NOVELTY - The system (100) has a set of thermal energy storage banks (102a-102h) among which each bank has a thermal energy storage material undergoing energy absorbing and/or releasing phase transition at temperatures or temperature sub-ranges. A control system of a set of multi-bank phase change material thermal storages regulates an amount and source of electricity drawn by a set of elements e.g. heat pumps, based on electricity grid stability, availability, cost and carbon intensity of electricity, signals from an electricity grid operator and system elements operation criticality.    USE - Thermal energy storage system for use in a heat recovery ventilation apparatus, for accepting, storing and releasing thermal energy at a temperature to/from thermal energy source/sink e.g. local combined heat and electrical power source such as fuel cell, diesel generator, hybrid photovoltaic collector, thermal collector and solar thermal electrical generator, in a smart grid (all claimed). Can also be used for accepting, storing and releasing thermal energy at a temperature to/from environmental source, natural source, solar thermal collector, ground source, air source, combustion system e.g. wood burning furnace, natural gas burner and oil burner, electrical heater such as electric water heater, and waste heat from fan coil, cooling tower, air conditioner, refrigeration system, heat pump, heating system or thermal electricity generation system or machine such as internal combustion engine and jet engine, active cooling system, heat sink, computer processor and LED lighting.    ADVANTAGE - The system selects a flow rate and/or circulation path of a heat transfer fluid to return the heat transfer fluid to the source at a temperature to enhance thermodynamic efficiency of the heat source. The system balances multiple match characteristics of the source/sink to maintain desired target quantities of thermal energy in each bank and/or desired target temperatures that are changed from time-to-time in a better and optimized manner by the control system.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (1) a heat recovery ventilation apparatus for providing mechanical ventilation with heat recovery, comprising a set of heat exchangers    (2) a combined thermal energy storage and thermal energy collector and/or radiator apparatus comprising a phase change material composite    (3) a phase change material and heat transfer fluid heat exchanger apparatus comprising a gas to heat transfer fluid heat exchanger    (4) a method for constructing a phase change material and heat transfer fluid heat exchanger apparatus.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a thermal energy storage system.    Thermal energy storage system (100)    Thermal energy storage banks (102a-102h)    Insulation unit (104)    Inlet (106)    Outlet (108)    Feed points (110)