Hot Heat Source Research Articles

Numerical Simulation of MHD Natural Convection and Entropy Generation in Semicircular Cavity Based on LBM

To study the natural convection and entropy generation of a semicircular cavity containing a heat source under the magnetic field, based on the single-phase lattice Boltzmann method, a closed cavity model with a heat source in the upper semicircular (Case 1) and lower semicircular cavity (Case 2) is proposed. The cavity is filled with CuO-H2O nanofluid, and the hot heat source is placed in the center of the cavity. The effects of Rayleigh number, Hartmann number and magnetic field inclination on the average Nusselt number and the entropy generation of the semicircular cavity are studied. The results show that the increase in the Rayleigh number can promote the heat transfer performance and entropy generation of nanofluids. When the Hartmann number is less than 30, the increasing function of the Hartmann number at the time of total entropy generation reaches its maximum when the Hartmann number reaches 30. As the Hartmann number increases, the total entropy generation is the decreasing function of the Hartmann number. The larger the Hartmann number, the greater the influence of the magnetic field angle system. Under the same Hartman number, with the increase in the Rayleigh number, the flow function of Case 2 increases by 29% compared with that of Case 1. The average Nusselt number of heat source surfaces in Case 2 increases by 5.77% compared with Case 1.

Experimental investigation of a quantum heat engine powered by generalized measurements

Generalized measurements may allow the control of its back-action on the quantum system by interpolating from a very weak to strong projective action. Such a measurement can fuel a quantum heat engine or extract work depending on the system-meter interaction. Here, we performed a proof-of-concept experiment using nuclear magnetic resonance techniques to investigate a spin quantum heat engine driven by non-selective generalized (weak) measurements without feedback control. Our prototype of a quantum thermal device operates with a measurement protocol and a single heat bath. The protocol is composed of two non-selective measurement channels with variable measurement strengths, one dedicated to fueling the device (analogous to a hot heat source) and the other committed to work extraction. The experimental results highlight that this kind of quantum thermal device can reach unit efficiency with maximum extracted power by fine-tuning of the measurement strengths.

Attainability of Maximum Work and the Reversible Efficiency of Minimally Nonlinear Irreversible Heat Engines

Abstract We use the general formulation of irreversible thermodynamics and study the minimally nonlinear irreversible model of heat engines operating between a time-varying hot heat source of finite size and a cold heat reservoir of infinite size. We find the criterion under which the optimized efficiency obtained by this minimally nonlinear irreversible heat engine can reach the reversible efficiency under the tight coupling condition: a condition of no heat leakage between the system and the reservoirs. We assume the rate of heat transfer from the hot to the cold heat reservoir obeys Fourier’s law and discuss physical conditions under which one can obtain the reversible efficiency in a finite time with finite power. We also calculate the efficiency at maximum power for the minimally nonlinear irreversible heat engine under the nontight coupling condition.

Measurement Analysis and Evaluation of Desiccant Air Handling Units with Various Heat Source

In hot and humid summer as in Japan, dehumidification is important process for air conditioning. Desiccant air handling unit is one of the major system for dehumidification in which absorbent or sorbent absorbs moisture in the air and hot heat source is required in order to regenerate them. Desiccant air handling unit is becoming common system, however there is little information about actual operation results and evaluation based on the measurement results. This study provides actual data set and benchmarks which are useful for designing desiccant air handling unit and to evaluate energy performance of various desiccant air handling systems. Measurement results show that coefficient of performance of desiccant air handling unit itself is around 0.7 in summer with polymer sorbent. In addition, the coefficient of performance of desiccant air handling system is defined and the value depends on the coefficient of performance of heat source and the distribution between chilled water and hot water.

Comparison of Performance of Desiccant Air Handling Unit with Solar Thermal System under Various Control Methods

In hot and humid summer as in Japan, dehumidification is important process for air conditioning. Desiccant air handling unit is one of the major system for dehumidification in which absorbent or sorbent absorbs moisture in the air and hot heat source is required in order to regenerate them. In order to use solar thermal energy efficiently, flexible control, which controls the distribution of the dehumidification at precooling coil and dehumidification wheel, is proposed and analysed its performance through simulation. Simulation results show that supply absolute humidity fulfils the set point even when solar thermal energy is not supplied to regenerating coil under flexible control. On the other hand, system with conventional fixed control can not fulfil the set absolute humidity when solar thermal energy is not supplied. The unprocessed latent heat load is around 49kW, which results in higher relative humidity around 56.5%. Solar thermal energy supplied to regenerating coil under flexible control is larger than fixed control.

The impact of selected heat pumps on CO2 emissions

The increase in energy consumption in the world, with the decreasing resources of conventional energy, is increasingly leading to campaigns aimed at limiting the acquisition of energy from non-renewable sources to intensify the use of alternative energy sources. The paper presents the possibilities of obtaining heat energy using a heat pump for heating purposes, ventilation and hot water preparation. Hot heat sources for the heat pump are discussed and their energy possibilities in relation to the geographical conditions of the territory of Poland. Variants of heat pump cooperation with renewable sources in the form of air, soil and water were adopted for analysis. Two cases of co-operation of the ground collector with the heat pump are also considered, in the form of vertical and horizontal wells of the heat exchanger type. Based on the known methodology of seasonal heat demand, the final and primary energy consumption was determined for each analyzed case together with auxiliary energy. Based on the calculations carried out, the final emission of carbon dioxide was determined for individual cases and they were compared with the emission that would be generated when producing heat using a gas boiler and heating network.

Natural convection in fully open parallelogrammic cavity filled with Cu–water nanofluid and heated locally from its bottom wall

The natural convection in fully open parallelogrammic cavity filled with Cu–water nanofluid and heated locally from its bottom wall is investigated numerically for the first time in the present paper. The finite volume method together with the body fitted coordinates system are used to deal with the governing non-linear partial differential equations. The top wall of the cavity is considered fully open to the surrounding, while a specific location in the bottom wall is subjected to a local hot heat source. The other regions of the bottom wall are kept adiabatic. Both right and left sidewalls of the cavity are considered inclined and maintained to an isothermal cold temperature. The nanofluid entering form the top wall is considered cold, while it leaves in adiabatic condition from the same wall. The flow and thermal fields together with the Nusselt number are investigated for various values of Rayleigh number (103≤Ra≤105), heat source location (0.2≤δ≤0.8), cavity inclination angle (0°≤θ≤60°), volume fraction of nanoparticles (0≤φ≤0.2). The results show that for both pure and nanofluids the highest counter-clockwise circulation occurs at (δ=0.8) and (θ=0°) for all Rayleigh numbers, while the highest clockwise circulation occurs at (δ=0.5) and (θ=30°) for Ra=103 and at (δ=0.8) and (θ=30°) for Ra=105. In order to increase the heat transfer for both pure and nanofluids in the cavity, it is recommended to select (θ=60°) and make the heat source adjacent to the left sidewall (δ=0.2).

Simulation of effect of bottom heat source on natural convective heat transfer characteristics in a porous cavity by lattice Boltzmann method

The influence of bottom heat source on natural convective heat transfer characteristics in a two-dimensional square cavity fully filled with a homogeneous porous medium is numerically studied by the lattice Boltzmann method. In this physical model, the upper wall of porous cavity is set to be a cold heat source, and the bottom wall is designed as a local hot heat source. Both the left wall and the right wall are set to be adibatic. Specifically, the effects of both the position and size of bottom heat source on the properties of natural convective heat transfer are analyzed. The numerical results show that the position and size of bottom heat source have great influences on the characteristics of natural convective heat transfer, and there also exist the best position (a=4/16) and optimal size (b=0.75) of the bottom heat source for the maximal convective heat transfer intensity (Numax 10.35) and heat exchange capacity (Qmax 5.69).

A novel pyroelectric generator utilising naturally driven temperature fluctuations from oscillating heat pipes for waste heat recovery and thermal energy harvesting

Low temperature thermal to electrical energy converters have the potential to provide a route for recovering waste energy. In this paper, we propose a new configuration of a thermal harvester that uses a naturally driven thermal oscillator free of mechanical motion and operates between a hot heat source and a cold heat sink. The system exploits a heat induced liquid-vapour transition of a working fluid as a primary driver for a pyroelectric generator. The two-phase instability of a fluid in a closed looped capillary channel of an oscillating heat pipe (OHP) creates pressure differences which lead to local high frequency temperature oscillations in the range of 0.1–5 K. Such temperature changes are suitable for pyroelectric thermal to electrical energy conversion, where the pyroelectric generator is attached to the adiabatic wall of the OHP, thereby absorbing thermal energy from the passing fluid. This new pyroelectric-oscillating heat pipe (POHP) assembly of a low temperature generator continuously operates across a spatial heat source temperature of 55 °C and a heat sink temperature of 25 °C, and enables waste heat recovery and thermal energy harvesting from small temperature gradients at low temperatures. Our electrical measurements with lead zirconate titanate (PZT) show an open circuit voltage of 0.4 V (AC) and with lead magnesium niobate–lead titanate (PMN-PT) an open circuit voltage of 0.8 V (AC) at a frequency of 0.45 Hz, with an energy density of 95 pJ cm−3 for PMN-PT. Our novel POHP device therefore has the capability to convert small quantities of thermal energy into more desirable electricity in the nW to mW range and provides an alternative to currently used batteries or centralised energy generation.

ハイドレート生成速度がCO2ハイドレート発電機のエネルギー貯蔵特性に及ぼす影響

In our previous study of a CO2 hydrate engine generator (CHEG), the analysis results were clarified that the net electricity output was 1.4 kWh per 1 m3 water. This results were evaluated under the condition where the temperatures of the cold heat source and the hot heat source were 253 K and 313 K, respectively. Generally, the temperature of the outside air is not less than 253 K. Therefore, it is necessary to consider how to enable the energy storage by using the cold heat source which temperature is higher than 253 K. Along with the rise of the temperature of the cold heat source, the hydrate generation speed decreases and the duration time for the energy storage increases. The hydrate generation speed is proportional to the pressure difference between the generation pressure and the phase equilibrium pressure of the hydrate. The phase equilibrium pressure is possible to be moved to the high temperature and low pressure side by adding cyclopentane and its derivatives (c-C5H10, c-C5H8O, and c-C5H9F) , as generation reaction promotors, to the simple CO2 hydrate. In this paper, the influence of the hydrate generation speed on the energy storage speed is investigated, by means of numerical analysis. The numerical results indicate that the suitable promoters are c-C5H8 and c-C5H9F, corresponding to the temperature of the cold heat source is 273 K.

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

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.

Effects of layering sequence on thermal response of multilayer fibrous materials: Unsteady-state cases

Heat transfer between multilayer fibrous materials under unsteady-state conditions is regarded as an intricate and unpredictable process, and the system thermal response with different layer stacking sequences is investigated in this paper. All six possible sequences for three-layer fabrics are measured by an apparatus set up in our lab and simulated by the finite element method, taking account of the effects caused by the internal natural convection and boundary heat loss. It is found that the fabric contacting the hot heat source is the key layer in affecting the system thermal response during the dynamic unsteady-state conditions, and its volumetric heat capacity is the major factor in determining the results. In addition, the effect of the inner natural convection in the multilayer system resulted from the buoyancy is marginal, and negligible under common boundary conditions. In general this study demonstrates that the layer stacking sequences should be considered as an important factor in thermal response while designing a multilayer thermal protective suit.

A223 複数の熱源を用いたカリーナサイクルの高効率化に関する研究(OS2 再生可能エネルギー)

Kalina cycle is a power generation system for waste heat source of low temperature. This cycle is mainly used to a single heat source. By the way, there are extremely large potential of waste heat having different temperature region simultaneously. Therefore, this research examined the system which can use two or more heat sources effectively by a single system. Three different systems for two waste heat sources were proposed. Their three system were evaluated for waste water(70℃) and exhaust gas(300℃) as hot heat source. As the result, the power output of system which warming the separator latter part of a cycle directly by an exhaust gas was increased about 40 percent than one of conventional system.

Dissipated energy and entropy production for an unconventional heat engine: the stepwise ‘circular cycle’

When some entropy is transferred, by means of a reversible engine, from a hot heat source to a colder one, the maximum efficiency occurs, i.e. the maximum available work is obtained. Similarly, a reversible heat pumps transfer entropy from a cold heat source to a hotter one with the minimum expense of energy. In contrast, if we are faced with non-reversible devices, there is some lost work for heat engines, and some extra work for heat pumps. These quantities are both related to entropy production. The lost work, i.e. , is also called ‘degraded energy’ or ‘energy unavailable to do work’. The extra work, i.e. , is the excess of work performed on the system in the irreversible process with respect to the reversible one (or the excess of heat given to the hotter source in the irreversible process). Both quantities are analysed in detail and are evaluated for a complex process, i.e. the stepwise circular cycle, which is similar to the stepwise Carnot cycle. The stepwise circular cycle is a cycle performed by means of N small weights, dw, which are first added and then removed from the piston of the vessel containing the gas or vice versa. The work performed by the gas can be found as the increase of the potential energy of the dw's. Each single dw is identified and its increase, i.e. its increase in potential energy, evaluated. In such a way it is found how the energy output of the cycle is distributed among the dw's. The size of the dw's affects entropy production and therefore the lost and extra work. The distribution of increases depends on the chosen removal process.

Design and Construction of a Standing-Wave Thermoacoustic Engine with Heat Sources Having a Given Temperature Ratio

A standing-wave thermoacoustic engine is essentially composed of a stack, two heat exchangers, and tubes; the stack has many narrow flow channels and is located inside one of the tubes. One end and the other end of the stack are thermally connected to hot and cold heat sources via the heat exchangers, respectively. When the ratio of temperatures of the heat sources exceeds a critical value, the gas inside the tubes spontaneously oscillates and the stack generates acoustical energy using heat from the hot heat source. In this study, the critical temperature ratio needed for exciting the spontaneous gas oscillation was numerically calculated by changing the stack's length, flow channel radius, and position. Further, the thermal efficiency with the critical temperature ratio was calculated. These calculations allowed us to design and construct the engine in accordance with the heat source temperatures. The constructed engine worked with the heat sources having the temperature ratio 1.7 and achieved 8% of the Carnot efficiency. These obtained values quantitatively agreed with the design ones.

C08 音響負荷が進行波型熱音響エンジンに与える影響(パルス管冷凍機,熱音響機器及び関連要素と応用システム(3))

A thermoacoustic engine has attracted attention because it can work without moving parts and use variety of heat sources. When the thermoacoustic engine receives heat from a hot heat source,, the gas inside it spontaneously oscillates and outputs acoustical energy. In general, this acoustical energy is converted into electrical energy or cold energy by the thermoacoustic coolers or linear alternators. The performance of the engine depends on the acoustic-to-heat or -electric converter. In this study, we characterized the converter consists of a tank and a valve, and constructed the traveling-wave thermoacoustic engine with the characterized converter. The measurements of the performance were shown.

Thermoelectric generator placed on the human body: system modeling and energy conversion improvements

This paper focuses on the production of electricity using a thermoelectric generator placed on the human body connected to a dc-dc converter. The small difference in temperature between the hot heat source (e.g. the human body, T b = 37 °C) and the cold heat source (e.g. ambient air, T a = 22 °C), associated with a poor quality thermal coupling (mainly with the cold source), leads to a very low temperature gradient at the thermoelectric generator terminals and hence low productivity. Under these use conditions, the present article proposes an analysis of various ways to improve productivity given a surface capture system. Furthermore, we demonstrated, in this particular context, that maximizing the recovered electric power proves to be a different problem from that of maximizing efficiency, e.g. the figure of merit Z . We therefore define a new factor Z E , depending on the physical characteristics of thermoelectric materials, that maximizes electric power in the particular case where the thermal coupling is poor. Finally, this study highlights the benefit of sub-optimization of the power extracted from the thermoelectric generator to further improve efficiency of the overall system. We show that, given the conversion efficiency of the dc-dc converter, the maximum power point of the overall system is no more reached when the output voltage of the thermoelectric generator is equal to half of its electromotive force.