Low-grade Waste Heat Recovery Research Articles

Data-driven identification and fast model predictive control of the ORC waste heat recovery system by using Koopman operator

Organic Rankine Cycle (ORC) has received its wide application in low-grade waste heat recovery (WHR) technology for its significant performance and easy access to its components. Given the highly fluctuating nature of the waste heat source, Model Predictive Control (MPC) is usually utilized to realize the reasonable adjustment of ORC based WHR systems and has performed satisfactorily. In order to apply MPC, a relatively precise model should be built up for the ORC system which ensures acceptable control performance. However, popular modeling methods based on the mechanism of the ORC establish the high-dimensional nonlinear model and suffer from computational costs when utilizing complicated nonlinear MPC. Even if linear MPC is employed for the purpose of reducing the calculation amount, the model obtained by linearization near the operating points usually makes it valid locally, thus bringing about suboptimal or unstable control performance. To address this problem, the Koopman operator is introduced for the data-driven identification and MPC of the ORC system. Koopman identification constructs a linear model in the lifted space in which the ORC system possesses nearly global linear evolution and enables the prediction of the nonlinear ORC dynamics from measurement data. In view of the possible online calculation burden increment caused by the rise of variable dimension through lifting action, the fast Koopman MPC (FKMPC) algorithm is thus proposed based on the invariance of the Hessian matrix of the optimization problem to shorten the computing time. Simulations on setpoint tracking and disturbance rejection are performed to verify the established model accuracy and the control effectiveness of the proposed strategy in comparison with other approaches.

High-performance thermomagnetic generator controlled by a magnetocaloric switch

Low grade waste heat accounts for ~65% of total waste heat, but conventional waste heat recovery technology exhibits low conversion efficiency for low grade waste heat recovery. Hence, we designed a thermomagnetic generator for such applications. Unlike its usual role as the coil core or big magnetic yoke in previous works, here the magnetocaloric material acts as a switch that controls the magnetic circuit. This makes it not only have the advantage of flux reversal of the pretzel-like topology, but also present a simpler design, lower magnetic stray field, and higher performance by using less magnetocaloric material than preceding devices. The effects of key structural and system parameters were studied through a combination of experiments and finite element simulations. The optimized max power density PDmax produced by our device is significantly higher than those of other existing active thermomagnetic, thermo, and pyroelectric generators. Such high performance shows the effectiveness of our topology design of magnetic circuit with magnetocaloric switch.

High-Performance Thermomagnetic Gd-Si-Ge Alloys.

Exploring low-grade waste heat energy harvesting is crucial to address increasing environmental concerns. Thermomagnetic materials are magnetic phase change materials that enable energy harvesting from low-temperature gradients. To achieve a high thermomagnetic conversion efficiency, there are three main material requirements: (i) magnetic phase transition near room temperature, (ii) substantial change in magnetization with temperature, and (iii) high thermal conductivity. Here, we demonstrate a high-performance Gd5Si2.4Ge1.6 thermomagnetic alloy that meets these three requirements. The magnetic phase transition temperature was successfully shifted to 306 K by introducing Ge doping in Gd5Si4, and a sharper and more symmetric magnetization behavior with saturation magnetization of Mmax = 70 emu/g at a 2 T magnetic field was achieved in the ferromagnetic state. The addition of SeS2, as a low-temperature sintering aid, to the Gd-Si-Ge alloy improved the material's density and thermal conductivity by ∼45 and ∼275%, respectively. Our results confirm that the (Gd5Si2.4Ge1.6)0.9(SeS2)0.1 alloy is a suitable composite material for low-grade waste heat recovery in thermomagnetic applications.

Thermo-Osmosis in Charged Nanochannels: Effects of Surface Charge and Ionic Strength.

Thermo-osmosis refers to fluid migration due to the temperature gradient. The mechanistic understanding of thermo-osmosis in charged nano-porous media is still incomplete, while it is important for several environmental and energy applications, such as low-grade waste heat recovery, wastewater recovery, fuel cells, and nuclear waste storage. This paper presents results from a series of molecular dynamics simulations of thermo-osmosis in charged silica nanochannels that advance the understanding of the phenomenon. Simulations with pure water and water with dissolved NaCl are considered. First, the effect of surface charge on the sign and magnitude of the thermo-osmotic coefficient is quantified. This effect was found to be mainly linked to the structural modifications of an aqueous electrical double layer (EDL) caused by the nanoconfinement and surface charges. In addition, the results illustrate that the surface charges reduce the self-diffusivity and thermo-osmosis of interfacial liquid. The thermo-osmosis was found to change direction when the surface charge density exceeds -0.03C · m-2. It was found that the thermo-osmotic flow and self-diffusivity increase with the concentration of NaCl. The fluxes of solvent and solute are decoupled by considering the Ludwig-Soret effect of NaCl ions to identify the main mechanisms controlling the behavior. In addition to the advance in microscopic quantification and mechanistic understanding of thermo-osmosis, the work provides approaches to investigate a broader category of coupled heat and mass transfer problems in nanoscale space.

Nano-engineering enabled heat pipe battery: A powerful heat transfer infrastructure with capability of power generation

Low-grade electronic waste heat lower than 100 °C widely exists in information technology equipment. Waste heat from data centers is particularly concerning. A micro heat engine (or Carnot battery) that is suitable for the direct recovery of low-grade waste heat during the cooling of electronic devices is promising. In this study, a novel looped heat pipe battery (LHPB) was proposed to achieve the aforementioned function. By absorbing heat through the evaporator of the proposed LHPB, the working fluid was transformed into high-speed vapor and blasted the generator rotor sealed in the LHPB condenser, which directly converted waste heat into electricity. The proposed LHPB can realize excellent thermal management and effective waste heat recovery simultaneously. The experimental results revealed that under the heat load of 150 W with a rated 12 V fan cooling, the device can maintain the temperature of the heat source at 72 °C, and the maximum output electrical power is 6.19 mW. The nominal power usage effectiveness is lower than 1.01. Even though the generated electrical power is still low in the present form, this pioneering study might open novel avenues for electronic cooling and waste heat recovery.

Mathematical model establishment and optimum design of a novel cesium thermionic–thermoelectric hybrid generator

Static power sources, such as thermionic energy converters (TEC), thermoelectric generators (TEG), thermophotovoltaic converters (TPV), and alkali metal thermal to electric converters (AMTEC), have enormous potential application in aerospace, military, electronic devices, and low-grade waste heat recovery and utilization. Aiming to further improve the power output and conversion efficiency, any two of them can be cascaded according to the difference in operating temperature. In this study, a novel mathematical model of the static hybrid power generator composed of a cesium thermionic energy converter (CTEC) and a TEG was proposed. The model considered the irreversible heat loss and the additional energy barrier of the CTEC module, and the Thomson effect of the TEG module, synchronously. To obtain the temperature distribution of CTEC and TEG modules, energy balance equations were determined by nonequilibrium thermodynamic analysis. Then, the effects of the output voltage and anode work function of the CTEC, the heat source temperature, and the electric current of the TEG on the output performance were investigated systematically by means of MATLAB. The results indicate that the maximum efficiency of the hybrid generator reaches 26.82% at heat source and heat sink temperatures of 1800 K and 300 K, respectively, which is significantly higher than that of the individual CTEC module (21.28%). This reveals that the waste heat released by the CTEC is utilized effectively in the TEG module, and the increased heat source temperature will further improve the hybrid generator’s power output and efficiency. The output power and efficiency are obviously improved by reducing the anode work function. When ϕC = 1.0 eV, i = 0.005, the power reaches a maximum of 21.61 W at 0.37 V, and the efficiency reaches a maximum of 25.90% at 0.55 V. Moreover, the curve of the output power as a function of conversion efficiency could be used to determine the optimal operating region of the hybrid generator, i,e., Pη < P < Pmax, ηP < η < ηmax, thereby obtaining the thresholds of the hybrid system parameters, i.e., VP < V < Vη, T1,P < T1 < T1,η, T2,P > T2 > T2,η, T3,P > T3 > T3,η and TCs,P > TCs > TCs,η. This study provides a reference for the optimal design and application of a CTEC–TEG hybrid generator in the future.

Low-grade waste heat recovery scenarios: Pyroelectric, thermomagnetic, and thermogalvanic thermal energy harvesting

Identifying reliable and sustainable sources of electricity is a significant challenge of the present time. However, most energy-generation mechanisms produce unavoidable low-grade waste heat as a byproduct while harvesting (or converting) electrical energy from conventional or renewable energy sources. Each year, over 60% of the primary energy is wasted as heat. Accordingly, considerable efforts are being made to convert this waste heat into usable electrical energy using diverse energy conversion materials. In this review, three actively investigated energy conversion mechanisms beyond thermoelectrics, i.e., (1) pyroelectric, (2) thermomagnetic, and (3) thermogalvanic generators, are reviewed and compared from the viewpoint of fundamental theories, critical parameters for high energy conversion efficiency, and current status of materials and devices. The challenges and opportunities in low-grade waste-heat recovery are discussed in terms of material science and structural design. This review provides an overview of the current progress in thermal-energy-harvesting research, future challenges, and opportunities.

Potential evaluation of integrated high temperature heat pumps: A review of recent advances

Industrial and high temperature pumps are a well-established, sustainable, and low-emission technology for processing temperatures below 100 °C, especially when driven by renewable energy. The next frontier in heat pumping is to enhance the economic working envelope to serve the 100–200 °C range, where an estimated 27% of industrial process heat demand is required. High temperature heat pumps (HTHP) are an effective technology for delivering heat and recovering waste heat from various industrial processes, hence reducing primary energy consumption and the resulting CO2 emissions. The integration of high temperature heat pumps into different industrial process networks provides significant environmental and performance improvements,an innovative and profitable solution for different decarbonizing sectors. Higher temperature heat pumps offer significant potential to enhance thermally demanding industrial processes due to their high temperature lift capability. This review looks at how future improvements in HTHP technology can take use of breakthroughs in high-temperature heat pump research to address important technical obstacles. This review primarily consolidates data from HTHPs integrated with various industrial processes applications such as thermal energy storage, low-grade waste heat recovery, membrane fuel cell, organic Rankine cycle, super-critical water desalination, co-generation and poly-generation, vapor injection, steam injected gas turbine, and solar absorption system. However, the widespread diffusion of HP technologies faces several challenges, including technological (limitation of the electrical network due to intensive electrification of the heating sector), economic (high investment and installation cost), regulatory (lack of standards and mandatory policies), policy (uncertainty in policy and lack of clear heat decarbonization pathways and technology uptake), and public acceptance issues (unwarranted fear, misperception, misinformation, and previous experiences on the reliability of heat pumps) are highlighted.

High-Performance Industrial-Grade p-Type (Bi,Sb)2 Te3 Thermoelectric Enabled by a Stepwise Optimization Strategy.

As the sole dominator of the commercial thermoelectric (TE) market, Bi2 Te3 -based alloys play an irreplaceable role in Peltier cooling and low-grade waste heat recovery. Herein, to improve the relative low TE efficiency determined by the figure of merit ZT, an effective approach is reported for improving the TE performance of p-type (Bi,Sb)2 Te3 by incorporating Ag8 GeTe6 and Se. Specifically, the diffused Ag and Ge atoms into the matrix conduce to optimized carrier concentration and enlarge the density-of-states effective mass while the Sb-rich nanoprecipitates generate coherent interfaces with little loss of carrier mobility. The subsequent Se dopants introduce multiple phonon scattering sources and significantly suppress the lattice thermal conductivity while maintaining a decent power factor. Consequently, a high peak ZT of 1.53 at 350 K and a remarkable average ZT of 1.31 (300-500 K) are attained in the Bi0.4 Sb1.6 Te0.95 Se0.05 + 0.10 wt% Ag8 GeTe6 sample. Most noteworthily, the size and mass of the optimal sample are enlarged to Ø40 mm-200g and the constructed 17-couple TE module exhibits an extraordinary conversion efficiency of 6.3% at ΔT= 245 K. This work demonstrates a facile method to develop high-performance and industrial-grade (Bi,Sb)2 Te3 -based alloys, which paves a strong way for further practical applications.

Exergoeconomic analysis and multi-objective optimization of ORC configurations via Taguchi-Grey Relational Methods

Recovery of low-grade waste heat in industrial processes is an essential energy management topic. Yet, most low-temperature heat sources discharge their heat directly into the environment. The Organic Rankine Cycle (ORC), which has the benefits of being energy-efficient, enabling investment savings, and being ecologically friendly, is crucial in recycling energy from low-temperature waste heat. Both the application of the optimum cycle design and the provision of optimum working conditions are the issues that need to be focused on efficiently using energy. This study performs the energy, exergy, and exergoeconomic analysis of four different organic Rankine cycle configurations operating with renewable or low grade waste heat. The effect degrees and ratios of selected control factors are calculated using Taguchi and variance analysis methods to compare thermal and exergy efficiencies, total system cost, and unit cost of electricity produced by the system. The objective function of the multi-objective optimization problem is defined, and its solution is realized with the Taguchi-Grey Relational Analysis method. The best thermodynamic and exergoeconomic performance result is calculated for the configuration of ORC with Feed Fluid Heater-Internal Heat Exchanger (IHE–FFH–ORC). As a result of Taguchi and ANOVA analysis, the factors that most affect the thermal efficiency of the system, the exergy efficiency, the total system investment cost, and the unit cost of the electricity produced are, respectively, the evaporation temperature (∼50%), turbine efficiency (∼25%), working fluid (∼20%), subcooling (∼4%), pump efficiency (∼0.05%), and superheating (∼0.05%). As a result of the optimization process, the thermal and exergy efficiencies, the total system cost and the unit cost of produced electricity for the IHE–FFH–ORC power system are calculated as 22.6% and 73.5%, 1.06 $/h, 0.039 $/kWh and 2.9 years, respectively.

Unsteady and three-dimensional computational fluid dynamics modelling of scroll expander for low-grade waste heat recovery transcritical carbon dioxide micro-scale power system

Recent developments in the field of renewable energy have led to a renewed interest in low-grade heat (< 500 K). The low-grade heat is widely wasted by the lack of efficient heat recovery technologies. It is also limited by the system size, which defines as the micro to small-scale (< 50 kW). Although ORC based unit has been implemented in this field, the CO2 based waste heat recovery units can be more capable in the size construction. The performance of the expander plays a vital role in the system’s efficiency. Thus, the current paper provides thermodynamic and CFD analysis of a scroll expander regarding a micro-scale T-CO2 recovery system (< 10 kW) with a 400 K low-grade heat source. In the current CFD model, all the fluid domains were constructed by structural mesh. It also successfully integrated with the thermodynamic table to simulate two-phase T-CO2. This model can be the first scroll expander model for T-CO2 power system and gap the bridge of utilising the scroll machinery in this field. The CFD methodology was successfully validated by the new-built testing platform and previous data. The energy performance of T-CO2 and ORC (R123) based scroll expanders are compared by isentropic and exergy efficiency. The results showed that isentropic and exergy efficiencies of T-CO2 were 7% and 14% higher than the R123. It also identified higher irreversibilities of T-CO2 by the exergy of the working fluids. The pressure and temperature distributions identified the over-expansion and reversed flow characteristics, and the pressure imbalance of the initial expansion chambers denoted the reversed flow.

Data-Driven model identification and efficient MPC via quasi-linear parameter varying representation for ORC waste heat recovery system

Organic Rankine Cycle (ORC) stands out in low-grade waste heat recovery (WHR) technology for its significant performance. Considering the system’s coupling dynamics, model predictive control (MPC) has adopted its wide application in realizing reasonable adjustment of the ORC based WHR system and has been proved to be effective. MPC is usually applied under the premise of having established a relatively accurate model, thus achieving a satisfactory control performance. However, the popular first principles model of the ORC system turns out to be high-dimensional and will result in computationally costly during typical nonlinear MPC adoption. While model linearization around operating points would enable the employment of linear MPC and reduce online calculation amount to some extent, the obtained local model loses global validity, leading to possible unstable control performance. To address these problems, a practical input–output data-driven quasi-linear parameter varying (QLPV) model is constructed for the ORC based WHR system by introducing the Koopman operator to ensure the global control effect. The corresponding MPC algorithm is thus presented via QLPV representation which solves the constructed constrained optimization problem iteratively in the form of a series of quadratic programming (QP) problems at each time step. Moreover, considering the possible lack of adequate training data covering the important dynamics of the ORC based WHR systems in practical application, an online updating mechanism that involves recursive equations is proposed to realize prediction accuracy improvement. Simulations on prediction, setpoint tracking and disturbance rejection are performed to verify the established model accuracy and the control effectiveness of the proposed strategy.

A Self-Stratified Thermally Regenerative Battery Using Nanoprism Cu Covering Ni Electrodes for Low-Grade Waste Heat Recovery

Developing a low-cost and high-performance thermally regenerative battery (TRB) is significant for recovering low-grade waste heat. A self-stratified TRB induced by the density difference between electrolytes is proposed to remove the commercial anion exchange membrane (AEM) and avoid ammonia crossover. The simulation and experiment results show the uneven distribution of NH3, verifying the feasibility of self-stratified electrolytes. For better power generation performance, nanoprism Cu covering Ni electrodes with a high specific surface area and a stable framework are adopted to provide more reaction active sites for fast charge transfer during discharge. A maximum power density (12.7 mW cm-2) and a theoretical heat-to-electricity conversion efficiency of 2.4% (relative to Carnot efficiency of 27.5%) are obtained in the self-stratified TRB employing nanoprism Cu covering Ni electrodes. Moreover, the cost-effectiveness, simple structure, and sustainable discharge operation indicate that it will be a potential choice for energy conversion from low-grade heat.

Experimental study on performances of flat-plate pulsating heat pipes without and with thermoelectric generators for low-grade waste heat recovery

Low-grade waste heat recovery has significant importance to energy saving and emission reduction. Due to the lack of cost-effective heat recovery technology, a lot of low-grade heat is wasted. In this paper, performances of 3D-printed flat-plate PHPs and computerized numerical control (CNC) machined flat-plate PHPs without and with thermoelectric generators (TEGs) under four recycling modes were studied. The effects of the type and number of flat-plate PHPs, condenser section length, recycling mode, flow arrangement, velocity and temperature of hot/cold air, the number and layer number of TEGs on the performances of the waste heat recovery system were studied to maximize its performance. Results indicate that the performance of the heat recovery system with CNC machined flat-plate PHP is generally better than that with 3D-printed flat-plate PHP, and their maximum effective thermal conductivities can reach 585 and 541 W/(m·K), respectively. Performances of flat-plate PHPs based waste heat recovery system can be improved by using slotted fins, increasing the condenser section length and flat-plate PHPs number. For the air-to-air heat recovery system with three flat-plate PHPs, the maximum heat transfer rate and heat recovery rate can be up to 303 W and 85.3%, respectively. As for heat-to-power waste heat recovery under Mode 3, flat-plate PHP coupled with three TEGs can achieve its maximum open-circuit voltage of 7.6 V and output power of 1.98 W. Preliminary economic analysis proves that flat-plate PHPs based waste heat recovery system displays good economic benefits. These results may be helpful in designing a cost-effective flat-plate PHPs based low-grade waste heat recovery system.

Enhancing low-grade waste heat recovery by lead-free ferroelectric Ba(Zr0.1Ti0.9)O3 with Sr and Ca isovalent dopants

Kim's novel electrothermodynamic cycle enables the Ba(Zr0.1Ti0.9)O3 ceramic to achieve a significantly high energy density at temperatures 20–120 °C by utilizing its mixed ferroelectric-relaxor behavior. This allows the material to replace lead-based compositions and provides a clean, environmentally friendly, sustainable power source from low-grade waste heat for technologies like the Internet of Things. However, this still does not represent the highest potential of this material. In this research, our group utilizes the smaller ionic radii of the isovalent dopants Sr and Ca to enhance the relaxor behavior, hence improving the pyroelectric conversion ability of Ba(Zr0.1Ti0.9)O3. Actual measurements using the Olsen and Kim cycles revealed enhancement by factors of 1.29 and 1.55 for each cycle due to the improvement in remanent and saturated polarization and temperature dependence of the pyroelectricity. The results suggested that lead-free Ba(Zr0.1Ti0.9)O3 has promising potential as a critical pyroelectric waste heat recovery-based element.

Physics-guided co-designing flexible thermoelectrics with techno-economic sustainability for low-grade heat harvesting.

Flexible thermoelectric harvesting of omnipresent spatial thermodynamic energy, though promising in low-grade waste heat recovery (<100°C), is still far from industrialization because of its unequivocal cost-ineffectiveness caused by low thermoelectric efficiency and power-cost coupled device topology. Here, we demonstrate unconventional upcycling of low-grade heat via physics-guided rationalized flexible thermoelectrics, without increasing total heat input or tailoring material properties, into electricity with a power-cost ratio (W/US$) enhancement of 25.3% compared to conventional counterparts. The reduced material usage (44%) contributes to device power-cost "decoupling," leading to geometry-dependent optimal electrical matching for output maximization. This offers an energy consumption reduction (19.3%), electricity savings (0.24 kWh W-1), and CO2 emission reduction (0.17kg W-1) for large-scale industrial production, fundamentally reshaping the R&D route of flexible thermoelectrics for techno-economic sustainable heat harvesting. Our findings highlight a facile yet cost-effective strategy not only for low-grade heat harvesting but also for electronic co-design in heat management/recovery frontiers.

Experimental evaluation of a variable-stage open absorption heat pump system utilizing aqueous KCOOH solution as absorbent

Open absorption technology is a potential alternative for the heat recovery of flue gas, which is one of the main carriers for the low-grade waste heat. A variable-stage open absorption heat pump was proposed to work on wide working condition ranges by adjusting its equivalent stage through a low-pressure generator-absorber assembly. We have demonstrated that with LiBr/H2O as absorbent, the system could achieve COP of 1.2–2, corresponding to an equivalent stage of 1–4.6. In this paper, an experimental prototype was established, including a solution distribution regulation, which aimed to optimize the solution distribution ratio between the spraying solution in the absorber and circulation solution in the cycle and further to enhance the system performance. KCOOH/H2O was selected as the absorbent to overcome the challenges of crystallization and corrosion. Experimental results showed a COP of 1.17–1.85 and heat recovery rate of 48.7–89% with heat source temperature varied from 86 to110 °C and hot water delivery temperature varied from 55 to 70 °C. A simulation model based on the thermophysical properties of KCOOH/H2O had also been established to supplement experimental results with a wider range of external sources. Results showed that compared to LiBr/H2O system, KCOOH/H2O system had a larger working temperature range and higher heat recovery rate at the expense of a lower COP, i.e., 6–11% and 12–15% lower than that of LiBr/H2O system under SDR of 0.2 and 0.5 respectively. A wider concentration working range of KCOOH/H2O (56.4%-79.8%), temperature range of 50–95 °C, and relative humidity range of 10–100% had been simulated to obtain performance maps of VS-OAHP system in the form of psychrometric chart. This work provided guidance for the practical operation and efficiency improvement for low-grade waste heat recovery.

Significant optimization of active thermomagnetic generator for low-grade waste heat recovery

Recently, thermomagnetic generator (TMG) based on the thermomagnetic effect is becoming a promising technology for low-grade waste heat recovery. However, poor performance still hinders the application of this novel technology. The performance of TMG is significantly affected by the key device parameters and material properties. Unfortunately, there is a lack of systematic research in this regard. In present work, for the first time, we systematically study the effects of different key device and material parameters on the TMG performance through a combination of experiment and finite element simulation. It demonstrates that the TMG performance is influenced largely by both device and material parameters, especially the cabin size and specific heat capacity. Furthermore, the key device parameters are optimized, i.e., the sample cabin size is shortened from 145 mm to 20 mm, the system temperature range is expanded from 288 ∼ 353 K to 276 ∼ 361 K, and the cycle period is reduced from 60 s to 10 s. In comparison with the non-optimized experimental peak I of 28.8 μA, the simulated peak I increases largely to 136.1 μA after the optimization. In addition, more I peaks are obtained by shortening the cycle period. The maximum power density Pd and average Pd are 3.6 mW/m3 K and 0.4 mW/m3 K, respectively, which are one order of magnitude higher than the values before optimization (0.16 mW/m3 K and 0.01 mW/m3 K), and even three orders of magnitude higher than those of other reported TMGs.

Advanced Exergy Analysis of an Absorption Chiller/Kalina Cycle Integrated System for Low-Grade Waste Heat Recovery

Exergy analysis and advanced exergy analysis of an absorption chiller/Kalina cycle integrated system are conducted in this research. The exergy destruction of each component and overall exergy efficiency of the cascade process have been obtained. Advanced exergy analysis investigates the interactions among different components and the actual improvement potential. Results show that among all the equipment, the largest exergy destruction is in the generators and absorber. System exergy efficiency is obtained as 35.52%. Advanced analysis results show that the endogenous exergy destruction is dominant in each component. Interconnections among different components are not significant but very complicated. It is suggested that the improvement priority should be given to the turbine. Performance improvement of this low-grade waste heat recovery process is still necessary because around 1/4 of the total exergy destruction can be avoided. Exergy and advanced exergy analysis in this work locates the position of exergy destruction, quantifies the process irreversibility, presents the component interactions and finds out the system improvement potential. This research provides detailed and useful information about this absorption chiller/Kalina cycle integrated system.

Parametric investigation of charging and discharging performances of a cascaded packed bed thermal energy storage system

The packed bed thermal energy storage (PBTES) system employing cascaded phase change material (PCM) is useful for low-grade waste heat recovery and utilization. The performance of a cascaded PBTES system is parametrically investigated in the present study. A one-dimensional concentric dispersion model is developed and validated by the experimental results. The average heat transfer rates of a cascaded PBTES system at various volumetric ratios of PCM capsules are numerically investigated, and the cascaded PBTES system of a volumetric ratio of hydroquinone: adipic acid: erythritol = 3:6:1 is found to be the best configuration in the cyclic charging and discharging processes. Furthermore, the comparison of the thermal behaviors of the cascaded and non-cascaded PBTES systems shows that the PCMs in each layer of a cascaded PBTES system complete the phase change at nearly the same time, leading to a larger heat transfer rate compared to the non-cascaded one. The average heat transfer rate of a cascaded PBTES system with the volumetric ratio of 3:6:1 is about 6.96 % higher than that of the non-cascaded one for the cyclic process. In addition, the results show that the average heat transfer rate of the cyclic process of the PBTES system is improved by employing the cascaded PCMs, while the enhancement ratio of average cyclic heat transfer rate is found to decrease from 10.57 % to −1.35 % as the mass flow rate increases from 20 kg·h−1 to 100 kg·h−1 and to decrease from 10.32 % to 0.91 % as the capsule diameter increases from 20 mm to 100 mm.