A Pictorial Diagram showing solar PV system output

Factors That Hinders The Best Performance Of PV Solar Output

You must carefully consider solar efficiency factors when designing solar PV systems. This will get the most out of your efforts and investment.

If you have inefficient appliances, you will require an extensive PV system (and a significant dent in your bank balance!). Even if you are filthy rich, it does not make sense.

Because fossil fuels are polluting and finite, alternative energy sources such as solar act as options. As a result, you want to make the best use of it possible.

Even by placing the electrical load with the most efficient appliances, one thing still stands out. You must keep in mind the PV system’s inefficiencies, which are always present.

Be aware of the various factors that may degrade your system to minimize them during the planning stage. Here are seven critical considerations.

The thickness of the cable

The cable length between the panel and the charge controller is 20 meters. A typical cable with a cross-section of 1.5 sq mm has a resistance of about 0.012 ohms/M of wire length. A 20-meter-long wire will therefore have a resistance of 20 x 0.012 = 0.24 ohms.

If a 24V system and a 10 amp current are flowing through this wire, you can calculate the voltage drop. The formula is (V = IxR): 2.4V. 

If a 10 Amp current flows, the voltage at the charge controller end of the cables will be 2.4V. This will be less than the voltage produced by the panels. This 10% drop in voltage is unacceptable.

What if we use a cable cross-section of 6 sq mm and resistance of 0.003 ohms per meter?

The total resistance for a 20-meter cable will now be 0.06 ohms. On the other hand, the voltage drop will be 100.06 or 0.6V. For a 24V system, the voltage drop is 2.5%, which may be acceptable. What about the higher cost of thicker cables?

Similarly, there will be wiring all around, and careful consideration must be given to the impact on overall system efficiency. As a result, cable length and size must be carefully considered during the planning stage.

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Another method for reducing resistance loss is increasing the system voltage to 48V. It will still produce the same power as before (48V x 5A = 240W). By doubling the system voltage, the voltage drop is cut in half.

While the size and length of the cables are determined by system design and installation, the Ministry of New and Renewable Energy (MNRE) in India requires that cables adhere to IEC 60227 / IS 694 or IEC 60502 / IS 1554 standards for quality (Part I & II). It is beneficial to become well-versed with the specifications and standards.

The Temperature.

Solar PV cells perform better in cold climates than in hot climates, and panels are currently rated at 25 C, which can be significantly different from the actual outdoor situation.

The panel output decays by about 0.25% for amorphous cells and about 0.4-0.5% for crystalline cells for each degree rise above 25 C.

As a result, on hot summer days, the panel temperature can easily reach 70 degrees Celsius or higher. This means the panels will produce up to 25% less power than rated at 25 degrees Celsius.

Solar panels are tested in a laboratory under STC (Standard Test Conditions) conditions, which include an irradiance (light) level of 1000W/m2 and a temperature of 25 C.

However, because these conditions constantly change in the real world, the panel output differs from the lab conditions.

As a result, NOCT specifications are reported (Nominal Operating Cell Temperature). Under the following conditions, it is the temperature reached by open circuit cells in a module:

Shading.

A photo of a solar panel shaded by a chimney.
Courtesy: Clean Energy Reviews

Solar panels should ideally be placed so that there are no shadows on them because even a small shadow on a small part of the panel can have a surprisingly significant effect on the output.

Usually, the cells in a panel are wired in series, and the shaded cells affect the current flow of the entire panel.

However, in some cases, it cannot be avoided, so the effects of partial shading should be considered when planning.

If the affected panel is connected in series (in a string) with other panels, the partial shading of one panel will affect the output of all those panels. In such a case, the obvious solution is to avoid wiring panels in series as much as possible.

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Charge Controller and Solar PV Cell IV Characteristics.

A common characteristic of solar PV silicon cells is that the current produced by a specific light level is virtually constant up to a certain voltage (about 0.5V for silicon) and then suddenly drops off. 

This means that the voltage varies primarily with light intensity. Typically, a solar panel with a nominal voltage of 12 volts would have 36 cells, resulting in a constant current of up to 18 volts.

Above this voltage, the current drops precipitously, resulting in maximum power output of around 18 volts.

When the panel is connected to the battery via a simple charge regulator, the voltage of the panel is reduced to near that of the battery. Therefore, the panel’s watt power (watt = amp x voltage) output was reduced.

When the battery voltage is near its maximum, the panel can produce its full power (fully charged). Consequntly, it is beneficial to design a system so that the batteries are never less than fully charged for an extended period.

The batteries may remain less than fully charged during rainy or heavily clouded days. This would further reduce the panel voltage, degrading the output.

MPPT chargers (Maximum PowerPoint Tracking) are also helpful. It tries to keep the panel at its maximum voltage while producing the voltage required by the battery.

A primary charge controller protects batteries from overcharging by cutting off the current from the solar panels (or reducing it to a pulse) when the battery voltage reaches a certain level.

A Maximum Power Point Tracker (MPPT) controller, on the other hand, performs an additional function to improve the efficiency of your system.

What is the function of the MPPT Controller?

An MPPT controller, in addition to performing the functions of a basic controller, includes a DC-to-DC voltage converter, which converts the voltage of the panels to that required by the batteries with minimal power loss.

In other words, it tries to keep the panel voltage near its Maximum Power Point while supplying the battery’s varying voltage requirements.

Efficiency of Inverters

An inverter is required when the solar PV system caters to the AC loads’ needs. As things stand, nothing in the real world is 100% efficient. Although inverters have a wide range of efficiencies, the most affordable solar inverters are between 80% and 90% efficient.

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For example, Su Kam’s 1000 VA inverter is typically 85% efficient; their 2KW – 5KW models are over 87% efficient. UTL’s Solar Hoodi Back Up (810VA – 3000VA) models are typically 80% efficient, while the Solar S-20 model is approximately 85% efficient.

Solar Battery Performance.

Batteries are required for charge storage whenever a backup is needed. In general, batteries are made of lead acid battery. 

All batteries discharge less than what goes into them; efficiency is determined by battery design and construction quality; some are unquestionably more efficient than others.

Because lead acid batteries are typically charged at a float voltage of approximately 13.5 V and discharged at about 12 V, the voltage efficiency is about 0.88. The coulomb efficiency is about 0.92 on average. As a result, the net energy efficiency is around 0.80.

The efficiency of a lead-acid battery is only 75-85%. (this includes both the charging loss and the discharging loss). From 0% SOC to 85% SOC, the average overall battery charging efficiency is 91%; the remainder is due to discharge losses.

The lost energy manifests as heat, which warms the battery. It can be reduced by keeping the charge and discharge rates as low as possible. It keeps the battery cool and extends its life.

We did not include losses in the battery charger’s electronic circuit, ranging between 60% and 80%. As a result, the overall efficiency of the battery system may be significantly lower.

Soiling On The Solar

Accumulation of material on PV panels can prevent light from reaching the solar cells, lowering the power generated. 

The amount of power lost due to soiling varies greatly depending on the type of soiling (such as dust or snow) and the frequency with which it is cleaned.

Dust deposited on the module’s light-receiving surface reduces the module’s light transmittance at first. Second, it will alter the incident angle of a portion of the light, causing it to spread unevenly in the glass cover.

According to studies, the output power of a clean solar module is at least 5% higher than that of a dust-accumulating module under the same conditions. And the greater the dust accumulation, the lower the module’s output performance.

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