Tuesday, November 20, 2012

Copenhagen's Seawater Cooling Delivers Energy And Carbon Savings

Copenhagen, Denmark – Situated in wind-swept, cloud-covered northern Europe, Copenhagen is probably not the first city to come to mind in need of air-conditioning. Summer high temperatures rarely exceed the mid-70s Fahrenheit, but, in buildings with large server rooms or where many people work or shop, there is a need for indoor cooling.
Copenhagen’s elegant solution is a district cooling system that takes advantage of free cooling provided by seawater drawn from the city’s harbor. Two years ago, Copenhagen Energy opened the city’s first district cooling plant, a 10-megawatt (MW) facility located on the site of a retired power plant on Adelgade, in the historic inner city.
The new cooling network complements the city’s existing district heating system, the world’s largest, which covers 98% of Copenhagen. On a recent reporting trip to Denmark, I was given a tour of the plant by Thomas Grinde, an engineer with Copenhagen Energy.
When the temperature of the water in Copenhagen Harbor is sufficiently cold, Grinde explained, the district cooling system draws water from an intake pipe located near the Royal Danish Playhouse and Nyhavn canal. Every degree saved by pre-cooling with seawater saves 15% on electricity at the chiller. (Mussels and other small sea life, in case you were wondering, are separated at the intake and re-established in suitable habitat, he says.)
The Adelgade plant is designed for three methods of cooling (more information about the district cooling system is available here [p. 35-37], and in this video):
1) Free cooling – Seawater temperature is below 5.5 degrees Celsius [42°F] and cooling demand low (less than 2,400kW). All cooling demands are covered by free cooling heat exchangers.
2) Combined operation – Seawater temperature is between 5.5°C and 11.5°C [52.7°F]. Heat exchangers are used for pre-cooling of the cooled water, before it is fully cooled by chillers to the desired temperature.
3) Chiller cooling – Seawater temperature is above 11.5°C. The seawater is too warm to be used for free cooling so absorption and compression chillers provide all cooling. Free cooling exchanges are bypassed completely.
The insulated pipes that carry chilled water to commercial and industrial buildings for indoor cooling can be located in the same underground tunnels that carry steam for the district heating system or in entirely new networks. The aim, Grinde says, is to target co-located buildings (department stores, commercial buildings, hotels, and facilities with data centers) with cooling demands of 150 kilowatts (kW) or more. Copenhagen Energy does not plan to offer district cooling to residential customers.
The decision to integrate the already efficient district heating and cooling systems into one network begets additional efficiencies. In summer, when space heating demand falls, surplus heat from Copenhagen’s combined heat and power (CHP) plants is re-directed to the steam-powered district cooling chillers. Sending steam to the district cooling plant helps balance the two systems and compensates for the lack of free seawater cooling.
Customers with large cooling needs pay a fee to connect to the network, Grinde told me, and they subscribe to the service based on their expected and actual load. The upfront fees help pay for the build-out of the system. Copenhagen Energy paid for the $14-million Adelgade cooling plant with proceeds from the sale of its electricity assets to state majority-owned DONG Energy.
There are additional benefits, Grinde explained, citing the example of hotels. Conventional compressor-based air-conditioning is not just more noisy, expensive, and energy-intensive than district cooling; it also takes up more space. Adopting district cooling opens up a parking space in the basement where the compressor once stood and opens up rooftop space that had been occupied by cooling towers. Grinde said he knew of one hotel that had installed a rooftop employee canteen in this found space.The City of Copenhagen expects district cooling will prevent the release of 14,000 tons of CO2annually by reducing carbon emissions by 67% and electricity consumption by 80% compared to conventional cooling.
The City of Copenhagen expects cooling demand to increase in the coming decades. Peak summer high temperatures, mild compared to warmer climes, are expected to rise by 2% to 3% by 2050, with average daily temperatures rising as well. Thomas Grinde told me that Copenhagen Energy estimates that winter cooling demand will eventually increase to about 40% of the summer load.
Copenhagen Energy envisions the need for as many as seven district cooling zones. The city’s second cooling plant, located near the central train station, is scheduled to come online in May 2013.
Note: This is the second in a series of posts on clean energy and climate solutions in Denmark. Other installments looked at Denmark’s newcommunity wind shareholder plan, the Project Zero carbon-neutrality plan, Copenhagen’s success as a cleantech leader, and what other cities can learn from Copenhagen’s climate adaptation plan. The reporting for this post was supported by a grant from the Danish Ministry of Foreign Affairs International Press Initiative.

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