A little bit of trivia here that i'm mainly bringing to you due to having just done some stuff about it for uni.
The average Rankine cycle has a thermal efficiency of around 40%, with some places getting 45% from memory. (a rankine cycle is your standard powerplant: pump some water up to high pressure, heat it so it turns into steam, send it throuhg a turbine to generate work, and cool the stuff that comes out of the turbine to send back to the pump). An efficiency of 40% means that, if you put 100MW of heat into the water heating it, you'll get 40MW* of that out as electrical power from the turbine and 60MW will be released into the enviroment via the cooling tower.
*this of course assumes a perfect transfer of heat, which is never the case. However it means that 60% of the coal we burn is really just going into heating the air around powerplants. Now, if we built a similar powerplant with the sun reflected from mirrors to heat up water, we could be using 40% of so of the suns energy (again, assuming everything works perfectly, which it won't quite). Using sodium as the working fluid instead and storing it like in the OP won't actually increase the efficiency, but will just add to the ability to store energy, which is nice.
However, its limited to the mid 40's% A little bit of improvement may be able to be made over time, however, your always going to be stuck with efficiencies around that area. Photovoltic cells, on the other hand, are increasing in efficiency all the time, wiht the best being in the mid 30's% from memory. When a 50% efficient solar cell (and a decent battery system) becomes available for a fair price, it'll make plants like the one in the Op completely useless and inefficient.