Photo: Starosta, KIT

Scal­able charg­ing sys­tem for elec­tric vehicles

With the sig­nif­i­cant increase in the num­ber of elec­tric cars, the impor­tance of charg­ing infra­struc­ture and con­cepts for grid secu­ri­ty is also increas­ing. When elec­tric cars are charged pri­mar­i­ly after work or after work, they cause peak loads in the pow­er grid at these times. These could be avoid­ed if the entire ser­vice life was used for load­ing. Accord­ing to a study com­mis­sioned by the Fed­er­al Min­istry of Trans­port, the aver­age oper­at­ing time of pas­sen­ger cars is only 45 min­utes per day. In the SKALE project, the Karl­sruhe Insti­tute of Tech­nol­o­gy (KIT) with its part­ners Robert Bosch GmbH and Pow­er Inno­va­tion Strom­stech­nik GmbH is there­fore devel­op­ing a scal­able charg­ing sys­tem with pho­to­volta­ic sys­tem, sta­tion­ary lithi­um-ion stor­age and medi­um-volt­age grid connection.

The SKALE project com­bines elec­tro­mo­bil­i­ty and sta­tion­ary bat­tery stor­age in the con­struc­tion of a DC charg­ing infra­struc­ture for semi-pub­lic to pri­vate spaces. Intel­li­gent charg­ing man­age­ment, the use of decen­tral­ized renew­able ener­gy gen­er­a­tors and sta­tion­ary bat­tery stor­age can pre­vent peak loads and con­tribute to the sta­bil­i­ty of the pow­er grid. The research project looks at the entire ener­gy flow chain in order to increase charg­ing per­for­mance and effi­cien­cy and reduce costs. All require­ments from the grid-side pro­vi­sion of ener­gy to demand-based caching, dis­tri­b­u­tion and con­ver­sion, to vehi­cle bat­tery and return to the grid are tak­en into account. “The new approach is intend­ed to pro­vide a for­ward-look­ing infra­struc­ture solu­tion for any park­ing space with a vari­ety of charg­ing points and to effi­cient­ly inte­grate decen­tral­ized ener­gy sources,” says Pro­fes­sor Marc Hiller of KIT’s Elec­trotech­ni­cal Insti­tute (ETI).

Local flex­i­bil­i­ty and high efficiency

Cur­rent­ly, elec­tric vehi­cles can be charged either via alter­nat­ing cur­rent (AC) or DC. When charg­ing with alter­nat­ing cur­rent, con­vert­ing to direct cur­rent in the vehi­cle reduces the charg­ing pow­er and effi­cien­cy of the charg­ing process. When charg­ing with direct cur­rent, the charg­ing elec­tron­ics are installed in the charg­ing sta­tions. This allows for an increase in charg­ing per­for­mance and effi­cien­cy, but there are sig­nif­i­cant costs on the part of the infra­struc­ture. “The prob­lem is that both charg­ing con­cepts focus either on the vehi­cle or only on part of the infra­struc­ture, but do not look at the entire ener­gy flow chain,” explains Nina Mun­zke, group leader at the ETI. In con­trast to con­ven­tion­al charg­ing meth­ods, the mains pow­er elec­tron­ics are to be par­tial­ly cen­tral­ized, a buffer tank is used, load flows are cen­tral­ized and the ener­gy is dis­trib­uted in a DC grid. This should lead to cost sav­ings, high scal­a­bil­i­ty, appli­ca­tion flex­i­bil­i­ty and high efficiency.

With­in the frame­work of SKALE, a demon­stra­tor of the charg­ing infra­struc­ture is to be built. The planned con­struc­tion includes around ten charg­ing sta­tions, a pho­to­volta­ic sys­tem with an out­put of about 100 kilo­watt peak and a bat­tery stor­age sys­tem with a capac­i­ty of approx. 50 kilo­watt hours. The aim of the demon­stra­tor is to gain prac­ti­cal expe­ri­ence for the con­struc­tion and oper­a­tion of the charg­ing infra­struc­ture. The mea­sure­ment data obtained will be used for ener­gy sys­tem opti­miza­tion and the con­struc­tion of future plants.

Sta­ble and safe operation

Concept of the scalable charging system: grid connection to the medium-voltage grid, connection of various charging stations, a photovoltaic system and stationary lithium-ion storage via a DC network. (Graphic: Starosta, KIT)

Con­cept of the scal­able charg­ing sys­tem: grid con­nec­tion to the medi­um-volt­age grid, con­nec­tion of var­i­ous charg­ing sta­tions, a pho­to­volta­ic sys­tem and sta­tion­ary lithi­um-ion stor­age via a DC net­work. (Graph­ic: Staros­ta, KIT)

Charg­ing points, ener­gy stor­age, decen­tralised ener­gy gen­er­a­tors and grid con­nec­tion points: In the SKALE project, the EIT deals with the sta­ble and safe oper­a­tion of the DC net­work. For this pur­pose, the con­nec­tion of the charg­ing infra­struc­ture is also about the devel­op­ment of a suit­able invert­er con­cept for the con­nec­tion to the medi­um-volt­age grid, which has a high effi­cien­cy and can be con­struct­ed as com­pact­ly as pos­si­ble, but at the same time is also eco­nom­i­cal­ly attrac­tive. Based on the con­cept, a lab­o­ra­to­ry demon­stra­tor will be set up at the Eti, scaled in cur­rent and volt­age, to pro­vide insights into oper­a­tional man­age­ment and com­pli­ance with net­work requirements.

Based on sim­u­la­tions, the KIT sci­en­tists cre­ate an inter­pre­ta­tion rec­om­men­da­tion for the over­all sys­tem in the project and devel­op an inter­pre­ta­tion tool that can design and opti­mize the charg­ing infra­struc­ture, includ­ing its com­po­nents, for a spe­cif­ic loca­tion. The mea­sure­ment data from the demon­stra­tor can be used to eval­u­ate the effi­cien­cy of the over­all sys­tem, includ­ing the effi­cient use of renew­able energies.

The SKALE research project, with a project vol­ume of around 4.3 mil­lion euros, is fund­ed by the Fed­er­al Min­istry of Eco­nom­ics and Ener­gy and start­ed at the turn of the year.

Details on the bat­tery tech­nol­o­gy: