6 Factors that Affect Solar PV System Efficiency
Energy efficiency factors must be carefully considered while designing any solar PV systems if you want to get the best out of your efforts and investment. If you have appliances that are not very energy efficient you will need a rather large PV system (and large dent in the bank balance too!). It does not make much sense, even if you are filthy rich. An alternate power source such as solar is considered because fossil fuel is dirty and is not ever lasting (looking at the galloping pace of increase in energy consumption across the globe). Therefore, you would like to use it in the best possible manner.
However, even after you have replaced the electrical load with the most efficient appliances, you still have to keep in mind inefficiencies of the PV system which are always lurking around. Hence, it pays to have knowledge of different factors that can potentially degrade your system, so that you can make efforts to minimize them right at the planning stage. Here are 6 important considerations.
1. Cable Thickness
We generally have electrical appliances working at 220V which is significantly higher compared with the usual PV system DC voltages of 12V, 24V or 48V. For the same wattage much higher currents are involved in the PV systems. This brings into picture resistance losses in the wiring.
Let us see how it can be significant.
20 meter is the length of cable between the panel and the charge controller. A typical cable with 1.5 sq mm cross section has resistance of about 0.012 ohms per meter of wire length. So a 20 meter long wire will offer resistance of 20 x 0.012 = 0.24 ohms.
If it is a 24V system and a 10 ampere current is flowing through this wire, then from the Ohm’s law (V = IxR), we can calculate voltage drop across this wire: 2.4V. It means the voltage at the charge controller end of the cables will be 2.4V less than the voltage produced by the panels if a 10 Amp current is flowing. This 10% voltage drop is clearly unacceptable.
What if we use a 6 sq mm cross section cable which has a resistance of 0.003 ohms per meter. The total resistance for 20 meter long cable will now be 0.06 ohms; and the voltage drop, 10×0.06 or 0.6V. It is 2.5% voltage drop for a 24V system which might be acceptable. But what about the increased cost of thicker cable? Likewise, there would be wiring all around and careful attention must be paid to know the impact on overall system efficiency. Thus, cable length and size needs careful attention right at the planning stage.
Another way to reduce resistance loss is to raise the system voltage, to say 48V. It will still give the same watt as above (48V x 5A = 240W). Doubling the system voltage reduces the voltage drop by 1/4th.
While the size and length of the cables is a matter of system design and installation, for the quality of cables the Ministry of New and Renewable Energy (MNRE) in India specifies that cables adhere to IEC 60227 / IS 694 or IEC 60502 / IS 1554 (Part I & II). It pays to familiarize what these specification standards say.
2. Temperature
Solar cells perform better in cold rather than in hot climate and as things stand, panels are rated at 25˚C which can be significantly different from the real outdoor situation. For each degree rise in temperature above 25˚C the panel output decays by about 0.25% for amorphous cells and about 0.4-0.5% for crystalline cells. Thus, in hot summer days panel temperature can easily reach 70˚C or more. What it means is that the panels will put out up to 25% less power compared to what they are rated for at 25˚C. Thus a 100W panel will produce only 75W in May/June in most parts of India where temperatures reach 45˚C and beyond in summer and electricity demand is high.
Solar panels are tested under laboratory conditions, called STC (Standard Test Conditions): at an Irradiance (light) level of 1000W/m2 with a temperature of 25˚C. But in the real world these conditions are constantly changing so the panel output is different from the lab conditions. So, another specifications are reported, called NOCT (Nominal Operating Cell Temperature). It is the temperature reached by open circuit cells in a module under the following conditions:
Irradiance (light) falling on the solar panel at 800W/m2; Air temperature of 20ºC; Wind speed at 1m/s; and the panel is mounted with an open back (air can circulate behind panel).
Most good quality panels available today in India have NOCT values of 47±2˚C. Lower the NOCT the better it is expected to perform in hotter climates.
Temperature coefficient of the rated watt power, Pmax, is another important parameter.
Example: EMMVEE solar panels have NOCT of 48±2˚C and temperature coefficient of rated power -0.43% per K. Moser Baer panels have NOCT of 47±2˚C and temperature coefficient of rated power -0.43% per K for panels up to 125Wp; their higher power panels have NOCT of 45±2˚C and temperature coefficient of rated power -0.45% per K.
3. Shading
Ideally solar panels should be located such that there will never be shadows on them because a shadow on even a small part of the panel can have a surprisingly large effect on the output. The cells within a panel are normally all wired in series and the shaded cells affect the current flow of the whole panel. But there can be situations where it cannot be avoided, and thus the effects of partial shading should be considered while planning. If the affected panel is wired in series (in a string) with other panels, then the output of all those panels will be affected by the partial shading of one panel. In such a situation, an obvious solution is to avoid wiring panels in series if possible.
4. Charge Controller and Solar Cell’s IV Characteristics
An inherent characteristic of solar silicon cells is that the current produced by a particular light level is virtually constant up to a certain voltage (about 0.5V for silicon) and then drops off abruptly. What it means is that mainly the voltage varies with light intensity. A solar panel with a nominal voltage of 12 volts would normally have 36 cells, resulting in a constant current up to about 18 volts. Above this voltage, current drops off rapidly, resulting in maximum power output being produced at around 18 volts.
When the panel is connected to the battery through a simple charge regulator, its voltage will be pulled down to near that of the battery. This lead to lower watt power (watt = Amp x Volt) output from the panel. Thus, the panel will be able to produce its maximum power when the battery voltage is near its maximum (fully charged). So it helps to design a system in such as way that the batteries normally don’t remain less than full charged for long. In times of rainy or heavy clouded days a situation may occur when the batteries remain in the state of less than full charge. This would further pull down the panel voltage; thus degrading the output further.
This is also where an MPPT (Maximum Power Point Tracking) Charge Controller comes into picture. It tries keeping the panel at its maximum voltage and simultaneously produces the voltage required by the battery. A basic charge controller simply prevents damage of batteries by over-charging, by effectively cutting off the current from the solar panels (or by reducing it to a pulse) when the battery voltage reaches a certain level. On the other hand, a Maximum Power Point Tracker (MPPT) controller performs an extra function to improve your system efficiency.
What does the MPPT Controller Do
Besides performing the function of a basic controller, an MPPT controller also includes a DC to DC voltage converter, converting the voltage of the panels to that required by the batteries, with very little loss of power. In other words, it attempts to keep the panel voltage near its Maximum Power Point, while supplying the varying voltage requirements of the battery. Thus, it essentially decouples the panel and battery voltages so that there can be a 24 volt system on one side of the MPPT charge controller and panels wired in series to produce 48 volts on the other. Thus, offering the ability to provide some charging current even in dull conditions when a simple controller would not help much.
Relevant standards specified for charge controllers and power conditioning units by the MNRE are: IEC 60068-2 (1,2,14,30) / Equivalent BIS Std., IEC 61683 / IS 61683.
5. Inverter Efficiency
When the solar PV system is catering to the needs of the AC loads an inverter is needed. As things stand, in real world nothing is 100% efficient. Although inverters come with wide ranging efficiencies but typically affordable solar inverters are between 80% to 90% efficient.
Example: Su Kam’s 1000 VA inverter is typically 85% efficient; their 2KW – 5KW models have over 87% efficiency. UTL’s UTL Solar Hoodi Back Up (810VA – 3000VA) models are typically 80% efficient and the Solar S-20 model about 85%.
6. Battery Efficiency
Whenever backup is required batteries are needed for charge storage. Lead acid batteries are most commonly used. All batteries discharge less than what go into them; the efficiency depends on the battery design and quality of construction; some are certainly more efficient than others.
The energy put in a battery during charging Ein can be given as
Ein = ICVC ΔTC where IC is the constant charge current at voltage VC for time duration ΔTC
Likewise after it is discharged at a constant current ID, at a voltage VD during a time ΔTD ; the delivered energy is
Eout = IDVD ΔTD
Now writing the energy efficiency as Ein/Eout = ICVC ΔTC / IDVD ΔTD
There are two types of efficiencies: voltage efficiency (VD / VC) and coulomb efficiency (IDΔTD / IC ΔTC)
Since lead acid batteries are usually charged at the float voltage of about 13.5 V and the discharge voltage is about 12 V, the voltage efficiency is about 0.88. In average the coulomb efficiency is about 0.92. Hence, the net energy efficiency is around 0.80
A lead-acid battery has an efficiency of only 75-85% (this includes both the charging loss and the discharging loss). From zero State of Charge (SOC) to 85% SOC the average overall battery charging efficiency is 91%- the balance is losses during discharge. The energy lost appears as heat which warms the battery. It can be minimized by keeping the charge and discharge rates low. It helps keep the battery cool and improves its life.
Here we did not include losses in the electronic circuit of the battery charger which may vary between 60% and 80%. Thus, the overall efficiency of the battery system can be much lower.
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