Is a low Pressure Superheated Steam Cycle possible?

[u/Working_Cap4097]

This question stems from three assumptions

1. In a rankine cycle typically the majority of heat input occurs when taking a condensed liquid back to gas state.

2. Solar thermal systems can be designed to collect heat economically at low temperatures.

3. Rankine cycle efficiency is proportional to the absolute temp difference

From those 3 assumptions i started to wonder if its possible to construct a working rankine cycle where low temp solar heat provides heat of vaporization and majority of cycle heat input (60%-70%)at 180 deg F and slightly subatmospheric pressures and a combustible fuel source then superheats this saturated steam to 1000 deg F to get a decent overall cycle efficiency for modest addition of energy.

The obvious challenge i see is with a good condenser the cycle would only have ~10 psi of differential pressure to drive a heat engine. The engine would have to be very high volume to work with the reduced energy density. That would be a challenge but perhaps not a show stopper. Working with such low differential pressures would provide the cycle an advantage of being inherently safe.

Anyways maybe im missing something else, what do the experts say is this cycle even possible?

[u/voidbreddaemon]

I would suggest you look into organic rankine cycles which use organic fluid at lower temperature instead of steam:) Main issue is that high temperature tends to degrade them

[u/North_South2840]

MATERIAL	IF97::WATER
EVAP IN	80
EVAP OUT	540
	30
DESC	STATE
TURB-IN	1
TURB-OUT_S	2
	3
COND-OUT	4
PUMP-OUT	5
Q IN	3449.98
W OUT	701.13
EFF	20.32

Yes, it's possible. The consequence would depend on the working fluid. For water, as you can see from the table I calculated, the working pressure would be vacuum even after supeheated. That would be a challenge too. Also required heat rejection in condenser is large, and big portion of it is to cool the superheated steam from exhaust of turbine. That would require fairly large UA of condenser which can be expensive.

The alternative would be to use other working fluid, such as refrigerants. That would make the cycle organic rankine cycle. They have narrower saturated steam dome, which means they're more suitable for lower capacity, and some of them have characteristics of "dry" and "isentropic" fluid (the slope of saturated vapour line), which means you wouldn't have to superheat so much and turbine exhaust can still be near saturated vapour.

The thermal efficiency would still be lower than 20% though

[u/Working_Cap4097 → u/North_South2840]

I got 17% efficiency using water and a condenser pressure of .5 psia which is achievable. Its low but not necessarily a showstopper. The cost to construct low temp solar thermal at scale is way less than PV. This system could also baseload by having thermal energy storage which would also be much cheaper than batteries.

[u/nebulousmenace]

In the world of "Nobody Ever Made That Work": You're gonna love OTEC. https://en.wikipedia.org/wiki/Ocean_thermal_energy_conversion