Just a thought but why not use the waste water/brine from the RODI unit as your cold reservoir? Direct this to a large insulated container (trash can, tote, old tank) and you have quite a large heat sink that would probably stay cold enough for quite some time if the system is only used when required.
I get drinking water from mine too, but I'd think that given the typical waste ratio on an RODI unit there'd be plenty to maintain an effective heat sink.
If you are using tap water as the heat sink, I'd adjust the flow so that your waste flow from your heat exchanger is the same temperature as the sump water, that way you know you've pumped as much heat into it as possible..
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