ASHRAE Journal - August 2013 - 70

technical feature

Assuming a cost of electricity of $0.10 per kWh for the
system in question, the difference in energy efficiency
of 10% is equal to annual
savings of approximately
$10,000. With the energy
prices constantly rising,
the lifetime energy savings,
when opting for the most
energy-efficient configuration, could be in excess of
$150,000.

Figure 2 Total Equivalent Warming Impact of benchmark and alternative systems (New York climate, β=0,676 kg CO2/kWh).
15.083
13.621

Indirect
Recycling

6.743
Direct

4.389

Alternative
System

Reference
System

(1) 
(1)
R-404A/CO2 R-134a/CO2
Cascade
Cascade

MT

R-404A DX

R-404A DX
CO2 DX

TEWI Analysis

TEWI calculations were
LT
R-404A DX
based on average charges
for the analyzed systems.
Assumed leak rates are:
• 15% for R-404A/R-134a distributed systems, which is a conservative estimate for
North American supermarkets.
• 25% for CO2. This figure is rather high
and is mainly attributed to the fact that currently there are no requirements to reclaim
CO2 during service.
• 5% for chillers.
The general relationship between TEWI
for different systems are only representative; actual system charges, leakage rate and
carbon emissions factor (β) will differ from
site to site. However, keeping these factors
constant, the same general pattern in the
New York climate, shown in Figure 2, is seen
in both Atlanta and Quebec as well. In areas
with hydroelectric power, the indirect component disappears.

ashrae.org

Au g ust 2013

(3)
Propane
Chiller

(3)
Ammonia
Chiller

2.661

(3)
R-134a
Chiller

(4)
Transcritical
CO2 Booster
CO2 DX
CO2 DX

Prop. Glycol Pumped CO2 Pumped CO2 Pumped CO2
CO2 DX
CO2 DX
CO2 DX
CO2 DX

COP
QuebeC
(COld)

COP
New yOrk
(mild)

COP
atlaNta
(warm)

meaSured
eFFiCieNCy

R-404A DX MT + R-404A DX LT

3.5

3.1

2.8

3,5 (Cold)5

(1) R-404A DX MT / CO2 DX LT

3.4

3.1

2.8

3,8 (Cold)5

(1) R-134a DX MT / CO2 DX LT

3.4

3.1

2.9

Not Available

(2) R-404A / P.Glycol MT / CO2 DX LT

2.8

2.4

2.1

2,8 (Cold)5

(3) R-290 / Pumped CO2 MT / CO2 DX LT

3.8

3.4

3.1

Not Available

(3) R-717 / Pumped CO2 MT/CO2 DX LT

3.8

3.4

3.1

Not Available

(3) R-134a / Pumped CO2 MT / CO2 DX LT

3.3

3.0

2.8

Not Available

(4) Transcritical CO2 DX MT / CO2 DX LT

4.1

3.3

2.8

5 (Cold)/
4(Moderate)3

SyStemS

Practical measurements can be both difficult to obtain
and compare due to the many external factors that can
affect efficiency. Climatic fluctuations, load fluctuations,
how well the system is tuned during commissioning
and how well the system is sized relative to the required
capacity are all real factors that make one-on-one comparisons difficult. However, a pool of data is beginning
ASHRAE JouRnAl

R-134a DX
CO2 DX

(2)
R-404A
Chiller

2.577

table 1 Comparison of the simulated and measured COP for different climatic regions.

Practical Confirmation

70

3.289

2.587

to form that can give a reasonable indication. In general,
the trends indicated by the simulation are confirmed by
the field studies (Table 1).
In a study of transcritical booster systems by Wozny,3
results indicated the annual average COP of the transcritical booster systems to be on the level of 4 to 5,
depending on the ambient conditions. In a similar study
from the discount chain Fakta, a population of 10 stores
with R-404A systems is compared to a population of 10
stores with transcritical CO2 booster systems, all with
ASHRAE Journal - August 2013

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