Ruhrguss Metallwerke melts around 34,000 tonnes of grey and ductile iron a year in Essen-Kray, on two coreless induction furnaces drawing up to 9 MW between them. Under the German network tariff, the site's demand charge is set by a single fifteen-minute maximum — one cold charge run in February can price the connection for the following twelve months. Three years of billing data showed the same pattern: the annual peak was never a production record, only a coincidence of two furnaces powering up while the sand plant and the compressor house happened to be running. The charge accounted for a fifth of the plant's energy spend.
Fixing it by scheduling was not on the table. Melt timing follows the moulding line, and the line follows the order book; the melt shop cannot hold a charge waiting for a quieter half hour without losing metal temperature. The distribution network operator had no spare capacity at the 20 kV incomer, so a larger connection meant a new transformer bay and a queue measured in years. The site itself offered no easy ground: the only free area was the old sand-reclamation slab, forty metres from the furnace hall, and the plant runs six days a week with a single scheduled August shutdown of nine days.
The solution
Twelve containerised LFP units sit on the reclamation slab — 12 MWh usable, liquid-cooled, with deflagration venting and a three-metre separation to the nearest structure agreed with the Essen fire authority. Three SMA Sunny Central Storage 2200 inverters give 6.6 MW of continuous power into a new ABB UniGear ZS1 bay, tied to the 20 kV incomer upstream of the furnace transformers so the battery sees the whole site load, not just the melt shop. Cable routing crossed a live scrap yard; the 400 metres of 20 kV cable went in on an elevated tray rather than a trench. Commissioning ran in four stages across nine weeks. No melt was stopped.
The control layer does the real work. A site controller reads furnace power set points and the moulding line schedule over OPC UA, forecasts the rolling fifteen-minute average, and discharges to hold the meter under a target ceiling — full power in under 200 milliseconds, which is faster than an induction furnace can ramp. The ceiling was walked down through the first quarter, 8.8 MW to 6.4 MW, with the plant free to veto any step. Peak demand charges fell 31% over that quarter, and the melt shop no longer sets the tariff — the annual maximum is now made by the compressor house on a summer afternoon. Between melts the same asset is prequalified for FCR.
Peak charge cut
First quarter, against the prior year.
Usable capacity
Twelve LFP units, 6.6 MW continuous.
Peak load removed
Shaved off the site's billing maximum.
Retrofit to handover
Built around a running melt shop.
I argued against the setpoint schedule for six weeks — I was certain we would trip a melt and lose a heat. We never did, and now the hard part is convincing accounting that the January bill is not a mistake.
