Showing papers on "Thermodynamic cycle published in 2014"

An electrochemical system for efficiently harvesting low-grade heat energy

Abstract: Efficient and low-cost thermal energy-harvesting systems are needed to utilize the tremendous low-grade heat sources. Although thermoelectric devices are attractive, its efficiency is limited by the relatively low figure-of-merit and low-temperature differential. An alternative approach is to explore thermodynamic cycles. Thermogalvanic effect, the dependence of electrode potential on temperature, can construct such cycles. In one cycle, an electrochemical cell is charged at a temperature and then discharged at a different temperature with higher cell voltage, thereby converting heat to electricity. Here we report an electrochemical system using a copper hexacyanoferrate cathode and a Cu/Cu(2+) anode to convert heat into electricity. The electrode materials have low polarization, high charge capacity, moderate temperature coefficients and low specific heat. These features lead to a high heat-to-electricity energy conversion efficiency of 5.7% when cycled between 10 and 60 °C, opening a promising way to utilize low-grade heat.

High-efficiency thermodynamic power cycles for concentrated solar power systems

Abstract: This paper provides a review of high-efficiency thermodynamic cycles and their applicability to concentrating solar power systems, primarily focusing on high-efficiency single and combined cycles. Novel approaches to power generation proposed in the literature are also highlighted. The review is followed by analyses of promising candidates, including regenerated He-Brayton, regenerated CO2-Brayton, CO2 recompression Brayton, steam Rankine, and CO2–ORC combined cycle. Steam Rankine is shown to offer higher thermal efficiencies at temperatures up to about 600 °C but requires a change in materials for components above this temperature. Above this temperature, CO2 recompression Brayton cycles are shown to have very high thermal efficiency, potentially even exceeding 60% at 30 MPa maximum pressure and above 1000 °C maximum temperature with wet cooling. An estimate of a combined receiver and power cycle operating temperature is provided for the cycles considered and compared to the traditional approach of optimization based on the Carnot efficiency. It is shown that the traditional approach to optimizing the receiver and turbine inlet temperatures based on Carnot is generally not sufficient, leading to an optimum temperature shift of more than 100 °C from the Carnot case under various conditions.

An electrochemical system for efficiently harvesting low-grade heat energy

Abstract: Efficient and low-cost thermal energy-harvesting systems are needed to utilize the tremendous low-grade heat sources. Although thermoelectric devices are attractive, its efficiency is limited by the relatively low figure-of-merit and low-temperature differential. An alternative approach is to explore thermodynamic cycles. Thermogalvanic effect, the dependence of electrode potential on temperature, can construct such cycles. In one cycle, an electrochemical cell is charged at a temperature and then discharged at a different temperature with higher cell voltage, thereby converting heat to electricity. Here we report an electrochemical system using a copper hexacyanoferrate cathode and a Cu/Cu(2+) anode to convert heat into electricity. The electrode materials have low polarization, high charge capacity, moderate temperature coefficients and low specific heat. These features lead to a high heat-to-electricity energy conversion efficiency of 5.7% when cycled between 10 and 60 °C, opening a promising way to utilize low-grade heat.

Review of thermal cycles exploiting the exergy of liquefied natural gas in the regasification process

Abstract: In the present paper, a review is carried out of the current state of thermodynamic cycles that exploit the exergy released during liquefied natural gas (LNG) regasification, with the objective of improving power plant efficiency. A study of the exergy available in the LNG is carried out and is divided in thermal and mechanical exergy. The mechanical aspect can only be recovered through the direct expansion of LNG. As for the thermal aspect, power plants based on Rankine, Brayton, and Kalina cycles, as well as combined cycles and cycles with CO2 capture are evaluated. The choice of proper cycle for a better exergetic use is linked with the quality of the heat source of the cycle. The Rankine cycle is most suitable when of low grade, while the Brayton is more appropriate when disposing of a medium or high grade heat source. When the temperature of the heat source is high, the combination of Brayton, Rankine and direct expansion is most favourable in order to obtain good efficiency. Also established is the selection criteria of the working fluids. Ethane and ethylene are put forth as the most suitable to operate with low temperature Rankine cycles and CO2 with high temperatures. In the case of Brayton cycles, helium and nitrogen are those recommended.

Membrane-Free Battery for Harvesting Low-Grade Thermal Energy

Abstract: Efficient and low-cost systems are desired to harvest the tremendous amount of energy stored in low-grade heat sources (<100 °C). An attractive approach is the thermally regenerative electrochemical cycle (TREC), which uses the dependence of electrode potential on temperature to construct a thermodynamic cycle for direct heat-to-electricity conversion. By varying the temperature, an electrochemical cell is charged at a lower voltage than discharged; thus, thermal energy is converted to electricity. Recently, a Prussian blue analog-based system with high efficiency has been demonstrated. However, the use of an ion-selective membrane in this system raises concerns about the overall cost, which is crucial for waste heat harvesting. Here, we report on a new membrane-free battery with a nickel hexacyanoferrate (NiHCF) cathode and a silver/silver chloride anode. The system has a temperature coefficient of -0.74 mV K(-1). When the battery is discharged at 15 °C and recharged at 55 °C, thermal-to-electricity conversion efficiencies of 2.6% and 3.5% are achieved with assumed heat recuperation of 50% and 70%, respctively. This work opens new opportunities for using membrane-free electrochemical systems to harvest waste heat.

Analysis of Advanced Supercritical Carbon Dioxide Power Cycles With a Bottoming Cycle for Concentrating Solar Power Applications

Abstract: A number of studies have been performed to assess the potential of using supercritical carbon dioxide (S-CO2) in closed-loop Brayton cycles for power generation. Different configurations have been examined among which recompression and partial cooling configurations have been found very promising, especially for concentrating solar power (CSP) applications. It has been demonstrated that the S-CO2 Brayton cycle using these configurations is capable of achieving more than 50% efficiency at operating conditions that could be achieved in central receiver tower type CSP systems. Although this efficiency is high, it might be further improved by considering an appropriate bottoming cycle utilizing waste heat from the top S-CO2 Brayton cycle. The organic Rankine cycle (ORC) is one alternative proposed for this purpose; however, its performance is substantially affected by the selection of the working fluid. In this paper, a simple S-CO2 Brayton cycle, a recompression S-CO2 Brayton cycle, and a partial cooling S-CO2 Brayton cycle are first simulated and compared with the available data in the literature. Then, an ORC is added to each configuration for utilizing the waste heat. Different working fluids are examined for the bottoming cycles and the operating conditions are optimized. The combined cycle efficiencies and turbine expansion ratios are compared to find the appropriate working fluids for each configuration. It is also shown that combined recompression-ORC cycle achieves higher efficiency compared with other configurations.

Potential of conformal cooling channels in rapid heat cycle molding: A review

Abstract: Rapid heat cycle molding (RHCM) can eliminate the weld line, increase flow length, and improve surface quality of the molded parts. However, the cycle time is much longer as compared to the conventional injection-molding process due to the time taken to heat up and cool down the mold. This paper reviews the applications of conformal cooling channels to reduce the cycle time in rapid and hard tooling for the conventional injection-molding process. The performance of conformal cooling channels in reducing the cycle time has been proven; however, the full potential of conformal cooling channels in RHCM is yet to be explored. © 2013 Wiley Periodicals, Inc. Adv Polym Technol 2014, 33, 21381; View this article online at wileyonlinelibrary.com. DOI 10.1002/adv.21381

Quantum Stirling heat engine and refrigerator with single and coupled spin systems

Abstract: We study the reversible quantum Stirling cycle with a single spin or two coupled spins as the working substance. With the single spin as the working substance, we find that under certain conditions the reversed cycle of a heat engine is NOT a refrigerator, this feature holds true for a Stirling heat engine with an ion trapped in a shallow potential as its working substance. The efficiency of quantum Stirling heat engine can be higher than the efficiency of the Carnot engine, but the performance coefficient of the quantum Stirling refrigerator is always lower than its classical counterpart. With two coupled spins as the working substance, we find that a heat engine can turn to a refrigerator due to the increasing of the coupling constant, this can be explained by the properties of the isothermal line in the magnetic field-entropy plane.

The Influence of Unsteadiness on the Analysis of Pressure Gain Combustion Devices

Abstract: Pressure gain combustion has been the subject of scientific study for over a century due to its promise of improved thermodynamic efficiency. In many recent application concepts, pressure gain combustion is used as a component in an otherwise continuous, normally steady flow system, such as a gas turbine or ramjet engine. However, pressure gain combustion is inherently unsteady. Failure to account for the effects of this periodic unsteadiness can lead to misunderstanding and errors in some performance calculations. This paper clarifies the accounting by presenting a consistent method of thermodynamic cycle analysis for a device using pressure gain combustion technology. The incorporation of the unsteady pressure gain combustion process into the conservation equations for a continuous flow device is presented. Most important, the appropriate method for computing the conservation of momentum is presented. It will be shown that proper, consistent analysis of cyclic conservation principles produces more reali...

New thermodynamic cycles for magnetic refrigeration

Abstract: Most of the existing prototype devices for magnetic refrigeration are based on a thermodynamic cycle with an active magnetic regenerator (AMR) that operates as a Brayton-type regenerative magnetic refrigeration cycle. However, there are several other cycles that may potentially influence not only the efficiency, but also the cost, compactness and simplicity of magnetocaloric devices. In this article we discuss the possibility of introducing new thermodynamic cycles. This is supported by information about, and a comparison of, the corresponding magnetic field sources. We present the results of numerical analyses and compare the characteristics of different thermodynamic cycles under different operating conditions and for different magnetic field intensities. Guidelines for future work on new magnetic thermodynamic cycles are presented.

Optimal performance of endoreversible quantum refrigerators

Abstract: The derivation of general performance benchmarks is important in the design of highly optimized heat engines and refrigerators. To obtain them, one may model phenomenologically the leading sources of irreversibility ending up with results that are model independent, but limited in scope. Alternatively, one can take a simple physical system realizing a thermodynamic cycle and assess its optimal operation from a complete microscopic description. We follow this approach in order to derive the coefficient of performance at maximum cooling rate for any endoreversible quantum refrigerator. At striking variance with the universality of the optimal efficiency of heat engines, we find that the cooling performance at maximum power is crucially determined by the details of the specific system-bath interaction mechanism. A closed analytical benchmark is found for endoreversible refrigerators weakly coupled to unstructured bosonic heat baths: an ubiquitous case study in quantum thermodynamics.

Work output and efficiency at maximum power of linear irreversible heat engines operating with a finite-sized heat source.

Abstract: We formulate the work output and efficiency for linear irreversible heat engines working between a finite-sized hot heat source and an infinite-sized cold heat reservoir until the total system reaches the final thermal equilibrium state with a uniform temperature. We prove that when the heat engines operate at the maximum power under the tight-coupling condition without heat leakage the work output is just half of the exergy, which is known as the maximum available work extracted from a heat source. As a consequence, the corresponding efficiency is also half of its quasistatic counterpart.

Investigation of a novel ejector expansion refrigeration system using the working fluid R134a and its potential substitute R1234yf

Abstract: To overcome the constraints related to the steady-state operation of the ejector and the separator in a basic ejector-expansion refrigeration system, a new refrigeration system with two evaporators using an ejector as a throttling valve is presented. A constant-area mixing model for the ejector requiring a small number of data and assumptions was established to perform a thermodynamic cycle analysis of the new system. The effects of the ejector area ratio and the condenser temperature on the performance of the ejector and therefore on the increase in COP of the new system compared to the conventional bi-evaporator refrigeration system were investigated for the same cooling capacities of the two evaporators. The refrigerants R134a and its substitute R1234yf were tested. It was found that the novel system has an important improvement in COP for both R134a and R1234yf. This increase in COP is higher for R1234yf especially at high condensing temperatures.