Page 166 - Hybrid-Renewable Energy Systems in Microgrids

150 Hybrid-Renewable Energy Systems in Microgrids

4 Characteristic features of ESS

Some of the features of ESS, which will be crucial in selection and analysis of ESS for
hybrid systems, are explained below [10,11]:
1. Energy density: Energy density gives the amount of energy the storage system is capable of
holding per unit volume/mass. It can also be more fittingly known as specific energy and is
3
expressed in Wh/m or Wh/kg. In physical systems like hydro and compressed air systems,
the energy density is given by the capacity of the storage area, that is, reservoir/cavern ca-
pacity and in batteries this is given by the mass of the electrolyte held inside it. Thus higher
the energy density of the storage, higher is its ability to store/deliver power over longer
periods. However, it is to be noted that this is just the capacity and is no way related to the
energy conversion efficiency and likewise no single storage system can be pointed out as
having best specific energy and it is varying on other parameters including applications and
operating conditions. According to Peukrt’s law, some batteries (especially lead acid) suffer
from degradation in energy capacity when subjected to longer discharges.
2. Power density/power rating: Power density gives the amount of power (fixed voltage of the
cell times the current) the storage system is capable of delivering per unit volume/mass. It
can also be more fittingly known as specific power or volume power density and expressed
3
in W/m or W/kg. This gives an idea of how powerful your storage can be and it has no
limitation in the aspect of holding capacity. SC, which have lesser energy capacity, have
power capacities of the range of 10–100 times greater than batteries of same capacity. Also,
high power storages will charge/discharge faster as they are capable of absorbing/delivering
higher powers in unit time.
3. Energy costs: Any system is successful only when it proves to yield profits exceeding its
investments. Storage systems indifferently are subjected to severe scrutiny and criticism re-
garding their high costs and economic feasibility. Hence both the capital costs invested and
the maintenance costs endured during its operation stages are to be considered. In batteries
though the investments costs are low, they suffer from frequent maintenance and replace-
ment costs over the course of a project. Thus overall costs to be incurred over the life of the
project are to be carefully estimated. Levelized cost of energy is another important factor,
which includes the net present value of the storage system taking into account the inflation
rates and discount rates.
4. Efficiency: The ratio of the amount of energy stored to the ratio of the energy delivered back
to the grid can be calculated as the round-trip efficiency of the storage system. It is an impor-
tant parameter for comparison and determination of usefulness of ESS. The lesser the losses
suffered in the charge–discharge process, higher will be the round-trip efficiency. The major
aim of resorting to hybrid power systems is to promote energy conservation and improve
energy efficiency. Hence grid operators demand at least about 80% round-trip efficiency as
a competitive benchmark for ESS.
5. Discharge time: How quickly an energy storage system can respond to deliver at rated pow-
er is its response time and how long it can maintain this output is defined as its discharge
time. Response times of ESS determine the rapidity with which the system can be brought
into action, which is crucial for grid integrating applications. Discharge times will be criti-
cal for applications like load shifting and backup supports where a longer steady power
discharge will be the needed.
6. Lifetime: Period of time over which energy storage continues to deliver energy determined
in either years or number of charge–discharge cycles (cycle life). Physical storage systems
have extremely long life and are well suited for seasonal storage of electricity also. Lifetime