On-Grid Solar Electric Systems

Governments are encouraging households and group housing societies and associations to install On-Grid Solar Electric Supply by offering tax cuts, subsidies, etc. This is being done with the intention of bringing about reduction of electricity generation from fossil fuels which will reduce carbon emission.

A home with solar electric supply installation has to face eventualities of receiving inadequate or no power at all from it when the sun is not shining in the sky; an alternative supply has to be available in order to maintain continuity of electric supply. This then results in what is referred to as on-grid solar electric system. It is also called as grid-tied or grid-intertied solar electric supply system. The two alternative schemes in the on-grid solar systems are described here.

On-Grid Solar Supply System Without Battery

Fig. 1 shows one such on-grid solar electric supply system schematically. The figure shows the solar panel mounted on the roof. The inverter is synchronized with the grid supply so that it works in conjunction with the grid supply to provide electricity to the household. The energy meter in this system is a bi-directional meter; it permits flow of energy from the grid to the household loads or from the solar electric system (through inverter) to the grid. The solar system supplies load upto its capacity. When the solar electric system generates more electricity than required by the household load, it supplies the extra power it generates to the grid; and when the household load is more than what the solar system can supply, the grid supplies the balance power.

In this scheme the surplus power generated by the solar system and supplied to the grid system earns a credit from the utility.

You will notice that in this scheme if the grid supply fails at any time, for any reason, this home will not get power even for emergency purposes from anywhere. This scheme does not have a back-up which could have been provided if a battery was provided to feed the inverter.

This scheme of on-grid solar power supply without a battery is cheaper than the next alternative we shall see, but is not very popular due to the above mentioned drawback.

The schematic in fig. 1 shows only the basic system components. The other minor but essential components such as the DC switch between the solar panels and the inverter, the various MCBs required for the various loads, the main utility breaker, etc. are not shown.

On-Grid Solar Supply System With Battery

In areas where continuity of grid supply cannot be depended upon, a battery backup is desirable. Fig. 2 shows the schematic of such a system. In the normal course, the battery gets charged from the solar electric system and remains in a standby mode. When the grid supply fails the battery can supply at least the essential loads of the household. (The last blog dealt with the requirements regarding the charge controller and the battery).

These on-grid solar supply schemes are called net-metering systems. Different states and utility companies have different regulations for operation of net metering systems. Obviously the utility does not give credit for the power you supply at the same rate at which the energy drawn from the utility is charged.

Therefore generally such systems are designed so that the capacity of the solar electric system is about half of the total load of the household. During the daytime when the solar electric system is able to generate to its full capacity, it supplies most of the power requirement of the household with the grid supplying a smaller proportion; rest of the time the grid supplies the entire power requirement.

The initial cost of solar electric systems is quite high. But its operational cost is very low. Moreover the solar system has a life of about 25 to 30 years. Therefore this On-Grid Solar Electric system tries to derive the benefit of reduced grid supply charges over a long period of time by making a large initial investment.

Stand Alone Solar Electric Installation

We have seen how solar panels can be used to supply our electrical needs and considered the basic precautions required to be observed in using them. We shall now consider the practical solar electric supply systems.

Solar electric systems are a boon to people living in remote areas where due to economic reasons utility companies (electricity distribution companies) do not supply power. Such a situation exists not only in under developed countries but even in industrially advanced countries. For a location to be suitable to derive the benefit of solar electricity the only requirement to be satisfied is adequate sunlight being available for reasonably long period of time. In such locations you could get electricity to run pumps, provide lights in the house(s), provide electricity to run radios and television sets, refrigerators. In under developed countries many villages use solar electricity to power community entertainment and communication equipments, provide village street lights and run pumps for their irrigation need. All this improvement in their quality of life is possible without depending on the utility or grid supply.

The Solar Electric System

Fig. 1 shows the basic ingredients of such an installation schematically. Roof mounted solar panel generates electric current when exposed to sunrays. This is a DC current. Solar panels can generate only DC current. This is used to charge batteries so that the energy generated can be stored for use when sun sets or when adequate sunlight is not available, as say in the early morning or late evening times. It also serves the purpose of supplying an almost constant voltage to other equipments.

The strength of the current generated by the panels, however, varies drastically depending upon the intensity of sunlight available; this unregulated current can damage the battery. So a charge controller is introduced between the panels and the battery. It avoids the battery being charged at a damagingly high rate; it also ensures that the battery does not get overcharged.

Most of the equipments we commonly use work on AC current, we shall refer to them as AC loads. So to provide AC current from a battery which can essentially give only DC current, an equipment called inverter is introduced. It accepts the DC current available from the battery and turns it into AC current of a level acceptable to the AC loads we use. Any DC loads may be supplied from the charge controller directly or from the battery, as necessary.

Solar Panels

Fig. 1 shows roof mounted solar panels. For roof mounted solar panels, you will have to ensure that the orientation of the roof and the tilt of the panel is appropriate. If this is not possible, you can put the panels on a rack, mounted on an appropriate frame out in the open, in garden or field as shown in Fig. 2. The type and size of the panel should be chosen to be able to provide adequate power as per your requirements. You may have to consult Metrological data to get information about the average intensity of light in your area as well as the average number of days in a year you may get sunlight.

Battery

One thing needs a special mention. Most of the people erroneously think that any battery, even a car battery could be used. Sorry, that is a wrong assumption! Only a Lead-Acid battery, the type used in car, but of the “Deep-Discharge type” is required. A car battery is required to supply a large current for a short time, for starting, and for the rest of the time it is getting charged when the car is running and it is hardly ever required to give any current.during the rest of the time. The battery used in the solar installation, on the other hand, is required to supply current in more or less a steady manner for a long period of time (may be for the whole night), when the solar panel is not supplying adequate current and is going to be discharged a lot before it receives charge again (in the daytime)– hence the name Deep Discharge Type. Sealed or vented type batteries can be used.

Lead Acid batteries, especially the vented type, give off a small amount of acid fumes in the surrounding air; these are very corrosive for materials around and very damaging for our respiratory system. The batteries therefore should always be installed in a well ventilated enclosure or room.

Other Components

The above description of the system covers all the basic items required in the solar electric installation; a few more minor components are necessary to be added from the point of view of safety. As required by the electricity regulations the AC loads must be supplied through appropriate individual circuit breakers to ensure safety and the entire load supplied by the inverter must be supplied through a mains circuit breaker. Common practice requires that the mains circuit breaker and the individual load circuit breakers are mounted in an enclosure at a convenient location for easy access. A DC disconnect switch also should be installed to cut off current from solar panel to the entire installation for safety.

As per the safety regulations, the entire installation requires to be provided with appropriate “earth” connection. This essentially means that in case a fault occurs in the electrical system, the fault current may be diverted safely to the electrical system through a connection made in ground so that the fatal accidents can be avoided.

This is, then, the Stand Alone Solar Electric Installation, in its basic form, that gives you independence from utility or grid supply.

Some Considerations in Use of Solar Panels

Effect of Light Intensity & Temperature

Current and power output of a solar panel is very sensitive to the intensity of light falling on it. Current and power output are proportional to the light intensity. Fig. 1 shows this effect on a typical photovoltaic panel.

A solar array that gives 1000 W peak output at full light intensity will be able to give only about 800 W peak output when the light intensity falls by about 30%. (Peak output of a panel is available at the knee point of the V-I characteristics.)

Operating temperature also affects the current output and the peak power output of a panel as shown in fig. 1.

Shading:

A solar panel is made up of a number of solar cells interconnected in series-parallel combinations to achieve the rated voltage and power output. This aspect has a strong bearing on the manner in which solar panels are installed. If even one of the cells in a panel gets shaded by the shadow of a branch of a tree (or even the droppings of a bird) it may pull down the output of the panel by as much as 50%. Extreme care needs to be taken in positioning solar panels, especially when used for urban home installations.

Tilt Angle:

You can ensure getting maximum power output from a solar panel by mounting it so as to catch light at maximum intensity. This is, however, easier said than done. Position of sun in the sky changes during a day; it also changes with the season. Moreover, the location of the installation on earth also influences the positioning of the solar panel.

A solar array (assembly of solar panels) should be tilted at an angle approximately equal to the latitude of the site, and facing within 15° due south (when situated in the northern hemisphere, as in the US) in order to be able to capture maximum solar radiation over a year. The sun is higher in the sky during the summer and lower in the winter. Therefore for optimum performance in winter the solar array can be tilted 15° more than the latitude angle, and for optimum summer performance, 15° less than the latitude angle.

Smaller installations of solar panels such as for homes are generally on the roofs of the homes. Naturally a south facing roof would be ideal. Flush mounted panels on roof would obviously be cheaper if the tilt of the roof is correct. Otherwise generally the solar array is mounted in a rack which has provision for changing the tilt, at least in two fixed positions to permit near optimum performance.

Large installations, where economics permits the added cost, generally pole- or ground-mounted arrays are another choice. Facility for adjustment of mounting racks enables you to set the angle of your solar panels as the season changes, keeping them aimed more directly at the sun. The solar electric system’s annual energy production increases by a few percent by providing adjustable tilt angle. The tilt angle of pole- or ground-mounted solar arrays can be changed more quickly and safely as compared to the roof mounted ones.

Solar cell modules are expensive. For large output, obviously a larger assembly of modules will be necessary with the attendant higher cost. A concentrator may be useful and possibly, more economical in overall sense, in such situations. Figure 2 shows typical basic construction of a solar concentrator that helps in reducing the cost while providing an efficient solar electricity generator.

It uses plastic Fresnel lenses mounted suitably above solar cells in housing. We all know the lens that is used as a magnifying glass. It can also be used to concentrate light from the sun to a sharp focus where it can even ignite material because of the concentrated solar power. The Fresnel lens is a rather special type of lens which is very light and can concentrate light from over a fairly large area to a small area much more efficiently than a conventional lens can.

Fresnel lens directs the rays from sun to the solar cell mounted underneath. The light beam goes through a further secondary concentrator before it falls on the solar cell. Thus a concentrator in effect brings more solar radiation to the PV modules increasing their output at a reasonably low cost.

Solar Electricity – Energy Source Of The Future

Solar electricity is a very versatile energy source and most of all it does not pollute, it does not contribute to carbon emission and its use can contribute towards checking global warming.

Solar electricity is used in numerous varied conditions and application. It is used in remotely located homes and villages where grid or utility provided power is not available. In urban areas it is used in modest sized installations for small homes (see fig. 1) and also in reasonably large sized installations to provide electricity in group housing. It is used to provide electricity on small boats and mobile communication applications, and to power small vehicles for limited range of travel. It is used to provide on-board power on satellites and also for co-generation in large power stations of 100s of MW capacity. It is an extremely versatile source of non-polluting power.

It does have its limitations though; chief among them being that solar panels can provide electricity only when sun shines, that its use is feasible only in areas where adequate sunshine is available (within a certain band along the equator). Of course, these are not insurmountable limitations. There are many applications where by using storage batteries, electricity can be stored in the day time and used at night; or by using the solar energy in the day time and using the utility supply in the rest of the period you can contribute to checking global warming. In any case, in remote inaccessible areas these limitations do not matter any way.

In areas where at present utility grid power supply from utility using fossil fuel is available, solar electricity does not compare very favorably in cost due to the high cost of solar cells and related equipment. But this situation is changing; petroleum oil prices are escalating rapidly which is bound to escalate the prices of utility supplies (see fig. 2), governments are increasing taxes & levies on power generation using fossil fuels, and making it mandatory for utility companies to use more alternative energy sources which will reduce carbon emission. All these factors will tilt the scales in favor of energy sources like solar electricity.

Moreover, many large corporations have sensed the inevitable and have launched programs of setting up significantly large production facilities for manufacture of solar panels. This will bring down the prices drastically within the next two years or so.

Solar electricity is definitely an energy source of the future!

What Is A Solar Cell (Photovoltaic Cell)

Bell Laboratory scientists are believed to have discovered photovoltaic cells in 1954. Successful use of photovoltaic cells (PV cells) in US space programs in the late 1950s generated tremendous commercial interest in this technology. Since then there has never been a looking back for this new technology of solar cells.

Their use in calculators, wristwatches is almost taken for granted now. Many remote and inaccessible areas of the world are using electricity generated by the solar cells (PV cells) as the only available energy source to run water pumps, communication equipments and domestic or even community lighting.

Use of solar electricity for homes is catching up and PV generated electricity for co-generation in large power houses is also gaining wider acceptance.

• Structure of a Solar cell and Solar panels

Majority of the solar cells in common use are based on the use of a metal Silicon processed in such a manner as to exhibit strong photovoltaic properties converting visible solar radiation into electricity. Some other materials have also been developed and new manufacturing technologies are developed to increase conversion efficiency, to reduce cost, convenience of use, etc – and developments will go on.

Without getting into technical details, let us understand what a photovoltaic cell is made of and how it operates. As we mentioned earlier, Silicon metal, refined to a great extent and processed in a particular manner, is used to make a solar cell which converts solar radiation falling on the device and generates electricity. But Silicon is a very shiny material and the sunrays falling on it may get reflected away and would hardly contribute towards generating electricity. So a solar cell (PV cell) incorporates some antireflective material to help the conversion process. Figure 1 alongside shows the basic construction of a photovoltaic cell.

As shown here, a solar cell uses two wafers of Silicon processed in different manners, so that their “sandwich” has the capacity to convert energy from sun’s rays into electricity. Of course, this “sandwich’ must have contacts on both sides to collect electricity produced, an antireflective coating has to be incorporated and a transparent physical protective layer of glass must be incorporated. Well, there is your solar cell!

For many practical reasons, a single cell cannot be very large and can give hardly 1 or 2 watts of power. For practical purposes a number of PV cells are assembled in a panel and interconnected in series/parallel combinations so that sufficient electricity can be generated by using them. For large output a number of such panels are assembled together in an array as shown in figure 2; arrays are large, rigid structure.

Before going further, we must once again stress that there are various new techniques, processes and materials being used now to produce PV cells. We mention only a few variations here as an illustration.

• Single-crystal cells are made in long cylinders and sliced into round or hexagonal wafers. This process is rather wasteful and uses a lot of power but produces solar cells giving efficiency of up to 25%. A fairly large proportion of present day photovoltaic cells are of this type;


• A process similar to that used in production of rolled steel sheets is also used; this results in what is called the polycrystalline cells. Solar cells produced in this manner are reasonably low in cost but, on the downside, their efficiency is rather poor- about 15%. These cells can be packed together more closely in making solar panels. Well over 50% of the global use of solar cells is of this type;


• A process in which silicon is essentially sprayed on to glass or metal surface in thin films produces what is called as amorphous silicon photovoltaic cell. Amorphous-silicon solar cells are the cheapest – unfortunately they are the least efficient also – hardly 5%.

Utilization of Solar Energy – Part 2

The ever increasing use of fossil fuels is currently threatening our very existence by polluting air and water, and bringing about catastrophic global warming. Now is the time to take advantage of solar energy as an abundant resource of non-polluting alternative source of energy. Solar energy is received through radiations covering a very wide spectrum of frequencies – from infrared through the visible to ultraviolet frequencies. Radiations in Infrared and Ultraviolet range cannot be seen, only visible range of radiations is seen.

There are two major application areas in which solar energy can be used as inexhaustible alternative source of energy: for heating, and for generation of photovoltaic electricity.

Solar energy for heating

The infrared radiation causes heating effect. For ages men have been utilizing solar energy for heating. Even in the medieval ages techniques had been developed to design houses in a manner to use the solar energy to keep houses warm in winter and provide cooling draughts of air to cool and ventilate houses in summer. For a long time various methods have been used to provide hot water for domestic use and for comfort heating of homes by using solar energy; solar cookers also have been used for cooking food. There was a time when vehicles driven by steam engines running on solar energy were being tried out.

In the recent years many developments have taken place; now solar energy is used not only to provide domestic hot water supply or space heating of large buildings but it is being used to provide hot water, steam, etc for industrial processes and even to supplement power generation in large thermal power stations.

Solar energy for electricity

Generating electricity by photovoltaic conversion is a relatively modern development. “Photo” in Greek means light and “Volt” relates to scientist Alessandro Volta, who invented electric battery and contributed greatly to the study of electricity. The term Photo-Voltaic, thus, refers to light-electricity or electricity from light. A PV (photovoltaic) cell is a device made of Silicon – a metal - processed in such a manner that it will exhibit strong photovoltaic properties converting visible solar radiation into electricity.

Small PV cells have been in use for long in low power applications such as calculators, watches, etc. Individual PV cells or solar cells are small and give very small amount of power. Now PV cells are assembled in various configurations as panels and arrays so that sufficient electricity can be generated by using them for practical purposes.

Recent developments have made it possible to use photovoltaic cells to provide clean power to remote and inaccessible communities, to run cars on solar power, to supply power on satellites and space stations and even to supplement power generation in large power stations of hundreds of MW.

Using solar energy as alternative source of energy in various applications can help us in cutting down pollution of our environment and contribute towards the efforts to check global warming.

Utilization of Solar Energy – Part 1

In the context of global warming caused by large scale use of fossil fuels, solar energy assumes tremendous importance as an alternative source of energy. Realizing the tremendous influence the Sun had on virtually every aspect of existence of all living things on earth, many ancient cultures such as the Egyptians, the Greeks and the Romans feared and revered the Sun.

With the spread of science, we now have a much great appreciation of this gigantic source of energy around which our planet earth orbits. Now we know the enormity of the Sun in relation to earth. The Sun is a ball of burning hydrogen gas and is about 1,500,000 times bigger than the earth. Its surface temperature is about 27,000,000 deg C. Due to this high temperature it emits energy in all directions The Solar radiations cover a very wide range of frequencies. A very small part of the energy is emitted as visible radiation – the light, as we call it. Radiations from the Sun extend from infra-red (lower in frequencies than the lowest visible radiation frequency – red) and well into the ultra-violet (higher in frequencies than the highest visible radiation frequency – violet).

Sun is the centre of our planetary system and the planet earth rotates around the Sun. As the earth rotates around the Sun, solar radiation carrying energy falls on the Earth. About 5% of the radiation coming towards the earth is reflected away from the earth and the atmosphere absorbs about 15%. Thus the maximum solar power we can receive at the equator is about 1000 W/m². Presence of clouds, dust, etc. reduces this further and on an average we receive only about 800 W/m² of solar power at the equator.

Solar energy gives us the light and heat, and plants convert the solar energy by photosynthesis into chemical energy for their growth. The plants and the trees provide fruits, grains and also the fuel – wood, grass, etc. The whole interdependent kingdom of living organisms – plants, birds, fish, animals and humans – draw their sustenance from this life giving source of energy. (Fuels such as oil, gas and coal that we use today were formed from fossilized plants and trees and in a way they represent the solar energy stored by nature). All the energy received from the Sun is not utilized in this manner and the excess energy results in the rise of temperature of the earth and the oceans. This causes infra-red radiation from the earth outward into atmosphere.

Solar energy—power from the sun—is free and inexhaustible. Human civilization has always used the energy of the Sun as far back as they have existed on this planet. However, the amount of this bounty we utilize is insignificant in comparison to what the Sun bestows on us. We receive enough energy from the Sun in barely twenty days of sunshine to make up for all the energy stored in Earth's fossil fuel reserves. We realize that currently we utilize barely 1% of this energy and it has a tremendous potential as an important inexhaustible alternative source of energy.

Initiatives for Alternative Energy Sources

It is two years now since the Kyoto protocol on Global Warming was ratified by over 140 nations. Under the protocol, 35 industrially advanced nations voluntarily agreed to cut down their carbon emissions by 2012. This was a landmark international agreement in the history of cooperation of nations – not withstanding the refusal of USA and Australia to join the other nations. Sadly a review of the actual achievements made by most of these nations in reduction of carbon emissions and development of alternative energy sources is not very heartening. There could be many reasons for this situation.

There is a general awareness of the dangers of global warming and agreement on the need to reduce carbon emissions. However, a significant reduction in fossil fuel consumption would harm the commercial interests of oil, gas and coal industries and most of them endeavor in every possible way to sabotage the policies aimed at reducing carbon emissions. In many of the countries these industries have used their money power to promote disinformation campaigns, to promote political lobbies to raise road-blocks for the governments in one manner or the other. It is commonly believed that even the refusal of the United States to sign Kyoto Protocol and its subsequent policy decisions on this issue have been largely due to the lobbying of these industries.

It is important to remember that most of the industrially advanced nations that are signatories to the Kyoto have democratically elected governments and their constitutions require general elections to be held at four or five year intervals. This means at least two consecutive governments must formulate and vigorously pursue policies that will get the people and the industries to actively join in combating global warming. Major initiatives in this direction on governmental level require large financial outlays and tightening of belts all around. Many decisions are involved which cause short term hardships to certain segments of the society. Such decisions make slow progress in the climate of political compromises essential in an elected government; this situation is exploited by certain lobby groups as mentioned above. Consider the following:

A large proportion – almost a third – of UK’s carbon emission is contributed by its power sector; and its carbon emission has gone up by about 30% over the last eight years. Serious concerns are being expressed whether the UK government’s target of a 26-32% cut in emissions by 2020 can be met. WWF report points at the “woefully inadequate” policies of the UK government in this direction.

(“ Power station emissions soar” ref. http://environment.guardian.co.uk/energy/story/0,,2044717,00.html#article_continue)

Norwegian greenhouse gas emissions rose by 8.5 percent from 1990 to 2005. …… The main reason why emissions are continuing to increase is our reliance on fossil fuels like oil, coal, and natural gas.

(“NORWAY Climate” Ref. http://www.environment.no/templates/themepage____2143.aspx)

IPCC reports have amply illustrated the disasters we are facing unless our reliance on fossil fuels is cut down drastically. It is essential for the world community to make serious concerted efforts to dissuade continued use of fossil fuels by legislative and revenue measures and promote development of more energy efficient equipments and renewable alternative energy sources.