You can look at a Thermoelectric generator - they work on the peltier principle like some active CPU coolers, only in reverse. You can find them on **broken link removed**, among other places. People have used them to build **broken link removed**.
Note that they need a temperature difference to work, so you will need to find a cold(er) object for the other side, which could just be a heatsink. Don't expect to get a lot of power out of them unless you have a lot of them and something quite hot (like a fire).
You'll almost certainly need some kind of controller / step-up / battery charger circuit between the TEG and your battery.
By my opinion to this, maybe is bether to collect heat from sun, on building and house roof there is lots of heat. Also each peltier have their max allowed temp.