Seawater used for district cooling in Stockholm - GNS Science

Seawater used for district
cooling in Stockholm
By Lisa Lind (GNS Science), Stefan Mroczek (GNS Science) and Jo Bell.
Key Benefits:
case
study 19
View over Stockholm city. Photo: Micke Lundström.
Central Stockholm takes advantage of a natural
cold seawater resource to provide cooling to
hundreds of customers.
Stockholm boasts one of the
largest district heating and
cooling systems in the world. By
utilising cold seawater from the
Baltic Sea, central Stockholm is
able to fulfil most of their cooling
needs year round.
The seawater based district
cooling scheme was established
in 1995 to initially supply central
Stockholm. Today, the cooling
system serves over 600 buildings
including offices, hospitals and
universities.
New restrictions in Sweden
placed on CFCs, atmospherically
destructive compounds used in
conventional chillers, were the
key driver in getting the scheme
underway.
Fortum (Stockholm Energi at that
time) invested in the new system.
“We studied a number of different
technologies for producing
cooling in the initial stages of the
project,” says Anders Hill, District
Cooling Strategist, Fortum, Heat
Scandinavia.
“A seawater cooling plant was
also considered in the city, where
the temperature conditions are
the same as in värtan - our final
choice of location.”
“But with the cooling plant in the
city we would have been forced
to build a completely new cooling
plant to satisfy demands when
the water temperature was
too high. The system we finally
selected uses existing heat pumps
that required no investment and
with running costs substantially
covered by the production of heat.”
• Significant decrease in
electrical energy useage
• No significant impact
on the ocean
• Atmospherically
destructive compounds
(CFCs) are not used
Key Features:
• Cools over 600
buildings in central
Stockholm
• 204 km distribution
network
• 220 MW capacity which
results in 350 GWh
per annum output

Pipes are placed into the water body. Photo: Eddie Granlund Photo: Micke Lundström
Pipe tunnel for the
district cooling system.
The cooling system in Stockholm
differs from other lake/sea sourced
cooling systems as it includes heat
pumps that are used to cool the
seawater further if required, which
usually occurs in autumn.
In Stockholm it is possible to
source water which is cool most
of the year (minus the period in
autumn) from very shallow depths.
“The fresh water has lower
density than the salt water, so
the fresh water ‘floats’ on top of
the salt water on its way through
the archipelago and out into the
Baltic sea. The counter current
carries cold sea-bed water to our
district cooling system.”
“The water reaching the inlet of
the cooling plant in July left the
surface of the archipelago in April
where the water temperature was
extremely low due to ice melts at
that time.”
Two sea water inlets feed the
cooling plant, located 4 km from
the city; one on the sea bed at
20 m depth and one at the surface.
Anders says that this is very
shallow, especially when compared
to deep water systems which can
require over 1000 m depth to reach
sufficiently cold water.
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After passing six titanium heat
exchangers the waste water is
either returned to the heat pumps
or is released back into the sea.
The cool water is pumped 4 km to
central Stockholm then through
the city’s distribution network.
“A significant benefit was the
ability to make use of the mainly
existing culvert system for
distribution within the city.”
With 220 MW of installed capacity
and 350 GWh per annum output
(2011), the cooling system
provides a significant saving in
electrical energy.
“Stockholm’s situation is unique.
Sufficiently cold water is available
at very shallow depths almost all
year round. In more mild climates
greater depths for inlets of about
750 m to 1000 m would have to be
achieved, increasing capital costs.
“But the use of seawater as a
source of cooling in more mild
areas is still a promising option.
As examples of this, there is one
system designed in Hawaii and yet
another one in Bahamas that is
under consideration right now. ”
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114 Karetoto Road,
Wairakei 3377
Private Bag 2000,
Taupo 3352
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Phone: +64 7 374 8211
Email: earthenergy@gns.cri.nz
“A SIGNIFICANT
BENEFIT WAS THE
ABILITy TO MAKE
USE OF THE MAINLy
ExISTING CULvERT
SySTEM FOR
DISTRIBUTION WITHIN
THE CITy.”
Anders Hill
District Cooling Strategist
Fortum Power and Heat AB
Phone: +46 (0)8 671 7000
Email: kundservice.varme@fortum.com
Website: www.fortum.com