The copper plate is about a million times heavier than the 48 microscopic cooling sandwiches and the aluminum heat-sinking elements extending from them.
Applying a voltage to the sandwiches sends hot electrons charging through the device, a trip that involves quantum tunneling through the insulated middle. Only the hottest electrons can make the one-way trip through this normally impermeable barrier. The process removes heat from the normal metal layer, which in turn drains heat from the copper plate. The removed heat is then deposited in the on-chip aluminum heat sinks.