Why Solar needs storage

And why this is important

Solar Energy is all the rage worldwide. And Sri Lanka is no exception. Not surprising, given that it is clean, abundant and popular.

However, some important characteristics of Solar Energy are not discussed, which make the broader conversation both incomplete and misleading. We must fill these gaps.

This article discusses one such gap: solar’s dependence on storage.

I don’t have an expert knowledge on Energy generation, and am at best an informed lay-person. Please read this article with that in mind.

Also, I’d be grateful for any feedback that would improve our collective knowledge and lead to more informed discussion on the topic.

Finally, for simplicity, I only refer to Solar Energy in this article. But much of its content also applies to Wind Energy.

Energy Storage

Energy Storage is everywhere. We humans, like many animals, store energy in our livers and elsewhere, as sugars and fat. A Penlight Battery stores energy as chemical energy, which converts to electrical energy and then light energy in a torch.

A coal-power station, like Lakvijaya (also known as Norochcholai), has stores of coal. Like sugars and batteries, coal has chemical energy stored in its chemical structure. When coal is burnt, the chemical energy converts into heat, which drives a turbine, and in turn, drives a generator. In other words, chemical energy converts to heat energy, which converts to kinetic energy, which finally converts to electrical energy.

Not dissimilarly, a reservoir like Victoria or Laxapana stores energy by holding water at a higher elevation. When this water flows to a lower elevation, the flow can drive a generator, producing electricity. Potential Energy converts to Kinetic Energy which in turn converts into Electrical Energy.

Why do we need Energy Storage?

Energy supply and energy demand are rarely aligned. When there is supply, there might not be sufficient demand. When there is demand, insufficient supply.

For example, we humans need energy throughout the day, even when we sleep. But most people don’t eat more than two to six times a day, and supply is restricted to these few instances. Without energy storage, humans would be dead, because the moment we stopped eating, we would not have energy for vital bodily functions.

Storage solves this problem. Our livers store sugar which we can use for several days. Our body fat stores last even longer.

Hydropower stations like Victoria or Laxapana are similar. It does not rain all the time, even in Sri Lanka’s wet zone. If these power stations produced electricity only when it rained, we would have power for only a few hours per day, and that only on rainy days. Like our livers that store sugar, the hydropower reservoirs store water, which can be released in smaller quantities to produce electricity, on demand.

Differences in Supply and Demand

As we saw above, hydropower stations like Victoria and Laxapana can provide power on at any time — not simply when it rains. The same applies to coal power plants like Lakvijaya and other thermal facilities.

This ability is vital because the electricity demand varies considerably during the day and across days. This (vastly simplified) diagram represents Sri Lanka’s electricity demand throughout a typical day.

The x-axis represents time. The y-axis: power. The area on the graph represents energy. Consumption is lowest during nighttime when most people are asleep. It picks up during the day and peaks in the evening.

Different energy sources (Hydro, Coal etc.) have different costs. Hence, when demand is lowest, we will use the least expensive source, and with growing demand, more expensive sources are added.

Why Solar is Different

We can only generate solar energy during daylight hours, whether a single solar panel on your roof or a vast solar farm covering many hectares. Hence, if we were to add solar power to a national system (like the CEB’s national grid), its contribution would look like this:

On the one hand, solar would add to total “energy” and help reduce some variable costs. On the other hand, it would not be able to contribute to maximum “power” capacity because it cannot run during peak hours.

There is also considerable variation in Solar Power generation even during daylight hours. Energy production could collapse to almost nothing if a dark cloud moves across the solar panel. There are also effects in the sun itself that affect solar energy production. These solar effects could affect Jaffna, Colombo, Batticaloa, and Matara (at the far ends of the country) simultaneously.

Hence, our diagram should look more like this.

Solar with Storage

If solar power plants had storage, either in the form of batteries or some other technology, we would be able to match energy supply with demand. We could “smooth out” the various “variation” problems, and Solar could also contribute to peak power generation capacity.

In an ideal world, our diagram would look something like this.

The giant “CEB Battery”

Sadly, few solar generation systems in Sri Lanka have storage capacity.

When solar power generation is low and less solar power is fed into the national grid, conventional sources (Hydro, Coal etc.) compensate by increasing production. The latter can do this because they have storage. Conversely, when solar power generation is high, conventional sources compensate by decreasing production.

Hence, all the conventional sources collectively act as a giant battery that “buffers” the variation of solar power generation. In other words, solar power generation systems use the rest of the CEB system as a battery.

Maintaining this “CEB Battery” is very expensive. For example, even on a bright sunny day when solar power generation is high, conventional sources need to run during the night, and also during the day because of daytime variation. Even if these may not run all the time, there are high fixed costs like maintenance for the CEB.

As of now, only a very small proportion of Sri Lanka’s energy comes from Solar. Hence, it is feasible for the “CEB battery” to support solar energy systems.

However, suppose the capacity of solar is to signifciantly increase. Then, new forms of storage, like batteries, or the capacity of conventional sources (Hydro, Coal etc.) will also have to proportionally increase.

I hope that the above clarifies solar’s dependence on storage. Armed with this knowledge, we can now ask more questions.

Questions

Storage is vital for solar. Hence, we should ask the obvious question for any new solar energy generation system:

• Where is the storage?

If there is no storage built into a storage system, we should ask

• Where is the storage actually coming from?
• Is the system using the “CEB Battery”?
• If it is, who is paying for the “CEB Battery”?

The storage capacity supporting a solar energy system must be proportional to the its power generation capacity.

For example, if we have a 300MW solar farm, we might have to support it with 100MW of storage capacity.

The exact ratio between power generation capacity and storage capacity is complex and requires considerable analysis to determine. Different scenarios might have different ratios. However, some storage capacity will be necessary.

Hence, if we read about a new solar farm, you should ask:

• Does the solar farm have corresponding storage capacity?
• Is the existing storage capacity in the current system sufficient to support the new generation?
• If not, how do we plan to construct it?