how BIG the collector??

[1stmate]

Hi folks-

I am in the process of assembling a solar domestic hot water system. I’ve made some test components, & I am comfortable enough with the data collected to proceed with a full size unit.

My question is simple, I think:

I’m planning on making a 4 x 8’ collector of 1/2" tubing with 3/4" header/foot; how many tubes (i.e. water holding surface area) should I plan for? I’ve seen collectors with a few as six vertical tubes, & some with what looks like twenty!

BTW, our requirements are small- 2 adults, so 2 showers a day, 1 clothes wash, dishwasher twice a week or so.

Thank you in advance for any advice.

Regards,
Jack in NH

[JAK]

Two parts to this question I think.

First part is whether or not you have enough area and how efficient the system would need to be. The best design depends alot on your climate, and whether you are designing this for 6 months or 12 months of the year. Second part has to do with efficiency considerations, material costs, overheating, circulation losses, and again climate considerations and whether it will be natural or forced circulation. If you are off grid you will want to keep you circulation energy losses to a minimum because electrical energy should be more costly than heat energy you are producing. In general, I think DIY projects should go with somewhat larger pipe diameters than commercial units, especially if you can shop around and find some suitable materials cheap, like from a salvage yard, or recycled cans or something. Aluminum cans with a deposit are probably worth returning though. Not sure.

I would follow a proven design from Solar Gary's site, and resize and modify it to suit your demand and climate.

[Clair_Schwan]

I have 4 by 8 flat plate collectors from Energia. They have 1 inch headers and 21 of the 3/8 inch risers. The units work well because they have glass that is prism like on the inside and smooth on the outside. There is also dark foil between the risers to help generate more heat. They are also well insulated on the back and sides.

I think you are going to have too much flow too fast if you use the 1/2 inch piping. The smaller 3/8 piping provides more surface area to heat up so you will gain efficiency.

As for size and number, I have installed only one collector to heat the floor of my greenhouse, and it can raise the water temperature in a tank of 20 gallons or so to about 106 degrees during a full sun day. Now, that includes heating up about 135 feet of piping buried one foot deep in the ground, and that piping is always releasing heat to the ground as the water in the tank heats up.

My plan is to install at least 3 collectors for domestic hot water in my house, but no more than 4. I'll have a little over 100 gallons of storage, and that combination should give me plenty of hot water for domestic use, with a buffer for when the sun doesn't shine for a day or two.

The number and intensity of sunny days has to be taken into consideration. Here in Wyoming we have no problem because we are sunny most of the time. That certainly isn't the case around the country.

Also, remember that the efficiency of the collector reduces as the water inside is heated. Solar gain isn't linear. The sun can "push" more energy into cold water, so heat gain should slow as the temperature of the water rises. Still, having too large a system can cause the water to boil.

If you can't find data to tell you about size and number, I would build one and test it. Make certain your installation location would allow you to add on another collector, and provides pressure relief. If during the summer the water tends to get too hot, then use shade cloth to cover a collector or two.

I bought my collectors used from a laundry that was upgrading. I was able to purchase mine for $100 each, so I bought all 20.

You might check around. I have seen several advertised in garage sales and in the local trading paper over the last year or two. It would sure beat trying to build one.

[1stmate]

Thanks for the input guys.

I think I'll be taking Clair's advice: I'm just going to plow ahead & build it, then see if it's big enough or too big.

THEN deal with it... ;D

Thanks again!

[SolarGary]

Hi folks-

I am in the process of assembling a solar domestic hot water system. I’ve made some test components, & I am comfortable enough with the data collected to proceed with a full size unit.

My question is simple, I think:

I’m planning on making a 4 x 8’ collector of 1/2" tubing with 3/4" header/foot; how many tubes (i.e. water holding surface area) should I plan for? I’ve seen collectors with a few as six vertical tubes, & some with what looks like twenty!

BTW, our requirements are small- 2 adults, so 2 showers a day, 1 clothes wash, dishwasher twice a week or so.

Thank you in advance for any advice.

Regards,
Jack in NH




32 sqft of collector is about right for 2 people.
It should give you 100% solar heating the summer and a good fraction in the winter.

Half inch risers are fine -- a lot of commercial collectors use them.

The risers should be spaced at around 5 inches apart. But, each riser needs to have a fin attached to it that spans half the distance to the next riser -- 100% of the 32 sqft has to be fin area to have an efficient collector.
The fins actually collect the heat and transfer it to the riser.

If the risers just sit there in the collector with no fins to transfer heat to them, the collector will not be efficient.

There has to be a good thermal bond between the riser and the fin. In commercial collectors, the bond is made by soldering or by some form of weld.
For homemade collectors with copper fins soldering works fine, but is a lot of work.

If you want a pretty good way to use inexpensive fins (aluminum), and a way to get a pretty good thermal bond, have a look at this:

www.BuildItSolar.com/Experimental/PEXCollector/Construction.htm

In this collector, I used PEX risers and aluminum fins, but you can use copper risers with aluminum fins bonded in the same way that I used the PEX. I've tested this, and it performs quite well -- its within a few percent of copper risers soldered to copper fins.
This is easy to build, performs well, and will hold up to high collector temperatures as long as you use silicone for the bonding.

There is quite a bit more on building collectors on BuildItSolar in the Water Heating and Space Heating area and the Experimental area.

Gary

[randallhilton]

If you want a pretty good way to use inexpensive fins (aluminum), and a way to get a pretty good thermal bond, have a look at this:

www.BuildItSolar.com/Experimental/PEXCollector/Construction.htm

In this collector, I used PEX risers and aluminum fins, but you can use copper risers with aluminum fins bonded in the same way that I used the PEX. *I've tested this, and it performs quite well -- its within a few percent of copper risers soldered to copper fins.

Gary, I appriciate very much your details on building a collector. I noted in the comments that you wondered about using a filled silicone to help improve efficiency. Here's a link to a heat sink paste (http://www.grainger.com/Grainger/items/5KH10) that I sometimes use when sweating pipes in walls. The stuff is pretty amazing so I figure it might work with the alum to pex contact. I just don't know if it's chemically reactive to aluminum.

While searching for that example, I found this as well:
heat transfer mastic (http://www.grainger.com/Grainger/items/4E847).

It seems to me, and I don't know - just pondering - that the paste or mastic would maintain contact as well as silicone. I would wager that expansion/contraction of the aluminum would soon break the seal between the aluminum and the pex anyway.

Excellent information you provided. Well done!

[MooseToo]

years ago, panels were not constructed using "fins" but rather all risers were soldered full-length to a copper sheet that was nearly as wide and as long as the overall collector - besides providing a great amount of heat absorbtion area, it also contributed much to structural integrity - the copper sheet was a very thin gauge so cost was not prohibitive -