Danfoss Heat Reclaim in Transcritical CO2 Systems Application guide
Pressure (Logarithmic scale)
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.
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.
These unique features are utilised when reclaiming heat that
is normally rejected or wasted by an air cooled gas cooler.
3
Expansion
4
CP
Condensation
Evaporation
2
Compression
1
Pressure (Logarithmic scale)
Specic enthalpy
Difference between a subcritical and transcritical refrigeration process
3
CP
Expansion
4
Evaporation
Specic enthalpy
Gas cooling
2
Compression
1
Efficient heat reclaim with CO
25° C
55° C
5° C
-10° C
Qo
Wgc Whr
40 Bar
Wc
25° C
55° C
5° C
-10° C
Qo
Wgc Whr
40 Bar
Wc
2
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.
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).
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.
2
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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