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Go with the flow

The benefits of ejectors in the refrigeration vapour compression cycle aren’t just applicable to CO2 systems, says Daniel Clark

Ejectors are being publicised as one of the initiatives that will improve the efficiency of transcritical CO2 systems operating in high ambient temperatures, but what are they? How do they work? And is there mainstream potential for the technology?

Put plainly, the objective of ejector technology is to capture and make use of the huge amount of kinetic energy, which is usually wasted through the liquid expansion process; therefore, recovering energy from the refrigerant that was induced at the compressor.

The vapour compression ejector concept isn’t a new idea. In fact, it’s received a substantial amount of academic and commercial research over the past 80 years.

To explain the ejector concept in layman’s terms, high pressure condensed (or cooled transcritical CO2 vapour) refrigerant is expanded though a nozzle; this accelerated flow acts to draw the flow of suction gas through a second nozzle.

The subsequent complexed flow then enters into a diffuser, where it is decelerated – a process that actually raises the pressure (boost) of the mixture above the pre-nozzle suction pressure.

The flow is then finally passed into a liquid separator. As the suction vapour pressure is now higher than it was, the required compression ratio is effectively reduced and consequently so too is the compressor energy consumption.

The thermodynamic and fluid mechanical processes are extremely complex within ejector technology – processes that are still little understood, even at an academic level in physics.

The principle, though, has significant theoretical potential.

Research papers suggest that in theory up to a 20 per cent increase in the coefficient of performance (COP) could be achieved. However, such is the complexity of the technology,

only a small proportion of this figure has ever been achieved in practice. In this case, every little counts.

It is very commendable that ejector devices are being skilfully applied in commercial CO2 refrigeration systems and I am sure they offer the published modest energy savings and benefits, but it is still early days in the application of this technology within commercial refrigeration.

There is no doubt plenty of scope for important continued development and investment in the ejector concept. For the wider industry, the area of development with the most potential is the variable nozzle ejector; these have been historically used in the Japanese CO2 heat pump market.

Variable nozzles are required because, in order to optimise ejector performance, the nozzles have to be engineered for specific operating conditions such as the mass flow rate and system pressures.

Obviously, in the real world such operating conditions are constantly changing – hence the need for variable nozzle ejectors, which can be adjusted dynamically to suit given operating conditions.

To put a perspective on this subject, ejectors are, in theory, a good concept all round and their benefits don’t just lie in transcritical CO2 systems.

Ejector technology could potentially be applied to any refrigerant. Natural refrigerants such as ammonia and Hydrocarbons would benefit, as could synthetic refrigerants.

However, CO2 transcritical operation raises the stakes because the system pressures are elevated so high that the amount of kinetic energy for potential recovery is even more substantial.

Ejector technology within the commercial CO2 refrigeration sector does not directly address the fundamental inefficiencies of high ambient transcritical operation, but it can be employed to make efficiencies elsewhere in the system.

As too can the parallel compression concept, which has a similar outcome of ultimately lifting the average suction pressure.

Technology application

Parallel compression is currently employed on both conventional DX (dry expansion) systems, or alongside the ejector system on a flooded evaporator circuit, however, I don’t think it’ll be too long before we see ejectors on DX systems as well, supporting or even replacing the parallel compressor.

Ejector technology is currently applied within flooded systems, but this may be a technology step change too far for the mainstream commercial and supermarket sectors, certainly in the short to medium term. I feel the ejector flooded system just adds layers of further complexity to the transcritical system, which is a system that already requires major, but manageable, training and skills reform.

Although both parallel compression and ejector suction gas technologies are technically detached from the condensing inefficiency issues of high ambient transcritical operation, they are certainly worth consideration. However, they do add cost.

The number of system choices available for CO2 refrigerant suited to medium and high ambient temperature conditions is growing quickly. However, the assessment criteria for deciding on such significant investments are highly complex.

Balancing the lifecycle running costs, capital cost and operational considerations such as reliability and serviceability, are all mixed in with the ambient temperature profile of the geographic location. It certainly isn’t a straightforward or a ‘one size fits all’ decision.

One thing is for certain though, recovering kinetic energy from high pressure refrigerant should remain an ultimate goal in the further development of the vapour compression cycle.

Daniel Clark is director of refrigeration consultancy Hamilton-Clark

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