Some thoughts in addition to what @juhist already said:

• AC will be the more energy efficient the colder the cold pool/heat sink to cool against is. @juhist has outlined a scenario (10 °C) that will allow for efficient transfer of heat on its own.
  Nevertheless, any heat sink that is cooler than the ambient air you are presumably cooling against will help. From that perspective, a water tank with less than ambient air temperature would help making your AC more efficient, regardless of what additional use you can put that then warmer water to.
  Even more efficient is evaporation (except in very humid conditions). That is what industrial cooling towers use (which btw. have been sources of important legionella infections in the past)

• Whenever the weather is such that you'd want to run AC, getting really hot water via solar thermal collector, and the proposed tank would maybe better be employed as heat buffer for an STE system.

• Ground source heat pump systems use either soil or e.g. a pond as heat source that keeps higher temp in winter than ambient air. That basically would also give an opportunity to use the thermal buffer as heat sink in summer.

  (I don't know whether it is acutally used this way - I'm in Germany where for residential buildings we still do fine without AC because masonry walls supply sufficient heat capacity that with thoroughly airing during the night and closing window lids during the day we get along without AC (unless you are living in a flat right under the roof). And heating installation over here is anyways water-based rather than hot air. These systems don't lend themselves well to cooling since you'd easily get into trouble with moisture condensing on the cool surfaces)

I think it likely does exist, though I cannot point you to any specific system. What you'd want to look for is

• a ground source heat pump system
• which works also in reverse mode, and
• on the house side an AC/air heating system (rather than hot water central heating)

Some thoughts in addition to what @juhist already said:

• AC will be the more energy efficient the colder the cold pool/heat sink to cool against is. @juhist has outlined a scenario (10 °C) that will allow for efficient transfer of heat on its own.
  Nevertheless, any heat sink that is cooler than the ambient air you are presumably cooling against will help. From that perspective, a water tank with less than ambient air temperature would help making your AC more efficient, regardless of what additional use you can put that then warmer water to.
  Even more efficient is evaporation (except in very humid conditions). That is what industrial cooling towers use (which btw. have been sources of important legionella infections in the past)

• Whenever the weather is such that you'd want to run AC, getting really hot water via solar thermal collector, and the proposed tank would maybe better be employed as heat buffer for an STE system.

• Ground source heat pump systems use either soil or e.g. a pond as heat source that keeps higher temp in winter than ambient air. That basically would also give an opportunity to use the thermal buffer as heat sink in summer.

  (I don't know whether it is acutally used this way - I'm in Germany where for residential buildings we still do fine without AC because masonry walls supply sufficient heat capacity that with thoroughly airing during the night and closing window lids during the day we get along without AC (unless you are living in a flat right under the roof). And heating installation over here is anyways water-based rather than hot air. These systems don't lend themselves well to cooling since you'd easily get into trouble with moisture condensing on the cool surfaces)

Some thoughts in addition to what @juhist already said:

• AC will be the more energy efficient the colder the cold pool/heat sink to cool against is. @juhist has outlined a scenario (10 °C) that will allow for efficient transfer of heat on its own.
  Nevertheless, any heat sink that is cooler than the ambient air you are presumably cooling against will help. From that perspective, a water tank with less than ambient air temperature would help making your AC more efficient, regardless of what additional use you can put that then warmer water to.
  Even more efficient is evaporation (except in very humid conditions). That is what industrial cooling towers use (which btw. have been sources of important legionella infections in the past)

• Whenever the weather is such that you'd want to run AC, getting really hot water via solar thermal collector, and the proposed tank would maybe better be employed as heat buffer for an STE system.

• Ground source heat pump systems use either soil or e.g. a pond as heat source that keeps higher temp in winter than ambient air. That basically would also give an opportunity to use the thermal buffer as heat sink in summer.

  (I don't know whether it is acutally used this way - I'm in Central Europe where for residential buildings we still do fine without AC because masonry walls supply sufficient heat capacity that with thoroughly airing during the night and closing window lids during the day we get along without AC (unless you are living in a flat right under the roof). And heating installation over here is anyways water-based rather than hot air. These systems don't lend themselves well to cooling since you'd easily get into trouble with moisture condensing on the cool surfaces)

Some thoughts in addition to what @juhist already said:

• AC will be the more energy efficient the colder the cold pool/heat sink to cool against is. @juhist has outlined a scenario (10 °C) that will allow for efficient transfer of heat on its own.
  Nevertheless, any heat sink that is cooler than the ambient air you are presumably cooling against will help. From that perspective, a water tank with less than ambient air temperature would help making your AC more efficient, regardless of what additional use you can put that then warmer water to.
  Even more efficient is evaporation (except in very humid conditions). That is what industrial cooling towers use (which btw. have been sources of important legionella infections in the past)

• Whenever the weather is such that you'd want to run AC, getting really hot water via solar thermal collector, and the proposed tank would maybe better be employed as heat buffer for an STE system.

• Ground source heat pump systems use either soil or e.g. a pond as heat source that keeps higher temp in winter than ambient air. That basically would also give an opportunity to use the thermal buffer as heat sink in summer.

  (I don't know whether it is acutally used this way - I'm in Germany where for residential buildings we still do fine without AC because masonry walls supply sufficient heat capacity that with thoroughly airing during the night and closing window lids during the day we get along without AC (unless you are living in a flat right under the roof). And heating installation over here is anyways water-based rather than hot air. These systems don't lend themselves well to cooling since you'd easily get into trouble with moisture condensing on the cool surfaces)