Heat storage unit, heat storage system, and heat storage method

A heat storage unit comprising:

• a heat storage material solution that contains water and polymers that exhibit hydrophilicity or hydrophobicity depending on a temperature;
• a heat exchanger to cause heat exchange to be performed between a heating fluid and the heat storage material solution to heat the heat storage material solution and store heat in the heat storage material solution, and cause heat exchange to be performed between a heat utilization fluid and the heat storage material solution to receive heat from the heat storage material solution and cause heat to be transferred from the heat storage material solution;
• and
• a container that is filled with the heat storage material solution and houses the heat exchanger,
• wherein the heat exchanger includes,
• a heating pipe through which the heating fluid flows, and
• a heat utilization pipe through which the heat utilization fluid flows, and
• wherein the temperature is a lower critical solution temperature for the heat storage material solution, and
• the polymers exhibit hydrophilicity at a temperature below the lower critical solution temperature, and exhibit hydrophobicity at a temperature above the lower critical solution temperature.