Using a supercapacitor for power management and energy storage with a small solar cell, Part 1

& -April 17, 2017

Why energy harvesting?

The environment has abundant energy but little power. Energy harvesting can tap the environment’s “infinite” source of energy, and avoids the cost of wiring to mains power or the time-consuming and environmentally-sensitive task of replacing and disposing of batteries. Many applications have cost and size limitations so this article will look at using small solar cells. Over recent years much development effort has gone into solar cells, making them the most efficient, effective, and available small energy harvesters. Energy harvesting provides a convenient and cost effective energy supply for autonomous applications such as wireless sensors, and wireless sensor networks (WSNs) are becoming ubiquitous. This article will focus on environmentally-harvested solar energy which can power motion sensors to turn off lights if nobody is detected in a room, dim lights depending on the light level in a room, sense and report temperature for air conditioning or heating, for exercise monitors and IoT generally, and monitor the security of remote locations.

Why supercapacitors?

Depending on the light level, whether indoors or out, small solar cells may deliver sub mW to 10's of mW. The peak application power may range from ~50mW for Bluetooth to ~7W for cellular transmission. So the problem becomes how to power wireless transmission, which requires higher power, from a low-power source. Supercapacitors solve this problem playing the roles of temporary energy storage and power delivery.

This article will examine how to use supercapacitors with small solar cells in two case studies:

  1. Relatively low power applications which only operate when there is indoor light, providing sub mW power and transmitting with BLE. The supercapacitor need only be sized for the energy and power to support the peak load burst.
  2. An outdoor solar cell for higher power applications which must run when there is no light, such as overnight and reports by SMS using GPRS. In this case the supercapacitor is sized for energy storage over the dark period as well as for the peak load.
Small, thin prismatic supercapacitors are ideal for the first case, particularly for small, space-constrained sensors where form-factor is important such as in wearables. Larger cylindrical cans up to 400F are best for the second case. I will use CAP-XX supercapacitors to demonstrate both cases.

 

Supercapacitors are an ideal power buffer

Prior to low-impedance supercapacitors, designers needed to size the entire power supply system for the load’s peak power. As an example, assume a remote location is reporting its status once per hour using an SMS that takes three seconds to transmit on a GPRS cellular network. The peak output power is ~7W. For GPRS class 8 (single slot transmission), the average power  = 7/8W and over three seconds ~2.6J of energy.  The only solution without a supercapacitor is to have a solar cell that can deliver 7/8W or to trickle-charge a battery that can deliver this power, possibly with the support of a tantalum or electrolytic capacitor for the 0.577ms 7W transmission peaks.

The supercapacitor’s high energy storage and high power delivery make it ideal to buffer a high-power load from a low-power energy-harvesting source, as shown in Figure 1. The source sees the average load, which with appropriate interface electronics, will be a low-power constant load set at the maximum power point. The load sees a low-impedance source that can deliver the power needed for the duration of the high-power event. The only constraint is that the average power from the solar cell > average load power = load power x duty cycle /efficiency of supercapacitor charger. The supercapacitor charger is also sized for the average load power, not the peak, so a smaller, lower-cost charger can be used.



Figure 1 Supercapacitor as a power buffer

Supercapacitors use physical charge storage rather than electrochemical so have “unlimited” cycle life. They also have a wide operating temperature range with Equivalent Series Resistance (ESR) at −40°C approximately twice room temperature ESR, so can be used in outdoor applications such as perimeter security in colder climates such as North America and Europe.

The physical charge storage and low ESR supercapacitors mean they have excellent round trip charge/discharge efficiency, another beneficial attribute for a power buffer. The losses in charging/discharging a supercapacitor are the I2 ESR losses.

 

 

Where t is the time to charge the supercapacitor from V1 to V2 at current I

 

A worst case would be a supercapacitor supporting GPRS transmission, discharging from 3.8V to 3.2V supplying 2A peak current. If the supercapacitor ESR = 100mΩ, then discharging efficiency = 95%. If the solar cell charges the supercapacitor at 50mA, which is the peak power current in Figure 4, for the second case study, then charging efficiency = 99.9%.

Supercapacitors are also very simple to charge, only needing charge current with over-voltage protection rather than a constant voltage constant current regime.


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