In the methods listed above, effectiveness is principally equated in terms of the total exergy content of an experimental TES, with comparison to an ideal and/or mixed system. Although this is useful when assessing the overall performance of a complete simulation, the indices are not particularly suited to highlight not only the effect of thermal mixing within a TES, but more importantly, clearly indicate at which point turbulent mixing effects become negligible, if at all. They also offer a varying degree of applicability for different TES simulations. Panthalookaran’s method, for instance, does not seem to apply easily to variable temperature inflow conditions. [7]

In support of the existing indices, a new characterization index has been derived and is proposed in eq. (4). The index, deemed the Exergy Charge Response (nCR), examines the rate at which the exergy stored within a real TES increases, and compares it to that of an ideal tank. The index is derived principally from eq. (3) above, which describes the exergy stored within a TES at a given time.

The applicability of the Exergy Charge Response is limited by the understanding that, given sufficient energy input with high inlet temperatures, a fully-stratified and a fully-mixed tank will eventually reach similar exergy levels:

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J£stored, strat(t) £stored, real (t) stored, mix (t) if! iS Estored (t) > 0 (6)

0 0 0

It is therefore possible that at some point towards the end of a simulation with high energy availability, n^CR must become greater than 100%. This will only occur, however, when the energy response efficiency of the tank, tfER, begins to decrease from 100%. Therefore, so long as (nER > 0), hcR will be affected primarily by internal mixing, particularly at the early stages of charging.

The particular advantage of the Exergy Charge Response applied to TES simulations is the ability to effectively monitor the transient relationship between the thermocline and the fluid motion within the TES itself. It is clear that when п^ск ~ 1, the inlet plume is not thermally mixing the TES, and oppositely so when ncR ~ 1.