Low Temperature Waste Heat Utilization Research Articles

Nonlinear System identification and Predictive Control for Waste Heat Recovery with Heat Pumps

The utilization of low-temperature waste heat, particularly from electrolyzers, in district heating networks via heat pumps presents a promising approach to accelerate the decarbonization of the heat sector. However, managing the electrolyzer’s specific temperature requirements and dynamic waste heat output, while simultaneously meeting the district heating network’s variable temperature demands, requires the implementation of an advanced control system for the heat pump cycle. For this, model predictive control is a promising approach since it not only ensures the satisfaction of constraints, but also facilitates a direct optimization of the heat pump’s operational efficiency. Nevertheless, model predictive control requires a dynamic heat pump model. In this context, first-principles models are often used. However, they are very complex and difficult to parameterize for real heat pumps. Therefore, in the present paper, a data-based system identification is carried out to obtain a reduced-order heat pump model from a high-fidelity first-principles simulation model. Based on the identified model, a predictive controller is implemented. The effectiveness of the obtained controller for operating the first-principles model is demonstrated in a numerical case study.

Thermal-Economic Analysis of an Organic Rankine Cycle System with Direct Evaporative Condenser

The organic Rankine cycle (ORC) system for power generation has proven to be an effective technology for low-temperature waste heat utilization. Accurate prediction and comprehensive comparison of system performance under different conditions are necessary for the development and application of suitable ORC configurations. This paper proposed an organic Rankine cycle (ORC) system using a direct evaporative condenser to realize performance enhancement and analyzed its dynamic performance based on the actual climatic condition, which is beneficial for the performance optimization of this system. This study begins with an introduction to the thermal economics model of the proposed system and evaluates the performance of the system based on the 3E (energy, exergy, economy) analysis method. Secondly, four candidate working fluids were compared and analyzed, leading to the selection of R142b as the best working fluid for the proposed system. Finally, the dynamic performance of the proposed system using the working fluid of R142b was analyzed based on the hourly environment temperature. The result showed that the net thermos-electric conversion efficiency of the system was negatively correlated with the ambient wet-bulb temperature. The annual average exergy efficiency of the system is about 65.79%, and the average exergy loss of the heat absorption unit, evaporative condenser, pump, and expander account for 61.07%, 6.92%, 2.99%, and 29.01% of the exergy loss of the system respectively. In the case 8760 h of operation per year, the payback period of the proposed ORC system using direct evaporative condenser is about 2.14 years.

Quantification of waste heat potential in China: A top-down Societal Waste Heat Accounting Model

The recovery of waste heat is important for energy saving and carbon reduction in China. Although numerous technologies for waste heat recovery in China have been analyzed, research is still insufficient to provide an overview of the waste heat potential within the entire energy system. To fill this gap, this paper applies a top-down energy efficiency analysis to develop a Societal Waste Heat Accounting Model to quantify the waste heat potential in China. By indicating the total amount, sector and temperature distribution, and the ability to perform work of waste heat for China in 2018, the results revealed that: a) waste heat occupies 42% of total primary energy inputs, and 26% of primary energy and carbon emissions can be theoretically saved by utilizing waste heat. b) the electricity generation sector is the main source of waste heat, accounting for 45% of the total, of which 88% is below 100 °C. c) the industry sector has a wide range of temperatures, mostly above 300 °C, highlighting the highest ability to perform work. Future studies can focus on the utilization of low-temperature waste heat, which is 66% of the total, for example, the Organic Ranking Cycle technology and the heat pump technology.

Constructal design for dual-pressure axial-flow turbine in organic Rankine cycle

Dual-pressure organic Rankine cycle (ORC) has great advantages in low-temperature waste heat utilization systems. There are few studies on optimizing the performance and structure of the dual-pressure turbine, which is a key part of the dual-pressure ORC. The constructal design is performed for a dual-pressure axial-flow turbine (AFT) in a dual-pressure ORC to improve the turbine and cycle performances. Total power is taken as optimization objective function, volume ratio of the high-pressure turbine (HPT) and inlet pressure of the low-pressure turbine (LPT) are chosen as optimization variables, and total volume is employed as a constraint. After obtaining the optimization results, the impacts of some flow and structure parameters are analyzed. Furthermore, performances of the dual- and single-pressure AFTs are contrasted. The results demonstrate that the total powers of the dual-pressure AFT after the primary and twice optimizations are increased by 0.97% and 1.69% compared with the initial value, respectively. After the twice optimization, the optimal construct of the dual-pressure AFT is that the volume ratio of the HPT and inlet pressure of the LPT are 0.072 and 695 kPa, respectively. The nozzle inlet hub to tip radius ratios of the two turbines and the working-fluid mass flow rate ratio positively impact the total power, while the total volume hurts it. Besides, the dual-pressure AFT has a better performance advantage than the single-pressure AFT, and the total power of the former is 16.59% higher than that of the latter. The results confirm that the dual pressure is more suitable for low-temperature waste heat utilization systems than the single pressure and provide strategies for improving the turbine performance.

Application Analysis of Organic Rankine Cycle in Low Temperature Waste Heat Utilization

Application Analysis of Organic Rankine Cycle in Low Temperature Waste Heat Utilization

Constructal thermodynamic optimization for a novel Kalina-organic Rankine combined cycle to utilize waste heat

Kalina cycle and organic Rankine cycle have different optimal heat source temperatures. To realize cascade utilization of low-temperature waste heat, a model of novel Kalina-organic Rankine combined cycle is established by using a dual-pressure Kalina cycle as top cycle and a dual-pressure organic Rankine cycle as bottom cycle, and constructal thermodynamic optimization is carried out by uniting constructal theory and finite-time thermodynamics. With the total turbine volume and total heat exchanger area being fixed, the optimal tube external diameters of the evaporators in the dual-pressure Kalina cycle and dual-pressure organic Rankine cycle are obtained by maximizing the net power output of the Kalina-organic Rankine combined cycle system. The results prove that the net power outputs of the Kalina-organic Rankine combined cycle system after four progressive optimizations are increased by 2.62%, 5.41%, 15.05% and 16.17%, respectively, compared to the initial one. In the quartic constructal thermodynamic optimization, the optimal tube external diameters of the high- and low-temperature evaporators in the DPKC are 0.024m and 0.023m, respectively, and the those in the DPORC are 0.020m and 0.024m, respectively. The results obtained will improve the performance of the KORCC and promote the efficient utilization of waste heat.

A new optimization method for cooling systems considering low-temperature waste heat utilization in a polysilicon industry

Cooling systems have been widely used in the industry. In order to satisfy cooling processes with large temperature ranges, cascade cooling systems included different cooling methods have been developed. However, waste heat in such processes could still be recovered instead of being cooled down in further operations. There are many waste heat recovery technologies, but a systematic method to implement such technologies in a cascade cooling system optimally has not been well studied. This paper presents a new cascade cooling system considering waste heat recovery via Organic Rankine Cycle (ORC) and absorption refrigeration (AR). An optimization method for minimizing the total annual cost has been proposed to distribute the cooling duty for different cooling methods. A case study is conducted to validate the economic advantages of the proposed method. From the results, the scheme using AR can reduce the total annual cost by 37.5% and the scheme using ORC can reduce the total annual cost by 42.8%. Results show that in cases where the heat source supply temperature is higher than 123 °C, ORC will achieve higher economic benefits. Otherwise, AR is economically superior to ORC. The method can also indicate how to distribute cooling duty between different cooling method.

Constructal thermodynamic optimization for dual-pressure organic Rankine cycle in waste heat utilization system

Dual-pressure organic Rankine cycle (DPORC) is a technology with great potential to solve energy problems. This paper performs constructal thermodynamic optimization (CTO) for the DPORC based on a combination of constructal theory and finite-time thermodynamics. The tube outside diameters of the heat exchangers, such as high- and low-temperature evaporators (HTE and LTE) and condenser, and the volume ratio of the high-pressure turbine are the design variables, the system net power output (POW) is the optimization function, and the total volume of the turbines and total heat transfer area (HTA) of the heat exchangers are the constraints. The influences of some parameters, such as the HTA ratios of the HTE and LTE, and the total mass flow rate (MFR) of the working medium on the CTO results are studied. The performance comparison between the DPORC and single-pressure ORC (SPORC) is carried out under the same conditions. The results demonstrate that the system net POWs after the four stepwise CTOs are improved by 1.70%, 3.01%, 5.80% and 8.84% over against the initial system net POW, respectively. Reducing the HTA ratios of the HTE and LTE and increasing the total MFR of the working medium can increase the system net POW. Compared with the performance of the SPORC, the system net POW and system net thermal efficiency of the DPORC are increased by 4.91% and 5.51%, respectively. The optimization results can be used to guide the optimal designs of the low-temperature waste heat utilization systems.

Integrated adsorption-based multigeneration systems: A critical review and future trends

This paper presents the state-of-the-art of the integration of adsorption chillers into different configurations of combined cooling, heating, and power (CCHP) systems. Adsorption chillers, as a thermally-activated cooling system, provide a great solution to the utilization of low-temperature waste heat, thus help to increase the efficiency of the existing power generating systems, and to reduce the greenhouse gasses emissions including the chlorofluorocarbons (CFC) that is used in traditional electric-powered chillers. In this study, a brief introduction to CCHP systems and the thermally-driven cooling systems is presented, and the working scheme of the adsorption cooling cycle is illustrated. The paper also focuses on the CCHP systems integrated with adsorption chillers that are driven by solar thermal energy via different solar collectors. Furthermore, multigeneration systems driven by internal combustion engines, gas turbines and fuel cells are highlighted. Finally, the integration of adsorption chillers in hybrid cooling systems and with Organic Rankine Cycles (ORC) is also presented.

Organic Rankine Cycle with Pure and Mixed Working Fluids for LNG Cold Energy Recovery

Liquefied Natural Gas (LNG) contains a large amount of cold exergy, which is normally wasted during the regasification process. Organic Rankine Cycles (ORCs), which have been widely employed for low-temperature waste heat utilization above ambient temperature, can also exploit LNG cold energy efficiently. Due to the low regasification temperature of LNG, the ORC for LNG cold energy recovery is quite different from the conventional ORCs recovering low-temperature waste heat. In this study, we investigate the working fluids for such an ORC recovering LNG cold energy. Both pure and mixed working fluids are studied and compared. According to open literature, mixed working fluids perform better than pure working fluids because the mixed working fluid can achieve a better match with the heat source. Therefore, the main motivation for mixed working fluids is to obtain gliding temperature for a better match with sensible heat sources. However, a few studies show that pure working fluid outperform mixed working fluids when the evaporating condition is close to the critical point. In addition, if the evaporation pressure is greater than the critical pressure, pure working fluids can also match well with a sensible heat source. Therefore, it is too general to claim that mixed working fluids will perform better than pure working fluids. For cases where the mixed working fluids perform better than pure working fluids, the optimal composition of the working fluid has to be determined. In this study, the performance of ORCs with pure and mixed working fluids are investigated for different types of ORCs. The optimal working fluid and composition for an ORC recovering LNG cold energy can be determined based on a simulation-based framework. The results show that mixed working fluids do not show significant improvement in net power output of the integrated system.

Experimental study of an adsorption chiller for extra low temperature waste heat utilization

Lots of low temperature waste heat exists in industrial fields which also requires cooling processes, i.e.: data center and glass and liquid crystal display panel manufacturing. Silica gel-water adsorption refrigeration can well match the operation temperature in waste heat utilization for these applications but its technical feasibility and experimental performance is not studied. In this paper, a silica gel-water adsorption chiller using mass recovery process is built and experimentally studied under the condition of lower than 80 °C driving hot water temperature. For the case of data center waste heat utilization, the adsorption chiller can be well operated under the condition of driving temperatures of 51.4–61.3 °C, with the COPs (coefficient of performance) of 0.285–0.477 and SCPs (specific cooling power) of 71.2–108.4 W∙kg−1 while producing 18.8–22.4 °C chilled water. For the cases of glass and liquid crystal display panel manufacturing waste heat utilization, COPs of 0.317–0.483 and SCPs of 95.2–146.7 W∙kg−1 are obtained under the conditions of driving temperatures of 56.2–74.9 °C and chilled water outlet temperatures of 14.2–16.8 °C. These experimental results indicate that feasibility in operation and good performance of silica gel-water adsorption chiller for extra low temperature waste heat utilization is confirmed.

Process assessments for low-temperature methane reforming using oxygen carrier metal oxide nanoparticles

The utilization of low-temperature waste heat in the endothermic hydrocarbon reforming reaction can improve the energy efficiency of the chemical product synthesis system. In this study, process assessments are conducted, and potential technologies to realize the reforming process using low-temperature waste heat are discussed. Based on the assessment and process design, a low-temperature methane reforming process using circulating fluidized beds is proposed. An investigation into the use of metal oxide nanoparticles fabricated by the supercritical method as an oxygen carrier serves as a partial proof-of-concept for the low-temperature reforming process. The requirements for the oxygen carrier in the process are calculated based on a target for the amount of methane reformed in the methanol production plant.

Process integration of organic Rankine cycle (ORC) and heat pump for low temperature waste heat recovery

Organic Rankine cycles (ORCs) have been a mature technology for low temperature waste heat utilization. However, the relatively low thermal efficiency and incomplete waste heat recovery limit the power output of ORC systems. The reason for the above two problems is that the temperature-enthalpy (T-H) profile of waste heat sources does not match well with the organic working fluid in an ORC system. Heat pumps may reinforce the match between the waste heat carrier and the organic working fluid of the ORC. The condensation heat released by a heat pump can be used to evaporate the organic working fluid during phase change. Thus, a better thermal match between waste heat and the organic working fluid in the ORC can be obtained. Due to the possibility to improve power output, the integration of an ORC and a heat pump is investigated in this study. Proper working fluids in ORCs and heat pumps are chosen simultaneously. The integration between ORCs and heat pumps presents significant power output increases for special cases and deserves to be popularized in practical applications. However, the integrated system does not always lead to an increase in power output. The integration is profitable only when the following pre-conditions are satisfied simultaneously: (a) poor thermal match between working fluid and waste heat for standalone ORC; (b) the evaporation temperature of the ORC is set appropriately; (c) the working fluid of the ORC has a small ratio of latent to sensible heat; (d) the COP of the heat pump is satisfactory. A systematic procedure to determine the optimal operating conditions of the system is proposed in this study. A case study adopted from literature demonstrates the potential benefits of the integration of heat pumps and ORCs. The results show that the net power output and the amount of waste heat recovered increase by 9.37% and 12.04% respectively.

New operating strategy for a combined cycle gas turbine power plant

The highly dynamic nature of power demand forces a combined cycle gas turbine (CCGT) power plant to often run at less-efficient part-load conditions. A new operating strategy called EGR-IGVC is proposed in this work to improve the part-load performance of a CCGT plant. The strategy partially recycles the flue gas from the heat recovery steam generator and manipulates the inlet guide vanes to regulate part-load operation and boost plant efficiency. It allows the CCGT plant to set its own acceptable gas turbine temperature limits at any time and thus maximize the plant performance. Our previous CCGT flow diagram in Aspen HYSYS [Liu and Karimi (2018)] is modified to accommodate EGR for evaluating EGR-IGVC. Then, using a case study CCGT plant, EGR-IGVC is compared in detail with the conventional inlet guide vane control (IGVC). The results show that EGR-IGVC improves the plant part-load performance significantly. It increases the plant efficiency by up to 1.2% (actual) and reduces the CO2 emissions by up to 13.8 kg MW−1 h−1. By enabling an effective utilization of low-temperature waste heat and reducing the propensity for NOx formation, EGR-IGVC promises to be a better strategy for the part-load operation of CCGT plants.

OPERATING PROCESSES PARAMETERS OF OPEN-TYPE SORPTIVE HEAT STORAGE DEVICES IN HEAT SUPPLY SYSTEMS

Operating processes of open-type sorption heat accumulator in heating systems were studied. The al-gorithm for calculation of its performance is developed. It includes computation of mass transfer cofficient, sorp-tion, useful heat sorption, heat input for heating the sorbent, device casing, water in the humidifier, evaporation of water, heating the sorbed water, desorption, and calculating efficiency coefficient. Operational performance of open-type heat storage device based on the sorbent composite ‘silica - sodium sulfate’ is estimated. Suggested com-posite sorbent is obtained from tetraethoxysilane, Na2SO4, ethanol (as a solvent), hydrochloric acid (as a catalyst) and polyionenes served as organic modifiers. The impact of vapor flow velocity on efficiency is taken into account by the coefficient equal sorption value. A monotonic increase of efficiency coefficient while increasing the vapor flow speed and its relative humidity is shown. The efficiency coefficient of heat storage device is shown to be with little to no dependence with regeneration temperature heightened from 90 ºC to 110ºC. The optimal operating con-ditions of the heat accumulating device which allow to operate with maximum magnitudes of efficiency coefficients 53 – 57 % are stated to be vapor-air mixture speed 0.6 - 0.8 m/s and relative humidity of 40 – 60 %. The results of simulative experiments and field trials are given. Field tests were carried out in two modes differed with start time. Heat storage device was switched on in the morning or in the evening for load-factoring of electric energy peaks, indoor temperature being kept within limits from 20 ºС to 22ºС during day or night hours. Correlation between effi-ciency coefficients deduced from experiments and calculated with suggested algorithm is confirmed. The possibility of reducing the power consumption by applying of the heat accumulators in 2,4 - 90 times versus decentralized heat-ing systems on basis of solid fuel boiler, gas boiler and electric boiler is stated when open-type sorptive heat stor-age device used. Results of the study can be applied to develop sorptive storage devices in decentralized heat supply and ventilation systems and sorption units for utilization of low-temperature waste heat.

System Design and Thermodynamic Analysis of a Sintering-driven Organic Rankine Cycle

Abstract By reviewing development of waste heat utilization in sintering process and sinter cooling process, two major difficulties are more and more apparent. A system of low temperature waste heat utilization based on organic fluids is proposed, which is composed of flue gas waste heat utilization heat exchanger, thermal storage subsystem and organic Rankine cycle (ORC) subsystem. The sintering flue gas and the sinter cooling air are adopted as the heat source. And on this basis, we establish the simulation analysis platform for the whole thermodynamic system. From a comprehensive perspective, the low temperature waste heat utilization could run well when the sintering heat source fluctuates within a certain range. It means that, the ORC-based system could overcome difficulties in utilizing waste heat in sintering process.

Simulation Optimization of a New Ammonia-Based Carbon Capture Process Coupled with Low-Temperature Waste Heat Utilization

Although ammonia-based CO2 capture has attracted global research attention, several inherent issues with this technology remain to be resolved. To address these problems, a new design for carbon capture using ammonia is proposed on the basis of anti-solvent crystallization, also known as precipitation crystallization. The crystallization of a low carbonized absorbent was found to be enhanced in the crystallizer using an anti-solvent process, which can maintain a high absorption rate and simultaneously prevent crystallization from occurring in the absorption tower. Energy consumption for sorbent regeneration is reduced by regenerating the crystal product rather than the rich solution. Energy-cascade utilization is an effective way to improve the use of energy. In this work, steam was used to drive a heat pump that extracts energy from discharged flue gas from a wet flue gas desulfurization system in a power plant to enable the recovery of low-temperature residual energy; this energy can be used in the crys...

Working Fluid Selection for Organic Rankine Cycle (ORC) Considering the Characteristics of Waste Heat Sources

Organic Rankine cycle (ORC) is a promising way for low temperature waste heat utilization. The performance of an ORC system is strongly related to the working fluid. Therefore, working fluid select...

Selection of organic Rankine cycle working fluids in the low-temperature waste heat utilization

In the current study, simulations based on the engineering equation solver (EES) software are performed to determine the suitable working fluid for the simple organic Rankine cycle system in different temperature ranges. Under the condition of various temperatures and a constant thermal power of the flue gas, the influence of different organic working fluids on the efficiency of the subcritical organic Rankine cycle power generation system is studied, and its efficiency and other parameters are compared with those of the regenerator system. It is shown that the efficiency of the subcritical organic Rankine cycle system is the best when the parameters of the working fluid in the expander inlet are in the saturation state. And for the organic Rankine cycle, the R245fa is better than other working fluids and the efficiency of the system reaches up to 10.2% when the flammability, the toxicity, the ozone depletion, the greenhouse effect and other factors of the working fluids are considered. The R601a working fluid can be used for the high-temperature heat source, however, because of its high flammability, new working fluid should be investigated. Under the same condition, the efficiency of the organic Rankine cycle power generation system with an internal heat exchanger is higher than that of the simple system without the internal heat exchanger, but the efficiency is related to the properties of the working fluid and the temperature of the heat source.

Thermodynamic analysis of a low-temperature waste heat recovery system based on the concept of solar chimney

The utilization of low-temperature waste heat draws more and more attention due to serious energy crisis nowadays. This paper proposes a low-temperature waste heat recovery system based on the concept of solar chimney. In the system, low-temperature waste heat is used to heat air to produce an air updraft in the chimney tower. The air updraft propels a turbine fixed at the base of the chimney tower to convert waste heat into electricity. The mathematical model of the system is established based on first law and second law of thermodynamics. Hot water is selected as the representative of low-temperature waste heat sources for researching. The heat source temperature, ambient air temperature and area of heat transfer are examined to evaluate their effects on the system performance such as velocity of updraft, mass flow rate of air, power output, conversion efficiency, and exergy efficiency. The velocity of air demonstrates a better stability than the mass flow rate of air and the pressure difference when temperature of heat source, ambient air temperature or area of heat transfer changes.