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Name: Eddie
Status: Student
Grade: Other
Location: N/A
Country: United States
Date: November 2008


Question:
My dad is adding solar heating to our house and is going to build a storage tank to hold water that is heated by the solar. he has some copper coils in the tank that are hooked to the solar. my question is: is it better to fill this tank with just water or water and gravel? He thinks the stone would be able to hold more energy so he would be able to store more energy in the same amount of space.



Replies:
Hi Eddie,

The short answer is you should use all water.

The key concept here is 'heat capacity', which basically means some materials take more energy than others to raise their temperature. Water is not only cheap and easy to handle, but it also has very high heat capacity, so it is a great material for storing energy from your solar cells.

To compare rock and water for heat storage, you need to know the heat capacity and the density of the two materials. Comparing water to stone, it takes a little over 4kJ of energy to raise 1kg of water by 1deg C. In contrast, granite takes a little less than 1 kJ to raise 1 kg by 1 deg c. Granite is about 3 times more dense than water, so for a given volume, it still stores less than 75% of the energy of water.

The only other factor to consider is the temperatures involved. Water is only useful between 0 and 100C -- in other words, your solar cells cannot deliver heat energy above 100C (or you will boil your water) and you cannot store your tank below freezing (or the water tank or lines may freeze). In industrial settings, usually different fluids are used that can tolerate very high or very low temperatures, depending on the situation. For home use, water is probably the best, safest, and cheapest option.

Hope this helps,
Burr Zimmerman


If you look up the "specific heat" value for those materials (a quick Internet search will find it), you will see that water has a remarkably high specific heat, much higher than nearly anything. This means it can store more heat for a given volume and would be your best choice. Another factor to consider is that convection currents in water allow heat to move around more quickly than in a solid, which is another vote for water. In passive solar design (which is different than the *active* solar project you are speaking of), masonry (e.g., tile flooring) is often used to stabilize room temperatures (otherwise temperatures vary during the day). However, a number of people have used tanks or tubes of water because it performs much better than stone (but it is not as attractive, usually). For the same reasons, water would be your best choice.

Another technique you may read about is using "phase change materials" by using a material's "latent heat". When a substance changes phases, for example when water changes from liquid to gas, extra heat called "latent heat" is absorbed or released. If you use a material that changes from a solid to liquid in between the temperature of the solar collector and your room temperature, you could store more heat that way, not in the form of heating the material as much, but in the form of forcing the material to change from solid to liquid. So-called "Glauber's Salt" is a well known material that changes phase at around 90 degrees F. But for several practical reasons (cost, corrosiveness, etc), it is not commonly used in residential solar design. So again, I would recommend plain water for your storage.

Paul Bridges


Hi Eddie,

The ability to store heat is related to a material's Specific Heat Value, and the weight of the material. If a material has a specific heat of 1 in the metric system, that means that it would take 1 Joule of heat energy to raise the temperature of 1 kilogram of this particular material by 1° Celsius.

Similarly, it means that if 1 kg of this material cooled by 1° Celsius, it would liberate 1 Joule of heat.

So the best material for storing the most heat (or energy) would be one that has the highest Specific Heat value.

Stone has a typical Specific Heat value of about 800 Joules per kg per degree Celsius. Water, however has a value to over 4000. In other words, as one kg of gravel cools by 1°C, it would release 800 Joules of heat energy. But the same 1 kg of water cooling 1°C will release over 4000 Joules of heat energy. The end result is that a tank of water will store well over 4 times more heat than the same weight of gravel.

But there is something else to consider.... gravel weighs more than the same volume of water. A container when filled with gravel will weigh about 2.5 times more than if the same container were filled with water. So when a container that holds 1 kg of water were allowed to cool 1°C, it will release about 4000 Joules of heat (as explained above). But if you fill the same container with gravel it will weigh about 2.5 kg, so it will store (800J/kg x 2.5kg =) 2000 Joules of heat energy. This is still only about half the heat energy that the same volume of water will store.

So it appears that you are correct.... water will store more heat than the same volume of gravel. The main problems with using water is that the container must be corrosion resistant and not leak, the water must be treated to resist bacterial growth (just pour in a jug of bleach, but make sure the system can handle it), and the container should be sealed to prevent evaporation.

Regards,

Bob Wilson



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