It is also more than twice as dense as water, so the amount needed to heat 100 liters of water with 53,3 K will only have 1/10th the volume of said water. So even if you only use a small area in your basement for energy storage, it should be enough to have hot baths all winter...
20kg of lye for a bath seems like a lot. That's 2000 kg just to get through the coldest 3 months. Multiply that by the number of people in the house. And then remember to add space for all the water you'll have to mix with the lye to extract the heat. And actually double the water space if you're going to keep it after recharging instead of dumping it.
2000kg of NaOH has a volume of less than one cubic meter. And I don't get for what reason I would have to store the water needed for the reaction instead of just connecting the device to whatever water supply the house uses.
A cubic meter per resident. For just 3 months of the years. And each unit is diluted 50% already.
As for the water, you're going to basically dump a bunch of water into this thing to generate heat. And when you're done, your NaOH will be more dilute and need a bigger storage vessel. So you need enough room to store all the water you'll ever add.
When you recharge, you'll produce a bunch of water, too. You either dump it or you store it for the next use. If you store it, then you double the amount of storage that you need for water because you have room premix and postmix.
You could use a cubic meter of solution and store the heat underground then use a heat pump to extract it. But that could be also done directly by heating the ground using excess solar. Lye seems better suited for daily storage - store during the day, extract at night. Mostly useful in the desert or during the spring/fall when day/nught temperature variations are high and solar irradiation is good enough.
