Which of the following can store large amounts of electricity even when unplugged?

7.1 Introduction

Distributed generation (DG) is getting more attention in recent years. This is mainly due to the various advantages of DGs, such as an electrical energy loss reduction in the distribution system, reduction of voltage fluctuations, increasing reliability, power quality improvement, energy cost reduction, and ultimately increasing customer satisfaction. Despite all the benefits associated with DG in power systems, interconnecting these new technologies to the national energy systems leads to some crucial problems such as changing the protection setting, power system stability, and islanding phenomena.

DGs may include different forms of electrical energy generation; renewable resources, mainly wind and solar power plants, or nonrenewable resources (conventional methods). Employing most of the renewable energy resources, such as wind farms and photovoltaic (PV) systems as DGs leads to the main challenges: changeability and uncontrollability of output power. Indeed, these main features lead to additional fears in DGs application in a power system. Using an energy storage system (ESS) is proposed and is one of the most appropriate solutions in this area. This new category enables engineers to manage the power system optimally.

Generally, the ESS operation is categorized as follows:

The charging period: This process is applicable using the network electrical energy, during the off-peak intervals when the electrical energy is available at lower prices,

The discharging period: In times of peak the stored energy in an ESS is used. It should be mentioned that in this period the network electrical energy has a higher price and use of DGs is more economical. Accordingly, application of an ESS system is mainly explainable for reducing or even eliminating the uncertainties of renewable DG.

It should be mentioned that the most commonly used methods in ESSs are based on the DC type, so using these systems is widely more intertwined with power electronic devices to connect with the national power grids.

Generally, a variety of ESSs can be provided in terms of technology, location, capacity, demand, and costs of investment.

In this chapter, different operational status related to DGs prescience in power system consisting of interconnected or isolated mode is presented. Modeling storage system devices considering practical small- and large-scale ESSs based on different applications is described. Furthermore, the governing relations with each technology are explained in detail. Finally, some important points related to the economics of ESS operation will be discussed.

7.1.4 ESS Capacity and Size

Up to now different ESS technologies have been proposed. Some of them are suitable for medium-scale applications such as pumped storage, compressed air energy storage (CAES), flywheel, and superconducting magnetic energy storage (SMES). Mainly they have the capacity in MW ranges [1].

A general overview of ESS is described in Fig. 7.1 in which the systems are classified based on their applications.

Fig. 7.1. Classification of the principal energy storage systems.

Also, the characteristics related to the most important ESS technologies are indicated in Table 7.1 [2]. Furthermore, Fig. 7.2 presents some useful comparisons about ESS technologies by their power and energy densities.

Table 7.1. Energy storage technologies

Technology	Power	Energy density	Response time	Efficiency
Pumped hydro	100 MW–2 GW	400 MWh–20 GWh	12 min	70–80%
CAES	110 MW–290 MW	1.16 GWh–3 GWh	12 min	90%
BESS	100 W–100 MW	1 kWh–200 MWh	Seconds	60–80%
Flywheels	5 kW–90 MW	5 kWh–200 kWh	12 min	80–95%
SMES	170 kW–100 MW	110 Wh–27 kWh	Milliseconds	95%
Super capacitors	< 1 MW	1 Wh–1 kWh	Milliseconds	> 95%

Fig. 7.2. Power and energy densities of different energy storage devices.

7.1.5 Power Electronic Interface

The electrical output power in all of the energy storage devices is in the form of DC power and it is required to be converted to AC using the power electronic devices and delivered to the power grid.

Generally, when the produced power is higher than the demand power, the extra power will be stored in the system (the system is charged).

Converting the AC power to DC power is inevitable in all of the electrical energy storage systems (EESSs). It means that when the power demand is more than the generated ones, the energy flows from the EESS to the power grid. This means that the system enters the discharging mode. This interval requires converting DC power to AC.

Fig. 7.3 briefly shows the interconnection of an EES to the network through unidirectional and bidirectional converters. Because using two unidirectional converters is more costly than using a bidirectional converter, bidirectional converters are preferred in practical applications. As indicated in Fig. 7.4, a bidirectional converter acts such as rectifier during the battery charging process [3]. This role will be changed to an inverter during the discharging process, in which the batteries are charged.

Fig. 7.3. (A) Unidirectional and (B) bidirectional converter.

Fig. 7.4. Interconnection scheme for storage devices.

8 Technological challenges for ESS

ESS market potential is much larger than the existing one and is mainly driven by renewable and microgrid developments in current power in infrastructure [24]. Batteries and SC have been available commercially and widely used for grid and electric vehicles too. Conventional storage systems such as PHS and CAES are also implemented across the world for large-scale storage. However, some of the challenges facing development of ESS can be listed as

Public Interest and support for ESS development

Incentives for ESS investment and development

Safety and flexibility in operations

Cost of energy deliverability should further reduce

Parallel dependence of ESS on power electronics based integrating systems.

Development of international standards for ESS selection and implementation

Weak grid and poor formed markets

To ensure sustainability and access to component materials.

Installation infrastructure and maintenance requirements.

5.2.1.3 Operation mode

ESS installed at the new energy power generation side can be operated under the following several modes, each independent from each other and not necessarily multifunctional:

Power variation rate limitation. Power variation rate is limited to a certain range relying on real-time measurement of the output power of a wind power plant. The ESS starts charging when the output power of the wind power plant climbs sharply; it discharges when the output power drops sharply. This aims at meeting the needs of the grid for a certain power variation rate. Under such a mode, the ESS should be installed close to the wind power plant.

Grid power variation support. The ESS charges and discharges power according to the grid's dispatch instructions to make extra adjustment for the grid. Under such a mode, the ESS does not target a certain wind power plant. Therefore, it is not necessary for the ESS to be installed near the wind power plant.

Voltage regulation. The ESS works on reactive power absorption or release according to real-time need for regulation of transmission voltage.

Frequency regulation. The ESS can charge or discharge power according to the signal received each second. Under such a mode, the ESS can maintain a long-term operation at a set State of Charge value.

Low-voltage ride-through. The ESS works in power electronic equipment for low-voltage ride-through. Most wind power plants have power electronic equipment. So there is no need for extra support.

Abbreviations

ESS

energy storage system

microgrid

photovoltaic

wind turbine

superconductor magnetic energy storage

flywheel energy storage

pumped hydro storage

hydrogen energy storage

thermal energy storage

polysulfide-bromine energy storage system

lead acid

lithium-ion

ampere hour

total operating cost of MG

carbon dioxide

harmony search algorithm

time of use

benders decomposition algorithm

genetic algorithm

mixed-integer programming

gray wolf optimization

hybrid energy storage system

loss of power supply probability

loss of load expectation

distributed generation

distributed energy resources

renewable energy source

double layer capacitor storage

compressed air energy storage

sodium-sulfur

zinc bromide

polysulfide bromide battery

vanadium redox

vanadium redox energy storage system

nickel-cadmium battery

ultra-capacitor

point of common coupling

electric vehicle

state of charge

particle swarm optimization

general algebraic modeling system

net present value

battery

operating cost

quantum-behaved particle swarm optimization

surplus of power supply probability

depth of discharge range

raditional distributed generation

Abstract

Hybrid energy storage systems can be created by combining multiple energy storage units. With hybrid energy storage systems, demand powers in the receiver can be met. The energy storage system structure consisting of battery and ultracapacitor is explained in this chapter. While the battery meets the long-term energy need, the ultracapacitor can meet the instant power demands. An ultracapacitor self-discharge experiment has been performed in this chapter. It has been shown how long an ultracapacitor can hold energy on it with this experiment. In addition, innovative energy storage technologies are explained in this chapter, these are gravity energy storage, flywheel energy storage, and superconducting magnetic energy storage systems. Gravity energy storage system with high-capacity energy storage feature is a very new technology. The application topology with gravity energy storage system and solar photovoltaic panels is explained.

2.3.2 ESS Overview

The two fields more involved in the development of ESS are the power system and the transport sector. The demand for especially lithium-ion battery systems rises rapidly due to the electro-mobility promotion (i.e., plug-in hybrid and full electric vehicles). The link between this sector and the electric grid integration of the ESS is quite straightforward. Electro mobility requires more efficient batteries and large-scale production is expected to give impetus to the usage of ESS in distribution systems thanks to cost reduction. Furthermore, of the V2G option that allows using the vehicle batteries as grid storage during times when the vehicles are plugged-in for charging; secondly because the utility companies necessarily start getting involved in the upgrading of their infrastructures to integrate the EV charging stations. However, the development of batteries dedicated to grid applications goes rapidly on and several technologies can be considered quite mature for these purposes.

The ESS technologies, and consequently their cost, strongly depend on the specific services that they are called to perform. ESS services can be: ancillary services (i.e., frequency control, voltage regulation, spinning and stand reserves, black start service, etc.), peak shaving, load leveling, islanding support, or other service mainly related to private uses of the ESS (e.g., residential use for increased self-consumption of DG production, industrial applications, uninterruptable power supply etc.). Different classifications can be applied to the ESS, but one of the most effective is that one related to the duration and frequency of power supply from the ESS:

short-term (seconds to minutes),

medium-term (daily storage), and

long-term ESS (weekly to monthly).

The short-term ESS (<0.25 hours) can be used for primary and secondary frequency control, spinning reserve, black start, peak shaving, islanding, electro-mobility, and uninterruptable power supply (UPS).

The medium-term ESS (1–10 hours) are able to provide services of tertiary frequency control, standing reserve, load leveling, islanding, electro-mobility, residential self-consumption increase, UPS. Finally, the long-term ESS (from 50 hours and typically less than 3 weeks) can be exploited for long duration services, during periods when there is no or scarce generation of electricity from wind and solar (“dark- calm periods”).

Super-capacitors, superconductive magnetic coils, or flywheels may offer short-term services. Pumped hydropower, compressed air ESS, thermoelectric storages, and electrochemical ESS, as lithium-ion, lead-acid, high temperature and flow batteries, are able to perform medium-term services. Long-term services can be offered by hydrogen or natural gas storage systems.

The main applications that can be suitably exploited by the smart distribution networks fall into the medium, or at least short-term services.

The pure electrical super-capacitors, superconductive magnetic coil, (since they have some strengths, as high efficiency, high power capability, and long life), are yet affected by the lack in the validation and experimentation for grid purposes and by their very high costs, due to the high innovation degree.

The mechanical systems can be subdivided into well-established technologies (i.e., pumped hydropower), the ones that need short time-to-market (i.e., compressed air energy storage), or those that are developed for other applications than the network operation (i.e., flywheels, that are well-established in UPS systems).

The technologies useful for distribution networks applications and that already reached a higher technical readiness level are the electrical-chemical batteries. Such technologies may have internal or external storages. Examples of the latter systems, disregarding the hydrogen or methane storages that are useful for long-term services, are the redox-flow batteries, which have the advantage that energy and power are independently scalable (energy capacity depends on the tank while the cell stack determines the power). Vanadium redox-flow batteries are commercially available with different modular scalable sizes but the still high costs of the electrolyte solution and the maintenance obstacle their large-scale diffusion. The internal storages systems, in which the energy and power depend each other, work at low (Li-ion, lead-acid, Ni-Cd) or high temperature (NaNiCl2, NaS).

In Table 2.1 the range of the most important parameters for some batteries are reported [12].

Table 2.1. Parameters for chemical storage systems with internal storage

Lithium-ion batteries have become the most important storage technology in different areas (e.g., portable devices and EVs) and can be an option for stationary applications also, due to their high energy density, high efficiency, and relatively long lifetime. Despite lithium resources being limited to only few countries, the current fervent development activity related to this kind of batteries could result soon in a significant cost reduction and improved lifetime and safety. Lead–acid batteries are one of the most developed and long time installed technologies. They are mainly used for cars but are widely used also for stationary grid application, e.g., in islanded grid. Their main disadvantage is the toxicity of lead and this fact causes social acceptance problems. However, the market opportunity is the low investment costs and the existing large number of manufacturers.

Sodium-nickel-chloride batteries (NaNiCl2, also called zebra-battery) and sodium-sulphur batteries (NaS) operate at high temperature, in a range from 270°C to 350°C. The temperature has to be maintained during the charging/discharging cycles, thus a suitable isolation has to be designed. Typical stationary applications can be peak shaving and load shifting. They are commercially available from few manufacturers, thus they are not much diffused around the world. For their characteristics high temperature batteries may be competitive with the lead-acid and lithium-ion batteries, but the main obstacle to their diffusion is related to safety issues (fire incident caused by NaS batteries).

4.2.4 Low-voltage ride through

LVRT requires that when the grid fails or disturbance causes a grid connection point voltage decline, the ESS can be ensured to operate and stay connected with the grid and provide certain reactive power to the grid at certain voltage decline scope and duration until the grid voltage restores, i.e., “riding through” the low voltage period (zone).

The ESS LVRT is normally required at the high-voltage, high-capacity energy storage station. The energy storage converter PCS device should have the LVRT capability. The technical requirements on the energy storage converter LVRT capability are shown in Fig. 4.27.

The ESS has the capability of ensuring on-grid operation for 625 s after the PCC voltage declines by 20% of the rated voltage.

When the ESS can recover the voltage to 85% of the rated value within 2 s after the PCC voltage drops, the energy storage converter can ensure continuous on-grid operation.

For different faults of the grid, the following requirements are imposed on the ESS LVRT:

When the grid experiences three-phase short circuit that causes the PCC voltage drop, and the voltage of different lines of the ESS falls in the area of the voltage contour line and above in the diagram, the ESS must guarantee on-grid continuous operation; when the voltage of any line of the ESS PCC is lower or partially lower than the voltage contour line in the diagram, the ESS is allowed to be off the grid.

It is the same when the grid faces two-phase short circuit that causes PCC voltage drop.

When the grid faces single-phase earthing short circuit that causes the PCC voltage decline, and the voltage of different phases of the ESS PCC fails in the contour line and above area in the diagram, the ESS must guarantee on-grid continuous operation; when the voltage of any phase of the ESS PCC is below or partially below the contour line in the diagram, the ESS is allowed to be off the grid.

5.3.4 Energy Storage System Constraints

The following equations model the energy storage system operation:

(5.21) 0≤PESS,tdis≤PESSRu1,t∀t

(5.22)−PESSRu2,t≤PESS,tch≤0∀t

(5.23)PESS,t=PESS,tdis−PESS,tch∀t

(5.24)u 1,t+u2,t≤1∀t

(5.25)CESS,t=CESS,(t−1)− PESS,tdisηESS−P ESS,tch∀t

(5.26)0≤CESS ,t≤CESSR∀t

The ESS charging and discharging power are limited by the rated power (5.21) and (5.22). The ESS power, PESS,t, is negative in charging mode, positive in discharging mode, and zero in idle mode. The binary variables u1,tandu2,t indicate the discharging and charging states, respectively. Equation (5.24) implies that the ESS cannot be simultaneously charged and discharged. The energy stored in the ESS at each hour is determined by Eq. (5.25) and limited by Eq. (5.26). For more realistic consideration, the ESS state of charge constraint (SOCmin≤SOC≤SOCmax) should be included.

3.2.3 Constraints on vehicle-level design based on ESS attributes

The ESS of PHEVs provides many constraints on vehicle-level design attributes. Unavoidable design tradeoffs exist between the ESS specifications and the vehicle-level design attributes such as passenger space, performance, and vehicle costs.

The ESS of PHEVs is generally heavier and more bulky than those of HEVs. Whereas HEVs have been designed with the ESS integrated into the chassis, body, or trunk of the conventional vehicle, PHEVs may require a vehicle redesign to accommodate the increased weight and bulk of the PHEV ESS. PHEV conversions require significant compromises to vehicle handling and interior volume to package the battery system. For example, the GM Volt uses a custom vehicle chassis with an integrated battery tunnel. This configuration centers the mass of the battery system to improve vehicle dynamics, reduces the lost interior volume, and protects the pack from accidental impact.

The increased mass of PHEV ESS elicits a cost in terms of vehicle fuel economy and performance. Because of the dynamic nature of vehicle fuel economy and performance testing, vehicle-level attributes such as tested fuel economy, 0–100 kph times, and dynamic handling are very sensitive to vehicle mass. As a result, PHEVs generally have slightly lower charge sustaining fuel economy and slightly worse dynamic performance when compared to conventional HEVs.

Finally, a PHEV ESS is intrinsically more costly than a conventional HEV ESS due to its increased capacity and power. PHEV studies agree that the manufacturer cost and purchase price of PHEVs will be significantly higher than comparable HEVs due to the costs of the PHEV ESS.

2.3.1 Generator Side Distributed ESS

An individual distributed ESS is smaller than an aggregated ESS, because it only handles a single (or a small group) renewable generation unit. Similar to aggregated ESSs, the major function of generator side distributed ESS is to smooth the output of renewables. The distributed ESSs are installed on-site with each renewable generation unit, as illustrated in Figure 2.2.

A distributed ESS is usually connected to the DC link of the renewable generation unit behind the grid-side inverter. For a wind inverter, the ESS connected to the DC link in the back-to-back converter. For solar PV generation, the ESS is connected to the output of PV through the DC/DC converter. In Figure 2.2, the distributed ESS is able to help the wind turbine inverter to have a stable DC link voltage, so the inverter can work properly. The distributed ESS outputs the desired power to compensate the fluctuation of renewable generation.

As mentioned before, a distributed ESS is not as powerful as an aggregated ESS for mitigating the fluctuation of renewables, but the distributed ESS is easier to be expanded and maintained.