By Vvon Engineering Team | Published | Updated
How rolling-mill load, billed demand intervals and power quality affect the value of a rooftop array.
Solar EPC & street lighting RFQ checklist (TXT)
A rolling mill may have a large roof and a much larger electrical demand. Establish the share an array could actually supply before quoting a percentage saving. The result depends on the mill process, roof and operating schedule.
That does not mean the answer is no. It means the case has to be built on the right line of the bill, and there are two lines that most proposals ignore.
A re-rolling mill has a very high connected load and a low load factor. The reheating furnace is usually fired on gas or furnace oil, so the largest energy input is not electrical. What is electrical is the rolling train: main drive motors that see a heavy transient every time a billet enters a stand, plus the roughing and finishing stands, the cooling bed, the shear, the descaler pumps and the water treatment. The demand swings hard and it swings fast.
An induction-furnace billet plant has a different load profile because melting is electrical. Calculate the likely solar share from measured consumption and feasible roof capacity; it may be modest without being economically irrelevant.
So the arithmetic goes in this order. Take twelve months of consumption. Take the roof area actually available after obstructions and setbacks. Work out the plant that area supports. Express it as a share of annual consumption, and put that number in front of the owner before anyone talks about payback. If the honest share is small, that is the starting position, and every subsequent claim has to live inside it.
There is a second consideration on the energy side that cuts the wrong way for heavy industry. Industrial tariffs in Bangladesh charge a higher rate in the evening peak hours. A solar plant generates in the off-peak daytime block, so it displaces the cheaper units and leaves the expensive ones untouched. For a business whose demand peaks in the evening, that weakens the case relative to a business that runs hardest at noon. It is worth stating plainly rather than being discovered in the first year's bills.
Check the actual tariff and bill to establish whether demand charges are based on contracted capacity, measured maximum demand, a minimum billing floor or another rule. Measured demand normally relates to a defined integration interval, not a single instantaneous spike. PV output does not automatically reduce the charge.
If billing responds to measured net demand, model PV output against the same interval data and charging rules. Only a reduction that changes the billed quantity produces a demand-charge saving.
Use timestamped interval data covering representative production and seasonal conditions. A midday historical peak alone is not enough: cloud or a different operating peak may still set the bill. Treat demand savings separately from energy savings.
Where the peaks do sit in daylight but the mill wants certainty rather than probability, the honest recommendation is not a bigger array. It is a demand controller that sheds a defined non-critical load as the integrating window approaches the threshold, with the solar plant reducing energy underneath it. Photovoltaics dispatch on sunlight; a controller dispatches on command, and only one of those can be relied on at the minute that matters.
Rolling mills run large induction motors with heavy transient loading, and the power factor is frequently poor enough to attract a penalty. Most sites deal with it through a capacitor bank that was sized for a load pattern the mill has since outgrown.
Some inverters support reactive-power control within model-specific operating limits. Confirm capability, control integration and utility approval before assigning a power-factor benefit; additional metering or control work may be needed.
Three things to establish before counting on it. Confirm the inverter model's reactive power capability and the range it will hold at, since supplying reactive power consumes part of the inverter's rating and reduces the active power it can deliver at the same time. Confirm with the distribution utility that operating at a non-unity power factor is permitted under your interconnection agreement, because the setting is one they approve. And check the interaction with the existing capacitor bank so that the two are not fighting each other, which is a real risk where the bank is switched on a controller with its own target.
Some manufacturers also offer reactive power support at night, with the inverter drawing a small auxiliary supply to provide correction outside generating hours. Whether it is worth enabling depends on the tariff and on what the bank already does, but it is worth asking the question at tender stage rather than discovering the capability three years in.
Rolling mill sheds are tall, wide and usually uninsulated, and the roof runs hot. That costs yield, so mount with a real air gap rather than flush to the sheet. The atmosphere inside carries mill scale and, near the furnace, oily particulate, which makes for a soiling deposit that is greasy rather than dusty and needs a detergent wash rather than water alone.
Light engineering workshops, incidentally, are much better candidates than the mills they serve. A workshop with CNC machines, welding sets, compressors, a paint booth and a single day shift has a daytime load with good coincidence to generation and a roof that stays comparatively clean. Their difficulty is the opposite one: the roof is often too small relative to what they would happily consume.
Consult the relevant edition and the documents for the exact equipment and site: SREDA: Net Metering Guidelines and current application documents; Schneider Electric: power-factor correction equipment and selection.
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