I was thinking of a way to increase the energy density of a material. I also read that plasma has a high specific heat capacity than water therefore we can store the heat energy for an extended period of time.
Water has a specific heat capacity of 4,200Jkg-1K-1. To store 1Tera joules of energy in one kilogram we shall heat it to a temperature of one billion degrees centigrade.
The heat energy can be converted to electricity by using the pressure to turn an electric generator or using a thermoelectric generator.
How feasible is my idea?
If I have done the calculation correctly that's about 500 Kev per atom of, say, titanium. I don't see how you're going to store that much energy without using nuclear forces. And we have NO IDEA how to store heat as nuclear energy. But if we did have that much control over nuclear forces I am pretty confident that using them to store heat would not even be on the horizon. Sorry to be a pessimist on this one! (Maybe someone could check my calculation , ?)
The problem lies in the state changes of the matter, in function of the temperature, moreover, at a certain temperature (million degree) it becomes a plasma. Then there is no material that may contain the matter at a high temperature such, here can only be within a magnetic glass.
You can certainly store energy as heat but the temperatures must be those that can be contained in reasonably priced physical systems, and are typically in the hundreds of degrees celsius. Now, the only way to get useful work from a heat store is to use a heat engine with all the thermodynamic limits, Carnot cycles and so on, that these are subject to. Heat is the lowest grade energy of all and the hardest to turn efficiently into electricity or other useful work. You might do a lot better in a spacecraft because facing away from the sun the sink temperature of your engine is only about 4K and that should enable energy recovery to get well on the way to 100%. But on the ground the sink temperature is ambient and that's typically 300K or so.
In space propulsion the problem is not only stored energy but also available propellant. Hydrogen can be accelerated by thermal energy (and fission energy) without any conversion
On the other hand, huge amount of energy can be contained in 1 kg of "matter" or "antimatter" and an adeguate engine is required to get thrust.
Assuming that Emmanuel is talking about conventional propulsion, a rocket using stored thermal energy and conventional propulsion was proposed by Parker & Humble and analyzed by Gany. Some improvement was proposed by King. You can find the references in this note
G. Colasurdo, L. Casalino, “Optimal Performance of Enthalpy Rocket”, Journal of Propulsion and Power, Vol. 15, No. 3, 1999, pp. 485-488.
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