Passive solar systems have been around since the 70's and while they offer a durable, low-cost means of offsetting heating costs, they rarely receive the same investor attention that their sexier Active solar (photovoltaic) cousins receive.  Water heating alone accounts for approximately 12% of total energy use in the US.  A reduction of 20% would mean saving 4.4 quadrillion BTU's (based on 2007 EIA data), the equivalent of about 4 trillion cubic feet of natural gas, enough to heat 45,000 homes (a typical Midwestern home uses about 88,000 cubic feet of gas for heating each year).

One Silicon Valley company wants to shave off that 20% through a simple "energy assistance device" that captures solar heat even on cold, overcast days and sends it directly to the cold water intake of your home's hot water heater. The device looks like a satellite dish on the outside, but is covered with hi-tech coated mirrors on the inside.  

Those mirrors reflect both thermal radiation and infrared radiation to a central point, where the energy is focused into a 'wave guide.' The wave guide is an opti-thermal transmission cable, like a fiber optics cable, but is engineered specifically to move both types of energy. This means even on cold and cloudy days, the infrared heat will reach your water heater, reducing the amount of natural gas required to bring the water to 150 degrees.

Here is a brief interview I did with Behzad Imani, CEO of Transoptic.  He explains how the wave guide works and how the basic technology could be scaled by bundling the cables together, super-heating water for industrial applications like power generation:

A typical residential unit costs about $200 and can pay for itself in energy savings in less than a year.

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