Alternating mood

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At the very begin­ning of glob­al elec­tri­fi­ca­tion, there was a dis­agree­ment. Thomas Alva Edi­son, best known as the inven­tor of the light bulb, was con­vinced that direct cur­rent grids were the best way to sup­ply peo­ple with elec­tric­i­ty. For his oppo­nent George West­ing­house, engi­neer and entre­pre­neur, it was clear that alter­nat­ing cur­rent was bet­ter suit­ed for trans­mit­ting elec­tri­cal ener­gy. And thus at the end of the 19th cen­tu­ry a com­pe­ti­tion about the future stan­dard began. In the end, West­ing­house famous­ly pre­vailed: Thanks to the trans­former, it was much more cost-effec­tive to trans­port alter­nat­ing cur­rent over long dis­tances. But could Edi­son still in fact win in the end? Today, a good 130 years lat­er, it appears that direct cur­rent grids could make a comeback—albeit on a small­er scale for now.

Solar energy is on the rise

The ener­gy tran­si­tion makes it pos­si­ble and also nec­es­sary. To stop glob­al warm­ing, activ­i­ties are increas­ing world­wide to pro­mote phas­ing out fos­sil fuels and replac­ing them as quick­ly as pos­si­ble with renew­able sources. Already today, about 50 per­cent of the elec­tric­i­ty pro­duced in Ger­many comes from solar or wind pow­er plants. And if the cur­rent Ger­man gov­ern­ment has its way, a total of 300 gigawatts will come from renew­able ener­gies by 2035, with a large part of it from solar ener­gy.

The Fraun­hofer Insti­tute for Solar Ener­gy Sys­tems in Freiburg has cal­cu­lat­ed that Ger­many the­o­ret­i­cal­ly has enough sur­face area to gen­er­ate 3,160 gigawatts of solar pow­er, and 400 gigawatts would already be suf­fi­cient to make gas, oil, coal and nuclear pow­er super­flu­ous. Whether the fed­er­al government’s goals will be met on time is uncer­tain. But time is press­ing, because demand is simul­ta­ne­ous­ly on the rise due to con­sumers such as elec­tric vehi­cles and heat pumps, which explic­it­ly require direct cur­rent. And to con­stant­ly meet dai­ly demand despite volatile pow­er gen­er­a­tion from wind and solar, more and more bat­tery stor­age sys­tems will be need­ed.

“In total, 12 per­cent of green ener­gy goes unused. Tech­no­log­i­cal­ly and eco­nom­i­cal­ly, that is com­plete­ly inef­fi­cient.”Stephan Rupp, grid expert at Rein­hausen

What they all have in com­mon is that they are DC sys­tems. Until now, how­ev­er, they have each been con­nect­ed to the AC grid via an invert­er. Stephan Rupp, grid expert and busi­ness devel­op­er at Rein­hausen, who is work­ing close­ly on the future of pow­er grids, explains: “Tech­ni­cal­ly and eco­nom­i­cal­ly, this is com­plete­ly inef­fi­cient.”

An exam­ple will illus­trate this: When the sun shines at noon, solar pow­er sys­tems pro­duce the most elec­tric­i­ty. How­ev­er, the great­est demand occurs in the evening hours, when peo­ple come home from work and charge their elec­tric vehi­cles, for exam­ple. There­fore, the mid-day elec­tric­i­ty ends up in bat­tery stor­age sys­tems in the mean­time. Real­ly a sen­si­ble con­cept.

But the path from the gen­er­a­tor to the con­sumer is cur­rent­ly still quite cir­cuitous: From the solar-pow­er sys­tem, the elec­tric­i­ty first goes to the AC grid, from there to the bat­tery stor­age sys­tem, then back to the AC grid, until it final­ly reach­es the charg­ing sta­tion. In oth­er words, the elec­tric­i­ty will already have four con­ver­sion stages behind it before it charges the vehi­cle bat­tery. “At every stage, there are loss­es of at least 3 per­cent. So in total, 12 per­cent of green elec­tric­i­ty goes unused,” says Rupp. Wouldn’t it be much eas­i­er if we could sim­ply draw direct cur­rent direct­ly from the pow­er sock­et?

Lacking standards for DC technology

In the future, most of the solar pow­er from roofs and facades will be gen­er­at­ed in the low-volt­age grid, i.e., local­ly. “Cou­pling the DC sys­tems at the low­est volt­age lev­el with­out going through the AC grid would make the most sense,” Rupp says. Such a DC net­work would also have the advan­tage that the lines could be much thin­ner with a slight­ly high­er DC volt­age com­pared to AC volt­age, and few­er lines would be need­ed. This would reduce the cop­per need­ed by about 40 per­cent.

“The solu­tion could ini­tial­ly be small, local, low-volt­age DC grids, known as DC micro­grids,” says Rupp. But norms and stan­dards are cur­rent­ly lack­ing for the nec­es­sary tech­nol­o­gy that these grids require and that would ulti­mate­ly make them afford­able. Mar­co Stienek­er, Project Man­ag­er DC Pow­er Appli­ca­tions at Rein­hausen, empha­sizes: “We need to get to the point where DC tech­nol­o­gy can be used accord­ing to the plug-and-play prin­ci­ple. Only then will direct cur­rent be suc­cess­ful.”

“Cou­pling the direct cur­rent sys­tems at the low­est volt­age lev­el with­out going through the AC grid would make the most sense.“Stephan Rupp, Busi­ness Devel­op­er, Rein­hausen

To research these paths, Rein­hausen is par­tic­i­pat­ing in a series of projects. These include the Coper­ni­cus pro­ject ENSURE (New Ener­gy Grid Struc­tures), fund­ed by the Ger­man Fed­er­al Min­istry of Edu­ca­tion and Research (BMBF), in which DC sta­tions are being test­ed, among oth­er things; the Ger­man Fed­er­al Min­istry of Eco­nom­ics and Cli­mate (BMWK) project HPC-Prime (High Pow­er Charg­ing), which is devel­op­ing fast-charg­ing columns for DC grids; and the DC-Indus­try project, which is work­ing on DC grids for indus­tri­al appli­ca­tions.

Stienek­er, who devel­ops the nec­es­sary tech­ni­cal solu­tions for DC appli­ca­tions at Rein­hausen, empha­sizes: “We are not start­ing from scratch. We already have many years of expe­ri­ence with pow­er elec­tron­ics, invert­er and bat­tery sys­tems. We have a sol­id under­stand­ing of sys­tems from the old world, and we know how to trans­form that exper­tise into the new world.”

DC voltage in the local grid

pecif­ic use cas­es for direct-cou­pled DC sys­tems include shop­ping malls and com­mer­cial areas. Here, there is plen­ty of room for solar pow­er sys­tems on build­ings or even above park­ing lots. While peo­ple are shop­ping or work­ing in offices, they could charge their elec­tric vehi­cles direct­ly with clean solar pow­er. Bat­tery stor­age sys­tems would ensure that this is pos­si­ble even when the sky is cloudy.

But how might such a low-volt­age DC grid work? In prin­ci­ple, it is very sim­i­lar to an AC grid. Sim­i­lar­ly to the local net­work trans­former, a DC sta­tion would have to be con­nect­ed to the medi­um volt­age. In a sense, it is the heart of the DC grid and tech­ni­cal­ly func­tions like a con­vert­er with a built-in trans­former, sup­ply­ing the DC grid with 1,500 volts DC with out­puts of up to 2,000 kilo­watts.

“We need to get to the
point where DC tech­nol­o­gy can be used based on the plug-and-play prin­ci­ple.
Only then will direct cur­rent be suc­cess­ful.“Mar­co Stienek­er, Project Man­ag­er DC Pow­er Appli­ca­tions at Rein­hausen

Rein­hausen has already devel­oped a solu­tion for the DC sta­tions that is cur­rent­ly being test­ed as part of the ENSURE project: the GRIDCON® Pow­er­Con­ver­sion Sys­tem. “This is a mod­u­lar AC con­vert­er sys­tem for DC and AC sup­ply in the low volt­age range,” says Stienek­er. If the DC grid rep­re­sents the exten­sion of an exist­ing AC grid, the DC sta­tion could be set up in addi­tion to the local grid trans­former. In the event of grid fail­ures or faults, it is then able to also set up an AC grid from the bat­tery stor­age sys­tems in the DC grid to ensure sup­ply. And it can also pro­vide AC con­nec­tions in the low volt­age to sup­ply AC equip­ment. This increas­es sys­tem auton­o­my and the sup­ply secu­ri­ty of the grids.

The industrial grids of tomorrow

DC grids also offer enor­mous poten­tial for indus­tri­al oper­a­tions. Elec­tric dri­ves account for 70 per­cent of indus­tri­al elec­tric­i­ty con­sump­tion. With­out them, con­trol­ling robots such as those used in the auto­mo­tive indus­try would be impos­si­ble. How­ev­er, the fre­quen­cy con­vert­ers required for pre­cise motor con­trol also cause con­ver­sion loss­es. When con­nect­ed direct­ly to a DC grid, these would be reduced. In addi­tion, the brak­ing ener­gy could be fed back in the form of direct cur­rent and—if the grid is intel­li­gent­ly controlled—could direct­ly sup­ply oth­er sys­tems. Robots, for exam­ple, have to brake very fre­quent­ly in their sequences of motion. How­ev­er, until now, the ener­gy gen­er­at­ed in the process has been dis­si­pat­ed in the form of heat. Anoth­er dis­ad­van­tage of fre­quen­cy con­vert­ers is that they gen­er­ate dis­rup­tive har­mon­ics.

If these are elim­i­nat­ed, the pow­er sup­ply in the plant also becomes more reli­able and pro­duc­tion down­times are avoid­ed. Over­all, indus­tri­al com­pa­nies can there­fore sig­nif­i­cant­ly increase their effi­cien­cy if they set up a DC grid in the plant. Experts esti­mate that five per­cent of elec­tric­i­ty could be saved in this way.

Direct current on all grid levels

In the future, direct cur­rent trans­mis­sion at medi­um-volt­age lev­el is also con­ceiv­able. Research projects such as AC2DC, spon­sored by the Ger­man Fed­er­al Min­istry for Eco­nom­ic Affairs and Cli­mate (BMWK), in which Rein­hausen is work­ing togeth­er with the Tech­ni­cal Uni­ver­si­ty of Dres­den, are already address­ing this issue. DC trans­form­ers are then need­ed to cou­ple the dif­fer­ent volt­age lev­els togeth­er. Here, too, Rein­hausen has devel­oped an inno­v­a­tive solu­tion with the GRIDCON® DCT. “This is an elec­tri­cal­ly iso­lat­ed DC-DC con­vert­er, which in prin­ci­ple behaves in exact­ly the same way as a reg­u­lat­ed AC pow­er trans­former,” explains Stienek­er.

So will there even­tu­al­ly only be direct cur­rent com­ing out of the sock­et at home? Grid expert Rupp doesn’t think this will hap­pen so quick­ly. “But I can imag­ine DC tech­nol­o­gy slow­ly becom­ing estab­lished as a co-infra­struc­ture in res­i­den­tial build­ings and fac­to­ry build­ings as well, cou­pling the solar sys­tem on the roof with the heat pump in the base­ment and the car in the garage, for exam­ple.” So it could be the case that DC and AC emerge from his­to­ry with at least a draw.

YOUR CONTACTS

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Do you have ques­tions about DC grids?
Stephan Rupp and Mar­co Stienek­er are hap­py to help:

Stephan Rupp: S.Rupp@reinhausen.com

Mar­co Stienek­er: M.Stieneker@reinhausen.com