The heat exchanger is adapted for cooling a coolant (e.g., water), used to cool a device or an area (e.g., building interior), such as during periods of peak energy cost and usage, to save energy and energy costs. The heat exchanger includes a coolant storage tank with one or more refrigerant circulators in contact with the floor of the tank. The circulators use a refrigerant having a freezing temperature colder than the coolant, with coolant on the floor of the tank forming a layer of ice thereon. A rotary scraper extends up through the tank floor from each circulator, with the scrapers operating to remove the thin layer of ice from the floor as the ice forms. The resulting ice chips are relatively small and thin, thus having a relatively large surface area for their volume in order to maximize melting and rapid cooling of the coolant.