Welcome, Sputnik
What you read about is a good idea, and storing energy in phase change materials has been mentioned here before. Using a large tank of water for heat storage (though without a phase change usually) is called a
heat accumulator or heat buffer and is very helpful in increasing a house's efficiency. I believe that member
osolemio has a large water tank under his house for seasonal heat storage.
But as Daox mentioned, proper insulation should be the first step, with a much better cost-to-return ratio than any other investment. Once you're insulated, your heat pump, or any other project, can be downsized.
Have fun digging through the info here