Grid Access of Renewable Energies - problems and solutions

The production of electricity from renewable sources like sun or wind fluctuates due to their intermittent character. This inevitably leads to an also fluctuating feed into the grid as shown in the right hand side figure. These fluctuations, though, are only predictable to a certain extent.

There are short fluctuations in demand on the consumer side, too. Production and consumption, however, need to be balanced constantly in electricity grids to prevent deviations from the desired grid frequency and voltage.

This is done with the help of dedicated power plants (balancing power plant) , which can increase or decrease their power output at very short notice. Furthermore, most conventional plants are run in a way that they can respond to a short notice request for more energy.

The intermittent nature of wind and solar energy creates a number of problems in respect to its integration into the existing grid-structure: the energy suppliersī load management orients towards certain patterns in consumersī demand. In contrast to conventional power plants, like coal or nuclear power stations, which can schedule their production following this demand pattern, renewable energy sources can not as easily be used as a schedulable element of a supplier's power plant outfit.

A rapid change of the meteorological situation, like shades on the solar PV panels or low winds, can cause a sudden drop in renewable energy production (see also figure). To keep the grid stable, pumped-storage hydroelectric plants or gas turbines need to be started up. They can balance load drops within seconds to minutes. These reserves are normally used to cover unforeseen short-term fluctuations on the demand side or peak-loads. This balancing power, though, is based mainly on fossil fuels and expensive.

If there is a wind surplus that exceeds the actual demand and cannot be fed into the grid due to an existing load from scheduled production wind turbines are shut down by the grid operator for safety reasons even today.

When integrating renewable energy systems with a high installed capacity (like offshore wind farms) into existing grids there is the additional problem of an insufficient expansion of those in the past. Regions with a high potential of wind energy or solar PV energy often have a poor electricity grid, which cannot take up large amounts of power. Furthermore, the grid is usually planned unidirectional, i.e. power is distributed from large central power plants "down" to the consumer, passing a number of levels. A feed of the grid from a lower up to a higher level is not always possible without works.

Operator of renewable energy systems as well as electricity companies are working on a solution for a better integration of renewables into the grid. One promising option is the storage of wind and solar PV energy with hydrogen. While feeding most of the energy into the grid, a certain proportion is being used for the production of hydrogen which can then later be converted back into electricity using a fuel cell. This gives the chance to decouple production and consumption and allows a more flexible energy management: by combining hydrogen with fuel cells renewables can smooth their energy output, drops in the production profile can be balanced and so the integration of renewable energy as a schedulable component of today's electricity supply becomes possible.

• the production of electricity from hydrogen can start within seconds and short-term supply lows can be balanced without using external fossil balancing energy. But that is not all: wind/solar PV systems can become balancing energy producers themselves and could deliver peak load electricity for unpredicted load peaks in the grid
• by producing hydrogen this energy is taken out of the system and less load needs to be managed and taken on by electricity grid; this could serve as a sensible alternative to costly grid extensions and refurbishments
• the entire production potential, which exceeds the current demand and could not be fed into the grid, is used
• hydrogen gives the option to deliver energy to markets outside the traditional electricity market, namely the transport sector and private homes, but also to serve as a raw product for the chemical industry (see section in Renewables)

Problems of fluctuation in production are also minimised when single renewable sites are integrated into larger systems and are spatially distributed. Research has shown that short-term fluctuations in production of single wind or solar PV sites are significantly compensated for if they stand far away enough from each other. The integration of spatially distributed sites also slows down the temporal variations. Similar averaging effects over prolonged periods of time can be observed at a region-, nation- or Europe-wide distribution of wind and solar PV parks.

There is also software available that can approximately calculate the renewable energy production of sites so that power plant operators can adjust their production schedule to a certain extent.

Another method to match general energy demand and renewable energy production is to influence the energy demand by changing consumer habits. Such demand-side-management focuses on governmental or private industrial consumers in particular and is realised through price structures.

Continue with Modern Energy Management with Offshore-Wind Farms and Hydrogen Storage