What is the actual difference between a dc/dc converter with and without a transformer? I have heard a little bit about bidirectional converter.... Should the bidirectional converter be always with a transformer?
The usual purpose of a transformer is either isolation or achieving a large voltage ratio with good efficiency.
As long as you don't tell your requirements, your question can't be answered.
I am considering a battery charging application, where power can flow from the battery also. The input voltage is (3-4Volts from fuel cell/PV) and the output voltage of the DC/DC converter should 12V/24V. The power wattage of about 100W.
Also my query is - is there reactive power consumption in the DC/DC converter? If yes, what supplies it?
What should be the purpose of running the converter bidirectionally with a fuel cell/PV voltage source?
However, for the moderate voltage ratio, a usual synchronous converter (a halfbridge with an inductor) can be used bidirectionally.
Reactive power is an AC property. It's not defined for DC circuits. Input/Output AC currents, that don't contribute to power
transmission would be described in terms of "ripple".