Saturday, July 28, 2012

Solar Thermal for the Masses

For a few years now, I've independently been trying to solve the problem of solar domestic hot water (SDHW) systems that are too expensive.  Here's what I've come up with: a simple and cheap solar hot water system design that at the very least should give manufacturers food for thought.

The Problem:  Even small SDHW systems cost way too much ($6-$8k on average)
Since gas heated water is still very cheap, the average family can only save $30/month with solar.

My Opinion:  The US SDHW industry is in deep trouble now, and will remain so until $2000 systems are available. 

A Solution?:  Bring back Recirculation for Freeze Protection (RFP) in all but the coldest climates in the US.

History:  RFP is still used in many non-freezing climates, so it's still a free option on most solar thermal controllers. It's reliability took a hit in the 80s when many systems froze because power failures often accompanied freezing weather. (Freezing rain will often take down power lines) Because of this, most installers eschewed this method for most climates. Historically, it can't be used with flat plate collectors because in the winter it takes too much energy to prevent the collectors from freezing. (They are pointed at the night sky which is usually colder than -100F on a clear night)

Why This Solution Can Work: A. If Evacuated Tube (ET) collectors are used, the heat required for freeze protection is almost negligible, and B. In case of a power failure, the system will self-freeze-protect by thermosiphoning. C. The outdoor piping is freeze and stagnation tolerant hose.

Yes, the ET collector header can still freeze and break, but the probability will prove to be too low to worry about. (Drainback systems have a few failure modes including freezing, but we still use them because the Mean Time Between Failures (MTBF) is long enough.)

I've tested this system for two winters now, and it works great in Denver. Here's a link to the YouTube tutorial about the system.

For more info and pricing, here's a link to my presentation at the 2012 ACEEE Hot Water Forum.

100% Off-the-Shelf:  Only  five of the items required must be purchased somewhere besides Home Depot:

1.   The pump:
2.   The Pump Controller:
5.   A 110V SPDT relay is also needed to control the ECV just right.


  1. Do you have pictures of the setup you've used the past two winters?

  2. Here's a video