Thermoelectric Generator System For Waste Heat Recovery Of Low And Medium Temperature

The energy crisis and air pollution due to the excessive consumption of energy have attracted more attentions on the improvement of energy conversion efficiency.The industrial production process consumes a majority of the energy capacity.Along with the speeding up of industrialization advancement,a considerable amount of energy in forms of gas,liquid and solid requires large scope of waste energy recovery.Currently,the high temperature waste heat can be directly utilized by driving steam turbine and gas turbine to generate electricity,but there are still difficulties in the utilization of waste heat in low and medium temperature range.Thermoelectric generator?TEG?technology shows great advantages in low-grade waste heat recovery,due to its entire solid state thermal-to-electricity energy conversion mode.Considering the decreasing cost of thermoelectric modules,the abundance of low-grade waste heat and low operating cost,thermoelectric waste heat recovery technology is demonstrating its advantages.Currently,the thermal-to-electricity efficiency of thermoelectric generator for low grade waste heat generation is the bottleneck.The main challenges of thermoelectric power generation application are improving the efficiency of thermoelectric materials and optimizing the system configuration.This thesis worked on the improvement of thermoelectric material performance and optimization of system configuration.Three thermoelectric generators for low grade waste heat recovery were designed and experimented.Metal foams were filled in the flow channels for the enhancement of heat exchange and power generation performance.The working parameters,connection mode of thermoelectric modules and types of flow metal foams were experimented and optimized.Based on the experiment results,the CFD simulation of plate heat exchanger and economic analysis of thermoelectric generator for low grade waste heat recovery were conducted.The main research contents and results were as follows:?1?Carbon nanotubes?CNTs?doped Bi_0.5Sb_1.5Te₃ nanocomposite thermoelectric materialsCNT/Bi_0.5Sb_1.5Te₃ composites of different CNTs doping contents were synthesized by high energy ball-milling method.It was demonstrated that adding CNTs and stearic acid does not change the crystal structure of Bi_0.5Sb_1.5Te₃.Doping of CNTs was demonstrated to decrease the average crystallite size of Bi_0.5Sb_1.5Te₃ and improve the bonding of composite powders.The TEM photos demonstrated the well dispersion of fractured CNTs particles in the Bi_0.5Sb_1.5Te₃ matrix materials.The thermoelectric performance testing samples were prepared by tableting and anti-oxidation heat treatment.Heat treatment temperature at high temperature was experimented to significantly enhance the thermoelectric properties.Composites of different CNT contents were showed to have higher Seebeck coefficiency,electrical conductivity and thermal conductivity.The tested results suggested that doping content of CNTs at 0.38 wt%could result in the maximum enhancement of thermoelectric performance.?2?Thermoelectric generator system by using heat exchanger surface temperatureA new type of open-cell metal foam-filled plate heat exchanger based thermoelectric generator system was proposed to utilize low grade waste heat.This system could realize waste heat recovery through efficient heat exchange and in-plane thermoelectric power generation.An experimental prototype was constructed to demonstrate the feasibility.At the hot air temperature of 163?,high heat exchange efficiency of 83.16%between heated air and cold water was achieved.Several open-circuit voltage and output power improving methods have been proposed and experimented,including adjustment of the cold water flow rate,enhancement of the heated air inlet temperature and increase of the number of TE couples.The CFD simulation results showed the temperature distributions of heat exchanger surface and temperature of inlet and outlet.When the hot air inlet temperatures were 163 ?,185?and 200 ?,the maximum open-circuit voltages were 1.61V,1.8V and 1.95V,respectively.By filling metal foams in the flow channel,the heat exchange efficiency was enhanced from 64.67%to 86.96%.The advantages of metal foam filled thermoelectric generator system were verified.?3?Sandwiched thermoelectric generator systemThis part proposed a type of metal foam-filled sandwiched thermoelectric generator for waste heat recovery.Metal foam inserts of three kinds of pore densities were analyzed,considering the heat transfer enhancing features of porous metal mediums.A flow channel detachable prototype was designed to experimentally investigate the influence of metal foams on the performance of thermoelectric waste heat recovery system.The operating parameters were further experimented to improve the thermoelectric power generation efficiency,including hot air inlet temperature,cold water flow rate,metal foam pore density and thermoelectric module?TEM?connecting mode.Moreover,the performance of the system was evaluated on power generation efficiency,heat exchange effectiveness and waste heat recovery rate,respectively.The results showed that filling metal foams in the flow channels could effectively enhance the performance of the system.The maximum power generation efficiency was 2.05%,when the TEG was filled with 5 PPI metal foams.That was 29.75%higher than the value of unfilled TEG.?4?High-temperature silocone oil based thermoelectric generatorThis part proposed a metal foam filled thermoelectric generator for the utilization of liquid waste heat resource.A prototype was designed and constructed to study the performance enhancement of metal foam inserts.High-temperature oil based experiment was conducted to investigate the TEG performance in higher temperature.The hot oil inlet temperature and cold water flow rate were optimized.Specially,net power output was presented to assess the overall net power benefit.The metal foam filled TEG was demonstrated to outperform the unfilled TEG in heat exchange ability and power generation ability.The maximum power generation efficiency and net power enhancement ratio of MF-filled TEG were 2.49%and 1.33,respectively.?5?The economic performance analysis of thermoelectric gererator systemThe economical analysis of thermoelectric generator system for low-grade waste heat recovery was conducted.When the ZT value of thermoelectric modules is 1,the payback time of thermoelectric generator is 7.31 year.The electricity generating cost of thermoelectric generator were 0.365,0.231,0.185 and 0.160 Yuan/?kW h?,when the ZT values were 1,2,3 and 4,respectively.Low grade waste heat recovery through thermoelectric generator is a clean energy conversion method.Considering its long lifetime and'low operating cost,thermoelectric generator technology has bright future for the improvement of energy conversion efficienc.