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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.

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Efficiency of Solar Panels

The more efficient the photovoltaic solar panel, the more energy output it will have per amount of light energy hitting the cell, which will in turn take up less surface area to meet your energy requirements.

Today, most solar panels provide an energy efficiency rating between 11 and 15 percent, which is the percentage of solar energy that is being converted into useable electricity.

While it may seem like a low percentage, advancements in solar energy technologies are continually being made, and modern panels can more than cover the energy requirements of most commercial and residential needs.

Today, researchers are continually trying to improve the efficiency of photovoltaic technology. Scientists have now achieved a record of 40 percent efficiency by utilizing multi junction cells that are tuned to capture different frequencies of light on the electromagnetic spectrum. While these are currently the most efficient solar cells that have been created, they are not yet available to the general public.

If you have a smaller roof and limited space, a more efficient solar panel may be the right choice for you. Because of their higher efficiency, these panels may be slightly more expensive, but they will still meet the energy requirements you need.

However, if you have a larger amount of space, you may be able to achieve your energy needs with less efficient and less expensive panels to help save on your installation costs. Considering the overall cost of the panels and the kilowatt production can help you pick which is best for your installation.

Photovoltaics and Types of Solar Panels

The industry standard for solar electric systems is based on photovoltaic, or PV, technology, which converts sunlight into electricity. Multiple solar cells are interconnected to create a module, which comprises the panel.

Each panel is usually connected together in a system in which electricity is sent to an inverter to provide the power needed to run household electric devices. Efficiency is often dependent on the design of the panel and how it is engineered to capture different frequencies of light energy. Consider the following designs:

• Monocrystalline Silicon Panels. The most efficient type of photovoltaic solar panel available today is currently monocrystalline, or single, silicon panel. Because of the higher silicon content used in the design, they are more expensive than alternative types of panels. energy is converted into electricity, so fewer panels are needed in most roof installations to meet the same power requirements. These types of square-shaped panels are ideal for roof mounted solar electric systems.
• Polycrystalline Silicon Panels. With lower amounts of silicon utilized in polycrystalline, or multi-layered, silicon panels, they are often cheaper than their more efficient counterparts. They implement a design to help reduce this efficiency loss, allowing them to be utilized on roof mounted systems. This makes them ideal for larger projects and installations because they cost less. Polycrystalline silicon panels are also resistant more resistant to heat.
• Building Integrated Photovoltaic Panels. Appearance is an important aspect of a building. As with many historic buildings, or ones that feature unique architectural designs, owners might be hesitant to alter the distinctive character of a structure by installing a solar electric system. To help mitigate the aesthetic changes that solar panels might bring, building integrated photovoltaics are available. While they may be able to keep the look of traditional roofing, these types of panels are more costly and less efficient than the alternatives.
• Thin-Film Solar Cell Technology – Thin film cells are a newer photovoltaic technology that consists of one or multiple layers of thin films of photovoltaic cells that are laminated to existing material, such as metal roofing or glass Windows. These photovoltaic films are very thin allowing them to be lighter and more flexible compared to other PV systems. While thin-film technology is extremely versatile, it comes at a cost. Thin-film systems are less efficient and can degrade faster than conventional solar systems, but are improving with technology advancements.

Factors That Can Impact Efficiency

Solar power efficiency depends on a variety of factors including proper installation and assessment of the structure. Hiring a professional to inspect the structural design and location of your building is essential in determining what type of installation will best meet your requirements and provide you with the long-term economic benefits of solar power. Empire Renewable Energy offers a range of services for commercial and residential applications:

• Most solar panels are designed with a layer of protective glass over the cells, which sunlight must pass through. The amount of energy harnessed is dependent on the angle in which light is passing through, as well as the reduction of reflectivity of the glass.
• Proper installation of a solar panel is significant to capturing the maximum amount of sunlight. The angle of the panel and the amount of light hitting it are both important factors that will help you maximize efficiency.
• For roof mounted panels, the slope of the roof will impact how much sunlight is hitting the panels throughout the day. Large commercial installations can compensate for the movement of the Earth by installing solar tracking systems, but due to the expense, they are not typically installed for residential use.
• Temperature can impact the overall output of a solar cell. Higher temperatures can reduce output and lower efficiency. Some solar panels are designed for warmer climates, where efficiency must be maintained as temperatures climb. Ensure you select the right panel that is best suited to your climate, so you can enjoy a better return on your investment.
• Even a small shaded area on solar panels can greatly reduce their output. Since panels are often wired together in a system, even a small shadow on one panel can drastically reduce the system’s overall energy production. It is ideal to install panels in a way that no shadows will fall on any of them. In some situations this may be difficult to avoid, so alternatives are available to help maintain efficiency.

What Is A Solar Panel Power Rating?

This principle of energy and power doesn’t just describe appliances that consume electricity. It also applies to products that produce energy, such as solar panels. As such, a solar panel’s power rating is also measured in watts.

The solar industry agreed upon standard ambient conditions for testing their panels to simplify things for the consumer. Looking at the datasheets of two different solar panels, you can compare their power ratings fairly quickly. This is true regardless of the panel design and construction.

Solar panels typically have a 30-year power warranty. The warranty gives you rights as a customer to claim back money or a repair or replacement if the panel is faulty.

What Is Solar Panel Energy Output?

The energy output of a panel is simply how much electrical energy your panel has produced over a given length of time — for example, one hour, one day, or a full year. Like the kettle example, a panel’s energy is measured in kilowatt-hours or watt-hours.

When comparing energy output from different panels, the output data must be for the same period of time, i.e., 24 hours or 1 week.

How To Calculate Solar Panel Energy Output

A solar panel, much like a garden, is sensitive to the changing environment throughout the year. Therefore, to calculate the energy output of a solar panel over a year, you need to consider the following factors:

• Panel Compass Orientation: i.e., is the panel facing north, east, west, or south? In the northern hemisphere, panels pointed south get more sun than panels pointed north.

• Panel Tilt: the tilt of a panel relative to the ground or roof. This affects how much sun a panel will receive.

• Shading: shading will reduce the output of the shaded panels. Trees, buildings, or other features that shade the panels will reduce annual energy output.

• Ambient Solar: energy available over the year at that location —sunny places are better for solar panel output!

• Ambient Conditions: air temperature and the ground surface around the solar array affect the array’s output.

• Solar Panel: the panel’s design and building materials affect its response to ambient temperature and its efficiency in converting sunlight to electricity.

The combined effect of these factors is calculated using complicated math. Fortunately for you, we have an online calculator that can assist with the grunt work!

Peak Sun Hour Method

An alternative method is the Peak Sun Hour method, in which you take the Peak Sun Hours (“PSH”) for a location and multiply your findings by your panel’s peak power output. The result is a quick and straightforward way of estimating the average energy output of a panel in your location.

To calculate the energy output of a panel, you need to look up the peak sun hours for the location and multiply the data by the power rating of the solar panel.

Energy (in kWh) = PSH (hours) x Solar Panel Power Output (in kW)

Variables Affecting Solar Panel Output

In addition to the amount of sunlight received per day, there are other factors that affect the output of your solar panel or system.

Dirt

Anything that builds up on the surface of your solar panel can affect the output. This can include dust, leaves, snow, or bird droppings. A clean solar panel can be 6.5% more efficient than a dirty and dusty panel.

Roof Direction and Angle

Solar panels are most efficient when directed in a south-east to south-west direction, at an angle of 30-45 degrees. Systems at other directions and angles can still work, but your outputs will be decreased.

Shade

Solar panels are very sensitive to shade, including trees, or a building next door. Minimal shading in the morning or evening is fine, but significant shading throughout the middle part of the day will significantly impact the amount of electricity a panel can produce.

Location

The amount of sunshine and Cloud cover will affect the amount of energy a solar panel can produce.

Time of year

Solar panels can produce electricity year-round, even on overcast days. Through summer, the days are longer which generates more output, but shorter days in winter mean your output will be lower over these months.

Age

As solar panels age, their efficiency decreases at around 0.5% each year. The life cycle of the system is approximately 25 years before performance has decreased to the point a new system is needed.

Temperature

The efficiency of solar panels is usually measured at 77 F, and temperatures above this can end up decreasing their efficiency. Solar panels can work well in cold weather, and can still generate power in snowy conditions, too.

How Much Electricity Does My Home Need?

One solar panel on its own isn’t going to create enough electricity to power your entire home, but a solar panel system can. To work out what size system you need, you’ll need to complete some basic calculations that we’ve covered in our article How Many Solar Panels Do You Need?

To fully power an average home using 11,000 kWh per year, a typical solar power system will need between 21-24 panels of 320 watts each. The exact number and wattage of panels, as well as the output they can produce, will depend on where you live and the setup of your specific system.

Types of Solar Panels and Output

There are three main types of solar panels used for domestic systems:

• Monocrystalline. These are the most popular type of panel, made with pure silicon. They have an efficiency of 24.4%, with a moderate cost and a long lifespan.
• Polycrystalline. These are made of silicon crystals that have been melted together. They have an efficiency of 19.9%, a low cost and a moderate lifespan.
• Thin-film. Made with a variety of materials including small amounts of silicon, thin-film panels have an efficiency of 18.9%, with a high cost and a shorter lifespan.

The output of each type of panel will vary depending on the individual manufacturer, but will always be stated as a power rating in watts. The higher the watts, the higher the output. You may also see a kilowatt peak rating, which is the maximum power the panel can produce under the standard test conditions mentioned earlier.

Cost vs. Value

The solar market is very cost competitive, but some brands will offer you a more efficient system for a slightly higher investment. These systems will generate more electricity over the life of the system, so in the long run, they will create more value with consistent increased output.

Remember to look into federal tax credits and other incentives designed to reduce the cost of solar panel installation.

Given your house gets about six hours of daily sunshine, a standard 250-watt solar panel would produce 1.5 kWh of energy in a day.

You would need about 20 250-watt solar panels to generate the amount of energy the average American home uses in a day.

You can increase solar panel efficiency by cleaning the dirt off your panels regularly, pruning any trees that could be shading the panels, optimizing the panels’ angle (ideally to a 30- to 45-degree angle facing south), or installing an automatic solar tracker that rotates the panel to keep it aligned with the sun.

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• Belyakov, Nikolay. Chapter Seventeen—Solar Energy. Sustainable Power Generation: Current Status, Future Challenges, and Perspectives. 2019, pp. 417-438., doi:10.1016/B978-0-12-817012-0.00031-1