Lake-source district cooling systems

ASHRAE Journal, Feb, 2008 by Robert Zogg, Kurt Roth, James Brodrick

Lake-source cooling (LSC), sometimes called deep lake water cooling (DLWC), uses cold lake water instead of chillers to provide cooling for large, multiple-building applications such as urban communities or college campuses. They typically extract cold lake water (from depths of 250 ft [76 m] or more) and circulate the lake water through heat exchangers to remove heat from a district (or campus) chilled-water loop (Figure 1). Auxiliary chillers may be used to supply additional cooling as needed.

Energy Savings Potential

LSC systems realize large energy savings primarily by eliminating (or reducing) the need for chiller-based cooling. * Higher pumping loads and thermal losses in the piping runs between the lake and buildings served by the LSC offset a portion of the energy savings. For example, a LSC at Cornell University uses about 5 miles (8 km) of transmission piping that have peak thermal losses of 800 tons (2 820 kW), or about 4% of the 20,000 ton (70 400 kW) total system cooling capacity. (1,2)

Limited data from recently installed LSC systems give insight into the energy-savings potential of LSC. An LSC system operating at Cornell University in Ithaca, N.Y. takes water from 250 feet (76 m) below the surface of Cayuga Lake, at a temperature of about 39[degrees]F (4[degrees]C). On an annual average, it consumes about 0.10 kW per ton of cooling delivered, an eight-fold reduction (i.e., 87.5%) in energy use in 2001 compared to the previous cooling system's 0.75 kW/ ton (0.21 kW/kW). (1) Similarly, a LSC system operating in Toronto using water from Lake Ontario (at about 272 ft. [83 m] below the surface) is reported to use one-tenth the electricity used by conventional chillers. (3)

In contrast, high-efficiency, water-cooled chillers currently available can achieve 0.5 kW/ton (0.14 kW/kW) or lower. Adding in 0.10 kW/ton (0.03 kW/ kW) for cooling tower fans and pumps results in about 0.6 kW/ton (0.17 kW/ kW) total. Assuming that 0.10 kW/ton (0.03 kW/kW) is representative of the performance of LSC, cooling energy savings would be about 83% relative to best available conventional chiller systems. Savings compared to air-cooled chillers would be higher.

U.S. commercial buildings consumed about 2.3 quadrillion Btu in 2005 for space cooling. (4) Of that, chillers consume about 31%, (5) or about 0.7 quadrillion Btu. Of course, not all commercial buildings are sufficiently close to a deep-water lake or an ocean to use LSC. The authors estimate that roughly 10% of the U.S. population lives in cities of more than 100,000 that are on or near the ocean, a large ocean bay, or a Great Lake. Assuming that LSC could address about 10% of the U.S. commercial-building chiller load, the annual energy savings would be about 0.06 quadrillion Btu, or about 8% of U.S. chiller energy consumption.

Market Factors

A seawater-source cooling system proposed for Honolulu is anticipated to deliver 25,000 tons (88 000 kW) cooling capacity (6) at a project cost of $145 million (7), or $5,800/ton ($1,650/kW). This includes the cost of piping throughout the service area (i.e., there is no preexisting distribution system). The fresh-water LSC installed at Cornell University cost $58 Million (8) ($2,900/ton [$825/kW]). In this case, a chilled-water distribution system already existed.

Based on 0.5 kW/ton (0.14 kW/kW) electricity savings and a $500/ton ($142/kW) avoided cost for high-efficiency chillers, and optimistically assuming 3,000 full-load-equivalent hours per year and a commercial electricity price valued at $0.20/ kWh ($0.05/MJ) for space cooling (taking into account electric rates during peak periods), the simple payback period would range from about eight to 18 years. Of course, incentives could lower the payback period significantly. Installation cost will vary significantly depending on location, water quality/depth, environmental considerations, and whether an existing district cooling system is in place. Disruption of the community during construction can also be a barrier.

Consideration of large-scale complex cooling systems such as LSC requires coordination among a large group of stakeholders, including local and state governments, environmental and regulatory agencies, a district cooling system operator, and local organizations that could use the cooling or heat delivered by the LSC. Recognition of the benefits of reduced energy consumption, reduced peak electric demand, and reduced [CO.sub.2] and pollutants may justify providing economic incentives for LSC installations.

One source estimates that there are 900 district energy systems (heating and/or cooling systems) in North America, including 5,000 miles (8000 km) of piping and serving 4.6 billion [ft.sup.2] (427 million [m.sup.2]) of customer space. (9) Only a small fraction of these currently use LSC.

[FIGURE 1 OMITTED]

Perhaps the energy and environmental benefits of LSC will encourage broader adoption of this energy-efficient approach to cooling.

References

(1.) Cornell University, Facilities Services, Utilities and Energy Management. "Lake-source cooling environmental facts and benefits." http://tinyurl.com/355tcv (or www.utilities.cornell.edu/ utl_lscfacts.html).