As you noted, thermoelectric conversion has low efficiency. Typically, it is between 2% and 10%. The low efficiency hurts the bottom line since the biggest expense are the heliostats. Heliostats are comprised of the mirrors, the support structure for the mirrors, and the tracking mechanism. If the efficiency is low, more heliostats are needed per kilowatt of generated capacity.

Furthermore, copper is expensive. While it has better thermal conductivity than aluminum, it is significantly more expensive. It is roughly 7500 per metric ton compared to about 2000 per metric ton for aluminum, but prices do fluctuate quite a bit. However, a thorough analysis would require us to determine the total amount of copper required, but building a tower with a solid copper metal core sounds like it would require a lot of copper. In addition, copper has a melting point of 1083 Celsius, but it degrades due to oxidation at temperatures lower than its melting point. Few metals can withstand oxidation and temperatures of 850 Celsius. I don't believe copper is one of these metals.