There are already many instances where surplus energy is stranded and there is no market for it. This is where Bitcoin mining is often viable. But even Bitcoin mining needs power to be available most of the time to justify the investment in mining rig. So still there are cases where surplus electricity is available but there's no demand for it at certain times. Hydro dams here in NZ have been overflowing lately due to heavy rain and limited demand. The same occurs with most forms of generation except that built specifically for backup when there is a supply shortfall and that is usually fossil fueled generation from gas or coal.