CH:
Peak demand is an interesting issue in energy efficiency because it is primarily a cost and
capacity issue, where businesses may be charged a higher rate for all of their electricity
because they have spikes where the demand is really high. These spikes also must be
within your rated supply capacity, and so spikes can push you to your allowable limit.
Kevin, what causes these spikes in demand?
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Thanks Celeste,
Up to now we have been looking only at average demand for energy – ie the energy
used over a period of time, for a production run or to produce a service.
But energy usage is not smooth, there are spikes in demand.
Obviously at times of high production, demand is high.
Across the grid nationally, there is a peak in demand during daylight hours, with an extra
peak around breakfast and dinner times. This is the ‘peak period’ and so consumption
outside this time is ‘off peak’ use and can be purchased at cheaper prices.
Particularly hot or cold days will add to this peak.
The same pattern of electricity use can also be seen in the demand for any user. This is
perhaps an obvious cause of peak demand.
However, the starting of equipment such as electric motors typically uses significantly
more electricity than running it. On start up an electric motor can draw up to 5 times (or
more) the current that it draws during normal operation.
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As we can see from the red line on the graph, starting current is significantly higher than running current. Turning one
motor on will cause a peak in demand. Turning them all on at once will cause a massive spike in demand for electricity
This often happens in a business – where the day starts and everyone turns on equipment, computers with fans, air
conditioning and so on.
This can have a couple of adverse effects:
• some power utilities will set your power rate of $/kWh based on your peak demand, and so a high peak can mean
paying more for every kWh used
• your power supply will be rated for a certain maximum load, and if you are close to it because of your peak
demand you may be restricted in not being able to install additional power using equipment without upgrading
your power supply at significant cost – a quarter of a million dollars for one company we talked to.
Reducing your peak demand, means this extra capacity is available for free.
Peak demand can be reduced by staggering the start time of equipment so spreading the spike caused by each item of
equipment. Some companies we talked to found that by doing that they actually found that some items don’t need to
be turned on every day, and some for only part of the day so also saving on the use of power that is not producing
anything. In one company, all computers were turned on at the same time first thing in the morning. They discovered
that the sales reps often did not get into the office until much later in the day, and so their computers only needed to
be turned on for half the day.
Scheduling overall production can help as well. One business we talked to realised that they could schedule work on
certain machines only on particular days then run jobs on the other machines on alternate days. This is more efficient
because the machines are fully utilised on those days, again not idling and waiting for work.
Peak demand can also be reduced by changing the way the motors start. This is often called a ‘soft start’ and while
soft starting still draws more power than normal running, it is much less. The typical soft start current draw is shown
as the blue line on the graph. Soft start motors reduce peak energy use and they also reduce damage to equipment
due to their lower starting torques. This is suitable and even beneficial for many situations, but not appropriate for
situations requiring high starting torque.
Where equipment takes a while to come on line and be ready for use, then turning it on might need to be part of the
production schedule. It may be discovered that some items of equipment do not need to be on all day, every day with
a bit of smart scheduling. This will reduce power demand and so cost.
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Managing the peak demand allows you to increase you productive capacity while staying
within your supply capacity without an expensive addition to your power supply capacity
if you are approaching your limit.
It can also reduce the cost of your power bill, by reducing the overall tariff you pay and
there can be some energy efficiency benefits.
Peak demand can be managed simply by staggering the start times for equipment,
particularly large equipment. It doesn’t need to be staggered by much to make an
impact.
Maybe it’s a matter of not turning everything on at the start of the day, but rather
turning them on when required. This may also save energy by not having ‘idle’ power
consumption on equipment that has been started before it is needed.
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Perhaps surprisingly computers have an impact as well – we use a lot of computers! So
you can get information from the hardware supplier – or sometimes on the net – about
energy consumption for start up and running computers – and don’t forget to look at the
rating plates on your computers, monitors, printers etc. As one business we interviewed
said you can work out whether it is better to turn everything on in the morning - even
though people are in meetings or getting coffees - and how much that contributes to
your peak demand. Or is it better to turn them on when you need them and perhaps the
person has to wait a couple of minutes for things to boot up, but they can get their
papers ready while they are waiting.
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CH
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