What is the Best Way to Store Solar Energy?
Why does solar power need storage? Delve into the intricacies of solar energy storage systems, from daily domestic needs to grid-scale demands.
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The importance of solar energy storage solutions

Solar energy storage is the conversion of solar power into another type of energy that can be saved to be used later.

More and more people use renewable energy sources to electrify their homes, institutes, and cities. This has a positive effect on both energy bills and our carbon footprint.

Solar panel technology is growing in line with this demand growth. It is now mature, its price is going lower, and its efficiency is increasing daily. But the sun falls and rainy days come, so we need some sort of tank to store this clean energy and use it when there is no sun.

The variability of solar power

Solar energy varies during the day. It’s zero at night, then it rises in the morning to reach its peak at noon, then it falls again.

Also, it has a seasonal variation

  • In the summer, the solar energy peaks when the sun is closer, the day is longer, and the irradiance is higher.
  • In the winter, the sun goes farther, the days are shorter, and the irradiance is lower.

There is other variation due to the clouds, rain, and snow coverage. 

All this variation in solar input leads to variations in the energy generated by the sun. 

To solve this problem, we need an energy storage system that smooths the energy variation throughout the day and year.

Figure 1: Small-scale battery storage (Source. US Dept of Energy)

Small-scale solar energy storage

Solar energy can be stored in different ways depending on the storage capacity, time, amount of energy, and the nature of the site.

Solar batteries

For daily storage of solar energy for houses and small businesses, the most suitable type of storage is batteries. 

They have small sizes, are portable, and are easily installed compared with other storage types.

Solar batteries convert the electrical power produced from the solar panels into chemical energy and store it. That is, they charge the battery. Then, the stored chemical energy is converted back into electrical energy and supplies the loads (discharging) of the battery.

Lithium or lead?

There are different types of battery technologies.

  • The old, well-known lead batteries were developed to become deep-cycle batteries to match the intensive discharge of solar systems. Lead batteries are cheaper, but they have a short life span of only 4 years with proper usage.
  • The new strong competitor is lithium ion batteries, which have a higher matter-to-energy ratio and a long life span of up to ten years. But, they are more expensive.

A solar energy storage system on such a small scale is necessary for off-grid systems where the solar system needs to fulfil all the energy requirements of the facility. On-grid systems don’t need solar energy storage solutions.

Utility-scale solar energy storage

At the utility scale, solar energy storage has different alternatives besides large-scale batteries. Solar generation can be stored as thermal energy, gravity, compressed air, or pumped water.

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Figure 2: Example energy storage system at Pillswood in the UK (Source: Harmony Energy)

Utility-scale batteries

Starting with large-scale batteries, it’s a well-known technology. It does not need any special site considerations like other large-scale storage and has a relatively small size but with a high cost and shorter life cycle.

We can store up to tens of megawatts using utility-scale batteries. Batteries can hold power for 1 to 5 days.

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Figure 3: Thermal solar energy storage (Source: Alnaimat and Rashid, 2019)

Thermal energy storage systems

In thermal storage systems, the solar energy is concentrated in a focal point containing the thermal fluid, which can be melted and stored in insulated tanks to be used for electricity generation using steam turbines.

The molten salt can remain in its liquid state for 14 hours. Thus, the solar thermal station can work 24 hours with solar energy only.

This type of thermal solar energy storage uses thermal solar energy with no solar panels, just mirrors or reflectors to collect the solar energy at one focal point.

It can also be a combination of solar thermal and solar photovoltaic by using the excess power from a solar photovoltaic plant to heat the thermal fluid.

The size of a thermal solar plant can reach hundreds of megawatts.

The advantage of thermal storage is its relatively low cost because no high technology is used, but it is only feasible in hot, dry locations. 

There is a need for direct irradiance from the sun at all operating hours, for which a solar tracker must be used.

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Figure 4: Gravity storage (Source: Science News Explores)

Gravity storage

Another way to store utility-scale solar energy production is through gravity storage. The excess energy from the sun is used to lift a heavy weight. When the stored energy is needed, the weight is released by moving a turbine to generate electricity.

The lifting can be done over the ground by building a tower for the weight or underground by digging a well for the weight. This is a new technology, but several investors have started to implement it commercially.

The advantages of this type of storage are that it doesn’t need much space and has a storage time between 2 and 18 hours. It can store up to hundreds of megawatts per plant.

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Figure 5: Compressed air storage (Source: Wang et al, 2017)

Compressed air storage

The fourth way to store solar energy for large-scale generation is compressed air. In this type of storage, the surplus energy from the sun is used to compress the air in a natural or artificial underground cave.

Up to hundreds of megawatts can be stored using compressed air technology. To retrieve the stored energy, compressed air is used to drive the turbine and generate electricity.

The storage time of compressed air technology is between 4 and 12 hours.

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Figure 6: Pumped hydro storage (Source: Drax)

Pumped storage

The last solar energy storage is the pumped storage hydro. The excess solar energy is used to pump the water from the bottom reservoir to the top reservoir.

The energy is retrieved from this kind of storage by opening the gate from the top reservoir and tuning the turbines in the same way as hydropower generation from dams.

The advantage of this type of storage is that the energy can be stored in large amounts—up to several gigawatts—and for a long time—days – depending on the reservoir size. The disadvantage is that it’s hard to find a suitable landform to implement the system.

Demand-side management

For all types of solar energy, demand-side management can play a huge role in finding efficiency and cost optimisation.

Smart grids with the support of AI can form a dynamic electricity grid with automatic tariff updates that can create a win-win relationship for the power grid and the consumer.

This grid type encourages the customer to match the load profile with the energy generation profile by consuming power during peak generation (at noon on sunny days), such as charging electric vehicles and turning on washing machines.

This will reduce the need for storage and lower the price of solar production.

What is the best solar storage system?

Finally, to answer the big question of what is suitable energy storage for your project, you need to reconsider all the mentioned storage systems, taking into account the size of the system, the landform of the site, the weather at the location, and your budget.

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Dr Samah Hashim
Dr Samah Hashim is a solar energy teacher and researcher. She shares her knowledge by writing articles and producing YouTube videos.

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