# Question #6d701

Because of big oil companies don't want you to.

#### Explanation:

Sorry, no grand physics explanation, thermal energy is much more efficient than burning fossil fuels. Money and political power is the only thing affecting that from happening.

Nov 27, 2017

Second Law of Thermodynamics comes in way of utilising this resource. Efficiency of conversion is so low as to not justify the cost spent in building machineries for this purpose.

#### Explanation:

Of the various forms of energies, Mechanical energy and Electrical energy are of high quality whereas Thermal energy is of low quality.

While the Energy Conservation Principle (First Law of Thermodynamics) allows one to convert energy from one form to another, the Second Law of Thermodynamics places constraints on the efficiency of this conversion, especially from low quality forms to high quality forms.

The machines that convert thermal energy into mechanical/electrical energies are called Heat Engines. Heat Engines draw thermal energy from a heat reservoir at high temperature (${T}_{h}$), convert a fraction of it to mechanical energy and dump the remaining into a sink at lower temperature (${T}_{c}$). The Heat Engine with the maximum possible efficiency allowed by the laws of physics is called the Carnot Engine.

Second Law of thermodynamics clearly rules out a 100% conversion. The efficiency of the Carnot Engine is related to the temperatures of the heat reservoir (${T}_{h}$) and the heat sink (${T}_{c}$) as follows:
$\setminus {\eta}_{\text{carnot}} = 1 - {T}_{c} / {T}_{h} = \frac{\setminus \Delta T}{T} _ h$ ...... (1)
Looking at this equation it is clear that a 100% efficient engine would require either a heat reservoir of infinite temperature (${T}_{h} = \setminus \infty$) or a heat sink of zero kelvin (${T}_{c} = 0$ $K$ - ruled out by the third law of thermodynamics).

It is fair to ask why 100% why not be happy with what you get. Conventional heat engines (automobile engines and conventional nuclear plant turbines) have efficiencies in the range of 25% to 35%. Here the temperature difference between the heat source and heat sink is of the order of $1000$ $K$.

Now if you consider ocean as huge reservoir of thermal energy, the temperature difference between cold ocean bottom and hot ocean surface is of the order of few kelvins and the surface temperature is only about ${27}^{o}$ $C = 300$ $K$. This gives a Carnot efficiency of less than 1%. Now remember that Carnot Engines are ideal engines with theoretical maximum efficiency. Real engines will have efficiencies less than this. So it makes no sense to build heat engines of this poor efficiency. Power required to make and maintain these engines will be of of the same order as the power delivered by them.

$$        SORRY about overshooting the word limits.