As designers of portable devices pack greater functionality into products, power management requirements have increased. Products require multiple supply rails, battery charging and a variety of power management capabilities – all of which are implemented to maximise battery life. The frustration of a cell phone dying or a laptop powering off is not uncommon in our tech-driven world.
Such irritations proved the impetus for new battery developments, power management ICs and new energy harvesting techniques (vibration transducers, thermoelectrical converters, RF converters and photovoltaic cells (PV)).
Today, PV conversion of light into electricity remains the most effective way of harvesting ambient energy.
Consider the possibilities. Cell phones, MP3 players, PDAs, batteries and chargers, which typically use rechargeable batteries (lithium ion, NiCd), could have a solar PV cell added to their PC boards and be charging constantly, when light is available. In each charging application, solar PV cells will provide just enough current to hit the voltage limit – extending battery life, overcoming self-discharge problems and providing an environmentally responsible solution.
Many low-power applications and devices spend the vast majority of their time in a wait mode, only waking up periodically to perform a required function. Static displays and intermittent sensors that utilise a very small "maintenance current" are well suited to the use of solar PV cells to supplement battery power, recharge the battery between higher current operations and extend battery life.
Even the smallest remote sensing and data collection devices require some amount of current to operate even in their "standby" mode. The need to provide small amounts of current over long periods has spawned new battery designs and this work continues today.
The other part of the power equation, however, is in providing additional power to maintain these batteries as long as possible once the device or sensor is installed in the field. The use of solar PV technology is becoming an increasingly common source to provide this energy, and is an excellent choice for sensors that are used where light is available at regular intervals.
Yet, for all of the various applications that could benefit from such energy harvesting, few manufacturers and designers have implemented solar charging for portable product batteries. Why is this?
- Industry detractors: Once the consumer electronics product is sold, manufacturers glean extensive revenues from all of the extemporaneous items, not the least of which is replacement batteries;
- Low efficiency: The efficiency of a PV cell is defined by the ratio of the electrical energy generated by the cell and the energy of irradiance incident at the cell surface. Due to mobile operation and indoor use, light capture in PV-powered consumer products can be quite low. And further hampering the “energy harvesting” effort is that small consumer electronics have limited space available to implant solar cells for enough light capture;
- Inability to conform: PV cells must be lightweight, flexible and capable of conforming to the design of the electronic device. Regular cells (typically designed in a package) can’t conform to curves; amorphous cells better conform to the design constraints of small consumer electronics. However amorphous silicon’s efficiency is lower than its crystalline competitor;
- Voltage: The PV power source must reach battery voltage of 4-12v to ensure adequate recharging of batteries. Individual solar cells typically produce between 0.3 volts and 0.6 volts, not nearly enough to meet recharging demands. Solar cells must be packaged in a series, such as the panels for homes, to produce usable energy amounts. Even when packaged in a series, each cell only produces energy equal to the output of the lowest power cell. If a single cell is shaded, it affects the entire series. Such requirements make solar cells inflexible for use inside cell phones, remote controls and other small devices;
- High cell costs: The need for a large series of solar cells to produce reach 4-12v of usable energy make solar cells cost prohibitive;
- No breakeven: Consumer products have a short lifetime in the range of 2 to 6 years, so PV systems in these products suffer a similar fate. In combination with low light capture, the energy payback time of PV systems in consumer products might exceed the product’s lifetime.
Could mono crystalline be the answer?
Monocrystalline, high-efficiency solar PV cells (with efficiencies of up to around 20%) that incorporate an enhanced light-trapping surface have the potential to generate the voltage necessary to power batteries in all types of applications. And manufactured with Silicon-On-Insulator (SOI) process technology, a single chip could hold multiple solar cells in a series, so that a wide variety of voltages is possible.
With 24 solar cells on a single chip, the semiconductor chip gives off 12 volts (when exposed to natural light) – enough to power both rechargeable (nickel cadmium) and non-rechargeable (alkaline) batteries.
Monocrystalline cells have a spectral sensitivity range from 300 nm (near-ultraviolet) to 1100 nm (near-infrared), which includes visible light (400 to 700 nm). Due to this wide spectral range, they can be used in both indoor and outdoor applications. Monocrystalline or single-crystalline material does not contain impurities, and as such the power conversion efficiency does not degrade over operating time.
With a cell efficiency of between 17%-20%, a solar chip gives the ability to extend run time even in “low light” conditions and increase battery life and run time on a small footprint, which can be accommodated in the design of portable products. Conventional rechargeable batteries can take several hours to recharge; using a solar cell, the battery can be continuously trickle-charged while under a light source.
Low efficiency currently dictates the need for PV cells that harvest energy under a variety of lighting conditions; engineers should use PV cells that respond even to low light levels.
Given the present high costs of use of rechargeable batteries (more than US$3.50 kWh), PV solar energy under optimal light capture is cheaper now (about US$0.40-US$0.50 kWh) and might become much cheaper in the future. Therefore, it is sensible to partly substitute batteries.
And further dampening the cost prohibitive argument, large scale grid integrations have made PV cells cheaper due to economies of scale. Hence as we approach grid parity in certain regions, costs will be less of an issue.
Furthermore, cheaper raw materials and improved manufacturing processes are pushing down prices for photovoltaic (PV) cells and panels still further. Many companies/analysts are predicting that solar power could become cost competitive with natural gas-fuelled electricity in some areas as early as 2011.
For the time being, other factors such as ease of use, increased personal mobility, reduced environmental impact, long stand-by times have added value that can’t be neglected, yet remain difficult to quantify.
Engineers have sought to package solar cells right into the device in an unobtrusive way (either placed under a device’s display or cover). This integrated design would allow the mobile device, be it a PDA or portable music player, to be charged from daylight without having a separate solar panel that needs to be plugged into it. Electricity generated from the cells would be fed to the device's rechargeable battery.
The circuit design is fairly simple: The battery charging current is generated by four series-connected monocrystalline, high-efficiency solar cells. Each cell generates 0.63 V open circuit voltage and 42 mA short circuit current. A Schottky diode prevents the battery from discharging through the solar PV cells when the output voltage from the solar cells is lower than the battery voltage, as happens when sufficient light is unavailable.
And with the level of efficiency and voltage capability inherent to the solar chip, it makes it a perfect fit for consumer products including cameras, digital music players and phones.
Widespread adoption of this available technology may take time; first movers are integrating the chip into myriad consumer/portable products in test environments. This technology has already met with excitement, as evidenced by Apple’s recent interest in PV within the iPhone. Until full-scale integration meets with broad success, the solar industry sits at a crossroads, utilising advanced technology to produce solar chargers.
About the author:
Stephen Krausse has served as the General Manager for IXYS COLORADO (formerly Directed Energy, Inc)., a division of IXYS Corporation, since January 2003.