Temperature as Quality Measure of Energy.

In ideal gas dynamics temperature appears as an intensive variable $T$ connected to internal energy $e$ and density $\rho$ by 

• $T=\frac{e}{\rho}$                          

with a corresponding pressure law 

• $p=\gamma e$

where $\gamma$ is a gas constant. Internal energy is viewed as small scale kinetic energy from small scale molecular motion. Internal energy can transformed into mechanical work in expansion, which without external forcing (or gravitation) is an irreversible process.  

For a solid body viewed as a vibrating atomic lattice temperature scales with total internal energy as the sum of small scale kinetic energy and potential energy, which can be transferred by radiation and conduction to a body of lower temperature.   

In both cases temperature appears as a quality measure of internal energy as an intensive variable. 

The maximal efficiency of a Carnot heat engine transforming heat energy into work operating between two temperatures $T_{hot}>T_{cold}$ is equal to $1-\frac{T_{cold}}{T_{hot}}$. 

Radiative heat transfer form a hot body of temperature $T_{hot}$ to a cold body of temperature $T_{cold}$, scales with $(T_{hot}^4-T_{cold}^4)$ according to Stephan-Boltzmann-Planck. 

Conductive heat transfer scales with $(T_{hot}-T_{cold})$ according to Fourier.

In both cases the heat transfer from hot to cold can be seen as transformation from high quality energy into low quality energy in an irreversible process in conformity with the 2nd Law of Thermodynamics. 

The Nobel Prize in Physics in 2008 was awarded to experimental detection of Cosmic Microwave Background CMB radiation with perfect Planck spectrum as an after-glow of a Bing Bang with temperature of  2.725 K and corresponding very low quality energy.  

With radiation scaling with $T^4$ the difference between 300 K as global temperature and 3 K as deep space CMB comes out with a factor of $10^{-8}$. The contribution to global warming from CMB thus appears to be very small. 

We see from $e=\rho T$ that low density and low temperature both connect to low energy quality making both wind and solar energy inefficient compared to fossil and nuclear energy.