Buffer cylinders are made in many shapes and sizes. They are predominantly used to store heat, and therefore lessen the impact of certain technologies starting to work.
Typically heat pumps use buffer cylinders when linked to heating systems. This means that when the heating switches on there is already a store of hot water to minimise the impact of all the heating demand.
A correctly sized buffer cylinder will stop a heat pump from buffering – constantly switching on and off, and therefore significantly increase the efficiency.