Supercritical CO2: Unexplored source of electricity

Summary

CO2 is more energy-dense which is its advantage. CO2 is about twice as dense as steam in its supercritical condition. All advantages put together result in a smaller plant footprint and could potentially result in lower capital costs. Typically, a steam turbine contains ten to fifteen stages of rotors. An analogous supercritical turbine would have four - that is the summary

The low critical temperature and pressure of supercritical CO2 are distinctive. Due to its lower specific volume at the critical point than steam and significantly smaller turbine footprint, supercritical CO2 has a significant advantage over steam. At the critical point, CO2 is denser than steam by 63%. CO2 is more energy-dense than other working fluids due to its high density and volumetric heat capacity, which enables the size of the majority of system components, including the turbine and pump, to be significantly reduced. This results in a smaller plant footprint and could potentially result in lower capital costs.

An important point

CO2 power cycles for waste heat recovery have drawn more interest in recent years. Due to carbon dioxide's relatively low critical point, such cycles are often run in trans-critical or supercritical conditions. These key benefits can be summed up as follows. First off, unlike the steam-based Rankine cycle, which is only effective at recovering waste heat at high temperatures, CO2 cycles are ideal for recovering waste heat from sources with a wide range of temperatures. Additionally, the Steam Rankine Cycle, which has a large working fluid steam volume and so exhibits considerable potential for system downsizing and light-weighting, takes up less space than the CO2 power cycles.

Definitions

Critical point: It is the endpoint of a phase equilibrium curve. It is the endpoint of the pressure-temperature curve in the phase diagram when a liquid and its vapor can coexist. At the critical point, defined by a critical temperature and a critical pressure, phase boundaries disappear. At higher temperatures than the critical temperature, the gas cannot be liquefied by pressure alone.

Supercritical fluid: A supercritical fluid (SCF) is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist.

Detail

The difference between CO2 and H2O

At the molecular lever level

At STP water [ molar mass 18 g/ mol] is liquid and CO2 is gas [STP is a temperature of 298.15 K (25 °C, 77 °F) and an absolute pressure of exactly 1 atm (101,325 Pa, 1.01325 bar)] Although carbon dioxide has a greater molecular mass than water, water is a liquid at room temperature while carbon dioxide is gas because of the strong intermolecular forces that hold water molecules together. Primarily, it is water's hydrogen bonds that keep it liquid at room temperature.

Boiling point

The normal boiling point of a liquid is the temperature at which its equilibrium vapor pressure, P, equals 1 atm. For water, this is 100 °C. For CO2, the liquid phase does not exist at 1 atm, so instead of boiling, solid carbon dioxide sublimes at a temperature of -78.5 degc.

At critical point

The image is a p-v diagram of carbon dioxide (CO2). Gas densities of interest are marked with dotted green lines. The critical temperature is marked with a solid yellow line.

Specific volume at the critical point

Water [374 °C, 22.1 MPa] is 0.003155 m3/kg.

CO2 [ (31.0 °C, 7.3773 MPa] is 0.002 m3/kg

At the critical point, CO2 is denser than water by 63%

The point worth noting is far higher specific volume of supercritical CO2.

What does it mean? It means a lot

Why does CO2 have a higher critical density than steam?

Explanation

A gas's density rises and its specific volume falls as pressure increases. The specific volume of CO2 [see above] is much reduced at the critical pressure of roughly one-third.

Carbon dioxide has a very high-power density because it is nearly twice [really 63 percent] as dense as steam in its supercritical stage from a thermodynamic standpoint. Because supercritical carbon dioxide is simpler to compress than steam, a generator can use a turbine to generate power at greater temperatures.

The end result is a turbine that is less complicated and up to ten times smaller than its steam equivalent. Typically, a steam turbine contains ten to fifteen stages of rotors. An analogous supercritical turbine would have four.

Supercritical turbines would also be a desirable improvement over steam systems on ships and submarines because they generate the same amount of power while taking up less space. These turbines would function well in locations suffering from drought since they use carbon dioxide instead of water as their working fluid.

A supercritical turbine could also be used in a cycle that is directly heated, in which a fuel like natural gas burns inside the turbine with only pure oxygen present, releasing only water and carbon dioxide as waste.

Final words

Just to be clear, CO2 has a comparatively low critical pressure of 7.4 megapascals (MPa) and a critical temperature of 31C. As a result, it is easily heated to a supercritical state before expanding and can be compressed directly to supercritical pressures. All advantages put together result in a smaller plant footprint and could potentially result in lower capital costs.

Process

Simple Closed-Loop Brayton Cycle

The working fluid (CO2) in a straightforward closed-loop Brayton cycle is heated indirectly from a heat source through a heat exchanger (much like steam would be heated in a typical boiler); energy is extracted from the CO2 as it is expanded in the turbine; the CO2 leaving the turbine is then cooled in a heat exchanger to the desired compressor inlet temperature; and after compression to the required pressure, the CO2 is sent back to the heater to complete the cycle. Maximum cycle efficiency is reportedly 34.5 percent at a turbine inlet temperature of 700C and a turbine exit pressure of 8.27 MPa.

Credit: Google