Application guide
Heat Reclaim in Transcritical
CO2 Systems
General description
When using CO2 as refrigerant the critical point is reached already at 31 °C (74 bar). At temperatures and pressures above this point there is no relationship between pressure and temperature (not tied together as seen in the sub critical refrigeration process) why temperature and pressure can be controlled individually. In this condition there are no phase changes and no liquid is present but the mixture is best described as a gas. Compressor discharge temperatures between 80 and 120 °C are typical reached and furthermore in the transcritical refrigeration condition it’s possible to reclaim most of the energy normally rejected by a gas cooler.
These unique features are utilised when reclaiming heat that is normally rejected or wasted by an air cooled gas cooler.
To be able to reclaim heat, one or two heat exchangers are mounted before the gas cooler. To ensure robust refrigeration process a superior control strategy is crucial to control pressure as well as temperature when reclaiming heat to avoid condensation of liquid in the heat exchangers and to avoid risk of liquid hammering. Also it’s important that the minimum temperature out of the gas cooler is at least 3 K higher than the receiver pressure (converted to temperature) to avoid condensation of gas in the receiver if cold liquid should be entering and thereby causing too low receiver pressure.
<![endif]>Pressure (Logarithmic scale)
3 |
CP |
2 |
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Condensation |
Expansion |
Compression |
4 |
Evaporation |
1 |
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<![endif]>Pressure (Logarithmic scale)
3 |
Gas cooling |
2 |
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CP |
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Expansion |
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Compression |
4 |
Evaporation |
1 |
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Speci c enthalpy |
Speci c enthalpy |
Difference between a subcritical and transcritical refrigeration process
Efficient heat reclaim with CO2
In theory all the heat rejected from a refrigeration process can be reclaimed. In a supermarket application this is typical all the heat removed from refrigerated display cases and cold rooms in addition to the electricity used by the compressors. Lets say the refrigeration capacity is 3 KW with a COP of 3 then the heat rejected would be 3 KW +1 KW compressor load equal 4 KW which as a rule of thumb could be reclaimed. Unfortunately the load of the refrigeration system is typical low in cold periods where there is a high load on the heating system, never the less it makes sense to reclaim heat especially for hot tap water where the consumption is normally constant throughout the year.
By using CO2 as refrigerant it’s attractive to invest in heat reclaim because of the high discharge temperatures (often 100 – 120 °C) and the high enthalpy compared to traditionally used HFC refrigerants meaning that a bigger part of the rejected heat (energy) can be reclaimed or more heat can be reclaimed at a higher efficiency compared to other traditional HFC refrigerants.
When using CO2 for MT refrigeration it’s possible to work with condensing temperatures down to 5 °C. As dimensioning criteria the minimum temperature out of the gas cooler is often set as low as 5 to 8 °C and minimum pressure in the gas cooler is typical set around 40 to 45 bar to ensure a receiver pressure to be kept at 35 to 40 bar to maintain sufficient pressure differential across the expansion valves in the display cases.
Heat reclaim without compromising COP
In practical the discharge temperature on the high temperature pack in a CO2 trans-critical booster system, is typically above 55 °C even at winter conditions. With above mentioned design criteria it’s possible to reclaim heat for hot tap water without increasing pressure making it possible to reclaim heat at highest possible refrigeration COP. The higher the ambient temperature is, the more heat can be reclaimed without compromising the refrigeration COP.
55° C
25° C
5° C
Wgc |
Whr |
40 Bar
-10° C
Qo |
Wc |
|
Qo: Refrigeration load
Wc: Compressor work
Whr: Heat that can be reclaimed (55° down to 25°C)
Wgc: Heat rejected by the gas cooler (25 down to 5°C)
Transcritical booster system running at winter condition (subcritical condition).
Increasing pressure in the gas cooler
If more heat is needed eg. for floor heating it’s possible to increase the pressure in the gas cooler which will increase the compressor work but as the COP is typical between 2 and 7 depending on running conditions, heat reclaim is often an attractive heat source compared to other heat sources even at winter time.
What can be seen is that by only adding a limited extra compressor work “Wchr” its suddenly possible to reclaim around 80 % of heat Whr2” (100 down to 25 °C), and only around 20 % is typically not possible to reclaim as the temperature of the gas is below 25 °C.
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