(57) A heat pump system has a refrigerant circulation system that is cooled by passing through a thermal storage heat exchanger. The heat exchanger is supplied by ice from an ice maker and can store heat energy by accumulation of ice in the heat exchanger. The ice is generated by cooling a secondary refrigerant, typically brine, at the wall of a ice making machine and scouring the wall prior to crystallisation of the brine mixture to disperse the super cooled liquid adjacent the wall into the body of the secondary refrigerant. Crystallisation is then promoted in the body of the liquid. The ice generator in a preferred form comprises a series of flat plates having primary refrigerant passing between the flat plates and secondary refrigerant passing over the surface of the flat plates. A series of scrapers rotate about an axis normal to the plane of the plates to disperse super colled secondary refrigerants throughout the body of the secondary refrigerant in the ice generator.