Anomalous Expansion of Water Upon Freezing at High Pressure Defying Carnot Theorem

At standard conditions (0°C and 1 atm), water expands by approximately 9% when transitioning from liquid to solid state. Moreover, when water is spatially constrained, upon freezing, it exhibits a remarkable pressure increase up to 220 MPa before it becomes another form of ice. Notably, increased pressure further lowers the melting point, thereby amplifying the expansion. In other words, the expansion becomes even more significant due to the reduction in melting point caused by the increased pressure. At a pressure of 200 MPa, water freezes at about 253 Kelvin and undergoes a remarkable 16.8% expansion. Contrary to typical liquids, water exhibits anomalous behaviour: its melting point decreases with increased pressure, as shown by the backward-sloping liquid-solid line in its phase diagram. This unique property enables a novel heat engine concept that leverages expanding ice to generate substantial work output. Notably, an engine can operate efficiently with extremely small temperature differences between its hot and cold reservoirs, as the phase change occurs at a constant temperature. By harnessing water's anomalous expansion, this discovery offers a new way to convert low-quality heat energy into work, challenging traditional thermodynamic limits.