Heat Pump Powered Thermal Water Softener

Fresh water as supplied to homes, business etc. varies in its composition widely across the globe. Where I grew up, the water is extremely soft (~25mg/litre) due to a combination of near constant rain and favorable flora & geology. Conversely, the south east of England has very hard water (~300 mg/l) probably because they deserve it. The situation is similar in most of the USA. Such water made "hard" by the presence of dissolved minerals, largely calcium carbonate, creates numerous problems. Soaps & detergents form calcium salts with hard water meaning much more has to be used. Calcium carbonates also precipitate out when heated, meaning any device that heats water is susceptible to "scaling". Calcium carbonate precipitating out due to heat can be seen in numerous thermal springs around the world. Conversely coastlines in the very northerly/southerly latitudes don't have sandy beaches since the cool waters dissolve away all the calcium carbonate sea shell fragments.

The standard solution to this problem is a water softener. This works by passing the water through an ion-exchange resin. The surface of this material is covered with negatively charged regions, occupied by sodium (Na+) ions. When dissolved calcium (Ca2+) passes through the resin, Ca2+ binds to the resin, displacing Na+ ions which move into the water. Eventually, the resin is saturated with Calcium and needs to be "recharged" by temporarily soaking the resin in high-concentration sodium chloride (NaCl) solution to knock all the Ca2+ ions off to be discarded.

This system works, but isn't perfect since it consumes salt and inevitably makes the water a little salty. To solve it, let's use heat. Our water softener is a two-chambered device, one chamber will be heated and one cooled by a heat pump. Pick your favorite type. Water from the standard domestic supply flows into chamber 1 where it is heated. Calcium carbonate is ~4x less soluble at 50C vs 15C <link>. So, in these conditions it will slowly begin to precipitate out. The chamber should be large enough to have enough dwell time for this to happen. The water then passes through a filter. This can be a simple particulate filter, or perhaps somewhat more sophisticated continuous flow device e.g. radial flow separator. The point is, solid calcium carbonate is removed.

The water then flows into a second chamber where it is cooled to normal temperatures by the heat pump, this recovers most of the energy used to heat it in the first place. The water then goes into the conventional water supply distribution of the house etc. with a much lower hardness.