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Electricity Saving and Cost Reducing Through Chiller System Optimization

Mahendra Dissasekera Department of Electronics and Telecommunication

University of Moratuwa, Sri Lanka Email: mahendrad@noyonlanka.com

Abstract-This research project mainly focus on proper coordination of chiller and supporting components in order to reduce it’s operating cost, improve temperature stability and improve equipment life. Control algorithm measures critical parameters and adjust variable outputs to optimize number of running compressors, chill water pumps and air handler units with improved performance. Case study was done at fashion lace knitting facility at Biyagama, Sri Lanka. Temperature stability is a one of the basic requirement of this warp knitting facility. Implemented PLC base controller is compatible to handle any type of high and medium capacity multi compressor chiller system. It has remote critical data and impotent parameter display. System can be control through remote logging and there is history data table and graphs. This controller eliminates KVA peaks which generated during chiller starting and chill water pumps starting period. Further electricity cost further reduces by 5% through three band tariff system.

Keywords- Chiller system, Air handler units, Chill water pumps, Programmable logic controllers, SCADA system, Compressor, Coefficient of performance

I. NOMENCLATURE

PLC Programmable logic controllers KVA Kilo Volt Ampere KWH Kilo Watt Hours SCADA Supervisory Control and Data Acquisition System E Compressor electricity consumption Pcf Compressor cooling load at full load Ef Compressor efficiency at full load Tf Full load operating time period Pcpi Compressor cooling load at part loads Epi Compressor Efficiency at part loads h1 Chill water enthalpy m/t Chill water flow rate Qch Heat delivered by chill water COP Coefficient of performance Tpi Part load operating time period

II. INTRODUCTION

Chillers systems use in all over the world to generate chilled water in order to provide space cooling of large air-conditioned buildings. Packaged chillers come with wide range of designs with different COPs. Chiller systems are maintaining building temperature by circulating chilled water through air handler units inside the building. Most of the cases there is no proper coordination in between each of these components. Implemented centralized control system can coordinate each of these components to save more electricity at part loads and improving performance of each of these components. Correct control algorithm obtained from period of analysis each and every component operation. Measured key parameters are production floor temperature profile, 24 hours chiller loading profile relative to the out door temperature and supporting

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components operating efficiency. Proposed controller keep the coordination in between chiller, chill water pumps and air handler units in optimum point for given dynamic load. Main idea of this research is to identify lagging point in chiller system and improve them through better coordination of each component.

Controller saves more electricity during part load operation by controlling pump speed and compressors no load run time span. Controller keeps stable chill water temperature in order to maintain production floor temperature without variations. Further it keeps all impotent parameters history backup for the future analysis. Case study was done at warp knitting plant at Biyagama called Noyon Lanka Pvt Ltd. Facility has warp knitting machines which require stable temperature at 23°C and humidity less than 65% for their optimum operation. Authorized person can logging the system through remote computer to find out online status and temperature history records. He can adjust control parameters in order to further optimize system for smooth operation.

III. OBJECTIVES • Electricity cost reduction through optimum tariff. • Electricity consumption reduced by proper coordination of

chiller, chill water pumps and air handler units. • Chiller starting KVA demand reduces through demand base

operation with controlled delays. • Eliminate knitting design defects which occur due to

temperature fluctuations.

IV. OPTIMUM TARIFF SELECTION

Fig 1 and 2 shows respectively electricity cost for different tariff systems which available for local industrial users and kW consumption variation throughout the day. Monitor electricity demand considerable time and apply it to deferent tariff to identify benefit out of it.

Fig. 1. Electricity cost for different tariffs

ICIAfS10

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Fig. 2. Electricity loadvariation within 24 hours

V. ENERGY ANALYSIS

Heat energy transfer at chiller system evaporator is defined by following equation (1). E denotes the total electricity consumption and right side of the equation gives full load and part loads each of the compressors electricity consumption.

E = [Pcf / Ef].Tf + ∑ i [Pcpi / Epi].Tpi (1)

Where Pcf cooling capacities in kW respectively, T is the

operating time in hrs/year and the subscripts f and p denote full and part load operation. Typically one would discrete time of operation to intervals such as 25-50%, 50-75% and 75-100% and sum the energy consumed during these intervals. As a norm 70% of the chiller operation time goes to 50-75% load category.

Most of the present chiller systems full load efficiencies

different from there part load efficiencies. But very recently developed water cooled chillers gives much better performance in part loads. Most of the older chiller systems give lesser performance in there part load operation. How ever chiller system part load efficiency depends on the number of compressors, their part load efficiency and existence of inverter controls with the sizing of condensers. In contemporary chiller systems, operating time of compressors can be retrieved from the chiller electronic control devices. But on line chill water temperature monitoring method is much more efficient and impotent for production flow temperature stability.

VI. EVAPORATOR HEAT TRANSFER EFFICIENCY

Most evaporators of large chillers are flooded shell and tube type design. Refrigerant boils outside of horizontal tubes, bubbles up through the liquid refrigerant and leaves the shell from the top. It is more convenient to model the heat transfer process in the evaporator using an overall enthalpy difference and mass flow rate of chill water. More details are given in referances 4. The heat balance equation is

Qch = [m/t] . [h1 – h2] (2)

It is very impotent to maintain chill water mass flow rate when compressor is in operation. Chiller system efficiency defined by COP value and it is directly proportional to the Qch which is given in equation (2). COP of a chiller is inversely proportional to the supply energy to the compressor denoted by E. It is impotent to maintain minimum required water flow in order to obtain optimum performance. COP can be easily deduced in equation (3)

COP = Qch /E (3)

By introducing automatic control system, chillier will start within shorter down time period after power shut down. It minimizes temperature increase at factory floor and it controls number of compressors switch on when power restore. It eliminates sudden temperature drop of chill water line due to switch on all the compressors once. Before improve the chiller system it operated by semi automatically and at that time there is variable manual starting delay. Starting delay varies from 5 minutes to 15 minutes and it cause considerable temperature fluctuation at production floor.

Fig 3 shows power graph of chiller which switch on with a delay

Fig 3 shows power graph of chiller which shows delayed start after power shutdown. Delay start gives temperature rise at production floor and short high KVA demand. Result of this manual starting operation is defective products and high KVA demand.

Fig 4 Chill water temperature graph of continuous capacity control compressor

Selection of high performance compressors is very impotent because it directly effect to the system efficiency. Latest screw compressors have profile rotors which consist of a five –lobe male rotor and a six-interlobe female rotor, to achieve high efficiency and reliability. Thermo-excel tube super slit fins and inner grooved tubes have been developed for more efficient the condenser and evaporator systems. More details can be finding in referances 1 and 2.

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Fig 5 Load variation of continuous capacity control and step capacity control compressor.

Step capacity and continuous capacity compressors performance and operation shown in fig 4 and 5. Continuous capacity control chillers gives smooth chiller operation with stable chill water temperature and save more electricity. More details are given in referances 3.

VII. CHILLIER AND CONTROLLER OPERATION

Power shutdowns cause unbalance in chiller systems and it can be minimize by reducing down time period. Due to the delay of compressor starting there is chill water temperature rise. It effect to production floor short period of time until chill water system temperature stabilize. Implemented controller starts chiller compressors 3 minutes and pumps 30 second further it starts last few compressors on demand basis. It eliminates sudden temperature fluctuations and high KVA demand.

.

Fig 6 KVA Demand peaks which come from Chiller

Partial load efficiency of a chiller system is very impotent

factor which need to maintain within acceptable level. Mean heat load through a year during cooling operation is far less than the total system capacity. Therefore, efficiency of the system should not only be evaluated at the full-load point, but also the partial- load points. Recently develop compressors give better part load efficiency but as a whole stem it may not that much efficient.

Through monitoring power demand of a chiller system considerable time period, chiller load profile can be identified.

After several years of operation it may be run over loaded or may be under load so it is better keep demand log which can be obtain from this system. History data prove that most of the time chiller consumes lesser power than its full load power. Most of the chillers operate 60 to 70 % load there 80% of running hours through out the year.

Variable speed drives provide variable load operation to two chillers, chill water pump and air handler units.

Fig 7 PLC base control panel

Chiller performance measured by using online power analyzer

and SCADA system Analyzed data shown in Fig 3 and 6. PLC controller monitors chill water inlet and outlet temperature through PT 100 sensors. Further it gets surround atmosphere temperature to measure average chiller load. PLC unit displays inlet, outlet temperatures and temperature difference. PLC controller and supporting components are shown in the fig 7. After implement the controller chiller load pattern is shown in

the fig 8. There are no high demand peaks at start and starting delay is 30 sec which is fixed.

Fig 8 Chiller electricity load variation according step capacity control method

Multi compressor system chill water pumps electricity saving can be further improved by automated chill water control valve system. These valves automatically close when that compressor is not in operation. Then water circulation goes through only the operating compressors evaporator coils. All the impotent parameters of chiller controller display its remote display. There is all impotent data history log and graph for further analysis. Figure 9 and 12 shows the remote use interface of proposed controller. Authorized person can login to this system remotely to monitor status

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Fig 9 Chiller status display and critical temperature s graph

Fig 10 Chill water circulation control valves and variable speed drive

Variable speed drive varies pump speed according to number of running compressors. Controller gets inputs from compressors to identify the running compressors. Number of compressors which allow operating is done based on chill water temperature and time of day. Fig 10 shows auto operated chill water flow control valve and chill water pump speed control variable speed drive. These two components are very much impotent to optimize chill water circulation system.

Fig 12 controller parameter display and temperature log.

During low heat load conditions Air Handler Units can be

operate low speed in order to save electricity. But there should minimum flow rate which is enough for draw air to far end of the duct line. More details are given in referances 5.

Fig 13 After power shutdown there is no high peak to increase KVA cost.

Fig 13 show the smooth operation of chiller without KVA peaks. Number of operating compressors depends on chill water temperature. It save electricity switching off unwanted compressors which runs under load.

VIII. RESULTS

A. Production floor temperature stability improvement

As a result of chill water temperature stability there is 6 Sigma level temperature stability improvements at production floor. Key factors which controlled this improvement are chiller starts shortest possible time after power failure and demand base compressor and chill water pump operation. Chill water flow rate controlled according to number of running compressors. Fig 14 shows production floor temperature variation and humidity variation before the improvement. Fig 15 gives the temperature stability improvement at production floor after fixing controller.

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Variable Prev io us data Present Data

Mean S tDev N 22.56 1.182 79 21.47 0.3750 79

Variable Var-O ld Var-N ew

Mean S tDev N -0.002564 1.193 78 0.002564 0.1651 78

Tem

pera

ture

Poi

nt

to P

oint

Var

iati

on

Tem

pera

t ure

Var

iati

on f

rom

Set

Poi

nt

Fre

quen

cy

Freq

uenc

y

Fig 14 Temperature and Humidity variation before fix controller Fig 17 after fix optimizer unit to the chiller, production floor temperature variation in continuous 3 weeks

Frequency Vs Temperature of Before and After the Improvement

70

60

50

40

30

20

10

0

20.8 22.4

24.0

25.6

27.2

Product ion Floor Temperature

Fig 18 Temperature variation histogram

Fig 15 Temperature and Humidity variation after fix controller

Control charts in the fig 16 gives temperature variation of continuous 3 weeks before the improvement.

Fr equency Vs Before and After Temper ature Point to Point Variation

90

Temperature Variation Vs Time

4

3

2

1

0

UCL=3.382 _ X=-0.003

80 70 60 50 40 30 20

10 0

-3 -2 -1 0

1 2 3

-1 Pr oduc tion Floor Point t o Point Temperatur e Var iation

-2

-3

1 -4

1 8 16 24 32

1

40 48 56

64 72

LCL=-3.387

Fig 19 point to point temperature variation histogram

B. Electricity Saving

Time 6 Hours blocks

Fig 16 before improves the system production floor temperature variation in continuous 3 weeks

Control chart in the fig 17 gives temperature variation of continuous 3 weeks after the improvement. It shows production floor temperature stability improvement by 6 Sigma level. Following histograms shows the improvement before and after status. Fig 18 gives the actual temperature variation before and after the improvement continuously 3 weeks. Fig 19 gives point to point variation before and after the improvement.

Temperature Variation Vs Time

• Chiller initial starting KVA demand reduced by delayed switch on compressors in demand base. This operation eliminates instant compressor starting high KVA demand and distributes the load evenly. Total KVA reduction is 20%.

• Electricity consumption reduced by control operation of

pumps and AHUs. Total electricity saving obtained from this improvement is 15%

• Chiller electricity consumption reduced by demand base

operation of chiller 1 and 2 compressors. It reduces partial load operation of compressors and improves full load operation. Obtained saving is 6% of chiller total power consumption.

0.50

0.25

0.00

UCL=0.458 _ X=0.003

• Knitting magnet faults due to temperature variation

eliminated. Out of all magnet faults 30% cause by temperature variations so temperature stability eliminates magnet faults due to temperature fluctuations.

• Moving to three band tariff reduce electricity cost by 5%

-0.25

-0.50 1 8

16 24 32

40 48

56 64 72

LCL=-0.453

IX. CONCLUTIONS

Through chiller system initial startup sequence controlling total KVA reduces by 20 %. Part of this saving by variable speed drives and other part obtained by controlled delay of compressor starting. Part of electricity consumption reduced by

Time 6 Hours blocks number of operating chill water pumps and their speed controlling. By minimizing no load running and partially

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loaded compressors operation further electricity saved. Total chiller electricity consumption reduced by 8%

Knitting magnet faults due to temperature variation eliminated. That is 30% reduction of total magnet faults.

Moving to three band tariff reduce electricity cost by 5%

Recently develop high efficient water cool chillers gives much better performance even in part loads. It is more impotent to introduce efficient part load operation to chill water pumps and AHUs through variable speed drives and multiple units.

ACKNOWLEDGEMENT

First my deep gratitude goes to Dr. Rohan Munasinghe for the given support to complete this project on time. And also to all the PG lecturers of the Department of Electronics and Telecommunication Engineering for bearing with us to give persistent guidance at all time when required.

My special thanks to Noyon Lanka (pvt) Ltd for the

support given to implementing the system and for the financial assistance.

REFERANCES

1) “Hitachi Air Cooled Water Chillers – Screw Type SCI- P02P,RCUP 245A,” Hitachi Air Conditioning Systems Co Ltd Japan, 2006, pp 2-20

2) “Hitachi Inverter – Driven Multi-Split System, Heat pump Air Conditioners - RAS-3FSVG,” Air Conditioning Systems Co Ltd Japan, 2007 pp 5-25

3) “York Air Cooled Liquid Chillers USA, YDAJ 99 MW6 RecipPak” York Air Conditioning Systems Co Ltd – 2005 pp 10-40

4) Grill Ashrae, “Improving the Efficiency of Chilled Water Plants” – Avery Journal 2005, pp 1-10

5) Grill Ashrae, “Controlling Chillers in Variable Flow Systems “ - Avery Journal 2004, pp 1-5