Environmental Engineering Reference
In-Depth Information
companies Urenco, Pentadyne and others are active in this area with flywheel
energy storage units of 200 MJ, 900 V available.
10.5 Pneumatic systems
Pneumatic ESS rely on dry nitrogen gas as the compressible medium whereby
energy is accumulated. For example, automotive and aerospace systems operate at
hydraulic pressures of 5,000-6,200 psi. Aerospace systems are now in the process
of converting from hydraulic and pneumatic to fully electrified systems. This is
because of the high containment system mass and cost associated with pneumatic
and hydraulic storage, whereas for electric systems the mass and cost are now
more competitive. A hydraulic actuator system on the A320 may weigh 200 kg, but
taking account of all plumbing, fluids and accumulators, it comes out to some
540 kg of closed system mass. Electric actuators have higher mass, but in a closed
system can be competitive with, or exceed, the mass and cost of hydraulic/pneu-
matic systems. For this reason further discussion of pneumatic systems will not be
pursued in this topic.
10.6 Storage system modelling
ESS can be modelled using high level metrics such as specific energy and power
densities. These approaches prove very beneficial in sizing and costing studies. For
electrical system, especially traction application, performance on a much more
detailed model is necessary. Lumped parameter models for various energy storage
technologies provide good results to real world experience. However, modelling is
also required to assess the state of charge (SOC) and state of health (SOH) of
storage components. Even more detailed models are needed for assessment of
available energy and fault prognostics. Some of the more accepted and promising
techniques are described in this section.
10.6.1 Battery model
Early attempts at modelling lead-acid batteries were empirical and based on
laboratory measurements of current, voltage and temperature. Figure 10.43 illus-
trates an example laboratory test for high power discharge and charge character-
ization. The battery is first conditioned by an overnight float charge to stabilize the
battery at or near 100% SOC. The pulse discharge testing is done at each SOC
followed by a timed discharge at constant current until a prescribed amount of
capacity is discharged and the testing repeated.
Battery modelling based on the pulse discharge and charge method is useful for
a coarse model definition consisting of battery internal resistance,
R
int
, double layer
capacitance,
C
dbl
, and bulk storage,
C
stor
. Each of these parameters is dependent on
battery charge/discharge history, present SOC and temperature, as well as current
magnitude. At the beginning of the current discharge pulse, the battery terminal
