Revisiting the Mareekh Process: Part 2 - The Exergy

Mareekh Process is our patented hybrid power generation concept through In-Situ Resource Utilization of Martian subsurface water ice for power generation augmenting auxiliary sources such as solar and limited nuclear.

Figure 1; The layout of the Mareekh Process from the patent document. Minor improvements include the redefining of 19, the router pump to MVR pump for condensate recovery and latent heat extraction.

The core concept is to bring the subsurface ice to the surface in the form of superheated water using auxiliary power and exposing it to Martian surface conditions and run a steam turbine through its explosive expansion. In the Mareekh Process, it is done through an intricate process of forming superheated water and supercritical water, evading the heat penalty of vaporization by avoiding boiling of water in any of the energy input processes. 

Our thermodynamics calculations suggest that the total output energy per kg conversion of subsurface ice into surface steam is higher than the energy input through auxiliary sources. The Mareekh Process achieves this through a very intricate process and clever use of thermodynamics principles and characteristics of water, steam and vacuum.

While much of the output is in the form of latent heat, our calculations suggest that the work output fraction of the total energy output is still > 60% of the energy input which is significantly higher than the work output from the solar or nuclear auxiliary alone. The latent heat generated is also utilizable in the cold environment on Mars for habitat internal heating, instead of using a fraction of the output work for this purpose. Nothing goes wasted on Mars, not even latent heat.

This brings to a very interesting discussion regarding the apparent paradox of getting higher output energy in the Mareekh Process than the input. We understand this is due to the entropy imbalance between the subsurface ice - a relic of Martian wet past and thicker atmosphere with higher pressure - and the present day cold and very thin atmosphere on the surface of Mars.

We need a robust thermodynamic model to explain this using deep understanding of the thermodynamics of entropy and can leave that for a later discussion.

Why do we find the prospect of running a steam turbine using superheated steam from recovered subsurface water so interesting? The core reason is the amount of work the steam can do which is much higher than what can be obtained through the use of the same superheated steam on Earth. 

The answer lies in the exergy potential of the steam. And this exergy potential is not only due to the inherent property of the superheated steam of particular parameters (temperature and pressure) but the attributes of Martian atmosphere vs Earth's atmosphere.

The Definition of Exergy

Exergy is the maximum useful work that can be extracted from a system as it comes into equilibrium with its surrounding environment.

Formally:

Exergy = the portion of energy that is available to do useful work, given a defined reference environment.

Equilibrium is reached when adiabatically expanding steam reaches its saturation temperature as per the atmospheric pressure where no further expansion can occur. On Earth where this point is reached at 1 bar pressure with steam temperature at 100C, continuing further through non-adiabatic expansion rejecting latent heat and finally condensing into water. On Mars, this process of adiabatic expansion will continue beyond 1 bar till it equilibrates with the Martian atmospheric pressure of ~ 0.006bar. This expansion continues adiabatically and generates potential work much further beyond the equilibrium point that is reached on Earth. Same superheated steam with same amount of input energy for its formation, enthalpy and entropy, but a different outcome on Mars as compared to that on Earth. Martian atmosphere imparts a much higher exergy potential to the superheated steam, resulting in a much larger amount of work output that is possible.

This is the core principle behind the Mareekh Process

The thermodynamic calculations in the previous articles to explain the principles and working of the Mareekh Process were based around generation of 1.5kg/second of superheated steam at 195C and 2bar at the inlet of the high pressure end of the steam turbine, with

• venting of 0.1kg/second to the exterior into the thin and cold Martian atmosphere at 0.006bar and temperature of -50c or below, and

• recovering the rest of the cooled steam or condensate through MVR and

• releasing the resultant latent heat into the interior of the Craterhabs for heating up of the Craterhab environment to liveable temperatures.
  

In real life scenarios, the actual superheated steam flow rate may be tens of kilograms per second, resulting in tens of megawatts of power, and nearly double the amount of latent heat. 

Both on Earth and Mars, creating steam from water (or ice as on Mars) will require an energy input, such as the use of fossil fuel. Creating a certain amount of steam with a set temperature and pressure will in principle require the same amount of energy on Earth and Mars. However, releasing them into the atmosphere of the respective planets will have different outcomes. On Earth, superheated steam for power generation is produced at a very high temperature and pressure. On Mars, superheated steam of relatively much lower temperatures will be sufficient to run a steam turbine which is relatively easily achievable and will still run a turbine with a very deep sink to expand into, and expand adiabatically for much longer until it reaches an equilibrium. In other words, the same steam will generate much more work on Mars than it can do on Earth due to its enhanced work potential or exergy on Mars.

Let's compare the direct exergy potential of superheated steam on Earth vs Mars…..

Imagine a cloud of superheated steam at 195C under 2 bar pressure in its initial state. On Earth in the standard atmospheric pressure (1 bar) and temperatures (e.g.15c), it will be subject to a rapid expansion and exert work on its surroundings (running a turbine or pushing a piston). It will continue to expand until it reaches an equilibrium with its surroundings (1bar, 15c). The adiabatic expansion will cease when it reaches 100c and 1bar pressure, and from that point onwards, it will simply condense into saturated vapour <100c and continue to cool off forming water and exchanging latent heat into the surroundings until reaching a condensate temperature of 15c. No useful work can be done from beyond the point of reaching its saturation temperature and pressure in the Earth’s atmosphere (100c, 1bar).

Let’s calculate the exergy potential of this scenario:

e_x ​= ( h − h₀ ​) − T₀ ​( s − s₀​ )

Where:

• h = enthalpy of the system

• s = entropy of the system

• h₀ ,s₀​ = properties at environmental reference state

• T₀​ = environmental temperature

Parameters: Superheated steam

Mass: 1kgTemperature: 195CPressure: 2bar or 200kPa

• h = 2861kJ/kg

• s = 7.4867 kJ/Kg-K

• 
  

Scenario 1: Earth

Figure 2: Amazon Rainforest. Bolivian Andes. Picture courtesy: M. Akbar Hussain

Ambient pressure: 1bar or 100kPa, Ambient temperature: 15C

• h₀  = 63.1kJ/kg

• s₀​ = 0.22445 kJ/Kg-K

• T₀ = 288.15K

Calculating exergy potential:

e_x​ = ( h − h₀ ​) −T₀​ (s − s₀ ​)

e_x​ = ( 2861 − 63.1​) − 288.15 ​( 7.4867 − 0.224​45)

e_x​ = (2797​) − 288.15 x 7.26225 

e_x​ = 2797 − 2092.6

e_x​ = 705kJ

Scenario 2: Mars

Figure 3: An AI generated picture of the Martian surface with thin, cold, and dusty Martian atmosphere.

Ambient pressure: 0.006bar or 0.6kPa. Ambient temperature: ~ -63C or 210K

At Martian temperatures, the dead state of water is best represented as ice.

The enthalpy of ice at these temperature should factor in the latent heat of fusion of ice (in the reverse direction) at (-) 334kJ/Kg and the energy loss with cooling factoring in the specific heat of ice from 0 to -63K (2.05kJ/kg to 1.68kJ/kg; avg. 1.865kJ/kg)

• h₀  = 0 - 334 - (1.865 x 63) = - 452kJ/kg

• s₀​ = - 1.71 kJ/Kg-K

• T₀ = 210K

  
  

Calculating exergy potential:

e_x ​= ( h − h₀ ​) − T₀​ ( s − s₀​ )

e_x​ = [2861 − (- 452​)] − 210 [7.4867 − (- 1.71)]

e_x​ = 3313 − 1931.3 

e_x​ = 2797 − 2092.6

e_x ​= 1381kJ/kg

Result

• Earth exergy: ≈ 705 kJ/kg

• Mars exergy: ≈ 1381 kJ/kg

  
  

Difference:

Δe_x ≈ 1381 − 705 ≈ 676 kJ/kg

Figure 4: A comparison of exergy potential of superheated steam on Earth vs Mars

So the same 195 °C, 2 bar steam has roughly 0.68 MJ/kg more maximum theoretical useful work potential on Mars than on Earth, under these chosen reference conditions.

Since our previous calculations revolved around the energy dynamics of forming 1.5kg of superheated steam at 195C under 2bar pressure, with an input energy as low as 2,710kJ (through a clever and intricate set of steps of Mareekh Process, evading the heat penalty of steam formation in the heat input steps and the trough the use of Supercritical water), these figures can be multiplied by a factor of 1.5x

• Earth exergy: ≈ 705 kJ/kg x 1.5 = 1057kJ/kg

• Mars exergy: ≈ 1381 kJ/kg x 1.5 = 2072kJ/kg

• 
  

Difference:

Δe_x ≈ 2072 − 1057 ≈ 1015 kJ/kg

It should be noted that the exergy or potential work output on Mars is

2072 / 2710 = 0.76

or at a whopping 76% efficiency, against a more moderate or realistic 1051/2710 = 0.38 or 38% on Earth. This percentage on Earth will be even lower, possibly around 20% or less, as the Mareekh Process cannot be replicated on Earth due to the need for flash steam generation in the LPFS under 0.6bar pressure, creating which on Earth will be an energy intensive process. 

One thing to note is that on Earth, once expanding steam reaches saturation near 100 °C at 1 bar, the remaining thermodynamic availability shifts from pressure-work potential to lower-grade thermal exergy associated with condensation and cooling toward ambient. This does not eliminate exergy, but it substantially reduces the fraction that can be converted into additional mechanical work.

On Mars, this doesn’t happen until the steam reaches a saturation point of near zero celsius, while the expansion work continues to that point. Below this point, steam starts turning into ice-vapour while continuing to release the latent heat with low grade thermal exergy, until it reaches an equilibrium at the ambient temperature of -60c or below.

In actual Mareekh Process, we suggest recovery of the saturated steam at 7C at 0.01bar pressure inside the low-pressure turbine housing and condensing it under pressure using MVR pump to release large amount of latent heat through that condensation process and use this heat to warm up the living spaces on Mars such as the Craterhab interior. This changes the calculations but the basic principle of exergy difference and work output remains more or less the same.

Figure 5: A schematic diagram of the Mareekh Process showing a breakdown of the energy input at various steps. Negative sign is used to depict energy going IN the system.

Figure 6: Energy output diagram of the Mareekh Process. A large amount of work output and latent heat can be extracted through the Mareekh Process, with total net energy output exceeding the net input energy, due to rapid release of unbalanced entropy.

Figure 7: A clever use of released latent or waste heat from the Mareekh Process, to warm up the interior of a Craterhab,

Figure 8: A pie chart of the comparison of input and output energies of the Mareekh Process

Conclusion:

Earth and Mars are two vastly different thermodynamic environments. Superheated steam would act very differently on Earth and Mars. The low atmospheric pressure and temperature of the Martian atmosphere acts as a very deep heat sink, giving a huge thermodynamic advantage to superheated steam by giving it a large exergy potential and ability to generate work much more than in Earth's atmosphere. Mareekh Process harnesses this potential through the use the vast subsurface deposits of water-ice on Mars to generate large amount of power for a power-hungry Martian colony on a planet virtually devoid of any reliable sources of energy harness-able at an industrial scale.