Device and method for converting solar PV energy into thermal energy storage using combined heat-pump and resistive heating elements in water heater

A controller comprising:

• a first output adapted to provide power to an electric heat pump-based water heater having an alternating current (AC) motor;
• a second output adapted for coupling to a resistive water heating element in the electric heat pump-based water heater;
• a multiphase variable frequency (VFD) motor drive coupled to provide power to the first output;
• an input adapted for connection to a solar photovoltaic power source;
• the second output coupled to receive power from a resistive power control apparatus selected from a pulse-width modulator and a DC-DC converter;
• a DC/DC power converter configured for connection to a solar photovoltaic system;
• wherein the DC/DC power converter is coupled to provide power to the VFD and to the resistive power control apparatus;
• wherein the DC/DC power converter, the VFD, and the resistive power control apparatus, are controlled by a microcontroller;
• wherein the microcontroller contains firmware configured to:
• operate the DC-DC converter to extract maximum power from the input adapted for connection to a solar photovoltaic power source using a maximum-power-point-tracking (MPPT) method,
• determine solar power available from the input adapted for connection to a solar photovoltaic power source and
• if solar power available is less than a first threshold, operate the resistive power control apparatus to use solar power available in the resistive water heating element;
• if solar power available is greater than the first threshold, use the VFD to use the solar power available to power the electric heat pump-based water heater at a frequency and an AC voltage increasing with solar power available, the frequency limited to a maximum frequency and the AC voltage limited to a maximum AC voltage;
• and
• if solar power available is greater than sufficient power to use the VFD to power the electric heat pump-based water heater at the maximum frequency and the maximum AC voltage, to operate the resistive power control apparatus to use that portion of solar power available greater than sufficient power to use the VFD to power the electric heat pump-based water heater at the maximum frequency and the maximum AC voltage to drive the resistive water heating element.

A system comprising:

• a solar photovoltaic system;
• a DC/DC power converter connected to the solar photovoltaic system to extract maximum power from the solar photovoltaic system and power a DC bus;
• an electric heat pump-based heating device coupled to heat a thermal mass;
• a resistive heating element in the electric heat pump-based heating device coupled to heat the thermal mass;
• a variable frequency (VFD) motor drive coupled to receive power from the DC bus and having at least a first phase output coupled to the electric heat pump-based heating device;
• the resistive heating element connected to resistive power control apparatus selected from a pulse-width modulator and a DC-DC converter;
• wherein the DC/DC power converter, the VFD, and the resistive power control apparatus are coupled to be controlled by a microcontroller;
• sensors adapted to measure current, voltage, and temperature coupled to the microcontroller;
• wherein, the microcontroller is configured by firmware to monitor solar power available;
• and
• further comprising firmware configured to operate the resistive power control apparatus to use the soler power available in the resistive heating element, the firmware configured to operate the resistive power control apparatus being executed when the solar power available is less than a first threshold;
• further comprising firmware configured to use the VFD to use the solar power available to use the VFD to use the solar power available to power the electric heat pump-based heating device at a frequency and an AC voltage increasing with power available, the frequency limited to a maximum frequency and the AC voltage limited to a maximum AC voltage, the firmware configured to use the solar power available to power the electric heat pump-based heating device being executed when the solar power available is greater than the first threshold;
• and
• further comprising firmware configured to use the VFD to use the solar power available to power the electric heat pump-based heating device at the maximum frequency and the maximum AC voltage and to operate the resistive power control apparatus to use that portion of solar power available unused by the VFD to power the electric heat-pump based heating device at maximum frequency and maximum AC voltage that is executed when the solar power available is greater than power required to use the VFD to power the electric heat pump-based heating device at maximum frequency and maximum AC voltage.