7/21/2019 Solar Battery Charger_Highlighted http://slidepdf.com/reader/full/solar-battery-chargerhighlighted 1/27 Solar Battery charger for portable devices application Filipe Távora, Analog Designer Andrea Sarmento Maia, Analog Designer trainee 2012, June
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o Solar panels are increasingly more powerful and cost has been
decreasing. This makes it increasingly more solar energy an economicallyviable 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 levelsof radiation have great potential for energy recovery (aneel_gov_br).
Figure 5: Daily insolation of Brazil (aneel_gov_br).
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• 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) couldhave, 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.
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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 efficiencya 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).
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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 meanmore 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.
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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 causedby 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 ofvoltage 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
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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 factorand efficiency are frequently used to characterize the performance of a solar cell. Thus,curves are independent of the solar cell type.
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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 ofNovember 2011 at Recife city, Brazil, using an 135 mm x 112 mm x 5 mm Monocrystalline solar
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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 bedelivered 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 5mm 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 isapplied, 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 open-
circuit voltage. Then, the MPP voltage is calculated based on predefined constant
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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 areported 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 untilthe 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
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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 andbattery 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,
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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 viathe 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.
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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 acommercial 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
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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, researcheshighlight 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.