2. Electrical Energy Sources and Storage#
In the 2nd lecture, we will study various types to store electrical energy. We again start our lecture with the structure of this electric car. As you can see, the battery pack occupies the largest space in the powertrain of this car.
Inside the electric car, the battery is the source of electrical energy. It is charged by either an on-board charger or an external charger, depending on the charging power, and is supplying power to the power electronics converters to drive the electric machines.
Today we will first go through various primary and secondary electrical sources and distinguish them, then discuss about various types of energy storage devices and their characteristics. In the end, we will summarise the take-away from this lecture.
Learning objectives are listed on this slide. As you can see, we have to not only be aware of the technology of electrical energy storage, but also should be able to do calculations for power and energy, and more importantly, distinguish power from energy.
2.1. Primary and secondary electrical energy source#
Let’s first check what is the primary source and secondary source for electrical energy are.
The distinction being that primary sources is a one-directional process where energy is transferred from one state to another without a viable option to reverse the process.
Primary sources are in essence uni-directional and are used to supply electric energy to the power grid or other systems.
We can divide the discussion on primary sources by making the divide between centralised sources and distributed sources.
A thermal power plant or a hydro power plant are centralised sources. Different sources of heat can be used as the primary energy source of a thermal power station. In large parts of the world, coal fired power stations provide the bulk of the electrical energy. Although coal power stations are a major contributor to unwanted emissions, however the relatively cheap price of coal makes it difficult to be decommissioned soon. Nuclear energy is also often used as a thermal source of energy, the difference compared to a coal one is that the heat is generated from nuclear reaction. Usually the efficiency from heat to water is about 90%, the thermodynamic efficiency from heat in water to mechanical rotation is limited to 47% for most thermal plants. Considering the efficiency of an electric generator is around 95%, the total efficiency from primary heat to electricity is usually below 45% for a thermal power plant.
In a hydro electric plant large amounts of water is flowing downhill to push the water turbine to generate electricity. A well designed water turbine can extract as much as 90% of the kinetic energy from the water, which makes the final efficiency from water kinetic energy to electrical energy to be around 83%.
Solar panels installed on the roofs of our houses are distributed primary sources. A solar panel is composed of many solar cells. In a photovoltaic cell the energy of a photon entering the cell is transferred to an electron which is then liberated from its valence band. If there is an external circuit connected, then the liberated electrons will produce electrical energy. The output voltage of the solar cell varies according to the amount of irradiation and also the power delivered by the cell. By connecting many cells together to form a panel, we are able to have electricity at higher voltage and high power. However, because of the fluctuating nature of the solar irradiation, we need power electronics to convert it to stable voltage (230 V, 50 Hz) for home appliances. Electrical energy storage (battery) should also be involved to smooth out the fluctuation or for usage at night.
A fuel cell converts the chemical energy to electrical energy by the electrochemical process. A fuel cell is composed of an anode, cathode, and an electrolyte membrane. A typical hyrdogen fuel cell shown in the slide works by passing hydrogen through the anode of a fuel cell and oxygen through the cathode. The byproduct is water and heat, which is a clean way to generate electricity.
Wind energy is considered to be another renewable source. It’s similar to a water power plant, but for wind energy, the kinetic energy comes from the air instead of water. So you can imagine, the mass density is lower, the unpredictability is higher, so there is more fluctuation. So to have stable output, we need power electronics to convert the variable voltage, variable frequency electricity generated from the generator to fixed voltage, fixed frequency output required by the power grid.
2.2. Electrical energy storage devices#
So far we have covered common primary sources we may encounter in practice. On contrary to the primary sources, secondary sources are in essence storage sources where we first store the energy we want to use and then extract it again at a later (and hopefully more convenient) time.
There are a few important properties we should know about the energy storage devices. First the energy can be stored (capacity) is in the unit of J or Wh, while the power discharged or charged is in the unit of W. The specific energy has a unit of J/kg, or Wh/kg. The specific power has a unit of W/kg. For the energy density and power density, the units are J/L (Wh/L) and W/L respectively.
Note
The unit of energy Wh (watt-hour) is related to J by the equation below:
To choose the energy storage solution for specific applications, there are both technical and economical considerations, as listed here. Here self-charging means the energy storage device loses its energy even when it is idle, e.g., a battery loses its charge even if it is not connected to external circuits.
In the coming slides we will compare various types of storage systems.
Here is a list of energy storage systems. As you can see here, the pumped hydro storage (PHS) has the highest capacity and relatively high efficiency, which makes it an appealing choice for storage to balance the power grid supply and demand.
This page lists different types of battery technologies. Lead-acid used to be the dominant technology and is cheap and technically more mature. However, the energy density makes it unsuitable for EV applications. Nowadays, Lithium-Ion battery technology is used in many places including EVs and consumer products (cell phones, laptop, etc.) because of the higher energy density and more and more developed technologies. But it is not suitable for a large amount of energy storage because of the high cost. The other types of batteries based on different chemical substances, e.g. Lithium-Sulfur, may have a higher energy density, but the technology is not ready enough to be applied massively.
This chart compares the power density vs. energy density of various energy storage systems. As we can see, the batteries give good energy density but lower power density, while the capacitors provide high power density. In many applications, it requires both high energy density and power density, e.g., in electric aircraft, it requires a burst of high power during taking off and a high amount of energy for long distance cruising. In such cases, a hybrid system of battery+capacitor can be used.
The storage technology is evolving fast because of high demands (application of renewable energies, electrification, etc.), and technology development. The Department of Energy of US have set their targets of 2030 for energy storage systems, as you can see, the aim is mainly on the cost reduction.
Energy can be stored in mechanical systems in the form of potential or kinetic energy. A flywheel is one way to store the electrical energy in the form of kinetic energy. A motor-generator set is connected to a spinning mass to accelerate or decelerate the mass. The electrical energy will be released or absorbed by the generator/motor. The energy contained in the spinning mass can be expressed as
where \(J\) is the moment of inertia and \(\omega\) is the rotational speed.
In order to prevent the spinning mass from disintegration because of high centrifugal force at high speed, lightweight fibre composite is often used to increase energy density by boosting the speed. The vacuum chamber is used to reduce the air friction, so that higher efficiency is obtained. With all these efforts, it’s able to reach an energy density of 0.05 MJ/kg and an efficiency of 80%.
This table shows various materials to build flywheels. To counteract the centrifugal force, the preferable material should have high strength to mass density ratio (specific strength), which makes composite material a better choice than metals.
The advantages and disadvantages of the flywheel energy storage are listed here. It’s important to note that, since a flywheel requires high speed spinning components and the bearings for that, it shows limited cycle life and reliability.
Inductors can be used to store electrical energy as a form of electromagnetic energy. Inductance can be treated as an “electrical inertia”. Inductor to current is like the moment of inertia to rotational speed. We are able to calculate the energy storage by integrating the electrical power flowing into an inductor. We will address the matter in detail in the Magnetics module of the course.
\( \begin{align} \newcommand{\diff}{\mathrm{d}} v &= L\frac{\diff i}{\diff t} \\ p &= vi = Li\frac{\diff i}{\diff t} \\ En_{inductor} &= \int_0^{\infty} Li \frac{\diff i}{\diff t} \diff t\\ &= \int_0^{i} Li \diff i = \frac{1}{2}Li^2 \end{align} \)
Battery can be defined as a device that stores chemical energy. Every battery comprises of a collection of cells. A cell is the basic power unit and consists of three main parts; two electrodes (electrical terminals) and a chemical electrolyte in between them. Of the two electrodes one is marked as positive while the other as negative. Depending on the type of battery, the electrolyte can be in liquid, gel or dry state. When a load is connected to the battery, chemical reactions take place at the electrodes to produce positive ions and free electrons. The ions take part in the chemical reactions in the electrolyte while the electrons provide the load with electric current.
Of course we are not able to make an electric truck in the way shown here, otherwise we are not able to carry anything else apart from the batteries. Therefore, the specific energy is critical for transportation electrification.
This page compares the specific energy of various battery technologies. As we can see, Lithium-Ion technology gives relatively the highest specific energy, while being more mature and safer compared to the Sodium Nickel Chloride technology.
We already mentioned a hybrid energy storage solution can be used to achieve both high power density and energy density. Here it is elaborated. This is an example applied in an electric car. When the power demand is low, the battery is used to charge the capacitor and supply the power converter, at high power demand, the battery and capacitor supply the power converter together. At regenerative braking, the regenerated power from the electrical machine is recycled by the power converter and is used to charge the capacitor and the battery.
There are two main types of batteries; primary and secondary. Batteries that can only discharge once are called primary batteries such as the well-known triple A and double A alkaline batteries. Secondary batteries can be recharged numerous times. These batteries have become relatively common in the world today. Examples are Lead-acid, Nickel cadmium, Nickel metal hydride and Lithium-ion batteries.
This page shows some real life capacitor examples.
Note
When applying capacitors, it is important to know that, some type of the capacitors, e.g., electrolytic capacitors, are polarised – they can only be connected one way in a circuit.
Here it shows a calculation example of an capacitor. Here \(C\) is the capacitance, \(A\) is the surface area of the two plates, \(\epsilon_0\) is the permittivity, and \(d\) is the distance between the two plates. To construct a super-capacitor with a large capacitance, activated carbon is often applied on the plates, so that the area can be maximised. By combining it with a small separation distance \(d\), we are able to obtain thousands of Farad, which is much larger than the capacitance of the earth!
It’s important to know that different energy storage devices have different characteristics and they are suitable for different applications. For example, for very fast discharging and charging in power electronics, we use inductors and capacitors, for day-night balancing in the power grid, we use pumped hydro storage.
We often use the state of charge to show the relative remaining energy inside the energy storage devices, which will be shown in the coming slides.
Here the energy stored equations in the flywheel, capacitor and battery are derived. As we can see, the speed of the flywheels, the voltage of the capacitors and the charge of the battery can be used to indicate the state of energy inside them. Similarly, the current in the inductor is an indicator of the stored energy inside it.
Here the plots are made based on the previously derived equations. As we can see, the energy stored is indeed in proportion to the aforementioned indicators.
Take-aways from the lecture on power and energy
Now let’s summarise what we should take from this lecture. The terminology to describe the energy conversion system is very important. Here we use a battery as an example since it is the most common energy storage device.
- Capacity
The rated amount of energy that can be stored in a device.
- Activated shelf life
The period of time (under specified conditions) that a charged storage device can be stored before its capacity falls to an unusable level.
- Ampere-hour
Unit used to describe the energy storage capacity. Abbreviated as Ah. The Ampere-hour is well suited for energy storage devices that deliver energy at a constant (or rated) voltage such as batteries. Some other types of energy storage devices can also be rated using the Ampere-hour as a measure if used in conjunction with a converter that keeps the output voltage constant.
- Cycle-life
The number of cycles (to a specified depth-of-discharge) a storage device can undergo before failing to meet the capacity of efficiency criteria. The cycle life of some storage devices is a strong function of usage criteria such as operating temperature, depth of discharge and discharge and/or charge rate. For this reason the cycle-life specification can only be used as a guideline since real-life operating conditions rarely match those used in deriving the cycle-life specification.
- Life
Usable life of an energy storage device, can be specified in cycles or years. The distinction between life and cycle-life hinges on the fact that some storage technologies do not exhibit a cycle-life but rather a maximum lifetime expressed in years.
- Charge
The process of storing energy in a storage device.
- Discharge
The process of removing energy from a storage device.
The depth of discharge (DOD) and state of charge are two important terminologies to show the relative discharging/charging status. A 100% DOD means a 0% SOC.
You should learn these terminologies by heart since you will encounter them very often as an electrical engineer working on electrical energy conversions.