Is it available? Or can it be created? The Dutch program “Nieuwe Energie Onderzoek” of Senternovem gave the Utrecht Center for Energy Research the task to find out. Their final report researches whether electricity storage can be of support for a durable energy generation. Considered are three applicational areas:

1. grid coupled storage systems
2. autonomous storage systems
3. electricity storage in transport media

Classification of available systems

Some systems are typically meant to deliver a lot of power in a short timeframe, such as flywheels and capacitors. Other systems can deliver a certain power for hours, such as pump accumulation plants and air pressure systems.
See below an image with many examples, with on its x-axis power and on the y-axis the discharge time.

(Source: Electricity Storage Association)

The different energy options are well explained in the final report, see for that pp 9, 10 en 11.

Besides, there are other ways to characterize storage systems, such as discharge losses when not charging or discharging to load (flywheels), the efficiency of a full charge and discharge cycle, the number of cycles in the total system lifetime, etc. A global overview of the characteristics of different energy storage systems is given in this table (see also the report, page 8).

storage system	capacity	lifetime	self-discharge	efficiency cycle	typical discharge time	costs
unit	kWh	cycli	%/mnth	%	hour	€/kWh
Lead-acid battery	1-40k	200-1200	2-5	75-80	0,5-5	200-900
NiMH & NiCd battery	1-40k	1k-3k	0,5-2	60-70	0,2-1
Lithium battery	15	3k-5k	1	95	0,5	500-2500
NaS battery	1-50k	1k-4k	None	80-85	8	225-400
Zink-Bromide battery	50-500	1k	None	65-75	0,5-3	1500
Vanadium flow battery	50-2k	5k-12k	None	80-87	1-8	100-500
Regenesys flow battery	120k	1k5-3k	None	70-85	1-8	600-3k
Metal-air battery	1-10	100	7-10	40-50	1-8
CAES	-3M	10s yrs	None	50-80	hours	30-115
Pump accumulation	-100M	10s yrs	None	70-85	hours	3,5
Flywheels	1-10	105-107	NB: 30-40/hour	90	NB: sec-min	1500
Supercapacitors	1-100	105-107	20-30	90	NB: sec	10k
SMES	0,1-3	10s yrs	<1	95	NB: sec	300-2k

With SMES is meant Superconducting Magnetic Energy Storage, and CAES means Compressed Air Energy Storage

Possibilities for Storage Systems

When reviewing all of the above mentioned options, see the following table showing the major possibilities lying within the area “Grid-coupled systems”.

application area	storage type	Role (current)	Role (future)
Grid-coupled systems	Pump accumulation	Maintain balance in grid	Maintain balance Introduce durable energy
	CAES	Maintain balance	Maintain balance Introduce durable energy
	Flow batteries		Power security Maintain balance
	Batteries	Power security (UPS)	Power security (UPS)
	SMES		Voltage quality
	Flywheels	Power security	Power security
Autonomous systems	Batteries	Portable systems Remote systems	Portable systems Remote systems
Systems in transport vehicles	Batteries	Start vehicle Alternative propulsion Fuel saving (hybrid vehicles)	Start vehicle Alternative propulsion Fuel saving (hybrid vehicles)
	Flywheels		Fuel saving (buses etc)
	Hydrogen		Alternative energycarrier

From a DTI study in 2004 I took the following figure, in which the development phase of different technologies is given as a function of their system power range and typical application area.

Source, DTI.

Big Scale Grid Coupled Systems

It appears from both overviews that for big scale grid coupled areas (> 100 MW) only pump accumulation systems are available as well as compressed air. In the future, the latter can obtain a bigger part of the market, especially when the adiabatic advanced storage (AA-CAES) is developed further with a higher efficiency.

Middle Scale Grid Coupled Systems

For middle size systems (0,1-10 MW) batteries (lead acid and NiCd) are still superior, but redox-flow batteries provide options as well. For short time power support in this range, flywheels are commercially available as well as SMES systems.

Small Grid Coupled Systems

For the small grid coupled storage systems (10-100 kW) the Ni-MH and Li-ion batteries are of increased importance, mainly for vehicle applications. Flywheels are used for trams and buses. Storage of hydrogen is still in a fundamental research phase.

Small Grid Coupled Systems

For the smallest storage systems (< 1kW) the Ni-MH and Li-ion systems are superior. In cases with high power demand of short duration, the use of super capacitors is appropriate. On this webpage we have an article on super capacitors, about one in development, and when this one becomes available, it will give a good potential for high power and long energy duration storage for use with wind turbine parks as well as potentially widely distributed (one in every home).

Support of the Durable Energy System

What is meant by durable energy system

In the above mentioned report, a durable energy system is defined as an energy system with the following characteristics:

1. clean (i.o.w. the climate will not change or disturbed)
2. safe (without big risks)
3. efficient
4. reliable
5. affordable
6. offers a long term perspective
7. acceptable to Society
8. supports local job opportunities and industrial development
9. does not recognize “lock-in” effects
10. preferably available all over the world

What is missing in this list, in my opinion, is the question of whether the __energy balance__ is positive. See also this article on energy balance. It may be implicit in point 6; where the energy system is required to offer a long term perspective. Also a remark on point 1, that it shall not disturb the climate. This is very subjective. One person might say a wind turbine disturbs the local climate, where the other will say the digging for new Uranium and disposal of used Uranium is disturbing the climate (see also the digging to Uranium.
My opinion about this list is that there are different priorities to be put on the different points measured, to be really meaningful. For instance, items 3 and 5 are not really of importance, and strictly speaking, perhaps not even points 4, 8,9 and 10. If we really focus on durable energy systems, then these shall fulfill the following items: 1, 2, 6, 7 and need definitely have a positive energy balance.

The Applicability of Grid Coupled Energy Systems

Different parties are interviewed and give their view on the applicability of different storage systems.

	Group / company	Directly applicable share	Indirectly applicable
Grid coupled systems	KEMA	Storage is needed to introduce bog wind powers into the grid
	Tennet	Storage is needed to introduce bog wind powers into the grid
	ECN		Storage to solve for imbalance gives cost reduction and indirect added value
Autonomous systems	Philips (and TU/e)	Extend lifetime batteries less toxic materials improve recycling
	TU Delft		Storage brings solar energy nearer
Transport vehicles	CCM	Flywheels deliver a 10 % reduction at buses and trams
	Ecofys		Connect electricity supply with mobility: durable sources will soon feed PHEV cars

Note: PHEV is a Plug-in Hybrid Electric Vehicle

The biggest contribution of electrical storage systems is to be expected in grid coupled systems, and this is also its biggest added value to the support of durable energy systems such as wind turbines and solar energy, that deliver a very variable power.