With global energy prices soaring to record high levels and no end in sight, appliance manufacturers struggle to find innovative and effective ways to improve energy efficiency. Among the most energy-consuming and expensive of all appliances are compressor-based heating and cooling systems. These include living space heating, cooling and comfort control equipment, as well as refrigerators and freezers that are present in millions of households and businesses throughout the world. And that is why any efficiency improvements to compressor-based equipment can have a significant impact on both the end-users and society as a whole.

With that in mind, Freescale Semiconductor developed a control system that can be used in compressor-based home and commercial appliances to optimize refrigeration cycle efficiency. Specifically, the control system targets an important parameter called evaporator superheat. The control accurately monitors and controls this parameter to provide near-perfect evaporator heat absorption across a wide range of thermal load conditions. The result of this more precise control is significant utility cost savings, as well as prolonged equipment life.

A typical refrigeration cycle schematic is shown in Fig. 1. This cycle is utilized in one form or another in all common refrigeration, air conditioning and heat pump appliances. The cycle uses a suitable refrigerant compound that is formulated to change phase from liquid to gas and vice versa, at suitable temperatures and pressures for a particular application. A typical home air conditioning application serves as a good example to illustrate how the cycle works.

Enlarge this picture Fig. 1. Typical Refrigeration Cycle.

The cycle starts at the system compressor. The refrigerant at the compressor input is a cool, low-pressure gas. The compressor physically compresses the refrigerant into a hot, high-pressure gas, which then enters the condenser coil. Here the refrigerant changes phase from a hot, high-pressure gas to a hot, high-pressure liquid. The key point is that the refrigerant phase change that occurs in the condenser causes a very large transfer of heat from the condenser coil to its surroundings, in this case, the outdoor air.

Hot, high-pressure liquid refrigerant emerging from the condenser coil is then passed through some type of fixed nozzle or metering device, exiting as a cold, low-pressure liquid. Finally, this liquid is passed through the evaporator coil, where it changes phase into a cold, low-pressure gas. Again, a very large amount of heat is transferred, but here the heat is absorbed by the evaporator coil from its surroundings, in this case, the indoor air. The entire cycle is then repeated until the indoor air is sufficiently cool and comfortable. In short, heat is absorbed from the indoors and rejected to the outdoors via this special arrangement of coils, refrigerant pressures, and temperatures.

The fixed-orifice metering device is a major contributor to degraded system performance. In the vast majority of air-conditioning systems, the metering device is sized for nominal design conditions (a certain quantity of heat energy transferred per unit of time). Unfortunately, heat loads can vary significantly in real-world applications, and system performance degrades significantly as actual load conditions vary from nominal. Factors such as outdoor temperature excursions, open windows and doors, sunlight, and the operation of other home appliances all contribute to significant heat-load variations within the home.

Enlarge this picture Fig. 2. Evaporator Superheat Control System.

When the heat load is higher than nominal, the liquid refrigerant in the evaporator coil starts to boil sooner. That is, when under nominal load, the last bit of evaporation occurs at the coil outlet, but under high-load conditions, this last bit of evaporation occurs upstream from the coil outlet. This condition is equivalent to reducing the effective size of the evaporator coil, resulting in reduced heat absorption and lower efficiency. Moreover, in the presence of extreme heat loads the system compressor can overheat, which impacts its reliability.

Conversely, when the heat load is lower than nominal, the last bit of liquid refrigerant boils downstream of the evaporator coil outlet. Evaporating downstream of the evaporator coil absorbs heat from some source other than the conditioned space, reducing the air conditioner’s cooling efficiency. Worse yet, this phenomenon can result in liquid refrigerant reaching the compressor. This can cause permanent damage to the compressor and incur very high service costs to both the equipment manufacturer and homeowner.

The Freescale evaporator superheat control system eliminates these anomalies by monitoring and controlling an important refrigerant parameter called superheat. Superheat is measured at the evaporator outlet and represents the difference between the actual refrigerant temperature and its boiling point temperature at the measured pressure.

Enlarge this picture Fig. 3. Control System Logic Flowchart.

The control system is comprised of a Freescale high-performance industrial pressure sensor (currently under development), a Freescale MC9S08GB60 microcontroller, a temperature sensor, operating software, and an expansion valve. A power transistor may also be needed, depending on the type of valve used in the application. An optional ZigBee transceiver may also be included to provide a wireless link to another device, such as a utility meter or service technician’s computer.

A schematic of the control system is shown in Fig. 2. As the heat load increases from nominal, the evaporator superheat rises. The controller senses this condition and adjusts the expansion valve aperture to allow more refrigerant into the evaporator, thus causing the superheat to fall.

Conversely, as the heat load decreases from nominal, the evaporator superheat falls. The controller senses this condition and adjusts the expansion valve aperture to allow less refrigerant into the evaporator, causing the superheat to rise. The result is optimized evaporation within the evaporator coil and maximum heat absorption from the indoor air. A logic flowchart for the control system is shown in Fig. 3.

Recall that the above description is specific to home air conditioning. However, the same control methodology can be applied to refrigerators, freezers, heat pumps or any other home, commercial or industrial appliance that uses a compressor-based refrigeration cycle.

The Freescale evaporator superheat control system is 100 percent compatible with all commonly used refrigerants, including the environmentally friendly R-410A and R-134A types.