Water is lighter than air and only needs a modest temperature to have it's vapor pressure exceed ambient pressure at high altitude. Bonus for self-inflating whe

Water is lighter than air and only needs a modest temperature to have it's vapor pressure exceed ambient pressure at high altitude. Bonus for self-inflating when you reach altitude/letting you accumulate thermal energy in some form and take off by dumping the heat into your liquid water bladder to inflate your water vapor balloon. Wonder how well/cheaply (in weight for $affordable budget) you could insulate the balloon to curb your thermal power demand. Maybe enough that you could use your liquid water bladder with a little insulation as a thermal mass to run an absorption heat pump (Type2: warm power, cold sink, hot target output) off of ambient temperature during the day, cooling the thermal mass back down during the night. Or lightweight solar panels, I suppose, feeding an air-source heat pump to boil the water. Normal Li-ion batteries sadly don't really have enough energy density (water takes 620 Wh/kg to boil; commercial energy-dense Li-ion is in the 200~300 Wh/kg range; at sea level and boiling point (=lowest possible temperature without it wanting to condense on every dust particle and other condensation nucleus) water vapor is half as dense as 20°C air so it could lift itself twice (and once it's used to lift itself already), and even a very good heat pump will have severe difficulty reaching a COP of 4.0). So I guess if you try really hard you could make a self-inflating steam balloon that takes off using Li-ion fed heat pumps and later stays airborne using solar cells to continue feeding the heat pumps...