District Cooling in Stockholm Using Sea Water

Author: Göran Fermbäck
Planning manager for district heating and responsible for district cooling business development
STOCKHOLM ENERGI AB 115 77 Stockholm SWEDEN

Abstract
In May 1995 Stockholm Energi started supplying properties in central Stockholm with cooling for comfort and for various processes from its new district cooling system. The project is unique in that most of the cooling energy is produced using cold water from the Baltic Sea. The following article describes the system and provides a summary of the considerations that resulted in our venturing to invest in sea-water cooling for such a large project. There is also a description of the hydrological conditions that made the system feasible in Stockholm and some speculations about the possibilities to use cooled sea water elsewhere in the world.
Background
Stockholm Energi already supplied district heating, electricity and gas in Stockholm before entering the cooling branch. It has not been a straight road and on several occasions we have refused when customers have come to us asking about cooling deliveries. However, in summer 1993 the time was finally ripe and we resolved in principle to built a district cooling system for central Stockholm. A factor that contributed greatly to the project getting off the ground at that particular time was impending restrictions in the use of CFC which came into force in Sweden in 1995. Initial market studies indicated, not surprisingly, that the largest concentration of potential customers was located in the heart of Stockholm. It was clear from the start that the use of sea water was a possibility but the final plans were approved at a relatively late stage in the project and the final investment decision was made in July 1994.
How does it work?
The cooling production plant is located in the immediate vicinity of our existing heat pump plant, VPIOO, in Vartan 4 km from the city. The plant comprises four heat pumps, each with a capacity of 25 MW, that obtain their energy from sea water. The heating energy, produced in the heat pump plant, is delivered to the district heating network. The plant has two sea water inlets, one at the surface and one on the sea bed at depth of 20 meters. Cooling is produced by cold sea water being drawn in through the inlet to the heat pump and then passing six plate heat exchangers that cool the water pumped out into the district cooling network. The heat-exchanger plates are made of titanium in order to withstand the corrosive, brackish sea water. The temperature of the cooling water leaving the plant is 6°C or lower and the return temperature from the distribution grid is 16°C at high load and a few degrees lower at low load. The district cooling system is designed for a maximum load of 60 MW.
After passing through the heat exchanger the heated sea water is released to the sea or returned to the heat pumps depending on current operating mode. During periods when the temperature of the sea water is not sufficiently low, the water entering the heat exchangers first passes the heat pumps to be cooled to a suitable temperature. See Figure I. Generally when the heat pumps are needed for cooling production they are in any case in operation to produce heat. Occasionally cooling determines the running of the heat pumps, which means that heat production becomes more expensive. This increase in the cost of heat production constitutes the production costs of the district cooling system. The district cooling water is conducted from the cooling station into the city via a transit pipe 4 km long and 800 mm in diameter. It was possible to make use mainly of the existing culvert system for distribution within the city.
Water temperature
In Stockholm we have a coastal climate with small temperature variations between summer and winter. The average temperature over the year is 7 "C, for July the average is +18°C and for January -3°C. The waters are normally covered with ice for two month during the winter and the surface temperature reaches +20°C in July. The favorable conditions for sea water cooling is based on the fact that Stockholm lies on the shores of the Baltic Sea and at the mouth of Lake Malaren, the largest lake in Sweden. The water in the Baltic is brackish, with an average salt content of 0.6 % in the area around Stockholm. The fresh water flowing into it has lower density then the salt water, which means that the fre5h water 'floats' on top of the salt water on its way through the archipelago and out into the Baltic. This surface current draws with it some of the salt water and in order to replace this a counter current is created along the aea bed into the waters surrounding Stockholm. It is this current that carries cold sea-bed water to our district cooling system. It takes about three months for the water in the current to travel from the archipelago into Stockholm. The water that reaches the inlet of the cooling plant in July thus left the surface of the archipelago in April. The water temperature in the archipelago in April is extremely low since the ice usually melts at that time. The natural current conditions cause the temperature of the water at the bottom and at the surface to vary during the year as shown in Figure 2. The figure also shows the temperature we require in the cooling system. We guarantee a maximum temperature of 6°C in the pipes leading to our customers. During high-load periods in the summer we lower the temperature in order to increase the capacity of the distribution network.
Heat Pumps in reserve
As shown in Figure 2, neither the surface water nor the water at the bottom is sufficiently cold during autumn to be used as the sole production resource for the cooling network and the existing heat pumps are then utilized to produce the cooling energy needed. These heat pumps were vital to the project and determined the location of the cooling station.
Why did we choose sea-water cooling?
We studied a number of different technologies for producing cooling in the initial stages of the project. In view of the high distribution costs for district cooling we first studied alternatives with production in close proximity to the market.
The production alternative that appeared the most natural was a heat pump which supplied its condenser energy in to our district heating network which is well spread in the area. However, the heat load was found to be too low during periods when the cooling requirement was great and it would have been complicated to make use of the limited temperature level from the heat pump during the winter period when we raise the temperature in the district heating network.
Another interesting alternative was a tunnel with purified and cooled waste water running below the central parts of Stockholm. The idea was to pump the cooled water to ground level and transfer the cooling via heatexchangers to a district cooling network. The reason the water is cold is that it is used to run heat pumps in a neighboring municipality to Stockholm. Difficulties in obtaining guarantees for the temperature of the waste water and district cooling plans of the neighboring municipality put a stop to the project.
A sea-water cooling plant was also considered in the city, where the temperature conditions are the same as in Värtan - our final choice of location. With the cooling plant in the city we would have been forced to build a completely new cooling machine in order to satisfy demands for cooling when the water temperature was too high. This demand is guaranteed in the system we finally selected, using existing heat pumps that require no investment and whose running costs are substantially covered by the production of heat.
The system we finally decided on required considerable investment in a long transit pipe but, on the other hand, we were able to use existing installations and a cooling production based on what nature provides.
From the environmental aspect the project is vastly superior to a conventional cooling plant and this fact has been successfully exploited in marketing campaigns. Environmental arguments have been very favorably received by our customers following a new trend in the property market - save the environment sells!
What conditions are required for sea-water cooling to be used in other places?
Access to cold water is of course a basic requirement, but the water must also be available during that part of the year when the need for cooling is greatest. Unfortunately I think the conditions in Stockholm are unique and would be difficult to copy in other places. However, there should be many places with good access to four-degree water from deep lakes. Even if the water is not sufficiently cold all the year it could be used for cooling the cooling-machine condensers, thereby reducing running costs.
The combined use of heat pumps utilized in Stockholm has also greatly contributed to the feasibility of the project. The combination of district heating production and district cooling production with the aid of heat pumps and good access to lake water is a concept that should be applicable elsewhere.
Future?
The sale of district cooling has exceeded all expectations and expansion plans are now under way. The power-limiting components in the system are the distribution pumps and the transit pipe. The idea is to build an accumulator in close proximity to the market, thereby enabling equalization of the great load variations during a 24-hour period. We also plan to connect a small number of conventional cooling machines to cover the peak periods. We expect these measures to enable us to supply environment-friendly district cooling to around two hundred customers with a combined power requirement of 95 MW.
The author: Göran Fermbäck, is head of the district heating planning department. He is also responsible for economy and business development in the business area of district cooling. He graduated with a masters degree in engineering in 1978. Most of his professional career has been spent as a power consultant, mostly with technologies relating to natural gas and liquefied petroleum. He has been employed with Stockholm Energi since 1993.