The importance of a high dielectric constant in single layered Ultracapacitors

By Nico van Dongen

A dielectric is any electrical insulator capable of being polarised by an electric field. Under the influence of an electric field, the charge distribution in the dielectric changes so that positive charges align with the field.​

Solar panel energy collection is an established technology. However, although prices have fallen hugely over the last decade, there are several challenges impeding its growth. One of these is the grid infrastructure which is ill-equipped to deal with the variability of renewable energy sources such as wind and solar energy, and the other is storage. While several storage solutions exist, such as batteries, electrolytic capacitors, and mechanical storage systems, they all have deficiencies.

Here we will look at how a recent development in solid state single layer energy storage devices (SSESD) offers a solution. The SSESD incorporates a high permittivity dielectric with a dielectric constant in the order of 16 million.

Dielectrics

A dielectric is any electrical insulator capable of being polarised by an electric field. Under the influence of an electric field, the charge distribution in the dielectric changes so that positive charges align with the field. The three primary polarisations mechanisms are:

• Ionic polarisation where positive ions flow with the field and negative ions flow against the field
• Orientational polarisation where the dielectric contains materials with a permanent dipole moment, in other words, molecules have an uneven charge distribution
• Interface polarisation, where free mobile charges within the material migrate to the dielectric/electrode interface; positive charges move to the negative electrode and positive charges to the negative electrode.

Capacitors

In its simplest form, a capacitor consists of a dielectric layer sandwiched between two conductors. Applying a voltage across the conductors creates an electric field. The dielectric constant or permittivity (K) of the dielectric is the ratio between the field without the dielectric (Eo) and the field with the dielectric (E).

K = Eo/E

In the single layer capacitor described, when a voltage (V) is applied across the conductors, a charge (Q) is induced in the capacitor. The ratio between the voltage and the charge is defined as the capacitance (C) of the capacitor.  

C = Q/V

The amount of charge stored depends on area (A) of the conductors, the separation (d) between them, and the dielectric constant.

C = K(A/d)

So, for capacitors with identical dimensions, the higher the value of the K, the greater is the amount of charge that can be stored.

The potential electrical energy (E) stored in a charged capacitor is a function of the capacitance (C), the voltage (V) across the electrodes. It is equivalent to the work done by charging it and can be expressed by:

E = ½ CV²

Power density

The power density of a capacitor is the amount of power it can produce for a given volume. For instance, capacitors have a greater power density than batteries as they are able to deliver energy much faster than batteries can. A small capacitor can have a much higher power density than a large battery, even though a battery may have a higher energy density. In other words, power is the rate of using energy. The power density of a capacitor is usually expressed as potential energy per gram or per unit volume.

Just as capacitors can deliver power far more quickly, they can also recharge much more quickly.  We will return to power density in relation to solar panel energy storage later, but first, we will other kinds of capacitors, namely ultracapacitors.

Ultracapacitors

As we have said, the higher the dielectric constant, the greater the amount of charge that can be stored. The first generation of supercapacitors was based on increasing capacitance by introducing an electrolyte. Typically, these consist of an anode on which a thin dielectric layer is deposited, an electrolyte, and a cathode. The electrolyte forms the true cathode. When polarised, ions in the electrolyte form double layers with negative ions attracted to the positive electrode.

Compared to conventional capacitors, electrolytic ultracapacitors have a far higher power density. A drawback, however, is they can only withstand low voltages before they break down. Additionally, progress on their development has slowed.

Typically, the highest power density that can be achieved with this kind of ultracapacitors is around 15 kW/kg.

Solid-state single layer energy storage devices

Unlike electrolytic ultracapacitors, solid state ultracapacitors do not use an electrolyte. Instead, they incorporate a solid dielectric with an extremely high dielectric constant. Referring to the equations given above, the potential energy of the capacitor is directly proportional to the dielectric constant.  Thus, by incorporating a super-dielectric with a dielectric constant in the order of 10⁶ capacitors with extremely high power densities can be constructed.

We have developed such a dielectric. Its dielectric constant of 16 million is the highest value yet reported. Typically a stack of 6,000 layers of 400 cm² and a charging voltage of 600 V would deliver 85 kWh and a power density of 7.78 kW/kg.

Charge time

A huge advantage of SSESD over electrolytic ultracapacitors is the charge time. Charge time is governed by the supply voltage and the equivalent series resistance (ESR) of the capacitor. The ESR is complex and determined by a range of factors including the materials used and the mechanical construction. In a conventional ultracapacitor, the ESR is relatively high, though polymer type  ultracapacitors can be constructed with lower ESR but still substantially higher than the SSESD. Typical charge times range from 1 to 10 seconds.

In the case of our SSESD, the charge is stored on the dielectric/metal interface. The ESR very much lower and fast charge times can be achieved. Currently, we are seeing charge times for several layers of less than one second.

Solar power

The current goal for renewable energy is to derive a third of total energy from renewable resources by 2020. We may well fall far short of that target unless improved technologies are developed.

Converting solar energy into electrical power is an established technology, and solar farms are a common sight worldwide. While there are various ways of converting sunlight into electrical energy, the most common are solar thermal energy plants and photovoltaics.

• Solar thermal energy plants concentrate solar radiation using lenses and mirrors and use the heat to drive steam turbines
• Photovoltaics exploit the phenomenon that when semiconductors absorb photons of a certain frequency the energy excites electrons from the valence into the conduction band leaving holes in the valence band. The photovoltaic cell consists of a PN junction, so the electrons move to the N side and the holes to the P side. When a circuit is formed between the two sides, electrical current will flow and can be used to power a load.

Here our focus is on photovoltaics and how in combination with ultracapacitors and in particular SSESD they provide an optimum solution for the generation of renewable energy.

Challenges facing solar power generation

In large parts of the world, solar power can make a large contribution to the grid, but significant  problems remain. Solar power is intermittent; when the sun shines solar power can contribute to the grid, but when a cloud appears that contribution is reduced substantially. This on/off effect can lead to grid instability and, to overcome this, alternative power sources are often required to kick in when solar power levels fall.

To avoid this some form of energy storage is required to even out the supply. Various solutions are possible including lithium-ion and lead-acid batteries, flywheels, electrolytic capacitors, and ultracapacitors. However, specific energy considerations suggest that only lithium-ion batteries, flywheels and ultracapacitors are realistic options.

Another consideration is lifetime. At least 10,000 cycles plus and a minimum lifetime of 10 years is a typical requirement. This eliminates lithium-ion batteries leaving just flywheels and ultracapacitors. A problem with flywheels is they must be built underground and require substantial investment, while ultracapacitors are a far simpler solution. Additionally, ultracapacitors have no moving parts and require very little maintenance.

SSESD for solar panel energy collection

As we have shown, SSESD with high dielectric constants offers many advantages over conventional ultracapacitors. They offer better power densities, higher charge rates, and have a potential cost advantage. Integration with solar panels is easy and provides an immediate solution to the intermittency of solar energy even at the panel level.

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