Integration of solar system and battery

 New solar system combines generation and storage, could store electricity outside the grid

Scientists have harnessed the abilities of both a solar cell and a battery in one device,  a 'solar flow battery' which soaks up sunlight and efficiently stores it as chemical energy for later on-demand use. The research could make electricity more accessible in remote regions of the world. Anyone with rooftop solar knows the awkward truth. Panels work hardest in the middle of the day while many homes and offices use the most electricity after sunset, when lights, TVs and air conditioners all put ON. The gap is usually filled by separate battery packs, which add cost and complexity to a system and, in many cases, to your electric bill too. Now researchers at Nanjing Tech University in China have built a lab-scale device which tackles both jobs at once. Their prototype solar redox flow battery collects sunlight and stores the resulting energy in liquid chemicals, then releases it later as electricity. In tests, it reached an average solar-to-electricity efficiency of about 4.2 percent and ran through more than fifteen charge and discharge cycles while being charged only by light.

While sunlight has increasingly gained appeal as a clean and abundant energy source, it has one obvious limitation, there is only so much sunlight per day, and some days are a lot sunnier than others. In order to keep solar energy practical, this means that after sunlight is converted to electrical energy, it must be stored. The work, led by Chengyu He with first author Kaige Ding and colleagues at Nanjing Tech University, focused on a small experimental device. They cut commercial triple junction amorphous silicon cells into tiny pieces about two centimeters on a side and coupled one of these chips to a flow cell with a carbon felt electrode and a Nafion membrane separating the two liquids. Before testing, the team bubbled argon gas through the electrolytes to remove dissolved oxygen which could interfere with the reactions. The team describes its invention as a solar redox flow battery, often shortened to SRFB. In simple terms, it is a small solar cell directly wired into a special kind of battery which stores energy in flowing liquids instead of solid electrodes. This means the same device can both turn light into energy and park that energy in chemical form for later use.

The solar flow battery has three different modes. If energy is needed right away, it can act like a solar cell and immediately convert sunlight to electricity. Otherwise, the device can soak up solar energy by day and store it as chemical energy to deliver it later as electricity when night falls or the sky grows cloudy. In a standard flow battery, two different liquids sit in separate tanks and are pumped through a central cell where they trade electrons. Those liquids contain redox couples, chemical pairs which can gain or lose electrons reversibly, which lets the system charge and discharge many times. The Nanjing Tech group chose an organic molecule called 2,6-DBEAQ on one side and a compound known as K4[Fe(CN)6] on the other, both dissolved in water. The twist in this design is the light absorber. Instead of using a normal solar panel which feeds a separate battery through external wiring, the researchers attached a triple junction amorphous silicon photo-electrode directly to the flow cell. When light hits this multilayer silicon structure, it generates enough voltage to drive electrons into the redox liquids, so the battery charges itself as soon as the sun shines.

For most rooftop systems, a single number gets a lot of attention. Modern commercial silicon panels usually convert around 15 to 22% of incoming sunlight into electricity, with an average a bit above 20%. On that scale, 4.2% may sound underwhelming at first glance. The comparison is not entirely fair, though. A regular panel needs a separate battery and power electronics to store energy for evening use, while this SRFB prototype handles conversion and storage in the same package. Earlier devices of this type using similar organic molecules managed efficiencies of around 1.7, 3.2 or 4.9%, and some alkaline systems reached only about 0.44 to 3.0% while suffering from corrosion or unstable chemicals. A key choice here is the operating environment. Many earlier experiments pushed the chemistry in very strong acid or very strong base, which can eat away at photo-electrodes and break down ferrocyanide-based electrolytes. The Nanjing Tech device instead runs at pH 12, a milder alkaline condition which aims to keep both the silicon and the K4[Fe(CN)6] solution stable during repeated cycling.

For charging tests, the device sat under a xenon lamp adjusted to mimic standard midday sunshine, about 100 milliwatts of light on each square centimeter of the cell surface. During this step it was charged only by light, with no extra power source pushing current into the system. In everyday language, the solar cell side acted like a tiny built-in charger for the battery. When the researchers switched to discharge mode, they pulled current from the device at 10 milliamps/square centimeter and repeated the charge and discharge cycle more than fifteen times. Across these experiments, the system delivered an average solar-to-electricity efficiency of roughly 4.2%, which the researcher report as one of the best results so far for solar redox flow batteries that rely on anthraquinone-based liquids. The main organic molecule in the new battery, 2,6-DBEAQ, has its own backstory. It was originally designed by a team led by Michael Aziz to create long-lived aqueous flow batteries at pH 12, and that earlier work showed it could stay stable over many thousands of cycles while paired with potassium ferrocyanide. The long lifetime is one reason it is attractive for a solar device which is required to charge and discharge day after day.

More broadly, scientists around the world are exploring organic redox flow batteries as a safer, potentially cheaper alternative for large stationary storage. Reviews of these systems highlight anthraquinones and related molecules as especially promising because their structures can be tuned, they dissolve well in water, and they avoid toxic metals. In that context, the new SRFB from Nanjing Tech slots into a growing family of designs which aim to move beyond traditional lithium ion packs for grid-scale uses. In day-to-day life, the appeal of a device like this is straightforward. If one piece of hardware could both capture sunlight and store it, small solar systems on homes or offices might someday need less extra equipment, which could ease installation and maintenance. Flow batteries also scale capacity simply by using larger electrolyte tanks, so a similar concept could be adapted to solar farms which want hours of storage without building huge banks of solid batteries. There are still big hurdles before anything like this reaches a rooftop or a commercial solar plant. The current prototype is tiny, its efficiency still trails far behind standard panels, and the team has tested only a modest number of cycles. Yet it offers a clear proof of concept for a hybrid device which might one day smooth out solar power so that the lights and appliances stay on long after the bright afternoon sun is no more available. We could eventually get to more efficient output using emerging solar materials and new electrochemistry. Manufacturing current solar flow batteries is still too expensive for real-world markets, but cheaper solar cell materials, and technological advances could help cut costs in the future. 

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Fuel of the future

Rare isotope helium-3 is the 'Fuel of the future', usually found on the Moon

Helium-3 is usually talked about as a Moon resource, something future astronauts might mine from lunar dust to power fusion reactors or cool quantum machines. The forests and wetlands in northern Minnesota stored helium-3, a fuel expected to be found as a resource on the Moon’s surface. Drilling beneath the Topaz Project site near Babbitt, scientists confirmed the presence of helium-3. A concentration of 14.5 parts/billion was found in the lab tests, which was similar to what was measured in Apollo samples. The discovery could be a game-changer for the future of clean energy and was led by exploration company Pulsar Helium. Geochemist Dr. Peter Barry at the Woods Hole Oceanographic Institution conducted gas analyses, which revealed the site in Minnesota to be one of the most unexpected helium-3 reservoirs ever found. Helium-3 was one of the rarest substances on Earth and was found at trace levels in the atmosphere.

At the Topaz Project near Babbitt, drilling has revealed measurable, usable amounts of helium-3 in gas trapped deep underground. Lab tests show concentrations similar to levels measured in Apollo samples brought back from the Moon. The gas analyses suggest Minnesota may host one of the most unexpected helium-3 reservoirs ever identified. Helium atoms occur in different forms called isotopes. These are atoms which have an identical number of protons but have different numbers of neutrons. They all count as helium, but some are heavier than others. The helium was freed from mineral grains by heat beneath the surface and old faults in the crust, which helped to travel upwards through the rock. Nitrogen-rich gas at the Topaz site functioned as a carrier fluid, which dissolved the helium and transported it without adding carbon-heavy hydrocarbons. The helium-rich mixture was prevented from leaking by the layers of tight rock which formed a barrier and also helped build up the concentration. Methods such as cryogenic distillation and adsorption columns were used by engineers to test the production of pure helium-3 gas streams.

Helium-3 (3He) has two protons and one neutron, whereas the far more common helium-4 (4He) has two protons and two neutrons. Most terrestrial 3He comes from the decay of tritium in nuclear weapons and reactors. This is supplemented by tiny amounts trapped in natural gas fields. In Earth’s atmosphere, 3He is present only at trace levels, which is many orders of magnitude lower than the gas in Minnesota’s new reservoir. This precious gas can command around nine million dollars/pound. This price makes it vastly more valuable than everyday helium, according to industry analysis. US agencies ration 3He for programs including neutron detectors and cryogenics, the science of working at low temperatures. Because Topaz helium does not depend on aging nuclear stockpiles, even modest 3He recoveries here could ease those long-term supply constraints. “We are thrilled to announce this remarkable helium-3 discovery,” said Thomas Abraham-James, President and CEO of Pulsar Helium. The Topaz helium did not depend on aging nuclear stockpiles, and even modest recoveries could aid long-term supply constraints. Labs in Ohio and Massachusetts analyzed the gas concentrations and ratios from the samples from the Jetstream 1. The ratio of normal helium and Topaz helium was constant across the gas with both contents in a single source. The ratios were measured with a specialized noble gas mass spectrometer, an instrument used to sort gas atoms by mass. The northern bedrock of the state was built from ancient, uranium-rich crust, which had been slowly generating helium for billions of years.

Scientists measure worldwide production of 3He in tens of thousands of liters each year. This is far below the expected demand from quantum computers and laboratories. Separating 3He from 4He in a gas stream is difficult because the two isotopes behave identically unless cooled to extremely low temperatures. Engineers test approaches such as cryogenic distillation and adsorption columns. However, until now, no company runs a plant which produces pure 3He from gas streams. Pulsar has invited universities and technology firms to treat Topaz as a test ground for separation methods which might unlock a 3He stream. The state’s northern bedrock is built from ancient, uranium-rich crust, which has been quietly generating helium for billions of years. Heat from below, and old faults in the crust, help free that helium from mineral grains, letting it migrate upward through the rock. At Topaz, nitrogen-rich gas acts as a carrier fluid, dissolving the helium and transporting it upward without adding carbon-heavy hydrocarbons. Overlying layers of tight rock form a barrier which prevents the helium-rich mixture from leaking and allows it to build up concentrations.

The decay of tritium in nuclear weapons and reactors created most terrestrial helium-3 and was supplemented by small amounts trapped in natural gas fields. The magnitude of the resource found in Minnesota’s new reservoir was much larger than what was seen in Earth’s atmosphere. The resource is 100,000 times the cost of common helium. The resource was rationed for programs, like neutron detectors and cryogenics working at low temperatures. Because 3He captures slow neutrons so efficiently, it underpins highly sensitive detectors which search for illicit nuclear material and monitor research reactors. In refrigeration systems, 3He mixes with 4He to reach extremely low temperatures. This is critical for quantum computing, a method which uses quantum physics to process information. Researchers explore 3He as a fuel for fusion, a reaction where light atomic nuclei combine and release energy. Helium-3 also cools specialized experiments in condensed matter physics and powers advanced imaging methods. These roles give the gas an influence far beyond its tiny volumes. Minnesota has never produced oil or natural gas commercially, so lawmakers are writing rules as companies prepare to tap the Topaz reservoir. Local residents and tribal governments are weighing questions about groundwater, wildlife and noise. They are also watching potential jobs and tax revenues gather momentum. Some community members worry that Minnesota lacks experience with gas wells. They insist that regulators must examine drilling, flaring and reclamation plans carefully before production. For Minnesota, supporters see Topaz as a way to supply helium without importing from politically sensitive regions. It will also generate significant revenue for counties.

Gas samples from the Jetstream 1 well were analyzed in laboratories in Ohio and Massachusetts. Both laboratories agreed on the gas concentrations and ratios found. Across gas with 4He contents between one and eleven percent, the ratio of 3He to 4He stayed constant, suggesting a single source. Scientists express this relationship as a 3He to 4He ratio of about 0.09 relative to air. This makes it significantly higher than in typical crustal gases. Those ratios were measured with a specialized noble gas mass spectrometer, an instrument which sorts gas atoms by mass. Startups planning to mine 3He from the Moon have signed contracts with firms making quantum computing refrigerators and with the Department of Energy. “They will need more helium-3 than is available on planet Earth,” said Gary Lai, chief technology officer of Interlune. For Pulsar, the next steps include drilling more wells and estimating recoverable helium. They also need to decide whether separation technology can support a profitable project. If helium from Minnesota can be produced at scale, this potential fuel of the future might come from forests instead of lunar soil. The state of Minnesota need to keep a lookout for potential jobs and tax revenues which may occur as a result of the reservoir. Communities demand regulators for the plans scheduled for the site before full fledge production starts.

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