Lots of volts in a ‘ tank’

A buf­fer bat­tery can give quick power to any car

The Witness - Wheels - - ELECTRIC -

ELEC­TRIC cars will only be truly com­pet­i­tive when it doesn’t take longer to charge them than it does to fill a fuel tank.

The stor­age ca­pac­ity of bat­ter­ies is im­prov­ing ex­po­nen­tially, but the power grid is the weak link: how could it pos­si­bly charge myr­iad cars at the same time?

This is es­pe­cially prob­lem­atic in the case of ul­tra- fast charg­ing, which re­quires more than 10 times more power. EPFL re­searchers have found the so­lu­tion: in­ter­me­di­ate stor­age.

It only takes a minute- and- ahalf to put enough fuel into the tank of a diesel car to run for around 1 000 kilo­me­tres. Af­ter be­ing charged for the same amount of time, the best elec­tric cars will only go six kilo­me­tres.

The only way to make the charg­ing process faster is to in­crease the power flow go­ing in. But such a quick charge would re­quire 4,5 MW of power — equiv­a­lent to 4 500 wash­ing ma­chines. This would bring down the power grid.

Low or medium volt­age

AL­FRED RUFER EPFL re­searcher Charg­ing sta­tions can dis­con­nect from the grid while still pro­vid­ing a high charge level for cars.

“We came up with a sys­tem of in­ter­me­di­ate stor­age,” said Al­fred Rufer, a re­searcher in EPFL’s In­dus­trial Elec­tron­ics Lab.

“With this buf­fer stor­age, charg­ing sta­tions can be dis­con­nected from the grid while still pro­vid­ing a high charge level for cars.”

And this can be done us­ing the low- volt­age grid ( used for res­i­den­tial elec­tric­ity needs) or the medium- volt­age grid ( used for re­gional power dis­tri­bu­tion), which sig­nif­i­cantly re­duces the re­quired in­vest­ment.

In­ter­me­di­ate stor­age is achieved us­ing a lithium ion bat­tery the size of a ship­ping con­tainer, which is con­stantly charg­ing at a low level of power from the grid.

When a car needs a quick charge, the buf­fer bat­tery promptly trans­fers the stored elec­tric­ity to the ve­hi­cle. The grid is not even used.

To prove the sys­tem works, the re­searchers at the EPFL En­ergy Cen­tre and In­dus­trial Elec­tron­ics Lab built a demon­stra­tor to­gether with their part­ners from the Swiss Fed­eral Lab­o­ra­to­ries for Ma­te­ri­als Sci­ence and Tech­nol­ogy ( EMPA), the Swiss Fed­eral In­sti­tute of Tech­nol­ogy in Zurich ( ETHZ) and the Bern Univer­sity of Ap­plied Sci­ences.

The demon­stra­tor is a trailer hold­ing the in­ter­me­di­ate stor­age bat­tery.

It draws power from the low­volt­age grid and, in the space of 15 min­utes, pro­vides the 20 to 30 kWh needed to charge a stan­dard elec­tric car bat­tery.

“Our aim was to get un­der the psy­cho­log­i­cal thresh­old of a half hour. But there is room for im­prove­ment,” said Mas­si­m­il­iano Capez­zali, deputy di­rec­tor of the En­ergy Cen­tre, who co- or­di­nated the pro­ject.

Fu­ture charg­ing needs

What is also in­ter­est­ing about this con­cept is that it can be used to de­ter­mine how much ca­pac­ity fu­ture charg­ing sta­tions will need.

Fuel sta­tions from the last cen­tury will grad­u­ally give way to elec­tric charg­ing sta­tions. And just as own­ers had to as­sess how big their fuel tanks needed to be, fu­ture sup­pli­ers of elec­tric en­ergy will have to es­ti­mate the needed ca­pac­ity for their buf­fer stor­age.

To help them in their task, the re­searchers de­vel­oped an equa­tion that fac­tors in a num­ber of pa­ram­e­ters, in­clud­ing traf­fic sta­tis­tics on a given stretch of road, the es­ti­mated num­ber of elec­tric cars, the charg­ing ca­pac­ity of the bat­ter­ies, users’ charg­ing needs, and so on.

The sim­u­la­tions, which are based on ac­tual fig­ures from French- speak­ing Switzer­land, show that the sce­nario is en­tirely re­al­is­tic.

A sta­tion able to quickly charge 200 cars per day would need in­ter­me­di­ate stor­age ca­pac­ity of 2,2 MWh.

This is the same or­der of mag­ni­tude as the en­ergy con­sumed by one home in one year. In vol­ume terms, it cor­re­sponds roughly to four ship­ping con­tain­ers.

“Elec­tric cars will change our habits. It’s clear that, in the fu­ture, sev­eral types of charg­ing sys­tems — such as slow charg­ing at home and ul­tra- fast charg­ing for long- dis­tance travel will co- ex­ist,” said Capez­zali.

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