The Problem of Variable Renewable Energy

Renewable energy like wind and solar, also known as Variable Renewable Energy (VRE), has gained huge pace in the utility sector as more and more demand is increasing for a green source of energy. Since the last decade, the price of solar panels has dropped astonishingly all thanks to mass production and industrialization. This has lead to a huge spike in the use of solar energy (also the same in the case of wind energy) on the grid.

But when the sun goes down, it reveals a dark side of solar energy. At night even the largest solar power plant in the world does not produce any electricity, the time when demand is at its peak. Similarly, wind energy is very unstable, it will not work if there is no wind blowing. For this reason, fossil-fueled power plants have to run side by side to provide a stable supply to the grid.


Why are Renewables to Blame

In the next decade, renewable energy sources particularly wind and solar will increase substantially and can even be a primary source of energy according to multiple projections.

Variable Renewable Energy is pretty unstable and unpredictable due to its dependency on climatic and geographic conditions. To increase the penetration of such renewable energy one must plan on how to manage the grid by balancing the demand and supply. Here are some characteristic features of a Variable Renewable system that should be kept in mind whilst planning

  • Unstable nature: Both wind and solar energy are highly unstable having their respective factors.

In the case of solar energy, it is dependent on the sun’s timeline, which varies throughout the year due to seasonal changes. Also, the variable intensity of the sunlight caused due to cloud cover, snow, dust, fog, etc. leads to fluctuations in production. But when the sun goes down there is zero generation of electricity.

Wind energy on the other hand is not directly influenced by the sun but is caused by the difference in temperature of the surface of the earth. It can work at any time of the day. Though one cannot predict with high accuracy when the wind will blow on a particular area in the next couple of hours.

  • Predictability and uncertainty: Though solar is more predictable than wind it is subject to more influencing factors like dust, snow, fog, smog, rain, etc., and is directly dependent on the sun. Predicting cloudy weather is possible with high accuracy than predicting the flow of wind. To run a wind turbine depends on several factors like the velocity of wind, air density, efficiency of the turbine, etc. Wind turbines are automatically shut when there is a storm or a cyclone to prevent damage to the turbine and the grid. Currently, it has not been possible to predict so many factors. Due to which it has to be integrated with solar or other sources of energy.
  • Geographical dependency: The intensity of the sun and wind varies from place to place. A solar plant cannot be economical in a tropical landscape having high rainfall but can be feasible for a wind farm.
  • The low-capacity factor of VRE: Capacity factor is the percentage of maximum energy produced at a given period. In this case, VRE has a very low capacity factor which is 36% for wind and around 33% for solar. This is quite low as compared to a nuclear power plant which can touch a staggering 98% and also can run 24*7.

The Increase in Complexity in Grid Management

Transmission lines

A major advantage of a fossil-fueled plant over a VRE is its stability and total control of power output. For the last 100 years, the gird managers have been in absolute control of the supply to meet the variation of total load on a given day. The work was only to increase or decrease the output of the generators accordingly.

But things are becoming way more complicated after the introduction of renewable energy like wind and solar due to its unpredictable nature that we have discussed above.

The grid managers aim to balance the supply to the demand. It was very easy to balance the grid using fossil-fueled plants due to their consistency. But as the penetration of VRE is increasing day by day there is a need for a greater flexible system.

As we know that the wind and the sun behave differently and the sun is more predictable than the wind, both complement each other in many scenarios. At night as the sun goes down so does the generation of power from solar panels but wind energy is more available.

The Windy Problem

Wind Turbine

The main disadvantage of wind energy is its unpredictability. We are not able to accurately predict when the wind will blow in the next one or two hours. So the question of ramping up of power when there is a peak in load is out of the question. There are instances when there is more supply than demand or vice-versa. Both the scenarios might be dangerous if not carefully handled. If the generation is more than demand then it can damage the grid and the transmission lines causing serious long-term damage. And if the net load (the difference between the total load and supply from wind energy) suddenly increases and is not rapidly compensated then there would be disruption of power.

The Sunny Dark Side

Unlike wind, we all know at what time the sun sets and rises. Though it is not constant throughout the year and varies for seasonal changes and geographical position. Solar energy has its unique problem and has a relationship with a duck.

The Duck Curve

In a pre-COVID era when everything was “normal”, we use to wake up in the morning travel to the office and by the end of the day reach home to relax. So, there is a clear load spike in the morning and the evening for a given day. If we map a graph of the load to time for a certain period, we will see a clear pattern. A spike in the morning and the evening. According, a utility company that generates electricity using a non-VRE source can ramp up its output.

But in the case of Solar power plants, they can only generate all their energies during the daytime. This means during the two spikes of load, power cannot be drawn from solar energy. Contrastingly during the day, there is more supply than demand due to solar energy. The cost of electricity per kWh during the day also reduces. More the supply less the demand.

Read more: Bhadla Solar Park: World’s Largest Solar Plant

The Duck Curve was originally coined by the California Independent System Operator or CAISO in 2012, where they studied and analyzed the load graph in the state of California as has a higher percentage of Solar energy supply.

So What’s the Solution to this?

There has been significate progress in managing and coping with the uncertainty of VRE. I have listed the solutions or I should say, precautions that have been used by the managers.

  • The Past: In the initial stages of VRE installation, somewhat similar problems occurred due to the unpredictability. There were situations where VREs produced more energy than the actual load. To prevent damage due to the overloading of the transmission lines and the distribution grid, the solar and wind sources were simply switched off.
  • The Present: There have been efforts to make the grid more flexible so that it can cope with the variability of renewable energies. Having a more connected transmission system has helped in transferring the excess energy produced to areas where there is high demand.
  • The Future: Some of the future aspects are already being used in the present but not to a level where it can be considered a significant amount.
    • Dispatchable Generation: Using more dispatchable sources like hydropower, biomass, geothermal energy, etc. can help to provide instant supply to the sudden peak of demand
  • Grid Energy Storage: Without wasting or switching off the VRE system, one can store the excess energy for later use. Storage of electricity for a layman might resemble batteries, which is correct. But in this case, it is not limited to Li-ion batteries.
    There are numerous types of Grid energy storage batteries, some are chemical-based, gravity-based, or thermal-based. Grid energy storage batteries at present are being used to store excess grid energy. The first was Pumped Storage Plants, but are very limited due to their geographic dependence. Li-ion batteries are also being used but are still very expensive. Many other types of chemical batteries like Redox Flow batteries have high potential. Gravity batteries similar to Pumped storage are in the works, which uses tonnes of solid weight with the help of a winch to store and release energy. One such type is being researched by Gravitricity in the UK.

Why cannot we just shut down all the Fossil-Fueled Plants

Fossil Fueled power plant

Technological advancements are being made to make the grid flexible enough for it to cope with the variability of a VRE system. Grid Energy storage is the future when we aim to replace fossil-fueled power plants. At present, the advancement of storage technology falls short in the race with the pace of VRE installations. This is also one of the reasons why a 100% renewable energy penetration is not possible. Fossil-fueled has to run at a base-load capacity to minimize cycle cost and also to ramp up generation when there is low renewable output. A cycle cost is a cost of a plant that bears in restating the turbines. Its cost varies depending upon the type of fuel and the time duration from stop to start.

The boiler-based plants have a very high cycle cost and the cost varies depending upon the boiler temperature. Coal-based power plants have the highest cycle cost but many power plants are designed to have less load cycle with a low response time such as hydropower plants.

Ending Note

There are no one or two solutions to the challenges faced with the higher penetration of renewable sources. Every country or region has its own unique characteristics and may vary from place to place. The aim is to provide both a clean and cost-effective source of energy, for which the most optimum solution may also vary. Real-time grid management might be the key in balancing the act of the variable supply to meet the demand.