Energy destruction or net power?

[micdib]

Hi,all. I'm an italian student writing my Bachelor's degree thesis. It's about rankine cycle coupled with csp solar.

I'm doing a few simulation using matlab+coolprop for the supercrtitical cycle. Using Toluene i get more net power and better thermodynamic efficiency but i'm not taking the most out of my solar source as the hot oil leaves the exhanger with a relatively high temperature. Using cyclohexane i get a sligthly lower net power but a much better matching in the heat exchanger beetween the 2 fluids. in both cases the pinch point is about 9K.

I'm guessing the power plant might have a better annual yield by taking the most out of the source even when the solar field is not at its design condition. But that would take too much effort to demonstrate and i'd like to know which is the best way to go in your opinion. Thanks a lot

 

[DEvens]

OK, I don't really understand some of your post. What do you mean by "a much better matching in the heat exchanger between the 2 fluids"? And what is a "pinch point"? And what does it mean that it's 9K?

Anyway. I'm guessing you have some sort of solar collector as the heat source to heat your working fluid. And that you intend to run some sort of turbine off the pressure created. Presumably you intend some sort of concentration of solar flux through mirrors or such, since you won't get much heat otherwise. You should also be thinking about things like re-heaters. They make a huge difference.

The efficiency of such a system will depend strongly on the net energy flux you can get on the heating side. Which will in turn depend on things like the area of your collection array, the efficiency of absorbing, cloud cover, day length, lattitude, and a bunch of other stuff I'm not thinking of just off.

To get something like a vaguely realistic picture, you should be taking these sorts of thing into account. Perhaps you can do analysis on the basis of net heat flux, number of hours per day at that flux, presumed shape of the heat flux curve (as the sun moves from horizon to horizon) and such. It probably isn't useful to try to model cloud shadows in any detail. Rather you want some sort of indication of thermal inertia in your heating side. So, what happens to your system if you get a 20 percent dip in solar flux for 10 minutes? Or what if cloud cover makes a day have 70 percent full flux for the entire day? And how does a 50 percent larger heating tank (for more thermal inertia) affect your startup in the morning?

Once you have some sort of handle on those, then you can try each one with different combos of working fluid. You might get that one working fluid was best below some lattitude, another at higher. That might be very interesting. Or you might get that a larger heating tank was better in some situations, smaller in others. Again, possibly interesting.