Polymer ISC Solar Collector Debuts

Just a few days after my post about passive solar thermal water heating, the grandson of the Arthur D. Little collector design came to my attention. Called the Solar Cache by Harpiris Energy, it has been designed only for non-freezing climates. With PEX pipe, which may be considered freeze tolerant, this will be a great product for economy installations, costing far less than the usual $6k solar DHW systems for freezing climates.

I just hope it doesn't get installed in colder climates, freeze, burst and give the product a bad name. When drain-down systems were used in Denver in the 80's, many an attic was flooded because of Murphy's Law. Even though there were freeze protection controls and backup freeze protection controls, they still failed.

Ironically, a common complaint from solar DHW system owners may cause pumps to appear in these passive systems. See, most systems have a solar preheat tank (with the Sun Cache the solar tank is on the roof), and a backup (fossil fuel) tank piped downstream in series. If the homeowner doesn't consume any hot water (like all day when he's at work) then the backup tank fires to make up for standyby losses, even though the preheat tank may be 140F or higher. In the summer, a solar system should be providing 100% of the hot water load on sunny days, so some homeowners get peeved when their gas bill isn't zero and the backup tank is coming on every hour or so. A really tiny recirculating pump with a simple control is a way to prevent the problem, but now you've added a pump to an otherwise elegantly simple system. The other beef with the "uninsulated" storage tank on the roof is that you'll never have any solar hot water left for the morning shower, the storage water will drop close to outdoor ambient by morning.

Rheem has also come out with a passive collector with some freeze tolerance. The collector loop is charged with antifreeze. Again, great care should be used on the supply pipe, even in "non-freezing" climates.