Thermal batteries: all about storing solar heat

Just as a regular battery stores electrical energy, a thermal battery stores heat. Solar heat can be collected, stored and distributed later as needed.

Thermal batteries for passive solar design
Thermal batteries are poised to be the next big thing in passive solar design ©

What is a thermal battery?

Thermal mass of any kind could by definition be called a thermal battery as it stores heat. In the context of a house, that means dense materials like bricks, masonry and concrete. Even a jug of water sitting in a sunny window could by definition be called a thermal battery as it captures and later releases heat from the sun.

A well-insulated concrete floor acts as a thermal battery as well; once you pump it full of heat it takes a long time to cool down (depending on the thickness), and it regulates temperatures during that time.

One practical use for getting the most from a radiant concrete floor as a thermal battery would be in areas with fluctuating energy costs - you can set your floor on a timer so it only comes on during low-rate hours (7pm to 7am in Ontario for example). During the twelve hours that it is off, it acts as a battery by slowly releasing the stored heat, and you avoid having to pay the higher rates during peak-hours. 

MIT Solar house
MIT Solar House via Wikimedia

As you move into the area of active heat-storage systems, one of the more common types of thermal battery (not that there are a lot of them) is a huge water tank buried in the ground that is heated by solar thermal panels.

Even this type of system is not new, the first house in the United States with an active solar heating system was built In 1939 on the MIT campus (Massachusetts Institute of Technology), and sat on top of a huge water reservoir that was heated by thermal solar panels.

MIT solar house thermal battery
MIT solar house thermal battery © Ecohome

Phase-change thermal batteries:

To get really serious about storing heat you can swap out water for a more beneficial phase-change material such as parrafin wax, meaing that it changes from a solid to a liquid and back. Water also changes between solid and liquid of course, but not in a functional way in the context of home heating. 

For wax to change from a solid to a liquid requires a much greater input of heat, so you can store several times more heat within the same volume of wax compared to water.

As you extract heat from your thermal battery (by pumping it into a radiant floor for example) it cools and changes back to a solid, and it has to dump a ton of heat to do so.

Water is obviously more accessible (and a bit cheaper we hear) than parrafin wax, so this is more about meeting demand within a limited space. You'd need three and a half tanks of water to store the same amount of heat as you could with one tank of wax. 

Applications for thermal batteries:

solar thermal battery
Thermal battery © Build-it-solar blog

If you had a heat-collecting solar panel (directly heating air or liquid rather than generating power with photovoltaics), you can use that to charge your thermal battery. Envision this - a large tank of wax (or water) that is warmed by heated coils from a solar collector. Through that same tank runs another coil that is extracting the heat to pump it through your radiant floor or whatever other heating distribution system you have. 

Specific Heat Capacity (AKA, going down the engineering rabbit hole):

If you take solid paraffin (heat capacity Cp = 2.5 kJ/kg·K and heat of fusion of 210 kJ/kg), let's say 1 kg, at room temperature, you will need 2.5 kJ (kilojoules) of heat to make the 1 kg block go from 20°C to 21°C. To make it go from 21°C to 22°C, you will also need 2.5 kJ (i.e. the same amount of energy). 

Paraffin melts at approximately 37°C. If it drops to 36°C, you will again only need 2.5 kJ  to bring it back to 37°C, but you would need 210 kJ (84 times as much) to go from 37 to 38°C.

This is because in order to melt, some chemical bonds in the solid lattice need to be broken, a process that requires extra energy. So overall, if a kg of paraffin is lying around at 20°C, you would need 252.5 kJ to bring it to 38°C.

One of the more common building materials with thermal mass benefits is concrete. In contrast to paraffin, 1 kg of concrete (Cp = 0.88 kJ/kg·K) would need 15.8 kJ to do the same. For water (Cp = 4.18 kJ/kg·K), the amount of energy required would be 75.2 kJ.

The amount of energy put in is the amount of energy STORED in a material, as this energy will later be released as the material cools back down to 20°C, or room temperature. While there are many materials that can be used in the application of heat storage, this is just a quick comparison of some of the more commonly available ones.

So to conclude, paraffin can store 16 times as much heat per kg as concrete, and  3.4 times as much as water. So while water may not be the best material to store heat, it certainly is the most affordably priced and easily accessible.

The Cp value referred to in the above text refers to the heat capacity of materials.
q = m Cp ΔT
where:
q = energy [J]
m = mass of material [kg]
Cp = heat capacity of the material [kJ/(kg·K)]
ΔT = temperature difference [K or °C]

Related topics:

Thermal battery diagrams are courtesy of Alternative Photonics.

Thermal battery diagram
Thermal battery diagram courtesy of alternative-photonics.com/