Paraffin is a usual PCM in these teperature levels. Check simply here https://en.wikipedia.org/wiki/Phase-change_material

Bellos's proposal is appropriate. Paraffin is used in PCM applications. You can see in many article. You can find it, easily and cheap.

Paraffin has been been utilised as a PCM between these temperature range. 

Paraffin has been been utilised as a PCM between these temperature range. 

A Phase Change Material (PCM) is a substance with a high heat of fusion which, melting and solidifying at certain temperatures, is capable of storing large amounts of energy.

In the attached papers, you find some PCM with their thermal properties

Article Review on Thermal Energy Storage with Phase Change Materials...

You can see PCMs for 19-32 degrees.

Vineet Veer Tyagi, D. Buddhi. PCM thermal storage in buildings: A state of art.

For a comprehensive list of PCMs available at various temperatures, visit Pluss® website. To go through all the temperature PCMs available and download technical datasheets you can visit the link given below.

https://www.pluss.co.in

http://pluss.co.in/product-range-PCM.php

Dear colleagues,

I would like to ask if we can incorporate pcm in polymer to encapsulate solar cells? Generally pcm is a layer at the pv module back side. What is the pcm characteristics for pv module?

Thank you for your advices?

Kamel

use a pcm with solidus temperature arround 5degree lower than the average surrounding temperature.