I did some research and found out that one of the applications of a heat pump is as a water heater which means that it could achieve a temperature of about 100-degree Celsius. Will changing the refrigerant or the compression ratio of the compressor lead to an increase in the temperature? (R718 refrigerant showing to reach 200-degree Celcius)
If yes what is the maximum temperature that could be achieved? and also will this affect its efficiency?
Even in gas household water heaters if you don't use a pressure relief valve the temperature will go above 100 °C rapidly if you bypass the thermostat. But it will explode!
If you have a water tank designed to take the required pressure you don't need any other refrigerant, just heat it up like a pressure cooker. Many cars' radiators circulate water above 100 °C for advantage of a higher temperature gradient hence more effective cooling in the radiator.
As a guide, the saturated vapor pressure at 121 °C is 200 kPa, 150 °C is 470 kPa, and 200 °C is 1,550 kPa.
This is not an answer but I couldn't fit it all in a comment.
First of all it will depend very much on the heat pump design. To my understanding, heat pumps are designed with a specific refrigerant in mind and with a specific range of operating temperatures. Changing the refrigerant you'd need to modify a lot of parameters (flow, exchange areas etc).
Theoretically, there is no reason why you can't do it. But you'd probably need to use a multistage heat pump system to raise the temperature in different stages and different coolants, in order to reach the desired temp. The main problem in this approach is that there will be the losses at different stages, and so the overall efficiency will drop significantly.
I don't know of a product that does what you are asking, but -to be honest- that doesn't really mean much.
A heat pump or refrigeration device is simply a machine designed to move heat from cold to hot (heat moves by itself only from hot to cold). All refrigeration equipment I am aware of uses the latent heat of fusion/vaporization to do this, as the temperature of vaporization/condensation changes with pressure. The range of temperature available will be set by the range of pressure increase we can induce via the compressor. What you will find is that the slopes of the saturation temperature and pressure curves slope up, so that getting higher temperatures require ever increasing pressures. This energy input will reduce the system's performance until we can no longer efficiently build a machine to do what we want.
Theoretically, we could do this at any temperature short of creating plasma in the gas phase. Aluminum has a melting and vaporization point, and quite a high heat of vaporization, which I assume changes with pressure. So we could design a heat pump that moved heat in the thousands of degrees range. The only thing preventing this is the materials required to both move the refrigerant and present it in a way to allows heat to transfer both to and from it (and in the case of aluminum keeping it from ever solidifying). I think that what you will find is that in the temperatures required for your example (100+ C) that conventional refrigerants will not work, and that the high temperatures in general will lead to a large heat loss throughout the machine.
