I found these amazing heat packs years ago.
Just click the button and boom! Heat! 130 degrees to be exact.
After you use them, all you have to do is put them into boiling to semi boiling water until they go back to a liquid state where they can be reused over and over and over again.
I used to carry them in my jacket when i would go snowboarding. When it's 9am, and 18 degrees out, the ski lifts would be a nightmare. What i would do is I would crack one and put it in my inner chest pocket right when i got on the lift. Worked like a charm. So if they worked in the snow, they would more than likely work well for the homeless community on the streets of Portland. It's hard to compete with the 4+ hour duration of heat from the throwaway kind. Hopefully these can at least reduce the amount of throwaway ones being used.
Here is an itemized list of my anticipated expenses for this project: