The present invention provides an energy storage device that utilizes a cold sink that undergoes cycles of freezing and thawing. The device converts electrical energy to stored thermal energy, and then re-converts the stored thermal energy to electrical energy, as needed or desired. The device can store energy on a large scale (e.g., on the order of megawatts or greater) and for an extended period of time (e.g., for at least 12 hours, or longer, as needed).