Flexible storage solutions needed to push renewables penetration

WORLDWIDE: A mix of flexible storage solutions will be needed to stabilise grids and prevent outages as the power system becomes more variable, according to a new research paper.

How flexibility sources may interact in the electricity supply system, according to DNV GL

Increases in penetration of renewable energy sources, power consumption and electrification of transportation, and the heating and cooling sectors, will increase variability and uncertainty in the power system, DNV GL explained.

Therefore, a ‘smart grid’ is needed in which renewable energy sources, flexible consumers and energy storage systems cooperate, researchers stated.

In most cases, single flexibility services will not provide a sufficient business case for deployment. They face technological barriers or have inherent weaknesses when deployed in isolation. Multiple flexibility resources will be required, with the best combination chosen for specific applications.

Flexibility could be increased by varying input from conventional energy sources, varying output on the demand side, deploying electrical energy storage systems (EES), reinforcing and expanding transmission and distribution networks, or by curtailing renewable energy sources.

These solutions face technological, cost, and regulatory barriers, however.

In its white paper, ‘Flexibility in the power system: The need, opportunity and value of flexibility’, DNV GL, suggested flexibility could be provided by:

Varying energy input from conventional generation units such as nuclear and coal-fired plants

Improving the flexibility of existing generators can help to deal with high levels of variable renewable energy inflow and increased peak demand events in the system, DNV GL wrote. This can be improved by increasing ramp rates and decreasing start and stop times of the generators, thereby negating the need for additional, expensive peak power generators.

As renewables achieve higher penetration in the energy mix, conventional sources will more often operate at part-load when renewables' output is high, or only at certain times of the year, driving down their utilisation rate, the researchers suggested.

This would be "relatively costly" and would lead to "issues related to the emission of greenhouse gases", however.

Increasing demand side management (DSM) or demand response (DR), in which the utility or retailer and consumer, respectively, controls demand

This is possible through time-based electricity tariffs and smart meters, for example.

However, consumer behaviour may render incentives-based unreliable, DNV GL concluded.

Large-scale flexibility on the demand-side would also necessitate development and investment in sensors, controls, communications and IT infrastructure – complicating the business case for deployment.

An EES which coverts electrical energy into another form of energy and stores energy internally

This could be electrical, electrochemical, mechanical, chemical or thermal storage.

Although it varies between different technologies, EES typically has a limited storage buffer, DNV GL stated, meaning continuous discharge is not possible. However, such systems have very fast response, are falling in cost, and can be scaled to provide both local and large-scale flexibility. Some systems, like batteries, are also fast to develop and install, the researchers added.

However, EES systems require a conversion of electricity, which means a loss of energy, have limited energy capacities and discharge durations, and integration is subject to "lagging" technical standards which currently prevents their deployment, DNV GL concluded.

Reinforcing and expanding transmission and distributing networks, including across borders

Control mechanisms are capable of managing rapid changes in the loading of transmission lines and, increasingly, to better direct and balance power flows, the researchers stated.

Reaping the full benefit of supranational grid connectivity requires high investment costs and the connected electricity markets to be "coupled and governed under the same regulatory framework", however.

Curtailing renewable energy when the source’s capacity is larger than that of the grid connection

The curtailed energy is lost and cannot be retrieved later on, decreasing the share of electricity produced by renewable sources.

RES curtailment is therefore "undesirable from a societal point of view", DNV GL wrote. In most cases, curtailment would only be a temporary measure and would also increase the average cost of renewable energy.

The paper’s lead author, Jos van der Burgt, senior researcher of power and renewables, in DNV GL's technology and research division, said: "Societal trends like the increased uptake of renewable energy and electric vehicles demand greater flexibility in our power systems, but they also open up new options for flexibility in the grid.

"And while energy storage – particularly through high-capacity batteries – have so far attracted most of the headlines, there are other possibilities such as demand-side management and network reinforcement.