How are refrigeration coils defrosted?

Since refrigeration coils are colder than air-conditioning systems, there needs to be a system for defrosting the frozen condensation. There are 3 general ways to defrost the coils 1). air defrost which is just a shut down of the unit while the coil defrosts 2). electric defrost which is electric heaters attached to the coils 3). hot-gas defrost which is provided by the compressor discharging refrigerant vapor which passes through water-storage tank heating coils.