## 1 Introduction  
In 2015, the Paris Agreement was adopted to reduce greenhouse gas emissions as climate change was becoming a more serious problem worldwide [1]. However, even after the adoption of the treaty, global greenhouse gas emissions have been increasing year by year, and are projected to increase in the future [2]. Greenhouse gases emitted worldwide mostly come from the energy sector (73%), and in particular, the energy use in industry accounts for 24%, which is the largest part [3, 4]. Therefore, it is urgent to replace fossil fuel energy with clean energy that does not produce carbon dioxide; but in 2019, renewable energy accounted for only 27.3% of the global energy production [5]. Hence, the importance of renewable energy is expected to increase further. Renewable energy, which is often referred to as clean energy, is defined to be energy collected from renewable sources. It includes hydropower, solar energy, wind energy, biomass energy and geothermal energy [6, 7]. In 2019, hydropower accounted for 50% of the total renewable energy capacity across the globe [8]. This indicates that hydropower has steadily gained ground in the renewable energy market. However, only a fraction of the hydropower energy potential is currently being developed and used [9]. Accordingly, it is necessary to further research hydropower. Hydropower generates electricity by rotating a hydro turbine using a head of the water, and it has the advantage that it does not produce carbon dioxide at all, while offering high energy density [10]. Micro hydropower, which is typically classified as hydropower with less than 100 kW of installed capacity [11, 12], has the advantage of longer life, lower operation costs, and fewer restrictions on space, relative to macro hydropower [13]. In particular, the in-line type hydro turbine for micro hydropower is highly safe and sustainable, because regardless of the external environment, it can supply water at a constant flow rate [13; 14]. In addition, it can be applied to wherever there is a pipeline, such as water and sewage, agricultural water, plant cooling water, and high-rise building drainage. Research on the in-line type hydro turbine for micro hydropower has been conducted for many years, and various types of hydro turbine have been proposed. Since the in-line type turbine is a reaction turbine that utilizes pressure energy remaining in the pipe, classical turbines such as [PERSON] and Kaplan turbines for small hydropower have been researched [15; 16; 17; 18]. A pump as turbine (PAT) is also known to be suitable for small hydropower applications [19] and has been investigated in a variety of forms such as mixed flow or centrifugal types [20; 21]. Furthermore, in addition to classic turbines, many new types of turbines have been developed. [PERSON] et al. [22] developed a vertical axis turbine to generate power from water pipelines, and conducted numerical analysis and experimental tests for over 20 models through design variables, such as drag-type or lift-type turbine, and deflector configurations. [PERSON] et al. [23] used a Savonius turbine as an in-line type hydro turbine, and improved the performance by adjusting design variables, such as deflector configurations and aspect ratio, through numerical analysis and experiments. A helical turbine, known as the Gorlor vertical axis turbine, was introduced into the LucidPipeTM system to generate energy [24]. [PERSON] et al. [25] studied a vertical axis turbine based on a helical turbine numerically, and investigated the effect of tip-speed ratios on the performance. A hydrocoil turbine, which is an in-line type turbine version of the Archimedes screw turbine, was developed by HydroCoil Power, Inc. [26]. [PERSON] et al. [27] adopted an in-line type crossflow turbine for pressure regulation and energy production, and verified the performance through numerical and experimental analysis. Research on contra-rotating turbines for micro hydropower was conducted, and the performance was evaluated through numerical analysis and experiments [28; 29].