1 Lecture 11 Utility Load Analysis 2010 ME 430 Thermal Systems Design Utility Loads and Demand • utility loading varies with time of day, week, and season • the capacity of a plant depends upon maximum power demand by energy consuming devices on the system, • loads include motors, lights, heating etc., “ the success of the central-station system of energy supply stems from the diversity of the demand of the various components of the total connected load”
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1
Lecture 11
Utility Load Analysis2010
ME 430
Thermal Systems Design
Utility Loads and Demand
• utility loading varies with time of day, week, and
season
• the capacity of a plant depends upon maximum power
demand by energy consuming devices on the system,
• loads include motors, lights, heating etc.,
“ the success of the central-station system of energy supply stems
from the diversity of the demand of the various components of the
total connected load”
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Prime Movers
Distribution Bus
Feeders
Station Bus
Generators
Elementary Power System
Principal elements of a power system (from Skrotzki & Vopat, Power Station Engineering and Economy)
Loads
Examples of utility loads
that vary over time
3
Residential electricity load distribution over year (this is a house
with natural gas heating, augmented by a little electric heat in the
loft) and summer air conditioning.
Summer Peaking Utility
Jan Mar Jun Sept Dec
Toronto, Electricity Demand, 2001
0
2500
5000
7500
10000
Ele
ctr
icit
y (
MW
)
4
• the utility must be able to meet this load and account for:
– plant shut-downs for repairs or maintenance
– unscheduled plant shut-downs due to equipment failure
• the utility must operate to ensure that they produce the least cost electricity and that they get the best return on the investment in their generation assets (power plants, hydro electric, etc)
• the cost of electricity generated depends on the type of power plant, its fuel, its operational life span, and the maintenance costs associated with the plant
• electricity must be transmitted from the power plant to the load and this results in transmission power losses and additional costs
• in a free market system, the cost of electricity is determined by supply and demand
• to meet Ontario’s electricity demands, a non-profit organization buys and sells electricity capacity (power & energy) and sets the price,
• see www.ieso.ca.
Utility Loads
5
Part of the supply may be made up by Ontario Power Generation or by
independent electricity producers within the province – some electricity
will be bought or sold across the provincial borders and to/from the
USA.
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Load Duration Curves
Load Duration Curves define the energy/power requirement of a load in terms of
i) maximum demand
ii) total energy requirement
iii) distribution of energy demand
- there are also a number of other indices used to describe the characteristics of a utility load.
Load Duration CurveChronological Load Curve
Load Indices
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Load Duration Curve
Quebec Load Data (Winter Peak)
0
5000
10000
15000
20000
25000
30000
1 1001 2001 3001 4001 5001 6001 7001 8001
Hour of year
Lo
ad
, M
W
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Load Factor
Lavg E/h
Load Factor = -------- = --------
Lmax Lmax
where:Lavg = average load for period
Lmax= peak load for period, for period ~ or <1 hour
E = total energy under the load cure, e.g., the integral
h = total number of hours in period
Load Indices
Load Factor: L/D Curves with same Max demand
and Load Factors
kW
hours
kW
hours
kW
hours
kW
hours
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Capacity Factor
Lavg
Capacity Factor = --------
Cap
where: Cap = rated capacity of plant
Utilization Factor
Lmax
Utilization Factor = --------
Cap
Load Indices
• the devices making up the load each have a maximum capacity for
absorbing (consuming) power
• if all devices ran at full load then the “maximum demand” would
equal the “connected load”.
• however, experience shows that the maximum demand of a
consumer is less that their connected load at the same time
• maximum demand is related to connected load by the “demand
factor”
Maximum demand
Demand factor = ------------------------------
Connected load
The Demand Factor depends on the type of the load
e.g., hotels 25%, refrigeration plants 90%
Load Indices
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• although each device may reach its own maximum demand at sometime,
the “demand factor” measures the extent it contributes toward the maximum
demand of the “group” of devices that it belong to.
• the maximum demand of individual consumers does not occur
simultaneously but is spread out over a period of time.
• this holds even for consumers whose activities and energy requirements
are very similar
• the time distribution of maximum demands for similar types of consumers is
measured by the “diversity factor”
Sum of individual maximum demands
Group Diversity Factor = ---------------------------------------------------
Actual maximum demand of group
• This is always greater than 1
– For residential customers it can be 5
– For industrial consumers it can be as low as 1.3
Load Indices
• the ”PEAK DEMAND” of a system is the sum of the loads of individual devices that are functioning at that time.
• at the time of the system peak demand, the demand of a particular group of similar consumers is seldom at the maximum value that it can reach at other times of the year.
• this effect is measured by the “Diversity Factor”