Efficiency in refrigerant circuits

Efficiency in refrigerant circuits

 

The efficiency of a refrigeration system depends only partly on the efficiency of the individual components.

This means that correct sizing and design of high-performance heat exchangers or efficient compressors are not enough on their own to achieve high overall system efficiency.

 

What on the other hand significantly helps increase efficiency is the combined operating mode of the components, which often does not simply refer to an individual refrigeration unit but rather the installation as a whole.

Consequently, one fundamental aspect is the control system, in other words the set of electromechanical and electronic components that acquire information from the refrigerant circuits via various types of sensors, process this using logic of varying complexity, often implemented by software, and then managing activation or operation of the various components.

 

For example, the control system on a split air-conditioner in the home uses an electronic board that measures room temperature using one or more probes, and, based on the data sent from the remote control, implements the logic programmed into the board's software to activate the fans at different speeds, the compressor and the louvers and thus reach the desired room temperature and maximum comfort. 

 

Rated and seasonal efficiency

 

Refrigeration units have historically been classified in terms of energy consumption based on rated efficiency, i.e. the ratio between cooling capacity delivered and power consumption in the worst-case operating conditions, typically calculated for operation in summer at full load.

In less demanding conditions efficiency decreases sensitively as the unit needs to switch on and off periodically; whenever it restarts it works for several minutes at low efficiency until reaching steady operation, and as it may need to restart 6-10 times an hour the consequence in terms of energy consumption is evident.

 

It's also easy to understand how operation at maximum load involves just a small fraction (around 1 to 5%) of typical total operating time. As a result, this classification gives little insight into actual efficiency.

 

In recent times, the drive to save energy around the world has led to more suitable classification methods being devised that categorise units based on the seasonal energy efficiency ratio, i.e. weighted to account for actual climatic conditions and typical load.

In these terms, traditional refrigeration units have a low seasonal energy efficiency ratio. This is leading to revolutions in design and the adoption of increasingly advanced technology.

 

The technologies that most significantly increase the seasonal energy efficiency ratio are undoubtedly electronic controllers, motor-driven expansion valves (also called electronic expansion valves) and capacity modulation systems for compressors, fans and pumps, with inverters clearly being the most efficient of these.

 

Electronic controllers use technology that acquires a complete set of information from the units or the system as a whole (temperature, pressure, power consumption, occupancy, etc.), then process such based on system rather than single-unit logic so as to ensure optimum control of all the components.

They thus save considerable amounts of energy.

To give some examples, automatic recognition of low load conditions (e.g. operation at night) means bottle coolers can save up to 30% energy by adjusting operating temperature or automatically switching off the lights. Serial communication between controllers on different units, such as multiplexed showcases, compressor racks and air-conditioning units in a supermarket, helps reduce peaks in consumption by preventing high power electrical loads from being activated simultaneously, and reduce energy consumption by producing only the "cooling" that's strictly required at any given time. 

 

Electronic expansion valves replace mechanical expansion devices (also see "REFRIGERANT CIRCUIT COMPONENTS"), improving control of refrigerant flow and consequently increasing unit efficiency by up to several percent.

Moreover, these also have statistical advantages again of several percent as the electronic system guarantees optimum calibration at all times, unlike mechanical devices.

Lastly, yet no less significantly in quantitative terms, is the contribution that this technology makes to optimising refrigeration units, guaranteeing average annual savings that may exceed 20%!

Mechanical expansion devices in fact restrict the unit to operate in the same pressure conditions - around rated summer values - at all times, as these devices cannot adapt to different conditions without the risk of damaging the compressor. Electronic valves do not have this problem as they're motor-driven and microprocessor-controlled and can consequently adapt refrigerant flow according to any conditions. They also have a range of movement that's 10-15 times greater and a modulation capacity that's 4-5 times higher than any mechanical device. These aspects let designers and installers have the units operate in the best possible conditions according to the season, reducing pressure in the condenser when the outside temperature allows and thus obtaining considerable savings in running costs.

 

The final component, yet only in terms of usage, is the inverter, a complex system of control hardware, power supply and software that adjusts the power supplied to a motor from the mains input in order to modulate operating speed.

As concerns refrigerant circuits, the main beneficiaries are compressors, as well as pumps and fans.

Focusing on compressors, inverters provide the best way to avoid inefficient on/off cycles that reduce seasonal efficiency. In part or low load conditions, an inverter-driven compressor slows down and decreases its cooling capacity without stopping completely. As a result, units with inverter-driven compressors have much higher efficiency in these conditions compared to rated efficiency. Indeed, as low load conditions are much more frequent than rated conditions, it's clear how a unit fitted with an inverter-driven compressor can exceed the average efficiency of a traditional unit by up to 60%!

Moreover, given that the most critical stage in the operation of a refrigerant circuit is when the compressor starts, using inverters significantly extends the life of components by minimising the number of starts.

Obviously, this technology is more complex and often more costly, both as regards construction of the compressors and the inverters themselves and the development of the refrigeration units.

Currently the most efficient technology for inverter-driven compressors is called BLDC (BrushLess Direct Current); this is used in top-end home air-conditioners and has recently been introduced in industrial applications.

 

It goes without saying that such advanced technology cannot be used without adopting electronic control systems that instantly calculate optimum compressor speed, and electronic expansion valves, the only expansion technology that can adapt to the variations generated by the compressor.