Renewable power generation from sources such as wind and solar energy are effective in directly powering applications or feeding power to the grid. As power generation from both sources is inconstant, a suitable means for temporarily storing energy is often desired. Various energy-storage mediums exist, depending on the scale and time response required for the energy delivery. For applications that rely on solar energy as a means for power generation, there are multiple options for energy storage.
Solar panels are constructed to provide various voltages and power ratings. The power output in all cases is delivered as a DC voltage. This DC voltage can be either delivered directly to the application or, if necessary, routed through an inverter to convert to AC. In either case, energy storage is often implemented to store excess generated power or for ride-through in low-light conditions.
Typically, 12 V batteries are used as the energy-storage device. Depending on the ride-through time or power delivery required, ultracapacitors may also be a suitable energy storage device. Ultracapacitors could be favored in remote applications or where battery maintenance is an issue.
In all applications using batteries or ultracapacitors as the energy storage medium, a solar charge controller is required. The solar charge controller is used to regulate the power going from the solar panels to the energy storage device. Regulation is required to prevent overcharging. The most basic form of a charge controller monitors the voltage of the energy storage device. When the voltage reaches the setpoint, charging stops.
More sophisticated charge controllers use pulse width modulation (PMW), which slowly lowers the amount of power applied to the energy-storage device as charge voltage is approached. An alternative controller topology is referred to as maximum power point tracking (MPPT). This topology is more complex, with the ability to convert excess voltage into amperage. The MPPT is an electronic DC-to-DC converter that optimizes the match between the solar array and the energy storage device.
A solar array is designed to provide a specific voltage. The efficiency of the solar array is influenced by the temperature of the solar array, and the hotter the array, the lower the voltage output. Conversely, the voltage of the battery or ultracapacitor fluctuates depending on the state of charge.
Consider a solar panel designed for 180 watts output at 16 V; the panel is capable of delivering a maximum of 11.25 A for use or charging of the energy-storage device. If the energy-storage device has a state of charge of 12 V, then only 135 W of the available 180 W is consumed for charging. In this case, the MPPT compares the output of the panel to the voltage of the energy-storage device, takes the available power, and converts to the voltage providing the maximum amps into the battery.
When considering batteries versus ultracapacitors within these control schemes, the batteries benefit from the more complex PWM or MPPT topologies as the batteries are more sensitive to the charge currents relative to the float voltage of the device. For battery systems, there is a 10 percent rule that states the battery should not be charged with more than 10 percent of its capacity as current. This means for every 1 Ahr (amp-hour) of battery storage, the battery should not be charged with more than 100 mA current. Otherwise, the battery could become damaged.
A beneficial attribute to the ultracapacitor technology is its relative immunity to charge currents relative to float voltage, which enables the opportunity to use less complex charge controllers in comparison. No special algorithms relative to charge current are required for ultracapacitor technologies.
Typically incorporated in the more complex charge controllers are provisions to prevent reverse current from flowing back into the solar panels. This is recommended to prevent the solar panels from draining the energy-storage device during low-light conditions or nighttime. In the case of ultracapacitors, where higher-level charge controllers may not be used, blocking diodes may be considered, Figure 1. A blocking diode will have voltage drop across the device so the overall voltage of the system must be considered relative to the efficiency losses associated with the use of the blocking diode.
Figure 1: Use of blocking diodes to prevent reverse-current flow
For a lower-voltage array, it may be more efficient to allow reverse current when not charging, compared to the losses associated during charging through the diode. If a diode is implemented, it should be connected at the positive leg of the circuit between solar panel and energy storage device as illustrated below.
Look at the output side
So far, the discussion has been limited to the interface between the solar array and the energy storage medium. Let’s consider the output or application side of the system. With the introduction of LED lighting, solar lighting applications have become more commonplace and suitable for larger market penetration. The efficiencies associated with the LED enable lighting to be supported with lower amounts of energy storage.
This has enabled the use of ultracapacitors for a broader range of solar lighting applications. This is especially true for a variety of ultracapacitors known as pseudocapacitors. Pseudocapacitors incorporate technology tailored for higher-energy, lower-power applications. Therefore, the combination of solar panels, LED lighting, and psuedocapacitor energy storage are an ideal combination of technologies.
The advantages of psuedocapacitors versus batteries in these types of applications are the long service lives. A battery technology located outdoors susceptible to the extremes of heat and cold could have expected life spans of two to three years at best. Ultracapacitor or psuedocapacitor technology would similarly be expected to last 10 to 15 years in the same application. This is well-suited for applications in which energy-storage replacement is undesirable.
When considering the life of the application, the lifetime of the LED bulbs should also be addressed. A typical rating for an LED bulb is 100,000 hours. If the specified current for this bulb is doubled, the life can be reduced to 100 hours. It is essential to regulate the current flowing through the LED to maximize the life of the device.
A resistor in series with the LED can be used to regulate the current. The use of a resistor is not the most effective solution if the power supply to the resistor is not stable. In the case of solar lighting, the power supply is the energy-storage device (battery or ultracapacitor). Since the voltage of the energy-storage device will drop as current is consumed, this unstable voltage will enable the current through the resistor to fluctuate, possibly within the range damaging to the LED.
To solve this problem, an adjustable voltage regulator can be used in series with a resistor. An LM317T, Figure 2, can be used to regulate the voltage on the "adjust" input to 1.25 volts. As a result, the current through the resistor connected in series sees this constant voltage, allowing sizing of the resistor for the desired current protection.
Figure 2: An adjustable voltage regulator such as the LM317 can be used to set current-protection.levels
In the case of batteries, the current should be limited to 10 percent of the battery amp-hours as previously described. In the case of the psuedocapacitor, it should be limited to the continuous current rating of the device, although these devices will typically have a much higher current rating than batteries, and are generally not a concern. .
In order to extract the greatest value and lifespan from solar-powered applications, product designers must select an energy-storage device to bridge the variable delivery of power. That selection should take into account the temperature extremes the application is likely to experience, as well as the possible effect of fluctuations in current. In many cases, particularly those that involve remote applications or where battery maintenance is an issue, ultracapacitors are the best option for energy storage.
About the author
Brendan Andrews is the vice president of sales and marketing at Ioxus, Inc. He is responsible for the leadership and coordination of Ioxus’ sales and marketing functions and for educating the global market regarding existing and future ultracapacitor technologies. Previously, Brendan served as Maxwell Technologies’ director of sales and marketing Americas.