Solar Battery charger for portable devices application

Solar Battery charger for portable devices application Filipe Távora, Analog Designer Andrea Sarmento Maia, Analog Designer trainee 2012, June

2 Abstract This paper highlights the evolution and techniques of harvesting renewable energy from the sun to power up portable devices, into clean and sustainable way. A small study of solar cell and how to improve its performance is presented, showing some practical experiments made at South-America. Finally, some implementations of solar charger for portable devices are analyzed, showing drawbacks and benefits for each architecture and a complete integrated solution is proposed.

3 Summary Abstract... 2 Summary... 3 Introduction... 4 Renewable Energy, what is the motivation?... 5 Solar Energy, what is the motivation?... 6 Mobile Devices... 8 About Solar cell... 9 Photovoltaic Technologies... 10 Equivalent Circuit... 12 Boosting solar cell efficiency... 15 How to design efficient solar panel system for portable devices... 18 SiliconReef s solution... 21 Operating Scenario: Battery-Charging through Photovoltaic cell only... 22 Conclusions... 25 About SiliconReef..26 Reference... 26

4 Introduction The emergence of alternative energy sources and new forms of exploration will rise with the technological evolution and development of societies. In the Eighteenth Century with rudimentary technology there was renewable energy. At the First Industrial Revolution, was the discovery of coal associated with the steam machine. In the Nineteenth Century is the World War I with the discovery of the principles of thermodynamics, development of transport, discovery of oil and natural gas in the Midtwentieth Century. With World War II, nuclear power appears later computer science, robotics, which together gives rise to the Third Industrial Revolution in the last decades of the twentieth century. Currently, renewable are being emerged with sophisticated technology to a new structure (minerva_uevora_pt). In 100,000 years, the world has remained stable with a billion people. However, in the last 200 years, there was an increase of 6 billion people, and the need for energy has grown unplanned. The impact of the Industrial Revolution and Technology was the dependence on electronic devices (fantastico_globo_com). Figure 1: Timeline (estudation_blogspot_com_br). To solve potential problems with mobile devices, SiliconReef - Brazilian Start Up that works with energy harvesting since 2009 - initiated a project titled EH01 (Energy Harvest 01). The goal is to reduce portable devices dependence of power grids. GPS navigators, smartphones and tablets, among many other applications, may work longer since the solar radiation is present everywhere.

5 Renewable Energy, what is the motivation? Renewable energy is every form of energy whose utilization rate is lower than its rate of renewal. The source can be terrestrial (thermal energy), gravitational (tidal power) and solar (energy stored in solar radiation, the kinetics of winds and waves). Moreover, the sources of agricultural waste, municipal and industrial renewable energy are considered. Figure 2: Wave, Wind, Solar, Mini-hydro, Geothermal and Biomass Energy (portal_energia_com). With various types of expression, availability of wide geographical range and varied possibilities for conversion, renewable are quite suitable for distributed or concentrated generation. The development and thus the usage of technologies for renewable will benefit areas of difficult access. Therefore, it can be stated that the use of green energy to replace fossil fuels is a viable and profitable direction. Well, besides being practically inexhaustible, renewable energy may have low environmental impact or almost zero, without affecting the heat balance and atmospheric composition of the planet, as is the case of fossil fuel energy. The renewable energy sources are appropriate to the strategy of sustainable development, contribute to improving the global competitiveness of industries; have positive effects on regional development and employment and, ultimately, end users are more conscious to consume and preserve the environment. However, the widespread deployment of renewable confronts the following obstacle: the attitude of general

6 resistance to change. It is necessary, therefore, a method of adjustment (minerva_uevora_pt). Figure 3: Renewable energy share of global final energy consumption in 2008 (ZEMAN). Solar Energy, what is the motivation? The energy sources as hydro, biomass, wind, fossil fuels and energy from the oceans are indirect forms of solar energy. Solar radiation, which focuses on the land for another 5 billion years, can be directly converted as a source of thermal or electric energy. It can also be converted into electrical energy by means of effects on materials, among which stand out the thermoelectric and photovoltaic. Besides the weather, the availability of solar radiation depends on the latitude of the place and position in time (time of day and day of year) (ZEMAN). Some advantages, therefore, the motivation for using solar energy are as follows (minerva_uevora_pt): o It is excellent in remote or difficult access location; o It is a totally clean and renewable source; o It is useful for small current loads;

7 o Solar panels are increasingly more powerful and cost has been decreasing. This makes it increasingly more solar energy an economically viable solution; o In tropical countries such as Brazil, use of solar energy is feasible in almost all territory. Figure 4: Applications of solar energy and world solar energy map (especifico_com_br). Most of Brazilian regions are located relatively close to the equator so that not observed large variations in the duration of solar day. The thermoelectric effect is most commonly found in South and Southeast Brazilian regions, due to climatic characteristics, as shown in Figure 5, and Photovoltaic (PV), in the North and Northeast, in isolated communities of the power grid. Importantly, even regions where lower levels of radiation have great potential for energy recovery (aneel_gov_br). Figure 5: Daily insolation of Brazil (aneel_gov_br).

8 Mobile Devices Mobility and mobile computing systems are systems that can easily be physically moved or may be performed while being moved. The small size, limited memory and processing power, low power consumption and limited connectivity are the main characteristics of mobile devices. The reliance on small devices and the ones which makes easier the daily tasks increases every day. With advances in solar technology, batteries and electronics in general, have been increasingly possible to develop new devices that require less energy to operate. Of course all mobile devices (without network connection) could have, because the power required for solar modules is suitable for battery use. Furthermore, mini charge regulators that reach the maximum power devices without causing overload of the batteries. Portable devices (mobile phones, tablets, notebooks and netbooks) have become increasingly popular, especially with the proliferation of access to wireless technology. One of its main characteristics is to rely on battery power for its operations. Techniques that allow energy savings, therefore, have been researched to meet the need for immediacy in which the world currently requires.

9 About Solar cell By relying on the PV effect, the conversion of sunlight into electrical energy can be said to be the conversion of PV. The photovoltaic effect, in general, means a potential difference generated at the junction of two different materials in response to the radiation. As a photo is a radical of the Greek word phõs which means light and "Volt" is an abbreviation of the name of Alessandro Volta (1745-1827), pioneer in the study of electricity, the literal meaning of photovoltaic is "electric light ". However, a common and popular term to refer to the PV solar energy is the solar electric power (ZEMAN). Figure 6: Schematic of converting solar energy into electrical (slide_ocw_tudelft_nl). The advantages of the PV solar energy are (ZEMAN): o Environmentally friendly; o No noise, no moving parts; o No use of fuels and water; o No emissions;

10 o Minimal maintenance requirements; o Long lifetime, up to 30 years; o Electricity is generated wherever there is light, solar or artificial; o PV operates even in cloudy weather conditions. The semiconductor device that converts solar energy into electricity is called a solar cell. The amount of energy provided by this device is characterized by an output voltage and a current. In order to utilize solar energy to practical devices, which require a particular voltage or current for its operation, a number of solar cells are interconnected to form a solar panel, also called a PV module (ZEMAN). Photovoltaic Technologies The first practical use of solar cells was the generation of electricity on the orbiting satellite Vanguard 1 in 1958. Since then, solar cells technology has improved dramatically in efficiency and cost. Today crystalline silicon cells, improved in efficiency a lot from 6% up to 33%, and it still dominates the market. Thin-film PV technology raised as a low-cost solution and currently account for 10% to 15% of global production. The conversion efficiency must be substantially increased in order to progress. The conversion of sunlight into electricity has currently low efficiency. For calculations based on thermodynamics, this limit is 93% and a single junction solar cell, such as a silicon wafer, is 33%. These new concepts are based on PV efficiency improved, since the performance of the solar cell can be increased by almost three times. Advanced concepts of conversion of new processes are often referred as a third generation of high efficiency thin film solar cells. They are currently the subject of basic research (ZEMAN).

11 Figure 7: Thin-film silicon, Wafer-based crystalline silicon, Cadmium Telluride, CIGSS and Polymer PV (ZEMAN) and (csa_com). Figure 8: Overview of solar cell applications and of several types of solar cells used in different applications (ZEMAN).

12 Equivalent Circuit Two important quantities to characterize a solar cell are: o Open circuit voltage (V oc ): The voltage between the terminals when no current is drawn (infinite load resistance); o Short circuit current (I ph ): The current when the terminals are connected to each other (zero load resistance). The short circuit current increases with light intensity, as higher intensity mean more photons, which in turn mean more electrons. Since the short circuit current I ph is roughly proportional to the area of the solar cell, the short circuit current density, J ph = I ph /A, is often used to compare solar cells. When a load is connected to the solar cell, the current decreases and a voltage develops as charge builds up at the terminals. The resulting current can be viewed as a superposition of the short circuit current, caused by the absorption of photons, and a dark current, which is caused by the potential built up over the load and flows in the opposite direction. As a solar cell contains a PN-junction, just as a diode, it may be treated as a diode. For an ideal diode, Figure 9, the dark current density is given by ( Here J 0 is a constant, q is the electron charge, V is the voltage between the terminals, T is temperature and k is Boltzmann s constant. Figure 9: The equivalent circuit of an ideal solar cell.

13 The solar cell can be electrically modeled as a current generator which generates the current (density) J ph. The dark current flows in the opposite direction and is caused by a potential between the positive and negative terminals. In addition we would have two resistances; one in series (R s ) and another in parallel (R p ). The series resistance is caused by the fact that a solar cell is not a perfect conductor. The parallel resistance is caused by leakage of current from one terminal to the other due to poor insulation, for example on the edges of the cell. In an ideal solar cell, you would have R s = 0 and R p =. When these so called parasitic resistances are included, Figure 10: The equivalent circuit of a solar cell with series and shunt resistance. Rs is the series resistance, Rp is the shunt resistance. In general, the power delivered from a power source is P = IV, i.e. the product of voltage and current. If we, instead, use the current density J, get the power density: P d = JV. The maximum power density occurs somewhere between V = 0 (short circuit) and V = V oc (open circuit).

14 Figure 11: Effect of parasitic resistances on the J-V characteristic of solar cell (ZEMAN). The four quantities: short circuit current density, open circuit voltage, fill factor and efficiency are frequently used to characterize the performance of a solar cell. Thus, curves are independent of the solar cell type.

15 Boosting solar cell efficiency Despite solar cells are used as power supply sources, they have a different behavior from conventional batteries. While a typical battery acts as a voltage source, solar cell current versus voltage (IxV) characteristic is different and can be characterized mostly by two parameters, the open circuit voltage (Voc) and short circuit current (Isc). While the latter is proportional to incident luminosity intensity and area, the former remains almost constant, as it was discussed on previous section. Figure 12 exhibits a characterization curve, made on November 2011 at Recife (Brazil), of a monocrystalline solar panel with 135 mm x 112 mm x 5mm. The blue curve illustrates the current versus voltage. Its generation method consist in systematically decrease a pure resistive load, from the open-circuit condition to short circuit, taking notes of delivered voltage and current to each load proof. The red curve, delivered power curve, was created by taking the product of current and voltage for each point of blue curve. Analyzing delivered power curve, it could be noticed the existence of a power peak (green triangle), called of Maximum Power Point (MPP). Figure 12: Solar cell characterization. Measures were made at 11:50 a.m. on sunny day of November 2011 at Recife city, Brazil, using an 135 mm x 112 mm x 5 mm Monocrystalline solar cell.

16 The particular behavior observed at Figure 12 is related with the luminosity condition. As it is already expected the more available light we have, more power will be delivered by solar cells. Figure 13 shows several power curves for different light conditions. It can be observed that all curves exhibit approximately the same shape, even for very low light conditions. Figure 13: Power curves for different luminosity conditions, using a 135 mm x 112 mm x 5 mm Monocrystalline solar cell. Also, the maximum power point (MPP) voltage is different for different luminosity intensities. Tracking the MPP of a PV cell is usually an essential part of a PV system. Several techniques have been developed to extract the maximum available power at cell terminals. In order to compare each one of them, a common used figure of merit is applied, called tracking efficiency, which is defined by the ratio between actual power given by the solar cell using a specific method and true maximum power that the cell could produce under a given temperature and irradiance condition (Esram, 2007). Constant voltage (CV) algorithm is based in the fact that, from the observation of power versus voltage curves, like those in Figure 13, MPP voltage is, in fact, a constant fraction of solar cell s open-circuit voltage. This constant fraction has a value reported between 0.73 and 0.8 according to (Hohm, 2002) and (Won C-Y, 1994). The method works initially disconnecting the battery or load from solar cell in order to measure opencircuit voltage. Then, the MPP voltage is calculated based on predefined constant

17 factor. After that, the solar cells are configured to operate at the calculated MPP and wait for next open-circuit reevaluation cycle. Although this method is simple, there is a reported difficult in find out the best constant factor value, once it can vary with temperature and luminosity conditions, besides the panel has to be disconnected periodically to recalibrate the MPP, interrupting the energy harvest during open-circuit measures (Esram, 2007). The basic concept behind this method is to periodically perturb the cell operation voltage and check if the power available was increased. Taking a look at Figure 12, it can be seen that a voltage increase will increase power, when operating on the left side of V mpp, and decrease power when operating on the right side of V mpp. Hence, if the measured power increases, the next voltage step will be kept in the same direction until the MPP is reached, if the measured power decreases the voltage step polarity will be reversed toward the maximum power point. Perturb and Observe (P&O) oscillates over MPP with low implementation complexity and is independent of solar cell type or characteristics (Esram, 2007), and for these reasons it is by far the most used in commercial applications.

18 How to design efficient solar panel system for portable devices In the past years solar energy has been considered for a much wider range of consumer application, than calculators. The industry is aiming increasingly at more power hunger and daily life devices, like mobile phones, tablets and so on. In this section we will discuss some approach on setting up solar panel system for such devices showing advantages and drawbacks. The major part of portable devices uses Li-ion batteries as power supply, and for most of us to be separated from our gadget is undesirable, even during battery charging process. So, at first, a good solar charger strategy would be use a solar panel to charge an internal battery to further recharge the portable device itself. This avoids heating the portable device, more than necessary, during charge cycles. So the basic components to build up a solar charger system are: the solar cell, a storage battery and a diode to prevent battery discharge to solar cell, as depicted on Figure 14. Figure 14: basic solar charger system. Although this is a good starting point, a direct connection between solar cell and battery is not very efficient and there are others drawbacks. For instance: Li-Ion batteries require voltage and current monitoring to avoid overcharge. So plug a battery at an unregulated power supply might not be a good idea, due to the risk of damage it permanently. Besides, at this configuration, cell operation voltage will be defined by the battery voltage summed to voltage drop across the diode, which will vary during charge and won t be fixed at the cell s maximum power point. This means this system is, in fact, wasting some amount of available light power.

19 A second approach could be done including a battery charger between solar cell and battery, Figure 15. Now the battery terminals are connect to a regulated and monitored power supply. This would probably solve overcharge or permanent damage problems for your portable charger. Yet, such kind of system is still wasting bright sun light, once it still does not control the solar cell operation voltage. Besides, the addition of a block into power path will inevitably add losses to overall efficiency. So the battery charger should be designed to be as efficient as possible. Figure 15: Solar charger system with battery charger. In order to overcome the efficiency issue, an extra circuit could be added between battery charger and solar cell. This circuit is the MPPT system and is usually the key component, along with the solar cell itself, of a portable solar charger system. There are several ways to design such systems and the first step is chose what MPPT algorithm it will implement. As discussed at the last section, each algorithm has advantages and drawbacks, so the choice according to the systems requirements. For commercial purposes, P&O (Perturb and Observe Algorithm) have been adopted by more than 90% of portable solar charger providers. To implement P&O method, one must measure the solar cell power and step-up or step-down its operating voltage, according to the measurements results, as described on previous section. Besides that, the MPPT system output should be compatible with battery charge conditions. Finally, the system efficiency must be high enough; otherwise MPPT will be just a waste of extra components and money. The trade-off between efficiency, output and input voltage control can be satisfied with the use of an DC/DC converter, regulating the input voltage at MPP, keeping output also regulated to improve battery charger performance. Figure 16, describe that scenario.

20 Figure 16: Improved solar charger system. Some commercial MPPT systems for portable devices application are already available, as integrated circuits components, and report efficiency results up to 95%. Although the high performance reported, this efficiency is related to MPPT system block alone and doesn t account to the battery charger or another component losses.

21 SiliconReef solution SiliconReef s EH01 is a complete power management solution for a variety of energy-harvesting applications. It contains two DC/DC converters, a Micro-USB Charging Port, a full-featured Li-Ion Battery Charger and numerous protection features. It provides a small, simple solution for obtaining power from three different sources: a wall adapter, photovoltaic cells and a Li-Ion battery. It also counts with an intelligent maximum power point tracking system that adjusts the solar cells operating point to keep them working at their maximum power point under any given lighting condition. EH01 s main advantage over competing solutions is its capability of not only harvesting energy from a PV cell, but also use an internal intelligent MPPT algorithm to dynamically adjust the operating point so that maximum power transfer is achieved. Such a tracking system can yield up to 20% more power than regular converters under similar conditions. This MPPT controller then constantly monitors input voltage and current to determine the maximum power point reference voltage, V MPP. The input switcher will then transfer this energy to the main power line. If this line voltage goes beyond the 4.3 V threshold or if the line is already being powered by a wall adapter via the MAX pin (Figure 17), the switcher is automatically disabled. The switcher is a Buck- Boost DC/DC converter that uses the PWM technique to reach the highest efficiency. To improve the efficiency even further, when the absolute difference between input and output is higher than 200mV, it will work either in a Buck-only mode or a Boost-only mode. The design also counts with numerous protection circuits, such as: o Under-Voltage Lock-Out: disables the switcher and connects PV to main power line when its voltage is under 1.65V. o Soft-start: limits any in-rush currents; o Inductor peak-current limit for SW2L2 pin: starts the discharge cycle as soon as the inductor peak-current has been reached during the charge cycle; o Reverse current protection: disconnects the PV pin from the main power line in case of negative current to the MAX pin (in case a wall adapter is connected); o Thermal Shut Down to protect against overheating; o Two Battery Status Pins to monitor the battery progress.

22 Operating Scenario: Battery-Charging through Photovoltaic cell only As an example of typical operation, consider the following situation: no light hitting the photovoltaic panel, no wall-adapter plugged-in, half-discharged battery and no load at the output. Initially, since the photovoltaic panel has no available energy, the input switcher is disabled and the main power line will be held at the battery voltage (minus a small PMOS threshold voltage drop). When light starts to hit the PV cell, the input switcher is enabled and starts to transfer energy from the PV cell to the power line. The soft-start circuits limit any in-rush current spikes. As the system starts to operate, the MPPT will start to sample input voltage and current to determine the best operating point. It will then gradually modify the V MPP reference voltage to match that of the PV cell s maximum power point. The switcher will respond to these changes and regulate the input voltage to the V MPP voltage. Power line voltage will start to increase and battery charging will take place. Once the battery is fully charged, it will be disconnected from main power line, allowing this line to increase at a much faster rate. Once it reaches 4.3 V, the input switcher will be disabled to prevent overvoltage.

23 Figure 17: EH01 typical application. Computing verification shows that EH01 exhibits a tracking efficiency up to 95%, for small solar panels. An electric model was created from the characterization of a commercial solar panel for portable device application, and used at EH01 s verification. Figure 18 shows the panel behavior for two light conditions. In this figure it can be seen that for high light conditions the panel can delivery up to 792 mw (see M22 marker at red curve), while only 35 mw can be delivered at low light condition (from dy between M22 and M23 markers of Figure 18). These two conditions can be experienced as the passage of clouds covering the sun temporarily at a bright day. The above scenario was simulated to evaluate the EH01 performance. In Figure 19 it could be seen that solar panel voltage (V PV, red curve) is kept around 5,2 V, which deliver 737 mw of average power, from 792 mw available (93% tracking efficiency). At 20 ms of simulation, solar panel changes its behavior to reproduce the passage of a cloud, as shown on Figure 18. Note that, at this new light condition, the MPP voltage is no longer at 5 V, but around 4 V (see M23 marker at yellow curve of Figure 18). If the system doesn t update the new operating point to new solar conditions, the solar panel could just stop to provide energy, and even consumes it, if no protection circuit is present (see M11 marker at blue curve of Figure 19). EH01 update the panel operating point in a couple of milliseconds and keeps it around 3.9 V, which deliver almost 34 mw of power (M6 marker of Figure 19), from 35 mw available (just about 96% tracking efficiency).

24 Figure 18: Solar panel simulation at two different light conditions. The same cloud issue occurs around 40ms of simulation, when the panel changes its behavior again, but this time, to reproduce the end of a cloud passage. If no MPPT system is present the power harvested from the panel would be almost 680 mw, while the total available power is 792 mw (difference between operating at 4 V and at 5 V over the red curve of Figure 18). Nevertheless, EH01 is able to quickly track the MPP and set the operating voltage back to around 5.2 V, which in turns harvest almost 740 mw of power from solar panel. Figure 19: Tracking simulation.

25 Conclusions Renewable energy is not a new concept, nevertheless at an exponential growing population, the development and improvement of them are essential to sustain world power hunger. In 2050 the population expectation on earth is about 9 billion people, where approximately 5 billion will use mobile phones. The application of renewable energy at portable devices starts to plays a significant role at global energy saving. Not due to the effective amount of energy saved, but as motivator of a more responsible individual behavior regarding energy consuming. Improving solar battery charger for portable devices applications is a way to start to unplug devices, not just portables ones, from the power grid. This scenario invite us to rethink the concept of electronic devices itself, as well as the energy supply chain. At the same time it also brings commercial opportunities for electronic and energy sectors along with. The domain of efficiency in energy harvesting process, especially with solar cells, gives competitive advantages for industrial companies. Experiments around the world have been improving solar energy technologies and making possible application of solar battery charger to portable devices at industrial level. In Brazil, researches highlight an intelligent maximum power point tracking (MPPT) system to support energy harvesting on portable devices boosting solar cells efficiency. With these paradigms in mind, this white paper was developed to provide more information about solar cells and system and motivate industry to invest on this sector. Here, it was also present some technical guidance about portable solar system and commercial initiative examples, of solar charger system for portable devices.

26 About SiliconReef SiliconReef is a Brazilian company driven by innovation in the microelectronics sector, which focuses on the development of integrated circuits (ICs) for harvesting alternative energies. By combining cutting-edge technology and sustainable energy, the company s mission is to develop efficient products and solutions for harvesting alternative energies. Over the past seven years, the SiliconReef team has been developing projects for local industry and the export market. Currently, the company is preparing to launch its first product in 2013, the microchip Energy Harvesting 01 (EH01). SiliconReef also offers design and project services for dedicated integrated circuits and the development of projects based on architectures for rapid prototyping (FPGAs). The company offers top-quality services in order to meet the needs of a number of industries, such as automotive parts, utilities, electronics and industrial automation, amongst others. SiliconReef s differential in the field of energy efficiency and alternative energy sources may be translated as greater autonomy for electronic devices, consolidating the promotion of renewable energy within the semiconductor industry, an indispensable alternative in a world that will not easily relinquish connectivity.