=energy =thermodynamic cycles
A simple Brayton cycle, which is what gas turbines in aircraft use, has the following steps:
- compress
gas
- heat the gas
- expand the gas
Heating the gas increases its volume, and thus expansion generates more power than compression uses. But the compression is still using a lot of power, and it has some inefficiency.
A Rankine cycle avoids compressing gas. With water, the steps are:
- heat water
- expand the resulting steam
- condense the steam to water
- pump the
water to a high pressure
This compresses less volume, and
liquid pumps are more efficient, avoiding some inefficiency. But
thermodynamically, the energy added by gas compression in a Brayton cycle
must come from somewhere. In a Rankine cycle, that energy comes from
increased heat capacity of the liquid relative to the gas, and from the
enthalpy of vaporization. As pressure and boiling point increase, more of
the enthalpy of vaporization is shifted to heat capacity, up to the critical
point where it's been completely shifted.
If using a heat exchanger
to preheat water, there is thus a mismatch between the heat capacity of
steam and water.
Supercritical CO2 cycles are
somewhere between a Brayton and Rankine cycle. CO2 near its
critical point
can be compressed a lot (~2x) with a small pressure increase. This reduces
total compression work and thus improves efficiency, but also implies a
larger heat capacity mismatch between the high-pressure and low-pressure
streams.
Compensating for that heat capacity mismatch is the basis of
supercritical CO2 recompression cycles. Basically, you split the flow from a
heat exchanger so that one side of it has more mass flow than the other,
which better balances the stream heat capacities.
Here's a paper with some diagrams of that.
As best I can tell,
this configuration was designed in the 1970s for NASA in an "Energy
Conversion Alternatives Study" and then gained interest more recently at
"International sCO2 Power Cycles Symposium" conferences. Chinese papers on
variants of supercritical CO2 recompression cycles oriented towards nuclear
power have been appearing, which is a reflection of Chinese government
interest in CO2 cooled nuclear reactors.
From this perspective, that
principle can obviously be applied in some other ways. For example, one
could do a recompression cycle with water, only condensing some of the
low-pressure steam, to avoid heat capacity mismatch between steam and water
in a heat exchanger. But that doesn't make economic sense, because the
turbines for such low-density gas are expensive.
Organic Rankine
cycles avoid the problem of low pressure at the cold end by using fluids
with a lower boiling point. Perhaps the idea of a recompression organic
Rankine cycle - like a recompression CO2 cycle, but replacing the cooler
with a condenser and the primary compressor with a liquid pump, and using an
organic working fluid instead of CO2 - is new; I'm not sure. If it is new,
the patent office might even consider that patentable, and it doesn't meet
my standards for patentability, so maybe it's good that I'm posting
it here. Well, obviously I have some thoughts of my own on new variations of
thermodynamic cycles, but I won't bore you with the details.
Because
heat capacity of fluids is variable and nonlinear, when using energy from a
stream of hot fluid with an organic Rankine cycle, it's possible to improve
efficiency by using a zeotropic mixture instead of a pure fluid, because the
heat capacity of the working fluid can be better matched to that of the heat
source. This has only been recognized relatively recently, so optimization
of such mixtures is still ongoing now.
Besides heat capacity mismatch,
there's another issue with using supercritical CO2 instead of a conventional
Brayton cycle: pressure. To get increased efficiency, the compression near
the critical point must be done first, to reduce volume compressed early on.
So, the pressure must be around 73 bar before compression, and the CO2 might
be compressed to 200 bar. That's a relatively high pressure, which reduces
turbine costs but increases some other costs. The optimal pressure from an
economic perspective is generally lower than that. But this is...acceptable;
the overall costs of supercritical CO2 systems are certainly lower than
those of systems using low-pressure steam turbines.
The relevant
properties of CO2 aren't magical or all unique. For example, ethane has a
lower critical pressure than CO2, and a similar critical temperature. So,
supercritical ethane has been considered for solar thermal power. Obviously,
hydrocarbons have worse stability than CO2 at high temperatures and inside
nuclear reactors, but for some applications ethane is fine. It can
potentially give slightly
better efficiency than CO2 in some circumstances.