A hybrid energy storage system (HESS) plays an important role in balancing the cost with the performance in terms of stabilizing the fluctuant power of wind farms and photovoltaic (PV) stations. To further bring down the cost and actually implement the dispatchability of wind/PV plants

there is a need to penetrate into the major factors that contribute to the cost of the any HESS. This paper first discusses hybrid energy storage systems

as well as chemical properties in different medium

deeming the ramp rate as one of the determinants that must be observed in the cost calculation. Then

a mathematical tool

Copula

is explained in details for the purpose of unscrambling the dependences between the power of wind and PV plants. To lower the cost

the basic rule for allocation of buffered power is also put forward

with the minimum energy capacities of the battery ESS(BESS) and the supercapacitor ESS(SC-ESS) simultaneously determined by integration. And the paper introduces the probability method to analyze how power and energy is compensated in certain confidence level. After that

two definitions of coefficients are set up

separately describing energy storage status and wind curtailment level. Finally

the paper gives a numerical example stemmed from real data acquired in wind farms and PV stations in Belgium. The conclusion presents that the cost of a hybrid energy storage system is greatly affected by ramp-rate and dependence between the power of wind farms and photovoltaic stations

in which dependence can easily be determined by Copulas.