Return to the Renewable Energy Forum | Post a Follow-Up

 o
open loop solar water heating in SF bay area

Posted by jcm55 (My Page) on
Tue, Nov 6, 07 at 11:18

Hi all,

I'm considering a solar water heating system for my home in Oakland, CA. I'd like to do the install myself, and I'm pretty handy with plumbing tasks, so I don't think that should be a problem.

I do want to get the design nailed down though before I begin. Right now I'm considering an open loop system (where the potable water is circulating through the solar collectors). For freeze protection I will use a drip valve and/or recirculation.

My question is, will that provide enough freeze protection for the climate here? Usually during the winter we get a handful of nights with frosty windshields, but no temps below 32 F. Last winter was an exception, and we got a few nights down to 29 F during the cold snap in January that damaged the citrus crop in southern California.


Follow-Up Postings:

 o
RE: open loop solar water heating in SF bay area

I wouldn't rely solely on the drip valve. I'd also have the system set up for recirculation to circulate a bit of warm water from the tank through the collectors when the temp got close to freezing.

I've thought about a drainback system, but there are two reasons why I'd prefer not to go that way:

1) A drainback system requires a bigger, costlier pump. Higher up-front cost, and it uses a lot more electricity which negates some of the energy savings of the system overall.

2) It seems to me that a drainback system requires a heat exchanger of some sort -- to keep the unpressurized solar loop separated from the pressurized potable water.

If I'm wrong on either of those counts, then maybe a drainback system is worth another look.


 o
RE: open loop solar water heating in SF bay area

I take it then that you're not interested in solar tubes?

I've been "kinda" looking at such a system here (Montana) but it uses glycol and depends on heat exchange into an electric hot water tank (which is obstensibly just for backup). Even though it's not cheap there are some credits available.

Good luck,

M


 o
RE: open loop solar water heating in SF bay area

Hi,
Yes, the drain backs for domestic water heating do need a heat exchanger. Its often a coil of copper pipe in the drain back tank. You also need the drain back tank itself.

The pump depends on how high the collectors are above the drain back tank. It also should be a bronze pump since the system is open to air. $200 buys a nice bronze Taco or Grundfos.
When you are collecting sun, you are pulling about 160 BTU/sf-hr of collector, or abut 8000 BTU/hr if you have 50 sf of collector. This is about 2400 watts being collected. An extra 50 watts to drive a bigger pump is pretty small potatoes compared to that?


I can see the appeal of the open loop system for you. I suppose that if you made a point of checking that the differential controllers circulate when close to freezing feature and the drip valve were working at the start of each season, it might work out OK. I would guess most failures come from owner inattention.

The collectors are looking at the cold night sky, and lose a lot of heat to sky radiation -- this will make them run colder than the air temperature on cold nights -- so if the air is near freezing, your collector panels are likely below freezing. So, you would really need to stay on top of maintaining your freeze protection features.

Gary


 o Post a Follow-Up

Please Note: Only registered members are able to post messages to this forum.

    If you are a member, please log in.

    If you aren't yet a member, join now!


Return to the Renewable Energy Forum

Information about Posting

  • You must be logged in to post a message. Once you are logged in, a posting window will appear at the bottom of the messages. If you are not a member, please register for an account.
  • Posting is a two-step process. Once you have composed your message, you will be taken to the preview page. You will then have a chance to review your post, make changes and upload photos.
  • After posting your message, you may need to refresh the forum page in order to see it.
  • Before posting copyrighted material, please read about Copyright and Fair Use.
  • We have a strict no-advertising policy!
  • If you would like to practice posting or uploading photos, please visit our Test forum.
  • If you need assistance, please Contact Us and we will be happy to help.


Learn more about in-text links on this page here