Transcritical CO2 Heat Reclaim

Transcritical CO2 Heat Reclaim - Important Considerations.

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Transcritical CO2 Heat Reclaim – Worth while or not?

When I speak to other people in the refrigeration industry about schemes with transcritical Co2 Heat Reclaim, I am often asked “Is it worth while ?”

The simple answer is – “absolutely, yes it is”.

However, there are two major provisos; they concern the reclaim water temperatures and keeping them as low as possible, whilst also making the most of the external ambient.

Importantly though, its only the extra energy used to go transcritical which should be used to calculate the efficiency of your Heat Reclaim system, because your refrigeration has a base running cost which your business has anyway.

Transcritical CO2 Heat Reclaim

Those who have been at certain events will know I talk about the importance of educating heating engineers to designing heating systems to use lower water temperatures when recovering heat. Heating Engineers most often use high water temperatures when designing their systems, this is understandable as burning natural gas can deliver high grade water temperatures and heat exchangers can consequently be sized much smaller. Conversely, for refrigeration heat reclaim to deliver a highly beneficial and efficient space heating output, water temperatures need to be kept substantially lower. Lower water temperatures are able to extract the maximum amount of heat from the CO2 refrigerant, prior to final cooling in the gas cooler, making the reclaim process more efficient. This does impact on the size of the heat rejection exchangers, however the lifetime benefits can be significant.

To provide a situation where heat can transfer effectively to the reclaim water, the CO2 plant usually has to operate transcritical, as the temperature of the CO2 is much higher. Operating a CO2 system in transcritical mode inorder to reclaim heat, does increase power consumption of the refrigeration plant. Importantly though, its only the extra energy used to go transcritical which should be used to calculate the efficiency of your Co2 Heat Reclaim system, because your refrigeration has a base running cost which your business has anyway.

Heat Recovery should be considered as ‘recovering heat picked up in the refrigeration cycle’ and delivering it at a higher grade with optimum efficiency; technically this is the same concept as Heat Pumps employ.

The eagle eyed among you though may be thinking that running transcritical is inefficient, however this is most exaggerated in high external ambients. Therefore, it is imperative that during the heating season – usually when external ambients are low that the transcritical gas goes on to leave the gas cooler only one or two degrees kelvin higher than the ambient temperature. This practice ensures that the generation of flash gas is either removed totally or reduced substantially, therefore mitigating the major inefficiency of transcritical operation.

When extra heat or higher grade heat is required from transcritical plant, I advise against bypassing the gas cooler or turning the gas cooler fans off, as some controls manufacturers enable. This practice further increases the running costs as it increases flash gas creation and therefore mass flow rate through the compressors. Any additional output is gained by almost nothing short of electrical heat energy delivered at the same efficiency as an electrical heater will provide, plus it stresses your plant. Heat Recovery should be considered as ‘recovering heat picked up in the refrigeration cycle’ and delivering it at a higher grade with optimum efficiency; technically this is the same concept as Heat Pumps employ. The practice of failing to exploit the low ambient temperatures increases the mass flow above the optimum point and the ‘Heat Pump’ effectiveness is compromised, therefore any extra heat is not recovered heat, it is developed heat!

there are two major provisos; they concern the reclaim water temperatures and keeping them as low as possible, whilst also making the most of the external ambient.

Transcritical CO2 Heat Reclaim is definitely worth while, however the application has to be well designed and integrated by engineers who fully appreciate both the issues effecting the thermodynamics and the practical building services requirements.

CO2 Heat Reclaim can’t be covered in one blog post, so keep an eye out for more on the same theme.

 

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