Canada, is an example. It uses cold lake water to cool the chillers, which in turn are used to cool city buildings via a district cooling system. The return water is used to warm the city's drinking water supply, which is desirable in this cold climate. Whenever a chiller's heat rejection can be used for a productive purpose, in addition to the cooling function, very high thermal effectiveness is possible. Vapor-compression chiller technology A vapor compression chiller typically uses one of four types of compressor: Reciprocating compression, scroll compression, screw-driven compression, and centrifugal compression are all mechanical machines that can be powered by electric motors, steam, or gas turbines. Using electric motors in a semi-hermetic or hermetic configuration is the most common method of driving the compressors since electric motors can be effectively and easily cooled by the refrigerant, without requiring fuel supply or exhaust ventilation and no shaft seals are required as the motor can operate in the refrigerant, reducing maintenance, leaks, operating costs and downtime, although open compressors are sometimes used. They produce their cooling effect via the reverse-Rankine cycle, also known as vapor-compression. With evaporative cooling heat rejection, their coefficients of performance (COPs) are very high; typically 4.0 or more. COP Current vapor-compression chiller technology is based on the "reverse-Rankine" cycle known as vapor-compression. See the attached diagram which outlines the key components of the chiller system. Key components of the chiller: Refrigeration compressors are essentially a pump for refrigerant gas. The capacity of the compressor, and hence the chiller cooling capacity, is measured in kilowatts input (kW), Horse power input (HP), or volumetric flow (m3/h, ft3/h). The mechanism for compressing refrigerant gas differs between compressors, and each has its own application. Common refrigeration compressors include reciprocating, scroll, screw, or centrifugal. These can be powered by electric motors, steam turbines, or gas turbines. Compressors can have an integrated motor from a specific manufacturer, or be open drive--allowing the connection to another type of mechanical connection. Compressors can also be either Hermetic (welded closed) or semihermetic (bolted together). In recent years, application of variable-speed drive (VSD) technology has increased efficiencies of vapor compression chillers. The first VSD was applied to centrifugal compressor chillers in the late 1970s and has become the norm as the cost of energy has increased. Now, VSDs are being applied to rotary screw and scroll-technology compressors. Condensers can be air-cooled, liquid-cooled, or evaporative. The