The disclosure describes a refrigeration device. The refrigeration device includes a cold store, positioned next to a reservoir having a head region and a body region, and a payload space positioned next to the reservoir opposite the cold store. The reservoir includes a cooling fluid (in many cases, water). A cooling element in the cold store absorbs heat from fluid contained in the head region of the reservoir. The fluid in the head region cools the fluid in the body region, which in turn cools the payload. In some embodiments, the body region has a greater volume than the head region which in turn increases the thermal resistance of the body region. The refrigeration device, once at an in use equilibrium, is resistant to temperature change in a payload container. The stable temperature is maintained at a critical temperature of the cooling fluid—the critical temperature being a temperature wherein the cooling fluid is of lower density both above and below the critical temperature. The natural critical temperature for water is 4 degrees Celsius.