Why can't heat energy be easily converted back into another form of energy? Excellent question. This belatedly introduces the first law of thermodynamics. Talk about being late to the party.
The first law of thermodynamics states that energy can be transferred from one form to another, but it can never be created or destroyed. All right, then how can heat energy be easily turned into another useful form of energy? It can't, really. As we mentioned earlier, not all types of energy are created equal. In fact, it is for this reason that we worry about depleting our fossil fuels; once they're gone, they're gone. No do-overs or takebacks.
Think of heat energy as basically the lowest guy on the energy ladder. This disordered form of energy cannot be easily converted back into another type of usable energy. Heat is at the bottom and has no promotions in his future. He's like the Milton Waddams of energy.
It is important to remember that although the first law of thermodynamics says that energy cannot be created or destroyed, this law applies to the total sum of energy in the universe. Which is a heck of a lot. Since a system is one piece of the universe, the change in energy in a system can be either positive, negative, or zero.
This concept can be written as a handy formula: ΔE = Q - W, where the change in energy (ΔE; delta signs, or Δ's, mean "change in") in a system equals the heat energy (Q) minus the work energy (W). If heat energy is lost to the surroundings, like when fossil fuel is burned, the Q of the system will be negative. If the energy content of the fuel is transferred to create work as well, the total change in energy (ΔE) will be negative and equal to the amount of work energy (W) done plus the amount of energy lost as heat (Q).
In summary, then, the first law of thermodynamics states that if a system loses energy, it has lost heat to the surroundings, or done work. In this way, energy is not created or destroyed in the universe as a whole. The catch is that all forms of energy are not equal, so the amount of useful energy in the universe can still be decreasing. Hey, we warned you that the universe has a pretty remarkable track record when it comes to winning these standoffs. (100% is pretty remarkable, right?)
Useful energy comes in many different forms. You probably know that you use the energy found in food to convert it to useful work energy in your body. However, other organisms, like bacteria, are able to use energy found in all sorts of things that would be toxic to us, like hydrogen sulfide, uranium, iron oxides, and of course, light.