EME 812
Utility Solar Power and Concentration

9.1. Options for energy storage


9.1. Options for energy storage

Because solar energy supply is variable in time, energy storage is an important issue. Energy storage is used to collect the energy generated by the solar conversion systems (thermal or photovoltaic) in order to release it later on demand. This can be a situation when sufficient power is produced during the day, and stored energy is used during the night. Also, when insolation conditions are ideal, the solar system may produce enough power for the target application, but on dull days, direct energy supply from collectors is diminished, and the energy from the storage is used to compensate the deficit. Energy storage devices help to smooth out differences and minor fluctuations in energy supply caused by shading, passing clouds, etc. Development of efficient and cost-effective energy storage is considered the main bottleneck of the universal development of solar systems.

Video: How solar energy got so cheap, and why it's not everywhere (yet) (7:53)

Credit: Seema Shah-Fairbank. "How solar energy got so cheap, and why it's not everywhere (yet)." YouTube. January 15, 2021.
Click here for a transcript of the How solar energy got so cheap, and why it's not everywhere (yet) video.


Narrator: Solar. It's astonishing. But clean energy from the sun. Solar energy has become the cheapest way to generate electricity. It's even cheaper than coal. And yet it produces only 3% of the world's electricity. Why aren't we using way, way more of it? How did it get so cheap? And what does all this have to do with ducks? Let's find out.

How solar energy got so cheap

Narrator: First, Let's take a look at how much the price for solar has fallen.

Jenny Chase, Head of Solar, BloombergNEF: I started this job as an analyst of solar in 2005, and then I thought, solar is ridiculously expensive.

Narrator: Jenny Chase is the head solar analyst at research firm Bloomberg NEF.

Jenny Chase: You'd pay about pay about $4 a watt for a solar panel, and today you'd pay about $0.20 for that same watt.

Narrator: And that is just the last 15 years. If you look further back, the price drop is even more impressive. How did this happen?

Gregory Nemet, Author, “How Solar Became Cheap”: It's been a long story, but it's unbelievable.

Narrator: Gregory Nemet has written a book about this.

Gregory Nemet: No one country did it. It was an exchange of one country building on another. One, the US created the technology.

Narrator: The modern day solar cell, made from silicon, was invented in the US in 1954. Back then, it mainly got used in the space industry and was still super expensive. But as the technology progressed, prices started to fall.

Gregory Nemet: Two, Germany created a market.

Narrator: In 2000, Germany passed a law to boost renewable energy development. This was big because it put a fixed price on energy generated from sources like wind or solar. That gave people and companies a reason to set up solar panels. And for them to do that, someone needed to build these solar panels.

Gregory Nemet: Three, China made it cheap.

Narrator: Once the German Lohat came into force, China really started to pump out those solar cells.

Jenny Chase: So basically, it built a whole industry for this on a scale that the west really didn't keep up with.

Gregory Nemet: China was almost a nonexistent player 20 years ago, and today they're the biggest producer of solar panels, about 70% of the world's production.

Narrator: So this is how we ended up where we are now, with clean energy. That also makes business sense. But if solar is so great, why don't we rely on it much, much more and just switch off all these dirty power plants? Well, solar has always had this one big problem. It only really works when the sun is shining, when it's cloudy, or even worse, dark. Even the best solar cells are pretty useless. And that's a real shame, because that's when we'd need them the most. Let's take a look at how we use energy. In the morning, when most people get up and get ready. We need energy. The so-called duck curve charts our demand for power from nonrenewable sources like coal and gas, throughout the day, first, in places without much solar. After the morning spike, it stays pretty level. When people come home in the evening, it goes up again and then drops at night. At this point, you might get an idea why they call it the duck curve, because it kind of looks like a duck. Anyway, in places with lots of solar, like California, this curve changes. The mornings are pretty much the same.

Then the sun rises and solar energy production kicks in. This lets demand for non renewable energy drop until the sun sets, that is. That is when conventional demand shoots up again, way steeper than in the first curve. Two problems with this. One, traditional power plants suck at ramping up this quickly. That means you have to keep them running at a certain output all day, even though there's lots of solar. And that means….

Jenny Chase: That you can end up with actually more power produced in the middle of the day than is used.

Narrator: And that leads to the second problem. There are limits to how much energy you can put into the grid. Too much solar could overpower it, so it needs to be thrown away. This has always made it super difficult to add lots of solar to power systems. But guess what? There is now a solution to this, and chances are you have part of it in front of you right now. A lithiumion battery.

Lithiumion batteries

Gregory Nemet: We're just taking that same construction, stringing together many, many of those cells, and making battery packs that we can use for cars. And then we can also scale that up to use for stationary power to go next to wind parks or solar farms.

Jenny Chase: What's been quite good over the last few years is that batteries have got a lot cheaper as well. And we're now seeing solar projects built with a couple of hours of storage in the battery, so that they could shift solar generation from the middle of the day to the evening, where there's often a peak in electricity demand.

Narrator: In the US, for example, the state of New Mexico just decided to shut down a coal plant and instead build new solar farms that store large amounts of the energy they produce in batteries. Lithiumion batteries have become a lot better and a lot cheaper than expected in the last few years. They're now a viable option for storing and shifting at least a few hours worth of solar energy as needed. So the storage problem that solar always had is actually not that much of a problem anymore. Sometimes, though, we might want longer term storage in places without much sunshine, for example. And that's why companies are offering other solutions. Let's just run through a few.


Narrator: Another type of battery, called a flow battery, separates the charge outside a cell. That has two advantages. It can store more energy and for longer. The problem is they're still relatively expensive. Then there's pumped hydro storage, which is already used quite a bit. You need two lakes, and one of them needs to be in a hill. During the day, you use solar energy to pump water from the lower lake up to the higher lake. When you need energy at night, you can just let it run down through a turbine.

But for that you need to find lakes and, well, a hill. Another solution using gravity comes from a swiss company. It's working on a tower that raises building blocks with solar energy and then releases the energy by lowering them again. But for this too, you need space. And there's also the option of using solar to produce hydrogen. And with that hydrogen, you could then do a number of things like fuel cars, or even make steel. But the whole process is still pretty costly.

Jenny Chase: I think that the storage will mostly be lithiumion. With some hydrogen and maybe a few other options.

Gregory Nemet: There are alternatives. It's just that lithiumion batteries are becoming so flexible and so inexpensive that it'll be hard for these alternatives to compete them. But they do have other attributes, like they hold a charge longer, which could turn out to be play a pretty important role in some applications.

Narrator: Solar has become cheap and has pretty much fixed its biggest problem. So what's next?

Jenny Chase: It's going to be big. It's going to be everywhere. We forecast that even with no further policy, solar would supply about 23% of global electricity by 2050. I personally think it's going to be much higher than that.

Gregory Nemet: I would not be surprised if by 2030, we're talking about solar doing a large part of the world's electricity supply.

Narrator: Solar has come a long, long way. But now that the technology is in place, it really looks like it's time to shine. Now we'd like to hear from you. What are your thoughts on solar energy? Let us know in the comments. And hit subscribe for more videos like this every Friday.

There are quite a few different technology options for energy storage, which are briefly outlined below:

  1. Grid. For grid connected solar systems, the most natural and cost-effective way would be to store energy in the grid. The main idea here is that the DC power from a solar facility (array or farm) is converted to AC power and is fed to the grid and further on is used for on-site or off-site applications. This way, the grid acts as the medium that collects energy from different power-making facilities (renewable or non-renewable) and redistributes it as necessary. Since a grid does not really represent a separate system that is part of a solar plant, it will not be discussed further in this lesson.
  2. Fluid. Fluid-based storage is typically used with solar thermal systems. Unlike grid, which stores electrical energy, fluids store thermal energy. Fluids, such as water, oil, molten salt or others act as a medium for absorbing heat. The main idea is that the solar radiation heats the heat-transfer fluid which is accumulated in the tank. The tank is insulated, so the hot fluid keeps its temperature for a substantial period of time. When needed, the heated fluid is used in a heat-exchanger to produce steam for the electric generator. This type of thermal energy storage was discussed in more detail in Lesson 8.
  3. Battery. A battery is an electrochemical device that stores chemical energy in internal components and releases energy as electricity, which is generated through electrochemical reactions. Batteries are reversible, i.e., can be charged and discharged, and the parameters of these processes are regulated to avoid damage by overcharging or over-discharging. Battery life is expressed in number of charge-discharge cycles. There are many different types of batteries, some of which will be discussed further in this lesson.
  4. Hydrogen. The idea behind hydrogen storage is that electricity generated by solar PV systems can be used to electrolyze water - to split it to hydrogen and oxygen. Further, hydrogen gas is collected and can be used as a fuel. One of the highly efficient devices "converting" hydrogen back to electricity is H2/O2 fuel cell, which has zero carbon footprint during operation.
  5. Compressed air. In this case, the electrical energy produced by a PV solar system is used to run compressors to compress massive amounts of air and store it in underground, above-ground, or underwater containers. Later on, when energy is needed, the air is decompressed and is supplied to a turbine to generate electricity. Compressed air energy systems (CAES) promise high efficiencies, although this technology is not yet widely implemented.
  6. Pumped storage hydropower. The available energy can be used to pump water into an elevated reservoir for storage. When power is needed, the water can be discharged under gravity to run a turbine, which is connected to a generator to produce electricity. The same as compressed air systems, the pumped storage technology has high energy return on investment, although it may require special topographical conditions and water availability in order to be used.

All of the above options for energy storage should be employed with understanding the facility needs and capacity. What energy storage is efficient for small residential systems may be insufficient or too costly when scaled up to the utility-size systems. Determining capacity of energy storage for a particular solar project is an important technical and economic issue. For example, if the capacity of the storage is too large compared to the energy produced by the solar conversion facility, the total system cost will be unnecessarily increased. On the contrary, if the capacity of the storage is too small, that leads to energy dumping and overall unsatisfactory plant performance.

In the following sections, we will discuss different energy storage options that can be possibly applied to utility scale solar systems.