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Special Report: Midea’s energy-efficient touch

Peck Zhao, marketing manager at Midea Commercial Air Conditioner, explains the science behind designing an energy-efficient HVAC system

Peck Zhao, marketing manager at Midea Commercial Air Conditioner.
Peck Zhao, marketing manager at Midea Commercial Air Conditioner.

The chiller system is still the best choice for large-scale buildings, as a single chiller cooling capacity can be up to 4,000 tonnes or above, which can reduce the installation space, says Peck Zhao, marketing manager at Midea Commercial Air Conditioner.

Significant improvement in operating efficiency can be achieved by replacing an older chiller with a new, high-efficiency chiller. “Centrifugal chillers that are new have a peak efficiency of 0.75-0.85 kw/tonne, whereas chillers that are 10-15 years old have a peak efficiency of 0.60-0.70 kw/tonne. However, with a comprehensive chemical water-treatment program and regular tube cleaning, those efficiencies have declined, resulting in peak efficiencies of 0.80-1 kw/tonne or lower,” says Zhao.

New-generation centrifugal chillers offer peak efficiencies of 0.50 kw/tonnes or higher. When coupled with variable-frequency drive, they can deliver higher efficiency over a range of cooling loads. High-efficiency chillers can reduce annual cooling energy requirements by 30%-50% in most applications compared with the existing old chillers.

Zhao says: “Midea’s high efficiency DC inverter falling film centrifugal chiller (Fig 1.) adopts core technologies such as aerospace aerodynamics, a high-speed inverter electric motor, two-stage complete enthalpy, and falling film evaporation. The chiller has overcome the energy efficiency bottleneck for small cooling tonnage, enhancing the IPLV to 10.69W/W (0.33kW/Tonne), lowering noise to 75 dB(A). Highly efficient and stable operation is achieved under multiple working conditions.”

In the HVAC industry, he says, energy conservation and restricting the use of hydrofluorocarbons (HFCs) with high global warming potentials (GWPs) are the current major issues for HVAC equipment. To improve the overall energy efficiency in chiller systems, there is more focus on larger oil-free centrifugal compressors with permanent magnet (PM) motors, variable speed control by Variable Frequency Drives (VFDs) and two-stage compression systems that maintain high efficiency with wide operating ranges.

Zhao says that Variable Refrigerant Flow (VRF) systems have been the HVAC system of choice in Europe, Japan, China and other parts of the world for quite some time. Over the past 10 to 20 years, VRF technology has become increasingly popular in Europe and Asia market and now in the Middle East.

He says: “The extraordinary 5000 villas project in Al Ain (Fig 2.) sets a very good example of VRF installation. This sophisticated new VRF HVAC technology is capable of providing not only cooling, but also heating, simultaneously to different areas within the space. These VRF HVAC systems are very quiet and energy-efficient because the variable-speed compressor runs only at the capacity needed for the current conditions by varying the flow of refrigerant to the indoor units based on the exact demands of the individual areas. For example, Midea recently launched its new generation full DC Inverter V6 series VRF system (Fig 3) in Dubai, featuring the world’s largest capacity single module outdoor unit at 38HP and combination capacity at 128HP. It also offers a 40% reduction in footprint.”

Natural refrigerants

On the subject of natural refrigerants, Zhao says that ammonia (R717) and propane (R290) are used in chillers mainly in Europe, and have advantages such as low GWP,  superior performance and low cost. However, toxicity and flammability exclude them from being installed in commercial buildings.

He adds: “Water (R718) as a refrigerant has been used for thermal-driven absorption chillers. Meanwhile, research and development has been carried out for a long time in Europe, the US, and Japan, with a view to using water for vapour compression chillers/heat pumps.”

As a refrigerant, water is fundamentally different from conventional HFC refrigerants. The required suction gas volume of water is about 150 times bigger than that required by R134a, and water has a compression ratio 3.4 times higher than that of R134a. Therefore, centrifugal compressors using water need to be much bigger.

“There’s still a long journey to find better alternative refrigerants that have lower cost and better performance in efficiency and capacity for the chillers, especially large capacity centrifugal chillers,” Zhao says.

Tips for designing an energy-efficient HVAC system

Zhao lists the following:

- Understand the variety of chiller options based on load requirements.

- Learn to calculate a simplified cost/tonne for estimating initial investment costs.

- Know the appropriate calculations for determining chiller plant operational costs.

Chilled water systems are cooling systems that circulate chilled water throughout a building for cooling and dehumidifying a building’s atmosphere. They are closed-loop systems, meaning that the system water is continually recirculated and not exposed to atmospheric pressure, similar to domestic water systems.

He says: “The first step in chiller selection is understanding the options available. A building’s block load will determine the overall capacity, whereas the part load will determine the number and quantity of chillers required, with multiple chillers providing the ability to stage chillers in response to load.

“The second is the type of chiller compressors. Most small chilled water plants use scroll compressors for production of chilled water. As the capacity increases, the chillers increase the quantity of scroll compressors, typically of equal sizes to provide the total chiller capacity required. The disadvantage is that the chiller capacity control is provided as a stepped control instead of modulating control. Although the multiple compressors may be a disadvantage for capacity control, generally they are piped with multiple refrigerant circuits which provide some system redundancy. Inverter driven scroll compressor have been widely used for chillers that can improve the part load efficiency quite a lot.

“Screw compressors are available in sizes from 50 tonnes up to about 500 tonnes. Screw compressors have the ability to vary the cooling output capacity from 100% to 20% via the use of a slide vane to limit refrigerant delivery to the compressor and provide a smooth, modulating transition between capacities. It is important to note that screw chillers have only one compressor, so a loss in the compressor would cause a complete loss in chiller capacity.

“The third type of compressor is centrifugal compressor that starts at approximately 100 tonnes and go up to thousands of tonnes depending on the number of compressors. A VFD also could be used for capacity control to vary the speed of the impeller rotation in conjunction with inlet vanes.”

Inlet vanes and VFDs accomplish different objectives: inlet vanes are used for buildings that may have a large load variation, while VFDs should be used for buildings that have large variations in lift, which equates to changes in condenser relief. VFDs are not always an appropriate option for chillers and their use greatly depends on their ability to vary temperatures. Centrifugal chillers operates at high speeds that are extremely reliable and robust devices. Centrifugal compressors have great efficiencies throughout their operating range and are relatively compact for the amount of tonnage that can be provided per square metre of mechanical room space.

Lastly, he says, control sequences for HVAC systems are very important. Using a BMS software can help to manage all the devices such as chiller, water pumps, air side terminals. In plants that use multiple chillers, the programmes can identify chillers that can help meet the current load at the highest operating efficiency.

“Taken together, the recent advances in chiller design has brought significant savings for institutional and commercial facilities. Designers that take advantage of new technologies and design features can expect these systems to use only 50%-60% of the energy required by systems installed as recently as 10-15 years ago,” concludes Zhao.

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