Bidjanga,
This is what we are working on and have been working on. I have 10 students looking at two different kinds of thermal storage and we have a publication (#5 on the research website). You could start building some of these. How long it retains heat depends on how much insulation there is.
There are many people doing installs of solar systems. Probably the best example is Robert Van Buskirk in Malawi.
The goal is for our present ISECs to be able to provide light at night, but charging a USB lighting system:
https://www.alibaba.com/product-detail/Usb-Led-Light-Usb-Motion-Sensor_1600310652805.html?spm=a2700.galleryofferlist.normal_offer.d_title.57a5454cxt01ny&s=p
Pete