METHODOLOGY FOR ENERGY AUDITS IN POWER PLANTS REGARDING ANALYSIS OF ELECTRICAL ENERGY CONSUMPTION

Transcription

1 METHODOLOGY FOR ENERGY AUDITS IN POWER PLANTS REGARDING ANALYSIS OF ELECTRICAL ENERGY CONSUMPTION Aleksandar NIKOLIĆ 1, 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 CO 2 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 selfconsumption, 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 selfconsumption network of thermal power plant, etc. 1 Aleksandar Nikolic, Electrical Engineering Institute Nikola Tesla, anikolic@ieent.org

2 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.

3 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 Short / Medium / Long term measure Fuel savings, metric tons / year Electricity savings, MWh/year Cost savings, million EUR / year Investment required, million EUR / year Simple payback period, years 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.

4 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 EUR for whole plant. On the other side, energy audit that comprised calibrated instrumentation with accuracy of 0.5% costs around EUR. Although initial costs are in the first case lower for EUR 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 tons of coal for the 200MW unit or EUR. 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 CO 2 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].

5 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 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 kw IE2 IE2 IE3 or IE2 + VSD 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 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 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

6 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 = 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 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 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.

7 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. Fixed frequency bus FR Variable frequency bus M 3~ M 3~ 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

8 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, Indo-German Energy Programme, Guidelines for energy auditing of pulverised coal/lignite fired thermal power plants, report, Sargent & Lundy, L.L.C., 2009, Coal-fired power plant heat rate reductions, DOE project EP-W 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: Pinero E, 2009, Future ISO for energy management systems, ISO Focus, Wachter, B. de, White Paper - Electric Motor Asset Management, ECI, Implementing Directive 2005/32/EC. 9. ABB, Technical note IEC standard on efficiency classes for low voltage AC motors. 10. EC, Full Impact Assessment (regard to ecodesign requirements for electric motors). SEC (2009) M. van Werkhoven, ISO Energy Management and Electric Motor Driven Systems, Motor Summit, Zurich, Switzerland 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.