A talk by Arvind Shah

Arvind Shah, professor and founder of the Photovoltaics (PV) Laboratory at IMT, University of Neuchâtel, also happens to be one of the co-founders of our very own Centre for Electronics Design and Technology (CEDT) at the Indian Institute of Science, Bangalore. He was here at CEDT yesterday to give a talk titled "Thin-film silicon solar cells/modules : prospects and bottlenecks" and I was one of the privileged ones in the audience. I'm not much into thin-film technology or for that matter into any of device physics or materials research. But I still garnered a few interesting take-home points from his discourse and the subsequent deliberations in my mind and some related information gathered from the internet. I have listed them down here in bulleted form for easy assimilation.

• The classification of the various solar cell technologies are shown in the following figures:

[Reference:http://www.techonline.com/showArticle.jhtml?articleID=212001473]

Mono-crystalline silicon solar cell

Poly-crystalline or multi-crystalline silicon solar cell

Amorphous silicon solar cell

• The major obstacles in the wide-spread use of solar energy are
  

1. Availability of raw materials
   
2. "Grey energy" or the energy investment that goes into the production of solar cells
3. Energy conversion efficiencies of all existing solar cell technologies are low hence requiring large areas for tapping substantial amounts of energy
4. Energy storage problems (Batteries simply aren't good enough)

• Talking of raw materials, it is interesting to know that the raw material that is the major bottle-neck in the wide use of wind energy happens to be Neodymium. A blog post titled "Wind and Neodymium" is certainly worth a read in this context.

• There exists a slow degradation mechanism in amorphous silicon (a-Si) solar cells because of which the energy conversion efficiency drops to two-thirds of its original value after about a hundred hours of exposure to sunlight. Subsequently the efficiency stabilizes and there is no further degradation. This phenomenon is referred to as the Staebler-Wronski effect or Light Induced Degradation (LID) in literature. Putting it in numbers, if I bought an a-Si cell/module/panel that gives me 12% efficiency initially, after about three weeks its efficiency would drop to 8%. Of course the good manufacturers should quote this final lower value on their products.

• Every solar cell technology has an associated temperature coefficient of efficiency. For most technologies, the temperature coefficient is negative which means that the efficiency decreases with increase in temperature. In the case of a-Si, it's the other way round i.e. they perform better as temperature rises. So, when they are getting cooked up by the sun's heat, while crystalline silicon (c-Si) cells might give an efficiency worse than their quoted values, a-Si cells might still be doing better that their quoted values.

• A "tandem cell" refers to an approach where two cells (or two junctions) are made to work in tandem by absorbing energy from different parts of the spectrum of the incident light. A typical tandem cell structure

• The major factors on which the efficiency of a solar cell depends are:

1. Intensity of light
2. Temperature
3. Spectral content of incident light

Most, if not all, of the records set in terms of solar cell efficiencies are under ideal laboratory conditions. The commercially achieved efficiencies tend to be a notch lower for any given technology.

Dr. Shah also discussed about indirect and direct band gap semiconductors, Transparent Conducting Oxides (TCOs), light trapping methods among other issues and answered several questions posed by an eager audience.

Nine Problems Worth Solving

Statutory Warning The following problems may or may not
> ...be worth solving!
> ...have solutions!
> ...have been solved already!


PROBLEM-1
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Scenario: We want to keep ourselves cool and comfortable irrespective of prevailing weather conditions

Existing solutions: Use a ceiling fan or an air conditioner

Problem with existing solutions: They tend to cool our surrounding objects as well leading to wastage of energy!

"Find a way to cool ourselves in a more efficient manner"

PROBLEM-2
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Scenario: You are traveling by a vehicle, say a car or a bus and want to write something legibly

"Find a way to enable writing smoothly in a moving vehicle"

PROBLEM-3
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Scenario: You are watching TV with all your lights switched on and then you fall asleep

"Find a way to detect sleep and accordingly take action - switch off TV, lights etc."

PROBLEM-4
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Scenario: You have your speakers on when you suddenly get to hear an annoying buzzing sound due to someone's cell phone coming near the speaker

"Why does the electro-magnetic interference between a cell-phone and a speaker sound the way it does? How to avoid it?"

PROBLEM-5
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Scenario: You do not switch off your PC when you go out for say 10 minutes since you do not want to waste 2-3 minutes booting it up again

"Find a way to boot a PC in a second!"

PROBLEM-6
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Scenario: You and your friend have a pen drive each. You want to give him some data from your pen drive. There are no PCs or laptops around

"Find a way to directly transfer data from one pen drive to another"

PROBLEM-7
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Scenario:

"Device a method to enable a physically challenged person to move up a staircase"

PROBLEM-8
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Scenario: You saw a dream that you want to share or you saw a criminal committing a crime.

"Is it possible to convert a dream into a video or a mental image into a photograph?"

PROBLEM-9
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Scenario: It is common to have two switches controlling a single light bulb that lights up a staircase - one switch at the bottom and the other at the top of the staircase.

"Is it possible to have n number of such switches at intermediate floors?"

What are Peltiers?

In any electronic circuit, keeping the components cool is very important from the point of view of reliability. With a small increase in temperature, the reliability goes down exponentially. Most electronic components show a degradation in performance with increase in temperature. While many of the components can keep their cool all by themselves, some of the power-hungry components cannot do without heat-sinks to efficiently remove the heat within. This is what you call passive cooling. When the ambient temperature in the vicinity of the electronic component is high enough not to allow any appreciable heat transfer from the heat sink to the surrounding air, one has to go in for active methods of cooling. Using a Peltier - also known as a Thermo-Electric Cooler (TEC) - is one such method.

A Peltier, as shown in the above pictures, has two surfaces. On forcing a DC current through a Peltier, it removes heat from one surface and passes it on to the other. Quite obviously, the first surface becomes cold while the other becomes hot. I can now place my electronic component on the cold side and a large heat-sink on the hot-side so that I have a neat way to pump out heat from the component. And yes, a thermodynamics guy would call such a device a heat pump.

The underlying principle - known as the Peltier Effect - is elegantly illustrated in the animation below [source: www.peltier-info.com]

While Peltiers can do an important job for you inside a high-end PC, especially if you are an over-clocker, they can also provide good amusement value if you wanted to freeze a few drops of water in a few seconds! I tried it out one sunny day with a solar panel driving just over an ampere of current through a small Peltier element mounted on a heat-sink. (You could as well use a good DC supply to drive your Peltier. But it certainly is funkier to use solar energy to freeze water!) It is advisable to apply a small amount of thermal grease between the Peltier and the heat sink to get better thermal conductivity. After making the connections, initially water vapour condenses over the cold surface and then freezes within a few minutes, if not seconds. Using a fan, taking hot air away from the fins, I got even better results. Under laboratory conditions of 26 degree celsius, a cold side temperature of -7 degree celsius was achieved! And yes the heat sink became quite hot, say 40-45 degree celsius. As the heat sink becomes hotter, the achieveable cold side temperature also goes up. This is where a larger heat sink helps as it will take a very long time to become hot. One more interesting thing. If you see that all of a sudden all the ice melts, it is mostly due to a passing cloud shading your solar panel :-)

Paul Peng gives a very neat and descriptive account of his experience with Peltiers on this page. And here's a video demonstration of the Peltier Effect:



One could ask, why dont people use Peltiers in refrigerators. The answer is Peltiers are expensive and yet quite inefficient. Cycling a refrigerant through a compressor, a condenser, an expansion valve and an evaporator (Refrigeration Cycle) is a far cheaper and more efficient way to cool our food stuff. But when it comes to meeting the cooling requirements in a small area and in a highly reliable fashion, nothing beats the Peltiers. After all, Peltiers are completely solid state devices with no moving parts.