Archives for category: Climate change

A subject that caught my attention during the 39th edition of the Photovoltaic Specialist Conference (PVSC) was a poster from Toshiba Corporation [1] about the study of a homojunction CIGS (Copper Indium Gallium Selenium) solar cell. CIGS solar cells are gaining more and more interest in the photovoltaic community as a thin film solar cell due to the material’s high absorptivity, low cost and relatively high power conversion efficiency. 

Standard CIGS solar cell consists of a p-type CIGS base, n-type CdS emitter and a ZnO transparent conductive oxide. This heterojunction between CIGS and CdS results in a conduction band offset. The heterojunction structure is used due to the fact that it is hard to get high enough levels of n-type doping in CIGS. P-type doping in CIGS is usually done intrinsically through Cu vacancies, which act as acceptors. To achieve n-type doping, a donor material would need to be introduced into CIGS. 

In their poster, the Toshiba corporation group reported achieving n-type CIGS with CdS doping up to a level of 1×1016 cm-2. They were able to achieve a high enough doping level to use a CIGS layer as the emitter in a homojunction CIGS solar cell. From this point, they have grown a homojunction CIGS solar cell leading to a power conversion efficiency of 17.2% (See J-V characteristics). This relatively high efficiency device shows the feasibility of a CIGS homojunction. Another talk about n-type CIGS was given by Professor Angus Rockett [2]. This talk was about nitrogen doped CIGS. He was mentioning the possibility in the future to achieve high level of doping in CIGS in order to eventually obtain CIGS tunnel junctions. From a modelling perspective, these two ideas could lead into very interesting designs of a CIGS tandem cell consisting of homojunction subcells. One of the main advantages of CIGS is the fact that it has a tuneable bandgap ranging from 1.0eV to 1.7eV which covers a good portion of the solar spectrum. The change in the bandgap can be achieved by varying the molar fraction x, corresponding to the gallium to indium ratio in CuIn1-xGaxSe2.  A high bandgap CIGS subcell on top of a low bandgap CIGS subcell connected together in series with a CIGS tunnel junction could lead into high power conversion efficiency while potentially reducing the cost compared to III-V based multi-junction solar cell. This type of technology is certainly not achievable in the short term, but it could definitely be an interesting exercise to model this type of device in order to have an idea of what level of efficiency we could potentially achieve from it.

Fred Bouchard

 

-Frédéric Bouchard

Undergraduate student, Year 4

Sunlab, University of Ottawa

References:

[1] N. Nakagawa, et al., “Feasibility study of homojunction CIGS solar cells”, 39th IEEE PVSC, 2013.

[2] A. Rockett, et al., “Nitrogen doped chalcopyrites as contacts to CdTe photovoltaics”, 39th IEEE PVSC, 2013.

Researchers at The University of Glasgow are attempting to mimic the process of photosynthesis to artificially produce a carbon-neutral biofuel that could potentially solve the problem of finding a liquid fuel suitable for a post-oil society.

Easy access to a relatively inexpensive source of liquid or gaseous fuel is indispensable to the functioning of a modern society. Inexpensive fuel means inexpensive transportation costs and the latter is one of the baseline assumptions of a global economy. Without the ability to ship goods around the world at low cost, the economy, as we have constructed it, will fail.  Thus, we need fuel.

Currently, our fuel of choice is oil and we can be sure of two things: (1) if we keep using it then we are going to run out and (2) the combustion of oil releases carbon dioxide into the atmosphere. Carbon dioxide emissions are, of course, the main culprit behind global warming. It then seems there are at least two good reasons to look at other liquid/gaseous fuel alternatives.

Two alternatives currently being researched are probably familiar to the educated layperson, namely, biofuels and hydrogen gas. Biofuels take organic matter and then chemically process it to get a liquid or gaseous fuel. One example is the production of ethanol from corn.

Hydrogen gas doesn’t occur in natural deposits and therefore, must also be produced. It can be made via electrolysis (ie. passing electricity through water), gasification of biomass, or other more advanced processes.

It is important to note that both biofuels and hydrogen gas require an energy input. If the energy input is from renewable resources than either could be a carbon-neutral liquid/gaseous fuel but if not, then their use could be doing more harm than good.

A different approach is currently being developed by researchers at The University of Glasgow and other universities around the world. The idea is to mimic photosynthesis, the process that allows plants to grow. In photosynthesis, water and carbon dioxide react under solar illumination to produce carbohydrate molecules and oxygen. Solar energy is converted into chemical energy and stored in chemical bonds of the carbohydrate molecule.

6CO2 + 6H2O (+ light energy)  C6H12O6 + 6O2

Photosynthesis is already exploited in the production of biofuels, during which the carbohydrate molecules (ie. corn) are further processed to produce ethanol.  However, using novel approaches, it may be possible to produce a useful biofuel like methanol directly from an artificial photosynthetic process rather than having a carbohydrate intermediary that needs further processing.

2CO2+4H2O → 2CH3OH + 3O2

The result could be an artificial solid-state “leaf” that uses solar energy to directly produce useful biofuels. This could have several benefits. The energy-intensive processing necessary for traditional biofuel production would no longer be needed. Issues with traditional biofuels, such as their effect on world food prices or climate sensitivities, could be avoided as the artificial photosynthetic process would not require anything to actually grow. The artificial leaf would be a fabricated self-contained unit. Furthermore, the process could be carbon neutral.

The field of artificial photosynthesis is relatively new but certainly has great promise as the potential benefits of such a technology are far-reaching. You can learn more about artificial photosynthesis at The University of Glasgow’s Solar Fuels webpage:  http://www.glasgowsolarfuels.com/proj.bio-insp.html.

-Erik Janssen

(Engineering Physics, MASc, Year 2 at McMaster University)

For those who were disenchanted with the results of the most recent United Nations Conference on Climate Change, a recent development gives at least one reason to be optimistic. The formation of the Climate and Clean Air Coalition to Reduce Short-Lived Climate Pollutants was announced on Thursday, February 16th by United States Secretary of State Hillary Clinton.[1]

It is a partnership between certain developed and developing nations with the aim of reducing the concentration of short-lived greenhouse gases (GHGs) in the atmosphere, thereby mitigating climate warming in the short-term.  It is the first effort to focus on short-lived GHGs collectively and it is intended to augment current efforts to reduce carbon dioxide emissions globally. The participating countries include Canada, Sweden, the United States, Mexico, Ghana, and Bangladesh.

Three GHGs are the focus of this initiative: Methane, Black Soot and Hydrofluorocarbons (HFCs).  Each is a contributor to climate change and is also short-lived in the atmosphere, from a matter of days to approximately 15 years. This can be contrasted with carbon dioxide, the most well-known GHG, which has an average atmospheric lifetime of longer than a century.

By reducing the atmospheric concentration of these short-lived GHGs, it should be possible to see strong and relatively quick climate change mitigation. A recent NASA study estimated that 0.5oC of global warming could be avoided by reducing the atmospheric concentrations of key short-lived GHGs like Methane and Black Soot.[2]  This is an important finding since the International Panel on Climate Change has determined the maximum allowable global temperature increase to avoid catastrophic climate change is 2oC. Furthermore, the study indicates that these emissions reductions could boost international crop yields and prevent hundreds of thousands of premature deaths related to these atmospheric pollutants.

The Climate and Clean Air Coalition to Reduce Short-Lived Climate Pollutants pledges to help reduce the atmospheric concentration of short-lived pollutants via a multi-faceted plan. It will work with already existing groups like the Arctic Council and Global Methane Initiative, create national policy priorities, mobilize funds, raise awareness, and support further scientific research into the atmospheric effects of these pollutants.

Tackling the problem presented by climate change is easily one of the most difficult and important tasks set before humankind. Any viable long-term plan will need to deal with all the issues—most importantly, our dependence on fossil fuels as an energy resource. However, with global action on climate change mitigation stalling, this seems to be a reasonable, albeit small, step forward.

-Erik Janssen

(Engineering Physics, MASc, Year 2 at McMaster University)