Semitransparent solar cells: a window to the future?

Looking through semitransparent cells – one day these could be big enough to make windows.
UNSW, Author provided

Matthew Wright, UNSW and Mushfika Baishakhi Upama, UNSW

Can you see a window as you are reading this article?

Windows have been ubiquitous in society for centuries, filling our homes and workplaces with natural light. But what if they could also generate electricity? What if your humble window could help charge your phone, or boil your kettle?

With between 5 billion and 7 billion square metres of glass surface in the United States alone, solar windows would offer a great way to harness the Sun’s energy. Our research represents a step toward this goal, by showing how to make solar panels that still let through enough light to function as a window.

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Solar is now the most popular form of new electricity generation worldwide

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The economics of renewable energy are becoming increasingly favourable. In Australia, and many other parts of the world, silicon solar cells already dominate the rooftop market.
Rooftop solar power offers an increasingly cheap and efficient way to generate electricity.

But while great for roofs, these silicon modules are opaque and bulky. To design a solar cell suitable for windows, we have to think outside the box.

When we put a solar panel on a roof, we want it to absorb as much sunlight as possible, so that it can generate the maximum amount of power. For a window, there is inevitably a trade-off between absorbing light to turn into electricity, and transmitting light so we can still see through the window.

When thinking about a cell that could be fitted to a window, one of the key parameters is known as the average visible transmittance (AVT). This is the percentage of visible light (as opposed to other wavelengths, like infrared or ultraviolet) hitting the window that travels through it and emerges on the other side.

Semitransparent solar cells convert some sunlight into electricity, while also allowing some light to pass through.
Author provided

Of course we don’t want the solar window to absorb so much light that we can longer see out of it. Nor do we want it to let so much light through that it hardly generates any solar power. So scientists have been trying to find a happy medium between high electrical efficiency and a high AVT.

A matter of voltage

An AVT of 25% is generally considered a benchmark for solar windows. But letting a quarter of the light travel through the solar cell makes it hard to generate a lot of current, which is why the efficiency of semitransparent cells has so far been low.

But note that electrical power depends on two factors: current and voltage. In our recent research, we decided to focus on upping the voltage. We carefully selected new organic absorber materials that have been shown to produce high voltage in non-transparent cells.

When placed in a semitransparent solar cell, the voltage was also high, as it was not significantly lowered by the large amount of transmitted light. And so, although the current was lowered, compared to opaque cells, the higher voltage allowed us to achieve a higher efficiency than previous semitransparent cells.

Having got this far, the key question is: what would windows look like if they were made of our new semitransparent cells?

Do you see what I see?

If your friend is wearing a red shirt, when you view them through a window, their shirt should appear red. That seems obvious, as it will definitely be the case for a glass window.

But because semitransparent solar cells absorb some of the light we see in the visible spectrum, we need to think more carefully about this colour-rendering property. We can measure how well the cell can accurately present an image by calculating what’s called the colour rendering index, or CRI. Our investigation showed that changing the thickness of the absorbing layer can not only affect the electrical power the cell can produce, but also changes its ability to depict colours accurately.

A different prospective approach, which can lead to excellent CRIs, is to replace the organic absorber material with one that absorbs energy from the sun outside the visible range. This means the cell will appear as normal glass to the human eye, as the solar conversion is happening in the infrared range.

However, this places limitations on the efficiency the cells can achieve as it severely limits the amount of power from the sun that can be converted to electricity.

What next?

So far we have created our cells only at a small, prototype scale. There are still several hurdles in the way before we can make large, efficient solar windows. In particular, the transparent electrodes used to collect charge from these cells can be brittle and contain rare elements, such as indium.

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Solar power alone won’t solve energy or climate needs

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If science can solve these issues, the large-scale deployment of solar-powered windows could help to bolster the amount of electricity being produced by renewable technologies.

So while solar windows are not yet in full view, we are getting close enough to glimpse them.

Matthew Wright, Postdoctoral Researcher in Photovoltaic Engineering, UNSW and Mushfika Baishakhi Upama, PhD student [Photovoltaics & Renewable Energy Engineering], UNSW

This article was originally published on The Conversation. Read the original article.

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States’ dummy-spit over the Murray-Darling Basin Plan clouds the real facts

The Darling River near Menindee, NSW.
Tim Keegan/Wikimedia Commons, CC BY-SA

Quentin Grafton, Crawford School of Public Policy, Australian National University and John Williams, Australian National University

Given the outraged reaction from some state water ministers to the disallowance of an amendment to the Murray Darling Basin Plan, you would be forgiven for thinking that a heinous crime had been committed against farmers in upstream states.

In fact, what happened was that the Senate voted for the Basin Plan to continue unchanged, rather than allow a modest increase in the water available to farms in the Murray Darling’s Northern Basin.

NSW water minister Niall Blair reacted by declaring that his state “will now start the process of withdrawing ourselves from the plan”, while his Victorian counterpart Lisa Neville angrily declared that “the plan is over” (despite Victoria not even being in the Northern Basin).

The political friction is generating a lot of heat, but precious little light. The debate could use a few more facts, so here they are.

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Is the Murray-Darling Basin Plan broken?

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First of all, the amount of water involved in the amendment represents less than 1% of the average annual surface water extractions allowed by the Basin Plan. This is roughly equivalent to a single day’s irrigation use throughout the basin during the growing season.

In other words, irrigators already use huge amounts of water, and ensuring that environmental water recovery on the Darling River is not reduced by 70 billion litres will make very little difference to irrigators.

Second, the delivery of the environmental water target of 390 billion litres in the Northern Basin, rather than 320 billion litres as proposed in the amendment, will be undertaken with full compensation. In other words, no individual irrigator will be made worse off by allowing the original target to be delivered. No one is “taking water” from anyone.

Third, let’s just reiterate that no one has changed the Basin Plan, so the “loss” of 70 billion litres simply represents 70 billion litres less in diversions that farmers were hoping to receive in future, but now won’t.

Meanwhile, there is another amendment under consideration, to be decided by May 7, that will potentially allow farmers across the Basin to divert an extra 605 billion litres from the river. These amendments are political compromises and not part of the scientific and economic assessments that led to the Basin Plan.

Fourth, the claims by the Murray-Darling Basin Authority that the reduction in the environmental water recovery will save 200 jobs does not bear scrutiny. Jobs in agriculture have everything to do the weather, with commodity prices and the value of the Australian dollar, and very little to do with environmental water recovery. We should not spend hundreds of thousands of dollars on a consultant to get an “answer” that does not pass proper peer review in academic journals if we want good public policy outcomes.

Fifth, and finally, it’s time for some maths. The Basin Plan that was passed in 2012 had, on average, a surface water diversion limit (that is, the total amount that farmers and other water users were allowed to take) of 10,873 billion litres per year. Before surface water diversions were controlled in the mid-1990s, the average annual surface water diversions in the Basin were 10,684 billion litres per year. Between 2000-01 and 2014-15, the average was 7,956 billion litres per year.

In other words, the water limits allowed by the existing Basin Plan represent an increase, rather than a reduction, on what water users have been taking, on average, for the past 30 years. For this reason alone, we should be very careful about letting them have even more.

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The Murray Darling Basin Plan is not delivering – there’s no more time to waste

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Politicians and vested interests are playing fast and loose with the facts.
Let’s be clear, the Basin Plan will not keel over because of this disallowed amendment. But it will die if the irrigators who have already received billions of dollars of taxpayers’ money, and who have billions allocated for them in the forward estimates, stop the Basin Plan from delivering on its original intentions.

It’s time our federal government stood up and defended the national interest and faithfully delivered on the original intent of the Basin Plan, and actually increase stream flows in the Basin by 2,750 billion litres per year.

As we and our colleagues argued earlier this month, that means establishing a truly independent scientific and expert body to evaluate the Basin’s health and what has been delivered in terms of increased net stream flows with the Basin Plan. It also means an end to further infrastructure subsidies and efficiency projects until the full facts are publicly known and scrutinised about what public benefits they provide.

Quentin Grafton, Director of the Centre for Water Economics, Environment and Policy, Crawford School of Public Policy, Australian National University and John Williams, Adjunct Professor Environment and Natural Resources, Crawford School of Public Policy, Australian National University

This article was originally published on The Conversation. Read the original article.

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