Over the past two decades, world leaders and scientists have made tremendous efforts to improve energy security and reduce greenhouse gas emissions while meeting rising energy demand. Their initiatives have increased the demand and proliferation for the widespread integration of renewable energy sources into the current electrical grids. Due to the intermittent nature of the most common renewable energy sources, such as solar photovoltaic (PV) and wind systems, their effective integration presents operational and technical challenges associated with network stability and reliability.

Critics often cite the main impediment to integrating renewable energy sources as their ability to generate energy only when the wind is blowing or the sun is shining. They also argue that the lack of a renewables-specific energy storage technology hinders their effective use. However, from a practical standpoint, many of the issues related to the intermittent nature of energy production from renewable sources are frequently overstated and infrequently discussed. The context of this article is, therefore, to highlight the challenges associated with integrating energy production from solar radiation into the grid and some of the solutions proposed to streamline this process.

Challenges associated with Integrating Solar Energy

Variability:

Fast variations in the wind or solar energy output affect the second-to-second balance of total electric supply and demand and the hourly load-following phase of grid planning. To ensure that the total power injected into the grid consistently equals the total power withdrawn, the grid operator typically provides a signal to power plants approximately every four seconds. The grid operator ultimately requires more reserve power and should be ready to go at a moment’s notice to ensure the grid remains balanced since wind and solar energy increase the magnitude of unexpected power generation shortfalls or excesses.

Uncertainty:

Grid operators must maintain an excess reserve running to mitigate against high demand because the output from solar plants cannot be predicted with perfect accuracy in day-ahead and day-of forecasts.

Location Specificity:

Energy from solar radiation is more potent (and therefore more economical) in some locations than in others, and not always in locations with the necessary transmission systems to deliver the energy where it is required.

Nonsynchronous Generation:

A power system’s frequency needs to be maintained close to its nominal value (either 50 Hz or 60 Hz based on the grid). Frequency deviations occur when there is a mismatch between generation and load. Adjusting the output of conventional generators directly impacts the economical operation of these systems and typically reduces the drastic fluctuation of PV array output caused by weather changes. The system’s inertia continues to decrease as PV generation becomes more and more prevalent, which poses a serious threat to the frequency stability of the system.

Low-Capacity Factor:

The average capacity factor, or production in relation to potential, for utility-scale solar PV in 2014 was around 28% (the wind was 34%), according to the Energy Information Administration. By comparison, US nuclear power plants have an average capacity factor of 92% as they almost always produce electricity.The low-capacity factor of variable renewable energy necessitates the use of conventional power plants to operate frequently and recover costs. However, due to the high output of variable renewable energy during peak hours, conventional power plants cannot always meet this requirement.

Solutions for Integrating Solar Energy

Flexible Demand and Storage:

We can manage demand somewhat similarly to supply. Demand response programmes gather customers willing to have their load ramped up and down or time-shifted. From the grid operator’s perspective, the outcome is equivalent to dispatchable supply. There are numerous demand-management tools available, and developers are constantly adding new ones. Energy storage can also aid in balancing the variable supply of variable renewable energy sources by absorbing excess generation when it is inexpensive and sharing it when it is more valuable. In this respect, these methodologies can make variable renewable energy sources dispatchable. Some concentrated solar energy power plants, for instance, have molten salt storage, making their power available 24 hours a day.

Flexible Conventional Generation:

Combining technology and improved practices can make many newer and retrofitted conventional power plants flexible and even more so. Older coal and nuclear plants are relatively inflexible, with prolonged shut-down, cool-off, and ramp-up times. Grid planners may therefore favour non-variable renewable energy source options like small-scale combined heat and power plants and natural gas that are typically more flexible. However, frequently cycling conventional power plants up and down incurs a cost, but it is generally lower than the fuel savings achieved from increased utilization of variable renewable energy sources.

Flexible Rules and Markets:

The National Renewable Energy Laboratory, USA, suggests that achieving access to significant existing flexibility can often be more cost-effective than implementing new physical flexibility options. This involves making changes to the rules and markets governing power plant scheduling, dispatching, reliability assurance, and customer billing. The most recent study by the Regulatory Assistance Project offers an overview of the necessary adjustments to market rules, market design, and market operations.  A recent Department of Energy study describes the best practices for utilities in ‘time-of-use pricing’, which changes the price of electricity throughout the day to encourage demand shifting.

Improved Planning and Coordination:

We should ensure the synchronization of variable renewable energy with appropriately flexible dispatchable power plants and transmission access to enable more fluid sharing of energy within and between grid regions.

The Future of Solar Integration within the Grid

Since current policies have not yet provided enough support for the development of energy flexibility and demand side services, the market development for energy flexibility services has lagged with respect to the technical and environmental opportunities provided by adopting variable renewable energy sources such as solar power. Numerous stakeholders with various perceptions of their needs influence the niche development of energy flexibility services. Innovation trajectories reveal numerous technical, social, legal, and market barriers where policies may play a role in easing the introduction of services. Flexible systems and markets should be in place to accommodate the growing capacity of grid-integrated solar energy power.
There are numerous ways to deal with issues brought on by the variability and unpredictability of energy production via solar radiation. The best ways to address integration vary depending on how different each electric grid is. Because of the generation mix (including the penetration of renewable energy), regulatory structure, presence or absence of markets, operational procedures, and institutional structures, the least-cost options available to individual grids depend on the flexibility of the grid as a whole. As higher penetrations of renewable energy sources are integrated, numerous tools and operational procedures have been implemented by utilities and system operators, both domestically and abroad.

Enhancing Solar Energy System Flexibility

Alternative solutions might necessitate more research and practical experience. For instance, markets could be created to encourage investment in the desired level of flexibility and to elicit the needed flexibility in real-time. Alternative methods of managing reserves may also be feasible and could lead to increased efficiency with greater use of renewable energy sources. Finally, for identifying and evaluating solutions, a better understanding of the least-cost mix of generation sources with higher levels of wind and solar energy as well as a better understanding of the cost of different flexibility options and metrics for assessing flexibility, may be helpful.