Energy density

SuperCapacitors Made From HEMP

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Scientists are reporting that fibres from the hemp plant can rival graphene in their ability to store energy, leading the way to supercapacitors made with hemp. Up until now, graphene has been the contender material in the race to build supercapacitors due to it’s ideal composition.

Hemp has one other standout adbantage over graphene in that it’s a far cheaper option.

Hemp Fibers SuperCapacitor

‘His team found that if they heated the for 24 hours at a little over 350 degrees Fahrenheit, and then blasted the resulting material with more intense heat, it would exfoliate into carbon nanosheets.’

David Mitlin, Ph.D., explains that are energy storage devices that have huge potential to transform the way future electronics are powered. Unlike today’s rechargeable batteries, which sip up energy over several hours, supercapacitors can charge and discharge within seconds. But they normally can’t store nearly as much energy as batteries, an important property known as energy density. One approach researchers are taking to boost supercapacitors’ energy density is to design better electrodes. Mitlin’s team has figured out how to make them from certain fibers—and they can hold as much energy as the current top contender: graphene.

Headline: Should You Get a Tesla Home Battery? Let Physics Explain

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Tesla announced a battery for your house, the Powerwall. I think this is a great opportunity to talk about batteries and physics. Let me answer some questions you might have.

Why would you want a battery for your house?
This is perhaps the most important question and one that has likely been addressed many times. In short, a house battery will let you be more power independent. If you have solar panels or electricity generated from wind, they don’t always produce the same amount of power. With a battery, you can store this energy during the day (or during wind) and then use it at night.

A house battery will also let you get power from the electric company at night when the rates are lower and then use it during the day. Really, that’s win-win. You win with a lower power bill and the electric company wins with lower demand during the day.

How is the Powerwall different than other batteries?

You could always have a battery for your house. The most common way was to use lead acid batteries, like the one in your car. However, this was not so simple. You would need to have a whole bunch of the batteries and you would have to connect them together. When one battery goes out, you have to replace it. Oh, the traditional battery is also expensive and takes up lots of space.

The Powerwall seems to make a home battery more like an appliance. It mounts on the wall and you don’t have to maintain individual batteries, and the price seems reasonable at between $3,000 to $3,500.

So, you could just get this battery and run your house?
Actually, no. Your house runs on AC current but the battery gives you DC current. This means that you need to take the DC current and convert it to AC current. You might have a device that does this in your car so that you can plug in household items like a computer or a coffee pot. The converter takes the DC current from the car battery and turns it into an AC current so that your laptop can then take this AC current and convert it back to DC. Yes, that seems silly but it’s true. The Powerwall does not include a DC to AC converter (or AC to DC if you want to charge from the power grid).

How long could you run your house with a Powerwall?
Tesla makes a 7 and 10 kilowatt-hour battery. Let’s look at the 10 kWh one—but Tesla says that you can stack these such that you could make a 20 kWh battery if that made you happy. But really, this comes down to the definition of power as the rate that you do work (or change energy).

We know the energy stored in the battery and we can estimate the average power the house uses. From that, I can solve for the time to use this energy stored in the battery.

What is the energy stored in a battery? The bigger Powerwall has 10 kWh. Yes, this is a unit of energy and not power. It says that you could get a power of 10 kilowatts for 1 hour. You can convert this energy to Joules if you like – it would be 3.6 x 107 Joules (1 watt is a Joule/second).

The next thing we need to calculate the run time is the power. How much power does your house use? I think 2 kilowatts is a good estimate. With that, we can calculate the time:

Five hours doesn’t seem like a long time, but I bet this would get you through the night if you are using solar power (you don’t use as much energy when you are asleep). Ok, actually you would get less than 5 hours. This calculation assumes everything is 100% efficient. In fact, the battery is only 92 percent efficient and the DC to AC converter would have some energy loss as well. If you aimed for 3 hours at 2 kW, I think you would be ok.

What is energy density?
If I talk about density, you will probably think of the mass of an object divided by its volume. This would be the mass-density. Energy density is the energy stored in a device divided by its volume. Simple, right? But why would you need this? Well, it tells you how large a storage device will need to be to store a certain amount of energy.

What is the energy density for the Powerwall? From the Powerwall page, the battery has dimensions of 130 cm x 86 cm x 18 cm. Assuming this is a perfect rectangular cube it would have a volume of 0.201 m3. With 3.6 x 107 Joules (or 36 MJ) of stored energy, the Powerwall has an energy density of 179 MJ/m3 or 0.179 MJ/L – I don’t know why people like energy densities in Joules per liter.
How does this energy density compare to other things? This Wikipedia page lists the energy density of various mediums. Looking at this data, the energy density for the Powerwall seems rather low since a lead-acid battery has a density of 0.56 MJ/L. However, maybe this is due to extra space in the Tesla battery. There is also a mass energy density (energy per unit mass). The Powerwall has a mass of 100 kg which puts the mass energy density at 0.36 MJ/kg. This value puts the Powerwall right in the rand for Lithium-ion batteries.

Can you assign some physics homework for the Powerwall?
Of course. Homework is my favorite part. Here are some questions for you to consider.

How long could you run your smart phone using the Powerwall as your battery? Here is some help on phone batteries.
What if you put this Powerwall in your Tesla car? What kind of range would you get? How does this Powerwall battery compare in energy storage to the battery in the electric cars?
Suppose you wanted to charge your Powerwall battery with a stationary bike connected to a generator. If you could pedal and produce 100 Watts, how long would it take to charge?
In his presentation, Elon Musk says that a small portion of the USA surface area could be used to provide solar power for the nation. Estimate the size of this square based on the power use for the USA and the power you get from the Sun. Also estimate the size of this area from the map that is shown in the presentation (I’m sure you can find a better image online). This is my kind of “trust but verify” question.
Elon Musk also claims that the battery for the whole nation would just be a tiny red dot. Estimate the size of land you would need to store a day’s worth of energy.
Telsa also announced the Powerpack. These are essentially industrial sized versions of the Powerwall that can store 100 kWh of energy. Musk also claims that with 160 million Powerpacks, you could move the USA to solar power with batteries. First, check this estimate of power storage. Second, how many Powerpacks would Telsa need to produce per day to make enough batteries to switch to solar in just one year.

COULD HEMP NANOSHEETS TOPPLE GRAPHENE FOR MAKING THE IDEAL SUPERCAPACITOR? 


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As hemp makes a comeback in the U.S. after a decades-long ban on its cultivation, scientists are reporting that fibers from the plant can pack as much energy and power as graphene, long-touted as the model material for supercapacitors. They’re presenting their research, which a Canadian start-up company is working on scaling up, at the 248thNational Meeting & Exposition of the American Chemical Society (ACS), the world’s largest scientific society.

The meeting features nearly 12,000 presentations on a wide range of science topics and is being held here through Thursday.

David Mitlin, Ph.D., explains that supercapacitors are energy storage devices that have huge potential to transform the way future electronics are powered. Unlike today’s rechargeable batteries, which sip up energy over several hours, supercapacitors can charge and discharge within seconds. But they normally can’t store nearly as much energy as batteries, an important property known as energy density. One approach researchers are taking to boost supercapacitors’ energy density is to design better electrodes. Mitlin’s team has figured out how to make them from certain hemp fibers — and they can hold as much energy as the current top contender: graphene.

“Our device’s electrochemical performance is on par with or better than graphene-based devices,” Mitlin says. “The key advantage is that our electrodes are made from biowaste using a simple process, and therefore, are much cheaper than graphene.”

The race toward the ideal supercapacitor has largely focused on graphene — a strong, light material made of atom-thick layers of carbon, which when stacked, can be made into electrodes. Scientists are investigating how they can take advantage of graphene’s unique properties to build better solar cells, water filtration systems, touch-screen technology, as well as batteries and supercapacitors. The problem is it’s expensive.

Mitlin’s group decided to see if they could make graphene-like carbons from hemp bast fibers. The fibers come from the inner bark of the plant and often are discarded from Canada’s fast-growing industries that use hemp for clothing, construction materials and other products. The U.S. could soon become another supplier of bast. It now allows limited cultivation of hemp, which unlike its close cousin, does not induce highs.

Scientists had long suspected there was more value to the hemp bast — it was just a matter of finding the right way to process the material.

“We’ve pretty much figured out the secret sauce of it,” says Mitlin, who’s now with Clarkson University in New York. “The trick is to really understand the structure of a starter material and to tune how it’s processed to give you what would rightfully be called amazing properties.”

His team found that if they heated the fibers for 24 hours at a little over 350 degrees Fahrenheit, and then blasted the resulting material with more intense heat, it would exfoliate into carbon nanosheets.

Mitlin’s team built their supercapacitors using the hemp-derived carbons as electrodes and an ionic liquid as the electrolyte. Fully assembled, the devices performed far better than commercial supercapacitors in both energy density and the range of temperatures over which they can work. The hemp-based devices yielded energy densities as high as 12 Watt-hours per kilogram, two to three times higher than commercial counterparts. They also operate over an impressive temperature range, from freezing to more than 200 degrees Fahrenheit.

“We’re past the proof-of-principle stage for the fully functional supercapacitor,” he says. “Now we’re gearing up for small-scale manufacturing.”

Fruit juice just another sugary drink?

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Inclusion of fruit juice as a fruit equivalent is probably counter-productive, researchers at the University of Glasgow have warned. File photo
APInclusion of fruit juice as a fruit equivalent is probably counter-productive, researchers at the University of Glasgow have warned. File photo
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Drinking fruit juice as a substitute for fruits could be counter-productive due to its high sugar content, researchers at the University of Glasgow have warned.

Writing in The Lancet Diabetes and Endocrinology journal, Professor Naveed Sattar and Dr. Jason Gill both of the University of Glasgow’s Institute of Cardiovascular and Medical Sciences, call for better labelling of fruit juice containers to make explicit to consumers that they should drink no more than 150ml a day.

They also recommend a change to the U.K. Government’s current five-a-day guidelines, saying these five fruit and vegetable servings should no longer include a portion of fruit juice.

Inclusion of fruit juice as a fruit equivalent is probably counter-productive because it fuels the perception that drinking fruit juice is good for health, and thus need not be subject to the limits that many individuals impose on themselves for consumption of less healthy foods.

Professor Sattar, who is Professor of Metabolic Medicine, said, “Fruit juice has a similar energy density and sugar content to other sugary drinks, for example: 250ml of apple juice typically contains 110 kcal and 26g of sugar; and 250ml of cola typically contains 105kcal and 26.5g of sugar.”

“Additionally, by contrast with the evidence for solid fruit intake, for which high consumption is generally associated with reduced or neutral risk of diabetes, current evidence suggests high fruit juice intake is associated with increased risk of diabetes.

“One glass of fruit juice contains substantially more sugar than one piece of fruit; in addition, much of the goodness in fruit fibre, for example is not found in fruit juice, or is there in far smaller amounts,” Sattar said.

Although fruit juices contain vitamins and minerals, whereas sugar-sweetened drinks do not, Gill argues that the micronutrient content of fruit juices might not be sufficient to offset the adverse metabolic consequences of excessive fruit juice consumption.

“In one scientific trial, for example, it was shown that, despite having a high antioxidant content, the consumption of half a litre of grape juice per day for three months actually increased insulin resistance and waist circumference in overweight adults,” Gill said.

“Thus, contrary to the general perception of the public, and of many healthcare professionals, that drinking fruit juice is a positive health behaviour, their consumption might not be substantially different in health terms than drinking other sugary drinks,” he said.

New device stores electricity on silicon chips.

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Solar cells that produce electricity 24/7, not just when the sun is shining. Mobile phones with built-in power cells that recharge in seconds and work for weeks between charges.

These are just two of the possibilities raised by a novel supercapacitor design invented by material scientists at Vanderbilt University that is described in a paper published in the Oct. 22 issue of the journal Scientific Reports.

It is the first supercapacitor that is made out of silicon so it can be built into a silicon chip along with the microelectronic circuitry that it powers. In fact, it should be possible to construct these power cells out of the excess silicon that exists in the current generation of solar cells, sensors, mobile phones and a variety of other electromechanical devices, providing a considerable cost savings.

“If you ask experts about making a supercapacitor out of silicon, they will tell you it is a crazy idea,” said Cary Pint, the assistant professor of mechanical engineering who headed the development. “But we’ve found an easy way to do it.”

Instead of storing energy in chemical reactions the way batteries do, “supercaps” store electricity by assembling ions on the of a porous material. As a result, they tend to charge and discharge in minutes, instead of hours, and operate for a few million cycles, instead of a few thousand cycles like batteries.

These properties have allowed commercial , which are made out of activated carbon, to capture a few niche markets, such as storing energy captured by regenerative braking systems on buses and electric vehicles and to provide the bursts of power required to adjust of the blades of giant wind turbines to changing wind conditions. Supercapacitors still lag behind the electrical energy storage capability of lithium-ion batteries, so they are too bulky to power most consumer devices. However, they have been catching up rapidly.

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Research to improve the energy density of supercapacitors has focused on carbon-based nanomaterials like graphene and nanotubes. Because these devices store electrical charge on the surface of their electrodes, the way to increase their energy density is to increase the electrodes’ surface area, which means making surfaces filled with nanoscale ridges and pores.

“The big challenge for this approach is assembling the materials,” said Pint. “Constructing high-performance, functional devices out of nanoscale building blocks with any level of control has proven to be quite challenging, and when it is achieved it is difficult to repeat.”

So Pint and his research team – graduate students Landon Oakes, Andrew Westover and post-doctoral fellow Shahana Chatterjee – decided to take a radically different approach: using porous silicon, a material with a controllable and well-defined nanostructure made by electrochemically etching the surface of a silicon wafer.

This allowed them to create surfaces with optimal nanostructures for supercapacitor electrodes, but it left them with a major problem. Silicon is generally considered unsuitable for use in supercapacitors because it reacts readily with some of chemicals in the electrolytes that provide the ions that store the electrical charge.

With experience in growing carbon nanostructures, Pint’s group decided to try to coat the porous with carbon. “We had no idea what would happen,” said Pint. “Typically, researchers grow graphene from silicon-carbide materials at temperatures in excess of 1400 degrees Celsius. But at lower temperatures – 600 to 700 degrees Celsius – we certainly didn’t expect graphene-like material growth.”

When the researchers pulled the porous silicon out of the furnace, they found that it had turned from orange to purple or black. When they inspected it under a powerful scanning electron microscope they found that it looked nearly identical to the original material but it was coated by a layer of graphene a few nanometers thick.

When the researchers tested the coated material they found that it had chemically stabilized the silicon surface. When they used it to make supercapacitors, they found that the graphene coating improved energy densities by over two orders of magnitude compared to those made from uncoated and significantly better than commercial supercapacitors.

The graphene layer acts as an atomically thin protective coating. Pint and his group argue that this approach isn’t limited to graphene. “The ability to engineer surfaces with atomically thin layers of materials combined with the control achieved in designing porous materials opens opportunities for a number of different applications beyond energy storage,” he said.

“Despite the excellent device performance we achieved, our goal wasn’t to create devices with record performance,” said Pint. “It was to develop a road map for integrated energy storage. Silicon is an ideal material to focus on because it is the basis of so much of our modern technology and applications. In addition, most of the silicon in existing devices remains unused since it is very expensive and wasteful to produce thin wafers.”

Pint’s group is currently using this approach to develop that can be formed in the excess materials or on the unused back sides of and sensors. The supercapacitors would store excess the electricity that the generate at midday and release it when the demand peaks in the afternoon.

When the researchers tested the coated material they found that it had chemically stabilized the silicon surface. When they used it to make supercapacitors, they found that the graphene coating improved energy densities by over two orders of magnitude compared to those made from uncoated and significantly better than commercial supercapacitors.

The graphene layer acts as an atomically thin protective coating. Pint and his group argue that this approach isn’t limited to graphene. “The ability to engineer surfaces with atomically thin layers of materials combined with the control achieved in designing porous materials opens opportunities for a number of different applications beyond energy storage,” he said.

“Despite the excellent device performance we achieved, our goal wasn’t to create devices with record performance,” said Pint. “It was to develop a road map for integrated energy storage. Silicon is an ideal material to focus on because it is the basis of so much of our modern technology and applications. In addition, most of the silicon in existing devices remains unused since it is very expensive and wasteful to produce thin wafers.”

Pint’s group is currently using this approach to develop that can be formed in the excess materials or on the unused back sides of and sensors. The supercapacitors would store excess the electricity that the generate at midday and release it when the demand peaks in the afternoon.

When the researchers tested the coated material they found that it had chemically stabilized the silicon surface. When they used it to make supercapacitors, they found that the graphene coating improved energy densities by over two orders of magnitude compared to those made from uncoated and significantly better than commercial supercapacitors.

The graphene layer acts as an atomically thin protective coating. Pint and his group argue that this approach isn’t limited to graphene. “The ability to engineer surfaces with atomically thin layers of materials combined with the control achieved in designing porous materials opens opportunities for a number of different applications beyond energy storage,” he said.

“Despite the excellent device performance we achieved, our goal wasn’t to create devices with record performance,” said Pint. “It was to develop a road map for integrated energy storage. Silicon is an ideal material to focus on because it is the basis of so much of our modern technology and applications. In addition, most of the silicon in existing devices remains unused since it is very expensive and wasteful to produce thin wafers.”

Pint’s group is currently using this approach to develop that can be formed in the excess materials or on the unused back sides of and sensors. The supercapacitors would store excess the electricity that the generate at midday and release it when the demand peaks in the afternoon.

“All the things that define us in a modern environment require electricity,” said Pint. “The more that we can integrate power storage into existing and devices, the more compact and efficient they will become.”

Graphene boosts energy storage.

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Monash University researchers have brought next generation energy storage closer with an engineering first – a graphene-based device that is compact, yet lasts as long as a conventional battery. 

Published today in Science, a research team led by Professor Dan Li of the Department of Materials Engineering has developed a completely new strategy to engineer graphene-based supercapacitors (SC), making them viable for widespread use in renewable energy storage, portable electronics and electric vehicles.

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SCs are generally made of highly porous carbon impregnated with a liquid electrolyte to transport the electrical charge. Known for their almost indefinite lifespan and the ability to re-charge in seconds, the drawback of existing SCs is their low energy-storage-to-volume ratio – known as energy density. Low energy density of five to eight Watt-hours per litre, means SCs are unfeasibly large or must be re-charged frequently.

Professor Li’s team has created an SC with energy density of 60 Watt-hours per litre – comparable to lead-acid batteries and around 12 times higher than commercially available SCs.

“It has long been a challenge to make SCs smaller, lighter and compact to meet the increasingly demanding needs of many commercial uses,” Professor Li said.

Graphene, which is formed when graphite is broken down into layers one atom thick, is very strong, chemically stable and an excellent conductor of electricity.

To make their uniquely compact electrode, Professor Li’s team exploited an adaptive graphene gel film they had developed previously. They used liquid electrolytes – generally the conductor in traditional SCs – to control the spacing between graphene sheets on the sub-nanometre scale. In this way the liquid electrolyte played a dual role: maintaining the minute space between the graphene sheets and conducting electricity.

Unlike in traditional ‘hard’ porous carbon, where space is wasted with unnecessarily large ‘pores’, density is maximised without compromising porosity in Professor Li’s electrode.

To create their material, the research team used a method similar to that used in traditional paper making, meaning the process could be easily and cost-effectively scaled up for industrial use.

“We have created a macroscopic graphene material that is a step beyond what has been achieved previously. It is almost at the stage of moving from the lab to commercial development,” Professor Li said.

 

Source: http://www.sciencealert.com.au