Cost of Electrical Energy

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Power Control and Energy Management Solutions

Power Limiting on Multi-Zone Furnaces using Thyristors

Energy cost is one of, if not the most important issue in the glass industry at the moment and improvements in efficiency of use or reductions in energy cost would have a substantial influence on the operational result of any glass producer.

In applications where an electrical energy user has to control multiple loads the Predictive Load Management features of Eurotherm EPower controllers will give better control over Peak Power Demand. In many countries the monthly Peak Power Demand is a critical factor in the cost that industrial users have to pay for electrical energy.

The Predictive Load Management* function of EPower controllers offers two features that can lead to a reduction in Peak Power Demand and therefore a reduction in cost. The features are:

Load sharing
Load shedding

In many applications the installed power capacity exceeds the required power during normal operation. Predictive Load Management uses this overcapacity by distributing loads on/off switching over time in such a way that the power required from the network is at any moment in time lower than the installed power capacity.

Load sharing

The load sharing feature takes care that a group of loads is switched on/off in such a way that the average power demand is flattened out as shown in the following diagrams.

In the first diagram the loads are switched on in a synchronised fashion.

The moment the furnace is switched on all loads will be switched on at the same time causing a peak demand equal to the installed power.

The second diagram shows the effect of desynchronising the switching of the loads and the effect on the peak demand.

The result is a decreased Peak Demand and flattened average power use. By switching on EPower controllers “Efficient Power” features the result becomes even better.

The average power demand shows an almost flat line, which results in less flickering and a reduced peak.

The efficiency of the use of power can be characterised by the Power Efficiency Factor K, a value between 0 and 1. The closer to

one the more efficient the use of power is.

K is calculated with the following equation:

In this example the installed power is 3138kW with a total power per modulation period of 1792kW.

The Power Efficiency Factor goes from 0 in the synchronised case to 0.84 in the desynchronised case and 0.97 by the use of “Efficient Power”

However, although Peak Demand is reduced significantly, it is not limited to a certain level. To limit the peak demand we can use the

load shedding feature of EPower controllers.

Load shedding

With load shedding a user can define a maximum allowable peak demand. Because this feature will actively reduce the power applied to certain loads it cannot be used under all circumstances. The possible effects of load shedding on the behaviour, the dynamic

stability of control and accuracy of control should be taken into consideration before load shedding is applied.

It is particularly useful when large thermal masses such as a lehr are controlled.

When all the signs are at green a user gets real dynamic control over its peak power emand.

In this case for instance the power demand during normal operation could be reduced by 20% to 1433kW without harming process dynamics. The limit of 1433kW is set in the Eurotherm EPower master controller and Eurotherm patented load shedding feature takes care that the limit is never exceeded.

So rather than a peak demand that is unpredictable and could be as high as the installed power of 3138kW load shedding in this case takes care of a controlled maximum peak demand of 1433kW.

To be able to predict the cost benefit for the user we would have to know how electrical energy costs are calculated and invoiced by the power company.

The tangle of rules, regulations, rates and conditions

Before the liberalisation of the power market life was easy. As a power user one had the choice out of a list of one power supplier, the power supplier active in the region and that was it. In each country all power suppliers applied comparable conditions and prices. That one company was responsible for generating and delivering the power at the user’s doorstep.

So how have things changed? The energy market in many areas is completely liberalised and a user can buy power from any supplier all over Europe, with power suppliers being totally free in setting their conditions and prices.

Rather than dealing with one supplier responsible for the complete supply chain, a user now has to deal with three companies and a tangle of rules, regulations, rates and conditions.

These three companies are:

The energy supplier. The company from which a user buys the actual kWh’s - a highly competitive market.
The network operator. The company that transports the actual energy from the generator to the customer. This company owns the grid in a region and often has a monopoly in that region.
The metering company. An independent company that meters the actual energy use, the monthly peak demand and the power factor.

Energy Supplier

The four main variables used to fix the price of a kWh for an energy contact are:

Contract power in kW: the maximum power expected to be

used at any moment i.e. equal to the total installed power

Annual use during off-peak hours: i.e. between 23.00 and 07.00 and weekends
Annual use during peak hours
Duration of a supply contract: in most cases a minimum period of 1/2 a year is applicable