Latent heat coolant thermos comparison (paraffin wax vs water)
 

So, with the cooler weather today, I was thinking about heating mods. I was curious what advantage using paraffin wax in conjunction with coolant would do for increasing heat storage.

We probably all know how the 1st and 2nd gen Prius used a coolant thermos that was warmed up during driving, and then used when needed during cold starts. What if we added paraffin wax to the mix? How much more heat could we store with that setup vs just coolant?

The calculations aren't too difficult to do, especially with Google's help. So, lets run through them and see what kind of heat storage boost we can get with taking advantage of some latent heat.

The Prius coolant tank stores ~3 liters of water (.79 gallons for us USers). 1L of water weights 1000g. So, we have 3000g of water for storing heat. I realize this isn't super accurate as coolant is not simply water, but we're going to use water in this example.

Coolant temp when fully warmed up is about 190F = 361 kelvin.
Lets say the coolant temp when cold is about 70F = 294 K.
This gives us a differential of 67K.


So, lets start with the only coolant version:

Water can hold 4.19 joules per gram per degree kelvin. We have 3000g and 67K. 4.19 * 3000 * 67 = 842,190 joules of heat.


Now, lets look at the paraffin wax / latent heat version:

Again, water can hold 4.19 joules per gram per degree kelvin. In this version lets say we have 1000g of water, and 2000g of wax. So, in this version we get 280,730 joules from the water.

Now lets figure out the wax part. Paraffin wax has a specific heat capacity of 2.14-2.9 j / g / k. We will use the median of that and make it 2.52. With 2000g of wax, that gives us 337,680 joules of heat.

But, we also have to calculate the latent heat (the energy required to change the wax from a liquid to a solid). The heat of fusion of paraffin wax is ~210 j / g. So, when we solidify 2000g of wax, we get 420,000 joules of heat.

When we add those all together we get: 1,038,410 joules.

So, the all water version gives us 842,190, and the wax version gives us 1,038,410. This shows us that we can increase the heat capacity by 23% using the wax version. Not too shabby of an increase for the same weight. However, I do think for a DIY version, it would be quite a bit easier to just increase the water capacity by 23% vs going through the trouble of incorporating wax into the design.

Anyway, I just ran through that exercise this morning and wanted to share. :)

Thank you for the calculations.

But when the wax resolidifies would it not clog the system?

While I was investigating waxes, their forte comes with being able to phase change at a certain temperature.

Say you want to heat the cabin, get a wax that phase changes at 40 deg c. (Or 30 deg C if it will be in the seats) and pump exhaust or coolant through it. It will maintain its temperature while being heated up and absorbing energy. Once heating atops It will stay at that temp untill it all resolidifies. Automatic thermal regulation.

The wax would certainly clog the system if it were just introduced directly into the system. If we did make a thermos with wax it would have to be isolated from the coolant to prevent this.

I have heard of manufacturers looking at integrating phase change materials into heater cores to prolong heating/cooling with start/stop systems so that they can maintain blowing heat or cool longer with the engine off. This is certainly another interesting possibility of using latent heat.

I was investigating it from a home heating cooling perspective.

But I was not able to find the wax that phase changed at 22 deg C.

I had also some scrap radiators. Fill them up, it absorbs the head during the day and releases it during the night. Solar heated water can be pumped through it during the day in the winter. And house water supply can run through it in the summer.

Or just fill beer bottles and stack them next to the walls :)


But that idea probably belongs in the home eco modding section of the forum.

And there was a company trying to integrate it to the walls.