|This section covers the design of the solar collector for the $2000
combined solar space and water heating system.
The collector is homemade and uses half inch copper riser tubes and aluminum heat collector fins. One of the key issues in getting this collector to perform well is a good thermal bond between the copper riser tubes and the aluminum fins. To insure a good thermal bond, a groove is formed in the aluminum fins that fits the outside of the copper tube snugly. The fin is then clamped tightly to the tube, and fasteners are used to hold the aluminum in tight contact with the copper. A thin bead of silicone is placed in the groove just before the clamping -- the silicone is ten times more conductive than air and will improve the thermal bond in locations where a paper thin gap exists between the aluminum and the copper.
The sequence of pictures show the fin being installed on the copper tube -- form more detail, see the Collector Construction page...
This design has a long history of successful use both in collectors made in solar association workshops as well as in commercial collectors.
The advantage of this design is that it can be made without special tools or special skills. Other thermal bonding methods such as soldering or welding the fins to the tubes either require special tools or are very long and tedious procedures.
Some pages that help to explain why the design is the way it is, and might answer questions you have about it:
- Making the fins: This page provides a number of different ways that people have worked out to form the fins, as well as some reasonably priced and good quality commercial sources.
- Preventing Corrosion: This page goes through the steps that are taken to reduce the chance of galvanic corrosion occurring between the copper and the aluminum, and provides the reasons that I think it unlikely that this will be a problem as long as the steps listed are followed.
- Optimizing the thermal bond between fin and tube: This page goes through the steps that are taken to achieve a good thermal bond between the copper tube and the aluminum fin, and goes through the reasons why the silicone is used in the joint.
- Performance testing relative to commercial collectors: This page goes through the tests that were done to compare the performance of our design with an all copper design with continuously soldered fins.
- Examples: This page provides a number of examples from people who built the $1K Solar Water Heating system, and sent in details on their project -- all use this copper/aluminum design or a variation on it.
The advantages of this design:
- It can be built with simple tools and ordinary DIY skills.
- The materials to build it are readily available.
- The cost of the finished product is about 1/5th that of a commercial collector.
- The performance is comparable commercial collectors.
- The life is likely to be long (but occasional painting of the outer collector case may be required)
Updated May 2011:
After a year of living with the system, I am less enthused about the vertical orientation of the collector that I used to be. The collector does VERY well in winter when the sun is low, but the performance in early fall and spring has been a bit disappointing. This spring (which has been unusually cloudy) the collector has kept up with our domestic hot water needs, but just barely -- I would like to see more spring heat production for both domestic hot water and for some late season space heating.
My feeling (based on last summer) is the mid summer through early fall performance will be OK. This is just due to more sunny days and the demand being only domestic hot water. So, basically, this is mostly a spring problem.
So, what to do:
- Could do nothing, and just live with the lower than ideal spring heat production. Overall the system performs well, and if being able to tuck the collector right up against the wall is aesthetically important, this could be a good option.
- Add a reflector below and in front of the collector for the spring period (maybe also in the early fall period while the sun is still fairly high in the sky). I am going to try this approach and see what the gain is like. My guess is that it will do the job. The negative is that the reflector might get in the way of whatever is in front of the collector, and may not be aesthetically acceptable.
- Tilt the collector. Using a collector tilt of about 70 degrees would still allow it to sit close to the wall and would dramatically improve spring output. I will probably give this a try after the reflector. The 70 degree tilt still fits the basic logic behind the system, which is that the steep tilt makes for very good winter heat production (when you want heat for both space heating and water heating), but produces much less heat in the summer when your collectors are way oversized for just domestic water heating needs. The earlier system with only about half the collector area and about a 70 degree tilt got through the spring domestic water heating load fine and did not overheat in the summer.... So, I think around 70 degrees should be about right.
So, I'll be looking into this over the next few months, and will report back on what I find, but wanted to provide an early head up for anyone building the system this summer.
Just a reminder on roof overhang above the collector: with a vertical collector, it does not take much roof overhang to result in a lot of shaded collector in the late spring and summer. When the sun is up at 60+ degrees in the summer midday, a roof that overhangs the glazing plane of the collector by 1 ft will shade about 2.5 ft of collector if the collector goes right up to the soffit. Software to look at this...
Further update July 10, 2011:
Just a quick update on summer performance. The system has been cruising along with the tank in the mid 130'sF almost all the time. So, with more summer sun and warmer air temperatures, the oversized vertical collector actually works just as hoped: it provides enough heat for 100% domestic hot water heating, but does not have any tendency to overheat.
I still plan to make some changes to improve performance in the spring.
I tested a pretty large reflector placed in front of and at the bottom of the collector... This was just a quick, tacked together reflector to see if it would work. It worked quite well, so I will do a "production" version of it that will be smaller, and (hopefully) look a lot better. I think that reflectors are under utilized -- they have the big advantage that (unlike growing the collector area) they add solar gain without adding to collector losses.
Further Further update September, 2012
I guess I've come back around to feeling that the vertical orientation is close to ideal all around. There is a time period in the late spring when still needing space heating that more output would be nice, but other than that, vertical seems near ideal. If circumstances allow you to use a slight slope, then that might perform just a bit better year round, but for most installations, the convenience of mounting right on the wall would (I think) outweigh any gain.
I may still experiment around with a reflector below the collector for next spring.
An advantage of the vertical tilt is that stagnation temperatures in the summer and early fall are less than for a tilted collector. This is because the sun is high in the sky in the summer, and the incidence angle that the sun makes with a vertical collector is large, so much less sun is absorbed. Here is a test of stagnation temperatures for this collector...
See the Cost Page -- rebates section for the ins and outs of buying your collectors instead of building them to collect the federal rebate.
Basically you have to buy SRCC certified collectors in order for the system to qualify for the federal rebate. Because commercial collectors cost about $30 per square foot plus expensive shipping and even high quality DIY collectors are only about $8 per sqft with no shipping costs, the system with commercial collectors will cost quite a bit more even with the rebate. But, you do save the labor of building the collector and you get a high quality product -- so, its something to consider.Gary February 16, 2011