A heat engine employing an organic Rankine cycle comprising: an organic refrigerant exhibiting an atmospheric boiling point below -35 degrees Celsius; a heat sink; a sealed, closed-loop path for the organic refrigerant, the sealed, closed-loop path having both a high-pressure zone which absorbs heat from a hot water heat source having a temperature of less than 82 degrees Celsius, and which contains a first portion of the organic refrigerant in at least a gaseous phase, and a low-pressure zone which transfers heat to the heat sink, and which contains a second portion of the organic refrigerant in at least a liquid phase; a positive displacement decompressor which provides a pressure gradient through which the organic refrigerant in the gaseous phase flows continuously from the high-pressure zone to the low-pressure zone, the positive displacement decompressor maintaining a pressure differential between the high-pressure zone and the low-pressure zone of between about 20 bar and about 42 bar, the positive displacement decompressor extracting mechanical energy due to the pressure gradient; an electrical generator coupled to the positive displacement decompressor which converts extracted mechanical energy to electrical energy; and a positive displacement hydraulic pump for providing continuous flow of the organic refrigerant in the liquid phase from the low-pressure zone to the high-pressure zone characterized by further comprising a vertically oriented refrigerant tank of downwardly tapering and downwardly decreasing cross-sectional area, the vertically oriented refrigerant tank configured to employ gravity to ensure maximum density of the organic refrigerant in the liquid phase as the organic refrigerant enters the positive displacement hydraulic pump. an organic refrigerant exhibiting an atmospheric boiling point below -35 degrees Celsius; a heat sink; a sealed, closed-loop path for the organic refrigerant, the sealed, closed-loop path having both a high-pressure zone which absorbs heat from a hot water heat source having a temperature of less than 82 degrees Celsius, and which contains a first portion of the organic refrigerant in at least a gaseous phase, and a low-pressure zone which transfers heat to the heat sink, and which contains a second portion of the organic refrigerant in at least a liquid phase; a positive displacement decompressor which provides a pressure gradient through which the organic refrigerant in the gaseous phase flows continuously from the high-pressure zone to the low-pressure zone, the positive displacement decompressor maintaining a pressure differential between the high-pressure zone and the low-pressure zone of between about 20 bar and about 42 bar, the positive displacement decompressor extracting mechanical energy due to the pressure gradient; an electrical generator coupled to the positive displacement decompressor which converts extracted mechanical energy to electrical energy; and a positive displacement hydraulic pump for providing continuous flow of the organic refrigerant in the liquid phase from the low-pressure zone to the high-pressure zone characterized by further comprising a vertically oriented refrigerant tank of downwardly tapering and downwardly decreasing cross-sectional area, the vertically oriented refrigerant tank configured to employ gravity to ensure maximum density of the organic refrigerant in the liquid phase as the organic refrigerant enters the positive displacement hydraulic pump.