Compact and flexible thermal storage

June 6, 2012
These zeolite pellets can bind steam within their pores, generating heat. Credit: Fraunhofer IGB

Biogas plants, combined heat and power plants don't just generate electricity, they also produce heat. However, unlike the electricity they yield, the heat generally dissipates unused. A new technology is set to change this: It will allow the heat to be stored lossfree in the smallest of spaces for lengthy periods of time, for use as and when required.

There's a growing trend towards from biogas. But these systems would be considerably more effective if better use could be made of the that is produced in the process. Roughly half of the total of the fuel is released as heat, which typically dissipates into the atmosphere unused. Large quantities of heat likewise escape from combined heat and power plants, not to mention many industrial installations. The root of the problem lies in the fact that the heat is not generally used at the time it is generated – and options for storing it are limited. Traditionally, water tanks have been used for this purpose, but they can only absorb a finite quantity of heat. And of course, the heat can only be stored for short periods of time, because although the water tanks are insulated, the water gradually loses its heat to the surrounding atmosphere.

Working together with industrial partners such as ZeoSys GmbH in Berlin, scientists from the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB in Stuttgart are currently developing a new type of thermal storage system. This new system can store three to four times the amount of heat that water can, so it only requires storage containers around a quarter the size of water tanks. Moreover, it is able to store the heat loss-free over lengthy periods of time and can even operate at temperatures well in excess of 100 degrees Celsius. The new system contains zeolite pellets, from the Greek zeō, meaning 'boil' and lithos, meaning 'stone'. Normally this material is used as an ion exchanger, for example to soften water. Because zeolites are porous, they have a huge surface area: A single gram of these pellets boasts a surface area of up to 1000 square meters. When the material comes into contact with water vapor, it binds the steam within its pores by means of a physicochemical reaction, which generates heat. The water is in reverse removed from the material by the application of heat and the energy is stored, but not as a result of the material becoming palpably warm – as when water tanks are used. What is stored is the potential to adsorb water and in the process release heat; the term 'sorptive thermal storage' is frequently used to describe these systems. And provided the dried zeolite material is prevented from coming into contact with water, it can store the heat for an unlimited amount of time.

Mobile test facility with a storage volume of 750 liters

Although the basic principle has been widely understood for some time, it had never before been translated into a broad-based technical application for storage systems. "We took the principle and confirmed it was technically feasible," says Mike Blicker, group manager, heat and sorption systems in the IGB. Initially, the researchers used a 1.5- and then a 15-liter reactor to demonstrate that the process really does work. Blicker explains: "First we developed the process engineering, then we looked around to see how we could physically implement the thermal storage principle – i.e. how a storage device has to be constructed, and at which locations heat exchangers, pumps and valves are needed." The institute's development partners were responsible for the material testing side of the project, investigating which of the various zeolites would be best suited for the purpose, how big the zeolite pellets needed to be, and whether or not the material would remain stable even after numerous storage cycles. They proved that heat could be stored and discharged many thousands of times without the system showing significant signs of wear and tear. The researchers subsequently up-scaled their operations to the current test facility, which has a storage volume of 750 liters and is mounted in a transportable container, along with all the additional equipment it requires. Its mobility allows the scientists to test the system in a variety of locations under realistic conditions.

The next stage of their work will be to reduce production costs, further optimize the system and adapt it for a variety of applications. Ultimately, the goal is to be able to store heat both in industrial installations and in small combined heat and such as those used in larger residential buildings. To start with, priority will be given to industrial applications. "It would be ideal if we were able to devise a modular system that would allow us to construct each storage device to suit the individual requirement," says Blicker. The Fraunhofer researchers will be using a model system to demonstrate the principles of sorptive thermal storage at ACHEMA 2012 in Frankfurt from June 18 through 22 (Hall 9.2, Booth D64).

Explore further: High water uptake capacity of mesoporous material ideal for use in heat transformation applications

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2.6 / 5 (5) Jun 06, 2012
THIS SOUNDS AWESOME. adsorbtion is a very powerful natural phenomena and usually it has been used industrially for gas , liquid and other types of separation. the novelty of using HEAT to force out adsorbed water at high temperature and then to reverse the process for extracting the heat sounds intuitively like it has great potential. ultimately it sounds like youre heating up dessicant that is specially designed not to DRY things, but store heat from a process.

it's as if you're not using the heat to dry out the dessicant, but you are using water to force out the physiochemically stored heat.

i like it.
1.8 / 5 (5) Jun 06, 2012
But how high a temperature do you need to drive out the water?

Because in a typical waste heat scenario, increasing the temperature of the outlet directly reduces the efficiency of the process. For something like a steam power turbine, the cooling system is well below 100 degrees C because it has to make the water condense at below atmospheric pressure.

That's why power and heat co-generation plants produce proportionally less electricity and more heat. The heat output is taken from the middle of the power turbine as 120 degrees steam, which then heats the pressurized water in the district heating pipe. The water has to be that hot in order to be hot enough as it reaches the last building of the loop. Otherwise you'd have to pump much more water around much faster, which would just use more energy and lead to bigger losses anyways.
2 / 5 (4) Jun 06, 2012
Also, as an estimate, water can store about 0.1 kWh of energy per kilogram from about room temperature to boiling, so if this thing stores four times as much heat, you'll still need quite a lot of it to store any appreciable amount of energy.

In comparison to things like heating oil for a house, you'd need 30 times as much to equal a single refill of the tank, so, instead of filling the tank 2-3 times a year, you'd be filling it every week, and sometimes twice a week. Even woodchips contain 8½ times more energy.

That's a big logistics problem right there. It's not really a product you can charge up and then deliver to a customer, because the delivery takes more energy than it's worth.
1 / 5 (3) Jun 06, 2012
in a typical waste heat scenario, increasing the temperature of the outlet directly reduces the efficiency of the process.
I cannot understand, why such a comment gets downvoted just at PhysOrg. Apparently most of physics wannabes here never heard of thermodynamics and Rankine cycle. They're only lead with single instinct like the bacteria: if someone doubts the results of new research, he must be a crackpot for sure...
1 / 5 (2) Jun 06, 2012
I cannot understand, why such a comment gets downvoted at PhysOrg. Apparently most of physics wannabes here never heard of Carnot cycle.

There's a nickname "kaasinees" who systematically downvotes nearly all of my posts for some reason or another. I've also spotted others like "Proton" who have no comments anywhere, and the account is simply used to downvote people.
1 / 5 (2) Jun 06, 2012
There's a nickname "kaasinees" who systematically downvotes nearly all of my posts for some reason or another.
Because he's a sociopath, who is known here under nicks Frank Herbert and thousands of another ones. He spends whole his free time with silent downvoting of posts here only. The bacteria is too noble denomination for such a personality.
5 / 5 (2) Jun 06, 2012
You know, if it weren't the Fraunhofer Institute behind this I would have suspected a hoax/scam upon reading it. I agree with the first poster. This sounds incredibly exciting.
1 / 5 (2) Jun 12, 2012
I agree with the first poster. This sounds incredibly exciting.

But why? Dealing with water, it obviously needs relatively high temperatures to dry it, which means it's not suitable for most waste heat scenarios where the output temperature is low, and being not very energy dense it seems pointless to transport it anywhere, so it's not a method of heat distribution either.

Maybe I'm being unimaginative, but how exactly do you propose to use it, and what for?

If I imagine a fuel cell that operates on gas to make electricity for the house, and a barrel of zeolite to store the waste heat in the summer to use it in the winter when the heat is needed, it still runs into the same problem as with boiling water - there's not enough of it. It's just four times denser in energy, while it needs to be 40 times denser to be practical.
5 / 5 (1) Jun 12, 2012
Dealing with water, it obviously needs relatively high temperatures to dry it

You could put the entire system under low pressure which would reduce the temperature needed - possibly to the point where a modest heat source would suffice.

it seems pointless to transport it anywhere

Agreed. This would probably be more of a 'home storage solution'. Possibly also for car batteries or fuel cells which require certain temperatures for startup/good performance. This would solve the problem such systems (which usually use insulation to retain generated during opration heat) have when they are left unused for a long time.

and a barrel of zeolite to store the waste heat in the summer to use it in the winter when the heat is needed

I'd see it more as a day/night climate control type of thing. Maybe could also be used on spacecraft to even out extremes?

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