Austria-based industrial design consultancy Salz designed this solar water heater for emerging markets.

While conducting research in an impoverished area of Mexico, Salz learned that residents were scraping by with two different sources of heat, and suffering for it:

"Most families we talked to had gas stoves for quick meals and reheating. The stoves were not used for daily cooking due to the high cost of gas. Outside wood fireplaces were used for cooking tortillas, beans and to heat water. Currently families bath every 3rd day. With more hot water available they'd bath every day and wash dishes with hot water."

The firm also observed that in this region, houses use rooftop gravity-fed water tanks.

They designed the heater to work with these, in this manner:

A startup called Suntap has commercialized the design and constructed 15 prototypes to date. They say that on a warm day, the Suntap heater can produce 15 gallons (56 liters) of 160°F (70°C) hot water; they also claim their system's output, on a daily basis, is the equivalent of displacing 4 pounds of wood, 8 ounces of gas or 30 square feet of solar panels.

The company says they can manufacture the units for $40 a pop in small-batch production, but can get that down to $15-$25 per unit in a 10,000-plus run. They've selected Mexico, India and Africa as their initial markets, where people are getting by on as little as $2 a day.

I think the company wouldn't do badly by targeting disaster preppers in America as well.