Capacitors - Energy Storage Application 

By on

sitarsUnit
Siemens, Sitars ESM 125 Water
Cooled Energy Storage Module

Key Specifications:

  • Rated Voltage: 120 ... 125 V d.c.
  • Rated Capacitance: 63 F
  • Usable Energy Storage: 105 Wh
  • Usable Current Range: 150 ... 170 A d.c.
  • Maximum Series Module: 8
  • Operational Cycles: 1,000,000

Note: if disconnected then the energy will self discharge due to internal resistance in approximately four to five days.

Capacitors have numerous applications in electrical and electronic applications.  This note examines the use of capacitors to store electrical energy.  The sidebar shows details of a typical commercially available energy storage module.

Advantages & Disadvantages

In deciding the appropriateness of using capacitors as an energy storage medium, it is worth looking at some of the advantages and advantages:

Advantages:

  • can charge and accumulate energy quickly
  • can deliver the stored energy quickly
  • losses are small compared to other storage medium
  • long service life and low (or no) maintenance

Disadvantages:

  • low energy capacity compared to batteries
  • limited energy storage per dollar cost
  • stored energy will eventually deplete due to internal losses

Note: some interesting schemes are being developed to overcome some of the disadvantages. 

For example, Shanghai is experimenting with super capacitor buses, called the Capabus. Siemens is developing a hybrid storage utilising both capacitors and batteries to be used in the powering of light rail systems. In both these applications, capacitors are quickly charged at stops or along the route by connecting to overhead charging points.

Energy Storage

Double Layer Capacitors

Many energy storage modules will use electric double layer capacitors, often referred to as super capacitors.

Super capacitors use a liquid electrolyte and charcoal to form what is known as an electrical double layer.  This greatly increases the capacitance. Capacitors with large Farad rating and small size can be obtained.

Note: due to having a large internal resistance, double layer capacitors are not suitable for a.c. circuits.

For anyone not familiar with capacitor theory or needing a quick refresher, please review the Capacitor Theory note.

The amount of energy (in joules) stored by a capacitor is determined by the capacitance (C) and voltage (V) and is given by:

  1 2 C V 2

The greater the capacitance or the voltage, the more energy it can store. 

When connecting capacitors in series, the total capacitance reduces but the voltage rating increases.  Connecting in parallel keeps the voltage rating the same but increases the total capacitance.  Either way the total energy storage of any combination is simply the sum of the storage capacity of each individual capacitor. 

Tip: one application of capacitors as part of a hybrid (capacitor/battery) energy system, is that they can help prolong battery life.   

Energy Stored Example

Consider the above energy storage module (63 F at 125 V).  What is the stored energy of one module by itself and then of two modules connected in series.

The energy of one module is:

  1 2 ×63× 125 2 =0.5 MJ

by connecting two modules in series (doubling the voltage, halving the capacitance), the energy storage can be doubled:

  1 2 ×31.5× 250 2 =1.0 MJ

Safety: capacitors store energy and will remain charged when disconnected from any supply.  Before working on any capacitive systems which have been isolated from the power supply, be careful to take all necessary steps to ensure the capacitors are fully discharged.

Hybrid Energy Systems

Hybrid energy systems (HES) employ capacitors in conjunction with batteries or to benefit from the advantages of both technologies while minimizing the disadvantages. 

Within a hybrid energy system the capacitors can charge and deliver energy more quickly, thereby reducing strenuous duties on the batteries.  The batteries can achieve greater energy densities and provide for longer running duration. 

The use of this type of hybrid energy systems is becoming more popular, particularly in transportation applications.

Hopefully,  everyone now has a better understanding of using capacitor for energy storage.  If anyone still has questions or any comments, please post below.

See Also



Steven McFadyen's avatar Steven McFadyen

Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author

myElectrical Engineering

comments powered by Disqus



Induction Motor Calculator

Just added a page to the tools, which will allow you to calculate the synchronous speed, slip and rated torque for an induction motor. Not a particularly...

9 power supply issues solved by using a UPS

Installation of a UPS can help in reducing problems due to issues with the power supply.  A lot of people relate this to nine key issues.  Depending on...

Understanding LV Circuit Breaker Fault Ratings

I think this post is going to be helpful to several of our readers. While the IEC low voltage circuit breaker Standard [IEC 60947-2, Low voltage switchgear...

Back to basics - the Watt (or kW)

When thinking about watts (W) or kilowatt (kW = 1000 W) it can be useful too keep in mind the fundamental ideas behind the unit. Watt is not a pure electrical...

Post Authorship

In 2011, with the introduction of it’s Panda search ranking algorithms, Google introduced tools for determining the original author of posts.  The intention...

Questions - Reputation and Privilege

Our question and answer system while letting you do exactly what it says, is much more.  It is a dynamic user driven system, where our users not only ask...

1,000 kV UHV First for China

At the beginning of the year China put the world's first 1,000 kV UHV transmission system into operation. Transmitting power at over a million volts is...

Photovoltaic (PV) - Utility Power Grid Interface

Photovoltaic (PV) systems are typically more efficient when connected in parallel with a main power gird. During periods when the PV system generates energy...

8 Motor parts and common faults

Straight forward list of some common motor faults.  If I have missed any other common faults, please take a bit of time to add them in as a comment below...

Getting Started with Patents

If you have a great idea or invent something the last thing you want is someone to steal the idea. One of the things you can do is protect the intellectual...

Have some knowledge to share

If you have some expert knowledge or experience, why not consider sharing this with our community.  

By writing an electrical note, you will be educating our users and at the same time promoting your expertise within the engineering community.

To get started and understand our policy, you can read our How to Write an Electrical Note