1.4.3.Characteristics of energy storage techniques
The main characteristics of energy storage systems are the following [1]:
storage capacity;
available power;
depth of discharge;
discharge time;
efficiency;
durability (cycling capacity);
autonomy;
costs;
feasibility and adaptation to the application;
self-discharge;
specific energy;
energy density.
Storage capacity is the quantity of available energy in the storage system after charging or the total energy stored Wst (Wh), which is superior to that actually retrieved (usable energy) Wut (Wh).
available power represents the maximum power of charge or discharge, Pmax (W).
Depth of discharge represents the percentage of the useful capacity. For example 20% depth of discharge means 80% of the energy capacity remained.
Self-discharge time is the time in which a fully charged, noninterconnected storage device reaches a certain depth of discharge.
Self-discharge represents the portion of the energy that was initially stored and which has dissipated over a given amount of non-use time.
Discharge time is the time needed to extract the stored
energy. It is given by the ratio between the total energy stored and
maximum power of discharge,
(s).
Energy efficiency is the ratio between released energy (energy
output) and stored energy or (energy input)
.
An image of storage techniques distribution as a function of energy
efficiency and lifetime is given in Fig. 1.
Figure 1. Distribution of storage techniques as a function of energy efficiency and lifetime [1].
Durability or cycle life is the maximum number of consecutive charge–discharge cycles a storage unit can undergo with maintaining the other characteristics within limits.
Autonomy is
the maximum duration the storage unit can continuously release the
energy and is given by the ratio between the usable energy and
maximum power of discharge,
(s).
The total costs of a storage system should not exceed the total gains. The investment cost can be estimated by using the following equation [1]:
(2)
where: C1 ($/kWh) – unit cost per total energy capacity;
C2 ($/kW) - unit cost per discharge power;
Pd (kW)– discharge power.
The operational costs (maintenance, energy lost during cycling, aging) are supposed to be proportional to the investments costs (in the order of 40% of investment over 20 years for electricity production systems) [1].
A distribution of energy storage techniques as a function of investment costs per unit of power or unit of energy is given in Fig. 2.
Figure 2. Distribution of storage techniques as a function of capital cost per unit of power and capital cost per unit of energy [1].
An energy storage system should be appropriate to the type of application in order to operate efficiently. Figures 3 and 4 show the distributions of storage techniques as function of their main characteristics which are useful in their selection for a given application.
Specific energy represents the amounts of energy accumulated per unit of mass of the storage unit.
Energy density represents the amounts of energy accumulated per unit of volume of the storage unit.
Figure 3. Distribution of storage techniques as a function of energy stored, time of discharge and power output [1].
Figure 4. Distribution of storage techniques as a function of discharge time and power output [1].