Stand-alone solar-powered devices have traditionally been operable only when sunlight is available or when surplus energy is stored in a rechargeable battery to ensure that the device stays powered in the absence of sunlight. Due to degradation of storage capacity over multiple charge cycles and limited ability to hold a charge for long periods of time, such batteries must be replaced frequently; the typical lifespan of a self-contained, low-maintenance rechargeable battery is typically 1–3 years. Longer lifespans are possible with certain types of batteries, but they greatly limit the functionality of the device. Overall, the high maintenance and replacement costs of rechargeable batteries over the lifetime of a device usually exceeds the cost of the device itself by a substantial margin, making energy storage the most critical component of the total cost of ownership (TCO) for stand-alone devices powered by the sun. Similar problems are common with devices powered by other renewable energy sources, where the cost of maintenance is the largest component of the TCO. In order to reduce these costs and promote installation of these devices in remote areas, a better energy harvesting scheme is required.
A promising alternative to batteries for energy storage is a new class of capacitors, known as supercapacitors. With the increasing volume production of supercapacitors each year, the price/performance gap between rechargeable batteries and supercapacitors has been narrowing. Supercapacitors can also be recharged up to 500,000 times and hold full charge for more than 10 years; with careful design can have a useful life of more than 20 years. Thus, supercapacitors might displace rechargeable batteries in applications where the power and energy storage characteristics of the supercapacitor can be efficiently matched both to the power generated by the renewable energy source and to the power consumed by the device.
A critical aspect of efficient power transfer is correct impedance matching between the energy source (typically fairly high, on the order of several kΩ for renewable energy sources) and the supercapacitor (typically a small value, on the order of a few mΩ). While it is common to match impedances by converting DC voltages with pulse-width modulation (PWM) circuits, PWM circuits don't work well with such a large impedance mismatch. For sub-Watt energy sources, the duty cycle is too short for a standard PWM DC/DC converter to match such an extreme range of impedances.
University of California researchers have found that a pulse-frequency modulation (PFM) buck regulator can efficiently charge a supercapacitor and is able to operate from a single mW-scale power source. Using a series of short pulses allows the converter to match the kΩ power source impedance to the mΩ load impedance of the supercapacitor. The UC PFM controller can lock the input voltage to a reference voltage which can either be a fixed voltage or a voltage from a maximum power point tracker (MPPT) circuitry that tracks the optimal operating point of the power generator. Depending on the power source and capacitor topology, the UC regulator enables the power source to generate at least 2–10× more power than if the power source were connected directly to the supercapacitor. Due to its sub-mW overhead, this regulator can operate with a wide variety of renewable energy sources, including vibration-powered piezoelectric ceramics, solar cells, and wind generators.
Suggested uses
The UC PFM regulator is generally useful for stand-alone devices powered by small-scale renewable energy sources; it might be employed in remote sensing devices for security, agricultural, structural, and environmental monitoring applications.
Advantages
As the underlying economics of supercapacitors relative to rechargeable batteries improves, the UC PFM regulator may play a unique role in adapting supercapacitors to a broader range of renewable-powered devices.
Inventors
- Chou, Pai H.
- Simjee, Farhan
