## Investigation of energy storage options for sustainable energy systems.

##### Abstract

Determination of the possible energy storage options for a specific source of
energy requires a thorough analysis from the points of energy, exergy, and
exergoeconomics. The main objective of this thesis is to investigate energy storage
options for sustainable energy systems. A technology description and illustration of
concerns regarding each system is presented. Moreover, the possibility of
implementing each option into different sources of energy is investigated. Thus,
integrated energy systems are developed, utilizing energy storage options with the aim
of achieving more efficient systems.
Energy and exergy analyses are performed for three novel, integrated renewable
energy-based systems. Energy storage methods investigated here include hydrogen
storage, thermal energy storage, compressed air energy storage, and battery. Solar,
wind, and biomass are the energy sources considered for the integrated systems. In
this research, a discussion on various energy storage systems is presented, and the
potential of each storage option in the current and future energy market is studied.
Each of the integrated systems is described and its operating strategy is presented.
The components of the integrated systems are first modeled to obtain their operating
characteristics. The energy, exergy, and exergoeconomic equations are applied to the
components to calculate the rates of energy and exergy flows. The efficiencies are
subsequently calculated. The results of energy and exergy analyses are combined with
exergoeconomic equations to report the unit exergy cost of flows in the components.
System 1 consists of a PV system, a water electrolyser and a fuel cell to generate
electricity for a house. Hydrogen and thermal energy storage are considered as the
storage options. The results show that the capacities of the components depend on
weather data and electric power demand. In System 1, the PV electric power output
exceeds demand during months with high-solar irradiance. The results of a case study
based on the weather data in Toronto, Canada, and the electricity demand pattern of a
Canadian house (5.74 kW maximum demand) are presented. The photovoltaic system
capacity and the electrolyser nominal hydrogen production rate are 37.17 kW and 4.5
kg/day, respectively. The economic investigation of the hybrid system reports an
average cost of electricity of 0.84 $/kWh based on 25 years of operation. The optimal nominal capacity of the fuel cell is found to be 1.5 kW, according to the optimization
results. The optimal exergy efficiency varies from 9.91 to 9.94%.
System 2 consists of a wind park, a PV-fuel cell and a biomass-fuel cell-gas
turbine system. This integrated renewable energy-based system is developed for
baseload power generation and utilizes wind, solar and biomass energy resources. For
a 64 bar compressed air storage system, and a 36 bar gas turbine inlet air pressure,
356 wind turbines are required. The lower the pressure difference between the
compressed air in the cavern and the gas turbine inlet air pressure, the fewer the
number of wind turbines required in the Wind-CAES system. The results also show
that 5.4×105 PV modules (covering 0.66 Mm2 of land) are required to generate 5 MW
of baseload electric power. Optimization of System 2 provides a range of optimal
points at which the exergy efficiency and the total purchase cost of the system are
optimum. At an optimal point, the overall exergy efficiency of the integrated system is
reported as 36.85%. At this point, the optimal values of compression ratio, gas turbine
expansion ratio, and CAES storage capacity are 8, 6.5, and 240 h, respectively.
System 3 consists of a biomass gasifier integrated with a gas turbine cycle
(biomass-GT). As another sub-part of System 3, a PV-electrolyser module is
integrated with a compressed air energy storage system. The overall hybrid system
supplies 10 MW baseload electric power, and 7730 MWh thermal energy. The PV is
accountable for 56% of the annual exergy destruction in the hybrid system, and 38%
of the annual exergy destruction occurs in the biomass-GT system. The overall energy
and exergy efficiencies of System 3 are 34.8 and 34.1%, respectively. The hybrid PVbiomass
system is sensitive to some parameters such as the steam-to-carbon ratio of
the biomass gasifier, and the gas turbine inlet temperature and expansion ratio. A 29%
increase in energy and exergy efficiencies is reported with the increase in SC from 1
to 3 mol/mol. The related specific carbon dioxide emission reduction is from 1441 to
583 g/kWh.