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Energy and Power Engineering, 2013, 5, 372-376
http://dx.doi.org/10.4236/epe.2013.55038 Published Online July 2013 (http://www.scirp.org/journal/epe)
Strategies for Energy Efficiency Improvement in
Zimbabwean Industries Using the Energy Audit
Wilson Mungwena1, Cosmas Rashama2
1Department of Mechanical Engineering, University of Zimbabwe, Harare, Zimbabwe
2Department o f Electrical Engineering, University of Zimbab we, Harare, Zimbabwe
Received March 20, 2013; revised April 22, 2013; accepted April 29, 2013
Copyright © 2013 Wilson Mungwena, Cosmas Rashama. This is an open access article distributed under the Creative Commons
Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is
Energy efficiency is a modern approach for using energy resources economically. Energy audit ensures that every unit
of energy gives the maximum in terms of production. This paper brings out the advantages of using energy audit to save
future installation of power generation capacity and load reduction of distribution systems. It also envisages the intro-
duction of energy conservation techniques to eliminate sub-standard equipment.
Keywords: Energy Efficiency; Energy Audit; Energy Conservation
As civilization grows, human beings consume more en-
ergy. Table 1 gives the chronological growth of energy
consumpt i on by man .
The above detrimental effects can be mitigated by sus-
tainable development. The objectives of sustainable de-
velopment are: as man consumes more energy, the eco-
nomic growth has caused rapid depletion of key natural
resources like fossil fuels, forest, fresh water and air
quality. There is also the possibility that large scale
deaths from pollution related diseases may occur, be-
cause basic life-support systems of the planet are af-
The Zimbabwean energy scenario is currently in a pre-
carious state with available capacity almost 50% of na-
tional requirements. This is because there has been no
new generation assets installed for the past thirty or so
years. The critical shortage of energy has affected all
sectors of the country with industry operations at 40%
partly because of shortage of electricity and obsolete
Abundant resources maybe a substitute for depleted
natural resources, e.g., solar energy for oil.
Economic growth leads to capital accumulation which
can be used as a substitute for natural resources such
as energy conservation requiring capital investment.
Continuation of technical augmentation, like more
efficient power systems, more efficient consumption
Table 2 summarizes resource options available for
electric utilities .
Energy consumed in the plant is determined by
whether we make or buy material. Materials used in in-
dustry have already an energy input built into it. It is re-
quired to estimate the energy content in products so that
less energy intensive materials could be promoted to de-
velop more energy effective products.
In industry, the reasons for high energy consumption
though not exhaustiv e may be listed as:
Inadequate modernization of plant.
Continue d use of obsolete technol o gy .
High specific energy consumption.
Less efficient lighting systems.
Table 3 lists typical values of energies consumed in
the manufacturing processes of materials in Zimbabwean
Table 1. Energy consumption by man.
People Time Energy consumption per day
Primitive man Before 1000 AD 4000 kilo calories
Man in agricultural
community Around 1000 AD 10,000 kilo calories
Man in industrial
society Aroun d 1870 AD 60,000 kilo calories
Modern man 20th century 350,000 kilo cal ories
opyright © 2013 SciRes. EPE
W. MUNGWENA, C. RASHAMA 373
Table 2. Resource options for utilities.
Supply side Demand side
Option Example Option Example
1. Conventional capacity Gas turbine, c ombined cycle, hydro,
pumped storage o r upgrading of
existing plants 1. Utility controls the load as needed Interruptible consumer load, appliance
2. Advanced technol ogy
Fluidised bed combustion of coal,
integrated gassifier combined cycle,
mini-hydro photovoltaics etc
2. Consume r installations encouraged
through utility incentives,
Captive generation must be mandatory
for power intensive industries like
cement, steel, aluminum, paper etc
Gas fired combustion combined
cycle, fluidized bed diesel combined
cycle 1. New technologies Computers, robotics, microwaves,
2. Elimination of energy theft Effective metering and checking,
vigilance on consumer connections
Table 3. Energy contents of materials.
Materials Energy Consumed
(Mj/Kg) % of Cost of Product
Attributable to Energy
1. Steels 20 - 50 30
2. Aluminum alloys 60 - 270 40
3. Copper 25 - 30 5
4. Magnesium 80 - 100 10
1. Glass 30 - 50 30
2. Plastic 10 4
3. Paper 25 30
4. Inorganic chemicals
(average values) 12 20
5. Cement 9 50
6. Waste 4 10
Nowadays, a total review of weight/strength needs is
undertaken by the automobile and aircraft industries us-
ing the Finite Element Analysis. As a result, more of
plastic and aluminum is used in the industry. This gives
better HP/weight ratio in the product and process of
manufacture as well as in the lifecycle energy costs of
Recycling of materials is energy economical. For ex-
ample, recycling of old/broken glass pieces requires ¼
the energy required for manufacturing new glass; recy-
cling of iron scrap requires ¼ the energy required to
manufacture new iron metal; recycling of aluminum
scrap requires ½ the energy required to manufacture new
aluminum and recycling of used paper requires about ½
the energy required for new paper manufacture.
2. The Energy Audit
The energy audit is a survey done on an organization to
ascertain the energy consumption and to examine energy
conservation optio ns. The typical objectives are:
To review energy consumption patterns so as to evolve
industry -wise norms and dat abase.
To classify consumers with respect to load demand
who 75% of total large supply industrial consump-
To make energy audit mandatory once a year with HT
consumers with maximum demand greater than 500
The activities of the energy au dit are: 
Electrical energy consumption month-wise vis-à-vis
the finished product.
Power bill study for each month focusing on KWh,
KVA, power factor and production throughput.
Analysis of the load curve to curtail/shift some loads
to off-peak perio ds.
Monitoring energy consumption of various equipment
separately to check efficiency, harmonics starting cur-
rents power factor and taking remedial measures to
achieve higher efficiency.
American Case Study 
Evidence of the importance of monitoring the consump-
tion of various equipment separately to check efficiency
was observed at Pacific Gas and Electric Company
(PG&E) that understood that businesses may need help
with understanding their consumption patterns, and cre-
ated the Pacific Energy Center (PEC) in San Francisco in
1991 to provide such assistance. The company initiated
the Tool Lending Library as a service to customers to
help customers understand and document their consump-
tion patterns. Lending Library contained an array of
measurement tools that were loaned out to California
utility customers free of charge for load studies up to 30
Copyright © 2013 SciRes. EPE
W. MUNGWENA, C. RASHAMA
days or more in length. In order to prove the importance
of a detailed breakdown of energy audit consumption
data, PG&E used its library equipment to measure the
spec ific energ y usage at the Pacific Energy Center build-
PEC staff monitored individual loads, and logged
power levels, and cross-checked the results against over-
all energy usage to verify that the building’s energy us-
age was indeed accounted for in the recordings. Once
that was done, PEC staff compared the loads of the
building to the common loads for other buildings of
similar size and type, looking for consumption patterns
that needed to be corrected. The energy audit found
higher than expected baseline energy usage over the
weekend. Analysis revealed that the high consumption
was as a result of the amount of safety illumination dur-
ing unoccupied hours, and the amount of refrigeration
needed in their commercial kitchen. To address this is-
sue, they reduced the wattage for safety illumination on
weekends, and identified more efficient refrigeration
equipment that would replace existing equipment when it
reached the end of its useful life. They also found a
boiler and an exhaust fan that were unnecessarily running
constantly during the monitoring period.
Further, in 2011, the PG & E Tool Lending Library
completed over 1250 test equipment loans to customers.
Borrowers estimated that the monitoring projects sup-
ported by these loans helped reduce energy demand by
157 megawatts and saved 92.5 million kWh of electrical
energy in the year 2011.
3. Achieving Energy Efficiency
3.1. Demand-Side Management
Here, the utilities seek to directly influence demand for
electricity in predetermined ways. The programs are load
management, strategic conservation, demand reduction
and development of captive power with cogeneration.
The main objective of demand-side management is to
influence the consumption patterns and behavior of con-
sumers towards efficient use of electrical energy. This is
achieved in the following strategic steps.
3.1.1. Load Management
Under this strategy, we direct load control in which por-
tions of the load are under the direct operational control
of the utility with the agreement of the consumers; indi-
rect load control where the consumers may control their
loads voluntarily & alter the use of electricity in response
to price signals and lastly power utilities implement tar-
iffs with inbuilt mechanism to discourage wasteful en-
3.1.2. Time-of-Day Pricing
Under this strategy, the price elasticity of demand for
electrical energy is assessed. This is quantified using an
economic model for the electricity demand and then lin-
ear regression techniques are applied to estimate the
price elasticity of demand. The tariff could indicate to the
consumers when electricity is cheap/expensive. The aim
is to produce tariffs which meet the utilities’ financial
targets. The price and demand can be coupled by the
elasticity factor to achieve load shifting away from the
peak time-of-day for different categories of consumers
Common forms of energy rate tariffs are given below:
a) Consumption limited tariff-consumer is only allowed
to consume electricity to a limited exten t.
b) Flat rate tariff-consumer is charged at a flat rate/unit
no matter how much electricity they consume.
c) Block rate regressive/progressive tariff-cost/unit can
decrease or increase per block of consumption.
d) Time-of day tariff-cost/unit depends on time of day or
month of the year.
e) Bulk-rate tariff-special tariffs for large consumers.
3.1.3. De mand Reduc tion
The leveling of demands will decrease the maximum
curre n t flow. As lo s e s v ar y wi th the squa r e o f th e cu rr ent,
the lower current will result in reduction of the total en-
ergy requirements of the consumer and reduction of loses.
Microprocessor base ‘demand controllers’ could be used
to supervise the operations of the consumers’ equipment.
The first step is to obtain the consumers’ demand profile.
If the profile shows a few sharp peaks, then the equip-
ment causing these peaks is identified and remedial
measures are taken. In this connection, the consumers’
major loads are classified into 4 categories like,
Those which can be r es ch eduled.
Those which can be deferred.
Those which can be c ur tailed or el iminate d.
Those which are essential base loads.
Demand controllers increase the effectiveness by re-
moving all the non-essential loads in addition to keeping
the demand under a preset level. The control function can
be by many types of systems namely,
Instantaneous—controls all loads at any time during
an interval if the rate of usage exceeds a preset value.
Ideal rate—controls load when they exceed the set
rate but allow a higher usage at the beginning of the
Converging rate—has a broad control bandwidth in
the beginning of the interval, but tightens control at
the end of the interval.
Predictive rate—the controller is programmed to pre-
dict the usage at the end of the interval by the usage
pattern along the interval and switches load to achieve
the preset demand level.
Continuous interval—the controller looks into the past
usage over a period equal to (or less than) the demand
Copyright © 2013 SciRes. EPE
W. MUNGWENA, C. RASHAMA 375
interval. Loads are switched in such a manner that no
time period of an interval’s duration will see an accumu-
lation of KWh that exceeds the preset value.
Before any of the above controllers can be installed, a
load survey should be made. This survey is an equip-
ment/process audit. Each process and piece of equipment
should be surveyed to find which loads can be switched
off and to what extent they can be switched. Any loss of
equipment life or mechanical problems associated with
switching each load should be evaluated.
The simple fact is that no energy is used when equip-
ment is shut off. Hence it is required to make sure that
unused, redundant and idling equipment is shut off.
3.1.4. C ogenerati on [5 ]
This is an important energy conservation strategy. En-
ergy savings from cogeneration do not necessarily imply
economic savings. Cogeneration will be an economical
investment for a firm if the value of the electricity pro-
duced is greater than the incremental capital and operat-
ing costs incurred by the firm. Packaged cogeneration
plants have potential application s in hospitals, hotels and
industries. Cogeneration systems can be classified into
two into 2 categories namely, topping systems and bot-
In topping systems, electricity is produced first in a
turbine and some of the energy is exhausted and used in
In bottoming system, high temperature energy is pro-
duced first for applications like steel reheating process,
cement kilns or aluminum furnaces; further heat is then
extracted from the hot exhaust waste steam and trans-
ferred to a work ing fluid. The f luid is vapor ized and us ed
to drive a steam turbine to produce electrical energy. The
figure below shows the fuel e ffecti veness.
a) Modern coal fired system
Maximum efficiency = 35%;
Losses in condenser = 48%;
Boiler losses = 15%;
Other losses = 2%.
b) Gas turbine cogeneration systems
Maximum efficiency = 90%;
Exhaust losses =10%.
c) Steam turbine cogeneration system
Maximum efficiency = 84%;
Boiler associated losses = 15%;
Other losses = 1%.
3.2. Efficient Energy Use in Lighting
Lighting constitutes an appreciable load and consists of
an inefficient system of lamps and luminaries. Use of
energy effective products will lead to the ultimate possi-
bility of halving this connected load, thereby avoiding
waste in a cost effective manner. The energy saving
measures could be:
Compact fluorescent lamps as replacement for GLS
light points in hotels, commercial, domestic and other
applications where 20 W/40 W tube lights are too
Electronic ballasts for fluorescent tube circuits as
replacement in existing tubes and for incorporation in
new lighting points. Th eses operate at low voltages &
have instant start with easy installation, high power
factor and immediate saving in connected load.
Upgrading of fluorescent tubes to high pressure so-
dium lamps as recommended for techno-commercial
considerations. Intelligent lighting as a practice can
be followed in terms of the following steps.
In many security situations, lighting is simply left on
throughout the high risk period which is generally at
night. Passive infrared protection systems are avail-
able which automatically sense occupancy and switch
on light to specific zones providing round the clock
security lighting with a huge potential for energy
saving. The infrascan device includes passive infrared
sensors, photoelectric sensors and timers. The inte-
gration of all these control elements provides an intel-
ligent solution which is capable of dealing with po-
tential security risks as well as continually adjusting
to daylight levels.
Energy saving, fully automatic controllers are avail-
able. These are designed to be installed in place of
existing wall switches and fir into standard wall boxes.
Electricity is wasted when people neglect to switch-
off when rooms are vacated or when daylight makes
artificial lighting unnecessary. The controllers re-
spond only to physical o ccupancy within the confines
of the room or area controlled by the individual unit.
Automatic switching is activated by means of a dou-
ble-dual-passive-infrared sensor system giving a 180˚
beam coverage over a 170 m2 area. The controllers
are fully programmable with internal switches allow-
ing adjustment at the time of installation to suit indi-
vidual situations. The time delay between a room be-
coming unoccupied and the lights being switched off
can be set to either 1, 4, or 16 minutes. Occupancy in
where full daylight does not penetrate can be con-
sidered by setting the ambient-daylight-sensitivity
3.3. Efficient Energy Use in Motors 
Motors are fairly efficient at rated loads. In general three-
phase motors are more efficient than single-phase motors
and larger motors are more efficient than the smaller
ones. Motor voltage unbalance will increase motor losses
due to the negative sequence voltage that the causes a
Copyright © 2013 SciRes. EPE
W. MUNGWENA, C. RASHAMA
Copyright © 2013 SciRes. EPE
rotating magnetic field in the opposite direction of the
motor rotation. A 2% voltage unbalance will increase
losses by 8%; a 3% unbalance will increase losses by
25% and a 5% unbalance will increase losses by 50%.
The power factor of three-phase motors is between
80% & 90% at full load & decreases as load is reduced.
The installation of capacitors for power factor correction
up to 0.95 or so) will decrease current requirements,
thereby reducing I2R losses in supply lines.
3.3.2. Oversized Motors
If a motor is operated at a reduced load; then its effi-
ciency begins to fall, it has higher starting current, lower
running power factor and higher capital costs. Oversized
motors lead to energy wastage.
3.3.3. Soft Starters for Induction Motors
These regulate the voltage at the motor terminals so that
the magnetizing forces just meet the load demand. This
boosts the efficiency of the motors operating below their
rated outputs. Energy savings are significant for motors
operating at less than 50% load for about 50% of the
3.3.4. Efficient M otors Desig n
These motors consume 5% to 8% less electricity than
standard motors but more material is used to reduce
copper and iron losses. These motors have a higher effi-
ciency because higher grade steel is used and have spe-
cial low friction bearings, added copper windings, close
tolerances & small air-gaps. They have a longer life be-
cause they run cooler than less efficient motors.
3.3.5. Del ta to Star C onnection
The winding of any under-loaded three-phase motor can
be reconnected in star. This reduces the voltage across
each winding to give 58% of its rated values. Motors
constantly running at less 58% of full load will benefit.
3.3.6. Variable Speed Drives
They adjust the speed of the motor replacing constant
speed motors. The variable-speed motors are energy effi-
cient at reduced loads and reduced speeds to meet dif-
ferent load requirements. These drives are well estab-
lished over a complete power rang e in all areas of Indus-
try like basic industries, material handling plants, trans-
port systems and utility companies for mechanical equ ip-
ment such as machine tools, extruders, pumps fans com-
pressors railways, elevators and conveyors.
Energy efficiency as a resource for saving future installa-
tion of power generation capacity & unloading of distri-
bution systems is a modern approach for using resources
efficiently, especially in Zimbabwe where there are an
acute power shortage and no investment in new generat-
ing assets. The energy audit aides this process by identi-
fying the deficiencies in the existing systems. Energy
conservation management, load management, time-of
day metering, electricity pricing & cogeneration efficient
technologies are the various methods to reduce system
demand and save system capacity. The benefit is an effi-
cient system, increasing plant capacity and a big save on
 M. Gown and L. Baine, “Energy Saving Lighting Con-
trollers,” Electrical Installation International, Vol. 3, No.
9, 1999, p. 14.
 Department of Energy and Power Development, “Zim-
babwe Energy Policy,” 2013, p. 49
 Norweigian Institute of Technology, “Economic & Fi-
nancial Analysis of Energy Systems,” 1998, p. 122.
 PG & E Tool Lending Library, “California Companies in
Measuring Energy Consumption,” 2013.
 W. Mungwena “Cogeneration in Zimbabwe Sugar Indus-
tries,” JESA, Vol. 23, No. 1, 2012, pp. 67-71.
 A. S. Pabla, “Efficient Energy Use in Motors,” 1996, pp.