For combined heat and power unit

water storage tank is proved to be effective in decoupling heat and power

in improving the unit's operation flexibility and new energy accommodation. In this paper

studies are conducted in a combined heat and power plant with two typical units of low pressure cylinder zero output and holing extraction. Firstly

operation optimization along with the heat net load is conducted which shows a mismatch between the lowest electric output case and the maximum ancillary services revenue case. Secondly

a water storage tank is added to the system

and the maximum capacity and peak regulation ability are studied in detailed

the characteristics between the maximum capacity

peak regulation and heat net load are obtained. Then

the structural parameters of the water tank are considered in designing the capacity. The tank number along with capacity is recommended. Lastly

varying water tank capacities are tested. The maximum heat net load of middle cold period is suitable for the water tank capacity design

and the lower tank capacity means the better investment income ratio.