Why steam is a more energy-dense material than air?

Steam is far superior to air on both counts, including internal energy and compressibility. Air’s much smaller enthalpy is a big limitation while delivering mechanical work.

There are two reasons

[1] small water [steam] molecules have smaller van der Walls forces than air molecules. The molecular weight of water is 18 g/mol vs air [mean] 28 g/mol. This moves steam molecules more rapidly and consequently a lower density than air i.e., higher specific volume than air and

[2] water molecules [ mol weight = 18 against air mol weight 28 g/mol are much smaller. That makes superheated steam more compressible than air [ emptier space/unit volume].

This implies that the specific volume of the steam is less than air during a compression process and higher than air in the expansion process.

Concept

Compressibility Z:

Z in thermodynamics is the compression factor or the gas deviation factor, which describes the deviation of a real gas from ideal gas behavior. It is simply defined as the ratio of the molar volume of a gas to the molar volume of an ideal gas at the same temperature and pressure.

Z = V actual / V ideal

Steam is more compressible

Image credit: Google

Superheated steam at 1500 kPa, has a specific volume of 2.75 m3/k mol and a compressibility factor (Z) of 0.95.

Thus, the compressibility of air z>1 and compressibility of superheated steam<1. In other words, under similar conditions, superheated steam is more compressible. Steam is the most compressible gas of all gases because water's stable form is liquid.

Steam always wants to go back to liquid. The specific volume of air at 100 bar/500dec is 0.023073 m3/kg while superheated steam specific volume is 0.032453 m3/kg which is one and a half times more than air. This suggests, that under similar temperatures and pressure, superheated steam does more work than steam.

Steam has more internal energy

Specific heat [ stored heat] of steam is twice of air. The internal energy of steam is 1.4 times of air. It is about 2.1 kJ/kg/K for superheated steam vs 1.00 kJ/kg/k for air.

Arising from the above two points, steam has higher specific enthalpy than air

Concept: Specific enthalpy

The specific enthalpy (H) of a substance is its enthalpy per unit mass. It equals the total enthalpy (H) divided by the total mass (m).

Note that enthalpy is the thermodynamic quantity equivalent to the total heat content of a system. The specific enthalpy is equal to the specific internal energy of the system plus the product of pressure and specific volume.

H = U + PV, U = Internal energy and PV is the capacity to do work

As explained, the fact that the specific heat of superheated steam is more than twice of air under identical conditions, and steam has higher internal energy. A very conservative estimate is superheated steam has internal energy of more than 1.4 times air. The second point as explained steam is more compressible than air, Z <1 at the same temperature and pressure gives the steam the ability to do more work. This explains why steam has more specific enthalpy or energy than air.

Enthalpy-Entropy diagram for air

Image credit: Google

The X-axis is entropy in kj/kg-k. Red lines are pressure lines in Mpa. The extreme right-side red line stands for 0.001 Mpa and the extreme left-side red line stands for 10 Mpa. Black bold almost horizontal lines are temperature lines. Two important observations are emerging from this diagram [1] at a constant temperature as pressure increases, enthalpy remains practically constant while entropy reduces, and [2] at constant pressure as temperature increases, the enthalpy, and entropy both increase. The most important observation is specific enthalpy of air 100 bar/500 degc is only = 795.9 kj/kg

Enthalpy-Entropy diagram for superheated steam

Image credit: Google

Y-axis is enthalpy expressed as kj/kg. The X-axis is entropy expressed as kj/kg-k. There are two sets of curved lines in the image. These are well explained what they stand for. The curves rounded upwards are temperature lines expressed as degc. The curves rounded downwards are pressure lines. The image suggests the following: [1] at a constant temperature when pressure increases the enthalpy reduces and also entropy reduces [2] At constant pressure when the temperature is increased enthalpy and entropy both increase. The most important observation is the specific enthalpy of superheated steam at 100 bar/500 degc = 3373.81 kj/kg.?

There is a huge difference in the enthalpy between superheated steam and air. Superheated steam has enthalpy at 100 bar/500 degc more than four times that of air.?Air cannot achieve superheated steam’s enthalpy even at 1700 degc and 100 bar.

So, air’s enthalpy is a big limitation while delivering mechanical work.