## 2 Background  
### Energy Harvesting  
Energy harvesting was first introduced in 1954 when Bell Laboratories in the United States conducted research on solar cells that convert sunlight into energy. Because the only way to produce electricity in a city is through a generator, natural energy, such as solar heat, is recognized as wasted energy, and the word 'harvest' is used to represent the generation of electricity by collecting wasted energy [21; 22]. The energy harvesting field is divided into two categories according to the scale of the energy source (Table 1). The first energy harvesting field is a macro-energy source that produces energy from natural energy sources such as sunlight, vibration, heat, and wind power. The second field of energy harvesting is technology that regenerates energy by harvesting or using wasted energy, such as pressure, thermoelectric, and electromagnetic energy with micro-energy sources [23]. In particular, energy harvesting technology that harvests and utilizes energy that is discarded based on the increase in demand for low-carbon and eco-friendly energy and the rapid use of mobile devices was developed. Energy harvesting, which produces energy from macro-energy sources, primarily utilizes solar, wind, and geothermal sources. Because energy harvesting originally started with the use of sunlight, energy harvesting technology using sunlight has become the most widely used. Solar-based energy harvesting uses solar panels to generate renewable energy. In today's solar-based energy harvesting, research on solar panel materials and batteries for effectively storing the energy produced from solar panels is being conducted to maximize \begin{table}
\begin{tabular}{c c} \hline \hline
**Macro-Energy Source** & **Micro-Energy Source** \\ \hline Solar, wind, and geothermal & Piezoelectric, thermoelectric, photoelectric, and electromagnetic \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Energy harvesting field according to the natural energy scale. the efficiency of energy production [24; 25]. In particular, the development of solar panel modules has resulted in their active use in architectural designs. The diversification of photovoltaic modules, such as multicolored and transparent photovoltaic panels, plays a significant role in the development of architectural designs, as photovoltaic panels are used as architectural design materials. Energy harvesting from wind power converts wind into electricity using a wind turbine. Wind power is currently used in many countries to produce relatively inexpensive and zero-emission energy sources [26]. Recently, the number of wind-power-generation-linked buildings in the field of architecture has been increasing worldwide [27]. Wind turbines in buildings are used not only to produce renewable energy through wind power generation, but also as eco-friendly design elements of the buildings themselves. Depending on how wind power is connected to buildings, it is divided into building-mounted wind turbines (BMWTs) and building-integrated wind turbines (BIWTs) [28]. BMWTs were first introduced in Germany in 2000, and they have been built worldwide since 2006. BIWTs were first constructed in 2008, later than BMWTs. Since the introduction of BIWTs, wind-power-linked buildings have been built using BIWTs rather than BMWTs [29; 30]. This is because BIWTs, which actively utilize wind turbines as an element of their architectural design, are preferred over BMWTs, which involve wind turbines being placed on the roofs of buildings. Energy harvesting, which generates energy from micro-energy sources, produces energy from piezoelectric, thermoelectric, photoelectric, and electromagnetic energy sources [31]. Piezoelectric energy harvesting has been developed focusing on the development of new materials for synthetic fibers that utilize the piezoelectric effect. Thermoelectric energy harvesting uses the temperature difference that occurs at the parts in contact with both ends of two types of metals or semiconductors and this is converted into voltage.