Heat capture and concentration.

Ok, we all know how difficult it is to capture heat from low temperature and low density sources (Solar, Geothermal, industrial waste heat) efficiently.

My idea is that instead of using water as a heat transfer medium, which will cool and cannot be stored for a long time or be concentrated use a chemical reaction instead.

For instance, heat is transferred to a gas circulating through pipes, radiator style.

This gas then decomposes into different components, having absorbed energy. These components are then separated and removed to prevent back reaction, accumulated, concentrated and then re reacted in some sort of power plant (possibly a fuel cell, or combustion). The gas is then collected and returned to the heat source.

My first thought was ammonia, NH3 splits into N2 and H2 after absorbing energy, they are then recombined to release energy. However, the energy barriers in both directions are too high.

My second thought was N202, which splits to form NO2, quite easily. However, as there is only one product, there would be no way to prevent back reaction, the mixture would essentially 'cool', losing energy.

My third, carbon dioxide decomposes into carbon monoxide and oxygen; the carbon monoxide could be stored and burnt. Once again, co2 is far too stable to decompose when exposed to these low density heat sources.

So I turn to the halfbakery community to help me find a gas (or alternative), which will readily absorb energy and decompose into heterogeneous products which can be separated easily. Preferably, it should be cheap and non toxic too...