Performance Of Thermoelectric Power Generation Coupling System Based On Pulsating Heat Pipe With Nanofluids

In the context of carbon peak and carbon neutrality,waste heat is regarded as one of the most important zero-carbon heat sources.Waste heat recovery is one of the important ways to indirectly achieve dioxide carbon emission reduction.When common waste heat recovery technologies,such as heat pump,organic Rankine cycle(ORC)and heat pipe heat exchanger,are applied to waste heat recovery,especially low-grade waste heat recovery,there are some limitations,such as the higher initial investment cost and lower efficiency.Thermoelectric power generation technology has unique advantages,such as zero carbon emission,long lifespan,no moving mechanical part,but it is limited by the lower thermoelectric output performance when recycling low-grade waste heat resources.The synergistic effect of heat pipes and thermoelectric generators(TEGs)can optimize performances of waste heat recovery systems.Pulsating heat pipe(PHP)is a new type of heat pipe with advantages of simple structure,low manufacturing cost and excellent heat transfer ability.PHPs can be used instead of traditional heat pipe in low-grade waste heat recovery and thermoelectric power generation systems.Therefore,for the actual demand of waste heat recovery and utilization,this paper carries out a series of experimental studies and theoretical analysis from the aspects of performance optimizations of PHPs and thermoelectric power generation coupling system based on PHPs.Surfactant-free multi-walled carbon nanotubes(MWCNTs)-based nanofluids were prepared by means of strong acid oxidation,chemical reduction and two-step methods.An experimental set-up with swing motions was first designed in order to study effects of working fluids,inclination angle,and swing motion on heat transfer performances of flat-plate PHPs.The experimental results indicate that the optimal working fluid is Ag-MWCNTs hybrid nanofluids,which has a enhancement ratio of up to 30% compared to absolute ethanol.Both swing motion and inclination angle increase the thermal resistance,and in the worst case,swing motion can reduce thermal performances of flat-plate PHP by 5.85%,which is mainly due to additional pressure drops induced by swing motion.In addition,swing motion also leads to periodic fluctuations in temperature differences between evaporation and condensation sections of flat-plate PHP.In order to explore the influence mechanism of different working fluids and operating conditions on thermal performances of PHP,a visualization experimental set-up based on flat-plate PHP was designed and built,and vapor-liquid two-phase flow characteristics and flow pattern evolution in flat-plate PHP were observed using a high-speed camera.Visualization experimental results show that the attached MWCNTs nanoparticles can modify the channel wall surface,enhance wettability and accelerate condensation liquid reflux,thereby enhancing heat transfer process.In order to investigate effects of different factors on oscillation flow characteristics of vapor plugs and liquid slugs,based on the mass-spring-damping model,an advanced oscillating motion model of vapor plugs and liquid slugs in symmetric and asymmetric rectangular channels of flat-plate PHP was developed considering the contact angle hysteresis and the transverse heat conduction.When developing a new working fluid or channel structure,the model can be used to predict the effectiveness and feasibility.Considering the advantages of flat-plate PHP in heat transfer performance and external flat shape,a novel thermoelectric coupling system based on flat-plate PHP with TEGs was designed to improve the thermoelectric output performance of TEGs.The influencing rules and mechanisms of the type,number,layer number,connection mode and installation mode of TEGs,and cooling water flow rate on performances of the coupling system under constant heating temperature and constant heating power conditions were comparatively studied,and the feasible methods to optimize the coupling performance were determined.The experimental results show that compared to the flat heat pipe,the maximum thermoelectric conversion efficiency of the coupling system is improved by 63.7%,and the thermal resistance of flat-plate PHP only accounts for 8.34% of the total thermal resistance of the coupling system.PHPs-based waste heat thermoelectric power generation system was designed and established based on heat exchange technology and thermoelectric conversion technology.The performance and economy of the system under recycling modes were studied and analyzed.Performances of PHPs-based waste heat recovery system can be enhanced by selecting appropriate recycling modes,using slotted fins,increasing condenser lengths and PHPs number.The maximum heat recovery rate of the system is 85.3%.The maximum deviation between the experimental effectiveness and the predicted effectiveness obtained from an effectiveness-number of transfer units(ε-NTU)theoretical model is about 15%.The system also displays good economic benefits with a payback period of about 2 years,and it is more suitable for low-grade waste heat recovery,such as industrial waste heat and domestic waste heat.These results can be used as data and theoretical support to guide practical applications of PHPs in the field of waste heat recovery and thermoelectric power generation,which lays a foundation for further promoting the technological innovation of waste heat utilization.With the perfection and development of performances and operating mechanisms of PHPs,low-cost and cost-effective utilization of waste heat resources will gradually become the reality,thereby significantly reducing the consumption of primary energy,such as coal,and effectively reducing carbon emissions,which will be helpful to the realizing of carbon peaking and carbon neutrality goals.