# Supercapacitor-Based Energy Storage in Elevators to Improve Energy Efficiency of Buildings
## 1 Introduction  
Energy consumption in commercial, residential, and administrative buildings accounts for a significant portion of total energy consumption [1]. In order to increase savings in buildings and meet future climate and energy goals, the concepts of zero energy buildings (ZEBs) and positive energy buildings (PEBs) have been introduced [2; 3]. The basic principle of these concepts is to optimize the energy consumption of buildings by using sustainable technologies and to generate energy by integrating renewable energy generation systems, resulting in buildings that can meet their energy needs and even generate more energy than is required. By implementing these concepts, the problems of energy-saving, environmental protection and reduction in CO\({}_{2}\) emissions in buildings are addressed, and reliability, independence from the power grid, environmental friendliness and cost efficiency are achieved [4; 5]. The main goal of energy-efficient buildings is to keep energy consumption as low as possible. However, little attention has been paid to energy-efficient elevator systems, which can lead to significant energy savings in buildings [6]. Elevators typically account for 2% to 10% of a building's energy consumption [7], but, in high-rise buildings, they can reportedly be responsible for 17% to 25% of total energy consumption [8]. During peak periods, elevators can consume up to 40% of a building's energy [9]. In addition, elevators can generate clean energy that can be used for consumers in the building instead of being wasted as heat, as is the case with conventional elevator systems. According to [10], there is significant potential for savings in elevator systems that can be realized through numerous measures: reducing standby consumption, adapting new motor technologies, using regenerative energy, improving dispatching systems, or introducing machine-roomless(MRL) approaches. By implementing these measures, energy savings of 40% or more can be achieved [11]. Research on the development of a net-zero energy elevator concept has also been reported [7]. Recently, energy savings in elevator systems achieved through the use of regenerative energy have attracted much attention. The most attractive solutions are energy storage systems connected to the DC-link of an elevator drive, where regenerative energy is stored and can be used to reduce peak loads or supply other equipment. The solutions proposed in the literature are mainly based on supercapacitors. Supercapacitors have a high power density, a fast charge/discharge rate, a high number of charge/discharge cycles, and a long lifetime, making them suitable for elevator applications characterized by a high number of starts and stops, short trips, and high braking power in regenerative mode [12]. Typical designs of energy storage systems in elevator applications use a bidirectional DC/DC converter and a supercapacitor bank [8; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23]. The research studies presented in the literature focus on control strategies [15; 17], storage system design [13; 14; 18; 19; 20; 21; 22], supercapacitor bank sizing [8; 19; 21], DC/DC converter topology [23; 24], and additional functions, such as emergency power supply [14] or power factor correction [16], with simulations and experimental tests used to validate the proposed concepts. However, a comprehensive estimation of the energy savings achieved by energy storage systems is missing. In addition, only a few recent research papers have highlighted the role of energy-efficient elevators in improving building energy efficiency. In [25], a hybrid energy storage system with an ultracapacitor energy storage system and a battery energy storage system was proposed to reduce the power and energy consumption of elevators in residential buildings. An ultracapacitor was used to reduce peak consumption, and a battery storage system was used to supply the general electrical loads in the building.