Let's say you pick s7-1215 or s7-1217 PLC and use only local I/O.  If 8 analog input modules are added, with 8 AI per module, that's only 64 AI of local analog inputs.

It really depends on how fast your data needs to be.  Remote analog IO _can_ be faster than local IO, especially for S7-1200 systems.  Choosing the right features and options, like which SCADA software will work best for this Rankine cycle, how fast the IO needs to be for proper (efficient, effective, and reliable) control, and how quickly the hardware configuration and logic and system integration can be accomplished... all play a part.

If it were me, I would probably recommend (or select) an s7-1500 based system.  After it was functional, I would then know if an s7-1200 could slip in and replace it for production use (or rolling out to end-users or interested integrators).

It is much easier to know "it can be done" and then go do it rather than, I don't know if it can be done, but I'll give it a go and see how close I can get to the target.

I would not recommend an s7-300/400 based control system system Siemens is slowly phasing those PLCs out in favor of S7-1200/1500 PLCs.

engco

I have research project which consists of 

Solar heating system which heat a water up to 50 degrees then I will feed this hot water to a boiler to get steam. This steam will run a turbine with generator to generate electricity. This output steam of the turbine will be fed into water desalination system to get pure water of it.

I will measure around 67 sensing points (pressure-temperature-flow-conductivity) and voltage , current of the generator.  And I will control solenoid valves for steam and water.I didn't calculat digital input/output yet. There will be scada system

I want your experience to help me to choose the right plc for this application.

Will s71200 can do it or I have to upgrade to s7300-400?

If you already have a design worked out, know the required update rates for all the I/O, including IO device selection and connection methods, you can better estimate whether an s7-1200 can do the job or not.  With s7-300/400, you would be designing with products already approaching their retirement years.

With s7-1500, you select a mid-to low-end S7-1500 PLC.  When it is all working, you can manually set the scan time to some value, like 1msc, 10ms, or 100ms as examples.  If the process runs fine with a 100ms scan time, an s7-1200 can handle the project, but it will need remote IO, possibly even "smart" remote IO (maybe multiple S7-1200 plc's acting as I-Devices).

If you set it to 1ms and it starts to run poorly, you can bump the PLC to the next higher level s7-1500 and it will run well at 1ms scan time.

With s7-1500, you get a true profinet master.   With S7-1200, you get something that can act as a profinet master, but cannot do all functions a PN master controller can do.

To me, it's a matter of risk/reward.  Save a small fraction of overall project costs on a less capable PLC, spend more time coercing things to act right, and risk the entire project failing.  Or, spend a small fraction more with the knowledge that the base hardware design is fully capable of making it work, or with a very minor PLC swap (upgrade) to get a faster processor and be running with much more ease.

Plus, there are quite a few more security options, connectivity options, Siemens hosted example projects, and more supported hardware options like ISO / IRT (for very high speed and synchronous data exchange).

If you are not getting paid to make this prototype and your time is considered free or donated, then sure, select an s7-1200 and shoehorn everything into it and make it work.  I've posted many times in these forums on some techniques useful for that type of project.  I just know that time-to-market is generally more valuable, and my personal opinion is s7-1500 will get the project up and running faster.

You did ask!

engco

You don't recommend s7_300,400?!! But as I know it is used for large applications. I still didn't get why you recommend s7-1200 or1500. Please elaborate more.

And how many remote IO could be added?

And why s7 1500over 1200?

I thought this was a research project?  Please do some.

S7-1500 is more expensive for the hardware.  For the reduced development time, it will be less expensive than if a s7-1200 were selected and forced to work.

That said, if you plan on making 100,000 of them, make the design parametric so the main controller can be replaced with relative ease.  Use of remote IO that works well with both S7-1200/1500 is ideal to support that since there will not be much local IO.

I recommend finding the TIA Selection Tool (online or downloadable package) to create several different hardware configurations.  The tool enables you to find your answers very quickly by selecting a PLC, and seeing what remote IO options exist.  Save the project, create new one, select different PLC, and see what remote IO options exist.  Compare projects / options, and see what common hardware would also meet the requirements for the project.

For example, the desalination system is to me a "standalone" system with some intercommunication between surrounding systems.  Having it standalone would allow water making to happen even if other parts are offline for maintenance.  After all, once the steam reaches the condenser, does the desalination system really care if the solar panels are online?

engco

if i compare s71200 vs s7 1500 for remote IO, what is the maximum remote  IO for both of them?

and what is the price? which one is more expensive?

Hopefully after you use the online TIA Selection Tool a few times building example projects, generating BOM lists and retrieving associated module and system documentation and manuals requests, I think you will quickly start to realize what the solution should look and feel like.

It starts with a good set of specifications.  If you do not have one, then for sure, the s7-1500 is the right solution.  You can specify a s7-1513 or s7-1515 (leaving enough room on the mounting rail for a larger PLC if required).

Once you have several BOMs from the Selection Tool, you can start "digitizing" the system of  integrated processes, using the PLCsim software and HMI simulator to visualize and test all the pieces, making sure they all will come together properly.

You might want to review some recent digitization webinars Siemens put on over the last year or so as you start importing BOMs into TIA portal projects, simulating and visualizing them.

If you want to go the next step, use PLCsim Advanced (latest version) so you can simulate the most types of devices and connection types in advance of actually ordering, building, interconnecting, and commissioning.

During the simulations, you can establish whether the selected s7-1500 is too much or too little PLC for the project _before_ purchasing.

If you use regular PLCsim basic for S7-1200, you can try the same types of things, but the simulation will be quite basic.  You would be better off visiting a local Siemens distributor's "lab" and use real hardware rather than a simulator - or purchase a starter kit with HMI (and VFD if they also offer that!).

engco

if i compare s71200 vs s7 1500 for remote IO, what is the maximum remote  IO for both of them?

and what is the price? which one is more expensive?

also time is a concern and i have to make the right choice for the plc. I am worried  that s7-1200 can't handle such large sensing readings.