**1. Introduction**

The increasing presence of humans in orbit over the last 50 years has shown that humans can adapt to short-term space flight. However, we still know very little about the long-term exposure to the space flight environment and its health-related consequences. Future missions to other planets and space objects, such as Mars and the moon and asteroids, provide a remarkable scientific opportunity for space biologists to explore life's ability to adapt to the environment of space flight during long-term missions.

More specifically, the study of animals in space allows us to investigate the effect of gravity on biological development, an area of research that is not open to humans. The ISS era promises an opportunity to observe and test various features of animal development during long-term exposure to microgravity, as well as access to centrifuges where specimens can be exposed to partial gravity loads. Space probes such as Neurolab provided evidence that frogs and rats need critical periods of gravity for biological development. The ability to store animals on the ISS for multiple life cycles allows scientists to determine exactly how, when, where, and why these gravitational dependencies exist [1]. By studying experimental animals aboard the space probe, scientists can better understand the adaptive response of animals or humans to long-term space flight. The results of these missions can also help determine the requirements for optimal human health in space [2,3].

There is no doubt that the Earth's gravitational field affects the physiology, morphology, and the behavior of life in almost every phenomenon. Space biology research covers a wide range of biological subfields, including gravity, developmental, and even radiation biology. It also focuses on advanced techniques, including molecular technology, genomics, DNA arrays, gene arrays, cell culture technology, and the study of related habitat systems [1]. Here, the most essential technique in life science research is polymerase chain reaction (PCR). PCR is used to make millions of copies of target DNA fragments, which have a wide range of specialized applications and are used by scientists in all fields of biology [4–6]. This is because PCR allows the identification and quantification of specific target species, even when very small numbers are present. Real-time PCR is based on the revolutionary method of PCR, an advanced laboratory technique in molecular biology, which monitors the amplification of target DNA molecules in real-time during PCR [7–9]. Thus, real-time PCR is an indispensable tool in modern molecular biology and has transformed scientific research and diagnostic medicine.

The PCR allows the polymerase to select a gene to amplify from a mixed DNA sample by adding small pieces of DNA that are complementary to the gene of interest. These tiny pieces of DNA are known as primers because the polymerase binds and prepares a DNA sample ready to begin copying the gene of interest. During PCR, temperature changes inside the PCR unit are of utmost importance, which is used to control the activity of the polymerase and the binding of primers. To start the reaction, the temperature is raised to 95 ◦C so that all double-stranded DNA is melted into a single strand. Then lower the temperature to 60 ◦C, which allows the primer to bind to the gene of interest. Accordingly, the polymerase has a place to bind and can start copying strands of DNA. The optimal temperature for the polymerase to work is 72 ◦C, so at this point, the temperature is raised to 72 ◦C to allow the enzyme to work faster. This temperature change is repeated over about 40 cycles. One copy continues until billions of copies have been made [10–12].

The efficient use of waste heat energy from ISS far from Earth is critical to the efficient operation of ISS. Since a large amount of thermal energy generated from spacecraft systems in space is released to the outside, applying a thermoelectric power generation system as a waste heat recovery device to utilize waste heat can increase the overall efficiency of the existing system. In this regard, we explored the applicability of the method of using thermoelectric power modules to utilize the temperature changes in the PCR process performed on spacecraft.

The main advantages of thermoelectric generators are direct energy conversion, no moving parts, and a relatively simple structure. However, the application of thermoelectric generators has a disadvantage that it is difficult to use variously due to the limitation of low conversion efficiency. Recently, the idea of using the TEG (thermoelectric generator) system as a waste heat recovery device has been under serious consideration in the automobile and incinerator industry. For waste heat recovery, it is unnecessary to consider the cost of the thermal energy input. The high conversion efficiency is not crucial. Rather, achieving higher power generation may be considered. Several types of thermoelectric generators for waste heat recovery were analyzed and developed by researchers with various approaches. Yu et al. developed a numerical model for a thermoelectric generator with a parallel-plate heat exchanger [13]. In this study, the optimization of the heat exchanger and thermoelectric module geometry was simultaneously performed by the numerical procedure. Crane et al. validated numerical heat exchanger models integrated with thermoelectric modules against experimental data [14]. The work suggests that the operating temperature range of the thermoelectric device should be delicately controlled to maximize the effectiveness of the system. Nuwayhid et al. developed a thermoelectric generator with natural convection cooling which recovered waste heat from a domestic woodstove [15]. In particular, the design factors that could achieve a low cost per watt were demonstrated. Esarte et al. researched the optimum heat exchanger system to maximize the performance of a thermoelectric generator [16]. In particular, theoretical expressions for heat exchangers were developed to compare with the experimental results and matched quite well in the low flow rate of the cooling medium. Saqr et al. reviewed the thermal design of thermoelectric generators for automobile exhaust waste heat recovery [17]. Four main factors that control the thermal

efficiency of the thermoelectric generator were presented: heat exchanger geometry, heat exchanger materials, the installation site of the thermoelectric generator within the car, and the coolant system of the thermoelectric generator.

Regarding the above works, the present study tried to hold focus on the other aspects of optimized conditions. As an example, thermoelectric generation performance was evaluated depending on the compressive force applied to the module. The compressive force is, in specific, a crucial means not only to lock TEG device to the heat source, but also to minimize the heat resistance, so TEG can perform maximum power. However, some optimal force exists where, putting more compressive force can reduce the performance because of heat transfer convergence to its maximum level, and putting more stress on the module can reduce the power because of the material's deformation. Moreover, putting more stress may result in mechanical failure. Additionally, the effect of the thermoelectric module surface temperature uniformity on the overall power generation performance was evaluated quantitatively. On the other hand, the experiment on the thermoelectric module power performance depending on the heat sink was conducted in a similar but partially different way from what Esarte et al. did [16]. The main difference was that in the present study, various types of heat sinks with different geometry (fin height, fin array) were applied whereas in Esarte's work, heat sinks were almost similar to one other with the same size except that fin thicknesses and numbers were varied from one another. After all, the main purpose of the present study was to simulate the waste heat recovery TEG system in automobiles and small-sized industrial facilities and thus, find the design factor that enhances the overall performance of the TEG system.
