A CHILLED WATER STORAGE SYSTEM AS PART OF AN ENERGY MANAGEMENT SYSTEM Abstract The Chilled Water Storage System at the Northern Territory University has now been commissioned and is fully operational. This system is the largest stratified chilled water storage system and the first transferring the full airconditioning load to off-peak in Australia This paper details the savings being achieved as well as the other benefits gained by a system such as this. In addition, the very successful ongoing energy management program will be discussed.

Steve Beagley Head of Works, Facilities Management Division Northern Territory University ________________________________________________________________________ Background The Casuarina Campus of the Northern Territory University is located in Darwin and comprises 42 separate buildings with a total floor area in excess of 80,000m2. Prior to implementation of the works described in this paper the majority of the campus air conditioning load was satisfied by two central chilled water plants effectively located at either end of the Campus feeding 3km of reticulation pipework. Peak electrical demand for the Campus was typically 4.8MW. Central Plant 1 (CP-1) was constructed approximately 20 years ago and comprised a conventional decoupled primary/secondary reticulation system. At the time of the investigation, the original chillers had been recently replaced with new Trane rotary screw machines with a total capacity of 2400kWR. Central Plant 2 (CP-2) incorporates four centrifugal chillers has a total capacity of 8,400kWR CP-2 served the larger and more recent additions to the Campus and supplied an extensive underground Campus reticulation system however it incorporated a primary pumping system only. Accordingly, it has suffered since its construction from poor flow control in the field and unnecessarily high operating costs. Low load operation has also proved a problem with CP-2 as the smallest available chiller (1,500kWR) is not capable of effectively handling the small loads existing at night during the “Dry” season.

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The NTU began investigation into the feasibility of using gas-fired turbines with heat absorption chillers. Problems associated with the supply of gas and the cost structure imposed by the Electrical Supply Authority (PAWA) for stand-by power limited the economic gains from the proposal. However, at the same time the growth in the Casuarina area of Darwin was imposing considerable strain on electrical infrastructure systems and PAWA had begun its own investigation into load control. As a result, PAWA offered financial incentives to the University if a full shift of refrigeration electrical load was obtainable MGF Consulting Engineers of Cairns were commissioned to carry out the necessary investigations on our behalf and produced a computer model to simulate the effect of various thermal storage approaches and establish projected savings for a variety of tariff structures. Ice and stratified chilled water storage options were investigated. Stratified chilled water storage proved the more viable option. The model was used as a basis for further negotiations with PAWA to establish a mutually acceptable tariff structure that provided 3-4 year payback period. Further analysis showed that the optimum solution comprised the construction of single stratified storage tank located adjacent to CP-2 combined with the provision of new secondary pumping systems in CP-2 and the interconnection of CP-1 and CP-2 systems. The Tank Various options were considered in respect to storage tank construction. The option selected has the tank made from stressed concrete with internally applied polystyrene insulation and internal flexible liner. This method yielded a lower estimated capital cost, mainly due to the reduced insulation and vapour barrier costs compared to a steel construction. It also reduced the risk of external condensation and vapour transmission to the insulation material and provided an improved appearance. Despite the 5-60C storage temperature and the high dewpoint conditions experienced in Darwin during the “Wet” season, no evidence of condensation has been observed on the tank to date.

Table 1 – The Tank Specification Type Construction Thermal Capacity Storage Volume Tank Height(Water) Tank Diameter Storage Temperature Storage ∆T

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Stratified Concrete 72,000kWh 8,600m3 12.5m 29.6m 5.0-6.0°C 7.5°C

For thermal storage systems the typical procurement approach is based on the use of a performance based specification to cover the thermal storage element. Under this arrangement the tender field is limited to a small group of Specialist Mechanical Services Contractors. In this case however the Project Consultants fully designed the tank and associated diffusers and the tender field was opened up to general Civil Contractors who are the obvious choice in construction of what are effectively large water tanks. This approach yielded considerable cost savings and it is significant that the tenders received from Civil Contractors were in the order of 15% below the specialist Mechanical Services Contractors who tendered on the project. Control System The NTU already had in place an extensive CSI Pacific Building Energy Management System (BEMS) covering its three campuses and the obvious solution was to incorporate the control function of this additional installation into the existing network. The cost of the controls was minimal in the overall project and includes alarms via an automatic paging system to Facilities Management Division staff and CSI Pacific technicians. Negotiated Tariff The tariff negotiated with PAWA is a time of use tariff with separate rates for both maximum demand and energy consumption elements during both peak and off-peak periods. Peak period is from 6am to 6pm, of-peak from 6pm to 6am. System Operation The tank is sized to satisfy the total chilled water requirements of the campus during the on-peak period designated by the Supply Authority (6.00 am to 6.00 pm). During this period all chillers and associated primary chilled water pumps, cooling towers and condenser water pumps are scheduled off. Chillers operate at night to charge the tanks and satisfy any coincident after hours load requirements. The operation of the tank has shifted some of our operational requirements away from the normal day time hours to after hours. At commencement of the charging phase appropriate chiller groups are selected to operate continuously at maximum efficiency points on the basis of residual storage capacity in the tank. The following graphs show the Water Usage in metres, Power Consumption and Chiller operation over two days, 13 July and 23 August this year. These days are typical, the 13th shows the requirements for an average “Dry” season day and the 23rd shows the increase in load as we enter the “Wet” season in Darwin.

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Chilled Water Tank Usage

Chilled Water Tank Usage 12

12

10

10

8

8

Tank

6

Tank

6

4

4

2

2

0

0

06 :0 0 0 8: 00 1 0 :0 0 1 2 :0 0 1 4:0 0 1 6 :0 0 1 8 :0 0 2 0 :0 0 00 :0 0 0 4 :00 2 2 :0 0 02 :0 0 0 6 :0 0 0 7/ 1 3 /9 9 0 7 /1 3 / 99 0 7 /1 3 / 99 0 7 / 13 / 9 07 9 / 1 3/ 9 0 9 7 /1 3 / 99 0 7 /1 3 / 99 0 7 / 13 / 9 07 9 / 1 3/ 9 0 9 7/ 1 4 /9 09 7/ 1 4 / 99 0 7 / 14 / 9 9 07 / 1 4/ 9 9

0 6: 00 08 :0 0 1 0 :0 0 1 2 :0 0 14 :0 0 1 6 :0 0 1 8: 00 2 0 :0 0 2 2: 00 0 0 :0 0 0 2 :0 0 04 :0 0 0 6 :0 0 0 8/ 2 3 / 99 0 8 / 2 3/ 9 09 8/ 2 3 / 99 0 8 / 2 3 /9 0 9 8 / 23 / 9 0 9 8 /2 3 / 9098 / 2 3/ 9 0 9 8 /2 3 / 9 08 9 / 2 3 / 99 0 8 / 2 4/ 9 09 8/ 2 4 / 99 0 8 / 2 4 /9 9 0 8 / 24 / 9 9

13 July 1999

23 August 1999

Within that 6 week period the airconditioning load had increased sufficiently to increase the chilled water usage by approximately 50%. This increase therefore resulted in the need to increase the chiller capacity required to operate during the off-peak period. To minimise the chance of requiring chillers to operate during the peak period we have adopted a policy of ensuring the tank is chilled completely each night to ensure we have spare capacity in the event of a chiller, power or system failure. In the middle of the “Dry” season we relax that requirement and ensure we have sufficient capacity for 2 day’s usage ie 8 metres. This involved the use of 3 chillers during the night but with a later start for the third unit as shown in the following graphs.

CP2 Chiller Loads

CP2 Chiller Loads

100

100

80

80

Ch1 Ch2 Ch3 Ch4

60 40

20

0

0

13 July 1999

Ch3 Ch4

40

20

06 :0 0 08 :0 0 10 :0 0 1 2: 00 1 4 :0 0 1 6 :0 0 1 8 :0 0 20 :0 0 22 :0 0 0 0 :0 0 0 2 :0 0 0 4 :0 0 0 6 :0 0 0 7/ 1 3 / 99 0 7 / 1 4/ 9097 / 1 4/ 9097 / 14 / 9097 / 14 / 9 9 0 7/ 1 3 / 99 0 7/ 1 3 / 99 07 / 1 3 /9 07 9 / 1 3 /9 097 / 1 3/ 9097 / 1 3/ 9097 /1 3 / 9097/ 1 3 / 99

Ch1 Ch2

60

06 :0 0 0 8 :1 5 10 :3 0 1 2 :4 5 1 5 :0 0 1 7 :1 5 1 9 :3 0 2 1 :4 5 0 0 :0 0 0 2 :1 5 0 4 :3 0 08 / 2 3 /9 90 8 /2 3 / 9 90 8 / 23 / 9 90 8/ 2 3 / 990 8 /2 3 / 9 90 8 / 23 / 9 90 8/ 2 3 / 990 8 /2 3 / 9 90 8 / 24 / 9 908 / 2 4 /9 90 8 /2 4 / 9 9

23 August 1999

These next graphs show the increase in power consumption for the two days, an increase of 150kW during the off-peak chiller operation and an increase of 240kW during the day. This latter increase equates to an increase in excess of $4,500 to the Peak Demand fee. By ‘staggering’ the starting times of the chillers, we are able to reduce the off-peak Maximum Demand Fee (MDF). If we ensure that additional chillers do not start at the time of peak demand, a saving in excess of $7,000pa is achieved.

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Casuarina Power Consuption

Casuarina Power Consuption

3000

3000

2500

2500

2000

kW

2000

kW

Max Demand

1500

Peak Demand Off Peak Demand

1000

Max Demand

1500

Peak Demand Off Peak Demand

1000

500

500

0

0

0 6 :0 0 0 8 :3 0 1 1: 00 1 3 :3 0 1 6 :0 0 1 8 :3 0 2 1 :0 0 23 :3 0 02 :0 0 0 4 :3 0 0 7 / 1 3 /9 9 0 7 / 1 3 /9 0 9 7 / 1 3/ 9 0 9 7 / 1 3/ 9 09 7 /1 3 / 9 0 9 7/ 1 3 / 9 0 9 7/ 1 3 / 9 0 9 7/ 1 3 / 9 9 0 7/ 1 4 / 9 9 07 / 1 4 / 9 9

06:00 08:30 11: 00 13:3 0 16 :00 18: 30 21: 00 23:3 0 02:00 04:30 08/ 23/99 08/2 3/9908/2 3/9908/23 /990 8/23/99 0 8/23/99 08 /23/99 08/ 23/99 08/ 24/99 08/2 4/99

13 July 1999

23 August 1999

Is the tank performing? At the end of August, if a comparison is made between the old and new tariffs, our savings estimated for the complete year are expected to be in excess of $530,000. If we compare our actual expenditure for the same period last year we will be achieving an annual saving closer to $600,000. Based on these results we are expecting to achieve a payback period for the complete project of 4 years. The de-coupling and associated pipe-work changes were required regardless of whether we proceeded with the Chilled Water Storage Project. If these costs are removed from the economic model then the pay-back period reduces to 3 years giving a further 7 years of savings under the current 10 year contract with PAWA. This could equate to a total savings well in excess of $3M during the remainder of the contract period. These following graphs show the results achieved compared to the economic model.

Cumulative Energy Cost Savings by Month - 1999 Comparison withh 1998 Costs

KWH Consumption by Month - 1999 900,000

$600,000

800,000

$500,000 700,000

$400,000

Electricity Cost

500,000

400,000

300,000

$200,000

On Peak - Actual Off Peak - Actual On Peak - Model Off Peak - Model

200,000

100,000

$300,000

98/99 Savings

$100,000

MONTH

MONTH

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DEC

OCT

NOV

SEP

AUG

JUL

JUN

APR

MAY

MAR

$0

FEB

DEC

OCT

NOV

AUG

SEP

JUL

JUN

APR

MAY

MAR

JAN

FEB

Model Prediction

0

JAN

KWH

600,000

Energy Savings As well as giving long term financial benefits to the University, this system provides energy savings. Reduced power consumption, and its associated benefit in the reduction of Greenhouse gases provides further incentives. The act of de-coupling the chillers and installing secondary pumps under VSD controls provides far more stable conditions in the field. The additional pressure drop imposed by the tank has increased pumping energy consumption. However, this has been off-set by the following savings • The chillers are now operating at close to their maximum efficiency during most of their operating times • We can better match the low load requirements. Previously the minimum capacity available in CP-2 was 1500kWR, this unit was well in excess of the requirements. Our operational capabilities now allow us to provide chilled water directly from the tank or during a charging cycle. • By operating our chillers during the cooler evening periods we obtain greater operating efficiencies from our chillers and cooling towers. • The efficiency of PAWA’s distribution system is improved as a result of lower line losses associated with a reduced maximum demand. We have achieved a 3% reduction in our consumption this year compared to 1998, this is significant achievement as additional load has been added due to some refurbishments completed since last year and earlier this year. Some of these savings however can be attributed to a cooler “Dry” season this year and this change in load due to climatic conditions needs to be remembered when assessing performance. Other Benefits Our demand on campus was such that we only had limited redundancy in the system, and any further development would have stretched our resources. The tank has been designed to have approximately 30% spare capacity, our “worse day” scenario uses approximately 9 metres of the water. Recharging of the tank can be achieved comfortably within the 12 hour off-peak window utilising three of the chillers, the fourth unit is held as back-up. In addition, we have the capability, albeit at an increased cost (approximately 30%), to generate chilled water during the peak periods provided such chilling does not affect the maximum demand. This, as shown on the previous Power Consumption graphs, can occur between 6 and 8am and 4:30 and 6pm. This ultimately gives the University approximately 45% total spare capacity in its current system. The installation of the tank and connecting of the two loops has allowed us to decommission CP1, sell one of the chillers and relocate the remaining two units to our Palmerston Campus thereby saving significant capital investment at that site. A recent failure to both the power supplies feeding CP2 which effectively reduced the power available from each of the two transformers to half their capacity would have caused the closure of a large number of our major facilities because of lack of

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airconditioning. However, the ability to generate sufficient chilled water over a longer off peak period and store this “energy” enabled the University to function without any interference to its operations and also without any increase in energy or demand costs. Although we now have shifted our operational requirement to nights, the maintenance and servicing of our major chiller plant can now be carried out during normal hours and is not as restricted in its scheduling because of our “increased capacity”. Partnering The development of this project and its ongoing operation in particular has been a success in partnering. This was not the standard form of partnering where parties to the contract share in the financial gains or sign documents detailing their support for the project etc. This was one in which the consulting engineers, the control technicians and the chiller and pump system maintenance staff have all worked closely together long after the commissioning and contractual obligations were completed to achieve these results. Energy Management The NTU has carried out a number of energy management projects. These have included; • The establishment of an Energy Management Advisory Committee comprising FMD staff, academics, student representation, and members from outside the university environment. • Charging for the use of airconditioning outside of “core” operating hours. • All buildings on our 3 campuses have been connected to the BEMS and a policy is in place which requires all new facilities to be connected. • Installation of automatic lighting control systems to the Library and a large workshop to make use of the available daylight. • Card Access systems via the BEMS to control airconditioning in individual classrooms, limiting the airconditioning operation times. • Selective switching of carpark and street lighting via the BEMS. • Control of Hot water Systems via the BEMS. The demand tariff negotiated as part of the tank project has further increased the need to conserve energy, particularly during peak times. The NTU has a number of further initiatives being implemented; • The connection of two more buildings to our chilled water system replacing standalone units. • Control the use of classroom lights by fitting timers to approximately 120 classrooms. Our major lecture facilities are already fitted with occupancy sensors. CSI Pacific are involved with NTU in monitoring the performance of this project, which has been sponsored partly by the Northern Territory Government Energy Management program. This report will be used to assess the feasibility of this system in Northern Territory Government schools. • The installation of translucent sheeting to workshops to reduce the use of lights. One workshop already completed now operates during the day using mainly natural light. A reduction in load of 12kW. Page 7 of 8

• •

Controlling the operation of air compressors, coolrooms and pre-cool units during peak periods of use via the BEMS. Developing a web page that will provide on-line information regarding our energy usage, and tank and system operation from our BEMS. This project is being jointly sponsored by CSI Pacific and MGF Consultants.

Facilities Management Division Northern Territory University Darwin NT 0909 Tel: 08 8946 6606 Fax: 08 8946 6562 Contact: Mr Steve Beagley Email: [email protected] Web: www.ntu.edu.au

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CSI Pacific Pty Ltd Unit 1A 27 Bishop St Stuart Park NT 0820 Tel: 08 8981 4566 Fax: 08 8981 9680 Contact: Mr Greg Holburt Email: [email protected] Web: www.csi-controls.com

MGF Consulting Engineers PO Box 797N North Cairns QLD 4870 Tel: 07 4051 0999 Fax: 07 4051 0526 Contact: Mr Tony Grijmans Email: [email protected]