The water source heat pump uses water (or some of other liquid) as the medium for heat rejection or heat absorption.
Instead of using an outdoor fan/coil in combination, a water/refrigerant heat exchanger (like the one pictured below) is used with a pump.
Liquid, usually water or glycol, is pumped into the heat exchanger. Heat is then absorbed (cooling mode) or rejected (heating mode) into/from this liquid.
In most geothermal systems, the water or glycol is then pumped through the geothermal loop where the heat is again, rejected or absorbed into/ from the ground.
So the ground (with the water flowing through it) is the "source" from which heat is absorbed (heating mode) or rejected (cooling mode)....
.....thus the expression water "source" heat pump.
The refrigeration system of a water source system is conceptually the same as an air source system.
The biggest difference is in the brazed plate heat exchanger pictured above.
Below is a water source heat pump schematic when in cooling mode.
As the air inside the building is cooled, water or glycol is pumped from a heat sink (medium for heat rejection) into the heat exchanger.
Heat is absorbed by the water from the hot refrigerant gas coming from the compressor. The water then flows to the heat sink for heat rejection.
During heating mode, water or glycol is pumped from the heat source (medium for heat absorption) to the heat exchanger.
At this point, heat is rejected from the water/glycol into the refrigerant. The heat moves through the refrigeration cycle before it is rejected into the air of the building.
Once heat is rejected from the water/glycol loop, it returns to the heat source (often a geothermal ground loop) to absorb more heat.
This process is shown in the diagram below.
There are many manufacturers for water source heat pumps. Air source heat pumps are more common....so there are more manufacturers for air source than water source. Below is a list of some common manufacturers:
Johnson Controls heat pump (also known as York)
Trane heat pump
Carrier heat pump
Lennox heat pump
Bryant heat pump
Mitsubishi heat pump
Daikan McQuay heat pump
Mammoth heat pump
Climate Master heat pump
FHP heat pump
Bosch heat pump
The Whalen Company heat pump
Aquacal pool heat pump
HH Systems heat pump
The Trane water source heat pump is one of the most popular on the market. It is very well known and is well-known by most water source HVAC technicians.