A normal pressure cooker is a bit OTT for fruit and veg and won't reduce your veg to concentrate it. Why not rig up a rocket stove ?
This summer we've had no solar panels to heat our water so we've been using a simple little rocket stove to heat directly under our back boiler (That's the cauldron in the photo). In the winter we heat the water with a normal fire but the chimney is designed to be a thermal mass and retain heat so the little rocket stove is doing a great job keeping the heat exactly where we want it.
You can use small branches of wood as your fuel but as well as wood we have thousands of light, clean, dry corn husks - we normally grow a couple of hectares a year for animal feed. We've been using them in the rocket stove with really good results. Today I was bottling veg and wanted to experiment with the stove instead of using gas, so we rigged up sort of flat plate (Which we grilled our lunch on!) with a chimney which led the flames up towards the back boiler. With one bucket of husks we slowly reduced 7 kilos of tomatoes (four hours) and now there's enough water for a bath !
