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[en] The time of beginning of heat pipe science was near 40 years ago with first heat pipe definition and prediction of most simple cases. Micro and miniature heat pipes have received considerable attention in the past decade. The interest stems from the possibility of achieving the extremely high heat fluxes near 1000 W/cm2, needed for future generation electronics cooling application. Now at the computer age some changes of basic equations are performed, more powerful predicting methods are available with increasing awareness of the complexity of heat pipes and new heat pipe generations. But even today heat pipes are still not completely understood and solution strategies still contain significant simplifications. Micro and miniature heat pipes have some additional complications due to its small size. A short review on the micro and miniature heat pipes is presented
[en] Highlights: • EHE is based on the reverse Carnot cycle and current heat transfer mechanisms. • EHE can decrease the return water temperature in the PHN to 35 °C. • EHE can increase the heating capacity of the existed PHN by approximately 43%. • The return water temperature in the PHN is much lower than that in the SHN. • EHE has a simpler structure, lower manufacture cost, and better regulation characteristics. - Abstract: As urban construction has been developing rapidly in China, urban heating load has been increasing continually. Heating capacity of the existed primary heating network (PHN) cannot meet district heating requirements of most metropolises in northern China. A new type of ejector heat exchanger (EHE) based on an ejector heat pump and a water-to-water heat exchanger (WWHE) was presented to increase the heating capacity of the existed PHN, and the EHE was also analyzed in terms of laws of thermodynamics. A new parameter, the exergy distribution ratio (EDR), is introduced, which is adopted to analyze regulation characteristics of the EHE. We find that the EHE shows better performance when EDR ranges from 44% to 63%. EHE can decrease the temperature of return water in the PHN to 35 °C, therefore, this can increase the heating capacity of existed PHN by about 43%. The return water with lower temperature in the PHN could recover more low-grade waste heat in industrial systems. Because of its smaller volume and lower investment, EHEs could be applied more appropriately in district heating systems for long-distance heating and waste heat district heating systems
[en] The necessity of the heating of the plasma and its importance are briefly discussed. Also, some of the methods used for heating of the plasma in fusion reactors are mentioned. Finally with appropriate examples, application of combinations of the different heating methods is suggested
[en] Using industrial excess heat in District Heating (DH) networks reduces the need for primary energy and is considered efficient resource use. The conditions of Swedish DH markets are under political discussion in the Third Party Access (TPA) proposal, which would facilitate the delivery of firms' industrial excess heat to DH networks. This paper estimates and discusses the untapped potential for excess heat deliveries to DH networks and considers whether the realization of this potential would be affected by altered DH market conditions. The results identify untapped potential for industrial excess heat deliveries, and calculations based on estimated investment costs and revenues indicate that realizing the TPA proposal could enable profitable excess heat investments. - Highlights: ► The paper identifies untapped potentials for industrial excess heat deliveries in Sweden. ► Unused primary and secondary heat potentials of circa 2 TWh/year and 21 TWh/year are identified. ► The paper indicates that realizing the TPA proposal could enable profitable excess heat investments.
[en] In this paper, recovery of waste heat from an industrial facility with a new Marnoch Heat Engine (MHE) is examined. The MHE can be operated with temperature differentials below 100 K. A flowing liquid transfers heat from the heat source into heat exchangers and then removes heat from cold heat exchangers. Compressed dry air is used as a working medium in the heat engine. In this paper, the mechanical configuration of the heat engine is presented and analyzed. A thermodynamic model is developed to study the performance of the heat engine under various operating conditions. The results show that the exergy efficiency of the MHE reaches up to 17%. The major sources of exergy loss are presented and discussed, in order to optimize the system performance. -- Highlights: ► To develop a thermodynamic model to study the performance of a Marnoch Heat Engine (MHE). ► To investigate the effects of changing operational conditions on the MHE's efficiency. ► To assess the application of the MHE's commercial viability
[en] Borehole heat exchangers are used to remove and/or add heat from/into the ground in different applications involving low-temperature heat usage, such as residential or social buildings heating by means of heat pumps. In such types of applications, seasonal heat storage is necessary since during the warm season an additional solar system is used to charge heat into the ground via the borehole system in order to regenerate the heat source for usage during the cold season. Heat transfer in the ground by means of the borehole results in both radial and axial propagations of the perturbation (ground heating/cooling). The magnitude of this perturbation and the added/removed heat are important parameters of the process that must be evaluated. The theoretical model presented in the paper uses the decomposition finite difference technique to determine the temperature distribution in the ground at different moments (1, 2, 4, and 6 months) during a charge/discharge cycle of the heat storage. The numerical results are necessary in order to evaluate the penetration depth of the thermal perturbation along the radial and longitudinal axes and the amount of stored/extracted heat. (paper)
[en] A new waste heat district heating system with CHP based on ejector heat exchangers and absorption heat pumps (DH-EHE) is presented to decrease heating energy consumption of existing CHP systems by recovering waste heat of exhausted steam from a steam turbine, which could also increase heat transmission capacity of the primary heating network (PHN) by decreasing temperature of the return water of existing PHN. A new ejector heat exchanger based on ejector refrigeration cycle is invented to decrease temperature of the return water of PHN to 30 °C under the designed case. DH-EHE is analyzed in terms of laws of thermodynamics and economics. Compared to conventional district heating systems with CHP (CDH), DH-EHE can decrease consumption of steam extracted from a steam turbine by 41.4% and increase heat transmission capacity of the existing PHN by 66.7% without changing the flow rate of circulating water. The heating cost of DH-EHE is 8.62 ¥/GJ less than that of CDH. Compared to CDH, the recovery period of additional investment of DH-EHE is about two years. DH-EHE shows better economic and environmental benefits, which is promising for both district heating systems for long-distance heat transmission and waste heat district heating systems. - Highlights: • Heating capacity of this new heating system increases by 41% by waste heat recovery. • Temperature of return water of the primary heating network can be reduced to 30 °C. • Heating cost of new heating system is 8.62¥/GJ less than that of conventional one. • The recovery period of additional investment of new heating system is about 2 years. • This new heating system shows better economic and environmental benefits
[en] Highlights: • An entransy analysis is adopted for the absorption heat exchanger. • Total entransy dissipation decreases then increases with increasing flow ratio. • The principle provides a reference for optimized design and operation. For substations in large-area district heating systems, the primary network supply water can be used as a driving force to realize heat transfer between the primary and secondary networks under an absorption heat-exchanger technique. An absorption heat-exchanger comprises an absorption heat pump (driven by hot water) and a conventional water-water heat exchanger. Generally, the return water of the secondary network is divided into two parts: one part is sent into the water-water heat exchanger, which is heated by the primary network; the other part is heated in the absorption heat pump. Finally, the water from each part is mixed and transmitted to the end user. The flow distribution of the secondary network affects the return water temperature of the primary network. This paper presents a theoretical analysis of the flow distribution principle for the secondary network. Here, an entransy analysis is adopted as an optimization method for the heat exchange process. The absorption heat-exchanger model is simplified and the mathematical representations of each entransy dissipation part are provided. The optimal flow distribution principle is obtained by calculating the minimum value of the total entransy dissipation. The principle provides a reference for optimized design and operation of the absorption heat exchanger.
[en] The maximum efficiency of a heat engine is able to be estimated by using a Carnot cycle. Even though, in terms of efficiency, the Carnot cycle performs the role of reference very well, its application is limited to the case of infinite heat reservoirs, which is not that realistic. Moreover, considering that one of the recent key issues is to produce maximum work from low temperature and finite heat sources, which are called renewable energy sources, more advanced theoretical cycles, which can present a new standard, and the research about them are necessary. Therefore, in this paper, a sequential Carnot cycle, where multiple Carnot cycles are connected in parallel, is studied. The cycle adopts a finite heat source, which has a certain initial temperature and heat capacity, and an infinite heat sink, which is assumed to be ambient air. Heat transfer processes in the cycle occur with the temperature difference between a heat reservoir and a cycle. In order to resolve the heat transfer rate in those processes, the product of an overall heat transfer coefficient and a heat transfer area is introduced. Using these conditions, the performance of a sequential Carnot cycle is analytically calculated. Furthermore, as the efforts for enhancing the work of the cycle, the optimization research is also conducted with numerical calculation. - Highlights: • Modified sequential Carnot cycles are proposed for evaluating low grade heat sources. • Performance of sequential Carnot cycles is calculated analytically. • Optimization study for the cycle is conducted with numerical solver. • Maximum work from a heat source under a certain condition is obtained by equations