TECHNICAL FEATURE

This article was published in ASHRAE Journal, August 2017. Copyright 2017 ASHRAE. Posted at www.ashrae.org. This article may not be copied and/or distributed
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INTERIOR VIEW OF THE RENOVATED GIMO ICE RINK.

Cooling and Heating

Ice Rinks With CO2
BY JÖRGEN ROGSTAM, MEMBER ASHRAE; SIMON BOLTEAU; CAJUS GRÖNQVIST

Ice rinks use a considerable amount of energy, and Sweden boasts more than
350 indoor rinks for ice hockey alone. An average Swedish ice rink uses about
1 million kWh of electricity and heat combined each year,1 about 40% of which is from
the refrigeration system. To reduce energy use, one municipality replaced its ice rink’s
old indirect refrigeration system with a direct 100% CO2 system that is combined with
a heat pump function. This article reviews the technology and how it reduced the ice
rink’s energy use by 50% to 60%.
Updated F-Gas Regulation Requires New Solutions Using CO2 Systems in Ice Rinks
One reason to consider CO2 as a refrigerant is updates Ammonia-based refrigeration systems that meet the
to the European Union (EU) F-gas Regulation in 2015. GWP requirements are well documented. Lately, how-
To further control emissions from fluorinated green- ever, attention has also been directed toward the appli-
house gases (F-gases), the EU updated the regula- cation of natural refrigerant CO2 (R-744). As discussed
tion in which refrigerants with high global warming by Rogstam,2 CO2-based technology is potentially well
potential (GWP) are to be gradually phased out and suited for ice rinks due to the combined refrigeration
replaced by substances that fulfill the environmental and heating demands of these facilities. The greatest
requirements. A group of refrigerants highly affected source for lower energy consumption lies in the use of
by the F-gas Regulation are synthetic hydrofluorocar- an optimized heat recovery system. CO2 has very good
bons (HFCs), which have been very popular over the properties in terms of heat recovery.
last decades. These refrigerants also have been used Figure 1 shows the share of used available heat on the
to some extent in ice rink refrigeration systems. This x-axis at corresponding temperatures on the y-axis. The
means many existing facilities will be facing renova- comparison is made at an ammonia (NH3) condensing
tions in the near future. New ice rinks should naturally temperature of 35°C (95°F) and a CO2 head pressure of 80
apply the most energy-efficient technology available. bar (1160 psi). The temperatures of each refrigerant are

Jörgen Rogstam is managing director, Simon Bolteau is project engineer, and Cajus Grönqvist is project engineer at EKA (Energi & Kylanalys) in Stockholm, Sweden.

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 TECHNICAL FEATURE

at the compressor discharge level when starting from the FIGURE 1  

Comparison of the heat available between CO2 and NH3.
left side, while getting cooled in the condenser/gas cooler
160 Compressor
as we move to the right. For example, when the refriger- Discharge
140
ant temperature is cooled to 35°C (95°F), only 19% of the

Refrigerant Temperature (°C)

120
available heat has been extracted in the ammonia (NH3)
case, whereas the corresponding figure for CO2 is close to 100 CO2 NH3
60%, which implies that more heat is available at a higher 80
temperature with CO2. This advantage allows the CO2 60
heat recovery system to cover all heating demands in a 40
35°C
“normal” ice rink, reducing the cost of operation consid- 20
erably, which coupled with lower service costs will yield 0
0 10 20 30 40 50 60 70 80 90 100
long-term benefits for the owner.
Portion of Used Available Heat (%)
CO2 was previously used solely as the secondary refrig-
erant in ice rinks, but CO2 today is also applied as the
primary refrigerant. Systems using CO2 as the primary refrigeration system (two 18.5 kW [two × 24.8 hp] pumps
refrigerant in this article are referred to as second- and calcium chloride, 25%) based on ammonia (two
generation ice rink CO2 systems. These systems can be reciprocating compressors with a total motor power
applied in direct or indirect system solutions. "Direct" of 165 kW [221 hp]) should be replaced with a modern,
refers to if the CO2 is used in the refrigeration loop as energy-efficient solution. A holistic approach was taken,
well as circulated in the rink floor. In "indirect" sys- resulting in the choice of a direct CO2 refrigeration sys-
tems the refrigerant is only used in the machine room, tem with full heat recovery. Its CO2 charge is 2000 kg
together with a secondary refrigerant such as calcium (4,400 lb) including distribution via copper tubes in the
chloride, glycol, or today (more commonly) ammonium rink floor. The system has the latest CO2-based technol-
hydroxide in Europe. ogy, in which the ice rink refrigeration is combined with
Until 2014 second-generation systems have been found a heat pump function (Figure 2).
especially in the Quebec province in Canada, where Furthermore, the system stores excess heat in geother-
the results have been very good. Currently, there are mal storage to be used when the ambient temperature
around 30 CO2-based refrigeration systems used in gets low or during the off-season. The ice rink used to
North American ice rinks, of which most are found in depend on district heat and electricity for heat, but is
Canada and just a few in the U.S. Since 2014 the interest now self-sufficient with the heat recovered from the
has grown in Europe as well, and today seven indoor ice refrigeration system; the heat recovery system is the sole
rinks use CO2. heating source, with no backup needed.
This ice rink refrigeration technology has evolved The ice rink became the first of its kind in Europe,
from, and is basically the same as, the one used in gro- where the good results together with its growing reputa-
cery stores. CO2 entered the market in the early 2000s, tion have sparked the concept “the ice rink of the future.”
and its application has grown exponentially over recent
years. This has positive effects on the ice rink market Lower Life-Cycle Cost With CO2
both from the technical and the financial perspective, as An important factor in the choice of the CO2-based
it guarantees the good availability of refrigeration pack- technology was the decision to conduct the procurement
aged systems manufacturers, components, and service process with a focus on life-cycle costs. The higher initial
companies. cost was still estimated to lead to a significant reduction
in operating cost. The energy consumption was esti-
Heat Recovery System Makes Ice Rink Self-Sufficient mated to be cut by half, i.e., the ice rink would annually
The 3500 m2 (37,674 ft2) Gimo ice rink in Östhammar, consume about 500,000 kWh less energy (electricity and
Sweden, which can hold 805 spectators, was renovated heat) than before. Lower energy consumption, lower
in 2014 after the roof partially collapsed during the win- service cost, and an optimized heat recovery function led
ter of 2013. It was decided that the existing old indirect to a calculated life-cycle cost for the CO2-based system

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TECHNICAL FEATURE

solution that was considerably FIGURE 2  

Overview of the refrigeration and heat recovery system in the Gimo 100% CO2 ice rink.
lower than modern competing
alternatives. The municipality Hot Water
of Östhammar could, therefore, Dehumidification 50°C to 70°C

only see benefits in choosing a

Resurfacing
CO2 system. Radiators 30°C to 50°C
Air Handling
Refrigeration System Freeze Protection
The refrigeration system is a Preheating Water 10°C to 30°C
Cold Snow Melt Pit
direct system, in which carbon Water In
dioxide functions both as the pri-
CO2 Vessel
mary and secondary refrigerant. Geothermal
CO2 has very good properties in Storage

ILLUSTRATION BY EKA
terms of cooling distribution, and
Ice Rink
its ability to recover heat is much
more effective than what is usu-
ally seen in today’s ice rinks. The
heat recovery system has been designed to embrace the PHOTO 1 The CO2 pack providing 250 kW (71 tons) cooling capacity and the
properties of CO2, maximizing the amount of heat that 2.5 m3 (88.3 ft3) CO2 receiver.
can be recovered from the refrigeration system.
The heart of the ice rink’s energy system is the tran-
scritical CO2 refrigeration unit (Photo 1), with a designed
cooling capacity of 250 kW (71 tons). Due to the unique
properties of CO2, this unit covers both the refrigeration
and the heating function of the facility. The technology

PHOTO BY EKA
is well-proven, as it is the same type of refrigeration
unit used in numerous grocery stores across Europe and
North America. warm and cold weather. When it’s warm, the storage can
The CO2 refrigeration system loop may be described be used to improve the subcooling of the refrigeration
starting with the compressors marked with (A) in Figure process, resulting in higher energy efficiency of the sys-
3. These evacuate the evaporated refrigerant from the tem, by storing excess heat in the boreholes. The same
accumulator tank and circulate it in the primary loop. heat can later be recovered during colder periods when
The working pressure on the high-pressure side is typi- there is an increased demand for heat.
cally above the critical pressure of CO2, making the sys-
tem transcritical. After the compressors, the refrigerant Distribution System Advantages
passes through a heat recovery heat exchanger (B). Using carbon dioxide as the secondary refrigerant
The refrigerant is then cooled further using a gas in the distribution system has several advantages over
cooler (C) and/or a subcooler connected to a geothermal traditional solutions, e.g., calcium chloride and glycol.
heat storage (D). After expanding the refrigerant back Perhaps the most prominent ones are that CO2 uses
into the accumulator tank (E), it can be passed through phase change (evaporation) and has a low viscosity,
the ice rink tube system, which is the evaporator (F). which leads to a considerably lower pumping power.
There is an additional evaporator (G) connected to the Measurements done in traditional ice rinks indicate that
geothermal storage as well, where additional heat can be about 20% to 25% of the refrigeration system's energy
provided to the system when the heat demand is high. consumption stems from the auxiliary equipment, such
As mentioned above, the combined refrigeration and as fans and pumps.
heat pump system is also connected to a geothermal By applying CO2 as the secondary refrigerant, pump-
storage, which provides further possibilities both during ing power can be reduced to about 1 kW (1.3 hp)

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TECHNICAL FEATURE

compared with traditional flu- FIGURE 3 Overview of the refrigeration system in the Gimo ice rink.
ids, which require 5 kW to 15
kW (6.7 hp to 20.1 hp). Thus, the (D) Subcooler
(C) Gas Cooler
energy consumption of the aux-
iliary equipment in the Gimo
ice rink’s refrigeration system
becomes less than 3% of the
grand total. (B) Heat Recovery
The pipes in the ice rink floor Heat Exchanger
(E) CO2 Accumulator
must be adapted to the proper- OS1
(F) Ice
Rink Tube
ties of CO2, as the substance has System
a much higher working pressure
than other secondary refriger- C1 C2 C3 C4
ants typically used. Welded steel (A) Compressors

pipes used to be the norm, but

(G) Additional Evaporator
are now often substituted with

COURTESY IWMAC
more convenient copper tubes LS1
designed for ice pads. However,
in renovations where the rink
FIGURE 4  
Heat recovery system.
floor is left untouched, very
good results can also be achieved
(A) Heat Recovery Heat Exchanger
by installing an indirect CO2- (B) High Temperature
system that uses, for instance, Accumulator Tanks
ammonium hydroxide as the
secondary refrigerant in the
existing plastic pipes.
(H) Freeze (C) Dehumidifier Circuit
Protection
Optimized Heat Recovery System Circuit
The design of the new heat (D) Radiators Circuit
recovery system separates the
Gimo ice rink from traditional
facilities by following what is (E) Resurfacing Water Heating
referred to as the “waterfall con- Accumulator
cept.” The idea behind the con-
cept is to extract heat to various Cold Water In
COURTESY IWMAC

(F) Ventilation Units’

heating systems in temperature Circuit
(G) Preheating Accumulators
steps, where the heating system
with the highest temperature
demand comes first and others with lower temperature throughout the facility for hot tap water and showers.
demands follow. This concept should provide the lowest The heat required for the dehumidifier is supplied
possible return temperature, which is beneficial from a using a heat-exchanger marked as (C), and the primary
heat recovery point of view. circuit water is then circulated as radiator water (D).
The heating system connects to the refrigeration In the following section (E), the resurfacing water is
system through a heat exchanger marked as (A) in heated. The ventilation units, which normally require
Figure 4. The primary circuit water first passes through slightly lower temperatures than the upstream func-
high temperature accumulator tanks (B) where hot tions, are supplied from system (F). Further, the pre-
water is heated to about 60°C (140°F) and distributed heating of cold water takes place in the accumulators

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TECHNICAL FEATURE

FIGURE 5  
Monthly average heat distribution for the 2015 – 2016 season.
JUL AUG SEP OCT NOV DEC JAN FEB MAR TOTAL TOTAL
2015 2015 2015 2015 2015 2015 2016 2016 2016 (MWH) (%)
Heat TOTAL HEAT AVAILABLE 67.4 140.5 142.7 121.4 117.6 111.3 118.8 115.6 59.0 994.3 100%
Heat Heat Rejected Via
Recovered Rejected Gas Cooler
(MWh): (MWh): 357.0, HEAT RECOVERED 24.0 65.7 79.1 83.3 83.2 81.5 91.8 84.2 44.6 637.3 64%
(MWh):
637.3, 64.1% 35.6% 344.3,

MWh
HEAT REJECTED VIA
34.6% GAS COOLER
43.3 74.7 63.5 37.6 33.5 29.0 24.0 27.7 11.0 344.3 35%
Heat Rejected in
Geothermal Storage HEAT REJECTED IN
GEOTHERMAL STORAGE
0.1 0.2 0.2 0.5 0.9 0.8 3.1 3.7 3.4 12.8 1%
(MWh): 12.8, 1.3%

(G) followed by the final step in which the fluid used FIGURE 6 COP of heat recovery vs. ambient temperature.
for the subfloor freeze protection is heated by a heat-
exchanger marked as (H). Finally, the water in the 4.00
primary loop is returned to the initial position (A) at 3.75

COP of Heat Recovery

about 25°C (77°F). 3.50
The temperature profile is, therefore, tailored to the 3.25
demands of the facility as well as to the properties of 3.00
CO2. The waterfall concept has the inherent advantage 2.75 COP of Heat Recovery Linear (COP of Heat Recovery)
that it cools the primary loop to the lowest possible 2.50
–12 –10 –8 –6 –4 –2 0 2 4 6
return temperature, resulting in an optimized heat
°C
recovery system for the ice rink.

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 TECHNICAL FEATURE

The optimized heat recovery system covers the Results on the Electricity Bill
entire heating demand of the ice rink, even during After the two first seasons, the results look very prom-
colder periods, without applying the geothermal ising. Before the ice rink was upgraded, the energy
storage, eliminating the need for any external heat consumption during an eight-month season was about
source. In fact, the system has proven so efficient that 900,000 kWh (electricity and heat), which corresponds
it will be used to export heat to a nearby swimming to about 3,700 kWh/24 hours. After the retrofit, the
pool facility. seasonal energy use has been less than 428,000 kWh,
which corresponds to 1,750 kWh/24 hours. This is a 50%
The Control System to 60% reduction, indicating that about 470,000 kWh
In terms of energy system interaction, not only is the energy is saved for an eight-month season. At the same
physical integration important but also the control time, by analyzing the refrigeration system during the
strategy and its implementation. The Gimo ice rink uses season, the amount of available heat per month can be
a programmable logic controller (PLC) control system, calculated.3
which functions as the brain of the facility. In addition to In Figure 5 (Page 54) the available heat has been com-
refrigeration, the PLC system also controls the functions piled with the recovered and rejected heat. The heat
of the heating, domestic hot water, dehumidification, recovery system has recovered 637 MWh (2174 × 106
geothermal storage, freeze protection, ventilation, and Btu) heat, which covers the total heating demand of
lighting systems. It is, therefore, possible to synchronize the ice rink. This amount corresponds to about 70%
and/or prioritize the energy systems when necessary, of the total heat available, so there is a fair share that
which basically is a prerequisite for the optimized heat also can be exported. Due to a pump control failure,
recovery process. the geothermal storage was not used much during the

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TECHNICAL FEATURE

second season, which explains the low percentage. or heat pump for that matter that would provide heat to
The available heat is relatively constant, an average of ice rinks.
110 MWh (375 × 106 Btu) per month throughout the year.
The available heat is mainly dependent on the load in Future Vision
the ice rink in the sense that a higher cooling capacity The rapid development of CO2-based refrigeration
results in a higher amount of heat available on the warm systems in supermarkets has allowed for an acceler-
side of the system. ated reduction of component and system costs. Today,
the investment cost for CO2-based technology is highly
Performance of Heat Recovery competitive when compared to traditional alternatives.
Tazi3 analyzed the heat recovery function through the There is also a considerable expansion of CO2-trained
COP of heat recovery, COPHR, which is defined as the service companies, which ensures the availability of
ratio between the heat recovered, Q HR, and the addi- skilled technicians offering the required service at a
tional compressor power used to provide that heat, competitive price.
where the latter is the difference between the power So far, the CO2-based systems have been very well
used by the refrigeration system in heat recovery mode, received in ice rinks, with Gimo serving as the prime
Ecomp, and floating condensing mode, EFC. example. The most relevant aspects regarding this spe-
COPHR =
QHR cific ice rink are evaluated and documented by Rogstam
Ecomp − EFC and Bolteau4 in an English-language public report
Figure 6 shows the COPHR for the heat recovery system financed by the Swedish Energy Agency. The results have
in the Gimo ice rink. This calculated value, between 3.5 propelled the interest to invest in CO2 technology, both
and 4, can be compared with any other heating system in new construction and as a replacement for obsolete
systems. At the end of 2016, Sweden had six CO2 second-
generation systems in operation, and the forecast indi-
cates another five to seven systems will be in operation
by the end of 2017.
The opportunity to export heat from the simple, yet
highly effective, heat recovery systems offered by CO2
technology has made facility owners connect the ice
rinks with nearby indoor swimming pools or other
sport-related facilities, maximizing the benefits even
further.
Advertisement formerly in this space. In conclusion, we might be witnessing a shift in how
ice rinks are cooled and heated. The results already
indicate that this most likely is the way forward. Systems
based on a natural refrigerant with such competitive
advantages are simply the way of the future.

References
1. Rogstam, J., C. Beaini, J. Hjert. 2014. “Stoppsladd fas 2—
Energianvändning I Svenska Ishallar” (in Swedish). EKA and
the Swedish Ice Hockey Association. http://ekanalys.se/assets/
stoppsladd_slutrapport.pdf.
2. Rogstam, J. 2016. “CO2 refrigeration systems evolution for ice
rinks.” ASHRAE Journal 58(10):34 – 48.
3. Bolteau, S., J. Rogstam, M. Tazi. 2016. “Evaluation of heat
recovery performance in a CO2 ice rink.” 12th IIR Gustav Lorentzen
Natural Working Fluids Conference.
4. Rogstam, J., S. Bolteau. 2015. “Ice Rink of the Future.” http://
tinyurl.com/yajgdb3n.

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