Technical Definition Ultracapacitors

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Ultracapacitors: The New Era of Energy Storage As our global population expands, so does our need for energy. Although efficient and renewable energy resources are an imperative focus, an equally key effort is to optimally store energy. For decades, batteries have been the forefront technology for storing energy. However, with an increasing need to store and release energy rapidly to meet demands, new developments have led to the ultracapacitor. The ultracapacitor, also known as an electrostatic capacitor (EC), allows for portable energy storage without several of the downsides of a battery. Basic design principles of the capacitor, comparisons with lithiumion batteries, and applications of the ultracapacitor will be overviewed in this document. What is an Ultracapacitor? An ultracapacitor is an energy storage device that stores electrical charge in two parallel electrodes. The primary basis for storing charge in the capacitor is due to a phenomenon known as the electric double layer. In order to understand the principles of the electric double layer, it is first vital to understand the design of the capacitor. The ultracapacitor is constructed in a ‘sandwich’ design, which is shown in detail in Figure 1. The Ultracapacitor ‘Sandwich’ 1: The Bread (Carbon Electrodes) The carbon electrode is the electrical conductor of the capacitor. The electrode connects the capacitor with the rest of the circuit to allow transfer of electrical charge. Carbon is chosen as the electrode material to allow for high surface area within the conductor. The high surface area permits more charges to be stored within the electrodes. 2: The Dressing (Electrolyte) The electrolyte is a mixture of both positive and negative ionic charges that is dissolved in a solution such as water. The liquid electrolyte 1 1 3 2 2 Figure 1: The Ultracapacitor ‘Sandwich’ 1

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Technical Definition Ultracapacitors

Transcript of Technical Definition Ultracapacitors

Page 1: Technical Definition Ultracapacitors

Ultracapacitors:  The  New  Era  of  Energy  Storage  

 As  our  global  population  expands,  so  does  our  need  for  energy.  Although  efficient  and  renewable  energy  resources  are  an  imperative  focus,  an  equally  key  effort  is  to  optimally  store  energy.  For  decades,  batteries  have  been  the  forefront  technology  for  storing  energy.  However,  with  an  increasing  need  to  store  and  release  energy  rapidly  to  meet  demands,  new  developments  have  led  to  the  ultracapacitor.  The  ultracapacitor,  also  known  as  an  electrostatic  capacitor  (EC),  allows  for  portable  energy  storage  without  several  of  the  downsides  of  a  battery.  Basic  design  principles  of  the  capacitor,  comparisons  with  lithium-­‐ion  batteries,  and  applications  of  the  ultracapacitor  will  be  overviewed  in  this  document.    

What  is  an  Ultracapacitor?    An  ultracapacitor  is  an  energy  storage  device  that  stores  electrical  charge  in  two  parallel  electrodes.  The  primary  basis  for  storing  charge  in  the  capacitor  is  due  to  a  phenomenon  known  as  the  electric  double  layer.  In  order  to  understand  the  principles  of  the  electric  double  layer,  it  is  first  vital  to  understand  the  design  of  the  capacitor.  The  ultracapacitor  is  constructed  in  a  ‘sandwich’  design,  which  is  shown  in  detail  in  Figure  1.      

The  Ultracapacitor  ‘Sandwich’    1:  The  Bread  (Carbon  Electrodes)-­‐  The  carbon  electrode  is  the  electrical  conductor  of  the  capacitor.  The  electrode  connects  the  capacitor  with  the  rest  of  the  circuit  to  allow  transfer  of  electrical  charge.  Carbon  is  chosen  as  the  electrode  material  to  allow  for  high  surface  area  within  the  conductor.  The  high  surface  area  permits  more  charges  to  be  stored  within  the  electrodes.    2:  The  Dressing  (Electrolyte)-­‐  The  electrolyte  is  a  mixture  of  both  positive  and  negative  ionic  charges  that  is  dissolved  in  a  solution  such  as  water.  The  liquid  electrolyte  

1   1  3  2   2  

Figure  1:  The  Ultracapacitor  ‘Sandwich’  1  

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contacts  both  the  carbon  electrodes  in  the  capacitor.    3:  The  Meat  (Separator)-­‐  The  separator  is  an  insulator  that  divides  the  two  carbon  electrodes  from  one  another.  The  separator  is  often  made  of  some  porous  material  such  as  paper,  plastic  etc.  Although  the  separator  prevents  the  electrodes  from  contacting,  its  porousness  allows  for  the  transfer  of  ionic  charges  between  the  electrodes.  

The  Electric  Double  Layer    One  of  the  basic  principles  of  electricity  and  magnetism  is  that  opposites  attract.  When  an  external  voltage  is  applied  to  the  capacitor,  positively  charged  ions  in  the  liquid  electrolyte  are  attracted  to  the  negatively  charged  electrode  and  vice  versa.  The  two  zones  where  the  opposite  ions  attract  in  Figure  2  are  known  collectively  as  the  electric  double  layer.  This  creates  an  electrically  neutral  gap  between  the  two  electrodes.  This  gap  creates  an  electric  potential,  which  is  synonymous  to  an  electric  charge.  The  longer  the  voltage  is  applied  to  the  capacitor,  the  larger  the  charge  that  builds  up.      

 Figure  2:  The  Electric  Double  Layer  2  

Ultracapacitor  vs.  Battery  -­‐  What  is  the  difference?    Batteries  have  been  the  dominant  energy  storage  technology  for  as  long  as  we  have  been  alive.  As  one  may  have  noticed  with  a  device  such  as  an  iPhone,  batteries  can  provide  complications.  The  lithium-­‐ion  battery  in  the  iPhone  can  take  hours  to  fully  charge,  and  it  is  depleted  rather  quickly  when  charged.  Also,  the  longer  a  user  keeps  their  iPhone,  the  shorter  the  charge  state  becomes  for  the  battery-­‐  a  correlation  that  

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is  due  to  cycle  time  fatigue.  Ultracapacitors  are  capable  of  providing  alleviations  to  these  common  problems.  When  analyzing  the  effectiveness  of  ultracapacitors,  it  is  key  to  understand  the  differences  between  capacitors  and  batteries.  Here  are  some  crucial  contrasts  between  the  two:    Table  1:  Batteries  vs.  Ultracapacitors  Characteristic   Lithium-­‐Ion  Battery   Ultracapacitor  Charge  Time   ~3-­‐5  minutes   ~1  second  Discharge  Time   ~3-­‐5  minutes   ~1  second  Cycle  Life   <5,000  @  1C  rate   >500,000  Specific  Energy  (Wh/kg)   70-­‐100   5  Specific  Power  (kW/kg)   .5-­‐1   5-­‐10  Cycle  Efficiency   50%-­‐90%   75%-­‐95%  Cost/Wh   $1-­‐2   $10-­‐20      Charging  Method:  Batteries  store  energy  via  chemical  reactions.  During  these  reactions,  the  battery  also  undergoes  physical  altercations  between  the  charged  and  uncharged  states.  As  the  battery  continues  to  supply  charge,  chemicals  are  used  up  in  the  reactions.  When  all  the  chemicals  are  used,  the  battery  is  dead.  Conversely,  capacitors  store  energy  physically  and  they  experience  no  key  transformation  in  physical  states  amongst  charges.    `  Charge/Discharge  Time:  The  energy  storage  and  release  time  for  an  ultracapacitor  takes  about  a  few  seconds  or  less.  During  this  short  discharge  time,  ultracapacitors  are  capable  of  releasing  great  amounts  of  power.    Conversely,  a  lithium-­‐ion  battery  can  take  several  minutes  or  even  several  hours  to  fully  charge.      Cycle  Life:  Although  there  have  been  countless  technological  developments  in  batteries,  the  chemical  changes  that  occur  in  the  cell  constrain  the  total  life  of  the  battery.  Some  batteries  proclaim  to  have  life  cycles  of  around  10,000  charge-­‐recharge  periods,  but  this  quantity  is  miniscule  compared  to  the  capacitor,  whose  life  cycle  is  measured  in  millions  of  cycles.    Energy  Density:  As  of  right  now,  batteries  have  a  much  higher  energy  density  than  capacitors.  Energy  density  is  the  amount  of  electrical  energy  that  is  able  to  be  stored  per  unit  volume.  This  is  an  extremely  important  measurement  when  dealing  with  portable  devices;  higher  energy  densities  lead  to  smaller  and  lightweight  charging  mechanisms.      There  are  several  other  slight  differences  between  the  two  that  also  influence  functionality  and  performances  of  the  devices.  For  quantifiable  differences  between  the  two,  see  Table  1.  

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Where  are  Ultracapacitors  Used?  Ultracapacitors  are  already  used  in  a  wide  variety  of  different  applications  that  span  unique  functions  throughout  the  globe.  These  capacitors  are  only  compatible  with  DC  (direct  current)  power  applications,  thus  limiting  the  ultracapacitor’s  functionality.  Ultracapacitors  are  typically  preferred  over  batteries  and  other  energy  storage  devices  in  three  different  scenarios:    

Ø A  sizeable  quantity  of  power  is  required  for  a  comparatively  short  time  

Ø A  high  amount  of  charge/discharge  cycles  is  needed  

Ø A  lengthier  lifetime  is  essential  

 Figure  3  displays  several  applications  where  ultracapacitors  are  an  essential  component.  

Challenges  for  the  Future    After  reading  this  far,  you  may  be  wondering:  “Why  aren’t  ultracapacitors  used  everywhere?”  The  answer  to  your  question:  Cost.  Despite  a  premiere  level  of  performance,  quality,  and  safety,  ultracapacitors  are  just  too  expensive  to  compete  on  the  free  market  with  lithium-­‐ion  batteries.  Also,  as  previously  mentioned,  the  relatively  low  energy  density  of  ultracapacitors  remains  an  improving  point  for  researchers  and  scientists.  In  some  instances  these  capacitors  are  too  large  to  be  an  adequate  choice  for  energy  storage.  Therefore,  as  technology  advances,  it  will  be  imperative  to  lower  cost  and  increase  energy  density  of  the  ultracapacitor  without  surrendering  the  redeeming  qualities  that  differentiate  it  from  traditional  batteries.  Hopefully,  in  the  near  future,  the  stomach-­‐turning  words  “my  battery  is  dead”  will  be  nothing  more  than  an  obsolete  saying.      

Power  Tools  Tools  such  as  electric  screwdrivers  or  nail  guns  require  a  high  pulse  of  power  in  a  short  instance  of  time.  Ultracapacitors  are  chosen  as  the  energy  storage  method  because  of  this  short  power  constraint  and  for  short  charge/recharge  times  

Forklifts  Similar  to  power  tools,  forklifts  require  extensive  power  in  a  short  amount  of  time.  Ultracapcitors  are  the  utilized  technology  because  they  can  meet  power  demands  and  are  allowed  to  be  volumetrically  large-­‐  which  permits  greater  energy  and  power  

Light  Rails  and  Trams  Trams  in  larger  cities  often  run  constantly,  therefore  needing  a  technology  with  a  high  cycle  time.  Capactitors  are  part  of  the  electrical  systems  in  trams  to  meet  the  longevity  of  the  application.  Cities  also  have  strict  pollution  rules,  and  capacitors  are  an  excellent  earth-­‐friendly  storage  device.  

Figure  3:  Applications  of  Ultracapacitors  

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References    

1-­‐ http://www.explainthatstuff.com/how-­‐supercapacitors-­‐work.html  2-­‐ http://www.intechopen.com/books/dynamic-­‐modelling/dynamic-­‐

modelling-­‐and-­‐control-­‐design-­‐of-­‐advanced-­‐energy-­‐storage-­‐for-­‐power-­‐system-­‐applications