EnergyTalk

Bell Laboratory scientists are believed to have discovered photovoltaic cells in 1954. Successful use of photovoltaic cells (PV cells) in US space programs in 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.

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 too much technical, 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 sun’s rays 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 below shows the basic construction f a photovoltaic cell.

               Fig. 1                                                                                     Fig. 2
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. 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 practical purposes. For large output a number of such panels are assembled together in an array (see figure 2 above)  which necessarily is required to be large and rigid.  

Before going further, we must once again stress that there are various new techniques, processes and materials are now being used 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%.  

With this basic information on photovoltaic cells (PV cells) we shall now proceed to see what considerations play part in putting them to use. 

Solar energy represents a viable alternative source of energy to the fossil fuels that are currently threatening our very existence by polluting air and water. Now is the time to take advantage of this abundant resource. Solar energy is received through radiation which covers 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.

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 utilize solar radiation to provide hot water for domestic use and for comfort heating of homes; 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 by which not only large scale domestic hot water supply or space heating of large buildings is done by use of solar energy but it is being used 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, this, 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. Individual PV cells or solar cells are small and give hardly 1 or 2 watts of power. PV cells are assembled in various configurations so that sufficient electricity can be generated by using them for practical purposes.

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

We shall be reviewing the developments that have taken place in utilization of solar energy as an important inexhaustible alternative source of energy.

In the context of global warming, solar energy assumes tremendous importance as an alternative source of energy. We shall review the present status of utilization of solar energy.

Many ancient cultures such as the Egyptians, the Greeks and the Romans considered the brilliant, searing hot ball in the sky as a God that gave light and warmth to earth. In those olden times, directly or indirectly, the Sun dominated human societies. Their subsistence depended on the energy given by the Sun, the life cycles of communities revolved around the seasons set by the Sun. The Sun was recognized as the life giver, the all- powerful provider. The Sun was worshipped in all the ancient civilizations of Egypt, Greece, India, etc. In the belief-systems of many communities of people such as the Zoroastrians, the Aztecs of Mexico, the Incas of Peru, Hindus, Native American tribes, etc. the Sun occupied an important position.

Magnificence of the Sun has been understood only in the modern times. Sun is the centre of our planetary system and the planet earth rotates around the Sun. 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 radiation 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).

Our planet Earth rotates around the Sun and the 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, 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 increase 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. Man’s ingenuity has always tried to find ways of using this free bounty of energy to his best advantage. Human civilization has always used the energy of the Sun as far back as they have existed on this planet. 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. .

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)

IPAC 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.

To understand anything about global warming let us start by understanding first what climate is and why it changes. Climate of a region is the long term average weather condition of the region in terms of temperature, humidity, and clearness/cloudiness. Temperature at a place may go through fairly large changes within the span of a day of 24 hours; that only constitutes a change of weather in the day not a climate change. A sharp shower at noon in summer rapidly brings down the temperature, people sigh with relief due to the pleasant weather after the shower; but it does not cause a climate change – because this is only a short term change.

Eruption of a volcano on the other hand not only causes a change in the weather conditions and cause large scale devastation immediately, but it slowly causes irreversible, permanent changes to the climate of that region. The eruption spews out tons of sulfur dioxide in the atmosphere and contaminates the atmosphere for hundreds of miles around. It causes acidification of sea in the region affecting the marine life; it pollutes the soil causing irreversible damage to crop patterns. In the region closer to the eruption, the droplets of sulfuric acid suspended in air cut down visibility and cut down energy from the Sun received by that area for a long period of time. All these effects of the volcano take place over a number of years; that region goes through drastic climate changes.

Deforestation of large areas of land is also known to cause change of climate in that region. What is presently engaging attention of scientists and leaders of the world is not the climate change of a small region but that of the entire planet earth. Since 1900, there has been an average increase of about 1 ۫C in temperature the world over, ringing alarm bells of impending disaster. Extensive investigations by renowned international scientists over two decades have established that the cause of this increase is mainly the human passion for burning fossil fuels to satisfy their hunger for energy.

When you burn oil or gas or coal a bye product of that combustion is carbon dioxide (CO2). It rises into the atmosphere and contributes to the greenhouse effect. Low temperature radiation from the earth, which could allow the earth to maintain temperature by throwing out the excess energy received from the Sun, is absorbed by the greenhouse gases; the temperature of the atmosphere rises and it radiates more energy back to the earth. This causes an overall increase in temperature of the earth; and we have global warming!

More fossil fuel you burn – whether to drive to work and back or to enjoy the cozy warmth of your fireplace - greater is your contribution to the greenhouse gases, and the consequent climate change and the global warming. We all have got addicted to using fuel recklessly and soon our children and grandchildren will be living the horrors of hot weather, shortage of water, flooding and devastations of large areas of the earth due to rising sea levels, more frequent hurricanes and storms, shortage of food……..