Machine Design

Scanning for ideas: Solar help for home water heaters

This closed-loop antifreeze-filled system from Butler Sun Solutions, Solana Beach, Calif. (, should supply half the hot-water needs of a family of four living in southwestern states or about 1,700 kW-hr annually.

It includes 40 sq ft of collectors and a solar-wand heat exchanger for about $2,500.

The solar wand is a double-walled copper-in-brass in-tank heat exchanger with about 2 sq ft of heat-transfer area. Inside of it, copper tubing runs side by side the brass outer tube, but plastic standoffs limit tube-to-tube contact. Tubes are also insulated with closed cell foam insulation from a collector-to-hot-water-tank umbilical, which also includes all electrical and fluid connections.

The system can be controlled with a conventional 115-Vac differential temperature controller or a photovoltaic-powered 12-Vdc pump. The working fluid is propylene glycol, a common antifreeze liquid.

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