The theory behind this kind of heat storage is fairly straightforward: if you pour water into a beaker containing solid or concentrated sodium hydroxide (NaOH), the mixture heats up. The dilution is exothermic where chemical energy is released in the form of heat. Moreover, the sodium hydroxide solution is highly hygroscopic and able to absorb water vapor. The condensation heat obtained as a result warms up the sodium hydroxide solution even more.
Heat exchangers from instant water heaters provided the solution: the sodium hydroxide solution. Spirals along a pipe, absorbs water vapor and emits heat. Image Credit: Empa. Click image for the largest view.
We are still far from a sustainable energy supply. In Switzerland during 2014, 71 percent of all privately-owned apartments and houses were heated with fossil fuels, and 60 percent of the hot water consumed in private households is generated in the same way. In other words, a considerable amount of fossil energy could be saved if we were able to store heat from sunny summer days until winter time and retrieve it at the flick of a switch.
Materials capable of storing heat include those such as bricks or concrete that slowly release the stored heat, and others such as water or ethylene glycol that take in heat when they transform from a solid to a liquid. However, none of these materials can store heat energy over a long period as they simply naturally release it slowly over time. A material that could store heat energy for a long time and release it at the exact timing desired would be a boon for the field of renewable energy.
Is there a way to do this? It certainly looks like the Swiss have it. Since autumn of 2016, following several years of research, Empa has a plant on a lab scale in operation that works reliably and is able to store heat for the long term. But the road to get there was long and winding.
When heat energy is fed into a dilute sodium hydroxide solution in the form of heat, the water evaporates, the sodium hydroxide solution will get more concentrated and thus stores the supplied energy. This solution can be kept for months and even years, or transported in tanks. If it comes into contact with water (vapor) again, the stored heat is re-released.
That’s how the theory works. But could the beaker experiment be replicated on a scale capable of storing enough energy for a single-family household? The Empa researchers Robert Weber and Benjamin Fumey rolled up their sleeves and got down to work. They used an insulated sea container as an experimental laboratory on Empa’s campus in Dübendorf – a safety precaution as concentrated sodium hydroxide solution is highly corrosive. If the system were to spring a leak, it would be preferable for the aggressive liquid to slosh through the container instead of Empa’s laboratory building.
Unfortunately, the first prototype didn’t work as anticipated. The researchers had opted for a falling film evaporator – a system used in the food industry to condense liquids, most commonly, orange juice into a concentrate. Instead of flowing correctly around the heat exchanger, however, the thick sodium hydroxide solution formed large drops. It absorbed too little water vapor and the amount of heat that was transferred remained too low.
Then Fumey had a brainstorm. The viscous storage medium should trickle along a pipe in a spiral, absorb water vapor on the way and transfer the generated heat to the pipe. The reverse – charging the medium – should also be possible using the same technique, only the other way round. The idea worked. And the best thing about it is the spiral-shaped heat exchangers are already available from existing inventory as the heat exchangers from flow water heaters.
Fumey then optimized the lab system further by asking which fluctuations in NaOH concentration are optimal for efficiency? Which temperatures should the inflowing and outflowing water have? Water vapor at a temperature of five to ten degrees C is required to drain the store. The water vapor can be produced with heat from a geothermal probe, for example.
During the heat recovery process a 50-percent sodium hydroxide solution runs down the outside of the spiral heat exchanger pipe and is thinned to 30 percent in the steam atmosphere. The water inside the pipe heats up to around 50º Celsius – which is just about right for radiant floor heating.
To recharge the heat inventory, the 30-percent, “discharged” sodium hydroxide solution again trickles down around the spiral heat exchanger pipe. But now inside the pipe flows 60º C hot water, which could be produced by a solar collector. Then the water from the sodium hydroxide solution evaporates; the water vapor is removed and condensed. The condensation heat is conducted back into a geothermal probe, where it is stored. The sodium hydroxide solution that leaves the heat exchanger after recharging is concentrated back to 50 percent again, i.e. “charged” with thermal energy.
Fumey said, “This method enables solar energy to be stored in the form of chemical energy from the summer until the wintertime, and that’s not all. The stored heat can also be transported elsewhere in the form of concentrated sodium hydroxide solution, which makes it flexible to use.”
The search for industrial partners to help build a compact household system on the basis of the Empa lab model has now begun. The next prototype of the sodium hydroxide storage system could then be used in Switzerland’s NEST building innovation project.
There have been many attempts at heat storage over the years, most all too vulnerable in heat loss to justify the investment, Here the Swiss have a fully static storage. This concept might well get to market. There are many questions yet to ask and answer, but this looks very good indeed.