Techniques to Maximize Solar Panel Power Output
Two recent articles, Energy Harvesting With Low Power Solar Panels and Solar Battery Charger Maintains High Efficiency at Low Light, discuss how to efficiently harvest energy with low power solar panels. Both of these articles mention a concept known as maximum power, which in the context of solar panels is the ability to extract as much power as possible from the solar panel without collapsing the panel voltage. When discussing solar panels and power, terms such as Maximum Power Point Tracking (MPPT) and Maximum Power Point Control (MPPC) are often used. Let’s look into the definition and meaning of these terms in more detail.
As can be seen in Figure 1, the output current of a solar panel varies nonlinearly with the panel voltage. Under short-circuit conditions the output power is zero since the output voltage is zero. Under open-circuit conditions the output power is zero since the output current is zero. Most solar panel manufacturers will specify the panel voltage at maximum power (VMP). This voltage is typically around 70 – 80% of the panel’s open circuit voltage (VOC).In Figure 1 the maximum power is just under 140W with VMP just under 32V and IMP just under 4.5A.
Ideally, any system using a solar panel would operate that panel at its maximum power output. This is particularly true of a solar powered battery charger, where the goal, presumably, is to capture and store as much solar energy as possible in as little time as possible. Put another way, since we cannot predict the availability or intensity of solar power, we need to harness as much energy as possible while energy is available.
There are many different ways to try to operate a solar panel at its maximum power point. One of the simplest is to connect a battery to the solar panel through a diode. This technique is described here in the article Energy Harvesting With Low Power Solar Panels. It relies on matching the maximum power output voltage of the panel to the relatively narrow voltage range of the battery. When available power levels are very low (approximately less than a few tens of milliwatts), this may be the best approach.
The opposite end of the spectrum is an approach that implements a complete Maximum Power Point Tracking (MPPT) algorithm. There are a variety of MPPT algorithms, but most will have some ability to sweep the entire operating range of the solar panel to find where maximum power is produced. The LT8490 and LTC4015 are examples of integrated circuits that perform this function. The advantage of a full MPPT algorithm is that it can differentiate a local power peak from a global power maximum. In multi-cell solar panels, it is possible to have more than one power peak during partial shading conditions (see Figure 2). Typically, a full MPPT algorithm is required to find the true maximum power operating point. It does so by periodically sweeping the entire output range of the solar panel and remembering the operating conditions where maximum power was achieved. When the sweep is complete, the circuitry forces the panel to return to its maximum power point. In between these periodic sweeps, the MPPT algorithm will continuously dither the operating point to ensure that it operates at the peak.
An intermediate approach is something that Analog Devices calls Maximum Power Point Control (MPPC). This technique takes advantage of the fact that the maximum power voltage (VMP) of a solar panel does not, typically, vary much as the amount of incident light changes (see Solar Battery Charger Maintains High Efficiency in Low Light for more information). Therefore, a simple circuit can force the panel to operate at a fixed voltage and approximate maximum power operation. A voltage divider is used to measure the panel voltage and if the input voltage falls below the programmed level, the load on the panel is reduced until it can maintain the programmed voltage level. Products with this functionality include the LTC3105, LTC3129, LT3652(HV), LTC4000-1, and LTC4020. Note that the LT3652 and LT3652HV datasheet refer to MPPT rather than MPPC, but this is largely because Analog Devices had not come up with the MPPC terminology when the LT3652 product was released.
A final note about MPPC and the LTC3105 – the LTC3105 is a boost converter that can start up at the exceedingly low voltage of 0.25V. This makes the LTC3105 particularly well suited for boosting the output voltage of a “1S” solar panel (i.e. a solar panel whose output voltage is that of a single photovoltaic cell, even if the panel has many photovoltaic cells in parallel). With a 1S solar panel, there will be only one maximum power point – it is not possible to have multiple power peaks. In this scenario, differentiating between multiple maxima is not necessary.
Table of contents
Let’s start from the beginning. Almost all energy we use here on earth, with the exception of nuclear power, was originally solar energy. Yep, even gasoline got its energy millions of years ago from the sun. Solar panels shortcut the millions of years waiting period and turn it into immediate electricity for our use.
Solar energy is simply energy that’s converted from the sun’s rays into DC (Direct Current) electricity. This happens when sunlight hits the cells on a solar panel (also known as photovoltaic cells).
The DC electricity can then either be used directly from the solar panel or stored within deep cycle batteries, like our Battle Born series of lithium, for later use. The panels can also invert the DC electricity to AC (Alternating Current) energy for household use.
Parts of a Solar Panel
Now that we understand what solar energy is, let’s explore all the different parts of a solar panel. This will help us understand exactly how they convert sunlight into usable electricity.
Solar Cell
Solar cells are the bread and butter of a solar panel: They actually do the dirty work of converting UV rays into DC electricity. The solar cells make up the face of the solar panel, and they typically have a black or bluish hue, depending on the type of the panels.
Solar cells use silicon or another material with photovoltaic properties in their construction. When sunlight hits these cells, it creates an electric current by a process called the Photovoltaic Effect.
Busbars/Internal Wiring
So, if the cells convert sunlight into electricity, how does that electricity get to where it needs to go? This is where busbars come in. Busbars are thin strips of copper or aluminum that run on and between the cells. They function as conductors to collect and channel the DC energy to the junction box and wires. The busbar layout also will determine the voltage of the panel. The more cells connected in series, the higher the voltage.
Encasement
An important thing to remember when thinking about how solar panels work is that none of these important elements remain exposed. Just like the wires that run through your walls, the busbars and solar cells sit encased within a protective covering that shields them from the elements so they can conduct electricity properly. Solar cells are typically encased in stiff sheets of glass or plastic that are unaffected by UV rays, while the busbars are coated in plastic, creating a route called busways.
Junction Box
Another essential part of a solar panel is the junction box. The junction box is where all the internal wiring meets. This box protects the wiring from the outside environment.
Bypass diodes within the junction box ensure the electricity flows in one direction and not back into the cells themselves. This is a critical part of the solar panel because it makes it possible to connect multiple panels together or directly to the solar inverter.
How Does the Solar Cell Work?
This is all great in terms of how a solar panel works, but what actually happens within the solar panel when light hits the cells? Essentially, the panel consists of layers of solar cells. Each layer is “doped” to create a positive/negative junction.
In other words, one layer will have a positive charge (and thus a lack of electrons), while the next layer will have a negative charge (and thus an excess of electrons)
W here the different layers meet is called a PN junction and creates a zone within the cell that lacks a charge, known as a depletion zone. When sunlight hits the cells, photons knock electrons across this barrier causing electrons to flow from negative to positive, generating voltage.
→ Need a refresher on volts? We suggest reading: What are Volts and Why Do They Matter?
Temperature drop
The researchers found that radiative cooling resulted in a 5–36 °C drop in the temperature of the system, depending on weather conditions, compared with the set-ups without radiative cooling. Bermel told Physics World that the largest temperature difference was recorded with radiative cooling on its own, but the lowest absolute temperatures occurred when it was used in tandem with convective cooling.
These temperature drops caused a relative increase in open-circuit voltage for the solar cells of between 8–27%. This is “roughly proportional to efficiency,” Bermel says. Using temperature data from the experiments the scientists also simulated the impact of cooling on the lifespan of the solar cells. This suggests that radiative cooling could extend the lifetime of concentrated photovoltaic cells by a factor of 4–15.
According to the researchers, the results demonstrate that radiative cooling provides benefits in all weather conditions. But Bermel’s colleague, graduate student Ze Wang, also at Purdue University, cautions that radiative cooling probably will not be suitable for cooling concentrated photovoltaic systems on its own. Other systems would be needed to ensure cooling in all conditions.
Auxiliary cooling mechanism
“Radiative cooling is a very good auxiliary cooling mechanism, which requires no extra energy, performs well at high temperatures, and adds little weight to the whole system,” Wang says. “However, in most cases, radiative cooling serves as an add-on to the existing cooling system utilizing convection or conduction, in order to improve the overall performance.”
However, radiative cooling does not perform well in low-temperature conditions, Wang explains. This is because the temperature difference between the solar cell and the air is too small to fully exploit the potential of radiative cooling. This is a particular issue when there are no low-temperature absorbers, such as a clear sky, around.
Radiative cooling materials are also not limited to the soda-lime glass. “We could work on the materials or structures of the coolers in the future to further improve the emittance profile,” Wang says.
Michael Allen is a science writer based in the UK
Quantum Dots
Quantum dot solar cells conduct electricity through tiny particles of different semiconductor materials just a few nanometers wide, called quantum dots. Quantum dots provide a new way to process semiconductor materials, but it is difficult to create an electrical connection between them, so they’re currently not very efficient. However, they are easy to make into solar cells. They can be deposited onto a substrate using a spin-coat method, a spray, or roll-to-roll printers like the ones used to print newspapers.
Quantum dots come in various sizes and their bandgap is customizable, enabling them to collect light that’s difficult to capture and to be paired with other semiconductors, like perovskites, to optimize the performance of a multijunction solar cell (more on those below).
Multijunction Photovoltaics
Another strategy to improve PV cell efficiency is layering multiple semiconductors to make multijunction solar cells. These cells are essentially stacks of different semiconductor materials, as opposed to single-junction cells, which have only one semiconductor. Each layer has a different bandgap, so they each absorb a different part of the solar spectrum, making greater use of sunlight than single-junction cells. Multijunction solar cells can reach record efficiency levels because the light that doesn’t get absorbed by the first semiconductor layer is captured by a layer beneath it.
While all solar cells with more than one bandgap are multijunction solar cells, a solar cell with exactly two bandgaps is called a tandem solar cell. Multijunction solar cells that combine semiconductors from columns III and V in the periodic table are called multijunction III-V solar cells.
Multijunction solar cells have demonstrated efficiencies higher than 45%, but they’re costly and difficult to manufacture, so they’re reserved for space exploration. The military is using III-V solar cells in drones, and researchers are exploring other uses for them where high efficiency is key.
Concentration Photovoltaics
Concentration PV, also known as CPV, focuses sunlight onto a solar cell by using a mirror or lens. By focusing sunlight onto a small area, less PV material is required. PV materials become more efficient as the light becomes more concentrated, so the highest overall efficiencies are obtained with CPV cells and modules. However, more expensive materials, manufacturing techniques, and ability to track the movement of the sun are required, so demonstrating the necessary cost advantage over today’s high-volume silicon modules has become challenging.
Learn more about photovoltaics research in the Solar Energy Technologies Office, check out these solar energy information resources, and find out more about how solar works.
