitate the process of waste-to-liquefied hydrogen generation for a marine engine for improved handling. In addition, the research includes an intelligent study that comprehensively analyzes and optimizes the proposed system in terms of long-term sustainability and economic feasibility. For this purpose, a data-driven optimization technique is implemented using artificial neural networks in combination with multi-objective grey wolf optimization. The aims of the research encompass the optimization of the sustainability index and unit cost of liquefied hydrogen. The research findings indicate that the vapor generator’s terminal temperature difference exerts the most substantial influence on performance metrics, as evidenced by a mean sensitivity index of 0.368. Also, the mentioned objective functions exhibit values of 1.139 and 7.60 $/kg, respectively. Besides, the optimum exergy destruction rate is 124.3 kW, the total investment cost rate is 4.94 $/year, the specific cost of products is 74.87 /GJ, the payback period is 2.2 years, and the net present value is 7.48 M$.

Synthesis of heat-integrated water networks: Integration of heat pumps

A typical heat-integrated water network (HIWN) synthesis problem aims to minimise freshwater and utility consumption while establishing trade-offs between operating and investment costs by exploring different water and heat integration opportunities. Heat integration between hot and cold water streams is performed to save external heating and cooling utilities required to achieve the target temperatures of these streams. This paper proposes a mathematical programming approach to integrate water-to-water heat pumps into the HIWNs. A simple cascade heat pump system with isobutane as a natural working fluid enables a larger temperature lift between the heat pump source and the sink. The objective function of the proposed mixed-integer nonlinear programming (MINLP) model is to minimise the total annualised cost (TAC). The TAC includes operating costs for freshwater, heating/cooling utilities, electricity, and investment in heat exchangers and heat pumps. A sensitivity analysis is performed to evaluate the economic feasibility of the system by varying the heating/electricity cost ratio and depreciation period for the capital investment. The results show that additional utility savings can be obtained by integrating heat pumps into the HIWNs. For threshold problems, a heat pump is feasible for heating/electricity price ratios from 1.175, and heating utility is replaced by electricity for a 1:1 ratio. However, for pinched problems, the feasibility of the heat pump is shifted to lower heating/electricity price ratios, from 0.3 to 0.5, depending on the annualisation factor for the investment.

Simultaneous Optimization and Integration of Multiple Process Heat Cascade and Site Utility Selection for the Design of a New Generation of Sugarcane Biorefinery.

Biorefinery plays a crucial role in the decarbonization of the current economic model, but its high investments and costs make its products less competitive. Identifying the best technological route to maximize operational synergies is crucial for its viability. This study presents a new superstructure model based on mixed integer linear programming to identify an ideal biorefinery configuration. The proposed formulation considers the selection and process scale adjustment, utility selection, and heat integration by heat cascade integration from different processes. The formulation is tested by a study where the impact of new technologies on energy efficiency and the total annualized cost of a sugarcane biorefinery is evaluated. As a result, the energy efficiency of biorefinery increased from 50.25% to 74.5% with methanol production through bagasse gasification, mainly due to its high heat availability that can be transferred to the distillery, which made it possible to shift the bagasse flow from the cogeneration to gasification process. Additionally, the production of DME yields outcomes comparable to methanol production. However, CO2 hydrogenation negatively impacts profitability and energy efficiency due to the significant consumption and electricity cost. Nonetheless, it is advantageous for surface power density as it increases biofuel production without expanding the biomass area.

Proposing an innovative polygeneration process based on biogas fuel: A comprehensive thermo-enviro-economic (4E) analysis

The utilization of biogas as a renewable fuel is feasible due to its high energy density, allowing for the development of polygonation structures in a cascade configuration. However, the high initial cost associated with biogas-based high-temperature applications is the central gap considered in this study. This paper introduces a novel heat integration model that employs a novel multi-stage cascade heat recovery and cost-effective operational approach. A biogas burner, an organic Rankine cycle, a Kalina cycle, an absorption chiller cycle, a heat supplier unit, and a proton exchange membrane electrolyzer-based hydrogen generating process are all part of the proposed system. Following the structure's modeling with the Aspen HYSYS program, thermodynamic, environmental, and economic evaluations are carried out along with a thorough parametric assessment. The simulation results indicate that the system has the capability to generate 2993 kW of electricity, 23350 kg/h of hot water at 80 °C, 68120 kg/h of chilled water at 10 °C, and generate 40.32 kg/h of hydrogen. This structure's energy, exergy, and electrical efficiencies are measured at 40 %, 23.71 %, and 16.68 %, respectively. Moreover, the exergy analysis indicated that the biogas burner and ORC evaporator account for 75.1 % of the total irreversibility (13800 kW).

Computational boundary improvement and performance optimization of the flue gas waste heat cascade utilization system of the ultra-supercritical 1000 MW generator set

Abstract There is no uniform performance test rule and standard calculation method for efficiency improvement of a system that utilizes waste heat from flue gas and it is mainly carried out through turbine parameters, without considering boiler system performance. Given the above problem, in this paper, the steam turbine, boiler and waste heat from the flue gas step-up utilization system are calculated by establishing a thermal model of ultra-supercritical secondary reheating 1000 MW unit. At the same time, unit design data and field performance test results are used for verification. Analysis comparing simulation outcomes with experimental data revealed that the discrepancies in the heat consumption rate per unit, the efficiency of the boiler, and the rate of coal consumption for power generation peaked at a marginal variance of 0.2%. The optimum operating parameters of the system for low-grade flue gas recovery in a cascading manner are obtained through variable operating condition calculations. The optimum value of high-pressure bypass feed-water flow rate is 52 kg/s at 970 MW operating condition. At the same time, the difference between high-pressure bypass effluent and high plus effluent temperatures should be controlled to be close to 0°C, and the optimal value of low-pressure bypass condensate flow rate is 53 kg/s. As the flow of flue gas in the bypass increases, the efficiency of heat exchange in the bypass economizer also increases, resulting in greater absorption of heat in the utilization apparatus for flue gas waste heat, leading to a more effective energy-saving outcome. However, it is necessary to regulate the temperature of both the primary and secondary air to levels that exceed the minimum requirements for the safe functioning of the coal mill.

Development and performance evaluation of a high solar contribution resorption-compression cascade heat pump for cold climates

Absorption-resorption heat pumps (ARHPs) have great potential in utilizing low-temperature solar heat for efficient winter heating, but are limited by their weak adaptability to cold climates relative to absorption heat pumps (AHPs) and vapor compression heat pumps (VCHPs). In this work, a resorption-compression cascade system consisting of ARHP and VCHP subsystems is proposed and investigated, with the goal of achieving efficient winter heating with improved solar contribution in cold climates. Thermodynamic models of the two subsystems and of the whole cascade system are developed for system feasibility verification and performance evaluations. Feasible operation temperature and pressure conditions are identified in terms of internal subsystem matching. Based on these operation conditions, the primary energy ratio (PER) – which is a key performance evaluation index – is investigated over a range of solar fractions (f). The results show that for cases without solar contribution, the PER can be generally >0.95 under feasible operating conditions. In addition, the primary energy saving ratio (PESR) and heating capacity lift ratio (εlift), as well as the heat source temperature (Th) and ambient temperature (Tamb) restriction, are all investigated and compared to those of other, competing heating systems. The results show that the proposed system can reduce the Th demand to 69 °C (with a minimum Tamb of −21 °C) and extend the operational Tamb to −31 °C (with a minimum Th of 90 °C), while importantly achieving PESR > 0.20 and εlift > 20 % under the above extreme conditions. Moreover, when integrating the system with solar heat with f > 43 %, the proposed solar-assisted system has a clear PER advantage over an equivalent VCHP system in the Tamb range from −31 °C to 10 °C, highlighting the possibility of achieving higher solar contribution in cold climates.

Performance analysis of a novel air source ammonia/water cascade heat pump for heating buildings in subarctic climate

Performance of available air-source heat pumps (ASHP) for buildings is extremely influenced by the outdoor air temperature (OAT). When OAT falls below –20 °C, especially in subarctic climate, most commercially available ASHPs lose their defined functionality. To address this issue, a novel cascade heat pump (CHP) is proposed that utilizes two natural (with zero global warming index) refrigerants: ammonia, aka R717, as the low stage (LS) refrigerant, and water, aka R718, as the high stage (HS) working fluid. The proposed system is equipped with two rotary vane compressors for LS and HS. To desuperheat the compressor discharge temperatures and increase the HS heating capacity, liquid refrigerant is injected into the chamber of both LS and HS compressors. The wet compression is thermodynamically modeled and validated against experimental data available in the literature. Optimum design parameters such as the maximum allowable temperatures during compression and the minimum vapor quality after injection are obtained where the COP is maximized. Total exergy destruction, number/location of injectors and the injected amounts are also investigated. The proposed CHP can deliver 6.35 kW heating load at 50 °C with a COP of 1.985 when OAT is –40 °C. From a broader perspective, the proposed CHP not only proves its efficacy in extremely cold subarctic climates but also emerges as a pivotal player in addressing the global warming issue. This is accomplished through a dual approach to decarbonization—firstly, by electrifying heating processes and secondly, by substituting phased-out refrigerants with environmentally friendly natural alternatives.

Design and thermo-enviro-economic analyses of a novel thermal design process for a CCHP-desalination application using LNG regasification integrated with a gas turbine power plant

Designing and studying processes with reduced thermodynamic irreversibility and environmental impact is imperative to improve the global applicability of the stand-alone gas turbine cycles. Hence, the present study introduces a novel cascade heat integration tool for a gas turbine cycle combined with a liquefied natural gas cold energy utilization and regasification process. A saltwater desalination subsystem, an absorption chiller, and an organic Rankine cycle are also included in the integrated system. Therefore, the developed system demonstrates an environmentally friendly combined cooling, heating, power, and freshwater production framework, improving the performance of the system in comparison to previous research. This configuration is simulated within the Aspen HYSYS software for comprehensive energy, exergy, environmental, and economic assessments. The findings demonstrate that the whole configuration yields energy and exergy efficiencies of 92.92% and 63.5%, respectively. In comparison, these efficiencies in the single-generation mode are found to be 45.92% and 43.72%, correspondingly. The environmental analysis reveals that the carbon dioxide footprint equals 0.1792 kg/kWh when operating in the multigeneration mode and 0.4591 kg/kWh in the single-generation mode. Furthermore, the economic evaluation exhibits that the cost of energy equals 67.2 $/MWh and 135.92 $/MWh for the multigeneration and single-generation operational modes, respectively.

Thermoeconomic, environmental and uncertainty assessments and optimization of a novel large-scale/low carbon hydrogen liquefaction plant integrated with liquefied natural gas cold energy

Abstract Presently, the liquefaction of hydrogen represents a promising solution to alleviate challenges associated with its storage and transportation. It is crucial to formulate methodological frameworks for scrutinizing hydrogen liquefaction routes to enhance energy efficiency. This paper endeavors to establish, assess feasibility, and refine a novel approach for a high-capacity hydrogen liquefaction facility, leveraging the cold energy from liquefied natural gas (LNG). This new route utilizes four hybrid refrigeration systems, each designed to handle 50 × 103 kg daily. Significant energy savings are achievable through the primary utilization of LNG’s energy in the precooling stage and the generation of electrical power during the vaporization phase. The architecture of this novel route is crafted around the principles of energy conservation, incorporating thermodynamic assessments alongside economic and environmental viability studies. Furthermore, the performance of this innovative hydrogen liquefaction method is thoroughly evaluated across both non-optimized and optimized scenarios. Advanced techniques such as composite curve and uncertainty analyses are employed to provide a detailed examination of heat cascades and cost differentials. The findings indicate that managing LNG’s cold energy is crucial for refining the hydrogen liquefaction route, potentially reducing the specific power requirement of the optimum route by 27.4% compared to its non-optimum counterpart. Moreover, in the optimized scenario, there is a decrease of ~4.72% in unit production expenses, 26.26% in CO2 emissions, and 21.85% in specific power usage for avoided CO2 emissions.

Thermal performance study of a solar-coupled phase changes thermal energy storage system for ORC power generation

The current solar organic Rankine cycle power generation (ORC) system cannot run smoothly under the design conditions due to the shortcomings of solar fluctuations, and thermal energy storage (TES) can effectively buffer the fluctuations of solar energy. Cascaded heat storage (CLTES) has been shown to be more suitable for solar heat storage than single-stage heat storage (LTES). The main objective of this study is to analyze the thermal storage characteristics of thermal storage systems under real-time solar energy fluctuations, and to improve the thermal storage efficiency and total thermal storage capacity of solar phase change thermal storage systems in distributed scenarios. The current research on solar CLTES system needs to focus on the heat storage characteristics under the actual solar fluctuations to provide guidance for the design and practical operation of CLTES system. Analyzing the effect of solar fluctuations on thermal storage performance is the focus of solar thermal storage research, which can provide guidance for the design and actual operation of CLTES systems. However, the current research on solar CLTES system lacks the analysis of thermal storage characteristics under typical solar energy fluctuations. Therefore, a simplified two-dimensional enthalpy-dynamic model of heat transfer for shell-and-tube latent heat storage system is developed in this paper to simulate the heat storage characteristics of single-stage and cascade heat accumulators containing different phase change materials (PCMs) at a fixed temperature of the heat source, and the heat storage performance of the system is simulated based on the solar fluctuations of typical four seasons. Finally, based on the shortcomings of large-capacity thermal storage, a dual-tank recirculation thermal storage model is proposed and the thermal storage characteristics of the improved recirculation model are analyzed. The results show that the optimized cycle heat storage mode can bring about a minimum of 10 % and a maximum of 17.2 % thermal efficiency improvement. On a typical summer day with the most abundant solar energy resources, four times of complete phase change heat storage and one incomplete phase change heat storage were completed (melting fraction = 81.83 %), and on a typical winter day with the least solar energy resources, two times of complete phase change heat storage and one incomplete phase change heat storage were completed (melting fraction = 75.81 %). The maximum difference of heat storage between them is 45.38 %.

Exergoeconomic Evaluation of a Cogeneration System Driven by a Natural Gas and Biomass Co-Firing Gas Turbine Combined with a Steam Rankine Cycle, Organic Rankine Cycle, and Absorption Chiller

Considering energy conversion efficiency, pollution emissions, and economic benefits, combining biomass with fossil fuels in power generation facilities is a viable approach to address prevailing energy deficits and environmental challenges. This research aimed to investigate the thermodynamic and exergoeconomic performance of a novel power and cooling cogeneration system based on a natural gas–biomass dual fuel gas turbine (DFGT). In this system, a steam Rankine cycle (SRC), a single-effect absorption chiller (SEAC), and an organic Rankine cycle (ORC) are employed as bottoming cycles for the waste heat cascade utilization of the DFGT. The effects of main operating parameters on the performance criteria are examined, and multi-objective optimization is accomplished with a genetic algorithm using exergy efficiency and the sum unit cost of the product (SUCP) as the objective functions. The results demonstrate the higher energy utilization efficiency of the proposed system with the thermal and exergy efficiencies of 75.69% and 41.76%, respectively, while the SUCP is 13.37 $/GJ. The exergy analysis reveals that the combustion chamber takes the largest proportion of the exergy destruction rate. The parametric analysis shows that the thermal and exergy efficiencies, as well as the SUCP, rise with the increase in the gas turbine inlet temperature or with the decrease in the preheated air temperature. Higher exergy efficiency and lower SUCP could be obtained by increasing the SRC turbine inlet pressure or decreasing the SRC condensation temperature. Finally, optimization results indicate that the system with an optimum solution yields 0.3% higher exergy efficiency and 2.8% lower SUCP compared with the base case.