Solar heat storage in ICF walls?
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If you dump the solar heat into the concrete between the EPS layers in a hydronic solar approach you'll be losing more of that heat to the outdoors than would be going into heating the space. The whole "storage" concept isn't really applicable, given the ratio of insulation area to thermal mass you have in an ICF, compared to that of a well insulated tank fully inside of conditioned space. You'll get some performance improvement, but not nearly what you'd expect for the effort & expense.
Thermal air panels (TAPs) mounted to or integrated into the south facing walls would deliver far more seasonal heat for a lower installed cost. It's not too tough to use snap-disc thermostats on the collector's heat exchanger metal to turn on blowers when the heat exchanger plate hits 110F and have it turn off when it drops to 90F, and put a line-voltage thermostat in series with the snap disc to keep it from overheating the place. With TAPs there are no freeze-up or leak issues to be concerned with too. In sunny high-altitude CO you should be able to offset at least 2-3 therms of natural gas (or 2+gallons of oil, or 3+ gallons of propane) per square foot of collector area per heating season with a decently designed active-blower TAP. If that's a couple hundred square feet of collector area it could knock off a a quarter to a third of your total heating fuel use. Additional gains from snow-reflected sun make vertical mounted collectors far more useful at altitude than in lower-dryer locations, and has the effect of less output when it's snowless/warmer out, which is generally what you would want.
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