Cold Source Temperature Research Articles

Thermodynamic, dynamic and flow friction analysis of a Stirling engine with Scotch yoke piston driving mechanism

This study concerns with the thermodynamic and dynamic analysis of an alpha type Stirling engine with Scotch-yoke piston driving mechanism. The thermodynamic aspect of the analysis is treated with a polytrophic nodal approximation. The pressure of nodal volumes is calculated with modified Schmidt formula which takes into account the pressure differences between nodal volumes caused by flow friction. The flow friction is calculated with adapted Darcy formula. The variation of the gas temperature in nodal volumes are calculated via the first law of thermodynamics given for unsteady open systems. The dynamic behavior of the engine is modeled via the motion equations of pistons and crankshaft. For a 2 kW nominal shaft power and 1400 rpm nominal speed, dimensions and working conditions of the engine were investigated by using realistic inputs. It was estimated that an engine having about 1.44 L swept volume, 1000 K hot source temperature, 400 K cold source temperature, 9050 cm2 total inner heat transfer area, 6 bar charge pressure, 2000 W/m2K inner heat transfer coefficient may produce more than 2 kW shaft power. For 142 rad/s average crankshaft speed the optimum thermal efficiency and torque of the engine were determined as 31% and 15.63 Nm respectively.

Анализ применения теплонасосных установок в системах теплоснабжения

The article discusses matters concerned with the use of heat pump units (HPUs) in systems for reclaiming the secondary energy resources of industrial low-temperature thermal technological processes and in heat supply systems of buildings. Data on the sales of HPUs in different European countries are given, from which it can be seen that there is a significant lag in the use of HPUs for heat supply systems in the Russian Federation. Different HPUs are classified by cycles and by low-grade heat sources. The basic circuit diagram of an HPU applied in the system involving the use of waste water low-grade heat is presented. As an example, a heat pipe can be used instead of an electrically driven pump for transporting intermediate heat carrier from the source of low-grade heat (LGHS) to the HPU evaporator. The graph of heat fluxes (constructed in consistency with the heat carrier temperatures) in transferring heat from the LGHS to the consumer and the heat balance for an element of the system containing an HPU are given. Methods for determining the energy efficiency from applying an HPU are analyzed through the use of the heat transformation (conversion) coefficient and the exergetic efficiency, which takes into account not only the amount of produced heat, but also its quality in terms of temperature level. It is shown that, if additional energy is spent for transporting heat from the low-grade heat source to the HPU evaporator, these energy expenditures must be taken into account in determining the energy efficiency of the system containing an HPU. In case of using electric and other types of energy, it is advisable to use such indicators as the cost of energy carriers and the expenditures of equivalent fuel for producing these kinds of energy in determining the system’s energy efficiency. It is shown that in case of using fossil fuel to produce electricity and heat for the operation of the HPU’s electrically driven compressor, the heat conversion coefficient in the HPU should be well above 2.32. The results of implemented projects in which the performance of HPU-based heat supply systems was investigated are given. An analysis of the obtained results shows that a number of implemented projects feature low energy efficiency, a circumstance that does not allow the cost of their construction to be compensated. It is shown that the energy conversion coefficient depends on the required temperature for a consumer, on the cold source temperature, on the working fluid thermodynamic properties and the HPU thermodynamic cycle features, on the HPU design technical perfection, on the type of energy expenditures for driving the compressor, and on the method for transporting heat from the low-grade source. For securing high efficiency of HPU-based heat supply systems (HSS), it is necessary to have a low-grade heat source with the highest temperature and a heat consumer with the lowest temperature of the used heat carrier. The availability of convenient low-grade heat sources that would (i) have a sufficiently high temperature both in winter and in summer, (ii) not require high expenditures for transferring them and (iii) not cause corrosion of heat exchangers and pipes is one of the most important conditions for using an HPU for heat supply purposes. Matters concerned with the use of HPU-based HSS under specific conditions require a thorough preliminary feasibility study. The heat transformation (conversion) coefficient in an HPU is by no means a universal factor that can be used as the determining one in assessing the energy efficiency of using an HPU-based HSS. It must also be borne in mind that the cheapest available type of energy should be used for driving the HPU compressor and for transporting low-grade heat from the source to the HPU evaporator. In a number of cases, the most efficient way of driving the HPU compressor consists in using the energy of excess pressure available in energy carriers (gas, steam, water, etc.) or the mechanical energy of internal combustion engines, gas turbine plants and similar devices. When using electrically driven HPUs, it is recommended to adjust the ratio of tariffs for electricity and heat in order to reduce the ratio of electricity prices to the price of heat.

Study on dielectric properties of BADCy/Ni0.5Ti0.5NbO4 composites fabricated by freeze casting combined with vacuum assisted infiltration process

Bisphenol-A dicyanate ester resin (BADCy)/Ni0.5Ti0.5NbO4 composites, within which Ni0.5Ti0.5NbO4 and BADCy are continuously directionally distributed, were fabricated using freeze casting combined with vacuum assisted infiltration process. The control of the microstructure of porous Ni0.5Ti0.5NbO4 preforms was systematically studied. Effect mechanisms of the preform microstructure on the dielectric properties of the BADCy/Ni0.5Ti0.5NbO4 composites were firstly researched. Results show that cold source temperature and sintering temperature have significant influences on the microstructure of porous Ni0.5Ti0.5NbO4 preforms. The microstructure of the preforms further greatly affects the dielectric constant and the loss of the BADCy/Ni0.5Ti0.5NbO4 composites. Due to the excellent dielectric properties, this kind of composites possesses a wide application in printed circuit board (PCB) field. This study is also very helpful for the design and fabrication of this kind of composites.

Dynamic model of capillary pumped loop with unsaturated porous wick for terrestrial application

This paper presents a dynamic model of a Capillary Pumped Loop designed for gravitational applications. The CPL model is divided into subsystems, for which transient mass, energy, and momentum balances have been developed. The heat and mass transfer inside the evaporator structure is precisely described by a 2D mathematical model, which takes into account the existence of a vapor pocket inside the porous wick. The liquid lines are discretized into small elements. The comparison of the numerical results and data experiment validates the proposed model in a transient regime. The model is then used to analyze the thermohydraulic behavior of a capillary pumped loop by modifying external conditions such as the cold source temperature and the operating conditions such as the reservoir temperature.

Development and application of a generic CFD toolkit covering the heat flows in combined solid–liquid systems with emphasis on the thermal design of HiLumi superconducting magnets

The main objective of this work is to develop a robust multi-region numerical toolkit for the modeling of heat flows in combined solid–liquid systems. Specifically heat transfer in complex cryogenic system geometries involving super-fluid helium. The incentive originates from the need to support the design of superconductive magnets in the framework of the HiLumi-LHC project (Brüning and Rossi, 2015) [1]. The intent is, instead of solving heat flows in restricted domains, to be able to model a full magnet section in one go including all relevant construction details as accurately as possible. The toolkit was applied to the so-called MQXF quadrupole magnet design. Parametrisation studies were used to find a compromise in thermal design and electro-mechanical construction constraints. The cooling performance is evaluated in terms of temperature margin of the magnets under full steady state heat load conditions and in terms of maximal sustainable load. We also present transient response to pulse heat loads of varying duration and power and the system response to time-varying cold source temperatures.

Experimental study on water separation process in a novel spray flash vacuum evaporator with heat-pipe

Based on the high heat transfer performance of heat pipe (HP) and large specific surface area of small diameter droplets, HP and spray technologies are combined in a novel single stage vacuum evaporator and artificial seawater with 3% saltness is studied experimentally. Parametric studies are carried out, such as cold source and heat source temperature, spray temperature, and spray flow. The results show that: the maximum heat flux density reaches 32Wcm−2 on evaporator bland plate; HPs absorb energy effectively from low grade heat source (40–80°C) then transfer the energy to the droplets already flashed, so as to maintain or even increase the superheat degree of droplets during evaporating process; This method can significantly improve the water separation rate (quality of condensed water/quality of dilute solution∗100%); Changing the parameters can regulate water separation from 0% to 65%. Thus it has important significance for utilizing lower grade heat source effectively, gaining brine, and promoting the fresh water yield per unit volume and adaptability to variable load in desalination project.

Comparison of Segmented and Traditional Thermoelectric Generator for Waste Heat Recovery of Diesel Engine

This paper established a mathematic model of segmented thermoelectric generator (TEG) based on low-temperature thermoelectric material bismuth telluride and medium-temperature thermoelectric material skutterudite. The performance of segmented TEG and traditional TEG has been compared using this model under different conditions, such as heat source temperature, cold source temperature, heat transfer coefficient, and length and cross section area of thermocouple. The results show that the maximum output power and conversion efficiency of segmented TEG are higher significantly using exhaust of diesel engine (DE) as heat source and coolant as cold source. The results also show that the trends of maximum output power and conversion efficiency are contrary with increment of thermocouple length. The trends of maximum output power is linear with increment of cross section area, and the conversion efficiency is constant. Finally, the segmented TEG has a more potential to recover waste heat than tradition TEG. The performance of segmented TEG is decided by the ratio of materials, the optimum design of segmented TEG should base on the crossing point of ZT value for two materials.

Experimental analysis with numerical comparison for different thermoelectric generators configurations

Thermoelectric (TE) energy harvesting is a promising perspective to use waste heat. Due to the low efficiency of thermoelectric materials many analytical and numerical optimization studies have been developed. To be validated, an optimization must necessarily be linked to the experience. There are a lot of results on thermoelectric generators (TEG) based on experiments or model validations. Nevertheless, the validated models concern most of the time one TE module but rarely an entire system. Moreover, these models of complete system mainly concern the optimization of fluid flow rates or of heat exchangers. Our choice is to optimize the number of these modules in a whole system point of view. A numerical model using a software for numerical computation, based on multi-physics equations such as heat transfer, fluid mechanics and thermoelectricity was developed to predict both thermal and electrical powers of TEG. This paper aims to present the experimental validation of this model and shows interesting experimental results on the location of the TE modules. In parallel, an experimental set-up was built to compare and validate this model. This set-up is composed of a thermal loop with a hot gas source, a cold fluid, a hot fin exchanger, a cold tubular exchanger and thermoelectric modules. The number and the place of these modules can be changed to study different configurations. A specific maximum power point tracker DC/DC converter charging a battery is added in order to study the electrical power produced by the TEG. The analysis of the influence of the number of thermoelectric modules and influence of electric currents on the produced electrical power was investigated. Different operating points of hot inlet gas airflow rate and of cold inlet source temperature were tested. Both experimental and numerical results show the necessity to optimize the position, the number of TE modules and the electrical currents.

Comparison and parameter optimization of a segmented thermoelectric generator by using the high temperature exhaust of a diesel engine

This paper proposes a segmented thermoelectric generator (TEG) that can be used to recover exhaust waste heat from a diesel engine (DE). A mathematic model of the segmented TEG was constructed based on the low-temperature thermoelectric material bismuth telluride and the medium-temperature thermoelectric material skutterudite. Performance was compared between segmented and traditional TEGs, and the performance of the segmented TEG was optimized based on the comparison. The model simulates the impact of relevant factors, including the exhaust temperature, cold source temperature, thermocouple length, and the length ratio between the two materials, on the output power and conversion efficiency. The results showed that the segmented TEG is more suitable than the traditional TEG for a high-temperature heat source and for large temperature differences. Moreover, the maximum output power was inversely proportional to the thermocouple length; however, the maximum conversion efficiency was directly proportional. The ratio of the two materials depended on the temperature of the heat and cold source. Finally, a comparison of application potential of the TEGs showed that the segmented TEG had greater potential for waste heat recovery compared with the traditional TEG.

A novel thermomechanical energy conversion cycle

This paper presents a new power cycle for direct conversion of thermomechanical energy into electrical energy performed on pyroelectric materials. It consists sequentially of (i) an isothermal electric poling process performed under zero stress followed by (ii) a combined uniaxial compressive stress and heating process, (iii) an isothermal electric de-poling process under uniaxial stress, and finally (iv) the removal of compressive stress during a cooling process. The new cycle was demonstrated experimentally on [001]-poled PMN-28PT single crystals. The maximum power and energy densities obtained were 41W/L and 41J/L/cycle respectively for cold and hot source temperatures of 22 and 130°C, electric field between 0.2 and 0.95MV/m, and with uniaxial load of 35.56MPa at frequency of 1Hz. The performance and constraints on the operating conditions of the new cycle were compared with those of the Olsen cycle. The new cycle was able to generate power at temperatures below those of the Olsen cycle. In addition, the new power cycle can adapt to changing thermal and mechanical conditions.

Transient thermohydraulic modeling of two-phase fluid systems

This paper presents a transient thermohydraulic modeling, initially developed for a capillary pumped loop in gravitational applications, but also possibly suitable for all kinds of two-phase fluid systems. Using finite volumes method, it is based on Navier-Stokes equations for transcribing fluid mechanical aspects. The main feature of this 1D-model is based on a network representation by analogy with electrical.This paper also proposes a parametric study of a counterflow condenser following the sensitivity to inlet mass flow rate and cold source temperature. The comparison between modeling results and experimental data highlights a good numerical evaluation of temperatures. Furthermore, the model is able to represent a pretty good dynamic evolution of hydraulic variables.

A Novel Optimization Method for the Electric Topology of Thermoelectric Modules Used in an Automobile Exhaust Thermoelectric Generator

Based on Bi2Te3 thermoelectric modules, a kind of automobile exhaust thermoelectric generator (AETEG) with a single-column cold-source structure was designed. To enhance its net power and efficiency, the output performance of all the thermoelectric modules was tested with a temperature monitoring unit and voltage monitoring unit, and modeled using a back-propagation (BP) neural network based on various hot-source temperatures, cold-source temperatures, load currents, and contact pressures according to the temperature distribution of the designed heat exchanger and cooling system. Then, their electric topology (series or parallel hybrid) was optimized using a genetic algorithm to achieve the maximum peak power of the AETEG. From the experimental results, compared with when all the thermoelectric modules were connected only in series or parallel at random, it is concluded that the AETEG performance is evidently affected by the electric topology of all the single thermoelectric modules. The optimized AETEG output power is greatly superior to the other two investigated designs, validating the proposed optimized electric topology as both feasible and practical.

Investigation of a two stage Rankine cycle for electric power plants

A two stage Rankine cycle for power generation is presented in this paper. It is made of a water steam Rankine cycle and an Organic Rankine bottoming Cycle. By using an organic working fluid with higher density than water, it is possible to reduce the installation size and to use an air-cooled condenser. In order to search suitable working fluids for our application, nine potential candidates from four different organic fluid families and ammonia are tested. The performances of the two stage Rankine cycle operating with those different working fluids are evaluated. The influences of design external temperatures and the steam turbine outlet pressure on the system are analyzed. Optimal points are found at different cold source temperatures and steam turbine outlet pressures for each fluid. System efficiency can also be enhanced by introducing a regenerator for some of the selected working fluids.

Entropy Analysis and Optimization of Phase Transition Heat Exchangers

Entropy analysis method was adopted to analyze and optimize a jacketed pipes-type heat pipe steam generator. In the entropy analysis process, the entropy production resulted from the total temperature difference across the heat exchanger, the entropy production from main heat transfer element – heat pipes and the entropy production caused by the pressure drop due to working media flow on cold and hot sides were taken into consideration. In the calculation, the entropy production in all parts of the heat exchanger was analyzed row by row in a discrete pattern, except for the entropy production due to pressure drop on flue gas side. Analysis results show that for the heat exchanger in this case, the proportions of entropy production in the above-mentioned three parts are respectively 74.79%, 21.75% and 3.46%. On this basis, the article also made optimizing analysis with the cold and hot source temperature, heat pipe length and No. of heat pipe rows as parameters, to provide theoretical basis for further improving the energy efficiency of heat exchangers.

Modeling of an ejector refrigerating system operating in dimensioning and off-dimensioning conditions with the working fluids R142b and R600a

This paper describes a whole simulation program which is useful to evaluate the performance and the characteristics of the operating cycle of an ejector refrigerating system with the working fluids R142b and R600a. The cycle is determined using the temperatures of the three thermal sources and local heat transfer coefficients for the boiler, the condenser and the evaporator. In addition, the simulation program includes a correlation of the ejector entrainment ratio established in different operating conditions at critical point from the conservation equations of the 1-D model available in the literature. Previously, all the components of the system were dimensioned for a refrigerating power of 10 kW, the hot source temperature is equal to 120 and 130 °C, whereas those of the intermediate and the cold sources are fixed at 35 and 10 °C, respectively (dimensioning conditions). Then, the system performance is investigated in dimensioning conditions and in off-dimensioning conditions using the simulation program. Particularly, the analysis of the results shows that at fixed cold source temperature, the intermediate temperature corresponding to the critical mode with ( P C < = P C ∗ ) , the system COP decreases when the hot source temperature is higher than that of its dimensioning. Consequently, it is recommended to dimension the system components at the highest possible temperature in order to guarantee better performance in the case of an operating at lower temperature of the hot source. Also, it is noted that R142b leads to better performance of the system in all cases. This can be due to the fact that R142b is a heavier fluid than R600a.

Cycles de frigopompes à absorption en cascades matérielles — détermination du nombre d'étages optimal pour le mélange ammonoiac–eau

This study is devoted to the modelling of absorption heat pumps using an ammonia-water mixture, for air conditioning. The mixture is separated in a distillation column and the components are remixed in a number of evaporation-absorption cells, linked in a material cascade. We compare the performance of three structures, with one, two or three stages, and we determine the influence of various parameters: final desorption temperature, cold source temperature, the number of theoretical separation plates, and the thermal pinch of the main exchanger. This last factor exerts only a small influence. the two-stage cycle appears to be the most efficient: it is even more efficient than the three-stage cycle. The higher rectification losses and recirculation rates for the three-stage cycle lower its performance. The three-stage cycle becomes the most efficient for the production of cold at a lower environmental temperature and with a greater number of theoretical separation plates.

Thermodynamic Cycle of the Atmospheric Upward Heat Convection Process

¶Closed ideal thermodynamic cycles are used to analyze the atmospheric upward heat convection process which is compared to the Brayton gas-turbine cycle. The heat to work conversion efficiency of the atmosphere is shown to be close to the Carnot efficiency calculated using the average temperatures at which heat is received and given up for hot and cold source temperatures, respectively. The efficiency is independent of whether the lifting process is discontinuous or continuous, and nearly independent of whether the heat is transported as sensible or as latent heat.

Simplified Theory of Ringbom Stirling Machines

This paper presents a first order analysis of four types of overdriven free-displacer Stirling machines. All the presented types of machines can work as refrigerating machines, prime movers or heat exchange accelerators depending on parameters such as the hot to cold source temperatures ratio, the nondimensional mass of working gas in the machine, the displacer rod to displacer cross sectional area ratio, the corrected dead space to piston cylinder volume ratio and the displacer to piston cylinder volume ratio. In its analytical form this theory holds for machines at low speed as it is assumed that the piston displacement can be neglected during the displacer movement duration. This analysis may be used to find the conditions and values giving either the best theoretical refrigerating cycle or the best theoretical prime mover cycle, the associated reference work, reference time, efficiency and heat quantities involved. A table gives the analytical expressions and the limiting values of the main parameters for the four different types of Ringbom machines considered. The preliminary design of a Ringbom prime mover is then presented. The main parameters influences are predicted and the magnitude of work, rotational speed limit and efficiency are obtained.

An entropic model of liquid vapour absorption cooling system: Second Law analysis

ABSTRACT This communication presents an entropic analysis based on combined first and second laws of thermodynamics for a vapour liquid absorption (VLAB) cooling system. The model provides an insight understanding of entropy production mechanisms of various irreversible processes inherent in absorption systems and also gives an explicit explanation of actual COP degradation with respect to the ideal reversible Coefficient of Performance (COP) in terms of entropy generation associated with irreversible processes. Numerical results for water-LiBr absorption system and given cold production, source and sink temperatures are presented for different effectiveness of solution heat exchanger and the components/parameters which affect most the system performance are visualised. It is ascertained that the solution circuit components are most sensitive and the individual entropy productions are inter-dependent on each other as well as on the solution recirculation ratio and the effectiveness of the solution heat ex...

Performance prediction of an absorption heat pump for utilization in industry

Recently, an absorption heat pump has been put on the market for industrial utilization. It is a H 2O-LiBr absorption machine which heats up water to 90°C with a cold source at 40°C. The capacity and COP of the machine have been studied by a computer program as a function of cold source temperature and heated fluid temperature. Also, a comparison with a compression heat pump is reported.