The technology of the combined heat and power(CHP) improves the energy utilization efficiency of units. However

this technology also restricts the adjustability of the power generation

limits the flexibility of the power system

hinders the introduction of large-scale renewable energy

and causes serious curtailment of wind power. Taking the total coal consumption of the system as the main reference

this study discusses the influence of CHP on the overall energy efficiency of the system

and compares the effects of four kinds of flexibility retrofit schemes: heat storage by electric heating

electric heat pump heating

heat storage by CHP unit

and extraction heating by high/medium pressure bypass of turbine. The results show that whether CHP improves the overall energy utilization efficiency of the energy system depends on the supply and demand condition of power and heat in the system. At the present

a high proportion of the CHP units

which are operated in the "heat-led" mode

have limited the overall energy utilization efficiency of the system. Extraction heating scheme by high/medium pressure bypass of turbine has less limitations and can improve the adjustability of the CHP unit effectively

which is the optimal flexibility retrofit scheme.