TARIFFS AND ENERGY MANAGEMENT
1a) Discuss briefly the factors governing tariff structures.
The tariff structure of energy is governed by the factors:
Demand
The demand level of energy will determine the tariff structure of energy, when the demand is high the tariff is likely to be low and when the demand is low the tariff is likely to be high.
The rate charge of unit consumption of energy will also determine the structure of energy tariffs; the rate is the unit charge of one unit of energy consumed.
Distribution voltage
The distribution voltage will also determine the structure of energy tariff when the distribution voltage is high then the tariff is likely to be high.
User application:
The other factor that determines the structure ids the user application, this is whether the consumption is for domestic application or for industrial or commercial purposes.
Accounting methods
The other determining factor is the accounting method used in determining the consumption level of electricity, the most used application of determining the consumption level is the use of electric meters that determine the watt consumption level over a period of time.
Social and economic factors:
The structure of energy tariff will also be determined through considering the social and economic factors in a region, the adoption of a certain structure will take into consideration other factors of a region.
1b) Describe a typical tariff for:-
i) Domestic consumers
The domestic consumer will pay for the number of units consumed per month, these charges may include a monthly standing charge, for example the rate will be charged per KWH, and for example a domestic consumer may be required to pay a twenty dollar standing charge and in addition to this pay 40 cents per KWH. The tariff will also change in terms of peak and off peak consumption, peak units will be charged higher than off peak.
ii) Industrial consumers
An industrial consumer will still pay a standing monthly charge if applicable, he will also be required to pay for the number of units consumed per month, for industrial consumption of energy the consumer will pay less due to bulk consumption than the domestic consumer, peak charges will be higher than the off peak charges and summer tariffs will also be different from the winter charges where winter charges will be higher than the summer tariffs.
2) A factory with a foundry and a machine shop has a maximum demand of 750 kW and a load factor of 0.2. The tariff is 20 Pound/kW of maximum demand + 4 Pence/kWh of energy consumed.
a) Calculate the annual cost of electricity.
Load factor = (KWH/ total KWH) X 100
0.2 = (750/ Y)
Y= 3750
Total consumption = 3750
Cost = 3750 X .04 = 150
Cost for maximum demand 750 X 20 = 15000
Total cost = 15000 +150 = 15150 pounds
b) Discuss briefly one way in which the energy cost of the factory may be reduced without reducing the number of kWh consumed.
Because most of tariffs are charged higher during peak hours then the firm may reduce the cost of energy by utilizing the off peak hours when the tariffs are lower. Lower energy costs can also be achieved by reducing the load factor of the firm, if the firm achieves a lower load factors then the cost of energy will be lower which can be achieved without reducing the number of KWH consumed.
3a) State one method of raising the overall power factor of an industrial load.
Use of an power corrector either passive or active (PFC)
3b) Discuss briefly the advantages and disadvantages of applying power factor correction at:
i) The main bus bars
Advantage:
It is a useful method in power factor correction
The use of a filter can reduce the harmonic current
Disadvantage:
The rectifier used in this case is non linear and therefore there is energy at the harmonics of the frequency of the input current.
The above problem also posses a problem to the energy providing company as they cannot rectify this problem, whereby they cannot use capacitors to rectify the harmonic current.
ii) Individual loads
Advantages:
It is easy to adjust the correction level because the correction acts as a variable capacitor.
4) A factory has maximum demand of 400 kW at a lagging power factor such that 600 kVAr of re-active power has to be supplied.
The tariff in operation includes a maximum demand charge of 12 Pound/kVA.
Consider the effect of adding capacitor banks to the system in SIX stages, each drawing 100 kVAr leading. The capital charges on each 100 kVAr bank are 400 Pound per annum.
a) Calculate the net saving and overall power factor at each stage.
Power factor = real power (p) watts/ apparent power (s) volt amperes
S2 =P2 + Q2
Where Q is the reactive power measured in VAR
reactive power Q P s power factor savings per year initial 600 400000 400000.45 0.999998875 0 stage 1 500 399999.5 399999.862 0.999999219 7.04999725 84.599967 stage 2 400 399999.5 399999.75 0.9999995 8.3999979 100.79997 stage 3 300 399999.5 399999.662 0.999999719 9.44999867 113.39998 stage 4 200 399999.5 399999.6 0.999999875 10.1999994 122.39999 stage 5 100 399999.5 399999.562 0.999999969 10.6499998 127.8 stage 6 0 399999.5 399999.55 1 10.8 129.6
b) Plot a graph of net saving (vertically) against overall power factor (horizontally)
c) Explain what your results and graph show.
From the above graph it is clear that as we increase the number of bank capacitors the reaction energy is reduced until it is zero, the firm makes a save on their cost of energy as shown above.
5) Energy-efficient lamps are miniature fluorescent lamp designed to replace ordinary GLS lamps.
a) Discuss briefly the advantages and disadvantages of energy-efficient lamps compared with other sources of illumination.
Advantages of miniature fluorescent
They consume less energy than GLS lamps
Longer life span
Disadvantages of miniature fluorescent
Costly to install and maintain
Cannot be used at low voltages
Do not lender desired color
Advantages of GLS lamps
Cheap to install and maintain
They lender good color
Simple to operate
Disadvantages of GLS lamps
Consume more energy
Short life span
b) Obtain manufacture’s data and compare the total cost of using an energy-efficient lamp with that of an ordinary GLS lamp, assuming illumination is provided for 8000 hours.
| | GLS | Florescent |
| initial cost | 2.8 | 9.3 |
| power consumption watt | 100 | 7 |
| voltage | 240 | 240 |
| per hour | 0.41666667 | 0.029166667 |
| constant 10 | 4.16666667 | 0.291666667 |
| 8000 hours | 33333.3333 | 2333.333333 |
| total cost | 33336.1333 | 2342.633333 |
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