Low-grade Waste Heat Recovery Research Articles

Dual-mode fast DMC algorithm for the control of ORC based waste heat recovery system

Organic Rankine Cycle (ORC) system stands out in low-grade waste heat recovery for its outstanding performance. Considering the fluctuating nature of the heat source, model predictive control (MPC) is used to control the ORC systems and has achieved good performance. However, with the increase of the number of variables and the expansion of the control/prediction horizon, MPC will take longer calculation time. This may bring about the incapacity to obtain optimization problem solutions in one sampling instant and finally, lead to the failure of the setpoint tracking. To address this problem, in this article, a dual-mode fast dynamic matrix control (FDMC) algorithm grounded on singular value decomposition (SVD) and dynamic matrix control (DMC) is proposed. The presented algorithm features in its calculation speed improvement on the premise of ensuring tracking performance. Two simulations on disturbance rejection and setpoint tracking are performed to verify the effectiveness and rapidity of the proposed algorithm in comparison with the traditional DMC approach.

A passive evaporative cooling heat sink method for enhancing low-grade waste heat recovery capacity of thermoelectric generators

In this study, we utilized a novel passive evaporative cooling heat sink (PECHS) to enhance the waste heat recovery capacity of a thermoelectric generator (TEG). When heat is applied to the PECHS placed across the TEG, convection heat dissipation and evaporative cooling occur on the fins, thus, greatly improving the TEG output capacity. We established a theoretical model of the PECHS cooled TEG, and investigated the effects of certain key parameters on the TEG output capacity. The results show that the TEG output capacity is positively correlated with the TEG hot side temperature (TH) and fin height (Lf), and inversely correlated with the ambient relative humidity (RH), ambient temperature (Tamb), and fin thickness (tf). However, as Lf increases, the rate of increase in the cooling capability of the PECHS decreases. Moreover, RH has a small impact on the TEG output capacity. Considering the heat dissipation capacity and cost, a PECHS needs to have an optimized number of fins (M). Consequently, the TEG open-circuit voltage (Uopen = 4.86 V) and maximum output power (Pmax = 4.1 W) were obtained under fixed conditions (TH = 150 °C, RH = 50%, Tamb = 20 °C, Lf = 35 mm, tf = 1 mm, M = 11). The results show that the Uopen and Pmax are 1.83 and 3.33 times of a normal heat sink case (Uopen = 2.66 V and Pmax = 1.23 W), respectively. The theoretical model was verified using the constructed experimental system. Our findings show that PECHS is an efficient passive cooling device and can potentially be used in cooling applications such as photovoltaic board cooling and building cooling.

Performance Assessment and Working Fluid Selection for Novel Integrated Vapor Compression Cycle and Organic Rankine Cycle for Ultra Low Grade Waste Heat Recovery

This paper presents the performance assessment and working fluid selection for a novel integrated vapor compression cycle-organic Rankine cycle system (i-VCC-ORC), which recovers ultra-low-temperature waste heat rejected (50 °C) by the condenser of a vapor compression cycle (VCC). The analyses are carried out for a vapor compression cycle of a refrigeration capacity (heat input) of 35kW along with the component sizing of the organic Rankine cycle (ORC). The effects of the operational parameters on integrated system performance were investigated. The integrated system performance is estimated in terms of net COP, cycle thermal efficiency and exergy efficiency by completely utilizing and recovering the heat rejected by the condenser of the VCC system. R600a-R141b with COPnet (3.54) and ORC thermal efficiency (3.05%) is found to be the most suitable VCC-ORC working fluid pair. The integration of the vapor compression refrigeration cycle with the organic Rankine cycle increases the COP of the system by 12.5% as compared to the standalone COP of the vapor compression system. Moreover, the sensitivity analysis results show that there exists an optimum operating condition that maximizes the thermal performance of the integrated system.

Thermo-fluidic characteristics and performance in a distribute heating bubble pump generator

In some thermally driven two-phase natural circulation systems, bubble pumps serve as the key driving powers for the cycles. Recently a type of distribute heating bubble pump generator (BPG) is gradually receiving attention due to its compact structure and great potentials to utilize solar energy and low-grade waste heat recovery. The BPG provides a variety of promising features (e.g., passive heat transfer, enhanced reliability), which can benefit the advancing of heat transfer technology. For the primary study, we performed an experiment in a distributed heating BPG. Through utilizing multiple lift tubes and partial visualization configurations, it provides accesses to observe the flow pattern transition and monitor the flow instability, and thus to explore some of the underlying mechanisms affecting BPG performance.Results showed that heat input and immersion height were crucial parameters to enable the operation of distribute heating BPG. With low heat input or high inlet water subcooling level, the flow within the pump was unstable with intermittent flow interruptions. As the heat input increased, the fluid flow became more stable, the vapor generation increased linearly, while the lifted liquid flow rate initially increased then decreased. Correspondingly, the flow pattern at the outlet section of lift tubes gradually changed from slug flow to churn flow, and then to annular flow. The higher of the immersion was, the higher heat input was needed for the flow pattern transition. It was in the churn flow regime at the outlet of lift tubes for the BPG to lift a maximum liquid. At lower immersion level, liquid reflux in the lift tubes was obvious and affected the flow stability as well as the lifting performance. At higher immersion level, the fluid flow was more stable and faster, which lifted more liquid while generated less vapor depending on the inlet subcooling. In general, the BPG showed better performance (both the lifted liquid and vapor generation increased) at smaller inlet subcooling level or lower system pressure. This study highlights the flow pattern evolution and flow stability, which is helpful to the reliable design and effective operation of the distributed heating BPG.

Nearly isotropic transport properties in anisotropically structured n-type single-crystalline Mg3Sb2

As a top candidate for competing with the conventional n-Bi2Te3 thermoelectrics, Mg3Sb2-based materials have attracted increasing attentions for low-grade (<300 °C) waste heat recovery applications, due to the high thermoelectric performance, low cost, abundance and nontoxicity. Because of its anisotropic crystal structure as that of Bi2Te3, possible property anisotropy and the resultant similar requirement of texturing for preferential performance remain not entirely clear Mg3Sb2-thermoelectrics at working temperatures. This motivates the current work to focus on the transport-property anisotropy of n-type Mg3Sb2, centimeter-sized single crystals of which are successfully grown by a flux-assisted vertical Bridgman technique with a post-annealing under a Mg vapor pressure. This enables a revelation of nearly isotropic transport properties in this anisotropically structured material, guaranteeing reasonably high performances in polycrystalline materials achievable by cheap and scalable processing approaches such as powder-metallurgy. In addition, this work uncovers the strong lattice anharmonicity and large band degeneracy are respectively the origins for the low lattice thermal conductivity and superior electronic performance.

Leveraging the entropy generation minimization and designed porous media for the optimization of heat sinks employed in low-grade waste heat harvesting

• A porous media model and optimization procedure to design heat sinks is presented • Different geometries of the heat sinks are treated within the same framework • Which geometry should be employed depends on operational conditions and size • Entropy generation is a reliable metric for low-grade waste heat recovery systems A comprehensive optimization procedure for the design of heat sinks used to harvest low-grade waste heat for secondary processes is proposed. A designed porous medium approach is used, providing modelling versatility and allowing for the treatment of several internal geometries within the same framework. Two applications, taken from the cooling of high concentration photovoltaic cells and membrane distillation, are used to illustrate the proposed procedure. Three internal heat sink geometries, namely plate fins, pin fins, and metal foams, are analyzed, and the selection as to which one is the more advantageous is shown to be dependent on size and operational conditions. Possible pitfalls of a traditional design procedure are pointed out, for heat transfer capability, under high heat load condition, should be emphasized over fluid friction avoidance. The entropy generation rate is then shown to be a reliable metric when designing for low-grade waste heat recovery, automatically focusing on either viscous dissipation or thermal resistance minimization without introducing any artificial figure of merit for that purpose.

Enhancing energo-exergo-economic performance of Kalina cycle for low- to high-grade waste heat recovery: Design and optimization through deep learning methods

The Kalina cycle has proven to be a reliable bottoming cycle for low-grade waste heat recovery. However, compared with other recovery cycles (e.g. the Rankine cycle), it is characterized by a lower efficiency rate and constraints on the heating medium inlet temperature. This study proposes a systematic method for configuring and optimizing three novel Kalina-trilateral-based systems to overcome those disadvantages. This accurate technique integrates thermodynamics with deep learning to accelerate the computation process. First, the actual thermodynamic-economic features of the alternative systems are modeled through the analyses of energy, exergy, and economy. Second, the surrogate models of the systems are developed through a long short-term memory (LSTM) network. Third, the direct and hybrid optimization algorithms are applied separately to the actual and surrogate models. The objective functions yield thermal efficiency, exergy efficiency, and distributed payback time. Moreover, the strength Pareto evolutionary algorithm (SPEA-II) is employed to solve the multi-objective optimization problem. According to the results, the LSTM network had considerable power to simulate and predict the energo-exergo-economic performance of an energy system. Furthermore, the computation time was much shorter for a hybrid algorithm with the maintained accuracy. Consequently, the heating source temperature constraint was eliminated in the final alternative cycle (KTS-36). Compared with the base system (KCS-34), the thermodynamic and economic objective functions were improved by 74.3% and 34%, respectively.

Enhancement of low grade waste heat recovery from a fuel cell using a thermoelectric generator module with swirl flows

This research analysed the enhancement to the power output of a thermoelectric generator (TEG) module in converting waste heat from a low temperature Polymer Electrolyte Membrane fuel cell into electricity assisted by swirl flows (swirl strength from 1.2 to 1.6). The module was constructed using a single TEG cell, a heat pipe and a finned heat sink. The study applied the scenario where the fuel cell operated as the power source for a mini fuel cell vehicle. This scenario provided a practical evaluation of the module performance as the hot stream temperature was varied due to the changes in fuel cell power while the cooling air varied according to the vehicle drive speed. The maximum power was enhanced from 200 μW (for non-swirl condition at 60 °C) to 3056 μW as the swirl strength increased to 1.6. Significant power enhancement factor by 8–17 in stationary mode and between a factor of 1.4–2.2 at the speed of 10 ms−1 were obtained with a non-linear relationship to the swirl strengths and cooling modes. Therefore, the introduction of swirl to the hot air stream is a viable approach to provide significant enhancements in low grade waste heat recovery using TEG.

Multi- objective optimization and life cycle assessment of a cascade system integrating LiBr/H2O absorption refrigeration with transcritical CO2 power cycle

Energy, economic and environmental (3E) analysis of a cascade system combining absorption refrigeration (AR) and transcritical CO2 power cycle (TCPC) for low-grade waste heat recovery (WHR) is conducted. Life cycle assessment (LCA) is applied to evaluate environmental performance. A novel comprehensive index Eco-indicator 16 (EI16) integrating seventeen environmental impacts was proposed which involves human health, ecosystem and resource. A multi-objection optimization model with thermal efficiency, annual total cost and EI16 as three objection functions is established and solved by Non-dominated sorting genetic algorithm II (NSGA-II). The final optimal solution is obtained after decision making with Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) and Shannon entropy method. The results show that payback time can be less than 7 years and there is a synergistic relationship between energy efficiency and environmental performance, while energy efficiency and economic performance are mutually contradictory. The final optimal solution achieves an increase by 13.17% in thermal efficiency and a decrease by 13.49% in Eco-indicator 16 at cost of an 8.94% increase in annual total cost. In all environmental impact categories, the contribution to the climate change is the largest, which is beneficial to the human health and ecosystem protection.

Cycle analysis and economic evaluation for seawater-LNG Organic Rankine Cycles

An Organic Rankine Cycle (ORC) has a variety of environmental and operating conditions, which result in different efficiency and net work of cycle. In this regard, it is important to determine the optimal operating conditions with the consideration of various operating variables since the outcomes of the low-grade waste heat recovery systems are low as well as highly dependent on the conditions. In this study, the ORC having seawater as a heat source and LNG cold energy as a heat sink is analyzed with cycle parameters as well as the environmental conditions such as seawater temperature. An ORC Economic evaluation Code (OEC) is developed to consider the effects of numerous parameters such as the pump outlet pressure, mass flow rate, pinch point, materials of the heat exchangers, the type of working fluid, cycle configuration, and seawater temperature simultaneously. Based on the analysis, an economic assessment is conducted, so we could find the optimal conditions for the designed ORC systems. Also, it is found that the seawater temperature variation could affect about 23.8% in terms of net work compared to fixed heat source temperature conditions.

Proposal and comprehensive analysis of an innovative steam generation system by deep recovery of low-grade waste heat

The waste heat generated by industrial processes is often not fully recycled, resulting in a significant amount of wasted energy and thermal pollution. Meanwhile, low-pressure steam plays an important role in industry as many processes consume a large amount of steam. An innovative steam generation system based on the deep recovery of low-grade industrial waste hot water is proposed. The waste hot water first flashes out low-pressure steam, and then drives an organic Rankine cycle to generate mechanical work, which is used to compress the low-pressure steam. A techno-economic model is established. The thermodynamic indicators are calculated under preliminary optimization conditions, and a comparative analysis between the proposed system and other waste heat utilization systems is carried out under the same heat source conditions. The techno-economic analysis shows that the proposed system is better for recycling wasted hot water. The steam generation mass ratio, exergy efficiency, and cost per ton of recycled steam are 2.50%, 44.31%, and 7.67 $/ton, respectively, under the optimized conditions. When the heat source temperature is 100 °C, the exergy efficiency of the proposed system is 37.01% and 60.59% higher than that of the steam generation heat pump system and waste heat power generation system, respectively. Advanced exergy analyses are conducted to determine the actual potential improvement for system components, with the avoidable exergy destruction of the expander showing the largest. This paper proposes an efficient and cost-competitive approach for the deep recovery of low-temperature hot water for producing industrial steam.

A fluidized-bed reactor for enhanced mass transfer and increased performance in thermally regenerative batteries for low-grade waste heat recovery

A thermally regenerative battery with a fluidized-bed reactor design (TRB-FB) is developed to enhance mass transfer and promote battery performance. The results demonstrate that the fluidized-bed design induces highly efficient mass transfer and improves the battery performance considerably. Comparative tests show that the highest power density (11.6 W m−2), total charge (274.6 C), and energy density based on the anolyte volume (834.2 Wh m−3) are obtained with the TRB-FB. Moreover, reversibility testing over successive cycles reveals that the TRB-FB generates a relatively stable power output over 15 batch cycles. Although a decreased particle diameter induces better fluidization, highly efficient mass transfer, and improved performance, an exceedingly small particle diameter results in decreased fluidization and a significant dip in battery performance. The optimal particle diameter is determined to be 187.5 μm. In addition, increasing the stirring rate within a certain range further enhances mass transfer and promotes battery performance.

District heating potential in the case of low-grade waste heat recovery from energy intensive industries

• Low-grade Waste Heat Recovery (WHR) increases the efficiency of cogeneration units. • District Heating (DH) potential of Italian pulp and paper sector is assessed. • Data of 50 Italian pulp and paper mills are considered in this study. • About 143 kTOE can be saved by using the low-grade waste heat of CHP units for DH. • Combining energy intensive industries and DH can encourage low carbon districts. Waste Heat Recovery (WHR) from energy intensive industries has a great potential in curbing CO2 emissions. Among the different solutions, District Heating (DH) is considered of major interest, satisfying the heating demand of users in the proximity of power plants. Considering the energy intensity of the pulp and paper industry, a method for evaluating the recovery potential of its low-grade waste heat from cogeneration plants in DH is presented. The proposed method allows to evaluate the thermal power from cogeneration plants to end users and to assess the potential maximum number of residential buildings that could be connected to each DH network. Based on the proposed method, the benefits of the WHR are evaluated from both energy and environmental points of view. More precisely, considering 50 pulp and paper mills in Italy under investigation in the present analysis, a yearly natural gas saving corresponding to 143.76 kTonnes of Oil Equivalent (TOE) and 333.11 ktCO2 is obtained. In case of WHR, the average Primary Energy Saving (PES) of the cogeneration plants increases from 0.14 up to 0.22. In particular, cogeneration units based on steam turbine technology show the greatest improvement, since its average PES moved from 0 up to almost 0.1.

Characterization analysis of dynamic behavior of basic ORC under fluctuating heat source

Organic Rankine Cycle (ORC) is widely investigated for low-grade waste heat recovery, but its performance deteriorates under heat source with thermal power fluctuations. As the effects of fluctuating heat source on ORC system performance is less investigated, the operating rules of ORC systems under fluctuating heat source are not clear. In this study, the effects of fluctuating amplitude and period for fluctuating heat sources on the performance of a basic ORC system are studied in detail, considering the proposed indicators including safety and efficiency. Afterwards, the allowable fluctuation ranges of heat source are obtained based on requirements of safety and efficiency. Finally, the impact of the average temperature of fluctuating heat sources on the allowable fluctuation range is systematically investigated. The results indicate that the safety limitations and efficiency of the basic ORC system are generally negatively related to fluctuating amplitude and period, and the allowable fluctuation range is generally positively related to average temperature of heat source. To ensure the efficient operation of basic ORC, the allowable relative amplitude is 25–40 K under large fluctuating period while it is higher than 35 K with the fluctuating period smaller than 200 s. This study contributes to provide valuable reference for evaluating the performance of an existing ORC system under different fluctuating heat source conditions.

Overview of the Development and Application of the Twin Screw Expander

With the development of society, the energy crisis has become increasingly prominent, which greatly affects the sustainable development of the economy of various countries. Industrial energy consumption accounts for more than 70% of China’s total energy consumption, of which more than 50% is converted to industrial waste heat, and recyclable waste heat resources account for about 60% of the total waste heat resources, while China’s current utilization rate of industrial waste heat only reaches about 30%. The development of renewable energy and recovery of low-grade waste heat in industry is the key to solve the problem. As a type of volumetric expander with full flow expansion, the screw expander is extensively applied in the industrial waste heat recovery and geothermal energy generation industry because of its effective utilization of low enthalpy energy. Improving the performance of the screw expander as the core, the paper concludes and summarizes the research status of the leakage, rotor geometry, sealing and lubrication, processing and manufacturing, which can affect the performance of the screw expander. In addition, it also introduces the application status and potential utilization of screw expander.

Performance analysis of a cascade lithium bromide absorption refrigeration/dehumidification process driven by low-grade waste heat for hot summer and cold winter climate area in China

Recovery of low-grade waste heat has great potential to reduce energy consumption around the world. In this paper, a novel cascade lithium bromide absorption refrigeration/dehumidification system is developed to recover low-grade waste heat with temperature range from 90 °C to 150 °C for dehumidification in hot summer and cold winter climate area of China. This system consists of two subsystems: lithium bromide absorption refrigeration system and lithium chloride liquid dehumidification system. The low-grade waste heat is applied by two subsystems in a cascade approach. The key parameters in terms of air temperature, air humidity, solution concentration, and solution temperature are investigated to obtain the optimization design of the proposed system. Performance of the proposed system is analyzed for different refrigeration temperatures. The results show that the max coefficient of performance for the proposed system is 0.6029. The moisture removal rate and the humidity efficiency are 20.20 g/kg and 0.9385. The novel coupling system develops a creative and efficient way to recover low-grade waste heat for dehumidification in hot summer and cold winter climate area of China.

Performance of a Thermally Regenerative Battery with 3D-Printed Cu/C Composite Electrodes: Effect of Electrode Pore Size

A thermally regenerative ammonia-based battery (TRAB) is a new electrochemical energy device used for the recovery of low-grade waste heat. The use of a three-dimensional (3D)-printed Cu/C composit...

Development and validation of a TRNSYS type to simulate heat pipe heat exchangers in transient applications of waste heat recovery

Heat pipe heat exchangers (HPHEs) are being more frequently used in energy intensive industries as a method of low-grade waste heat recovery. Prior to the installation of a HPHE, the effect of the heat exchanger within the system requires modelling to simulate the overall impact. From this, potential savings and emission reductions can be determined, and the utilisation of the waste heat can be optimised. One such simulation software is TRNSYS. Currently available heat exchanger simulation components in TRNSYS use averaged values such as a constant effectiveness, constant heat transfer coefficient or conductance for the inputs, which are fixed during the entire simulation. These predictions are useful in a steady-state controlled temperature environment such as a heat treatment facility, but not optimal for the majority of energy recovery applications which operate with fluctuating conditions. A transient TRNSYS HPHE component has been developed using the Effectiveness-Number of Transfer Units (ɛ-NTU) method and validated against experimental results. The model predicts outlet temperatures and energy recovery well within an accuracy of 15% and an average of 4.4% error when compared to existing experimental results, which is acceptable for engineering applications.

Design and Operational Control Strategy for Optimum Off-Design Performance of an ORC Plant for Low-Grade Waste Heat Recovery

The applicability of organic Rankine cycle (ORC) technology to waste heat recovery (WHR) is currently experiencing growing interest and accelerated technological development. The utilization of low-to-medium grade thermal energy sources, especially in the presence of heat source intermittency in applications where the thermal source is characterized by highly variable thermodynamic conditions, requires a control strategy for off-design operation to achieve optimal ORC power-unit performance. This paper presents a validated comprehensive model for off-design analysis of an ORC power-unit, with R236fa as the working fluid, a gear pump, and a 1.5 kW sliding vane rotary expander (SVRE) for WHR from the exhaust gases of a light-duty internal combustion engine. Model validation is performed using data from an extensive experimental campaign on both the rotary equipment (pump, expander) and the remainder components of the plant, namely the heat recovery vapor generator (HRVH), condenser, reservoirs, and piping. Based on the validated computational platform, the benefits on the ORC plant net power output and efficiency of either a variable permeability expander or of sliding vane rotary pump optimization are assessed. The novelty introduced by this optimization strategy is that the evaluations are conducted by a numerical model, which reproduces the real features of the ORC plant. This approach ensures an analysis of the whole system both from a plant and cycle point of view, catching some real aspects that are otherwise undetectable. These optimization strategies are considered as a baseline ORC plant that suffers low expander efficiency (30%) and a large parasitic pumping power, with a backwork ratio (BWR) of up to 60%. It is found that the benefits on the expander power arising from a lower permeability combined with a lower energy demand by the pump (20% of BWR) for circulation of the working fluid allows a better recovery performance for the ORC plant with respect to the baseline case. Adopting the optimization strategies, the average efficiency and maximum generated power increase from 1.5% to 3.5% and from 400 to 1100 W, respectively. These performances are in accordance with the plant efficiencies found in the experimental works in the literature, which vary between 1.6% and 6.5% for similar applications. Nonetheless, there is still room for improvement regarding a proper design of rotary machines, which can be redesigned considering the indications resulting from the developed optimization analysis.

Energy Storage of Low Potential Heat using Lithium Hydroxide Based Sorbent for Domestic Heat Supply

With the increasingly energy consumption and environmental degradation, the recovery of low-grade industrial waste heat and the development and utilisation of renewables have become urgent needs. At present, the recovery of waste heat is mainly at high temperature, and the recovery rate of waste heat at medium and low temperatures is low. Adsorption thermochemical energy storage (TCES) is a pivotal technology applied for long-term thermal energy storage based on the reversible gas-solid reaction at mid-to low-temperature. LiOH·H2O is a potential thermochemical material (TCM) candidate because of the high energy storage density (ESD). In this paper, the expanded graphite (EG) is used as supporting matrix to impregnate LiOH to further improve its heat and mass transfer. The water vapour adsorption properties such as sorption capacity and sorption isotherms of the consolidated composite sorbents with EG contents of 0, 5, 8, 12, and 15 wt% were tested. It was found that the samples with 5% and 8% wt% EG have superior sorption capacity and kinetics at various relative humidities (RHs), and the maximum water uptake substantially decreased with increasing temperature. The cyclability results showed 90% and 92% of the original ESDs are retained after 12 cycles for the samples with 8% and 15% wt% EG, suggesting good stability of composite sorbent. The ad/desorption isobars processes also proved the existence of the sorption hysteresis loop, and this phenomenon is more obvious at low pressure. The data fitting results revealed that the kinetics coefficient of the promising composite sorbent containing 8 wt% EG is 0.01671 s – 1 at 30 °C with 58% RH. Compared with the sensible or latent energy storage, the heating temperature produced from the sorbent is adjustable for different purposes. Considering the low emissions and pollution of LiOH-based heat production compared to the coal-fire system, i.e., 1262 kg pollutants are theoretically reduced for a household per year, it is a clean alternative for the residential heat supply.