This equation is very useful when one wishes to compute by how much one should change Td, for the EWH operating with a given value of Wd, in order to change PEWH by a certain value in steady-state. The limit values for PEWH and Td are those shown in Table 3.
Another important aspect of the operation of an EWH for active power balancing is the amount of power it can drop or take under transient conditions. Since the EWH operates in an ON/OFF mode, its instantaneous power consumption is either rated or zero. This cannot be changed. However, one can during transient condition values for ton and toff significantly larger than those obtained for steady-state conditions.
Let’s consider first the case where the EWH should take additional load. From Fig. 1(a) one sees that TH increases almost linearly when the EWH is ON and ton is the time required for TH to increase by 2Δ, 5 °F in this study, when Td remains constant. As shown in (4), ton increases as Wd increases but it does not vary significantly with Wd since Q is the dominant element in the denominator of (4). If Td is suddenly increased by a value larger than the tolerance band (ΔTd>2Δ), the EWH will be turned ON immediately and remain ON until the value of TH increases by at least ΔTd. Based on this, one can estimate that the increase in ton during transient conditions, with respect to the previous value in steady state, for a given ΔTd on average, for TH = Td, as