Thin-film Photovoltaic (PV) Cells

In some of my previous posts I have mentioned thin-film photovoltaic cells and in this article I'll give a brief overview of them.

Solar panels based on the photovoltaic effect have been used for more than thirty years and have traditionally been built using wafers of crystalline silicon, which requires expensive processing and results in ridged, heavy and fragile solar panels.

Crystalline silicon PV cells are still the mainstream products in the PV cell market because they have high conversion efficiencies. However, their output is increasingly being bogged down by shortage of raw material, high production cost and difficulty of processing. These factors have given rise to rapid development of second generation PV technology known as thin-film PV technology.

Thin-film solar cells are generated by coating a substrate (glass, thin flexible metal or plastic substrate) with layers of conductive and semi-conductive materials of a few micrometers in thickness. The individual layers of material are deposited by various processes.

The key materials for the thin-film solar cells are semiconductor elements such as amorphous silicon (a-Si, still silicon, but in a different form), cadmium telluride (CdTe) and copper indium (gallium) diselenide (CIS or CIGS).

Amorphous silicon (a-Si) was the first thin-film material to be commercialized, although, the PV cells built from amorphous silicon are invariably less efficient than crystalline PV. These PV cells have low efficiency and limited lifetime (approximately 10-15 years). Initially, a-Si was mostly used in consumer items such as calculators. Amorphous silicon is the most widely used for the creation of thin-film solar panels. It has a sun energy conversion rate as high as 9%.

Cadmium telluride (CdTe) is a highly useful material in the making of solar cells. Cadmium telluride PV (CdTe PV) is the first and only thin-film photovoltaic technology to surpass crystalline silicon PV in the marketplace in terms of lower system price for a significant portion of the PV market – large (multi-kW) systems.

CdTe PV cells structure includes a very thin layer of cadmium sulfide that allows most sunlight to pass through to the CdTe layer. These characteristics provide the potential for high-efficiency modules with low-cost manufacturing processes. CdTe cell efficiencies are over 16% in the laboratory; commercial module efficiencies are likely to be in the 9% range in the first manufacturing plants.

Copper indium gallium diselenide (CIGS) cells create more electricity from the same amount of sunlight than does other thin-film PV and therefore has a higher "conversion efficiency". Besides that, CIGS conversion efficiency is very stable over time, meaning its performance continues unabated for many years.

CIGS cells use extremely thin layers of semiconductor material applied to a low-cost backing such as glass, flexible metallic foils, high-temperature polymers or stainless steel sheets. They are of interest for space applications and the portable electronics market because of their light weight. CIGS cells are also suitable in special architectural uses, such as photovoltaic roof shingles, windows, siding and others. CIGS thin-film solar cell recently reached 19.9 percent efficiency, setting a new world record for this type of cell.

Thin-film PV technology has attracted a lot of interest in the recent years. The main reason for this interest is that thin-film PV cells are less expensive than other PV systems. Rather than being manufactured laboriously piece by piece, thin-film can be mass-produced in cheap rolls like packaging - in any colour. Thin-film PV cells also can harvest as much energy from the sun with far less semiconductor material. They can be made with flexible substrates which allow them to be used in more locations than silicon cells, such as clothing and sails. A number of applications are being pursued using thin-film PV technologies, including roof-top applications (such as rooftop shingles, roof tiles), building-integrated photovoltaics (BIPV), the glazing for skylights or atria, and utility-scale applications.

Thin-film PV cells represent the most promising technology for providing more affordable solar cells for residential and other uses in the future. According to NanoMarkets, the thin-film photovoltaics (TFPV) market will produce 26GW by 2015, generating over $20 billion in revenues.

Silicon-Based Solar Cells for Flexible and Transparent Solar Applications

Scientists at the University of Illinois at Urbana-Champaign have developed new high-efficient silicon-based solar cells that are flexible enough to be incorporated on a curved surface or fabric, and transparent enough to be used to tint windows on buildings or cars.

The finding, reported in the journal Nature Materials, offers a new way to process conventional silicon by slicing the brittle wafers into ultra-thin layers and carefully transfers them onto a flexible surface.

"We can make it thin enough that we can put it on plastic to make a rollable system. You can make it gray in the form of a film that could be added to architectural glass. It opens up spaces on the fronts of buildings as opportunities for solar energy.” said John Rogers of the University of Illinois at Urbana-Champaign, who led the research.

Many international companies are making thin-film solar cells, but they are usually less efficient at converting solar energy into electricity than conventional cells.

Rogers' team uses a special etching method to slice very thin solar chips off the surface of single crystal silicon wafers which are highly efficient but, in their current form, rigid and fragile. The sliced chips are 10 to 100 times thinner than a normal silicon wafer, and the size can be adapted to the application. Once sliced, the bits of silicon chips are picked up by a special device and deposited on the target surface "like a rubber stamp".

“These silicon solar cells become like a solid ink pad for that rubber stamp. The surface of the wafers after we’ve done this slicing become almost like an inking pad,” said Rogers. “We just print them down onto a target surface." "The final step is to electrically connect these cells to get power out of them," he said.

Adding flexibility to the material would make the cells far easier to transport. Rogers envisions the material being “rolled up like a carpet and thrown on the truck.”

The technology has been licensed to a startup company called Semprius in Durham, North Carolina.

Sources: Reuters & Energy Efficiency News

Solar Fashion: Solar-Powered Purse for Ladies

Since I'm interested in solar energy and all information related to it, I was curious to find out if there are any cool solar-powered gadgets/accessories available out there that would be of interest for the ladies. So, after doing a little research on the net I came across Solarjo's Power Purse - a fashionable-looking solar-powered purse.

Actually, this is a black rectangular handbag (6" x 12" x 3"), the brainchild of Joe Hynek, a PhD student of mechanical engineering, who designed the bag in 2005, during a handbag competition in an experimental garment design class at Iowa State University. The initial inspiration for incorporating solar cells into clothing and accessories, Joe Hynek got when he worked at a solar panel manufacturer in Ames, making solar panels that work with military applications.

This very creative, eco-conscious high-tech handbag is covered with laminated solar panels made of thin-film photovoltaic cells, which absorb the sun’s rays and create electricity. The purse design incorporates six solar panels outside and two batteries tucked away inside its lining connected to a USB port, so that you can charge your small electronic gadgets that you carry around with you. Electronic gadgets such as cell phones, ipods, Blackberrys, etc. can be attached directly to the USB port, or by using an adaptor.

The stylish purse needs to be exposed to 2 hours of sunlight in order to fully charge a mobile phone battery. The purse’s fully charged battery can last for up to a month.

The Power Purse's current price is about $300 (£150) and it is likely to be available for purchase by the end of 2008.

Joes Hynek's website: Solarjo.com

Sources: Daily Mail & Environment News Service

Solar Hot Water Systems in Israel

Solar Hot Water Heaters in Israel
Photo credit: www.flickr.com/photos/juliepics

I continue my series on solar water heaters with the interesting information that more than 90% of Israeli households have solar water heating systems. Today, Israel is the world’s largest per capita user of solar water heaters. Actually, solar hot water systems are so popular in Israel because by law, all houses constructed in Israel must have them installed.

These heaters were first installed during the 1950's when Israel experienced a fuel supply shortage and severely restrictions were placed on the times when people could heat their water. As a result, many Israelis started to purchase and install solar water heaters on the rooftops of their homes.

In 1983, about 60% of the population heated their water with the sun. Eventually, a law was passed making solar heating mandatory for new residential buildings. The law has saved Israel and its citizens millions of shekels in energy costs. And each year it is also saving between 2% and 3% of the country's electricity needs.

Over the years, the solar hot water systems improved and mass production lowered the cost, so that more companies entered the field. Today purchasing and installing (usually by the builders) a solar water heating system is common practice.

The country’s leader in solar water heating sector is Chromagen with a range of thermo-tanks from 30 to 300 liters capacity linked to solar collector panels. The company produces small and large systems for domestic and industrial purposes. The systems are suitable for households, hotels, hospitals, senior citizens homes, swimming pools and other commercial uses.

Solar Water Heaters Types

Active indirect solar water heating system
Image credit: www.southface.org

Solar water heaters are also known as solar domestic hot water systems or residential solar water heaters. There are different types of solar water heaters and the choice depends on the temperature required and the climate.

Basically, solar water heaters can be either active or passive. Active systems use electric pumps, controls, and other such equipment to circulate water or heat-transfer fluids through the collectors. They also require electric power to activate pumps and/or controls.

Passive solar water heating systems require no moving parts and rely on thermodynamics rather than electric power to circulate the household water or a heat-transfer fluid through the system. Passive solar water heaters are the simplest solar water heating systems and they are also called bread box or batch heaters. These systems are most common in regions that do not experience extensive periods of below freezing temperatures.

Active solar water heaters can be further characterized as direct (open loop) or indirect (closed loop) systems. These terms explain the way the water is heated. With direct heating system household water from the storage tank is circulated through the solar collector, heated by the sun and returned to the storage tank. Direct active systems use pumps to circulate household water through the collectors. Direct solar thermal systems work best in warmer climates where the system is less prone to freezing.

Indirect systems use a heat-transfer fluid (usually a glycol-water antifreeze mixture) to collect heat and a heat exchanger to transfer the heat to household water. Indirect active systems also use pumps to circulate heat-transfer fluid through the collectors. Heat exchangers transfer the heat from the fluid to the household water stored in the tanks. Indirect systems are popular in areas subject to extended freezing temperatures because they offer good freeze protection.

Direct systems are more efficient that indirect ones but they require more maintenance to keep the pipes clear of mineral deposits.