I always wondered if, instead of using solar panels in the deserts, we could use very long and black pipes running water, heated by the sun. Then the heat is moved to the ground for storage, and once there is enough heat, we use a turbine to generate electricity.

I don't think you ever get enough of a temperature difference just by having a passive black pipe in the sun, to do any useful work besides potentially keeping someone warm in the winter. You could do useful work if you concentrate the energy from the sun somehow, like with mirrors.

Heat pumps do magic by changing the pressure at which a working fluid changes phase, so you can boil the fluid over here, have it absorb an enormous amount of energy then compress it back to a fluid elsewhere and push that heat back out -- this works pretty well because you're just moving the heat and only pushing the temperature on the "hot" side up a relatively small amount. I don't think, for instance, you could make an oven with heat pumps.

To do useful work you need a _substantial_ energy gradient -- it's hard to live in the sun even though its got lots of free energy floating around. The sun is very useful to the earth because the energy it provides is so much more energetic than the ambient environment.

Edited to add:

There are discussions of using exotic working fluids like compressed CO2 -- that'd allow you to manage the phase change maybe to a region where you could concentrate the energy in the fluid then expand it elsewhere at "room temperature" temperatures -- but I think things like compressed (to a _fluid_) CO2 are really hard to work with.

It's done a bunch, all over the place: https://en.wikipedia.org/wiki/Solar_thermal_energy. Most commonly people use them either for batch pre-heating of hot water or for full heating. I looked in to getting something like that for the hydronic heat in my house because currently it's propane-fueled and the deliveries are quite expensive in the winter.

>the heat is moved to the ground for storage

..or into a giant sand-filled barge which transports the thermal energy up north for the winter.