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METHODOLOGY FOR ENERGY AUDITS IN POWER PLANTS

REGARDING ANALYSIS OF ELECTRICAL ENERGY

CONSUMPTION

Aleksandar NIKOLIĆ1, El.eng. Institute Nikola Tesla, University of Belgrade, Serbia

INTRODUCTION

Electricity production accounts for 32% of total global fossil fuel use and around 41% of total

energy-related CO2 emissions [1]. Improving the efficiency with which electricity is produced

is therefore one of the most important ways of reducing the world’s dependence on fossil

fuels, thus helping both to combat climate change and improve energy security. Additional

fuel efficiency gains can be made by linking electricity generation to heating and cooling

demands through high efficiency combined heat and power (CHP) systems (e.g. in industry

and for district heating, esp. in thermal power plants).

The paper presents some guidelines for performing energy audits in power plants regarding

analysis of electrical equipment and its consumption. Presented methodology gives directions

for locating places with highest electrical energy usage and consumption, precautions while

performing measurements and analysis of gathered data. Instructions for data presentation in

the form of executive summary is given in the paper with summarize the key points of the

energy audit study such as energy saving potential, recommendations, cost savings,

investment requirement etc., for each sub system for which energy audit is done.

Methodology will involve characteristic systems in thermal and hydro power plants, like self-

consumption, pumps, compressors, fans, mills, coal handling plant, motors, lighting, etc.

Finally, the result of performed energy audit should be proposals for energy efficiency

improvements. Several contemporary solutions are presented in the paper, like application of

motors with improved efficiency, synchronous transfer for pumps using frequency converters,

ISO50001 usage for supervision of electrical drive systems, voltage control in the self-

consumption network of thermal power plant, etc.

1 Aleksandar Nikolic, Electrical Engineering Institute Nikola Tesla, anikolic@ieent.org

ELECTRICAL ENERGY AUDIT IN POWER PLANTS

Significance of energy audit

Energy audit represents a most important tool for current energy efficiency analysis in power

plant in order to propose adequate measures for energy efficiency improvements. Such an

audit involves:

Systematic data gathering about energy production and consumption,

Identification of power flow through the plant,

Defining the measures for improving energy efficiency,

Economical and technical justification of proposed measures,

Rating the proposed measures according to the established criteria.

The importance of carried out energy audit regarding electrical energy usage in power plants

could be summarized in following:

Determine locations in power plant with the highest electrical energy losses,

Cost reduction for electrical energy production using measures proposed in energy

audit,

Increasing of electrical energy production by improvement of efficient usage of

turbine cycle and reduction of self-consumption,

Maintenance planning and improvement of availability,

Using on-line monitoring for important systems and equipment,

Benchmarking of most important electrical equipment and systems.

Energy audit report structure

The mandatory nature of energy audit requires not only establishing guidelines for energy

auditing procedures but also calls for standardization of energy audit reports. Power plants

consist of equipment of varied nature and functionality performing different functions. There

is, therefore, need for establishing procedures for conducting energy audit on different types

of equipment at site operating under different conditions according to the process of operation

of the power plant [2],[3]. The structure of the energy audit report is governed basically by the

directives [4]-[6]. The energy audit reports are required to highlight:

Details of energy consumption, their costs, and specific energy consumption,

Energy efficiency / performance analysis of various equipment,

Suggested energy conservation measures energy savings, benefits, cost economics,

monitoring and evaluation.

Each report may include the following:

Title page,

Table of contents,

Acknowledgement,

Auditor firm and audit team details and certification,

Executive summary,

Introduction to the energy audit and methodology,

Description of the plant / establishment,

Energy consumption profile and evaluation of energy management system,

Equipment / systems specific section reports,

Summary of recommendations and action plan,

List of suppliers of retrofits / vendors,

Annexures / references, software tools used.

An executive summary provides an overview of the energy audit report. The purpose of an

executive summary is to summarize the key points of the energy audit study such as energy

saving potential, recommendations, cost savings, investment requirement etc., for each sub

system for which energy audit done. The executive summary shall draw the entire information

from the main report. The one of the most important parts of executive summary is summary

list of energy saving measures along with classification. A typical format of energy measures

summary list is given in table 1.

TABLE 1 – SUMMARY LIST OF ENERGY SAVING MEASURES

No. Energy saving

measure

Fuel

savings,

metric tons

/ year

Electricity

savings,

MWh/year

Cost

savings,

million

EUR / year

Investment

required,

million

EUR / year

Simple

payback

period,

years

Short / Medium /

Long term

measure

1

2

3

The executive summary shall highlight the impact of implementation of energy saving

measures in energy savings, cost savings, improvement in efficiency / performance and heat

rate. As final result of the executive summary is the action plan with rank of proposed energy

saving measures as per table 1.

MAJOR AREAS FOR ENERGY AUDIT IN THERMAL POWER PLANTS

The major areas for conducting energy audit in thermal power plants are [2], [3]:

Boilers and associated parts,

Turbines and associated parts,

Insulation,

Draft system /fans (ID fans, FD fans, PA fans and other fans),

Cooling system (condensers, cooling towers and cooling water pumping system),

Water pumping systems (boiler feed water pumping system, condensate extraction

pumping system, DM water pumping system, make up water pumping, raw water

pumping system, etc.),

Fuel handling system (e.g.: coal handling system, coal mills, fuel oil handling system),

Ash handling system,

Compressed air system,

Air conditioning system,

Electrical systems,

Electric drives and motors, esp. those of high power (>50kW) and high voltage (6kV),

Plant lighting system.

There may be some other sections /equipment in addition to those mentioned above which

may need to be added.

Required instruments for performing analysis of electrical energy consumption

The following instruments are required for conducting the electrical energy audit in thermal

power plant on previously defined equipment / systems of interest:

Power analyzer for measurement of electrical parameters such as kW, kVA, pf, V, A

and Hz of class 0.5 accuracy (for all systems),

Stroboscope to measure the speed of the driven equipment and motor (pumps, fans,

mills, compressors),

Tensiometer for belt tension check for belts in coal and ash handling system and

compressed air system,

Temperature indicator & probe (coal and ash handling system, boiler, compressed air

system),

Lux meter for lighting system analysis,

Available on line instruments at the site (calibrated).

Influence of instrumentation accuracy on the energy audit

There are situations where it is suitable to use on site available instruments for the energy

audit. But, some precautions should be noted in order to have accurate results of the

performed audit. Such plant on line instruments could be of accuracy around 3.0%. On the

other side, accurate calibrated instruments could have accuracy of 0.5%. Furthermore, error in

energy audit could be also from the procedure of performing energy audit and it could be

from 2% for boiler up to 3% for turbine system. In such cases total error of instrumentation

and procedure could be up to 6%.

Importance of accuracy in energy audits could be emphasized on the following example. In

one power plant in India with 200MW unit two different energy audits are performed.

Difference in those audits was only regarding used instrumentation. The first one uses mainly

existing non calibrated instrumentation on site and the overall cost of that audit was around

120.000EUR for whole plant. On the other side, energy audit that comprised calibrated

instrumentation with accuracy of 0.5% costs around 200.000EUR. Although initial costs are

in the first case lower for 80.000EUR it should be noted that such approach yield to the error

in results of just 1%. But, on the annual base, even small error of 1% in results produces

losses of 25.000tons of coal for the 200MW unit or 700.000EUR.

EXPLORATION FOR ENERGY CONSERVATION POSSIBILITIES

In this section some possibilities for improving energy efficiency and reducing electrical

energy usage in power plants are proposed. Those solutions are mainly focused on motors and

drives, since electric motors account for about two thirds of electricity consumption in the

industry [7]. About 110 million low-voltage AC motors are operational in the European

industrial and tertiary sector and about 10 million are sold every year in Europe. The

associated electricity consumptions amount to roughly 1119 TWh/a in 2010, or 97.2 billion

Euro and 513 Mtonne of CO2 emissions. It has been predicted that the electricity consumption

of motors will increase to 1252 TWh/a in 2020 if no measures to limit the consumption are

taken [8].

Application of motors with higher efficiency

Motors are grouped into four efficiency classes, IE1 being the standard efficiency and IE4 a

super premium efficiency. The efficiencies depend primarily on the size, as shown in Figure 1

[9].

Fig. 1. IE efficiency classes of a 4-pole motor at 50 Hz

Currently, the majority of the electric motors in the EU are still of the class IE1; representing

more than 82% of European market share in 2009. Obviously, there is a considerable potential

for energy saving by increasing the efficiency of operational motors, not just waiting for

existing ones to be replaced. Motors are usually run to failure, and even then it is common to

repair rather than replace them.

The Eco design directive set standards for the efficiency of new motors sold on the EU market

[10]. The implementation of these standards follows a step wise approach and is shown in

table 2.

TABLE 2 – ECODESIGN REQUIREMENTS FOR THE IMPLEMENTATION OF

ELECTRIC MOTORS

Rated output: From 16 June 2011 From 1 Jan 2015 From 1 Jan 2017

0.75 – 7.5 kW IE2 IE2 IE3 or IE2 + VSD

7.5 – 375 kW IE2 IE3 or IE2 + VSD IE3 or IE2 + VSD

It is estimated that the electricity savings by Eco design amount to 135 TWh in 2020 [10].

This estimate is based on a stock turnover rate assuming average lifetimes of motors per size.

Electric motors have an average lifetime of 10 – 20 years, but in most companies are run to

failure, which can be much longer. Some motors will be repaired or rewound multiple times,

extending the lifetime beyond the 10 – 20 years. As rewinding of motors usually also results

in an efficiency loss, it will take much longer before the minimum efficiency requirements on

new equipment lead to a substantially more efficient stock.

Accelerated replacement of electric motors

It is common practice in industry to rewind burnt-out motors. Careful rewinding can

sometimes maintain motor efficiency at previous levels, but in most cases, losses in efficiency

result. Rewinding can lead to deterioration of motor efficiency due to factors such as winding

and slot design, winding material and insulation performance.

The real question (ignoring for now the benefits of using a more efficient motor as explained

previously, and of the reduction in capital expenditure by possibly repairing rather than

replacing), is the difference between buying a new now, or buying the same new motor at a

time in the future. The overall picture will vary according to the individual company

circumstances and motor application, but in many cases it will represent a positive cash flow.

Let us consider the following simplified example:

11kW motor with an IE1 efficiency (87.6%) or IE3 efficiency (91.4%)

Motor is to be replaced after 15 years

4000 operating hours per year, no load factor compensation

IE1 motor cost of €450, IE3 motor cost of €675, electricity price of €0.09/kWh

The first approach we use is taking into account the full cost difference between the IE1 and

IE3 motor. Annual energy costs of the IE1 motor amount to €4520 and €4332 for the IE3

motor, saving €188 a year. The difference in motor costs is a mere €225, implying that the

higher investment costs are compensated by a reduction in energy costs within two years.

The second approach is that we consider is bringing forward the investment in a new motor

for two years. We can use the net present value to calculate this. We assume that the motor

will be linearly depreciated over five years, so after two years the motor still has a value of €

405. The net present value with a discount rate of 10% is € 582. The costs for bringing

forward this investment are € 675 - € 582 = € 93. This cost difference is compensated by the

savings on electricity costs within half a year. Obviously, this outcome depends strongly on

the assumptions of the discount rate and the depreciation method.

Another case we need to consider is comparison of rewinding and replacing. Motor rewinding

generally reduces motor efficiency and comes at two thirds of the costs of a new motor. In the

above case, with a rewind caused efficiency loss of 0.5%pt, the difference in annual energy

costs would rise to €214 a year. The difference in investment costs would be €350 (rewind vs.

new more efficient motor), resulting in a less than two year simple payback period. The

assumed 0.5% efficiency loss is based on a high quality rewind. Usually motor rewinds are

carried out under time-pressure and therefore generally result in typical winding loss of

efficiency of about 2%pt. Such a loss in efficiency would further reduce the simple payback

period.

Using ISO 50001 standard to drive forward energy management

The idea of using ISO50001 standard [5], [6] to drive forward energy management is a ―hot

topic‖, and so should be considered carefully. The Dutch programme is probably the most

advanced in Europe, and explains in some detail the mechanics of how this works [11].

It is shown that small savings could be obtained on motors, but the large savings could be

result of considering all parts of the motor driven system as integral system. Such a system is

defined as Electric Motor Driven System, or EMDS. For instance, heating system driven by

electric motor is considered completely with motor, transmission (gear), variable frequency

drive, pipes and pump.

In this way, ISO 50001 standard procedures in a company are used to encourage:

Adoption of more efficient components within EMDS,

Better sizing tasks of EMDS,

Optimisation of the ensemble of components within EMDS,

Use of variable frequency drives (VFD) for variable-load applications,

Better in-field management of EMDS.

As a result, supervising of EMDS is performed continuously as it is defined as activity of a

company’s quality procedure. Rewinding and motor replacement are also defined as a clear

procedure – they are performed on a protective basis and not after motor failure. This gives a

solid framework for the introduction of an Energy Management system, which can include a

motor management policy. Estimated energy savings using this approach are 20% to 30%.

Application of variable frequency drives

There are several points in power plant where VFD driven motors could significantly help to

reduce energy consumption. The largest savings could be expected on pumps and fans,

regarding their power characteristics where reducing the motor speed by 20% gives power

reduction of 50%, since power is proportional to speed cubed. In pumps applications, further

savings and installation cost reduction could be obtained using so called synchronous transfer

where one VFD is used to start up several pumps and control speed of one pump [12].

A pump system using synchronous transfer allows a single drive to be used to separately

control several pump motors in variable speed such that the total output is continuously

variable. As the demand increases, the drive will increase speed and output to the maximum

for each single pump, transfer the single pump to a fixed frequency source, and start the next

pump at a low speed (frequency) to provide additional output. When demand decreases, the

single drive will slow down its pump until the demand decreases below the output for the

remaining constant speed motors. The drive then shuts off its motor and desynchronizes one

of the motors running on fixed frequency, controlling it at a lower frequency to reduce overall

output. The output is continuously variable from zero to the total output of all pumps. The

control system associated with the drive usually handles switching between pump motors,

allowing the drive to synchronize and desynchronize pump motors depending on the demand

in the process. Each pump motor has contactors connecting it to the drive output and to the

fixed frequency source (Fig. 2). Contactors also allow the isolation of each motor, pump, or

drive for maintenance purposes.

M 3~

M 3~

FR

Fixed frequency bus

Variable frequency bus

Fig. 2. Synchronous transfer principle

The economic advantages with a synchronous transfer system are in both installation costs

and operating costs. When comparing the synchronous transfer system to a multiple drive

system, the initial capital outlay and installation costs for electrical equipment are

approximately a 33% reduction for a two-motor system to a 60% reduction for a four-motor

system. A synchronous transfer also allows for a bumpless process startup versus a control

valve. Starting a second unit at a pipeline station with a control valve will introduce a pressure

surge on the pipeline until the control valve can react and catch the increasing pressure. If

operating close to maximum operating pressure of the pipe, the increase of pressure from the

second pump can cause excessive pressure on the system. A synchronous transfer does not

produce this pressure surge due to the slow ramping of the second pump to a control speed.

CONCLUSION

Guidelines for performing energy audit in power plants are presented in the paper. Along with

a given structure of such an audit, several important tasks are emphasized in order to

accomplish a successful energy audit. The most important is accuracy of used instrumentation

and some possible errors and its influence are shown. As a final result of presented

methodology for performing energy audit, an adequate energy saving measures are proposed.

In the paper some specific and up-to-date energy conservation possibilities are shown and

analysed in the field of electrical motor and drives, due to the fact that the largest part of

electrical energy in industry today is consumed by electrical motors.

Proposed methodology could be also used for other similar industrial systems, like heat

production/distribution companies, municipal water systems and large industries (cement,

metal, paper, etc.).

LIST OF REFERENCES

1. Taylor P, Lavagne d’Ortigue O, Trudeau N, Francoeur M, ―Energy Efficiency Indicators

for Public Electricity Production from Fossil Fuels‖, International Energy Agency,

OECD/IEA report, 2008

2. Indo-German Energy Programme, ―Guidelines for energy auditing of pulverised

coal/lignite fired thermal power plants‖, report, 2002

3. Sargent & Lundy, L.L.C., 2009, ―Coal-fired power plant heat rate reductions‖, DOE project

EP-W-07-064

4. Energy Efficiency Policies around the World: Review and Evaluation, 2008, ―WEC‖

5. International Standard, 2011, ―Energy management systems — Requirements with

guidance for use‖, ―EN 50001:2011‖

6. Pinero E, 2009, ―Future ISO 50001 for energy management systems‖, ―ISO Focus‖, 18-20

7. Wachter, B. de, ―White Paper - Electric Motor Asset Management‖, ECI, 2001.

8. Implementing Directive 2005/32/EC.

9. ABB, Technical note IEC 60034-30 standard on efficiency classes for low voltage AC

motors.

10. EC, 2009. Full Impact Assessment (regard to ecodesign requirements for electric motors).

SEC (2009)1014.

11. M. van Werkhoven, ―ISO 50001 Energy Management and Electric Motor Driven

Systems‖, Motor Summit, Zurich, Switzerland. 2012.

12. Seggewiss J. G., Kottwitz R. G., McIntosh D., ―The process and economic benefits of

synchronizing applications with medium-voltage drives‖, IEEE Industry Application

Magazine, 58-65, July/August 2003.