Defrosting methodology for heat pump water heating system

A vapor compression system comprising:

• a compression device to compress a refrigerant to a high pressure, said compression device including a discharge;
• a heat rejecting heat exchanger for cooling said refrigerant;
• an expansion device for reducing said refrigerant to a low pressure, said expansion device including an inlet;
• a heat accepting heat exchanger for evaporating said refrigerant;
• a refrigerant line bypassing said heat rejecting heat exchanger between said discharge of said compression device and said inlet of said expansion device;
• a valve located on said refrigerant line to control a flow of refrigerant between said discharge of said compression device and said inlet of said expansion device, wherein said valve includes a first port in fluid communication with said inlet of said expansion device, a second port in fluid communication with said heat rejecting heat exchanger, and a third port in fluid communication with said discharge of said compression device, and said control closes said second port to prevent said refrigerant from said heat rejecting heat exchanger from flowing through said expansion device and opens said third port to allow said refrigerant from said compression device to flow through said expansion device along said refrigerant line when said sensor detects said defrosting condition and said control opens said second port to allow said refrigerant from said heat rejecting heat exchanger to flow through said expansion device and closes said third port to prevent said refrigerant from said compression device from flowing through said expansion device along said refrigerant line when said sensor does not detect said defrosting condition;
• a sensor that detects a defrosting condition of said heat accepting heat exchanger;
• and
• a control that opens said valve when said sensor detects said defrosting condition to allow said refrigerant to flow through said valve.

A method of regulating a high pressure of a transcritical vapor compression system comprising the steps of:

• compressing a refrigerant to the high pressure in a compression device including a discharge;
• cooling the refrigerant by exchanging heat with a fluid, and the fluid accepts heat from the refrigerant;
• expanding the refrigerant to a low pressure in an expansion device including inlet;
• evaporating the refrigerant in a heat accepting heat exchanger;
• sensing a defrosting condition of the heat accepting heat exchanger;
• directly flowing the refrigerant along a refrigerant line from the discharge of the compression device to the inlet of the expansion device;
• and
• melting frost on the heat accepting heat exchanger when the step of sensing the defrosting condition indicates the defrosting condition is necessary.