Answer to Question 1
In a direct expansion system, the primary refrigerant evaporates and expands, absorbing both sensible and latent heat. In contrast, a secondary refrigerant does not evaporate and therefore only absorbs sensible heat, which makes the temperature difference and surface area requirements greater than those needed for a direct expansion system.
Answer to Question 2
The water acts like a secondary refrigerant, absorbing heat from the soil and transferring it to the refrigerant in the heat pump.