scholarly journals Optimal Heat Exchanger Area Distribution and Low-Temperature Heat Sink Temperature for Power Optimization of an Endoreversible Space Carnot Cycle

Entropy ◽  
2021 ◽  
Vol 23 (10) ◽  
pp. 1285
Author(s):  
Tan Wang ◽  
Yanlin Ge ◽  
Lingen Chen ◽  
Huijun Feng ◽  
Jiuyang Yu
Keyword(s):  
Low Temperature ◽  
Power Output ◽  
Heat Sink ◽  
Heat Exchangers ◽  
Power Plants ◽  
Maximum Power ◽  
Carnot Cycle ◽  
Design And Optimization ◽  
Maximum Power Output ◽  
Temperature Heat

Using finite-time thermodynamics, a model of an endoreversible Carnot cycle for a space power plant is established in this paper. The expressions of the cycle power output and thermal efficiency are derived. Using numerical calculations and taking the cycle power output as the optimization objective, the surface area distributions of three heat exchangers are optimized, and the maximum power output is obtained when the total heat transfer area of the three heat exchangers of the whole plant is fixed. Furthermore, the double-maximum power output is obtained by optimizing the temperature of a low-temperature heat sink. Finally, the influences of fixed plant parameters on the maximum power output performance are analyzed. The results show that there is an optimal temperature of the low-temperature heat sink and a couple of optimal area distributions that allow one to obtain the double-maximum power output. The results obtained have some guidelines for the design and optimization of actual space power plants.

Thermodynamic Optimization of Irreversible Power Cycles With Constant External Reservoir Temperatures

1991 ◽  
Vol 113 (4) ◽  
pp. 505-510 ◽  
Author(s):  
L. W. Swanson
Keyword(s):  
Heat Exchanger ◽  
Low Temperature ◽  
Power Output ◽  
Heat Exchangers ◽  
Power Plants ◽  
Cycle Phase ◽  
Change Processes ◽  
Power Cycles ◽  
Global Power ◽  
Temperature Heat

An extended version of the Bejan model of irreversible power plants is proposed using a log-mean temperature difference (LMTD) representation for both the high and low-temperature heat exchangers. The analysis focuses on minimizing the irreversibilities associated with the hot and cold heat exchangers. The results indicate that the maximum power output, external conductance allocation ratio, and second law efficiency are functions of the number total heat exchanger transfer units (N), and are asymptotic to Bejan’s original results as N → O. This asymptote represents a global power output maximum and occurs for either extremely high cycle flow rates or cycle phase change processes in both heat exchangers. The LMTD representation also shows that under optimal conditions, more conductance should be allocated to the low-temperature heat exchanger as N increases.

scholarly journals OTEC Maximum Net Power Output Using Carnot Cycle and Application to Simplify Heat Exchanger Selection

Entropy ◽  
2019 ◽  
Vol 21 (12) ◽  
pp. 1143 ◽  
Author(s):  
Kevin Fontaine ◽  
Takeshi Yasunaga ◽  
Yasuyuki Ikegami
Keyword(s):  
Heat Transfer ◽  
Heat Exchanger ◽  
Pressure Drop ◽  
Power Output ◽  
Heat Exchangers ◽  
Power Plants ◽  
Seawater Temperature ◽  
Carnot Cycle ◽  
Ocean Thermal Energy ◽  
Net Power Output

Ocean thermal energy conversion (OTEC) uses the natural thermal gradient in the sea. It has been investigated to make it competitive with conventional power plants, as it has huge potential and can produce energy steadily throughout the year. This has been done mostly by focusing on improving cycle performances or central elements of OTEC, such as heat exchangers. It is difficult to choose a suitable heat exchanger for OTEC with the separate evaluations of the heat transfer coefficient and pressure drop that are usually found in the literature. Accordingly, this paper presents a method to evaluate heat exchangers for OTEC. On the basis of finite-time thermodynamics, the maximum net power output for different heat exchangers using both heat transfer performance and pressure drop was assessed and compared. This method was successfully applied to three heat exchangers. The most suitable heat exchanger was found to lead to a maximum net power output 158% higher than the output of the least suitable heat exchanger. For a difference of 3.7% in the net power output, a difference of 22% in the Reynolds numbers was found. Therefore, those numbers also play a significant role in the choice of heat exchangers as they affect the pumping power required for seawater flowing. A sensitivity analysis showed that seawater temperature does not affect the choice of heat exchangers, even though the net power output was found to decrease by up to 10% with every temperature difference drop of 1 °C.

Maximum Power From Fluid Flow: Results From the First and Second Laws of Thermodynamics

2012 ◽  
Author(s):  
German Amador Diaz ◽  
John Turizo Santos ◽  
Elkin Hernandez ◽  
Ricardo Vasquez Padilla ◽  
Lesme Corredor
Keyword(s):  
Heat Transfer ◽  
Fluid Flow ◽  
Power Output ◽  
Power Plants ◽  
Cross Sectional Area ◽  
Maximum Power ◽  
Sectional Area ◽  
Cross Sectional ◽  
Maximum Power Output ◽  
Piston Cylinder

The heat transfer principle of power maximization in power plants with heat transfer irreversibilities was cleverly extended by Bejan [1] to fluid flow, by obtaining that the energy conversion efficiency at maximum power is ηmax = 1/2(1 − P2/P1). This result is analog to the efficiency at maximum power for power plants, ηmax = 1 − (T2/T1)1/2 which was deduced by Curzon and Ahlborn [2]. In this paper, the analysis to obtain maximum power output delivered from a piston between two pressure reservoir across linear flow resistance is generalized by considering the piston cylinder friction, by obtaining relations of maximum power output and optimal speed of the piston in terms of first law efficiency. Expressions to relate the power output, cross sectional area of the chamber and first law efficiency, were deduced in order to evaluate the influence of the overall size constraints and fluid regime in the performance of the piston cylinder system. Flow in circular ducts and developed laminar flow between parallel plates, are considered to demonstrate that when two pressure reservoirs oriented in counterflow, with different and arbitrary cross sectional area, must have the same area in order to maximize the power output of the system. These results introduce some modifications to the results obtained by Bejan [1] and Chen [3]. This paper extends the Bejan and Chen’s work by estimating under turbulent regime the lost available work rate associated with the degree of irreversibilities caused by the flow resistances of the system. This analysis is equivalent to evaluate the irreversibilities in an endoirreversible Carnot heat engine model caused by the heat resistance loss between the engine and its surrounding heat reservoirs. This paper concludes with an application to illustrate the practical applications by estimating the lost available work of an actual steady-flow turbine and the layout pipes upstream and downstream of the same device.

Enhancement and Optimization of Planar Impingement Heat Transfer for Thermoelectric Power Generation

2015 ◽  
Author(s):  
Kazuaki Yazawa ◽  
Stephen D. Heister ◽  
Timothy S. Fisher
Keyword(s):  
Heat Transfer ◽  
Power Generation ◽  
Power Output ◽  
Heat Exchangers ◽  
Fluid Flows ◽  
Impinging Jet ◽  
Maximum Power ◽  
Counter Flow ◽  
Maximum Power Output ◽  
Impingement Heat Transfer

We present an analytic model and optimization of impingement heat transfer in fluid-to-fluid heat exchangers with integrating a thermoelectric (TE) generator between the hot and cold fluid flows. In power generation systems, designing for maximum power output generally involves balancing the external thermal resistances while the generator contacts the hot and cold temperature reservoirs. In fluid-to-fluid heat exchangers, fluid temperatures are not constant or uniform. They gradually change along the flow direction. In general, counter-flow heat exchangers outperform parallel flow configurations in maximizing TE power generation using internal fluid flows. We show here the performance of our impingement model compared with a counter-flow configuration as the base line. To obtain the maximum power output from practical thermoelectric materials (ZT values are 1.2–1.8), the enhancement of liquid-to-wall heat transfer is significant. An array of traditional impinging jet orifices provides a uniformly planar and focused heat transfer process that spatially targets the TE elements. This approach provides more uniform hot and cold side temperatures among the TE elements. We investigate the impact of introducing impingement orifices directly at the locations of the TE elements. The major focus of this work is the trade-off between the advantage of increasing power generation by impingement and the disadvantage of introducing additional pressure drop. Decreasing the external thermal resistances yields not only a larger maximum power output but also requires thinner TE elements. This enables lower cost per power generation capacity approaching the 0.2–0.3 $/W range as well as a more lightweight design. We report here the associated cost impacts for the impinging jet arrangement. Design optimization depends on the specific constraints and parameters, such as TE material and substrate thickness, flow design to avoid the stagnation, and required exit temperatures. In some cases, active pumping by an additional actuator can augment the enhancement, while a fraction of generated power is consumed for the actuation. In the paper, we show examples of gas and liquid flow cases.

scholarly journals Effect of Different Operating Conditions on Performance of Commercial Low-Temperature Thermoelectric Modules

2019 ◽  
Vol 9 (2) ◽  
pp. 1781-1791
Keyword(s):  
Low Temperature ◽  
Conversion Efficiency ◽  
Power Output ◽  
Waste Heat ◽  
Thermoelectric Module ◽  
Operating Conditions ◽  
Maximum Power ◽  
Thermoelectric Modules ◽  
Temperature Range ◽  
Maximum Power Output

In the field of waste heat recovery, thermoelectric generators (TEG) are used to convert waste heat to electric power. This system attracts the attention of researchers to make it more and more efficient. The performance of thermoelectric module (TEM) plays a crucial role for thermoelectric system. Appropriate selection of thermoelectric module is one of the important criteria for enhancing the power output and conversion efficiency of thermoelectric generator. In this work, the effect of various operating conditions on performance of thermoelectric modules was experimentally investigated. Three commercial bismuth telluride (Bi2Te3 ) thermoelectric modules (TEM1, TEM2, and TEM3) were experimentally tested to find the best performance module for low-temperature waste heat. The open-circuit voltage, power output, and conversion efficiency were measured at various operating conditions. Different operating parameters such as water mass flow rate, heater voltage, hot and cold side temperature of thermoelectric module, and external load resistance were considered for this work. An electric heater was used as a heat source and water used as a cooling fluid at heat sink side. It was observed that the TEM1 shows maximum power output of 0.31, 0.71 and 1.25W, for temperature ranges of 80-100, 100-150, and 150-200 oC respectively. TEM3 achieved maximum power output 0.81W for temperature range of 100-150 oC. TEM1, TEM2 and TEM3 have the maximum conversion efficiency of 1.37, 0.60, and 1.64 % respectively. The TEM2 having less power output and conversion efficiency for temperature range of 80-200 oC compare to TEM1 and TEM3. However, the TEM1 is more appropriate for temperature range of 80-200 oC and the TEM3 is also suitable for the temperature range of 80-150 oC.

Improved Layouts and Performance of Single- and Double-Flash Steam Geothermal Plants Generated by the Heatsep Method

2020 ◽  
Vol 142 (9) ◽  
Author(s):  
Giovanni Manente ◽  
Andrea Lazzaretto
Keyword(s):  
Power Output ◽  
Power Plants ◽  
Internal Heat ◽  
Maximum Power ◽  
Saturated Steam ◽  
Heat Integration ◽  
Geothermal Resources ◽  
High Enthalpy ◽  
Maximum Power Output ◽  
Double Flash

Abstract Single- and double-flash steam power plants are commonly used in the utilization of high enthalpy liquid-dominated geothermal resources. In these plants, the expansion line in the wet steam region results in significant penalties of turbine isentropic efficiency and power output. Accordingly, the “self-superheating” and “interstage heating” plant modifications have been recently proposed in the literature, where the saturated steam at turbine inlet is superheated by using the heat of the geothermal liquid, which is cooled before the flashing process. In this study, the aforementioned and additional new flash steam plant layouts are generated by using a systematic method, called Heatsep, for the optimum design of energy systems. All the thermal connections between consecutive basic plant components are “cut” to let these temperatures vary and in turn generate additional hot and cold streams, which are combined to enhance the overall performance of the system. It is demonstrated that the single-flash plant with self-superheating is simply obtained by cutting two out of five thermal links. In the double-flash plant, the higher number of components allows for a higher number of thermal cuts and heat integration options. Unlike the existing literature, the maximum power output is not constrained by a predefined heat transfer network. The optimization results show that the maximum power output of the novel single- and double-flash steam plants exceeds by 5.5–9.2% and 3.9–7.7% the maximum attainable by the corresponding traditional plants without internal heat integration.

scholarly journals Analysis and Comparison of Some Low-Temperature Heat Sources for Heat Pumps

Energies ◽  
2019 ◽  
Vol 12 (10) ◽  
pp. 1853 ◽  
Author(s):  
Pavel Neuberger ◽  
Radomír Adamovský
Keyword(s):  
Heat Transfer ◽  
Low Temperature ◽  
Heat Source ◽  
Heat Pump ◽  
Heat Exchangers ◽  
Ambient Air ◽  
Heat Transfer Fluid ◽  
Heat Sources ◽  
Ground Heat Exchangers ◽  
Temperature Heat

The efficiency of a heat pump energy system is significantly influenced by its low-temperature heat source. This paper presents the results of operational monitoring, analysis and comparison of heat transfer fluid temperatures, outputs and extracted energies at the most widely used low temperature heat sources within 218 days of a heating period. The monitoring involved horizontal ground heat exchangers (HGHEs) of linear and Slinky type, vertical ground heat exchangers (VGHEs) with single and double U-tube exchanger as well as the ambient air. The results of the verification indicated that it was not possible to specify clearly the most advantageous low-temperature heat source that meets the requirements of the efficiency of the heat pump operation. The highest average heat transfer fluid temperatures were achieved at linear HGHE (8.13 ± 4.50 °C) and double U-tube VGHE (8.13 ± 3.12 °C). The highest average specific heat output 59.97 ± 41.80 W/m2 and specific energy extracted from the ground mass 2723.40 ± 1785.58 kJ/m2·day were recorded at single U-tube VGHE. The lowest thermal resistance value of 0.07 K·m2/W, specifying the efficiency of the heat transfer process between the ground mass and the heat transfer fluid, was monitored at linear HGHE. The use of ambient air as a low-temperature heat pump source was considered to be the least advantageous in terms of its temperature parameters.

Oxidation of combined ingestion of glucose and fructose during exercise

2004 ◽  
Vol 96 (4) ◽  
pp. 1277-1284 ◽  
Author(s):  
Roy L. P. G. Jentjens ◽  
Luke Moseley ◽  
Rosemary H. Waring ◽  
Leslie K. Harding ◽  
Asker E. Jeukendrup
Keyword(s):  
Power Output ◽  
Statistical Significance ◽  
Random Order ◽  
Carbohydrate Oxidation ◽  
Maximum Power ◽  
The Other ◽  
Cycling Exercise ◽  
Oxidation Rates ◽  
Maximum Power Output ◽  
Exercise Period

The purpose of the present study was to examine whether combined ingestion of a large amount of fructose and glucose during cycling exercise would lead to exogenous carbohydrate oxidation rates >1 g/min. Eight trained cyclists (maximal O2consumption: 62 ± 3 ml·kg-1·min-1) performed four exercise trials in random order. Each trial consisted of 120 min of cycling at 50% maximum power output (63 ± 2% maximal O2consumption), while subjects received a solution providing either 1.2 g/min of glucose (Med-Glu), 1.8 g/min of glucose (High-Glu), 0.6 g/min of fructose + 1.2 g/min of glucose (Fruc+Glu), or water. The ingested fructose was labeled with [U-13C]fructose, and the ingested glucose was labeled with [U-14C]glucose. Peak exogenous carbohydrate oxidation rates were ∼55% higher ( P < 0.001) in Fruc+Glu (1.26 ± 0.07 g/min) compared with Med-Glu and High-Glu (0.80 ± 0.04 and 0.83 ± 0.05 g/min, respectively). Furthermore, the average exogenous carbohydrate oxidation rates over the 60- to 120-min exercise period were higher ( P < 0.001) in Fruc+Glu compared with Med-Glu and High-Glu (1.16 ± 0.06, 0.75 ± 0.04, and 0.75 ± 0.04 g/min, respectively). There was a trend toward a lower endogenous carbohydrate oxidation in Fruc+Glu compared with the other two carbohydrate trials, but this failed to reach statistical significance ( P = 0.075). The present results demonstrate that, when fructose and glucose are ingested simultaneously at high rates during cycling exercise, exogenous carbohydrate oxidation rates can reach peak values of ∼1.3 g/min.

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