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Energy Efficient Retrofits Guide: HVAC - Chiller Plants
   

2.4.6 Seasonal Chilled Water Temperature Reset

Why?

§  relatively low cost
§  reduces building cooling load

Where does it work best?

Office Buildings (Types 1 & 2)

Hotel Buildings

Image Source: ASHRAE

Overview

Raising the chilled water temperature increases the chiller plant performance, reduces the cooling output, and increases the supply air temperature of the cooling delivery terminal units. For a typical 7°C -12°C chilled water system, the energy consumption can be reduced by 5% when the supplied water temperature is increased from 7°C to 8°C, or even up to 10% if the temperature is raised to 9°C.

Chilled water temperature reset will not face any physical constraints as equipment does not need to be replaced. However, there could be a potential reduction in cooling capacity, and an increase in supply air temperatures for certain equipment. After the reset, the capacity of terminal units could be reduced by roughly 10% to 20%, which means, to achieve the same cooling load, they will need to run for longer durations.

The energy savings will be largest during spring and autumn, where maximum cooling is not required, and at sites that require cooling in winter. The temperature reset can be done manually by a facilities manager, or automatically through the building management system (BMS).

Modelled Costs and Saving Potentials (assumptions are listed in Appendix A)

Seasonal Chilled Water Temperature Reset

Building Type

Office

Hotels

Type 1

Type 2

Capital Cost (per m2 building area)

1

1

0.3

Payback Period (years)

0.1

0.1

0.0

Saving Potentials (per m2 building area)

Cost

5 HK$/m2/year

7 HK$/m2/year

9 HK$/m2/year

Energy

5 kWh/m2/year

6 kWh/m2/year

8 kWh/m2/year

Carbon

3 kg CO2-e/m2/year

4 kg CO2-e/m2/year

5 kg CO2-e/m2/year

Advantages

  • significant energy savings, as chillers represent a significant proportion of energy consumption
  • zero cost for buildings with building management systems or chiller controls in place

Limitations

  • the dehumidification performance will decrease for buildings that rely on chillers for humidity control
  • may not be suitable for buildings that require constant temperatures, such as data centres or industrial processing plants
  • could be expensive if a new chiller or BMS is involved
  • frequent monitoring needed, especially in manual operations

Additional Considerations

  • may need to check tenants’ requirement on chilled water temperatures
  • this can be an energy saving opportunity in a retro-commissioning study

Relevance to Landlords and Tenants

  • landlords are responsible for central chiller plant upgrades
  • cost savings from reduced chiller plant energy use are directly financially beneficial to landlords

2.4.7 Variable Speed Drive (“VSD”) Chillers

Why?

§  high coefficient of performance (COP) at part-load conditions
§  their capital costs have been decreasing as technology matures

Where do they work best?

Office Buildings (Types 1 & 2)

Hotel Buildings

Image Source: York

Overview

Traditional chillers with constant speed motors in compressors are most efficient at peak load, but have reduced efficiency as the load ratio drops. They run at maximum speed until the thermostat sensor shuts off the machine, and then turn it back on when the room heats up again. This constant on-and-off process both wastes energy and increases wear and tear.

VSD chillers are able to change the speed of compressors to maintain the temperature of the chilled water. They are more efficient at part-load conditions than traditional constant speed chillers, and as efficient at peak load. Provided that chillers operate at part-load more than 90% of the time, VSD chillers can improve energy performance in air conditioning significantly.

Modelled Costs and Saving Potentials (assumptions are listed in Appendix A)

VSD Water-Cooled Chiller

Building Type

Office Type 1

Office Type 2

Hotel

Scenario

End-of-Life

Total Cost

End-of-Life

Total Cost

End-of-Life

Total Cost

Capital Cost (per m2 building area)

HK$68

HK$241

HK$140

HK$409

HK$156

HK$456

Payback Period (years)

3.0

10.5

7.0

20.5

6.0

17.6

Saving Potentials (per m2 building area)

Cost

HK$23/year

HK$20/year

HK$26/year

Energy

20 kWh/year

18 kWh/year

23 kWh/year

Carbon

12 kg CO2-e/year

11 kg CO2-e/year

14 kg CO2-e/year

Advantages

  • the capital cost has been decreasing as technology matures
  • significant energy savings
  • can be installed in like-for-like replacement situations
  • VSD chillers have both air- and water-cooled models

Limitations

  • higher capital cost than traditional chillers
  • water-cooled chiller plants have additional requirement such as cooling towers, water tanks and associated spacing and structural consideration

Additional Considerations

  • VSD chillers will need an additional control logic set-up to be efficient
  • water-cooled chillers must be in the Fresh Water Cooling Towers (FWCT) Scheme areas[1]
  • the payback period will be a lot shorter when the existing chiller plant reaches its end-of-life and needs replacement

Relevance to Landlords and Tenants

  • landlords are responsible for central chiller plant upgrades
  • cost savings from reduced chiller plant energy use are directly financially beneficial to landlords, and in the longer term potentially beneficial to tenants

2.4.8 Oil-free Magnetic Bearing Chillers

Why?

§  extremely efficient at part-load conditions
§  require significantly less maintenance

Where do they work best?

Office Buildings (Types 1 & 2)

Hotel Buildings

Image Source: LG

Overview

Oil-free chillers are considered to be one of the most cost-effective energy saving technologies. They are air conditioning systems that use magnets to levitate parts that normally require lubrication. This reduces friction between the parts and lowers the energy use compared to a traditional chiller that uses lubricants.

Similar to VSD chillers, the coefficient of performance (COP) of oil-free chillers is very high at part-load conditions, and their peak load efficiency is as good as traditional chillers. Therefore, the key for oil-free chillers is to maximise part-load opportunities. Oil-free chillers are also quieter and have longer plant life due to less vibration and friction of bearings.

Modelled Costs and Saving Potentials (assumptions are listed in Appendix A)

Oil-free Magnetic Bearing Chiller

Building Type

Office Type 1

Office Type 2

Hotels

Scenario

End-of-Life

Total Cost

End-of-Life

Total Cost

End-of-Life

Total Cost

Capital Cost (per m2 building area)

HK$130

HK$303

HK$200

HK$469

HK$224

HK$523

Payback Period (years)

4.3

9.9

6.0

14.2

6.0

14.0

Saving Potentials (per m2 building area)

Cost

HK$31/year

HK$33/year

HK$38/year

Energy

27 kWh/year

29 kWh/year

33 kWh/year

Carbon

16 kg CO2-e/year

18 kg CO2-e/year

20 kg CO2-e/year

Advantages

  • the capital costs reduced as technology matures, resulting in reasonable payback periods
  • significant energy savings as they are extremely efficient at part-load
  • require less maintenance in terms of replacing worn bearings and lubricants
  • quieter and have longer plant life due to less vibration and friction in bearings
  • both air- and water-cooled models are available

Limitations

  • higher capital cost than traditional chillers
  • require control optimization for maximum efficiency

Additional Considerations

  • chiller control is the key for oil-free chiller and this will require very careful control logic planning, with input from manufacturer and site operation team in order to achieve the expected efficiency
  • the chiller plant can utilise standby capacity to enable the chiller to operate at part-load conditions for maximum efficiency

Relevance to Landlords and Tenants

  • landlords are responsible for central chiller plant upgrades
  • cost savings from reduced chiller plant energy use are directly financially beneficial to landlord and potentially beneficial to the tenant in the longer term

Case Study – Oil-free, VSD, Water-cooled Chillers

In 2013, variable speed oil-free centrifugal type water-cooled chillers were installed at the rooftop of an office building in North Point, replacing old air-cooled chillers with evaporative cooling tower.

Prior to the installation in 2013, a feasibility study for the chiller plant upgrade was carried out in 2011 to 2012, assessing the various upgrade options. The key factors of consideration were performance, stability, ease of maintenance, and having a short payback period. The variable speed oil-free centrifugal type water-cooled chillers fulfilled these requirements, and were selected as the retrofit option as they were more advantageous in terms of energy efficiency and payback period, even though the investment cost of the new chillers was 20-30% higher than conventional type chillers.

The new chillers came into full operation in 2014. Compared to the old air-cooled chillers, the new chillers consumed 30-40% less energy. This has resulted in an annual saving of more than 1,000,000 kWh, which is equivalent to a 700-ton reduction in CO2 emissions.

Note: The coefficient of performance (COP) of the old chillers were about 3.5, with relatively high maintenance and operation costs. 

 

Information provided by Towngas

 

Case Study 2 – Oil-free, VSD, Air-cooled Chillers

In 2014, an oil-free air-cooled chiller was installed at an office building in Kowloon Tong, replacing an old air-cooled chiller of similar capacity.

Compared to the old chiller, electricity consumption of the oil-free chiller is 30% less, resulting in an annual energy saving of more than 65,800 kWh.

With this retrofit, there is an additional annual maintenance cost HKD$20,000 incurred to the building owner with the new chiller installed. However, the retrofit also resulted in a number of benefits in other areas. The oil-free chiller is smaller and lighter than the old chiller, hence there were no issues in regarding the building’s space and structural requirements. Furthermore, saving space adds value to the retrofit work as the space can then be used for other applications. Also, as the oil-free chiller is more silent, there was no need to add silencers or an acoustic enclosure to the chiller. This can save costs if the chiller is installed near residential buildings.

 

Information provided by Business Environment Council


2.4.9 Variable Speed Drives (“VSD”) on Chilled Water Pumps

Why?

§  significant energy savings
§  relatively low equipment and maintenance costs

Where do they work best?

Office Buildings (Types 1 & 2)

Hotel Buildings

Image Source: Southerns Water Technology

Overview

Similar to VSD chillers, VSD pumps modulate pump's motor speed based on actual demand, thus saving energy from unnecessarily high pumping powers. There are numerous applications for VSD pumps but they are most commonly used in HVAC plants.

VSD pumps require corresponding control and piping systems to be set up, for example, parallel pumping through multiple pumps to deliver the same flow rate, but at a lower pressure and pump speed to achieve energy savings. These pumps are particularly efficient when coupled with primary flow chiller plants.

In addition, VSD pumps can also be used as a commissioning tool to set a desired pump flow rate instead of traditional resistance-based hydraulic devices that waste energy on unwanted pumping.

Modelled Costs and Saving Potentials (assumptions are listed in Appendix A)

VSD on Chilled Water Pumps

Building Type

Office Type 1

Office Type 2

Hotels

Scenario

End-of-Life

Total Cost

End-of-Life

Total Cost

End-of-Life

Total Cost

Capital Cost (per m2 building area)

HK$5

HK$9

HK$5

HK$9

HK$3.4

HK$7

Payback Period (years)

0.4

0.8

0.3

0.7

0.2

0.4

Saving Potentials (per m2)

Cost

HK$11/year

HK$13/year

HK$17/year

Energy

10 kWh/year

12 kWh/year

15 kWh/year

Carbon

6 kg CO2-e/year

7 kg CO2-e/year

9 kg CO2-e/year

Advantages

  • technology is mature
  • significant energy savings
  • relatively low cost
  • reduced maintenance costs with reduced pump wear

Limitations

  • would need parallel pumping through common headers to maximise the cost savings of VSD pumps
  • primary variable flow may cause system instability in certain chiller plants, so building managers are advised to seek input from manufacturers when setting up their control strategies
  • may require recommissioning of existing plants
  • likely require other upgrades to work, such as a chilled water pipe header arrangement

Additional Considerations

  • can be upgraded together with chiller plants and building management system (BMS) controls to achieve maximum efficiency
  • existing pumps may be able to adopt new VSD controller to achieve similar effects

Relevance to Landlords and Tenants

  • landlords are responsible for central chiller plant upgrades
  • cost savings from reduced chiller plant energy use are directly financially beneficial to landlords and potentially to tenants in the longer term.

 

 

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