Extraterrestrial energy harvesting for all-day electricity generation using thermoelectric generators assisted by latent heat storage

Abstract

The increasing need for sustainable energy solutions has spurred development of renewable technologies like solar energy and radiative sky cooling. While thermoelectric generators (TEGs) show promise for solar conversion, their adoption faces challenges including inaccurate performance prediction, intermittent output, and low efficiency. Current research also overlooks the potential of combining multiple heat sources. This thesis investigates hybrid solar/radiative cooling TEG systems through innovative modeling, design optimization, and performance analysis. By integrating spectral-selective thermal management, radiative cooling, and phase change materials, the study develops novel approaches for efficient 24/7 operation while examining heat transfer mechanisms and system dynamics. The work provides theoretical models, experimental validation, and multidimensional analysis to advance next-generation thermoelectric energy harvesting. Firstly, a novel spectrally selective thermal-electrical-coupled model based on thermal resistance theory is developed and validated to accurately predict the heat transfer and electric performance of the TEG system. Unlike traditional models that often overlook spectral effects, this new model comprehensively evaluates solar thermoelectric generator systems, considering the full spectrum's influences. Subsequently, the five main impacts of various ambient & design parameters on system performance were thoroughly examined, and then the influence level of each factor on the system was determined by an orthogonal calculation analysis. Results indicate the TEG with ideal selective absorber (ISA) provides the best power generation performance, with a power generation of around 2.06 W/m² and a temperature difference of about 3.18 K. The maximum power generation is 2.89 and 2.87 W/m² for the ISA and ideal broadband absorber (IBA) systems when bottom cooling temperature is 275 K, respectively. For every 100 W/m² increase in solar irradiance, there is a temperature difference increment of about 0.31 K. The IBA and ideal selective absorber and emitter (ISAE) systems have the function of generating electricity 24 hours a day. Moreover, the sky radiative cooling capacity provided by the ISAE system is greater than that of the radiative cooling TEG system. Subsequently, to enhance thermoelectric generator system performance through the combined use of solar energy and radiative sky cooling, the heat transfer mechanism was analyzed. This analysis revealed the impact of ambient factors on system heat losses, emphasizing the significant role of wind speed in convective heat loss. Findings indicated that for every 1 m/s increase in wind speed, convective loss increased by an average of 8.76 W/m². For the ideal selective absorber and emitter system, the main heat losses from the absorber occur due to radiative cooling to the sky as well as for the ideal broadband absorber system, as opposed to convection and ambient radiative losses. These sky radiative cooling losses account for approximately 83.8% and 73.7% of the total heat losses, respectively. Results also highlighted the importance of selective absorber and emitter systems for reducing radiative cooling losses. Elevated ambient temperatures were found to decrease heat loss overall. Recommendations included incorporating a PE film to mitigate convection losses and using phase change materials to store heat efficiently during peak solar radiation for nighttime applications. Furthermore, to achieve the goal of continuous power generation throughout the day, this study experimentally and theoretically scrutinizes the performance of an auto-switching solar-heating TEG/PCM unit under simulated solar irradiation, encompassing both the experimental configuration and theoretical model for in-depth system analysis. Results show that the total electric generator (EG) over 24 hours relies on the EG during the lighted operation phase (LOP). Once the PCM in the aluminum box (AB) fully melts, a secondary temperature rise near the bottom of the top cover occurs during the transition to static phase (TSP). Unit 80-AB achieves the highest total EG under a total solar irradiance of 4 kW·h/m², with values of 3.72 and 0.15 W·h/m² for the LOP and TSP, and when exposed to 8 SSs for 5 hours, it peaks at 4.4 and 0.19 W·h/m² for the LOP and TSP, respectively. The open-circuit voltage of unit 80-AB reaches approximately 110 mV, with the maximum output power amounting to 0.34 W/m² when the load resistance is 5 Ω. Finally, aiming to effectively synchronizing with the environment and society, a comprehensive 4E analysis was conducted for the proposed units, focusing on solar parameters, cooling methods, and the environmental advantages facilitated by PCM integration. The results demonstrate that despite consistent total solar irradiation, higher irradiation intensities correspond to increased power generation. The dollar-per-watt (DPW) values fall below 20 $/(W·m²) during the LOP, range between 10 and 10⁴ $/(W·m²) at the TSP, and span from 10⁴ to 106 $/(W·m²) during the full static state. Once solar irradiation intensity is set, the initial maximum power outputs for each unit are fixed. Unit 80-AB attains a maximum exergetic efficiency of about 0.124% under approximately 968 W/m² solar irradiance from 8 solar simulators (SSs), and a minimum DPW value of around 0.5 $/(W·m²), leading to an annual reduction of 0.43 kg/year in CO2 emissions. It shows the lowest DPW among all units, ranging from around 0.43 to 0.67 $/(W·m²) over various durations, with a peak after ten hours. Additionally, it achieves the highest annual CO2 savings, approximately 1.11 kg/year for ten hours of irradiation. Experimental unit 80-AB consistently saves 2 to 3 times more CO2 annually than control unit 80-ABC across all conditions. In summary, the key academic contributions derived from this thesis are summarized as follows: a novel spectrally selective thermal-electrical-coupled model has developed for improved system assessment, enabling accurate predictions of heat transfer and electrical performance; the model was utilized to analyze heat transfer mechanisms and optimize system performance; an innovative auto-switching solar-heating TEG/PCM unit was designed and experimentally validated for continuous 24-hour power generation; and a comprehensive 4E analysis was conducted to assess sustainable energy generation and environmental impact mitigation.