Emerging EE air-conditioning technologies

All Variable Speed Chilled Water Plant Control

Technology outline:
The All Variable Speed Chilled Water Plant Control (VSCWPC) methodology is a new control technology for determining the best operation sequence and loading of equipment of a chiller plant with respect to the kW/ton characteristics curves. The operating principle of VSCWPC is, by means of demand base control, to optimize the operation of the chillers, chilled water pumps, condensing water pumps and cooling tower fans. Therefore the major equipment of the chiller plant is required to equip with variable speed drives.

How it can save energy:
The VSCWPC technology controls the variable speed chillers in a way such that they are always operating as close as possible to its maximum efficiency for a given entering condenser water temperature (ECWT). Optimization of ECWT is achieved by maintaining the greatest active surface area on the cooling towers by slowing down the condenser water pumps tower fans. The best combination of condenser water pump speed and tower fan speed is determined by the VSCWPC. The VSCWPC controller will also work out the optimum speed of the variable speed chilled water pumps and optimum position of the chiller water bypass valve. As a result, the operation of the all variable speed chiller plant is optimized based on the actual demand.

Adaptive or Fuzzy Logic Control

Technology outline:
Adaptive or Fuzzy Logic Control Algorithms improve upon classic control approaches by allowing for much better flexibility. In HVAC system control, it is common to use proportional-integral (PI) feedback control algorithms. However, the default control parameters used in the PI algorithms are not appropriate and can lead to poor control performance because of the existence of time-varying dynamics parameters. On the other hand, Adaptive or Fuzzy Logic Control provides continuous adjustment of the control parameters as the system dynamics change. However, much equipment uses conventional non-electronic control and would first require conversion to electronic control to allow implementation of fuzzy or adaptive control. Technicians also require more training to properly troubleshoot electronic control system.

How it can save energy:
Adaptive or Fuzzy Logic Control Algorithms eliminate the need for tuning of the controllers, which is required and not done properly for classic PID controllers. In addition, energy use can be reduced through advanced knowledge of weather conditions offered by this kind of control algorithms. Further, thermostats that learn from building occupancy patterns can optimize delivery of heating and cooling.

How much energy can be saved?
Claims of energy savings resulting from Adaptive or Fuzzy Control vary widely. Literature reports energy savings in a number of applications, but the range of savings potential associated with advanced controls is not very well understood in general. A rough preliminary estimate of energy saving potential is around 5% [1].

Novel thermal energy storage

Technology outline:
This kind of thermal energy storage system has the same principle as the traditional thermal energy storage system (ice-based type). Instead of ice, this system employs other kinds of phase change materials (PCM) with transition temperature approximately equal to the chilled water generation temperature as the energy storage medium.

How it can save energy:
In addition to the benefits of the ice-based TES system, the PCM-based TES system provides a more energy efficient option because the phase change materials have a temperature closer to the chilled water temperature (around 8.3oC) than water (0oC). This results in smaller chiller lift, reducing the energy required to create cooling.

How much energy can be saved?
Owing to the temperature lift of the PCM-based cycle (i.e. the water changes phase at 0oC versus 8.3oC for the PCM), the PCM-based TES consumes about 30% less energy than the ice-based TES. However, TES may experience 1-5% thermal loss from tanks per day [1].

Oil-less Centrifugal Compressor

Technology outline:
The oil-less centrifugal compressor mainly comprises the followings:

(a) Variable speed drive (VSD)-controlled magnetic bearing compressors
(b) Control program to control the chiller operation including load sharing among   compressors
(c) Electronic expansion valve

Instead of using conventional oil-lubricated bearings, the newly-developed oil-less compressor uses magnetic bearings, which eliminates high friction losses, mechanical wear and high-maintenance oil management system. At the same time, the energy efficiency of the compressor is optimized by the VSD. The diagram below shows the structure of an oil-less centrifugal compressor.

An oil-less centrifugal compressor (Source: Danfoss, Turbocor). The text above describes the image.
An oil-less centrifugal compressor (Source: Danfoss, Turbocor)

How it can save energy:
The oil-free feature of this compressor provides high-efficient operation because the frictional loss of the traditional bearing is eliminated. The VSD also enhances the efficiency of the compressor. The operating speed range of the VSD is from 18,000 to 48,000 RPM.
The oil-less centrifugal compressor uses magnetic bearings and variable speed drive to achieve better energy efficiency.

Solid State Refrigeration

Technology outline:
Solid State Refrigeration uses thin, efficient, and small thermoelectric cooling devices for the cooling purpose. Thermoelectric cooling moves high-energy electrons (hotter electrons) across a junction between two semi-conductors using electric current. Solid state refrigeration uses nanotechnology to move the hot electrons to one side across a gap and keep the cold electrons at the other side of the gap. This prevents heat migrates back to the heat source and results in cooling coefficient of performance (COP) that is twice of conventional mechanical cooling system. The diagram below shows the principle of this technology. [2]

Working principle of solid state. The text above describes the image.
Working principle of solid state

How much energy can be saved?
According to the manufacturer of this technology, laboratory results show efficiencies of 50-55% of the theoretical maximum Carnot efficiency, but the manufacturer claimed that efficiency will ultimately rise to 70-80%, approximately 50% better than conventional refrigeration devices now in use. [2]


  1. Roth, K. W., et al, 2002, Energy consumption characteristics of commercial building HVAC systems volume III: Energy saving potential, Building Technologies Program, Department of Energy, United States.
  2. Sachs, H., et al., 2004, Emerging energy-saving technologies and practices for the buildings sector as of 2004, American Council for an Energy-Efficient Economy, United States.
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