High Concentration Photo Voltaic Thermal Systems

On Earth Day, scientists have announced a collaboration to develop an affordable photovoltaic system capable of concentrating solar radiation 2,000 times and converting 80 percent of the incoming radiation into useful energy. The system can also provide desalinated water and cool air in sunny, remote locations where they are often in short supply.

A three-year, $2.4 million grant from the Swiss Commission for Technology and Innovation has been awarded to scientists at IBM Research, Airlight Energy, ETH Zurich and Interstate University of Applied Sciences Buchs NTB to research and develop an economical High Concentration Photo Voltaic Thermal (HCPVT) system.

Based on a study by the European Solar Thermal Electricity Association and Greenpeace International, technically, it would only take two percent of the solar energy from the Sahara Desert to supply the world’s electricity needs. Unfortunately, current solar technologies on the market today are too expensive and slow to produce, require rare Earth minerals and lack the efficiency to make such massive installations practical.

“The design of the system is elegantly simple,” said Andrea Pedretti , chief technology officer at Airlight Energy. “We replace expensive steel and glass with low cost concrete and simple pressurized metalized foils. The small high-tech components, in particular the microchannel coolers and the molds, can be manufactured in Switzerland with the remaining construction and assembly done in the region of the installation. This leads to a win-win situation where the system is cost competitive and jobs are created in both regions.”

According to Bruno Michel, manager, advanced thermal packaging at IBM Research, the group intends to use triple-junction photovoltaic cells on a micro-channel cooled module which can directly convert more than 30 percent of collected solar radiation into electrical energy and allow for the efficient recovery of an additional 50 percent waste heat.

“We can achieve this with a practical design made of lightweight and high strength concrete, which is used in bridges, and primary optics composed of inexpensive pneumatic mirrors—it’s frugal innovation, but builds on decades of experience in microtechnology,” added Michel.

With such a high concentration and a radically low cost design scientists believe they can achieve a cost per aperture area below $250 per square meter, which is three times lower than comparable systems. The levelized cost of energy will be less than 10 cents per kilowatt hour (KWh). For comparison, feed in tariffs for electrical energy in Germany are currently still larger than 25 cents per KWh and production cost at coal power stations are around 5-10 cents per KWh.

Scientists envision the HCPVT system providing sustainable energy and potable water to locations around the world including southern Europe, Africa, Arabic peninsula, the southwestern part of the United States, South America, and Australia. Remote tourism locations are also an interesting market. A prototype of the HCPVT system is currently being tested at IBM Research – Zurich.

Solar-powered plane to set out to cross U.S. in early May

The first crossing of the United States by a solar-powered plane is expected to start in just over a month, its creators said on Thursday, as they make final preparations for an attempt two years from now at the first round-the-world flight without any fuel. Swiss pilot Bertrand Piccard and project co-founder and pilot Andre Borschberg, whose Solar Impulse made its first intercontinental flight from Spain to Morocco last June, aim for their plane to take off from near San Francisco in early May and land at New York’s John F. Kennedy airport about two months later.

With the wingspan of a jumbo jet and weighing the same as a small car, the Solar Impulse is just a test model for the team as they build a new aircraft they hope will circumnavigate the globe in 2015.

The project began in 2003 with a 10-year budget of 90 million euros ($112 million) and has involved engineers from Swiss lift maker Schindler and research aid from Belgian chemicals group Solvay — backers who want to test new materials and technologies while also gaining brand recognition.

Unveiling the current plane at a news conference at Moffett Field on San Francisco Bay, Borschberg highlighted the cramped conditions of the cockpit in the Solar Impulse.

“That’s a bad economy seat – you would not fly on this airline,” he joked. “The next one should be good business class.”

While the current plane was set up for 24-hour flights, the next one would have to allow for up to five days and five nights of flying by one pilot – a feat never yet accomplished.

Meditation and hypnosis were part of the training for the pilots as they prepare to fly on very little sleep, Borschberg said, adding that some sort of autopilot system would have to be built on the next plane to allow for some rest.

The plane runs on about the same power as a motor scooter, he explained, powered by 12,000 solar cells built into the wing that simultaneously recharge the batteries – with storage equivalent to that of a Tesla electric car.

The plane has already flown a 26-hour flight, back in 2010, to prove continuous flight was possible with charging taking place in the day and battery power working at night.

Piccard, asked about the downside of solar-powered flight, agreed that there is a price paid for the small carrying capacity and massive wings.

“In that sense, it is not the easiest way to fly,” he said. “But it is the most fabulous way to fly, because the more you fly, the more energy you have on board.”

The first stop for the Solar Impulse as it crosses the United States will be Phoenix, followed by Dallas and then one of three cities: Atlanta, Nashville or St. Louis. It will then stop outside Washington D.C. before heading on to New York. “It carries one pilot and zero passengers, but it carries a lot of messages,” Piccard said. “We want to inspire as many people as possible to have that same spirit: to dare, to innovate, to invent.” Piccard has a pioneering legacy to maintain. His grandfather helped his father, Jacques, build a revolutionary submarine that Jacques co-piloted on the deepest-ever dive. Bertrand said he believes the basic idea behind this spirit is to find out what you deeply believe, and then try the opposite. “Innovation is not about new ideas, it’s about getting rid of old ideas.”

Thin Film Solar Cells

Hanyang University in collaboration with Stanford University has succeeded in fabricating peel-and-stick thin film solar cells (TFSCs). The Silicon (Si) wafer is clean and reusable. Moreover, as the peeled-off TFSCs from the Si wafer are thin, light-weight, and flexible, it can be attached onto any form or shape of surface like a sticker.

Professor Dong Rip Kim of the Department of Mechanical Engineering has succeeded in fabricating peel-and-stick thin film solar cells (TFSCs) with the collaboration of Stanford team led by Professor Xiaolin Zheng. This method makes possible the overcoming of hardships related to working with traditional solar cells, namely the lack of handling, high manufacturing cost, and limited flexibility while maintaining performance.

Kim is currently in charge of the Hanyang University Nanotechnology for Energy Conversion Lab. His research interests are solar cells, energy conversion devices using nanomaterials, flexible electronics, nanoelectronics, and nanosensors. Among Kim's recent publications are "Peel-and-Stick: Fabricating Thin Film Solar Cell on Universal Substrates" in the journal of Scientific Reports, "Shrinking and Growing: Grain Boundary Density Reduction for Efficient Polysilicon Thin-Film Solar Cells" in the journal of Nano Letters, and "Thermal Conductivity in Porous Silicon Nanowire Arrays" in the journal of Nanoscale Research Letters.

Most solar cells are now fabricated on Si wafers or glass substrates. The biggest issue for commercialized solar cells is their high price. In addition, due to their fabrication on the Si wafer, the cells are rigid and heavy while being fragile. While they are recognized as one of the most crucial alternative sources of energy, such limitations have prevented wider application of solar cells.

Fortunately, Kim and his colleagues devised an idea to produce a light-weight flexible solar cell on non-conventional or universal substrates that overcomes the limitations of traditional methods while maintaining performance. By doing so, Kim believed that his new cells could broaden the application spectrum of solar cells.

The success comes from using the same traditional fabrication method while adding a metal layer between the fabricated a-Si:H TFSCs and the underlying Si/SiO2 wafer. After numerous attempts and trials, Kim and his colleagues found a method to reliably peel the fabricated TFSCs from the Si/SO2 wafer by using water penetration between the metal layer and the SiO2 layer on the wafer.

The Si wafer is clean and reusable, which is a big cost-saving factor for solar cells. Moreover, as the peeled-off TFSCs from the Si wafer are thin, light-weight, and flexible, it can be attached onto any form or shape of surface like a sticker. Although others have successfully fabricated TFSCs on flexible substrates to realize the flexible solar cells, many efforts have been driven to modify the existing processes for solar cell fabrication, due to the rubber-like properties of the flexible substrates. Importantly, Kim and his colleagues made the light-weight flexible solar cells without modifying any existing fabrication processes, and their performance was maintained even after the transfer. Kim states that their novel technology is not limited to the solar cells only. Numerous other appliances like flexible displays can adopt his method.

"I will continue to focus on creating highly efficient but low costing energy conversion devices with nanotechnology," Kim said. Moreover, his future research will focus on applying his method in other types of solar cells and in other applications.

Lightning and Overvoltage Protection

Highly excessive voltages and currents can threaten the operation of a PV plant. Such surges are mainly caused by lightning strikes, but also by faults in the grid.

In principal, a PV plant does not generally increase the risk of a building being struck by lightning. A separate lightning protection system does not necessarily need to be constructed simply because a PV plant has been installed. Nevertheless, VdS (the German institute for fire protection and security) recommends installing a lightning and overvoltage protection system for all plants with a capacity of ten kilowatts or more. In a given case, the risks should be assessed in order to enable a decision in favor of or against the construction of a lightning and overvoltage protection system. If the building on which the PV plant is constructed is already equipped with a lightning protection system (e.g. a public building), the PV plant must be integrated into the protection concept.

The standard DIN EN 62305 (VDE 0185-305):2006-10 provides a comprehensive approach to internal and external lightning protection for buildings and systems. In particular, the supplementary sheets to this European standard offer practical support when deciding whether or not to install a lightning protection system, as well as details on how to install such systems properly. Photovoltaic installations are primarily discussed in Supplement 5 "Lightning and surge protection for PV power supply systems". 

External lightning protection includes all measures for arresting lightning and conducting it to ground, and consists of a lightning current arrester, a down lead capable of carrying lightning and a grounding system which distributes the lightning current in the earth. 

Priority must be given to preventing the lightning from directly hitting the modules. This is first and foremost necessary when the PV generator has been installed in an exposed area (elevated on a flat roof, for example). Rods or wires are used as lightning current arresters, and the core shadow of these should not be cast on the modules as far as this is possible. Somewhat smaller air terminal rods are, therefore, placed in front of the solar modules and somewhat larger ones are placed behind the modules. The exact number and spacing of the air terminal rods is given by the class of protection desired and is calculated using methods such as the "rolling sphere method".

Indirect effects

The probability of indirect lightning effects occurring is significantly higher than that of a direct lightning strike. This is because every lightning strike within a one kilometer radius can generate current flow in the modules, module cables and in the main DC cable by means of induction. Conductive and capacitive coupling are also possible and can equally cause overvoltage. 

An integrated lightning protection system comprising measures and equipment within the PV plant and in the building is, therefore, required. Its fundamental purpose is to prevent inductive coupling and provide a path to earth for currents caused by overvoltage. 

In order to keep coupling in the module cables to a minimum, the area of the open conductor loops in the generator circuit must be as small as possible. The outgoing and return lines of the strings are, therefore, laid as close as possible to each other. The use of shielded single lines also reduces the risk of lightning effects. 

Surge protection devices (SPD) not only prevent inductive coupling but also the occurrence of grid-side overvoltage, and are normally built into the generator junction box. Because varistors used as voltage dependent resistors can age due to leakage currents, the combination of two varistors and a spark discharger in Y connection is considered the safest long term protection against overvoltage.

Reverse current and electric arcs

Increased currents can also occur if there is a voltage drop in a string, caused for example by shading or a short circuit. If this happens, the parallel-connected strings will function like an external power source which drives a fault current in the direction of consumption (reverse current) through the modules of the defective string. If the reverse current resistance of the modules is exceeded they will start to heat up, so string diodes are used to prevent such reverse currents. Many PV plants today are, however, built without string diodes, as most modules now have higher reverse current resistance and will easily withstand reverse current of 10 to 20 A. 

Since direct current and DC voltage are generated in a PV plant, there is a danger that non-self-extinguishing arcs could be created, which could cause fire. This danger is not present in an alternating current circuit because the regular zero crossing of the alternating current’s sine curve immediately extinguishes any electric arc created. The electrical connections in the DC circuit of a PV plant must, therefore, be extremely secure, because a loose connection can lead to sparking and, consequently, trigger an electric arc. As a result, when laying the DC cables of a PV plant it is standard to protect them from short circuit and ground leakages. This is achieved by tidy cable routing (e.g. not running unprotected over sharp edges) and the use of separate positive and negative cables, as well as double cable insulation. The DC cables used should be tested to "PV1-F" standards and marked accordingly. 

String fuses in the GJB can also generally prevent the cables from becoming overloaded in the event of faults. These are intended to reduce the risk of electric arcs.