Search

Solar energy can be transformed to storable chemical fuels through high temperature thermochemical processes driven by concentrated solar radiation. Solar thermochemical reactors (STRs) are the key equipment to these processes. An STR integrated with heat pipe technology is proposed. The core part of this reactor is a high temperature special shaped heat pipe (HTSSHP), which is composed of a flat disk-shape evaporator and multiple cylindrical condensers. A second generation HTSSHP with sodium as the working fluid was fabricated and tested in this work. The evaporating behaviors of sodium and isothermal characteristics were investigated at various operation conditions. A temperature pulsation phenomenon on heating surface was observed lasting for a short period during its startup when the heat flux beyond 47.2 kW/m2, implying the generation of film boiling. Further, the operation characteristic of HTSSHP was experimentally proven steady under various heat fluxes and inclination angles, and it showed a good ability to spread temperature at the cooling side where the thermochemical processes take place. This work will facilitate the preliminary understanding of the operation characteristics in HTSSHP. [ABSTRACT FROM AUTHOR]

• Development of a pressure-based phase change model for CFD modelling of heat pipes. • Self-adjusting ability of heat input and output of heat pipes. • Considering the influence of hydrostatic pressure of liquid on boiling behaviors. • Validation of simulation results based on experimental results. • Sensitive analysis on key parameters of pressure-based phase change model. In order to overcome the problem of thermal balance self-adjustment and consider the hydrostatic pressure influence on boiling in CFD modelling of vapor/liquid phase change heat transfer of heat pipes, a pressure-based phase change model is proposed in this work. This model was then employed to develop a CFD model for the wickless heat pipes, or known as thermosyphons, based on VOF method, which was validated by experimental tests of ammonia thermosyphons. The results show that the variations of predicted thermal parameters during start-up are in agreement with experimental results with reasonable accuracy, and the recommended empirical coefficients of vaporization and condensation in pressure-based phase change model are 0.5 s−1 and 1.5 s−1, respectively, due to the simulation errors of thermal parameters at steady state are all within 3.7%. The CFD model has a decent self-adjusting ability for heat input and output of thermosyphons. In addition, the effect of hydrostatic pressure of liquid on boiling behaviors is well predicted with the use of pressure-based phase change model. This work develops a new vapor/liquid phase change model for the CFD modelling of heat pipes, showing a higher rationality and a wider application range comparing to Lee model. [ABSTRACT FROM AUTHOR]

• A compact solar collector was designed and optimized for integration with buildings. • Optical/thermal performances were analyzed through theory, simulation and experiment. • Annual optical and thermal efficiencies are 57.6 % and 30.6 % @ 250 °C, respectively. • The tilt angle within latitude ± 10.5° has little effect on the optical efficiency. • The efficiencies can improve by manufacturing accuracy, vacuum degree and emissivity. Process industry consumed 15 % to 30 % of the global energy, and 40 % of it is thermal energy demand in the range of 80 °C-250 °C. To produce mid-temperature heat, a new solar collector was designed with heat pipe evacuated tube, including several parallel micro-parabolic troughs, heat pipes, double-layer vacuum tubes contained in a compact frame and an external mechanical solar tracker. In addition, the selective absorbing coating (SAC) was applied on the inner surface of the vacuum tube, and the gap between the heat pipe and the vacuum tube was filled with heat transfer oil. Since the height of the designed collector was only 124.5 mm, it can be easily integrated with buildings. Genetic algorithm (GA) was used to optimize the structural parameters based on maximum obtained solar energy of the collector, and theoretical analysis and numerical simulation are carried out for optical and thermal performance of the collector. The analytical and numerical results showed that the annual average optical efficiency is 57.6 % and the thermal efficiency is 30.6 % at 250 °C when the normalized temperature difference is 0.23. By exergy analysis, the collector has a maximum exergy efficiency of 15.5 % with operating temperature of 204.4 °C, which is the station of the best thermal performance of the collector. Furthermore, theoretical model was verified to be valid by a prototype. Overall, the proposed collector is suitable for mid temperature industrial applications and integrated with rooftop. [ABSTRACT FROM AUTHOR]

A high temperature special-shaped heat pipe (HTSSHP) coupling the flat plate heat pipe (FHP) and cylindrical heat pipes (CHPs), respectively as the thermal receiver and heat transfer unit, is proposed as the key component in a solar thermochemical reactor. The startup characteristics, isothermal performance, and thermal resistance variation of HTSSHP were experimentally investigated and analyzed. The results show that heat transfer limits are not encountered in tests and HTSSHP can startup effectively. The continuum flow transition temperature of sodium vapor in FHP and CHPs are 426.0 °C and 442.3 °C respectively and the startup behavior of HTSSHP differs from the flat-front phenomenon in a traditional high temperature cylindrical heat pipe with a constant transition temperature each part. However, the temperature difference response in startup can be employed to indentify the sodium melting and the flow regime transition in FHP and CHPs, validating its startup law. HTSSHP possesses the potential to inhibit the hotspots on heat absorber surface and meanwhile improve the temperature distribution in reaction chamber. The overall thermal resistance in HTSSHP reduces with increasing operating temperature, ranging from 0.12 to 0.19 °C/W that is in the same order of magnitude in a typical heat pipe. Furthermore, the various heat inputs and cooling rates have considerable effect on the thermal resistance in cooling side. This work is helpful for establishing the preliminary understanding of the operating characteristics on HTSSHP and provides some suggests for its normal operation and structure optimization. [ABSTRACT FROM AUTHOR]

High-temperature two-phase loop thermosyphons (TPLTs) have the potential to be applied in high-temperature thermochemical conversions, concentrated solar energy, and nuclear energy. However, there has been limited investigation on the thermal performance of TPLTs so far. This work experimentally examines the thermal performance of TPLTs using alkali metals (NaK alloy and pure Na) as working fluids, aiming for internal energy recovery of a double-riser reactor. The effects of filling ratios (FR s), heating temperature, and altitude difference between the evaporator and condenser are investigated regarding start-up characteristics and steady-state performance. The experimental results show that the TPLT with NaK as the working fluid can successfully complete the start-up when the heating temperature is set to above 650 °C under the test conditions. However, the TPLT using pure Na as the working fluid can't successfully complete the start-up from a frozen state without a preheating operation. According to the variations in thermal resistance, an FR of 38% in the TPLT allows for the best heat-transfer performance under steady state when compared to the conditions that FR s are equal to 25% and 50%. Additionally, placing the condenser slightly higher than the evaporator is beneficial for improving the steady-state performance. However, if the condenser is lower than the evaporator, its steady-state performance is remarkably degraded. • High-temperature two-phase loop thermosyphons (TPLTs) for internal energy recovery in a double-riser reactor. • Experimental investigation on thermal performance of TPLTs using NaK and Na as working fluids. • Effects of filling ratios, heating temperature, and altitude difference between evaporator and condenser. [ABSTRACT FROM AUTHOR]

• Experimental investigation of sodium heat pipe under swing conditions. • Thermal tests under static state, and various swing amplitudes and speeds. • Comparison of swing state tests to static state. • Analysis of swing influence on start-up and thermal resistance. High-temperature heat pipes (HTHPs) have a potential of being used for passive heat dissipation of marine nuclear reactors in emergency. In order to understand the influence of wave action on the performance of HTHPs, a sodium heat pipe is fabricated and experimentally tested in this work. The effects of low-frequency swing on start-up and thermal performance of this heat pipe are examined and compared with the static state tests. The results show that swinging motion has a negligible influence on the whole start-up performance of the heat pipe, however, it would lead to the small-amplitude periodic temperature fluctuations especially at the evaporator. Temperature fluctuation frequency is nearly corresponding to the swing, and the temperature fluctuation amplitude is increased with the swing amplitude. The steady-state thermal performance is slightly decreased with increasing swing speed and decreasing swing amplitude in terms of thermal resistance under test conditions, but the influence seems negligible as well. [ABSTRACT FROM AUTHOR]

Highlights • Thermal performance investigation of thermosyphons at shallow geothermal temperature. • Development and validation of CFD modelling for ammonia thermosyphons. • Two-phase flow filed distribution in ammonia thermosyphons. • Relevance of inner phase-change heat transfer behaviors to thermal performance. Abstract The steady thermal performance of ammonia thermosyphons at shallow geothermal temperature, with the feature of low heat flux, are experimentally investigated in this work. In addition, a CFD (Computational Fluid Dynamic) modelling considering the phase-change heat transfer in the thermosyphon is established to reproduce the inside heat transfer behaviors and explore the corresponding relevance to thermal performance of these thermosyphons. The experimental results show that the influence of flow rate of cooling water q cooling on the total thermal resistance is significant, but the heat transfer power Q practical is mainly dependent on the temperature of cooling water T cooling. The maximum Q practical for the thermosyphon of 30% filling ratio under testing conditions is about 38 W, which is obtained with the heating length at 0.6 m. Both the increase and decrease of l e can reduce Q practical and raise R total. For filling ratio at 20%, the heat transfer power almost halves comparing to 30% filling ratio. The simulation results reveal that dropwise condensation is the main heat transfer mechanism at the condenser of these thermosyphons, and a better thermal performance is expected when the steady boiling liquid pool is nearly identical to the evaporator length for these thermosyphons operating at shallow geothermal temperature. This work provides a deep insight into the inner heat transfer behaviors of ammonia thermosyphons at low heat flux and can be used to facilitate the performance optimization of thermosyphons in shallow geothermal energy utilization. [ABSTRACT FROM AUTHOR]

A hybrid ground source heat pump system employing the super-long flexible heat pipes to extract heat from ground is proposed in this work. To evaluate its energy performance and the temperature recovery characteristics near boreholes during space heating season, a comprehensive simulation model was developed considering the dynamic performance of heat pump and the heat transfer of heat pipes and surrounded soil. The results show that the depth of boreholes should be not less 70 m to guarantee a higher long-term energy efficiency. The seasonal coefficient of performance increases with increasing groundwater advection velocity and decreasing building load, with a maximum value up to 4.2 under examined conditions. The short-term variations of ground temperature near boreholes are sensitive to the borehole depth and building load. The overall ground temperature continuously decreases under the condition with no groundwater advection throughout the heating season, but with the presence of groundwater advection, the variation tendency would almost level out after several days of decrease. In addition, it is also revealed that this system has a potential to reduce the ratio of the electrical consumption of circulation pumps to the total consumption, with a reduction of about 7%, showing an advantage of saving energy over the traditional ground-source heat pumps. [ABSTRACT FROM AUTHOR]

Loop thermosyphon, as an efficient heat transfer device, is considered as a potential receiver for high power solar thermal applications. A horizontal loop thermosyphon with metal foam (MF-HLTS) was proposed for further improve the thermal stability when operated in horizontal position, and experimentally investigated compared to that without metal foam (S-HLTS) in this work. The experimental results show that the filling ratio of 57.5% produced the lowest total thermal resistance of 0.056–0.093 K/W, with a maximum increase of 836% in terms of thermal performance at steady-state. Meanwhile, the start-up performance of MF-HLTS with the progressive start-up is superior to that of the S-HLTS and EF-HLTS with the overshoot start-up in terms of start-up stability and time under various filling ratios and heat fluxes. Additionally, the bidirection-oscillation flow is easy to occur inside S-HLTS, resulting in temperature and pressure fluctuations with amplitudes reaching 12.53 kPa and 2.67 °C. For MF-HLTS, the internal flow pattern exhibits stable unidirectional flow, with a maximum improvement of 78.5 times. All results demonstrated that the horizontal loop thermosyphon with the auxiliary of a metal foam in evaporator and straight section can greatly improve thermal performance and flow stability, which is promising in the energy utilization applications. • Enhancement horizontal loop thermosyphon with metal foam was developed. • Both the thermal performance and flow stability were significantly improved. • Thermal and flow performance and with a maximum improvement of 836% and 78.5 times. • A new flow pattern distribution for the enhancement HLTS was established. [ABSTRACT FROM AUTHOR]

• Development of a comprehensive CFD model for liquid-metal heat pipes. • Model validation by heat output and temperature distributions in steady state. • CFD investigation on effects of hydrogen permeation in a sodium heat pipe. • Hydrogen migration and distribution along with phase change of sodium. • Further structure optimization suggestions for the hydrogen window. The use of hydrogen window is a potential way for liquid metal heat pipes (LMHPs) to avoid hydrogen inactivation when operated in a hydrogen-containing atmosphere. To reveal the effects of hydrogen permeation on heat transfer in LMHPs in a visual way, we develop a comprehensive CFD model in this work for the first time, and experimental tests are also carried out for model validation. The experimental results show that the top end of an LMHP reduces to below 400 ℃ after hydrogen inactivation, when the heating temperature of the evaporator is maintained at 950 ℃. It takes about 150 min for the LMHP to recover performance by flushing its hydrogen window with nitrogen controlled at 50 mL/min. The CFD model is demonstrated reliable by comparing the predicted heat output and temperature distribution with the experimental results in steady state. The CFD model is capable of reproducing the liquid up-throwing phenomenon, and revealing the impact of hydrogen-buffer formation on heat transfer. In addition, the hydrogen migration along with the returning condensate leads to the formation of a gas blanket covering the inner surface in the condenser, which has a negative effect on the heat transfer as well. [ABSTRACT FROM AUTHOR]

• Super-long flexible thermosyphon (SFTS) fabricated by metal corrugated pipe, 32m in length. • Field tests of SFTS for shallow geothermal utilization. • Start-up and steady-state thermal performance of SFTS. • CFD simulation of long-term underground temperature variation near SFTS. • A method to avoid the heat transfer problem due to super-high liquid column of working fluid in super-long thermosyphon. A super-long flexible thermosyphon (SFTS) fabricated by the corrugated pipe, with a total length up to 32m, was field-tested in a drilling, in order to evaluate its potential for use in shallow geothermal utilization. The seasonable dependence of local underground temperature and air temperature, the corresponding temperature response of SFTS, and the its heat transfer characterists were investigated. The underground temperature recovery characteristics under the operation of uni-directional heat extraction were also conducted by a combination analysis of field tests and CFD simulation. The temperature response characteristics of SFTS in start-up of field tests show that the SFTS could operate effectively as the vaporization of working fluid occurs all over the whole evaporator. This indicates that employing a corrugated pipe prevents the formation of a super-high liquid pool at evaporator, which would inhibit the boiling of working fluid at the bottom. The maximum heat transfer power of SFTS obtained in field tests of this work is approximately 305W under the conditions with the inlet temperature of cooling water controlled at 5.5°C and the flow rate at 1500mL/min. The analysis of steady-state heat transfer results reveal that the underground temperature plays the most important role in the overall thermal performance of SFTS, followed by the flow rate and the temperature of cooling water. The long-term uni-directional heat extraction of SFTS from underground for heating is feasible and however the heat extraction rate is recommended to be maintained below 30W/m at the drilling site aiming to make sure a good sustainability. This work reports field tests of a super-long flexible thermosyphon in geothermal utilization for the first time. [ABSTRACT FROM AUTHOR]

• Ground temperature variation characteristics of a GSHP system by employing SFHPs through CFD technique. • Based on the coupling of thermal resistance model of SFHPs and the heat pump performance analysis. • The influences of hydrogeology conditions and operational strategies on ground temperature. • Thermal interference among SFHPs and cold accumulation phenomenon near boreholes were analyzed. • Recommendation of optimal layout and installation of GSHP systems. In this work, the ground temperature variation characteristics of a ground source heat pump system equipped with super-long flexible heat pipes for the extraction of shallow geothermal energy were investigated by computational fluid dynamics for the application of space heating. The study was based on the dynamic analysis results from the coupling of the thermal resistance model of super-long flexible heat pipes and the heat pump performance. The effects of groundwater advection, soil thermal properties, the layout of super-long flexible heat pipes, and the operational conditions were considered. Results show that groundwater advection plays the most important role in the variation of soil temperature. With no activity of groundwater advection, the ground temperature near boreholes continuously decreased throughout the heating season from approximately 290.6 K to 279.3 K, and it cannot recover to the initial temperature before the end of the transition season. The area of thermal plume of the upstream boreholes is much smaller than the downstream under the influence of groundwater advection. The thermal interaction of boreholes in the vertical direction of groundwater advection almost disappeared when the advection velocity exceeded 0.5 m/d. The cold accumulation near the boreholes can be alleviated under the intermittent operation by reducing the working time. The high soil thermal conductivity weakened the cold accumulation phenomenon. This work provides a theoretical basis for the optimization of the design and installation of ground source heat pump systems equipped with super-long flexible heat pipes. [ABSTRACT FROM AUTHOR]