**1. Introduction**

With the emergence of the Internet of Things (IoT) concept, the number of interconnected devices is rapidly growing. An issue that arises is the power autonomy of the nodes. Often, the energy harvesting concept is adopted [1]. The main idea is that ambient energy is harnessed and converted to electrical energy in order to power up the connected electrical loads. The most used environmental sources are light, heat, RF energy and mechanical stresses, exploited by piezoelectric or triboelectric devices [2–7]. Considering that an autonomous system should be functional, even in periods of input energy absence, the integration of an energy storage unit is crucial.

The type of the comprised storage medium should be carefully considered. Batteries offer very high storage capacitance (high energy density) but low power density, elevated cost and limited charge/discharge cycles. On the other hand, supercapacitors have numerous benefits, such as high-power density and long lifetime, with low degradation between charging cycles, however, they present lower storage capacitance [8]. Hybrid energy storage solutions, which exploit the benefits of both types of storage devices, have been proposed [9]. The most common approach relies on a battery for long-term energy storage, combined with a supercapacitor element, connected to the power output. This way a storage scheme is created, which presents all the advantages of the battery and also high-power density for short periods of time. Such circuits find use in multiple applications, such as healthcare assistive tools [10], DC microgrids deployment [11] and electric

**Citation:** Gogolou, V.; Kozalakis, K.; Koutroulis, E.; Doumenis, G.; Siskos, S. An Ultra-Low-Power CMOS Supercapacitor Storage Unit for Energy Harvesting Applications. *Electronics* **2021**, *10*, 2097. https:// doi.org/10.3390/electronics10172097

Academic Editors: Shailendra Rajput, Moshe Averbukh and Noel Rodriguez

Received: 27 July 2021 Accepted: 27 August 2021 Published: 29 August 2021

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vehicles [12]. A similar approach employs a battery at the power output and makes use of a bidirectional voltage converter and a supercapacitor to increase the power density of the energy storage unit. This concept is often adopted in energy harvesting applications for autonomous nodes [13–15]. However, the above works are discrete and bulky solutions. Since today's world demands miniature implementations for portable devices, on-chip integration becomes imperative.

Based on the energy availability conditions (continuous or interrupted) the integration of a battery can be omitted. In the case of continuous energy flow, a battery-less scheme can be used. A relative work is a four-supercapacitor CMOS storage bank, which offers high energy utilization [16]. In case of interrupted energy flow, the supercapacitors are not able to provide long-term storage, and a battery should be used. However, the simultaneous integration of a supercapacitor, along with the necessary control circuit, besides increasing the output power density, can also significantly extend the life expectancy of the battery, minimizing its charge/discharge cycles. Such an approach is presented in [17] where the proposed unit utilizes additional switching voltage conversion circuits (i.e., charge pumps) for the battery charging and discharging operations, which offer high power conversion efficiency but present limited working power range and increase the volume of the system.

In this work, a novel ultra-low-power integrated storage unit is proposed, suitable for a plethora of energy harvesting autonomous applications (Figure 1). This design is an improved and more versatile version of previous work [18] and presents experimental results. It can be connected to the output of various energy harvesting circuit types (DC-DC converters, charge pumps, etc.) and transfer the harvested energy to the storage media, providing regulated voltage supply to the internal control units of the harvesting circuit and the output loads (e.g., low-power sensors).

**Figure 1.** Block diagram of an energy harvesting system.

The proposed unit achieves minimization of the internal power consumption, deployment area and design complexity. The main storage element is a supercapacitor of small value, while a second larger supercapacitor can be used to provide energy to high-power modules. Depending on the energy availability conditions (continuous or interrupted), a backup battery can be used to avoid the energy starvation of the system during time periods of low input energy. Any type of battery can be used, depending on the use case application. Due to technology restrictions of the proposed unit, its maximum voltage must not exceed 3.6 V. For example, in Section 5.1, two 1.2 V Ni-MH AAA batteries are used to validate the unit's operation.

The proposed unit provides self-startup operation and sub-µW consumption, highly desired properties that contribute to high energy utilization and power autonomy of the applied harvesting system. Furthermore, it presents enhanced adaptability since it can be integrated into a wide range of energy harvesting systems, considering that the control parameters (supercapacitor thresholds, produced supply voltage) can be modified by the user. Moreover, significant versatility is offered, since external control, e.g., a microcontroller unit, can be added to the topology.

This paper is organized as follows. Section 2 presents the proposed supercapacitor storage unit and its operational principle. Section 3 describes the control logic of the unit. Sections 4 and 5 show the simulated and experimental results, respectively. Section 6 discusses the utilization of the storage unit in wireless sensor nodes (WSN) applications. Finally, Section 7 concludes this paper.
