The amount of generation from renewable energy-based sources has increased considerably over the years. It is expected to increase further in the coming years in line with the sustainable development goals that aim to provide access to affordable, reliable and sustainable energy for all by 2030.
Energy is a major contributor to climate change, accounting for around 60 per cent of the total global greenhouse gas emissions. Reducing the carbon intensity of energy is, therefore, a key objective of long-term climate goals. As such, the share of renewable energy in the global energy mix is expected to increase substantially. India alone aims to install 60 GW of wind power capacity and 100 GW of solar power capacity by 2022, which is a significant jump from the current installed capacities of 24.7 GW and 4.7 GW respectively.
However, increased renewable energy-based generation has its own set of challenges. The biggest challenge is the intermittent nature of renewable generation, which leads to integration issues with the grid. In this respect, the deployment of microgrids, energy storage systems and automation technologies can play an important role, as they enable greater integration of renewable energy while ensuring system safety at all times.
Renewable energy sources
The typically used sources for distributed generation and microgrids are solar PV, wind, hydro and biomass. While all of these provide clean and green energy, each has its own advantages and disadvantages. Solar energy is the most abundant and widely available source of energy. Solar PV panels are easy to install and are noise free. Moreover, their cost has already declined significantly with advances in technology. Wind has enormous potential with low associated operational costs and is ideal for remote locations, same as solar PV. Biomass is again an abundant source of energy, which can be used in various forms like biogas and ethanol, and helps use waste. Hydropower is the most flexible and reliable source of renewable energy. At times when power consumption is low, water flow can be easily reduced to balance the supply.
A major drawback with respect to solar and wind energy is their intermittency and unpredictability, which make them less reliable. Moreover, wind speeds can be too low to support a wind turbine, making it a location-specific resource. While solar energy is available almost everywhere, biomass is not; and transportation of biomass to the plant can be critical. Access to biomass “raw” material is essential to operate continuously. Also, the process of conversion of biomass into heat or electricity is more complex, costly and the time taken to supply to the grid is more as compared to solar PV or wind. Hydropower, on the other hand, is expensive, highly dependent on the availability of water and severely affected by events of drought. A judicious choice needs to be made regarding the renewable energy sources to be used.
Issues in renewable energy integration
The uncertainty and variability of renewable energy generation can pose serious challenges for grid operators. Greater flexibility in the system may be needed to accommodate the supply-side variability. For instance, in cases where renewable energy generation increases when load levels fall (or vice versa), additional actions are needed to balance the system. The system should have sufficient resources to accommodate significant ramp-ups/downs in order to maintain the balance.
When wind or solar generation is available during low-load levels, conventional generation plants may need to back down to their minimum generation levels. Utilising all of the renewable energy would require conventional power plants to meet only the net load (demand minus renewable energy), which would require them to operate at a low-output level. During periods when the net load changes very rapidly, the operators must ensure rapid ramping-up/down of generation. Moreover, frequent turn-offs of conventional power plants can lead to equipment damage and decreased efficiency from thermal stress because of changes in output. Also, such generation typically turns out to be expensive since the generation plant is used only for a short time. Further, most renewable energy sources are location-specific and the energy thus generated may need to be transported over considerable distances.
Microgrids as a solution to renewable energy integration challenges
Microgrids provide ways to use renewable energy in an efficient way. A key challenge in the deployment of renewable sources is their intermittent nature. Large-scale renewable-based plants pose bigger challenges to the grid with a higher risk of creating an imbalance in the system. Microgrids with energy storage and automation and controller systems keep the system stable even with the variable nature of renewable energy. Such systems offer very quick absorption and discharge. Renewable energy can also be made reliable by using hybrid systems where more than one source of energy is used, which can complement each other. Clustering of microgrids having different renewable energy resources such as solar, wind, biomass and small hydro also helps in stabilising the system. Microgrids not only enable the maximum integration of renewable energy, but also provide for greater load flexibility and better demand management. Also, microgrids make it easier to ensure a higher share of renewables in the generation mix, since they do not require any changes in the transmission network.
Energy storage systems
Energy storage systems and controls can play a significant role in addressing the issues of grid stability in renewable energy generation. Energy storage systems are an attractive option for both grid-connected and off-grid renewable energy systems.
In order to utilise the energy from renewable sources, a power conversion system is necessary. A voltage source is required to synchronise the distributed generation, which can be a back-up generator or an energy storage system. Using a diesel generator involves CO2 emissions; hence energy storage systems are a much cleaner means of synchronising the renewable energy source.
Energy storage also helps in streamlining the intermittent power flow from renewable energy sources. It captures the excess energy generated from renewable energy sources during low demand times in order to despatch it during high demand times (also known as renewable energy time-shifting). Storage, moreover, helps in mitigating rapid changes in generation from renewable energy sources, which could be due to wind speed variability affecting generation or lower solar generation due to clouds. In such cases, the stored energy offsets the outage (also known as renewables capacity firming). In the case of grid-connected microgrids, energy storage systems can help with islanding, regardless of whether the renewable energy source is active or not.
Energy storage separates generation from demand and thereby increases both grid flexibility and performance. Storage can reduce outages, lower pollution from fossil fuels, and eventually enable a complete reliance on wind and solar. Storage-based renewable energy projects also have the potential to be used as peaking power plants.
The various storage options available are batteries (lithium-ion, sodium-ion, vanadium redox, etc.), flywheel energy storage, pumped hydro storage, compressed air energy storage, thermal storage, etc. While a number of options are available for energy storage, they are not widely used owing to their high costs. Other barriers in the deployment of energy storage systems include the lack of awareness, appropriate business models and a supportive policy framework.
The way forward
To ensure that renewable energy makes a greater contribution in meeting energy demand and providing access to the unelectrified, merely installing renewable energy-based generation systems will not suffice. Installation may not translate into successful integration with the grid without enabling technologies in terms of storage systems, grid stabilisers and automation systems. This is necessary for maximising the integration of renewable energy and enabling better demand and supply management. However, using a storage system considerably increases the cost of the entire system and hence deters its adoption. There is a need to make energy storage options more competitive. At the same time, there is a need to demonstrate the application of storage systems so as to increase awareness and understanding, and thereby promote efficient development of renewable energy technologies as well.