26 / 52
PLANT MAINTENANCE, TEST + MEASUREMENT
R
emember… Power is the rate at which energy is consumed by
a load at any instant in time. In other words, the load ‘demands’
a certain rate at which the energy is transferred. Watt is the
measurement of Power, describing the rate at which electricity is be-
ing used at that particular moment. For example, a 100 Watt (W) light
bulb ‘demands’ 100 W of electricity at any moment when turned on.
On the other hand, energy or consumption is measured in watt-
hours which describes the total amount of electricity used over time.
Watt-hours are a combination of how fast the electricity is used (watts)
and the length of time it is used (hours). For example, a 100 W light
bulb, which demands 100 W at any one moment, uses 100 watt-hours
of electricity in the course of one hour.
A simple way to determine the difference between demand and
consumption is by examining these two examples.
In
Figure 1
, a 100 W light bulb burning for 10 hours consumes
1 000 watt-hours or one kilowatt-hour (kWh). The entire time it is
turned on, it ‘demands’ 100 W from the power station. That means
the power station is required to have 100 W available whenever the
customer switches on the light.
Figure 1: One 100-watt light bulb burning for 10 hours consumes
1 000 watt-hours or 1 kilowatt-hour (1 kWh).
Similarly in
Figure 2
, ten 100 W light bulbs burning for one hour also
consume 1 000 watt-hours or 1 kWh, but it now requires or ‘demands’
1 000 W or 1 kW. Observe that in both instances the consumption
is 1 kWh. However, take a look how differently the second scenario
impacts the power station from a demand point of view. It is now
required to supply ten times as much power in response to the ‘de-
mand’ of the 10 light bulbs operating at the same time. These two
clients will receive identical bills because both of them used 1 kWh
of energy. However, the utilities have an ace up their sleeve, as we
shall see shortly.
Figure 2: Ten 100 watt light bulbs burning for one hour also consume
1 000 watt-hours or one kilowatt-hour (1 kWh.)
Demand is measured in kilovolt-amps (kVA), which is broadly speak-
ing the same as kilowatt (kW) but a phenomenon known as Power
Factor complicates the equation. This article is not the forum to explain
that, so for the sake of this discussion we will assume a Power Factor
of 1, which makes kW = kVA.
In most instances the amount of electrical energy (kWh) required,
is not the problem. The biggest problem arises when the demand
(kVA) escalates. Consumers are frequently requested to turn off their
geysers and other unnecessary equipment during the morning and
evening peak (see
Figure 3
). When the demand outstrips the supply,
load shedding occurs, which is Eskom’s style of Demand Manage-
ment, albeit an inefficient, but necessary, method.
Figure 3: Daily Peak demand occurs between 06:00 and 09:00 (morn-
ings) and 17:00 and 20:00 (evenings).
Peak Demand
Management
Facility Case Study
Hannes Roets, Power Optimal
To understand the concept of Demand Management, we have to refresh our memory of electricity basics a little.
Electricity+Control
September ‘16
24