Model development and performance evaluation of thermoelectric and radiative cooling module to achieve all-day power generation
Graphical abstract
A novel and simple radiative cooling driving thermoelectric generator is developed to achieve all-day power generation and the developed module achieves significantly greater power generation.
Introduction
Electricity is indispensable for human production and life in the 21st century. However, there still exist 1.3 billion people worldwide lacking reliable access to electricity, especially in developing countries [1]. Photovoltaic technology can address part of the challenge [2,3], but note that lighting demand, one of the main electricity consumption for developing rural areas, generally occurs at night, when photovoltaic technology can't provide the corresponding solution. Thus, it is essential to employ other passive energy technology to develop the novel device to achieve all-day power generation.
Thermoelectric generator (TEG), harvesting energy from heat flux to convert into electricity via Seebeck effect, can help achieve all-day power generation, when providing continuous temperature gradient [4,5]. Due to the superior reliability and durability, TEG has been widely employed to recovery power from industrial waste heat [6], vehicle waste heat [7] and solar energy [8,9]. However, industrial waste heat and vehicle waste heat stem from active energy input and solar energy has severe time dependence, thus nearly all these heat sources can't provide passive and continuous heat flux to achieve all-day power generation.
Passive radiative cooling technology, dissipating terrestrial heat to outer space with a temperature of 3 K via the 8–13 atmospheric window [10,11], has the potential to provide reliable and continuous heat flux to drive thermoelectric generator. Especially with the realization of daytime radiative cooling, this field has drawn extensive research efforts [12,13]. In 2014, the milestone work by Fan and coworkers [14] achieved the daytime radiative cooling performance with the photonic structure for the first time. Under a solar radiation of 850 W/m2, a maximum temperature drop (MTD) of 4.9 K and a radiative cooling power at ambient temperature (RCPAT) of 40.1 W/m2 were successfully achieved. Zhai et al. [15] proposed a SiO2 metamaterial, with a solar reflectivity of 97% and an infrared emissivity of 93% in the atmospheric window and they achieved an average RCPAT of 93 W/m2 over several days of testing. Kou et al. [16] employed fused quartz and polydimethylsiloxane to further improve the mid-infrared emissivity and achieved a MTD of 8.2 K and a RCPAT of about 127 W/m2. Leroy et al. [17] developed the polyethylene aerogel with ultralow thermal conductivity to further improve the radiative cooling performance and they achieved a MTD of 13 K and a RCPAT of about 96 W/m2. Additionally, Chen et al. [18] developed a near vacuum radiative cooling device with the ZnSe rigid wind cover and selective emitter. They achieved a MTD of 42 K, an amazing temperature drop.
Despite the rapid improvement in radiative cooling performance, there still lack systematic investigations on radiative cooling driving TEG (RC-TEG) to achieve all-day power generation. Byrnes et al. [19] investigated the power generation potential with radiative cooling technology based on Carnot limit. They reported an average power generation of 2.7 W/m2 with radiative cooling in Oklahoma, indicating a Carnot-limit efficiency of 1–2%, far lower than the theoretical Carnot-limit efficiency with radiative cooling technology (about 15%). Mu et al. [20] investigated the power generation performance of RC-TEG device with the thin polyethylene (PE) wind cover. But the experimental device with PE wind cover can only achieve a nighttime MTD of about 4 K and nearly no daytime temperature drop in humid Shanghai, thus leading to an almost unobservable power generation. Raman et al. [21] also employed PE wind cover to improve the performance of RC-TEG. The developed RC-TEG device can achieve a MTD of about 4.5 K at night and a power generation of about 0.025 W/m2, enough to drive the LED with 10% of the maximum brightness. Simple model results revealed that a power generation of 0.5 W/m2 can be achieved via improving radiative cooling performance in Stanford.
From the above discussions, the existing RC-TEG devices with PE wind cover can only achieve an extremely low power generation. The main reason is that the RC-TEG devices with PE wind cover can't effectively reduce the temperature of TEG cold side, thus leading to a low temperature difference between the TEG cold and hot side. Note that Raman et al. [21] pointed out the power generation of RC-TEG device can be increased by about 20 times via improving radiative cooling performance. And the available temperature drop in radiative cooling field has been improved to about 13 K with polyethylene aerogel wind cover [17] and 42 K with ZnSe rigid wind cover [18]. Thus, it can be expected that the power generation performance of RC-TEG can be significantly improved via optimizing the experimental device.
In this work, a novel and simple RC-TEG module is developed based on optically selective and thermally insulating polyethylene aerogel (PEA). Compared with the RC-TEG module with thin PE wind cover, the proposed RC-TEG module has much greater power generation performance and applicability, due to the low thermal conductivity of PEA. Subsequently, the mathematical model is developed to optimize the RC-TEG module and investigate the impact factors on the performance of RC-TEG systematically. Additionally, case studies of the RC-TEG module in the inland remote areas and island off-grid areas are conduct to investigate the operation performance of RC-TEG module. Lastly, the potential of RC-TEG technology is explored based on ideal condition.
Section snippets
Systems description and material properties
In this work, the proposed RC-TEG module includes radiative cooling sub-module and TEG sub-module. Radiative cooling sub-module provides cooling for the cold side of TEG sub-module, and heat sink provides heating for the hot side of TEG sub-module from the ambient, thus forming temperature gradient and generating electricity. PEA with high solar reflectivity and high mid-infrared transmissivity [17] is employed in the proposed RC-TEG module (Fig. 1a) to reduce the convective heat exchange, thus
Model development
Due to the complicated structure of the developed RC-TEG module, the simplifications are essential to develop the one-dimensional and steady-state heat transfer models:
- (1)
The temperature and heat flux are uniform in every layer of the RC-TEG module.
- (2)
The TEG parameters are independent of temperature.
- (3)
The energy losses only occur in the top and bottom of the RC-TEG module, i.e. PEA or PE wind cover and heat sink. And energy losses from module side are neglected. Meanwhile, due to ultrathin PE film,
Power generation of different radiative cooling materials
Before discussing the power generation of the developed RC-TEG module, it is essential to investigate the radiative cooling difference between the RC-TEG module with PEA and the module with the PE wind cover. Thus, this work firstly employ ideal broadband emitter and ideal selective emitter to explore the radiative cooling performance (without TEG module). The both emitters are exposed to an environment with a solar radiation of 1000 W/m2, an ambient temperature of 300 K, a TWC of 1500 atm-cm
Conclusion
To address the challenge that a lot of people worldwide lack reliable access to electricity in developing rural areas, thermoelectric generator driven by radiative cooling technology is investigated systematically via the developed RC-TEG modules and the corresponding mathematical model. The work draws the following conclusions:
- (1)
For the radiative cooling materials discussed in this work, the optimal area ratio of radiative cooling module and TEG module is between 6.2 (ideal broadband emitter)
Credit author statement
J. Liu, J. Zhang and D. Zhang developed and performed the modeling. J. Yuan and J. Xing reviewed the corresponding literature. J. Liu and Z. Zhou edited and reviewed the manuscript. All authors contributed to the work. Z. Zhou is the corresponding author.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by Tianjin Science and Technology Commission, China (Contract No. 18ZXAQSF00030).
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These authors contributed equally to this work.