How it Works

The problem EnPot solves

Until now, aluminium smelters have required an 'always on' electricity supply, to keep the electrolysis process running stably. Due to this lack of flexibility, aluminium smelting accounts for more than 3% of the world’s total electricity use.

These limitations have a significant effect on the cost of producing aluminium because smelters cannot vary their energy use to match electricity price fluctuations. This also creates power supply issues for national grids trying to use more renewable energy and means the industry cannot respond to oversupply issues.

The EnPot technology gives most smelter operators the ability to turn energy consumption up or down by as much as 30%. This enables them to take advantage of off-peak power prices and intermittent renewable energy sources to better match supply with demand, in cooperation with electricity generators and grid operators.

"Currently the energy input of a smelter cannot be varied by much more than a few percent, so smelters operate at full capacity 24/7, 365 days a year, for their entire lifespan." Karyna Young, CEO, EnPot

Functionality

The EnPot system covers the sidewalls of each pot with patented 'heat exchangers', connected to an external ducting and suction system.

The airflow to each bank of exchangers can be varied using precisely controlled extractor fans. When power to the pot is reduced, the airflow to the exchangers is also reduced, effectively insulating the pot to maintain the required heat balance.

Similarly, at higher power the airflow is increased to boost heat transfer and maintain the balance. This allows power usage to be changed at any time while maintaining pot temperatures and preventing process disturbances. Operationally, once the line current has been set, EnPot modulation is essentially hands-off.

Capability

The EnPot technology enables

20% Stable modulation of +/-20% power consumption at any time, for any duration.

30% Up to +/-30% power for long term modulation with some process changes, for any duration.

This has been proven through extensive development and plant trials through to full commercial installation¹.

¹ Depree N., Düssel R., Patel P., Reek T. (2016) The 'Virtual Battery' - Operating an Aluminium Smelter with Flexible Energy Input. In: Williams E. (eds) Light Metals 2016. Springer, Cham.
Düssel R., Mulder A., Bugnion L. (2019) Transformation of a Potline from Conventional to a Full Flexible Production Unit. In: Chesonis C. (eds) Light Metals 2019. The Minerals, Metals & Materials Series. Springer, Cham.

Smelting cell and heat loss without EnPot

First, let's look at how smelters work without EnPot installed.

Half of the energy used in an aluminium 'reduction cell' is used to make metal, the other half is lost as waste heat to preserve the delicate energy balance of the cell.

Heat loss from the top is by fume gas extraction to the Gas Treatment Centre. Heat loss from the sides and bottom is by natural convection to the pot surrounds.

EnPot installation - arrangement

With EnPot the delicate heat balance required for stable, efficient pot operation is achieved. This enables smelters to vary their energy consumption indefinitely, to produce exactly as much aluminium as required.

The system is designed for maximum sidewall coverage; therefore, the number and size of heat exchangers is smelter specific.

EnPot technology: The breakdown

Shell heat exchangers
Branch ducts
Main duct
Suction fan

EnPot installation - detail

Suction fan draws ambient air from the potroom basement through the shell heat exchanger.
The (now) heated air is extracted from the shell heat exchanger exit, through the ducting network and expelled to the atmosphere as low grade waste heat.
Note: The EnPot system does not use this waste heat, making it available to the smelter for other purposes.

EnPot patented Shell Heat Exchanger units

The internal design of the EnPot heat exchangers create enhanced turbulent heat transfer, for maximum cooling effect.
The exchangers also incorporate an insulating layer so when air flow is reduced below the neutral point (see Figure 6 below) they act as insulators, decreasing heat loss from the cell.
Hence, the EnPot system can act in either cooling mode or insulating mode with the turn of a dial.
In the case of a power outage, the EnPot system can be used at maximum insulation mode to significantly extend the viable recovery time of the potline.

Base load energy usage without EnPot

A smelter without EnPot requires fixed energy input to maintain constant heat balance. Even small variations in energy input will upset the heat balance causing harm to the process and may require compensatory actions by the smelter. Read more about Energy Modulation (PDF).

Base load energy usage without EnPot

Natural heat loss only
Power setpoint with very limited flexibility

Base load energy usage with EnPot - neutral mode

The EnPot system requires a small amount of airflow to maintain heat balance to the pre-EnPot level.

Neutral Mode

Minimal airflow provided by suction needed to maintain existing heat balance

Increased energy usage with EnPot - cooling mode

An increase in amperage results in an increase in production and an increased cooling requirement. This requires more air flow to maintain heat balance and to extract the additional excess heat.

Cooling Mode

Increased smelter power consumption
Increased fan speed means increased cooling to maintain heat balance

Decrease energy usage with EnPot insulating mode

A decrease in amperage results in a decrease in production and smelter power consumption. The decreased cooling requirement decreases air flow to insulate the pot and maintain heat balance. This prevents cooling or catastrophic freezing of the pot's contents.