Thermal batteries: the next big step in passive house design.

Just as a regular battery stores electrical energy, a thermal battery stores heat. By capturing solar energy in a thermal battery, a passive solar house can compensate for night time heat loss, greatly increasing the percentage of its heating that is provided by the sun.

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

'Thermal battery' is a relatively unknown term in the building industry at the moment, but with increasing attention being paid to passive solar home design, that could soon change. It is also a term that can sound more complicated than it is.

A jug of water sitting in a sunny corner of your house would by definition be considered a thermal battery. It captures heat during times of abundance (in the midday sun for example) and releases it as the air around it cools.

One of the key elements to successful passive solar home design is taking advantage of thermal mass to store heat and regulate temperatures. Thermal mass of any kind could by definition be called a thermal battery, as is stores heat. That simply means dense materials like bricks, masonry and concrete.

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 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 solar panels.

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

Compared to many other solid materials, water has one of the greatest capacities to store heat and is an excellent convection coefficient as well, making it an ideal substance for thermal storage.

Heat capacity specifics:

Another way of storing energy is using the latent heat of fusion of a material. For instance, 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


q = energy [J]

m = mass of material [kg]

Cp = heat capacity of the material [kJ/(kg·K)]

ΔT = temperature difference [K or °C]

Thermal battery diagrams are courtesy of Alternative Photonics.

Thermal battery diagram
Thermal battery diagram courtesy of