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Essay on Energy Storage System
Essay Contents:
- Essay on the Introduction to Energy Storage System
- Essay on the Characteristics of Energy Storage System
- Essay on the Storage of Mechanical Energy
- Essay on the Electrochemical Energy Storage (Battery)
- Essay on the Thermal Energy Storage
- Essay on the Storage System for Solar Plants
Essay # 1. Introduction to Energy Storage System:
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An energy storage system is required to meet the difference between the energy demand by the customer and energy supply by the power plant. It is very important for solar, wind energy and other renewable forms of energy because these are intermittent in nature.
The energy storage systems can be classified as follows:
i. Mechanical energy storage. Flywheel energy storage, pumped hydro power storage and compressed air energy storage.
ii. Thermal and thermochemical energy storage, and
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iii. Electrical energy storage. Battery storage and magnetic field energy storage.
Essay # 2. Characteristics of Energy Storage System:
An energy storage system is characterised by the following performance characteristics:
1. Storage capacity.
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2. Energy density.
3. Charging and discharging rate.
4. Storage duration.
5. Storage efficiency.
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1. Storage Capacity:
An energy storage system consists of a reservoir which contains a storage medium. Storage capacity is the maximum energy quantity stored in one cycle.
A hot water storage system has the following capacity:
E = m Cp (ti-tf) [kJ]
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where,
m = mass of water [kg]
Cp = 4.186 kJ/kg-K
= Specific heat of water
ti = initial temperature [°C]
tf = final temperature [°C]
2. Energy Density:
Energy density of a storage system is the energy stored per kg or m3 of storage medium:
e = E/m [kJ/kg]
The energy density of various storage media and systems are given in Table 20.1.
3. Charging and Discharging Rate:
Charging rate is the amount of energy supplied to the storage system per unit time. The discharging rate is the amount of energy withdrawn from the storage system per unit time.
4. Storage Duration:
The operating cycle of an energy storage system consists of:
i. Charging from an energy source,
ii. Energy storage in the system,
iii. Withdrawal of energy from the system and supply to the consumer.
The total storage cycle time:
τ = τc + τs + τd
where,
τc = charging duration
τs = storage duration
τd = = discharging duration
A hot water storage system may have storage duration for 8 to 24 hours. In other solar heating plants, storage duration may be for a few months.
5. Storage Efficiency:
The storage efficiency may be defined as the ratio of discharged energy to charged energy.
where,
Ed = Discharged energy [J]
Ec = Charged energy [J]
ΔEl = Loss of storage energy [J]
Essay # 3. Storage of Mechanical Energy:
1. Flywheel Energy Storage:
It consists of a rotating mass (flywheel), a motor generator set and an enclosure. The rotating mass acts as storage medium and energy is stored as kinetic energy of flywheel. During charging the motor adds energy to the flywheel and during discharging, the energy is generated to cover the peak load. Various shapes of flywheel include a circular disc or a circular ring or a thin long shaft.
The flywheel energy:
where,
k = shape factor from 0 to 1
σ = allowable yield stress of flywheel [N/m2]
ρ = density of material [kg/m3]
The value of energy density depends upon the size, shape and material of flywheel. A disc of uniform strength has the shape factor equal to one. Titanium gives the highest energy density of the value 220 kJ/kg. Composite materials are used for higher energy densities. A flywheel made of circular rings of glass/epoxy composite can give an energy density of 450 kJ/kg and graphite/epoxy composite ring a value of 540 kJ/kg.
2. Pumped Hydro Power Storage:
A pumped hydro power plant as shown in Fig. 20.2 consists of an upper reservoir and a lower reservoir connected by a pressure pipe line through a pump-motor-turbine-generator set. When electrical power demands are low, the water is pumped from lower reservoir to higher reservoir by spare power. During peak load, the potential energy of stored water is converted into its kinetic energy and is used to generate extra power by turbine-generator set.
The pumping power required:
where,
g = 9.81 m/s2 = acceleration due to gravity
ρ = Water density [« 1000 kg/m3]
Qp = Pump water discharge [m3/s]
hp = Pump head [m]
ƞp = Overall pump-motor efficiency
The turbine power available:
PT = g ρ QT hT ƞT ƞG [W]
where,
QT = Turbine water discharge [m3/s]
hT = Turbine head [m]
ƞT = Turbine efficiency
ƞG = Generator efficiency
The overall efficiency of pumped hydro storage power plant:
3. Compressed Air Energy Storage:
A compressed air energy storage system as shown in Fig. 20.3 consists of the following components:
i. An air compressor (C)
ii. Motor-generator set (M/G set)
iii. Air cooler
iv. Underground air reservoir
v. Gas turbine (GT)
vi. Combustion Chamber (CC).
When electrical demand is low, the spare power is used to drive air compressor with the help of motor. The cooled air is stored in the underground reservoir. When demand is high, the compressed air from the underground is used to drive gas turbine to meet the peak loads.
Presently, there are only two compressed air storage systems operating in the world.
The specifications of Huntorf of (Germany) storage system are:
The turbine power:
PT = mT ΔhT ƞT [kW]
where,
mT = mass flow rate through gas turbine [kg/s]
ΔhT = isentropic enthalpy drop through gas turbine [kJ/kg]
ȠT = isentropic efficiency of turbine.
The power required to drive the air compressor:
Pc = mc ΔhG/ƞc [kW]
where,
mc = mass flow rate in the compressor [kg/s]
Δhc = isentropic enthalpy drop in the compressor [kJ/kg]
ƞc = isentropic efficiency of compressor
The enthalpy drop through compressor or turbine can be calculated as:
Δh = CP ΔT [kJ/kg]
Cp = Specific heat of air [kJ/kg-K]
ΔT = Change of temperature in compressor or turbine [K]
The overall efficiency of compressed air storage plant:
Ƞ = PT/PC
Essay # 4. Electrochemical Energy Storage (Battery):
An electrochemical cell or a battery consists of two electrodes and electrolyte in a box. When the cathode and anode are interconnected externally through a resistance, electrochemical reaction takes place in the cell. Lead-acid batteries, nickel-cadmium batteries and other batteries are used to store electrical energy.
A lead-acid battery has anode of porous lead, cathode of lead oxide and aqueous solution of sulphuric acid is used as electrolyte.
The overall reaction is:
The batteries have the following specifications:
Power density = 50 W/kg
Energy density = 200 Wh/kg
No. of cycles = 1000
Life span = 4 to 6 years
The principle of lead-sulphuric acid battery is shown in Fig. 20.4.
Example 1:
A 12-V lead-acid battery supplies a current of 60A. Calculate the power output and consumption of H2SO4.
Essay # 5. Thermal Energy Storage:
There are three types of systems:
1. Sensible heat storage.
2. Latent heat storage.
3. Thermal chemical energy storage.
The thermal energy storage systems can also be classified as:
1. Low temperature, below 100°C.
2. Medium temperature, 100-500°C.
3. High temperature, above 500°C.
These systems can use:
1. Liquid as storage medium.
2. Solid as storage medium.
The main parameter is specific heat of storage medium and change in temperature.
The latent heat storage system uses sensible heat as well as latent heat. There is a phase change, i.e., melting and freezing. The heat is supplied from a solar collector which melts the storage medium. When heat is withdrawn, the storage medium is frozen again.
The storage material should be physically and chemically stable in the operating range of temperature and should possess high heat of fusion. The latent heat storage system has higher energy density than sensible heat storage system.
Hot Water Storage System:
A hot water storage system is shown in Fig. 20.5. Water is used for storage below 100°C. For high temperatures steam, organic fluid, molten salts or molten metals can be used as storage materials.
Packed Bed Storage System:
Concrete, earth, rock, gravel and granite are suitable materials for low temperature heat storage. In a solar air heater vertical pebble bed heat storage is used. During charging hot air from solar collector flows from the top of pebble bed. There is thermal stratification and the upper and lower beds are at different temperatures, i.e., the upper bed is at higher temperature than lower bed.
During discharging the cold air enters at the bottom of the bed and hot air leaves at the top. The thermal stratification helps to get air at higher temperature from the bed top. The temperature difference between air and pebbles is small due to large surface area of the bed and high air-to-pebble heat transfer coefficient.
The storage capacity is given by:
Qs = Q (1 – ԑ) ρp – C(tmax – tmin) [J]
where,
Q = Storage volume [m3]
ԑ = Pebble bed porosity
C = Specific heat of pebbles [J/kg-K]
ρ = Density of pebble [kg/m3]
tmax = max. Temperature of storage [°C]
tmin = temperature of storage [°C]
Latent Heat Storage System:
The storage capacity can be calculated as follows:
Qs = m[Cs (tm – tmin ) + hm + Cl (tmax – tm )] [J]
where,
m = mass of storage material [kg]
CS = Specific heat of storage material in liquid state [J/kg-K]
Cl = Specific heat of storage material in liquid state [J /kg-K]
hm = heat of fusion of storage material [J/kg]
tmax = maximum storage temperature [°C]
tmin= minimum storage temperature [°C]
tm = fusion temperature of storage material [°C]
The storage capacity includes major part of latent heat and some part of sensible heat.
The charging process consists of:
1. Heating of storage material in solid state from initial temperature (tmin) to melting point (tm). This is sensible heat of storage.
2. Melting of storage material at tm. This is latent heat of storage.
3. Heating of liquid storage material to maximum temperature (tmax), this is also sensible heat of storage.
The discharging process follows the reverse process and consists of:
1. The cooling of liquid from tmax to tm.
2. Solidification (crystallization) at tm.
3. Cooling of material in solid state from tm to tmin.
Storage Material:
The efficient operation of latent heat storage system depends upon proper selection of storage material.
1. It should have large heat of fusion.
2. It should have large specific heats.
3. The melting point of material should be within the working range of storage system.
4. The material should have physical and chemical stability.
5. The volume change during melting and fusion should be minimal.
6. The thermal conductivity should not be very low.
The main advantage of latent heat storage system is reduction in the mass and volume of the material and the system.
The comparative indices of latent heat and sensible storage system are given in Table 20.2.
The following problems are faced in the low-temperature latent heat storage system:
1. There can be sub-cooling of liquid below the melting point during discharging process.
2. There are substantial changes in the volume during phase change.
3. Heat transfer rate can be very low between the working fluid and storage material.
These difficulties can be overcome by the use of hybrid system.
There can be a hybrid heat storage system. The sensible heat storage has water as storage material and latent heat storage system has paraffin.
The total storage capacity of a hybrid plant can be calculated as follows:
Qhs = [ms Cs (tmax – tmin) + ml[c’s(ts – tmin)] + hm + Cl (tmax – tm)] [J]
where,
ms = mass of sensible heat storage material [kg]
ml = mass of liquid [kg]
Cs = Specific heat of sensible heat storage material [J/kg-k]
c’s = Specific heat of latent heat storage material in solid phase [ J/kg- k]
cl = Specific heat of latent heat storage material in liquid phase [J/kg-k]
tmax = maximum storage temperature [°C]
tmin = minimum storage temperature [°C]
tm= melting point of latent heat storage material [°C]
hm = heat of fusion of latent heat storage material [J/kg]
Medium and High Temperature Storage:
Thermal energy storage system for medium and high temperature applications are very important for solar plants used for power generation, process heat, refrigeration and space cooling. The high temperature storage plant (above 500°C) can enhance the performance and availability of a solar power plant. The storage capacity should be sufficient to ensure full-load operation of solar power plant for ½ to 3 hours.
1. Medium Temperature Storage Systems:
The following materials and systems can be used for medium temperature storage systems:
The thermo-physical properties of storage materials used for medium temperatures are given in Table 20.4.
2. Steam Storage System:
Hot water is used as storage material. Steam is supplied through nozzles and pressure of storage tank increases from p1 to p2. The steam gets condensed due to mixing with water. There is a temperature rise from t1 to t2 and enthalpy rise from h1 to h2.
The mass of steam supplied can be determined from the equation of balance of storage energy.
where,
mw1 = Initial mass of water in the storage tank at and p1 and t1.
h1 = initial enthalpy of water [kJ/kg]
h2 = final enthalpy of water [kJ/kg]
hs = enthalpy of steam supplied [kJ/kg]
The mass of boiling water after charging.
mw2 = mwx + ms [kg]
It is available at p2 and t2.
During discharging, steam is withdrawn from the storage tank. There is an adiabatic pressure drop Ap. A portion of water is vaporised and temperature is changed to saturation temperature. The saturated steam is available for further processing.
A steam storage system used for a nuclear power plant of 400 MW capacities has the following specifications:
Storage capacity: 64 units of 580m3 capacity each
Steam super heaters: 8 units of 180m3 capacity each
Charging pressure: 20 bar and 48 bar
Charging temperature: 212°C and 260°C
Storage duration: 2 hours
Essay # 6. Storage System for Solar Plants:
The heat storage system of a parabolic trough solar power plant as shown in Fig. 20.6 consists of twin tanks. In one tank, hot oil is supplied from solar collector which is fed to second tank. The cooled oil is returned to the solar collector again. The maximum allowable oil temperature is limited to 400°C to avoid its decomposition.
Essay # 7. Thermochemical Energy Storage System:
Heat of reaction of reversible chemical reactions is used to store energy. The system is suitable for storing solar energy. An endothermic reaction is used for energy storage. Steam reforming of methane or dissociation of sulphur trixoide, methanol or ammonia is used and reaction takes place at 500°C and more in the presence of a catalyst. The reaction products can be stored locally or transported in a pipeline to the energy consumer location. At the consumer point, an exothermic reaction takes place and heat is released due to recombination of reaction products.
This system has the following advantages:
i. It has high energy density
ii. The energy losses during storage and transport are negligible.
Examples of reactions:
Chemical Heat Pipe:
The following endothermic reaction takes place at 960°C:
CH4 + H2O = CO + 3H2 – 6020 kJ/kg of CH4
The reaction products (CO + H2) carriers the energy through a pipe over long distances, from North Africa to Europe. The exothermic reaction at consumer point.
CO + 3H2 = CH4 + 6020 kJ/kg of CH4
The catalysts used are rhodium or nickel.
This system is very useful for transportation of solar energy over long distances.