Why should I go for LED Lighting

led bulbLED based lighting is the newest innovation in the lighting industry. It has revolutionized energy-efficient lighting. LED is short for Light Emitting Diode: it is a diode which emits light when current flows through it. These small solid state lights are extremely energy-efficient. LED light assemblies normally combine several such diodes to get the desired brightness. They also consume considerably less electricity for same brightness. The LED technology is still improving very fast and is the lighting solutions of the future.

Low power consumption and long life pays-off in the long-run, apart from being a green technology. Well-designed LED lighting fixtures can retain 70% of their initial output for 50,000 hours or more, depending on operating conditions. LEDs are more efficient than both incandescent and CFL lamps for two reasons: One, they emit light in a targeted direction – instead of scattering it in all directions which is wasteful. Two, they don’t emit great amounts of heat. In comparison, incandescent bulbs and the CFL lamps waste a large portion of power as heat: 70 percent or even higher.

light source comparison

LED’s are now commonly available with efficiencies of 100-120 Lumens per watt compared to 65-80 Lumens/watt of CFL, 45 Lumen/Watt of Mercury vapor and 75 Lumen /watt of metal halide or 94 Lumen / watt of sodium Vapor.

In dozens of nations, green initiatives and energy-efficiency directives are hastening the shift towards LED lighting systems, which have the lowest energy consumption and environmental impact, the longest useful life, and the lowest operational cost. LED lamps are now widely used in solar systems because they offer superior light output per watt which reduces total cost of the system.

advantages of LEDs

Light Quality

Lumens doesn’t describe the quality of the generated light – its color, tone or other variables. People often describe the lights as “warm,” “cool,” “pale,” “dim” and so on The biggest challenge for LED manufacturers is creating lightings that mimic the light quality of conventional lamps. There are two parameters that are often talked about while discussing LED lightings: Color Rendering Index (CRI) and Color Temperature (K)

Color Rendering Index is a subjective indication of how well a lamp will reproduce colors. Lights at the low end of the index, such as low-pressure sodium lamps (CRI 20-30) tend to wash out colors and are best used in applications like street lighting, where accurate color rendering is not important. An incandescent light bulb, on the other hand, is considered to have a “perfect” CRI of 100.

Linear fluorescents and compact fluorescents (CFLs) usually fall in the 80 to 90 range. Metal Halides only has a CRI of about 70, so only 70% of the colors will be rendered correctly. HPS has a CRI of 22. In general, CRI values higher than 80 are considered good for indoor lighting, and higher than 90 are good for visual inspection tasks such as those required in printing or the textile industry.

Color Temperature is not how hot the lamp is. It is the relative whiteness of a piece of tungsten steel heated to that temperature in degrees Kelvin. While CRI represents how accurate a light source is, color temperature (CCT expressed in degrees Kelvin (K)) represents the character of the light source. At the low end, a color temperature of 2600-2700K creates a warm light character like that seen in incandescent bulbs; a higher color temperature of 4100-5500K creates a whiter light like most often seen in office buildings. HPS has a warm (red) color temperature of around 2700K as compared to MH at 4200K, which has a cool (blue) color temperature.

It is important to remember that two light sources can only be compared if their color temperatures are equal. You cannot compare the CRI of HPS (2700K) and Metal Halide (4200K). Also note that at extremely low or high color temperatures, the color rendering can be very poor despite high CRI scores close to 100.

Illumination and light quality

Need to Think Differently

Because of long experience with incandescent light bulbs, most people have learned to correlate brightness with the wattage of the light source: a 100-watt lamp puts out more light than a 60-watt lamp. Thus, when people look for lighting source, they think in terms of 40, 60, or 100-watt bulbs. Generally, incandescent lamps use the same filament material heated to the same temperature, so the only way to increase their light output is to increase the wattage. It is different with LED light assemblies. They involve no filament or heating but merely current is passed through diodes which results in the emission of light. Usually several tiny LED devices are mounted alongside to get the desired illumination. Different LEDs may use different materials, each with its own light emission efficiency. For this and other reasons, two different LED sources can consume the same number of watts but give out different light output.

Another issue is the directionality of light. An incandescent light bulb illuminates in a 360-degree spherical pattern regardless of the shape of the bulb. When we light an area with such a ‘spherical bulb’ source, even with a reflector, only a small proportion (50% or less) of the light is delivered to the surface we are lighting – the rest is lost. Light is wasted by diffusers; or through filtering or lensing; or when directed away from the target area.

The way LEDs are shaped and designed, light is not emitted in all directions; with proper design over 80 percent light can be directed where it is needed. LED fixtures deliver light in the desired direction and create brighter illumination, allowing efficient use of the produced light. Thus, an LED lighting fixture with lower rated Lumens (a measure of illumination explained later) may deliver the same or more useful light to a targeted area than a comparable fluorescent lighting fixture with a higher rated light output.

Thus, people should now learn to think differently when they talk about lighting. With LED lights it is more useful to talk in terms of brightness delivered rather than brightness produced by the fixture.

An Important Note

Heat dissipation is an important design factor for efficient LED performance.

The LED must be connected to a Resistor in series for safe operation, otherwise the LED will “burn” in a short time.

The resistor Value, R is given by       R = (VS-VL)/I

VS= the Supply Voltage,       VL= the LED Voltage       I = the LED current

How to choose the resistors? Choose the nearest standard resistor value which is greater than the calculates values, if the calculate value is not available. The higher resistor value will decrease the LED current and increase the LED life but it will decrease the LED brightness.

Use led with resistors

Why should I Choose LED lights

Advantages of LED Based Lights

Dimmable LEDs have a Bright Future

Energy Efficiency in Homes

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10 Frequently Asked Questions on Solar PV Systems

1. What is a solar PV system?

Solar photovoltaic system uses solar panels to convert sunlight into electricity. The electricity generated can be stored in the batteries or used directly as DC supply or through an inverter for running Ac appliances. Solar PV systems have no moving part; thus they work silently and there is no mechanical wear and tear requiring regular maintenance. They are ideal for remote locations.

2. How long do the solar PV systems last?

Good quality solar panels are designed to resist environmental conditions such as rain, sun and strong winds. They generally have a 20-25 year performance warranty, but should last even longer. Manufacturers normally assure at least 90% efficiency up to 10 years and 80% up to 20 years.

Standalone PV systems have another important and costly component: the batteries. These are deep discharge cycle batteries, not of the types used in the cars or trucks. The average life of a battery depends on several factors: system design, battery quality, depth of discharge, number of charge-discharge cycles, temperature of battery and maintenance. Quality deep cycle solar batteries like those offered by Exide can last 5 – 10 years.

When the solar DC power is converted into AC inverters are involved. Inverters are also available in various designs, capacity, quality and price. They need to be replaced after some years depending upon the quality. Thus, if a PV system is well designed and good quality batteries and inverters are used, the system is virtually maintenance free.

3. Is my site suitable for solar PV power?

In India (as anywhere in the northern hemisphere) the site must have clear southern exposure during the day. The PV modules must not have shadows of trees, mountains, and buildings at any time of the day, especially during 9 am to 4 pm, during the whole year. The sun travels a lower path during the winter creating the possibility of shadowing even if the modules remain clear of shadows in the summer. Also consider the possibility of the growth of trees and some high rising structure coming up in the future that may cause shading problems. Further, a flat, grassy site is appropriate, whereas a steep and rocky hillside is not.

4. How does weather affect PV module output?

Sunlight is the raw material for the PV power systems. Weather pattern and seasonal factors have directly impact of the sunshine and hence they play crucial role in determining how much power your PV system produces. For any location the potential of PV power generation is given by the annual average “sun hours”. The other important factor is the temperature: in hotter climates panels produce lesser power. Solar modules have a negative temperature coefficient of about 0.40% per degree of rated watt power. Thus, solar panels perform best under bright sun in cooler climates.

5. What is the best way to use solar electricity?

Efficiency is the key word. Before installing a solar power system, it is good to replace current electrical appliances with energy efficient models. Investment in more efficient appliances pays back within months or years. Lights can be replaced with more efficient LED based lighting. Electronic chokes should be used with tube lights. Significantly more efficient fans are available at present; for instance, Crompton Greaves is soon coming up with a 35W ceiling fan (1200 mm size). Already ceiling fans are available in the 45 – 50W range. Star rating system is being used to mark the efficiency of air conditioner and refrigerator. These systems with higher star rating should be used. Laptops consume much less power as compared to desktops PCs (about 70 Watt as against 250 Watt), therefore use of laptops should be promoted.

6. What will determine the size of my solar PV system?

The size of a PV system depends on how much electricity is required. This is usually measured in watt-hour consumption of power. It is obtained by multiplying the wattage of appliances (fans, lights, laptop etc) by average number of hours of daily use and adding them all. For example, if you want to run a 10 watt fan for 5 hours and a 20 watt CFL light for 10 hours every day, then the total daily consumption will be (10 x 5) + (20 x 10) = 250 watt-hours. If you get about 5 hours of average daily sunshine, then a 50 watt panel (250 watt-hours / 5 hours) will be required. This is assuming 100 percent efficiency but there are losses in real systems, so somewhat higher panel wattage will be required. A battery will be required to store the power to run the light and fan after sunset. If AC appliances are to be run, an inverter will be required.

7. What loads can PV run and not run?

With solar Photovoltaic power you can run any electrical load. However, air conditioning and gadgets with electric heating elements (electrical stove, water heater, etc) use large amounts of electricity which will drive the system cost very high. Therefore, it is better to plan for a system that can take care of lighting, fans and laptop, TV etc and continue to run other high power appliances on the regular grid power supply. The solar system cost generally pays for itself within few years and savings on power bill continue for a long time. It will be still better if all lights are replaced with energy efficient LED lights that consume much less power for same illumination.

8. Can I use solar PV power for heating water?

No, it is a bad idea. The photovoltaic mechanism converts sun’s energy into DC electricity with a rather small efficiency of 13 – 16 percent. Therefore, trying to operate a high power electric heating element from PV would be very inefficient and expensive. There are solar water heaters that directly heat water from sun’s energy which is much more efficient and appropriate.

9. Can I use a car battery in my solar system?

It is not a good idea. Automotive batteries are shallow-cycle batteries and are designed to provide very high current for short durations and can be discharged only up to say 20 percent of their capacity. If repeatedly discharged beyond 20% more than a few dozen times, they are likely to be damaged or die down soon.

Unpredictability of electricity generation is inbuilt in the solar systems because not every day is a sunny day or sunny enough! Then there are cloudy and rainy days. Thus, you need the kind of batteries that can withstand vagaries of the panel power output, and hence of the climatic mood. It means, batteries must be able to cope with unpredictable charging and discharging. To provide electricity over long periods, PV systems require deep cycle batteries. These are usually lead-acid batteries and are designed to go through cycles of discharge up to 80% and recharge hundreds (even thousands) of times.

Lead-acid batteries used in solar systems are either low maintenance flooded type batteries which require addition of water every 8-10 month or are sealed type which are spill proof and do not require periodic maintenance (addition of water). Sealed batteries are ideal for remote locations and require no water addition.

Another class of batteries are alkaline Nickel-Cadmium type. Due to high cost they are only recommended where extreme cold (sub zero) temperatures are encountered or for certain critical applications requiring their advantages over the lead acid batteries. Their superiority over the lead acid batteries include tolerance of freezing or high temperatures, low maintenance requirements, and the ability to be fully discharged or overcharged without harm.

10. Can the solar panel be directly connected to the battery?

Batteries need protection against overvoltage and overcharging. Therefore connecting them directly to the solar panel is not a good idea and it is wise to use a charge controller. A charge controller regulates the voltage and current coming from the solar panels going to the battery. Most controllers also control the power to the load, disconnecting it as the battery becomes depleted; reconnect again when the battery is discharged to some low level. A charge controller is an essential part of PV systems with battery storage and keeps the batteries safe for long life.

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Basic Solar Energy Math

The basic unit of power or electricity is Watt. It is actually a measure of rate of energy. Larger units of power are measured in multiples of 1000. For example,

1000 watts     =       1 kilowatt (kW)

1000 kW        =       1 megawatt (MW)

1000 MW       =       1 Gigawatt (GW)

1000 GW       =       1 Terawatt (TW)

If an electrical appliance consumes 1000 watts for one hour, it has consumed 1 kWh of energy or 1 unit of electricity. So, 1000 watt hours = 1 kilowatt hour (kWh). If you run a 100 watt bulb for 10 hours, it again consumes 1 kWh.

Solar panels are characterized by number of watts (Wp) they can produce under Standard Test Conditions (STC) of 1000 W/m2 irradiation, cell temperature of 25 degree Celsius and air mass of 1.5. This is their peak performance. However, the amount of power they actually produce in outdoor conditions depends upon the amount of sunshine.

Air Mass

Air mass is a measure of the distance traveled by sunlight through the Earth’s atmosphere. Since light intensity is attenuated by scattering and absorption, the more distance it passes through the atmosphere, the greater is the attenuation. Consequently, the sun appears less bright at the horizon (morning and late afternoon) than when at the zenith (noon). An air mass of 1 means the sun is looking straight down on the sea surface when it is directly overhead. At any location with latitude greater than 23.5 degrees, the sun is never directly overhead and so air mass will be always greater than 1. The number 1.5 has been agreed upon for the STC (Standard Test Condition) for testing solar panels.

Solar Irradiance and Solar Constant

Solar irradiance is the amount of sunshine incident on a unit area and is typically expressed in watts per square meter (W/m2) or kilowatts per square meter (kW/m2). Irradiance is measured through an instrument called ‘pyranometer,’ which displays the instantaneous power available from the Sun.

Solar constant is the solar irradiance outside the earth’s atmosphere on a 1 square meter surface oriented normal to the sun’s rays. It is about 1367 W/m2. This is attenuated by the atmosphere and the peak solar insolation on a earth’s surface oriented normal to the sun on a clear day is of the order of 1000 W/m2.

This irradiance of 1000 W/m2 corresponds to Standard Testing Conditions (STC) and is called “peak sun” or “1 sun”. If the incident radiation is concentrated 10 times using a lens or a mirror assembly and the incident power increases to 10,000 W/m2, then the irradiance is called “10 Suns.”

Solar Insolation

Insolation is the amount of solar irradiance that is incident on a fixed area over a period of time, and hence is a unit of energy. It is typically expressed in watt-hours per square meter per day (Wh/m2/day) or kilowatt-hours per square meter per day (kWh/m2/day) or even (kWh/m2/year) for a particular location, orientation and tilt of a surface.

Since 1000 W/m2 is “1 sun”, one hour of this ideal irradiance produces 1,000 watt-hours per square meter (1 kWh/m2). This is also known as “1 sun hour.” Colorful maps of solar potential display solar energy in kWh/m2/day, which is equivalent to the number of full sun hours per day. This is a useful parameter for sizing solar panels in the PV systems. More “sun hours” means more potential for solar power.

Global Horizontal Insolation (GHI): It is the solar insolation received by a fixed flat horizontal surface.

Global Tilt Insolation (GTI): The fixed solar panel or collector is generally inclined at an angle roughly equal to the latitude of its location (facing south in India or any place in the northern hemisphere) to maximize the annual insolation received. The insolation received by such an oriented surface is called the Global Tilt Insolation (GTI).

How much energy does one panel produces?

The unit of electrical energy consumed is generally measured in kilowatt-hours (kWh). If an array of solar panels rated at 1000 Wp produce electricity for 1 hour under good sunshine, they have produced 1 kWh or 1 unit of electricity. The total amount of energy they produce during the day is governed by things like solar latitude which is associated with latitude and season, and atmospheric conditions such as cloud coverage, temperature and degree of pollution apart from panel orientation and shading.

For same sunshine, panels produce more power in cooler climates than under hot temperatures. In India, ideal orientation for solar panels is slight tilt towards true south; in South India placing panels flat (horizontal) will also do.

How much space is required to install 1 kW solar panels?

Under clear skies and good sunshine each square meter is receiving about 1000 watts of solar energy. At typical 15% panel efficiency, a 1 sq m area will generate 150 watts of power. For 1 kW power output about 7 sq m area will be required. After leaving some free space, about 10-12 sq m clear roof area will be required.

How much power a 1 kW solar PV system will annually produce in Delhi?

New Delhi has average daily sunshine of 5.5 hours. If we assume loss of 30 days due to rains and clouds every year, then total annual sun hours are 5.5 x 335 = 1843. Ideally 1843 kWh of energy can be produced. But the actual performance will be less than 100 percent because the outdoor conditions are different from standard test conditions of the panels. So, for a 80 percent system performance the annual power production will be 1474 kWh (ie 1843*0.8). In locations where there are shadows and panel tilt is not towards true south, it is often advisable to consider loss of another 10-15%.

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How the Sun Moves Through the Sky

Solictices sThe Sun appears to be constantly moving across the sky. How does its path changes from month to month? We know that the Sun rises in the east and sets in the west, but does it rise exactly east and set exactly west every day? We also know that the days are longer in the summer and shorter in the winter. But how is it connected with Sun’s changing path across the sky?

These are some common questions we are all confronted with. And they become all the more relevant when we talk about solar panels’ performance which depends upon their orientation and the angle of tilt. These questions are in fact more relevant for capture of solar thermal power where Sunrays’ are to be concentrated. Life would be very simple if the Sun were to arise and set precisely in the same directions and if it followed exactly same path through the sky everyday throughout the year.

The Path of Sun in Summer and Winter in the Northern hemisphere

summer winter solicticeIn the summer season the days are long and the Sun is high in the sky. The figure at the left shows the path of the Sun through the sky on the longest day of the year, the Summer Solstice (June 21). This is also the day when the Sun is the highest in the southern sky. Because the day is so long the Sun does not rise exactly in the east, but rises to the north of east and sets to the north of west allowing it to be in the sky for a longer period of time.

After the Summer Solstice the Sun follows a lower and lower path through the sky each day until it reaches the point where it is in the sky for exactly 12 hours again. This is the Fall Equinox (September 21). Just like the Spring Equinox, the Sun will rise exactly east and set exactly west on this day and everyone in the world will experience a 12 hour day. After the Fall Equinox the Sun will continue to follow a lower and lower path through the sky and the days will get shorter and shorter until it reaches its lowest path and then we are back at the Winter Solstice.

In the winter the days are short and the Sun in low in the sky. The shortest day of year falls on December 21 when the Sun is also lowest in the southern sky as shown in the diagram at the left. Each day after the Winter Solstice, which falls on December 21, the Sun’s path becomes a little higher in the southern sky. The Sun also begins to rise closer to the east and set closer to the west until we reach the day when it rises exactly east and sets exactly west. This day is called the Spring Equinox (March 21).

During the short winter days the Sun does not rise exactly in the east, but instead rises just south of east and it sets south of west.

The Spring and Fall Equinox in the Northern hemisphere.

EquinoxAs explained above the Sun is at its lowest path in the sky on the Winter Solstice. After that day it progressively follows a higher and higher path through the sky each day until it is in the sky for exactly 12 hours (the Spring Equinox, March 21). On this day the Sun also rises exactly in the east and sets exactly in the west. Every place on earth experiences a 12 hours day twice a year on the Spring and Fall Equinox.

After the Spring Equinox, the Sun still continues to follow a higher and higher path through the sky, with the days growing longer and longer, until it reaches it highest point in the sky on the Summer Solstice.

Altitude and Azimuth

As explained above the path of the Sun changes throughout the year but its position in the sky can be defined by two angles: altitude and azimuth. Altitude tells how high the Sun is above the horizon: at Sunrise and Sunset, the Sun’s altitude is 0˚ above the horizon and about 90˚ around noon when the Sun is overhead. Azimuth describes the Sun’s position from east to west; it is the angle between true south and the point on the horizon directly below the Sun.

The Sun’s path also varies with latitude due to earth’s tilt; at locations further north (locations of greater latitude) the Sun’s path is lower in the sky. However, the arcs of Sun path are symmetrical about true south and solar noon. Solar noon is different from the clock noon: Solar noon is exactly midway between Sunrise and Sunset, which can be taken from the local newspaper of the same day. By definition, at solar noon the Sun shines from true south and thus the shadow cast by any object at solar noon will be along true south to true north and it is also the shortest.

The Solar Window

Solar photovoltaic modules must have a clear view of the sky as defined by the Sun’s apparent daily crossings. This is referred to as the solar window which should be kept free of shading from 9 AM to 3 PM solar time, when the maximum solar radiation is captured.

Looking at the year-long perspective, the altitudes of the Sun on December 21 and June 21 (the Winter and Summer Solstices) determine the upper and lower boundaries of the solar window. It can be safely assumed that if the long winter shadows near December 21 (when the Sun has its lowest altitude) do not shade the solar module, the shorter summer shadows will not shade the module either.

At low solar altitudes of the winter, the atmosphere, clouds, smog, and air pollutants can absorb or deflect considerable amounts of solar radiation. Solar altitudes below 15˚ (in the morning and late afternoon) are essentially useless for collecting solar power.

Tilt Angle

A solar collector or photovoltaic module collects the maximum solar radiation when the Sun’s rays strike it at right angles. As the solar collector or module is tilted away from perpendicular alignment to the Sun, less solar energy is received. However, small deviations away from the ideal tilt will not affect energy output much, and may be preferable from an appearance (as along the roof slope) or stability standpoint.

The optimal tilt angle for a solar energy system depends on both the latitude of the location and on nature of the application. Fixed modules and collectors that need to produce electricity and heat on a year-round basis are usually tilted at an angle equal to the latitude of the site. This angle points the collectors and modules directly toward the Sun in the spring and the fall when the Sun is at its midpoint position in the sky. Energy from the low winter Sun and the high summer Sun is not collected as efficiently, but the average yearly collection of energy is maximized.

Locating True North South with a Shadow Plot

shadow plot for true southA compass uses the earth’s magnetic field to find north and therefore points toward magnetic north, which is not in the same place as geographic north. But true north-south can be easily located with the help of a shadow plot for 3-4 hours around solar noon. This is based on the idea that at solar noon the shadow of an object is shortest and points along true north south direction.

A shadow plot can also help gain a feel for how the Sun’s path changes across the sky during the day. If it is plotted for several days (or weeks), one can gain first-hand knowledge that the path of Sun is always changing.

How to make a Shadow Plot

It involves planting a 2-3 feet long pole on the ground and marking its shadow at frequent intervals of time, say every 30 minutes. If solar noon is at 12.30 pm, it is best to start marking the shadow edge 1.5 – 2 hours earlier and continue 1.5 – 2.0 hours after the solar noon. This will give 6 – 8 points to reveal the pattern of shadow movement. It will appear something like the figure shown at the left. You will notice that the shadows are of different length. The shortest shadow points to geographic north. It will also be near the solar noon time. So this is the N-S line; a line perpendicular to it will be the E-W line.

Useful Page: Solar Power

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What should be the Orientation of Solar Panels

india map lat longIndia is located in the northern hemisphere but closer to the equator between latitudes 6˚ and 36˚N (the longitude boundaries are 68˚ and 98˚E). The central-most state, Madhya Pradesh, is confined within the latitudes 21˚ and 27˚N. The distance of a degree of latitude is about 111 km (69 miles).The terrain of India includes Deccan plateau, the plains land beside the rivers, the Himalayan Mountain ranges in the Northern part and the desert area in the West. The southern region has tropic rainy climate and the northern part is temperate.

In order to get the most from solar panels, they must point in the direction that captures most sunshine. In India, as anywhere in the northern hemisphere, solar panels should face southwards. However, in the southern hemisphere, for example, in Australia panels should point towards north. Here North means the true north – not the magnetic north as pointed by the compass needle.

Magnetic compass does not always point to North. Actually, there are only a few locations on Earth where it points exactly to the True (geographic) North. The direction in which the compass needle points is known as Magnetic North, and the angle between Magnetic North and the True North direction is called magnetic declination.

Fortunately for India, the magnetic declination is rather small. For example, at Delhi the declination is only 0.41˚ east and at Mumbai the declination is 0.58˚ west. It means that the compass needle gives the direction of geographic north is given quite accurately.

How to Determine the True South

There is an easy method to determine the true south: At solar noon, by definition, the sun shines from true south and thus the shadow cast by any object at solar noon will be along true south to true north. The exact time of solar noon is different from the clock noon and changes slightly throughout the year. Solar noon is exactly midway between sunrise and sunset, which can be taken from the local newspaper of the same day. Apart from running from true south to true north, shadows cast at solar noon have the additional distinction of being the shortest shadows of the day.

Panel Orientation

solar-panel-mounting-systemsFor fixed tilt angle throughout the year, the angle of the latitude is preferred. This is one fixed orientation where the panel almost always intercepts the greatest amount of solar radiation during the year. Reference 4 claims that minor tweaking can yield 3 – 5 percent extra gain and generally recommend somewhat lower angle for fixed tilt.

However, in general the horizontal tilt of the panels may be adjusted 4 times a year: at the latitude angle in spring and autumn, (Latitude – 15˚) in summer, and (Latitude + 15˚) in winter. Alternatively, one can choose the angle depending upon when the power requirement is greatest. If power shortages are high in summer and the requirement goes high due to the need of running fans, then latitude – 15˚ should be the right choice. Another good way is to adjust angles twice a year for summer and winter seasons. The best time to adjust for summer angle is mid March and mid September for the winter angle. Following the 15˚ plus/minus rule, for Mumbai and Delhi you can set the panel angles as follows:

For Mumbai (latitude: 18˚ 55’N) summer angle 3˚ and the winter angle 33˚.
For Delhi (Latitude: 28˚ 38’N) the summer angle will be 13˚ and the winter angle 43˚.

Online Resources

There are various online tilt angle calculators, mostly optimized for US or western conditions where people are more concerned with high energy requirement in the winter, and are based on different assumptions. In India, on the other hand, people are more worried about power shortage in the summer months and rising power demand mainly due to fans, coolers and ACs. Therefore, one should think before applying these models in Indian conditions. Obviously they give differing optimum angles depending upon the assumptions inherent in their models. Some of them are:

1. http://energyworksus.com/solar_installation_position.html
2. http://www.solarpoweristhefuture.com/how-to-figure-correct-angle-for-solar-panels.shtml
3. http://solarelectricityhandbook.com/solar-angle-calculator.html
4. http://www.macslab.com/optsolar.html

If you apply these models for Mumbai and Delhi, you will get varying angles than the above common rule of ±15˚ compared to latitude. One model even gives a negative angle for Mumbai summer angle, implying slight tilt towards north rather than south! The only point of agreement is that in summer the panels are mounted flatter than latitude and in winter steeper than latitude.

How Critical is the Tilt Angle

Model calculations of the 4th reference mentioned above shows that if 2 axis tracking is taken to give 100% energy, fixed angle panels generate about 71% energy, adjusting two season gives 76% and adjusting four season does not produce any additional gain. Thus, for fixed mounting adjusting the angles twice a year is simple and good enough for maximum gain.

As far the accuracy of angle is concerned, a difference of few degrees does not make any significant difference in the energy collected by the panels.

Explore Further

Optimum Tilt of Solar Panels

How the Sun moves Across the Sky

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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 (ID ΔTD / 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.

Understanding Solar PV Batteries

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4 Things that Reduce Life of Batteries in Solar PV Systems

Deep cycle lead acid batteries are used in the PV systems to store energy from the panels during the day for use when there is no sunshine. Thus, the buffer charge stored in the batteries bridges the time gap between power generation during the daytime and demand after sunset hours. Since unpredictability is in-built in the solar PV systems due to varying sunshine intensity during the day, systems must be designed so that batteries are not adversely impacted. General problems encountered with the batteries in the solar PV systems are:

  • Difficulty to maintain them in fully charged condition,
  • Difficulty in recovery from deep discharged conditions during non sunny days,
  • Putting additional loads by the user,
  • Low acceptance of charge by the batteries at low charging currents.

People normally recommend low maintenance flooded tubular LA batteries which require less frequent water addition. For instance, EXIDE offers their TORR range of Tubular batteries with plates casted at high pressure (100 bar) to ensure void free structure and consistent grain orientation. The batteries have low self discharge rates of less than 3% per month at 30˚C and water topping is required every 8 – 10 month. EXIDE claims that these batteries can sustain partial state of charge for up to six months and have long lives of 5 – 10 year.

For people preferring sealed batteries, EXIDE has EP series of Powersafe batteries. These are VRLA AGM batteries with 3 – 5 year life depending upon how they are used (or abused!).

Although batteries come in various designs, preventing the following 4 conditions enables them to perform optimally for a long time. These are well known issues in battery circles which the PV system designers should also be familiar with.

  • Electrolyte Loss
  • Sulphation
  • Stratification
  • Very deep Discharge

Electrolyte Loss

In flooded or non-sealed batteries, conditions of high temperatures, high charging rates, and over-charging can lead to loss of electrolyte which is sulfuric acid. It can result in a situation where parts of the plates are above the electrolyte level which will lead to reduced battery performance. In sealed batteries, high charging currents and over-charging will cause an increase in temperature and pressure which can eventually result in the release of gas (and possibly loss of electrolyte) from valves. Even permanent damage can occur. Thus, the sealed (or maintenance free) batteries are particularly vulnerable to temperature and over-charging effects.

Excessive overcharging increases the water consumption in flooded or vented batteries and consequently the batteries require more frequent water addition. The danger is greater in the valve regulated (or sealed) lead-acid batteries which may overheat or dry out resulting in a loss of capacity. Overcharge and charging rates can be controlled by use of proper charge controllers. A controller senses the battery voltage and reduces or stops the charging current when the voltage gets high enough.

Sulphation

During lead acid battery discharge, lead sulfate crystals are deposited on the plates as part of the normal electrochemical reaction. During charging, the chemical reaction is reversed and the lead sulfate crystals are converted back to lead on the negative electrode and lead oxide on the positive electrode. But if the battery is left in partial state of charge (not fully charged) for sometime, or is in use but not reaching a fully charged state, the lead sulfate crystals can harden and will not convert back to lead or lead oxide during charging. When this happens, the battery capacity is reduced.

This effect will occur more quickly at higher temperatures. Thus, a PV system should be designed in a way that partially state of charge does not sustain for long and the battery gets back to full state of charge as quickly as possible.

Electrolyte Stratification

The lead acid battery chemistry depends upon the electrolyte which is a sulfuric acid solution in water. Needless to say, the solution should be uniform and homogeneous throughout the battery cells or compartments. However, since sulfuric acid is denser than water, a situation can arise where the acid concentration is higher at the bottom of the battery than at the top – this is stratification. It will reduce the battery performance.

In flooded batteries, stratification can develop if a battery is not being fully charged; when a battery is fully charged some gassing takes place which mixes the electrolyte layers. Stratification is not a permanent effect and a single full charge should sufficiently mix the electrolyte, but if it persists for some length of time it can result in sulphation at the bottom where the acid concentration is high.

Very Deep Discharge

Although deep cycle batteries can normally withstand up to 80% discharge, it is recommended to allow only 50% as the maximum discharge and leave 30% for emergencies. The less deeply these batteries are cycled, the longer they will last. Therefore, for maximum battery life it is best to shallow cycle the deep cycle batteries! For instance, for its TORR range of solar Tubular batteries EXIDE gives the following cycle lives:

1500 cycles at 80% DoD,

3000 cycles at 50% DoD, and

5000 cycles at 20% DoD

As seen from above discussion, allowing a battery to stay in a state of not being fully (or nearly fully) charged can lead to the sulphation problem. What is still worse is discharging beyond 80%; it creates possibilities of causing irreversible changes to the battery’s chemistry and can possible cause severe permanent damage.

You may like to read Battery Charging Basics

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