Impact of Storage Technologies upon Power System Losses
✁ ✂ ✄ ✁ ✂ ☎ ✆ ✝ ✞ ✟
Ioan
11
PhD Student, Technical University of Cluj-Napoca, Romania Department of Automation, Faculty of Automation and Computer Science G. Baritiu St., no. 26-28, 400027, Cluj-Napoca, Romania, dulau.lucian@gmail.com
Abstract ✠ The paper describes the main characteristics of storage technologies. The most important storage technologies are the batteries, hydrogen, pumped hydro, flywheels, compressed air, super-capacitors and superconducting magnetic devices. The storage technologies can be classified based on the function principle into electrochemical, mechanical and electromagnetic devices. The storage systems can also be classified based on their capacity to store power into short and long term devices. A power flow analysis is performed for the situation with and without a storage unit. The storage unit is inserted into the IEEE 14 bus test system.
Keywords: storage technologies; distributed generation; distributed energy resources; renewable energy sources; power flow analysis
I. INTRODUCTION
Energy storage technologies have attracted a significant interest and attention because they are enabling the integration of the growing capacity of renewable energy sources (RES) into the electric grid. These RES can be used to match the supply with the demand, have low CO2 emissions and can be operated on a small scale.
The RES are usually located in remote areas, with the units connected at the distribution level (medium voltage) or at consu✡ ☛ ☞ ✌✍ ✎☛ ✏ ☛ ✎ ✑✎✒ ✓ ✏ ✒ ✎✔✕ ✖ ☛ ✗✘
One disadvantage of these RES is that they their generation output is variable, therefore unpredictable in comparison with the large power plants. This unpredictability leads to stability, reliability and security of supply problems in the electric grid. One way to counter this disadvantage is to use energy storage systems.
Electricity storage offers the potential of storing electrical energy once generated by the RES and to subsequently match supply and demand as required. Storag☛ ✔☛ ✙ ✚ ✛ ✒ ✎✒ ✖ ✜☛ ✍ ✙ ✒ ✢ ✎✣ ✔✚ ☛ ☞☛ ✤✒ ☞☛ ☞☛ ✎✕ ✥ ✔✚ ☛ ✖ ☞✜✣ ✌✍ matching constraint by decoupling energy production and consumption.
Such storage options are not only essential to expand renewable energy sources, but also to ensure continuity of supply, increase energy autonomy and mediate against intermittent power production.
Energy storage can provide multiple benefits, such as:
· improves stability and reliability of transmission and distribution systems;
· improves the availability of distributed generation sources;
· improves power quality and the reliable delivery of electricity to customers;
· eliminates the increased cost of upgrading the power grid installations.
In this paper the impact of a storage unit upon the
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✒ ✓ ☛ ☞ ✤✎✒ ✓ analysis is performed for the situation with and without a storage unit, for the IEEE 14 bus test system in which several distributed generators have been inserted.
II. OVERVIEW OF ENERGY STORAGE TECHNOLOGIES
The storage technologies can be classified in many ways: function principle, storage capacity, power rating, discharge times and others.
Considering the function principle, there are:
· electrochemical (all types of batteries and hydrogen based energy storage);
· mechanical (pumped hydro storage, compressed air energy storage and flywheels);
· electromagnetic (superconducting magnetic energy storage and super-capacitors).
· Considering the energy storage capacity, there are: · short-term storage systems, that include the flywheel energy storage, super-capacitor energy storage, and superconducting magnetic energy storage;
· long-term storage systems, that include battery energy storage, hydrogen energy storage, pumped hydroelectric energy storage and compressed air energy storage.
An overview of these storage systems ([1-17]) is presented further.
A. Pumped Hydro System
A pumped storage system is composed of an upper-level reservoir and a lower-upper-level reservoir and turbine/generators which can be used in both ways, as turbines and pumps. The system works in the following _____________________________________________________________________________________________________________
way: during periods of high electricity demand the water is released to the lower reservoir so the turbines generate electricity, while during periods of low electricity demand the water is pumped in the upper reservoir, so it can be used later.
The efficiency of this system is between 70% and 80%, 250-1000 MW power and 10 hours (h) discharge time.
The main disadvantages are geographical restrictions and high investment costs.
B. Compressed Air Energy Storage System (CAES)
In this system compressors are used to pump air into a cavern. In order to recover the power the compressed air is released, heated using a natural gas-fired combustion turbine, and used to drive a turbine generator.
A CAES is composed of a motor, compressors, recuperators, air coolers, sealed cavern, turbine and generator.
The efficiency of this system is between 45% and 60%, 100-300 MW power and 3-10 h discharge time.
The main disadvantages are the need for a sealed cavern and high investment costs.
C. Storage Batteries
Storage batteries are rechargeable electrochemical systems used to store energy. They deliver, in the form of electric energy, the chemical energy generated by electrochemical reactions. These reactions are set in train inside a basic cell, between two electrodes plunged into an ☛ ✎☛ ✙ ✔☞✒ ✎✧ ✔☛ ✪ ✓ ✚ ☛ ✛ ✕ ✎✒ ✕ ✣ ✜✍ ✙ ✒ ✛ ✛ ☛ ✙ ✔☛ ✣ ✔✒ ✔✚ ☛ ✙ ☛ ✎✎ ✌✍ terminals. The reaction involves the transfer of electrons from one electrode to the other through an external electric circuit or load.
There are three main types of storage batteries: lead✫ acid batteries, nickel based batteries and lithium based batteries.
The discharge time is between a few seconds and several hours.
The main disadvantages are high costs and limited cycle life.
D. Super-Capacitors
Super-capacitors (or ultra-capacitors) are very high surface areas activated capacitors that use a molecule-thin layer of electrolyte as the dielectric to separate charge. The super-capacitor is similar to a regular capacitor except that it offers very high capacitance in a small package. Super-capacitors rely on the separation of charge at an electric interface that is measured in fractions of a nanometer, compared with micrometers for most polymer film capacitors. [8,9]
The efficiency of this system is 90%, 10 MW power and less than 30 seconds (s) discharge time.
The main disadvantages are low energy density and high costs per installed energy.
E. Hydrogen storage
Hydrogen is one of the promising alternatives that can be used as an energy carrier. The universality of hydrogen implies that it can replace other fuels for stationary generating units for power generation in various industries. Hydrogen is free of harmful emissions when used with dosed amount of oxygen, therefore reducing the greenhouse effect.[1]
Essential elements of a hydrogen energy storage system comprise an electrolyzer unit which converts electrical energy input into hydrogen by decomposing water molecules, the hydrogen storage system itself and a hydrogen energy conversion system which converts the stored chemical energy in the hydrogen back to electrical energy.[1]
The main disadvantage is the high costs for electrolyzers.
F. Flywheels
A flywheel is a mass that rotates around an axis. The flywheel stores energy mechanically in the form of kinetic energy. Energy is necessary in order for the flywheel to accelerate and rotate.
The amount of energy that can be stored by a flywheel depends on its rotational speed and its moment of inertia. Therefore, the faster it rotates the more energy it stores. The stored energy can be recovered by slowing down the flywheel via a decelerating torque and returning the kinetic energy to the electrical motor, which is used as a generator.[8]
A typical flywheel is composed of a flywheel, electric motor/generator, power electronic devices, bearings and casing.
The efficiency of this system is higher than 85%, 0.1-10 MW power and 15s-15 minutes (min) discharge time.
The main disadvantages are the need for a vacuum chamber and low energy density.
G. Superconducting magnet energy storage (SMES)
The principle of SMSE employs the storage of energy in the magnetic field around the coil carrying direct current.
SMES is achieved by inducing DC current into a coil made of superconducting cables of nearly zero resistance, generally made of niobium-titane (NbTi) filaments that operate at very low temperature (-✬ ✭ ✮ ✯✰ ✗✘ The current increases when charging and decreases during discharge and has to be converted for AC or DC voltage applications.[9]
SMSE is perfect for situations where great power is needed for a short period time, as the basic disadvantages of SMES are its low capacity and short storage period.
The efficiency of this system is higher than 90% and a discharge time between several seconds and few days. _____________________________________________________________________________________________________________
The main disadvantages are high cooling demand, expensive raw materials for superconductors and complicated inverter design and measurement circuits.
All these energy storage technologies have advantages and disadvantages. The technology choice often depends on several factors, such as the size of the system or the the power sources. Pumped hydro systems are currently the most used in the world, with 95% of the global storage capacity. The choice between large-scale storage facilities and small-large-scale distributed storage depends on the geography and demography of the country, the existing grid and the type and scale of renewable technologies entering the market.
III. CASE STUDY
A power flow analysis is performed for the situation with and without a storage unit, for the IEEE 14 bus test system (Fig. 1) in which three distributed generators (DGs) have been inserted.
Fig. 1. IEEE 14 bus system
The power flow is calculated for the cases in which the distributed energy sources and storage unit are connected to the grid (on-grid) or disconnected from the grid (off-grid). The analysis is performed using the Neplan software [18].
The distributed generators characteristics are presented in table 1.
TABLE 1. Distributed generators characteristics.
Generating type unit
Installed power [MW]
Bus
Wind turbine (WT1)
3 14
Photovoltaic power plant
(PV1)
0.7 10
Hydro generator 4.3 10
The power losses are presented in table 2.
TABLE 2. Power losses of IEEE 14 system.
Distributed generators
Active power losses [MW]
Reactive power losses [MVAr]
Off-grid 13.59 27.43
On-grid 13.01 24.77
In this system a storage unit (flywheel) is inserted that has 3 MVA and is installed at bus 10, where is the most available power from the DGs.
The results of the analysis are presented in table 3.
TABLE 3. Power losses of IEEE 14 system with storage unit.
Distributed generators
Storage unit
Active power losses [MW]
Reactive power
losses [MVAr]
Off-grid Off-grid 13.59 27.43
Off-grid On-grid 9.21 4.19
The power losses for the IEEE 14 bus test system are lower if the distributed generators are connected to the grid. Also, the power losses are considerable lower if the distributed generators are off-grid and a storage unit is inserted into the grid. This proves that the storage unit helps reduce power losses and increase the stability of the power system.
The storage unit stores the power produced by the renewable energy sources. The storage unit can also be used to store the power produced by the regular generators. An analysis of this situation is presented further, without any DGs taken in consideration. Multiple calculations are performed for the 3 MVA storage unit installed at each bus, in order to determine what is the best location for this unit in order for the losses to at the lowest level. The results are presented in table 4 and graphically represented in Fig. 2.
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Fig. 2. Power losses with a storage unit on-grid at different buses
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TABLE 4. Power losses of IEEE 14 system with storage unit at different locations.
Storage unit installed
at bus
Active power losses
[MW]
Reactive power losses
[MVAr]
4 5.11 -3.23
5 6.99 3.90
7 6.57 4.53
9 6.84 -0.69
10 9.21 4.19
11 11.33 9.55
12 13.65 14.23
13 10.78 7.76
14 10.98 7.79
IV. CONCLUSIONS
Storage is the weakest link of the energy domain, but with the increase of renewable energy sources (RES) it became more important. The storage systems can be used to store the energy produced by the RES, therefore facilitating the integration of these sources into the electric grid.
Also, storage can improve the stability and reliability of transmission and distribution systems, improve the availability of distributed generation sources, improve power quality and the reliable delivery of electricity to customers and can eliminate the increased cost of upgrading the power grid installations.
Storage is absolutely necessary when the power source is intermittent and located in an isolated area which can't be connected to the distribution network, storage becomes crucial.
The case study emphasizes that the power losses are lower when the DGs are on-grid. A storage unit is added to case study system that stores the power produced by the DGs when they are on-grid or when the generation output is higher than the demand. The power losses are reduced considerably when the DGs are off-grid and a storage unit is on-grid. The best location for the storage unit, considering the reduction of the power losses, is at bus 4.
The storage units can also be used to store the excess power produced by other generators in a power system.
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