2016-10-05 / Life @ HFE/ O&M
Re-Viewing Solar Cells
With each passing day, solar energy is flexing its muscle in the energy mix and empowering the world. This natural source of energy is lauded as an inexhaustible fuel source. A report from International Energy Agency (IEA) echoes that by the mid of this century, solar will be the largest source of energy globally. Wondering what is transpiring the success of this clean source? It’s the technology – the solar panel components or Photovoltaic (PV) cells, which convert radiance into electricity. Today, PV cells are recognized as a promising renewable energy development source and have gradually evolved from their use in small applications to large power generation source.
Deeper look into solar cells
Whenever we think of electricity generation, the most common visual that comes to our minds is that of large rotating parts. However, since past few decades, battery fundamentals have captured a pivotal position on this subject.
I am specifically indicating at solar cells. Earlier batteries were used for the purpose of energy storage. Silently these solar cells are replacing those fundamentals in terms of generation. PV solar cells are thin silicon slices which are utilized to convert sunlight into electricity. Solar cells combined into a group of arrays are called solar panels, which are utilized to set-up large solar plants for potential electricity generation.
Batteries are generally utilized as a secluded electricity source, which after consuming their entire charge become empty or powerless. To regain further supply of power (charge) it requires recharging through any charged carrying source.
Now think about a scenario, where a battery gets constantly and continuously charged through some kind of limitless natural source, then it is likely to become an endless electric supply source for mankind. The secret lies in the innovation of solar cells. During the decade of 1950, Bell Laboratories constructed the first solar cell which converted sunlight into electricity. Since then a lot of modifications and improvements in quality and efficiency have been done. Although the process of solar cell construction remains same.
Thanks to Scottish Scientist James Clerk who first in 1874, realized that Selenium was sensitive and had good conductivity in sunlight. This finding led him to investigate the reason behind this conductivity, whether it was heat or light. He placed the Selenium bar in a capillary glass tube filled with water, in a manner that heat gets absorbed by the water. He detected equal effects on conductivity when the Selenium bar was kept under the sun. He had conducted the initial tests which helped in comprehending the dynamics of electricity generation through sunlight. This was later ably corroborated by Albert Einstein in his study of ‘photovoltaic effects’. Einstein in his discovery of Photon clarifies that light comes with packets of energy i.e. bundles of electrons carrying huge amounts of energy, which can be absorbed and retained by metals like Selenium. As a result, charged Selenium rod if surrounded and infused with wires, produces electricity. This experiment became popular as a phenomenon of the Photovoltaic Effect. Post which, serious exercise started to capture sunlight as a useful natural source of electricity.
Next progress was made when few years’ later scientists at Bell Laboratories accidentally searched conductivity in silicon after infusing it with some impurities in the form of electrons, which was either in excess or deficient quantity. That phenomenon of the introduction of impurities transformed silicon from a poor to the preeminent conductor of electricity.
As part of the research, a piece of silicon rod containing a small concentration of gallium made silicons positively charged. When this rod was dipped into a hot lithium bath, the portion of silicon immersed became negatively charged. A permanent electrical field was developed, called p-n junction, where positive and negative silicones met. This p-n junction formed the heart of the transistor and solar cell, where all electronic activity occurs. This transformed the designing of Integrated Circuits (ICs). The solar cells work exactly in reverse order of p-n junction diode, when exposed to sunlight, it acts like a battery. But that was completely a different inner world, where the power fed externally and circuitry controls internally, was derived as per the design. Post successful progress, a different world was exposed in a reverse manner, with the thought that power produced from inside controls the external circuitry. Thus solar cells came into existence. Solar cells work exactly in a reverse manner of p-n junction diode when exposed to the sunlight. This acts like a battery.
For a continuous and reliable source of an electricity generator, this battery needs to carry firm amounts of power in fixed or linear variation in the open variable atmosphere. Secondly, it must be constantly charged for endless production. For this successful conversion of sunlight into electricity, the three basic requirements have been defined as: 1. Constant absorption of high energy Photons; 2. The process of separation of Electron/ Hole pairs; 3. Raise the Electrons to energy level for conduction.
What lies beneath the inscrutable glass surface?
The PV cell directly converts the energy in sunlight into electrical energy through the photovoltaic effect. The functioning of the technology and its parts are graphically described below:
- The glass is tempered and has a transparent layer, which protects the electronics and PV cell from environmental damage and facilitates passage of sunlight. The generation efficiency is highly influenced by the quality of glass, as it can prevent transmittance (reflection & refraction) and surface ionization.
- The EVA is a transparent silicon rubber lining on both sides of PV cell, providing cushion and bonding to them with glass and a back sheet. The basic aim is to prevent the cell from direct atmospheric contact, by acting as a lamination on cells.
- PV cell is the core electronic instrument where photovoltaic phenomenon occurs.
- Front bus and back ribbons are the metal contacts which collect the charges in conduction region from n & p sides of the cells.
- Back sheet finds similar utility as the glass. This peculiarity protects it from moist weather and allows a better dissipation of heat from the panel.
The process to extract more from each new iteration of the solar panels to improve efficiency is continuing. But are technological innovations, only solution to reduce solar costs and make it available at granular levels? Or will it take something more for a macro level of implementation?
Maintaining Efficiency: For the constructed plants the only way to maintain efficiency is the ‘CLEANING’. The continuous and qualitative cleaning is the most efficient way to generate more and designed power from solar farms. Generally, it was seen that carelessness in cutting vegetation or inefficient cleaning led to generation loss. As per my experience, recently I have observed that due to manual cleaning the Anti-reflective coating got damaged and affects its efficiency by 2-3%. Now we are seriously thinking about cleaning the panels through microfibers so that the anti-reflective coating can be protected for a lifetime. This problem is serious as the ARC cannot be done again in-situ. So lost generation cannot be regained for the future.
Design Efficiency: The present solar market is highly under pressure to develop low-cost solar panels with high efficiency. Some more efficient technologies are required to produce high-end solar cells. Another concern is ‘solar system with tracker’. The presently available technologies of trackers are massive and is inclusive of high cost of steel. Also, it was noticed that present tracker technologies are unable to handle the undulation largely if the panels are placed in a group. So light weight trackers with the ability to handle high undulated area is need of the industry.
Land Challenges: Other big challenge for solar projects is land. The significant cost of any solar project is land, the availability of large stretch is also a challenge for big MW size projects. Some kind of reflection technologies needs to come up, so that land requirement for construction of solar farms gets drastically reduced.
Although solar presently is facing constraints, largely because it’s in on expansion phase, but it is also evident that solar is the way forward to meet the electricity needs worldwide. With proper storage solution, the day is not far when solar will be considered the most prudent and dependable technology for fulfilling energy needs. A long way to go…
Contributed by Atul Raaizada, Vice President, Operation & Maintenance, Hero Future Energies