high specific volume at low temperatures, high pressure ratios required across the compressor, and high temperatures at the outlet of the compressor, exist as barriers to the use of water and water vapor as a refrigerant. Additionally, some applications can find issues with sediment build-up and bacterium breeding, although these issues can be minimized with techniques such as adding chemicals to fight the bacteria and softening the water used. Water is commonly used at higher temperatures in lithium-bromide absorption chillers, but the coefficient of performance (COP) in these applications is just one fifth of typical electric drive centrifugal chillers. Vapor compression refrigeration cycles are a rare application but do have the potential to yield high COPs due to the thermophysical properties of water. Beyond absorption chillers, water can be used in desiccant dehumidification/evaporative cooling, adsorption chillers and compression chillers. Water has also been proposed to be used in special rotary compressors, although the dimensions and price of these systems can become very large. In typical heat pump systems water can be an ideal refrigerant substance, with some applications yielding COPs that exceed 20. This makes it an obvious choice for industrial applications with temperatures above 80 °C. Water has also shown to be viable as a refrigerant in ground source heat pumps Air as a refrigerant Air is free, non-toxic, and does not negatively impact the environment. Air can be used as a refrigerant in air cycle refrigeration systems, which work on the reverse Brayton or Joule cycle. Air is compressed and expanded to create heating and cooling capacities. Originally, reciprocating expanders and compressors were used, which created poor reliability. With the invention of rotary compressors and expanders the efficiency and reliability of these cycles has improved, and alongside new compact heat exchangers makes air possible to compete with more conventional refrigerants. Noble gases as refrigerants The noble gases are rarely used as refrigerants. The primary uses of noble gases as refrigerants is in liquid super coolant experimental systems in laboratories or in superconductors. This specifically applies to liquid helium, which has a boiling point of 4.2 K. They are never used for industrial or home refrigeration. Other natural refrigerants These natural refrigerants are substances that can be used in refrigeration systems but are not used or are only used very rarely because of an availability of compounds that are either less expensive or are easier to handle and contain.