From park­ing garage to bat­tery storage

In the age of elec­tro­mo­bil­i­ty, park­ing garages are trans­form­ing into charg­ing sta­tions. In Schwäbisch Hall, research is being con­duct­ed into how a park­ing garage can even become a large, vir­tu­al bat­tery stor­age facil­i­ty that can be con­trolled to serve the grid. The cal­cu­la­tion of the load fore­cast, which is influ­enced by a vari­ety of fac­tors, is of cen­tral importance.

As co-oper­a­tor of the Langer Graben park­ing garage, which dates back to the 1980s, Stadtwerke Schwäbisch Hall faced the chal­lenge of installing con­tem­po­rary e‑charging infra­struc­ture dur­ing the refur­bish­ment. Instead of a com­plex and expen­sive pow­er grid rein­force­ment out­side and inside the prop­er­ty, the ener­gy sup­pli­er opt­ed for an intel­li­gent charg­ing and load man­age­ment sys­tem from its asso­ci­at­ed com­pa­ny enisyst GmbH. As a sci­en­tif­ic part­ner for the devel­op­ment of a solu­tion for mon­i­tor­ing and opti­miz­ing charg­ing and load man­age­ment, the Stuttgart Uni­ver­si­ty of Applied Sci­ences is involved via the “Smart_E_Park” research project. This is being fund­ed by the state of Baden-Würt­tem­berg as part of the “Intel­li­gent grid con­nec­tion of park­ing garages and under­ground garages” pro­gram. The long-term goal is to turn the park­ing garage into a grid-serv­ing ener­gy stor­age facil­i­ty, with the bat­ter­ies of the parked elec­tric vehi­cles act­ing as a bilat­er­al­ly usable ener­gy buffer, for exam­ple for Redis­patch 2.0.

Bal­anc­ing and dis­trib­ut­ing load­ing loads

What chal­lenges did and do the project part­ners face? The new trans­former at the Langer Graben park­ing garage is designed for a max­i­mum of one megawatt of pow­er. In the park­ing garage, around 100 of the total of almost 500 park­ing spaces have been equipped with charg­ing box­es, each of which allows up to 22 kilo­watts of charg­ing pow­er. If all 100 charg­ing park­ing spaces were occu­pied and charg­ing at 22 kW, the trans­former capac­i­ty would be exceed­ed by a fac­tor of about 2. The charg­ing sys­tem must there­fore be able to man­age charg­ing oper­a­tions in such a way that elec­tric­i­ty sup­ply and demand, as well as charg­ing capac­i­ty, are in bal­ance and the sys­tem as a whole remains sta­ble. This is ensured by the cas­cad­ing con­nec­tion sys­tem with intel­li­gent dis­tri­b­u­tion box­es from enisyst. The sys­tem is designed to bal­ance and dis­trib­ute loads. This means that if more vehi­cles need to be charged, the spe­cif­ic charg­ing pow­er is throt­tled. But the sys­tem would also be able to pri­or­i­tize indi­vid­ual charg­ing points, for exam­ple, for peo­ple who are will­ing to pay a pre­mi­um for faster charg­ing. The charg­ing infra­struc­ture has been in oper­a­tion since the reopen­ing of the park­ing garage in Octo­ber 2021 and is func­tion­ing as planned. How­ev­er, the uti­liza­tion of the e‑charging park­ing spaces is only around 25 per­cent today.

The par­tic­u­lar chal­lenge — and so far there have been at best rudi­men­ta­ry find­ings on this — lies in deter­min­ing a typ­i­cal load curve for a park­ing garage with e‑charging spaces. A reli­able load fore­cast is an indis­pens­able pre­req­ui­site for the planned use of the bidi­rec­tion­al­ly con­nect­ed e‑mobiles in the park­ing garage as ener­gy stor­age. Math­e­mat­i­cal­ly, 100 vehi­cles with 80 kWh charg­ing capac­i­ty could store up to eight megawatt hours of elec­tric­i­ty in the Langer Graben park­ing garage, which is more than twice the annu­al elec­tric­i­ty con­sump­tion of an aver­age house­hold. Although only part of this could be used as con­trol reserve, this would still be an attrac­tive sce­nario for the grid operator.

Fore­cast­ing challenge

To cre­ate the load curve, there were and are many para­me­ters to explore: When do e‑mobilists charge? What types of vehi­cles and bat­tery capac­i­ties are involved? In which bat­tery charg­ing sta­tus do they reach the charg­ing sta­tion? How long and how much pow­er is charged? The hard­ly sur­pris­ing answer is that the refu­el­ing behav­ior of elec­tric vehi­cle own­ers in park­ing garages is com­plete­ly dif­fer­ent from that of com­bus­tion vehi­cles. With the lat­ter, the tank is often dri­ven almost emp­ty and then filled again com­plete­ly. Elec­tric vehi­cles, on the oth­er hand, are charged more often and usu­al­ly only par­tial­ly and — depend­ing on the park­ing time — even when the charge lev­el is still high. The mul­ti­tude of volatile fac­tors makes fore­cast­ing complicated.

Nev­er­the­less, it has been pos­si­ble to gen­er­ate a uti­liza­tion fore­cast that is quite close to real­i­ty. On the one hand, an app was devel­oped that records which vehi­cles are charg­ing where, for how long, and with what pow­er. The sec­ond input source was pub­lic data from park­ing garages in North Rhine-West­phalia, which pro­vid­ed infor­ma­tion on how long vehi­cles typ­i­cal­ly stay in a park­ing garage and what the occu­pan­cy rate is at what time of day. The data from both sources were com­bined and a load curve was mod­eled from them with the sup­port of learn­ing algo­rithms and arti­fi­cial intel­li­gence devel­oped by the Stuttgart Uni­ver­si­ty of Applied Sci­ences. This work has not yet been ful­ly com­plet­ed, but the fore­casts already achieve an approx­i­ma­tion of the actu­al sit­u­a­tion of over 90 percent.

Fur­ther hard­en­ing of the data basis

It must be tak­en into account — and this is one of the rea­sons why fur­ther research is nec­es­sary — that the load fore­casts to date are based on low occu­pan­cy rates of the wall­box­es. With fur­ther pen­e­tra­tion of elec­tric vehi­cles and more fre­quent use of the e‑parking sys­tem, some para­me­ters of charg­ing behav­ior and sys­tem load may change. Very dynam­ic devel­op­ment can be expect­ed in the elec­tro­mo­bil­i­ty mar­ket, and fac­tors such as urban and neigh­bor­hood devel­op­ment, for exam­ple through changes in shop­ping behav­ior, can also have an influ­ence on user behav­ior. Fur­ther data must be con­tin­u­ous­ly col­lect­ed and insights gained for these sce­nar­ios. In the end, a data­base should be avail­able that makes it pos­si­ble to use the bat­ter­ies con­nect­ed to the charg­ing sys­tem in the park­ing garage as a vir­tu­al ener­gy store. The fact that tech­ni­cal rules still have to be cre­at­ed for this is anoth­er mat­ter. The oper­a­tor, Stadtwerke Schwäbisch Hall, says it wants to be ready to act when elec­tric vehi­cles become the stan­dard means of transportation.

What is already cer­tain is that the data obtained in Schwäbisch Hall will not be direct­ly applic­a­ble to oth­er park­ing garages that are being con­vert­ed into mul­ti-charg­ing sta­tions: It makes a dif­fer­ence whether peo­ple park their e‑mobiles in a big-city park­ing garage dur­ing the day while they go shop­ping, or whether the park­ing garage is locat­ed in an old res­i­den­tial dis­trict with­out asso­ci­at­ed under­ground garages and func­tions as a kind of fill­ing sta­tion for the night, so to speak.

Fur­ther steps in planning

The next project steps in the Langer Graben park­ing garage address, among oth­er things, the fur­ther broad­en­ing and val­i­da­tion of the data­base and bidi­rec­tion­al charg­ing. enisyst GmbH is already deal­ing with this future sce­nario in prac­tice else­where. Fur­ther­more, it is about the prac­ti­cal con­nec­tion of the E‑Parkhaus Langer Graben to the net­work con­trol cen­ter of the Schwäbisch Hall munic­i­pal utility.

Also on the to-do list is the inte­gra­tion of the charg­ing process­es into the con­sump­tion billing of Stadtwerke Schwäbisch Hall. Until now, users have paid for the tapped elec­tric­i­ty with their park­ing tick­et on site or by using a mobil­i­ty card from Stadtwerke Schwäbisch Hall. In the future, it will also be pos­si­ble to invoice the park­ing garage charg­ing cur­rent via nor­mal con­sump­tion billing. Con­sid­er­a­tion is also being giv­en to the imple­men­ta­tion of flex­i­ble charg­ing rates at quar­ter-hourly inter­vals. They are also a tool for grid load man­age­ment, because cus­tomers can fill up on elec­tric­i­ty at low cost when the gen­er­a­tion sup­ply is cur­rent­ly high.

Con­clu­sion: Imple­ment­ing intel­li­gent charg­ing man­age­ment in park­ing garages and con­trol­ling it to serve the grid is a com­plex task. Their real­iza­tion requires a lot of tech­nol­o­gy and know-how. To be able to con­trol an e‑parking sys­tem in advance and use it as a large bat­tery stor­age sys­tem, a lot of infor­ma­tion and data must be processed and eval­u­at­ed. There is no way around this grass­roots work, and there is still a long way to go. It is there­fore to be wel­comed that a munic­i­pal util­i­ty like Stadtwerke Schwäbisch Hall is doing pio­neer­ing work here and play­ing a key role in paving the way for a new world of mobility.