E14: Air Conditioning With Water Cooler Chiller

Requirement Summary

If the project includes a water-cooled chiller, the actual COP of the system should be entered in the software (even if the COP is lower than the default value). Savings can be achieved if the air conditioning system has a Coefficient of Performance (COP) greater than the base case, as set out in the Key Assumptions for the Base Case in the Design section. The COP must be determined under ARI conditions.

Intention

Water-cooled chillers are typically more efficient than comparable air-cooled chillers. A water-cooled system is the best option when reducing operating costs is of paramount concern and the project can invest in a system with a longer payback period. Water cooling does involve a higher initial investment since both a chiller and a circulating tower system are required, which in turn require additional pumps, piping and tanks. Also, water cooling systems consume considerable amounts of water due to evaporation, purging and bleeding.

Approach/Methodologies

EDGE uses the Coefficient of Performance (COP) to measure the efficiency of an air-conditioning system. The COP is the total output of cooling energy per electricity input. The COP for cooling is defined as the ratio of the rate of heating energy removal to the rate of electrical energy input, in consistent units, for a complete air conditioning system or some specific portion of that system under designated operating conditions. The formula to calculate the COP is explained below. For consistency, ARI conditions must be used for the comparison of COP values.

Where:

Q out = heating energy removal (kW)

W in = electrical energy input (kW)

To claim this measure, the design team must demonstrate that the chiller(s) achieve a COP greater than the base case. For large buildings with centralized systems, more than one chiller may be installed. If these chillers have different COPs, the weighted average COP must be calculated.

In some cases, the air conditioning system (chiller/s) for the cooling system can be centralized, serving a combination of buildings/dwellings within the development, for example, in a district cooling system. In these cases, the central plant will need to be included within the site boundary of the project, or managed by a company within the control of the site owner. This is to ensure the continuous sustainable management of (and access to) the facility by the site owner.

However, when the chiller for the cooling system is located off-site, then a contract with, or letter from the management company in charge of the chiller must be provided as part of the documentation for the post-construction stage. The document must include the efficiency of the system.

If air conditioning is not specified, any cooling load will be displayed as “virtual energy.”

Potential Technologies/Strategies

This technology is similar to air-cooled chillers, the primary difference being that water is used to cool the condenser instead of air.

The cycle begins in the evaporator where a liquid refrigerant flows over the evaporator tube bundle and evaporates, absorbing heat from the water circulating through the bundle. The refrigerant vapor is drawn out of the evaporator by the compressor. The compressor compresses the refrigerant raising its pressure and temperature, and pumps the refrigerant vapor to the condenser. The refrigerant condenses in the condenser tubes, giving up its heat to the water that is cooling the condenser. The high-pressure, liquid refrigerant from the condenser then passes through the expansion device that reduces the refrigerant pressure and temperature as it enters the evaporator. The cold refrigerant again flows over the water coils absorbing more heat and completing the cycle.

Relationship to Other Measures

The local climate, heat gains and the internal temperatures based on the building design impact the cooling load. A more efficient system will not impact other measures, but several measures will impact the total energy use of the cooling system.

In addition, when a water-cooled chiller is selected as an energy efficiency measure, total water consumption is increased for both the base and the improved case, as the chiller will require water to operate.

Assumptions

The base case for air conditioning system efficiency is listed in the Key Assumptions for the Base Case in the Design section.

The default value for the improved case COP for the water-cooled chiller varies as per ASHRAE 90.1-2007 based on the area and number of floors; if the system efficiency is different from the measure’s default value, then the actual performance must be entered, and the energy savings will be recalculated.

Compliance Guidance

To demonstrate compliance, the design team must describe the specified system and provide documentation to support the claims.

Design Stage

Post-Construction Stage

At the design stage, the following must be used to demonstrate compliance:  Mechanical layout drawings/schematic showing the location of the external and internal units; and  Equipment schedule or Manufacturer’s data sheets (with the project specific info highlighted & noted) for the water-cooled chiller system specifying COP information; and Average COP calculation for systems including more than one chiller unit.

At the post-construction stage, the following must be used to demonstrate compliance: As-built mechanical drawings with air conditioning schematics for all floors if there are changes; and Delivery notes showing that the specified chillers have been delivered to the site; and Manufacturer’s data sheets for the water-cooled chiller system specifying the COP information; and  Photographs of installed external and internal units including chillers and cooling towers; and/or Contract with the management company if the system is centralized or off-site.

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