Hello again Joe,
My background is as a mechanical engineer doing projects for private companies and also for several government agencies.. including the US Dept of Energy...several projects in the multi million dollar range. Nation wide. I will not be blowing smoke at you here.
You do not need any engineers approval... that said no engineer I know would go to the trouble of heating sand to 1,000F in order to heat a green house to 80F ... that would be like building a howitzer that shoots 10 inch shells, at very great expense, and using it to shoot flies.
Fly swatters are already on the market, and a lot cheaper.
Specific heat is not related to how many tons the material is involved... it is the amount of heat required to raise one pound of the given material 1 degree F. . that is one of several confusions that is throwing monkey wrenches into your thought process here... there are several others that have gotten you off on the wrong foot so to speak.
These physics need to be understood, and without that a person can get lost in the tall weeds.
In the context of 'just answer', that level of extensive training is not a fit for our low cost format....If you want to get involved in this sort of work, a junior college course in 'Heat transfer and thermodynamics' would get you off a more solid footing. (the course has some math pre requisites, algebra 1a and 2a)...and it is affordable. Amazon has books on the topic, including the economics of heat transfer also valuable on projects such as what you have in mind.
You are correct about phase changing salt however... the phase change stores about 9 times more heat per cubic foot than water, and 80 times more heat than sand.
All that said, I want to assure you, and you can tell your potential investors that you can indeed use 1,000 F sand as a heat storage medium to heat green houses... but you and they will need to know for sure, that the costs will be 10 times higher than current methods in wide use by growers world wide.
(solar heat to 200F with thermal storage in insulated underground pools is cost effective. co-generation produces power for the grow lights, carbon dioxide for fertilizer, and the waste heat to warm the building).
I will opt out, so that others here can see your question, but I will keep tabs on the progress and you can post to my attention and I will pick the thread up again, but only if you agree to at least rate my first hours work here positively.