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Limited experimental research has been conducted on organic Rankine cycle (ORC) systems that use wet working fluids. Therefore, the present study examined how the performance of an ORC system that uses a wet working fluid (R134a) was affected by the superheat degree ratio (SDR) under various scroll rotation speeds. The SDR is the dimensionless ratio between superheat degree and evaporation temperature at a given heat source temperature. Experimental results indicated that at scroll rotation speeds of 900, 1350, and 1800 rpm, the maximum output power of the aforementioned system was 1103, 1464, and 1537 W, respectively, with SDRs of 0.49, 0.49, and 0.54, respectively. The maximum net efficiencies at these speeds were 5.87%, 5.91%, and 5.32%, respectively, which occurred at SDRs of 0.61, 0.49, and 0.48, respectively. This level of system performance was attributable to the high enthalpy at the expander inlet and the high mass flow rate at the high evaporation pressure under an SDR of approximately 0.5. Although increasing the SDR did not enhance the scroll expander's isentropic efficiency, this efficiency decreased considerably when the SDR fell below 0.2. These findings emphasize the importance of optimizing the SDR of ORC systems to improve their performance. [ABSTRACT FROM AUTHOR]

A newly developed technology, thermo‐osmotic energy conversion (TOEC), is supposed to convert low‐grade heat into power. However, the performance of existing TOEC experiments is deficient. This paper discusses the feasibility of strengthening TOEC by applying negative pressure to the feed liquid, which can reduce air pressure in the membrane pores and molecular diffusion resistance. Theoretical calculation shows that when the cooling and heating temperatures are 40 and 80 °C, respectively, and the transmembrane pressure difference is 5.0 MPa, the TOEC system with a negative pressure of 0.5 bar at the feed side can approach an efficiency of 3.01% and a power density of 16.85 W m−2, which increases by 20% and 27% compared with no negative pressure, respectively. Given the nonuniformity in the real system, computational fluid dynamics simulation is used to obtain the correction factor, which is then used to revise the theory prediction results for the first time. Moreover, a lab‐scale experiment also proves that a negative pressure at the feed benefits the performance of the TOEC device. Overall, this research presents a feasible method to enhance a TOEC system, which may promote the development of a more‐efficiently TOEC system for low‐grade heat utilization. [ABSTRACT FROM AUTHOR]

Thermal energy is abundantly available in our daily life and industrial production, and especially, low‐grade heat is often regarded as a byproduct. Collecting and utilizing this ignored energy by low‐cost and simple technologies may become a smart countermeasure to relieve the energy crisis. Here, a unique device has been demonstrated to achieve high value‐added conversion of low‐grade heat by introducing redox‐active organic alizarin (AZ) onto N‐doped hollow carbon nanofibers (N–HCNF) surface. As‐prepared N–HCNF/AZ can deliver a high specific capacitance of 514.3 F g−1 (at 1 A g−1) and an outstanding rate capability of 60.3% even at 50 A g−1. Meanwhile, the assembled symmetric proton capacitor can deliver a high energy density of 28.0 Wh kg−1 at 350.0 W kg−1 and a maximum power density of 35.0 kW kg−1 at 17.0 Wh kg−1. Significantly, the thermally chargeable proton capacitors can attain a surprisingly high Seebeck coefficient of 15.3 mV K–1 and a power factor of 6.02 µW g–1. Taking advantage of such high performance, a satisfying open‐circuit voltage of 481.0 mV with a temperature difference of 54 K is achieved. This research provides new insights into construction of high value‐added energy systems requiring high electrochemical performances. [ABSTRACT FROM AUTHOR]

Liquid‐based thermoelectric cells integrate energy harvesting and storage technology, becoming the focus of energy fields. However, the simultaneous implement of high output voltage and heat‐to‐electricity efficiency is still challenging. Herein, we propose a lithium‐ion thermal charging cell using lithium anode and electrospun zinc vanadate@carbon nanofiber cathode. Benefitting from the superior performance of electrode and the significant difference between the size of cation and anion in electrolyte, such system can continuously realize the harvesting and conversion of low‐grade heat into electricity. Consequently, an open‐circuit voltage of about 2.1 V can be generated at a low temperature gradient of 25 K together with an ultrahigh thermopower of 47.9 mV K−1 and a remarkable power density of 9.2 W m−2. Moreover, an impressive Carnot‐related efficiency (5.2 %) can be achieved under same conditions. This work confirms the superiority of organic thermoelectric system and provides a new insight to develop promising thermal charging cell for sustainable applications. [ABSTRACT FROM AUTHOR]

A thermodynamic model of basic and recuperative transcritical organic Rankine cycles (TRCs) associated with using five pure and six mixed fluids as working fluids has been developed. This model can be employed to investigate the effects of recuperators at inlet expander temperatures (Texp,in) of 150–200°C and inlet expander pressures (Pexp,in) of 3.7–6.9 MPa. The ratio of change (ROC) of the first‐ and second‐law efficiencies (ηI and ηII) was positively correlated with the heat transfer rate of the recuperator and exhibited an opposite trend for a specific volume ratio. ROC was substantially affected by operating parameters and working fluid. However, the recuperator heat transfer rate was negligibly affected by the mixture temperature glide (Tglide). A universal empirical equation of ηI,II was proposed for both TRC configurations. The equation, a function of the specific volume ratio, can predict the system efficiencies for pure and mixed fluids, even if a mixture has an arbitrary mole fraction. As Tglide of R600a‐base mixture and R245fa/R134a was lower than 16 K, they had low error rates and low standard deviations. Finally, the equation was highly accurate in ηII prediction, particularly at a Texp,in of 160°C and 170°C and a Pexp,in of 4.9–5.8 MPa. [ABSTRACT FROM AUTHOR]

Large amounts of waste heat, below 120 °C, are released globally by industry. To convert this low‐temperature waste heat to power, thermally regenerative flow batteries (TRFBs) have recently been studied. Most analyses focus on either the discharging or the regeneration phase. However, both phases have to be considered to holistically assess the performance of the flow battery. Therefore, a dynamic, open‐access, full‐cycle model of a Cu–NH3 TRFB is developed in Modelica and validated with data from the literature. Based on the validated model, a trade‐off between power density and efficiency is shown that depends only on the discharging strategy of the flow battery. For a sensible heat source with an inlet temperature of 120 °C and heat transfer at a thermodynamic mean temperature of about 90 °C, the power density reaches 38 W m−2 over a complete cycle, and the efficiency reaches 20% of Carnot efficiency. In a benchmarking study, the power production of the flow battery is shown to already achieve 34% of a fully optimized organic Rankine cycle. Thus, TRFBs require further optimization to become a competitive technology for power production and energy storage from low‐temperature waste heat. [ABSTRACT FROM AUTHOR]

Harvesting heat from the low‐grade heat (below 100 °C) into electricity has the signification to enhance the utilization of energy and lower carbon emissions by a simple device. Herein we demonstrate a thermally chargeable ammonium ion capacitor (TAIC) by employing graphene‐polyimide (rGO‐PI) synthesized through polycondensation of 1,4,5,8‐naphthalenetetracarboxylic dianhydride and ethylenediamine as cold electrode, N‐doped hollow carbon nanofibers as hot electrode to directly convert waste heat into electricity. Combining thermodiffusion effect of electrolyte with thermogalvanic effect of a redox couple (−C=O/−C−O−NH4+), as‐assembled TAIC can deliver a high output voltage of 624 mV, power density of 82 μW cm−2 and average Seebeck coefficient of 9.07 mV K−1 at temperature difference of 45 K. Meanwhile, with the introduction of polyacrylamide‐polyacrylic acid‐based gel electrolyte, the assembled flexible device can well serve in various bending states, and the power density can attain a satisfying value of 1.92 μW cm−2. This quasi‐solid‐state TAIC shows great potential as one promising candidate for high value‐added conversion from low‐grade heat into electricity as well as wearable applications. [ABSTRACT FROM AUTHOR]

Second‐order and first‐order magnetocaloric materials (MCMs) not experiencing hysteresis are characterized by a reversible temperature change when exposed to an applied magnetic field. Due to this property described by the magnetocaloric effect, MCMs are used in magnetic cooling applications. Conversely, a rapid variation of the MCM's temperature around its specific Curie temperature causes a fast change in its magnetic permeability. Cycling the MCM's temperature within a magnetic field allows the possibility of inducing voltage, which would be higher for rapid cycling rates. Herein, latent heat transfer is introduced as an approach to obtain higher heating/cooling cycle frequency in a magnetocaloric generator that converts heat to electricity. In practice, rapid condensation/evaporation cycles of the water on a first‐order magnetocaloric La(Fe,Si) alloy are observed in a high vacuum system. This leads to the fast change of magnetization in an applied magnetic field from which an induced voltage is picked up. With the constructed set‐up, a peak induced voltage 2.75 mVp is obtained from low‐grade heat having reservoir temperature differences of △T = 56 °C. At higher temperature differences, a peak‐to‐peak voltage of around 3.5 mVpp at a cycle frequency of 2 Hz is achieved. [ABSTRACT FROM AUTHOR]

Summary: Thermoacoustic technology has drawn increasing attention due to its advantages such as reliability and environmental benignity. Aiming at low‐grade heat recovery, we developed a travelling‐wave thermoacoustic electric generator consisting of a looped travelling‐wave thermoacoustic engine and a linear alternator. In order to explore the operating characteristics of the electric generator, we numerically analyzed the acoustic field characteristics with a modified model. The analysis shows that high acoustic impedance appears in all three stages, and the travelling‐wave component dominates the acoustic field of the loop, which is significant for both thermoacoustic conversion and acoustic power propagation. Furthermore, we also investigated the effects of external electric compliance, resistance, and hot end temperature on the output electric power, thermal‐electric efficiency, and other related parameters. In the experiments, a thermal‐electric efficiency of 3.7% with an output electric power of 24 W has been achieved, when the hot end temperature is 120°C. The relative Carnot efficiency can exceed 14% when the hot end temperature is between 120°C and 190°C. The promising results demonstrate the significant potential of thermoacoustic electric generation in low‐grade heat recovery. [ABSTRACT FROM AUTHOR]

Summary: The industrial aerobic bioprocess of baker's yeast production requires large amounts of water to cool down large bubble column bioreactors. As result, an appreciable quantity of low‐grade heat is generated. Because of the low temperature level of the water (~25°C) exiting the bioreactors cooling system, very little attention has been dedicated to heat recovery and conversion from this stream, which is usually released in rivers, streams, and canals. In this work, we simulated the generation of low‐grade heat (up to 14.4 MW) from an industrial baker's yeast production plant consisting of seven 150‐m3 bioreactors. Subsequently, a dedicated transcritical carbon dioxide heat pump system for the conversion of this low‐grade heat into fourth generation district heat (~16.2 MW) was successfully designed. Fourth generation district heat employs low‐temperature water (30‐70°C) as heat carrier and is expected to play a major role in future sustainable energy system. Finally, an economic study confirmed the feasibility and the applicability of our approach and a concept for long‐term energy storage including state‐of‐the‐art phase change material (PCM)–units was discussed. The conversion of low‐grade heat from industrial baker's yeast production into fourth generation district heat is technically and economically feasible. The investigated process is able to generate up to 140 GWh and a potential turnover up to €9.2 million in Germany. [ABSTRACT FROM AUTHOR]

Summary: This paper focuses on the influence of regenerator on the performance of a single‐stage looped thermoacoustic engine. Numerical simulation has firstly been carried out to investigate how the regenerator efficiency, generated acoustic power, and acoustic field in the regenerator are affected by the cross‐sectional area, regenerator length, and mesh number. Based on the numerical optimization, an experimental setup with a normalized cross‐sectional area of 14.5 and a normalized regenerator length of 0.0036 has been established, and the influence of the hydraulic radius of regenerator (determined by regenerator mesh number) on the onset process and the stable oscillation of the system has been studied. The results show that the ratio of the hydraulic radius of regenerator over thermal penetration depth should be in the range of 0.2 to 0.3, in order to achieve a low onset temperature difference and a high thermal efficiency. In the experiments, an onset temperature difference of 84°C and a thermal efficiency above 4% at a temperature difference of about 200°C were obtained, showing the potential to recover low‐grade heat. Numerical and experimental investigations have been conducted to investigate the optimal length, cross‐sectional area and hydraulic radius of the regenerator in a single‐stage looped thermoacoustic engine driven by low grade heat. In the experiments, an onset temperature difference of 84°C and a thermal efficiency above 4% at a temperature difference of about 200°C can be obtained, showing the potential in low‐grade thermal energy recovery. [ABSTRACT FROM AUTHOR]