Page 354 - Renewable Energy Devices and System with Simulations in MATLAB and ANSYS

Batteries and Ultracapacitors for Electric Power Systems with Renewable Energy Sources 341

PV output
power Output
power

= =
~ ~
Unidirectional
PV panel Utility grid
DC–DC converter Facility
load
Energy storage
output power DC/DC Energy
converter storage

FIGURE 13.29 Illustration of a PV system integrated with energy storage (Based on the concept pro-
posed in Esmaili, A. and Nasiri, A., Proceedings of IEEE Annual Conference of Industrial Electronics
(IECON), pp. 3957–3962, Portugal, 2009.)

13.6.5 Microgrids and Islanded Grids
One of the main challenges for microgrids is to provide a stable and reliable power system for
grid-connected and islanded operation. Voltage and frequency stability are major concerns, espe-
cially if generation includes renewables, such as solar and wind, and the load demand is variable.
EESs can provide voltage and frequency stability for short-term and long-term applications.
Practical large installations have been demonstrated in combination with wind turbines and PV
systems, and ongoing research is conducted for optimizing the location and size of microgrid
EESs in order to minimize the system cost and energy loss, while ensuring improved system
reliability.
In microgrids, EESs can be employed, distributed, or centralized ESSs. In a distributed installa-
tion, EESs are capable of actively managing and supporting local loads. A centralized configuration
for ESSs is typically employed in smaller microgrids with critical loads, where a main backup power
supply is required in case of power outages [98–100].
Energy storage technologies have been developed also in order to meet utility-level require-
ments. Some applications, such as load shifting for duration of hours, require an ESS with high
energy, while voltage and frequency regulation need high power density. Lifetime, response time,
DOD, and efficiency are other significant factors for the selection of energy storage type for a given
application.
Typical electrochemical batteries have a lifetime in the range of 5,000–10,000 cycles, while
pumped hydro, compressed air, flywheels, and capacitors are rated at 10,000–100,000 cycles (see
Table 13.2). On the other hand, pumped hydro and compressed air are classified as having a rather
slow response time in order of minutes, while batteries, capacitors, and flywheels are characterized
as fast responders in order of fractions of seconds (Table 13.4). In terms of technology maturity
and commercial availability, pumped hydro, lead-acid batteries, and sodium sulfur batteries have
the highest rankings.