A Salt and Battery: How to Store Sunlight

It has become common knowledge that humanity needs to change the sources of our energy at an unprecedented rate if we are to avoid the worst effects of climate change. Renewable energy systems are the most promising means available to reduce our impact on the earth without giving up the comforts of readily available electricity. However, an issue with some renewables like wind and solar is that the energy is only available sometimes. There is no solar power without sunlight and no wind power without wind. In this article I’ll be looking at a type of solar power plant which avoids this problem in a most ingenious way.

One way to solve the storage problem might be to connect all our renewable energy infrastructure to a massive international grid. What this would achieve is that excess solar power from a hot day in San Francisco could be used to power Beijing in the middle of the night, or excess wind power from blustery Ireland could be used to power gustless Brazil. This is a very good idea in theory but it has its drawbacks. Consider the sheer quantities of copper and rubber required to connect every solar and wind farm in the world to every home or business which requires their energy. And what about the time it would take for such an ambitious project to reach completion? Climate change is already here and will soon become entirely irreversible without swift and decisive action.  

So how else can we store and distribute renewable energy? The answer seems very simple; build a battery. If you need solar power at night, why not store the electricity generated during the day rather than transporting it to the other side of the world? This, however, is far easier said than done. The current generation of lead-acid (car) and lithium-ion (phone) batteries are remarkable works of engineering. They are not, however, up to the task of storing the amount of energy we need them to store without seriously depleting natural resources like rare-earth metals. We are badly in need of a breakthrough. Lead-acid batteries have been working on the same basic principle since their invention by Gaston Plante in 1859 and are still one of the most widely used rechargeable batteries on the market. In this article, I’ll be looking at a new way of storing solar power that may revolutionise the energy grid of the future.

‘Concentrating solar power’ (CSP) plants have been providing more and more people with electricity ever since they were first built on an industrial scale back in the 1980s. The difference between these solar plants and standard photovoltaic (PV) plants is the way in which the electricity is generated. In PV panels, solar energy is converted directly into electricity. In CSP, the heat energy from the sun is used to make steam which spins a turbine and this is what generates the electricity. This is roughly the same process used to generate power from coal, oil, natural gas, nuclear fission, incineration, plasma gasification and thermal wave power so the proof of concept is definitely there. The major advantage of CSP over PV is storage. If your plant is generating electricity directly from the sun, you need somewhere to store the electricity when it is not needed; a battery. If you are generating electricity from heat, on the other hand, you can store the sun’s energy in something called a heat transfer fluid (HTF). This is any fluid, like mineral oil, which retains heat well over time.

The most basic and widely used form of CSP is known as a ‘parabolic trough power plant’ (PTPP). The first documented use of this technology was Auguste Mouchout’s ‘solar steam engine’ in 1866. In PTPPs, mirrors focus sunlight onto tubes which contain a HTF. The mirrors are curved like those you might see in a house of fun and are arranged in troughs with the tubes of HTF running down the centre. Picture a hot dog but with mirrors rather than bread and tubes rather than a highly questionable meat-like substance. The hot HTF is transported through the tubes to a series of heat exchangers where it evaporates water to spin a steam turbine. If electricity is not needed at that moment, the hot HTF can instead be transported to a storage chamber from which it can be removed when the need arises for electricity. Once the heat has been converted into electricity, the HTF returns to the troughs to begin the process again. 97% of the CSP plants currently producing energy are PTPPs.

Parabolic mirror with tubes of HTF

PTPPs, however, are not the only type of CSP available. Back in 2011, a company called Solar Reserve received a loan of $737 million for a project called ‘Crescent Dunes’; a massive solar plant in the Nevada desert which can provide electricity to 75,000 homes, night and day. Crescent Dunes is what is known as a ‘power tower’ CSP plant. Power towers operate on the same basic principle as PTPPs, but rather than each mirror focusing sunlight onto a different section of tubing, all the sunlight is concentrated on one central tower. Focusing all the sunlight on one place means that the plant operates at much higher temperatures, greatly increasing efficiency. This design also does not require expensive curved mirrors like PTPPs. The plant instead uses ‘heliostats’, flat mirrors which track the sun and change their position to maximise the amount of sunlight hitting the tower.

The real genius of the project is what is contained within the tower. Inside the tower is a mixture of potassium nitrate and sodium nitrate; also known as salt! More specifically, saltpeter. Sodium nitrate is currently used to preserve certain foods and is the reason bacon goes green if left uneaten for too long. In power towers, the salt is heated by the sunlight reflected off the mirrors until it is molten and packed to the brim with energy. The salt is cheap and extremely good at retaining heat, acting as a kind of thermal battery. This means that power towers can continue to provide energy long after the sun has stopped shining. What’s more, salt can be used at much higher temperatures than any of its competitors. One issue with using molten salt is that it can freeze in the pipes. For this reason, new types of solar salt are being developed which have much lower melting points.

One apparent issue with this design is the effect on birds. If you have thousands of mirrors concentrating the blazing sunlight of the desert into one spot, any bird that is unfortunate enough to fly through the firing line could be killed by direct heat. There have even been reports of birds bursting into flames mid-air then crashing down to earth like meteorites. We have decimated insect populations around the world, depriving many birds of their food source, and scientists estimate that between 100 million and 1 billion birds die each year by flying into buildings in the US alone. Given these facts, it could be argued that bird deaths are an unacceptable side-effect of power towers However, recent studies of bird deaths in a number of power towers have shown that initial estimates may have been wildly exaggerated.

Back in 2014, a conservationist by the name of Shawn Smallwood very roughly estimated that Ivanpah, the world’s largest CSP plant, could be killing 28,380 birds per year. That number or anything close to it would indeed be unacceptable. However, at the same time that Smallwood made his estimate, a large-scale study was being carried out at the Ivanpah plant to see just how many birds were actually dying. After 8,935 person hours and 281 dog-hours of searching, the team found just 695 dead birds and 35 dead bats. Adjusting for the bodies that weren’t found or were carried off by scavengers, the team estimated that around 3,500 birds had died in the plant’s grounds over the course of the year. They estimated that only around 1,500 of those deaths were caused by birds flying into the tower or being burned by the mirrors. That’s nearly twenty times fewer deaths than the number predicted by Smallwood which tarnished the plant’s name in the media. The other 2,000 deaths were listed as ‘unknown causes’ which could have nothing to do with the power plant at all. To put these numbers in context, it is estimated that in the US alone, domestic cats kill 1.3 to 4 billion birds per year.

Another consideration is that the negative effects suffered by birds if climate change goes unchecked greatly outweigh the effects they will suffer from concentrated solar, particularly given the recent assessments which show that the damage to bird populations from CSP is far less severe than was previously thought. There is certainly merit to this argument. We need to develop and roll out effective energy alternatives very soon or else birds, mammals, fish and insects alike will all suffer the worst effects of climate change.

It seems that CSP plants are getting better and better at mitigating the risk to bird populations. Each year the number of deaths goes down as adjustments are made to what is still a very new technology. It may seem cold and calculated to talk of flaming birds like mere teething pains, but we need to make these kinds of hard decisions if we are to ensure that we leave a habitable planet to future generations of people and birds alike.  

In PTPPs, the sunlight is concentrated on a massive number of different points which are at ground level, meaning that the threat to birds is greatly reduced. However, there are a number of drawbacks. First and perhaps most important is that power towers are far more efficient at converting heat into electricity. This is partly due to the higher operating temperatures but is also affected by the surface area on which heat-loss can occur. If you concentrate all the sunlight onto one point, there is a much smaller area in which heat can radiate out into the atmosphere. Another major factor is how much of resources like oil, metal, water or salt are required for the process. In power towers, you only need enough HTF at any given moment to fill the relatively small space at the top of the tower. If you are constantly heating several kilometres of pipes, on the other hand, you will lose a lot more heat to radiation and use a lot more resources in the process.

Like many sustainable technologies, there are a number of advantages and disadvantages to CSP. When it comes to large-scale energy production, CSP seems to have PV beat, but If you are just looking to power your own house, PV rooftop solar panels are far easier to install and provide you with a personal energy supply. In the US, you can also make money from producing excess energy for the grid, with the UK set to follow suit in January of 2020 after much controversy and tomfoolery on the part of the government. Right now, good PV panels convert roughly 20% of sunlight into electricity but researchers think that number could theoretically be brought as high as 80% with a few breakthroughs. When it comes to deciding which type of CSP is best, I will leave that up to you.

Power towers are far more efficient and require far fewer resources to generate the same amount of energy. Despite initial exaggerations, however, power towers do pose a threat to birds, particularly if new plants keep being built. What’s more, they do not have a proven track record as long as their rival. What is certain is that if we do not transition to cleaner forms of energy ASAP, the consequences will be far more severe than most people think.

We will see an acceleration of biodiversity loss and an increase in the frequency and severity of natural disasters like hurricanes and floods. Large areas of land will become inarable, greatly reducing our food supply, and hundreds of millions of people will be exposed to extended periods of drought. Depending on which predictions are correct, the emissions reductions brought about by technologies like CSP could easily end up saving more lives than were lost to the holocaust. If that is not worth investing in, then I truly don’t know what is.

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2 Comments on “A Salt and Battery: How to Store Sunlight

  1. Very interesting. And very easy to read. (I often get boggled by too many facts!) That sounds like it’s the solution.

    Like

  2. Pingback: Getting High on Grass – Can Plants Really Fuel a Plane? – Small Change

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