Tuesday, April 28, 2009

Advanced Anerobic Digestion


At a time of heightened concerns about waste, climate change and the need for cleaner energy, it is worth pointing out that not all the news is bad. Technologies are redressing the balance -- and one of these is Advanced Anaerobic Digestion (AAD).

AAD will not turn muck into brass, or gold, but it does offer the potential to transform the sewage treatment process from a simple clean-up to one that recovers significant quantities of energy.

In the Northumbrian Water region, in the north-east of England, there are more than 400 (437 to be exact) sewage treatment works that all produce varying amounts of sludge. This material has to be removed from every works but, inevitably, it is difficult to handle and, to say the least, rather smelly.

To make this sludge stable to further degradation and (nearly) odour free, Northumbrian Water Ltd (NWL) has long employed anaerobic digestion techniques for about 10% of its total sludge.

These technologies harness natural oxygen-free decomposition by which organic materials break down to produce biogas – roughly made up of 65% methane and 35% carbon dioxide – along with a much reduced residue of stabilized organic material. The latter can be safely deployed as fertilizer. In fact, by returning it to the soil in this way, nutrient and organic matter cycles that occur naturally are completed.

In the last five years, however, technology has advanced significantly and a technique has been perfected that can do much more.

Advanced Anaerobic Digestion significantly enhances the benefits of anaerobic digestion by separating and optimizing the key process stages used in more conventional digestion systems.

A More Sophisticated Process

There are two main pre-digestion processes used in AAD in the UK — thermal hydrolysis (the Cambi process) or enzymic hydrolysis (the Monsal process). Currently there are examples of each in operation and under construction.

Regardless of which process is used, the key to the AAD process is a phase that significantly enhances the breakdown of organic materials by, for example, breaking down cell walls. With thermal hydrolysis this is achieved by an initial high temperature of 165°C combined with high pressure (6 Bar) for less than one hour, or with enzyme hydrolysis this is achieved by phasing an increased temperature from 42°C to 55°C over several days.

The result is a far greater conversion of organic matter into biogas when the material is transferred into the anaerobic digestion phase. Following this digestion phase, there is a 50% reduction in sludge volumes, combined with the additional biogas/CHP- derived energy being produced, and ultimately a better quality bio-solids fertilizer.

One of the major benefits of this, of course, is that energy from biomass, including sewage sludge, are classed as renewable and therefore contribute to meeting Britain’s international commitments to address climate change.

Using AAD reduces the mass of material that is required to be transported off site and offers the benefit of nutrient recovery from materials that are presently wasted.

Indeed, some particularly difficult materials, such as food wastes under the Animal By-products Order (ABPO), need the conditions of AAD to render them safe.

One other benefit that is not to be sniffed at, AAD results in reduced odour.

The digested sludge cake remaining after the process will be a Class A biosolid – a safe and low odour product containing no detectable levels of pathogens, such as E. coli, and may be used as a valuable agricultural fertilizer.


Friday, April 17, 2009

The promise of Organic Solar Cells


In the race to renewable energy, organic solar cells are now really starting to take off. They can be manufactured easily and cheaply, they have low environmental impact, and since they are compatible with flexible substrates, they could be used in many applications such as packaging, clothing, flexible screens, or for recharging cell phones and laptops.

Teams at the Laboratoire in Angers (CNRS/Université d'Angers) and at the Laboratoire in Strasbourg (CNRS/Université Strasbourg 1) have recently obtained record conversion efficiency with solar cells based on organic molecules.

Photovoltaic solar energy works by transforming a fraction of solar radiation into electricity by means of solar cells, which are connected together to form a photovoltaic solar cell module. The solar cells currently on the market are made up of inorganic materials such as silicon.

A great deal of international research is aimed at developing solar cells made up of organic (carbon-compound based) semiconductors. Although their performance is still considerably lower than that of cells based on crystalline silicon (around 5% efficiency as compared with 15% for silicon cells), they present numerous advantages. Unlike crystalline silicon, which has to be produced at very high temperatures, they can be manufactured cheaply with low energy cost and environmental impact, arguments which are by no means insignificant when it comes to renewable energy.
Moreover, the fact that they are made using solution processes (for instance from inks or paints) makes it possible to cover large areas and flexible substrates such as films and fabrics.

Organic solar cells are not intended to compete with silicon, but rather to be used for specific applications, such as packaging, clothing, flexible screens, and recharging cell phones and laptops. However, in the longer term, they could make a significant contribution to the photovoltaic conversion of solar energy, as long as there is major investment in research into new, more efficient and stable materials.

Over the past ten years or so, most research has focused on developing organic cells in which the active light-absorbing material is made up of long conjugated polymer chains. Although these cells are the most efficient yet discovered, the use of polymers poses a certain number of problems: synthesis, purification, control of the molecular structure and mass, and the distribution of different lengths of chain (polydispersity).

In order to overcome these obstacles, Jean Roncali's team of researchers at the Laboratoire d'Ingénierie moléculaire in Angers (CNRS/Université d'Angers) has developed a novel approach based on replacing polymers by conjugated molecules with a clearly defined structure. Whereas the conversion efficiencies of the initial prototypes published in 2005 were of the order of 0.20%, a collaboration between the Angers team and Raymond Ziessel's team at the Laboratoire des matériaux, surfaces et procédés pour la catalyse in Strasbourg (CNRS/Université Strasbourg 1), supported by CNRS's Energy program, has recently succeeded in reaching conversion efficiencies of 1.70%, which are among the highest known for this type of cell until now.

New classes of active material specifically adapted to such cells are currently being synthesized in these laboratories. In this way, the researchers are hoping to improve their results very rapidly. Industry will no doubt be keeping close watch on their progress.

Wednesday, April 08, 2009

Protecting Wind Generators during Voltage Dips


Wind turbines experience problem due to sudden drop in voltage in a portion of the electric grid. Industrial engineer and member of INGEPER Research Team at the Public University of Navarre, Jesús López Taberna is trying to tackle this problem. His team has come out with a rotor model which predicts how the wind power units will behave under these circumstances. Mr López has already patented two techniques and one of which is already with a manufacturer who will utilize it internationally. They have designed generator turbines in such a way that when voltage dips occur, generators continue to function uninterrupted.

In his PhD, Jesús López Taberna presented two protection techniques so that wind generators continue to be operative despite breaks in electricity supply. The title of the PhD is: “The Behaviour Of Wind-Powered Generators With Double-Fed Asynchronous Motor During Voltage Dips”.

Over the past few years wind energy converters were not performing optimally. The most important difficulty is how wind generators will behave during voltage dips. A voltage drip is an unexpected cutback in the potential in the electric grid followed by a rapid return to its normal value. Voltage drips can be caused by lightening, falling of trees on power cables or any commercial unit consuming a huge power chunk suddenly. This voltage drop lasts for a few milliseconds. But these few milliseconds are very crucial for a machine. In fact, an interruption of half a second in a productive process can cause the whole process to block and it may have to be reinitiated.

Voltage drops can destroy the electronic part of the unit or it can get burnt, hence the need for a protection system. Currently wind generators have a “Crowbar” protection system which can protect the machine but its biggest disadvantage is that the machine comes to a halt. If a large commercial unit suddenly devours a lot of power the voltage dips and wind power units get disconnected and stop producing electricity. This situation makes it difficult for the voltage to reach its normal level. If problem persists for a day or two then one can imagine the seriousness of the problem. The research team is working to make the air generator behave as a conventional power station. A conventional station doesn’t disconnect during power failure but helps in getting grid voltage back up.

The research team is trying to find out solutions that enhance the behavior of the machine without any need to change anything except the control. Analogy can be drawn from a digitally generated text document. A new software version is introduced.

Jesús López Taberna has specifically proposed two systems of protection in his PhD thesis, and both have been patented. The first, only requiring changing the control of the machine converter, has been transferred to a manufacturer for introduction into wind parks worldwide; the other requires changing elements inside the machine and continues to be developed for applications in new creation wind generators.

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