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Technology

The major resources air conditioning systems demand are water and electricity. 

Conventional air conditioning

Air conditioning (AC) systems of large buildings commonly have a network of pipes through which chilled water flows. Fans blow over these pipes to supply cool air to the rooms. Two major components of a conventional system are the chiller and the cooling tower. Chillers are made up of two main components: the evaporator and the condenser. Water is cooled by removing heat from it, through the evaporation of a refrigerant at low pressure. As the refrigerant in a liquid state enters a gaseous state, it absorbs surrounding heat. This process occurs in the evaporator. The gas then enters a condenser, which raises the pressure and in the process of returning to a liquid state, the refrigerant heats up. The heat from the hot liquid is transferred to a heat sink where fans help expel this excess heat before the refrigerant circulates back to the evaporator. In a conventional air conditioner, the evaporator and condenser are powered by electricity.

Source: David Culler (used with permission)

In large AC systems, cooling towers conserve water and significantly improve cooling efficiency by lowering the temperature of the water that is heated up by the chiller's heat exchangers. Pumped up to a cooling tower, the water trickles over a mesh and is cooled down by air circulation and evaporation. The volume of water that is evaporated off depends on the ambient temperature and relative humidity in the atmosphere. The system must be replenished with freshwater, as water gets lost through evaporation or as droplets blown out by the fans. The remaining water has an increased concentration of dissolved solids. Once the concentration reaches to a level that can impair system performance, the mineralized water, known as blowdown water, is drained from the system and fresh water is added (Martin & Heaney, 2008).

Overview of district cooling technology

In a district cooling system, a cooling station supplies cold water to a collection of proximally located buildings. Cooling on a large scale can significantly reduce energy consumption. The upfront cost of system installation is high, but the total operating costs are lower than of those for typical local air conditioning systems. District cooling systems make it possible to better utilize renewable resources such as cold water from lakes or seas (Levine et al., 2007). When seawater is used for district cooling, required components include piping and pumps to bring up the deep ocean water, a counter-current heat exchanger, piping to establish a freshwater network so that cold water can be circulated to buildings included in the system, and additional piping to create a discharge pathway for the return of seawater to the ocean. A SWAC system could utilize freshwater AC distribution systems already in place in many commercial buildings and bypass existing chiller units.

Examples of systems that use either lakes or the ocean for their cold water sources

At the end of the 1980's two SWAC systems were in existence. In the 1990's, just one more system was built. By 2010, at least another nine systems had been developed with many more in development. SWAC, as an alternative to conventional air conditioning, is not limited to the use of seawater. Other such developments can also make use of cold lake water cycled from depth, in which case they might be called lake source cooling or deep lake water cooling projects.

Stocklholm, Sweden

With a cooling capacity of 80,000+ tons, Stockholm has the largest seawater cooling system in the world (Rezachek 2005). Stockholm Energy built the system in 1995 and prior to that, the company was operating a district heating system for the city. An important component of Stockholm's seawater project is the use of a cold-water storage facility. At night, when the demand for cooling is lower, the facility can store any excess cold water and later supply that water when demand increases during the warmer hours of the day.

Highlights of the project so far:

  • Reduction of 74,546 metric tons of GHG emissions annually

Amsterdam, Netherlands

Amsterdam has two lake water district cooling systems. The first started operating in 2004, and the second, in 2009. With a combined supply of 35,000 tons of cooling, the systems cool the city's business districts, Zuidas and Zuidoost. The cold water (41-43°F) is drawn from two man-made lakes at depths of 150 feet. Nuon, the company operating the system, reports that similar lakes in the area allow for the expansion of the systems.

Highlights of the project so far:

  • Estimated electrical savings of 69.9 GWh/year
  • Reduction of 40,000 metric tons of CO₂ emissions
  • 75% savings in energy costs

Toronto, Ontario, Canada

Toronto's deep lake water cooling (DLWC) system can cool 32 million square feet, making it the largest system of its kind in North America. The system uses 41°F water from Lake Ontario to cool buildings in the downtown commercial district. The system was commissioned in 1997, construction began the summer of 2002, and it was completed two years later. In addition to providing cooling, the system's exhausted supply water is used as city drinking water (Eliadis 2010). Because the supply water becomes the city water supply, there is no returned water and thus, no heat pollution added to the lake.

Highlights of the project so far:

  • Annual reductions of 80,000+ tons in GHG emissions
  • 62,500 tons of base load cooling and 75,000 tons of total cooling capacity
  • 65 MW of deferred peak load electricity demand
  • 85 million kWh of electricity saved annually (Eliadis 2010)

Halifax, Nova Scotia, Canada

Halifax has two SWAC systems, one at Purdy's Wharf and one at the Alderney 5 complex. The Purdy's Wharf system has been in operation since 1986 and is one of the world's first. It supplies cooling to a 700,000 square-foot office complex. The seawater near the surface is already quite cold so the intake pipe was submerged to just a 70 foot depth. Having just gone on line in February 2010, the Alderney 5 complex's has one of the world's newest SWAC systems. It is a small system, built to cool just over 330,000 square feet of building space. The unique feature of the system is its underground thermal storage facility. This cold storage battery is an array of 80, 4.5-inch wide holes bored 500 feet deep. After constructing the storage system, a parking lot was built on top of it. During the winter, the 'battery' is charged with very cold seawater. In the summer, the cold water is pumped out for cooling. It has reduced the cooling costs of the buildings by CAD$400,000 (Waldron; Cruikshanks 2008).

Cornell University, Ithaca, NY

Since July 2000, Cornell University's Lake Source Cooling Plant (LSCP) has supplied cooling for the university campus and the Ithaca City school district. Similar to other district cooling systems, the lake water is not pumped directly into buildings to cool them; rather, it flows through heat exchangers, which cool the supply water for buildings. The LSCP provides more than 18,000 tons of cooling by drawing water from a depth of 250 feet. Similar to the seawater system in Stockholm, the LSCP also features a thermal storage tank. Providing 4,000 tons of cooling at peak demand times, it is an effective demand side management tool. By recharging the tank at night, 4MW of electricity use can be shifted to off-peak hours (Cornell University Facilities Services Utilities and Energy Management 2006).

Highlights of the project so far:

  • Over 20,000,000 kWh/year in electricity savings, a reduction of 86% in AC energy costs
  • 19 million pounds reduction in coal consumption
  • Annual GHG emissions reductions of: 56 million pounds of CO₂, 645,000 pounds of SO₂, and 55,000 pounds of NO
  • 40,000 pound reduction in CFC emissions
     
References:  

- Cornell University Facilities Services Utilities and Energy Management. (2006). Cooling - Chilled Water Plants. <http://energyandsustainability.fs.cornell.edu/util/cooling/production/cwp.cfm>.
- Cruikshanks, F. (2008). Alderney 5 Complex BTES Cold Store Project [Slide presentation]. Halifax, Nova
  Scotia: Environment Canada.
- Culler, D. (n.d.). <http://www.eecs.berkeley.edu/~jortiz/gridos/site/modeling.html>.
- Eliadis, T. (Producer). (2010, December 12, 2011). Enwave Deep Lake Cooling. [Powerpoint Presentation
  Webinar].
- Levine, M., Ürge-Vorsatz, D., Blok, K., Geng, L., Harvey, D., Lang, S., . . . Yoshino, H. (2007). Residential and
  commercial buildings. In B. Metz, O. R. Davidson, P. R. Bosch, R. Dave & L. A. Meyer (Eds.), Climate Change
  2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental
  Panel on Climate Change
(pp. 396-403). Cambridge, United Kingdom and New York, NY: Cambridge
  University Press.
- Martin, J., & Heaney, J. (2008). BMP 9: Cooling Towers: Conserve Florida Water Clearinghouse.
- Rezachek, D. (2005). Docket No. 05-0145. Honolulu.
- Waldron, L. (n.d.). Deep Water Cooling. Interactive Case Studies in Sustainable Community Development.
 <http://www.crcresearch.org/case-studies/case-studies-sustainable-infrastructure/energy/deep-
  water-cooling
>.