The grid must maintain a full balance between generation (supply) and load (demand) at all times. In small-scale networks, imbalances occur more often and are generally more difficult to manage when compared to large-scale networks that often incorporate interconnector links.
Any imbalance situation between generation and load must be counteracted instantly in order to prevent wider issues, which can include a grid blackout in a worst-case scenario. In order to cope with larger amouts of distributed energy, networks should consider introducing or expanding energy storage to meet higher grid stabilizing demands and energy surpluses.
All power-generating sources, renewable and non-renewable, are characterized by specific inherent technology and operating features. In classic conventional electricity systems such as coal or nuclear, plants provide baseload energy because their operating output levels can only be varied gradually. Much faster-reacting natural gas-fired power plants are therefore necessary for managing rapid network demand fluctuations.
Utilities recognize and address the characteristics of each energy technology. For example, in Europe wind farms produce the bulk of their yield during autumn and winter, whereas PV plants yield best in summer and inherently do not produce during night hours. Combining these two complementary sources in an electricity network offers some grid stabilization and other benefits without actual storage.
The Stability Phenomenon
Biomass plants that burn sustainable dry fuels like wood pellets fit into the baseload category, whereas liquid biofuels can add to both baseload and fast-response variable capability. Geothermal has also been proven to be both a baseload and responsive technology. Combining biomass and geothermal with wind and solar in a single network inherently adds storage capacity through the contained energy in the biomass and geothermal resources.
Public discussions about network stability and grid disturbance often overlook the fact that fluctuations in demand are an inherent system phenomenon independent of the generating resource. There will always be demand differences during daytime and night hours, on work days versus weekends, between months and seasons, and during special events like electricity demand peaks during football match breaks.
Wind power is wrongly blamed as a primary electricity network disturbance that endangers stability. PhD research conducted under specific Dutch conditions showed that “natural” network demand fluctuations are far greater compared to the variability caused by wind power. The study also showed that a need for energy storage would only arise when wind capacity, as a share of total generating capacity, exceeds around 33 percent — it currently stands at about 4 percent. Nevertheless, below we explore some potential energy storage applications that could work in parallel with wind power.
Rather rudimentary “island” wind-diesel systems comprised of one or more fixed-speed stall-regulated turbines and diesel generators do not incorporate energy storage, and wind surpluses are simply “burned” via a dump load. More sophisticated modern wind-diesel systems that include modern (active) pitch-controlled variable-speed wind turbines do not require dump loads.
A highly sophisticated island power plant developed by German turbine supplier Enercon and Norwegian energy company Norsk Hydro during 2003/4 represents the other end of the technology spectrum. This unique stand-alone wind and hydrogen renewable energy system was installed in Utsira municipality, Norway in the winter of 2004/2005. The plant produces hydrogen through an electrolyser with excess wind energy. When there is insufficient wind, it releases the stored energy again as electricity via a fuel cell and hydrogen combustion engine. In contrast, last year German utility E.ON introduced a power-to-gas unit, which feeds hydrogen into a regional natural gas system.
However, this technology does have a major disadvantage. When converting wind electricity via electrolysis into hydrogen and back into electricity via a fuel cell, there is a huge cumulative efficiency loss estimated to be in the range of 45 to 55 percent.
Enercon’s containerized stand-alone system contains (up to) 900-kW wind turbines, in-house flywheel storage technology and a power management system, plus a third-party master synchronous machine, diesel-generator sets, and battery storage.
The benefits of flywheel storage include minimal efficiency loss and rapid response time, but there has yet to be a large-scale commercial breakthrough for these systems mostly due to cost. Battery storage, too, is still expensive.
A potentially interesting battery option is to reuse discarded electric car batteries in a wind power and PV energy storage application, which is currently being studied by Germany’s Münster University. It might also be possible to use surplus wind power for charging car batteries on a large scale, which together with smart grids might create a new energy storage form (vehicle-to-grid storage).
In mountainous regions with favourable geographic conditions for creating large elevated water storage basins, it may be possible to combine large-scale coal/lignite/nuclear power generation with pumped hydro storage for a long-term storage solution. In situations where electricity supply exceeds demand, surplus energy is used for pumping water from a lower level into the higher-level basin. When power demand exceeds supply, water from the basin is released to driving a hydro-turbine. However, its combined efficiency loss is at least 15 to 25 percent, and some sources claim cumulative losses of 30 percent and higher.
Compressed air is another storage option that has been studied for many years. From an energy efficiency point of view, adiabatic compression, which takes place without the exchange of heat, is the preferred process. When compressed air is stored in underground enclosures such as caverns, there might be competition with alternative uses including carbon capture and storage (CCS) or natural gas storage.
In early 2013 GE unveiled a 2.5-MW wind turbine that incorporates battery storage capability in the power converter’s DC bridge. Here, AC generator power is converted into DC and back to 50Hz/60Hz AC grid power. The solution offers short-term energy storage to help ensure reliable, predictable power, according to GE. The actual installed storage capacity depends upon local grid/market conditions, including whether or not utilities are prepared to pay for increased grid stability with voltage regulation.
Energy storage requires careful analysis, which should at least consider grid stability requirements, energy efficiency loss, and lifecycle-based investments set against overall cost savings.
Lead image: Turbines and Transmission via Shutterstock