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Automated Electrical Demand Controller

Electrical energy contracts for power sold to industrial electrolysis plants frequently have charges based upon a demand (MW) and an energy (MWh) component. Electrical demand is a measure of the maximum power draw of megawatts on the electrical system. It is more or less equivalent to a "Capacity Charge" and is usually a fixed fee per billing period. Often there are substantial penalties for exceeding a contracted maximum demand entitlement at any time during the contract year. Sometimes a single spike in demand for a single hour will "ratchet" the billing demand upward for the entire next year. Since demand charges can approach 35 to 40% of the entire power bill, controlling the maximum demand imposed upon the system is a high priority to avoid the "ratchet". The typical method to avoid setting a new demand peak is by reducing the normal operating level of the plant a safe distance away from the maximum demand limit. Often, the so-called "load factor" of the plant will be in the range of 90%, meaning that up to 10% of the available demand is wasted.

On the other hand, the energy component is a true variable cost depending upon how much energy (in megawatt hours) is taken each month. For electrolytic processes, the energy component is directly proportional to the quantity of product made. Thus, reducing the average level of energy consumption to avoid setting a new maximum demand reduces the amount of the main product made by the plant.

The challenge in operating an electrolytic process at maximum profit with minimum cost is therefore a balance between keeping the average rate of production as high as possible while avoiding setting a new maximum demand. The incentive to raise the load factor is very high because the incremental tons of production made will incur only the variable cost of the energy because the maximum demand charge is a fixed cost. The incremental profit is very high. However, without some automated system in place, this goal is nearly impossible to reach.

The Solution

Electrolytic processes, such as chlorine, aluminum, or chlorate manufacture have a unique ability to implement automated demand control since the consumption of electrical energy for accomplishing the chemical reaction is very large in comparison with the electricity used for motors or lighting. Universal Dynamics has developed automatic demand control systems that reliably control the load factor to as high as 99.9% for many consecutive years. Payback of the investment for such a system is often measured in months rather than in years.

The principle of the control is to predict the integrated periodic demand by monitoring the rate of electrical consumption and keeping track of the minutes remaining in the billing cycle (usually one hour). Small automatic increases or decreases in the production rate are made to keep the integrated demand precisely on the set point at the end of the cycle. Changes in the system voltage or load fluctuations due to starting or stopping of motors are automatically detected and corrections are made. The maximum allowed rate of rise and fall in the production rate is customized for each customer to minimize other process impacts.

Using Universal Dynamics's Demand Control results in the maximum production with the minimum cost. This system can yield a substantial improvement in plant profitability. The two graphs illustrate the concept. The area shown in red shows the wasted demand entitlement.