As the saying goes, “the cobbler’s children have no shoes.” Thus it sometimes feels at my home. I am usually busy helping others consider, design, install, troubleshoot and optimize their renewable energy systems and tend to neglect my own. This is the premise of my excuse for taking 2 years to complete my solar hot water system for domestic hot water heating. The second component of that excuse rests in my basic incompetence for sweated copper plumbing joints and I attribute one year of (very) part time work to chasing leaks in my system. With the help of some very patient friends, I finally filled the system with Cryotek and water and when the sun hit the tubes, presto, we were in business for making hot water from the sun.

What I Installed

I chose evacuated tube solar collectors over flat panel collectors for three reasons:

1. I continue to want for real installed performance data on evacuated tube collectors and felt obliged to develop the data for myself and the industry.
2. My only true solar south exposure is a 150 year old slate roof which I did not want to tear up or work on in order to install the system. This south facing roof also had no attic space beneath it, which would have made plumbing more difficult. This resulted in my installing the collectors on my east facing garage roof which is 1 year old galvalum with decking underneath exposed inside the garage.  Facing a solar collector to the east is a bit like driving backwards.  I knew going into this that my performance per square foot would be sub-optimal.
3. We live in up-state New York where the average winter daytime temperatures are relatively low.  Evacuated tubes outperform flat panels in low temperatures, and given my sub-optimal installation noted in #2, I knew we would need every bit of extra collector performance we could muster. The interested reader should expore the SRCC ratings on various panels.  Look specifically at the heat duty capabilities of the panels as Ti-Ta (inlet temp minus ambient temp) becomes large (i.e. cold weather performance).

Specifically, I chose SunMaxx Solar SunMaxx-20 collector arrays. SunMaxx is the hot water division of Silicon Solar in Bainbridge, NY.  I had just recently completed an install of these tubes for a client and was pleased with the customer service, the product, and the fact that the company was relatively local.

The collectors are connected to a hot water storage tank in the basement of my home via a 100 ft run (one-way) of 3/4″ Watts Black Onix reinforced tubing (100 psig, 180 deg F rating).  I installed two “future” manifolds in the garage with shut-off valves and trim valves.  The idea is to perhaps someday install a hydronic unit heater to help take the edge off in the unheated garage in the winter using any surplus hot water heading back to the collectors after it has been through the storage tank in the basement.  My thinking is also that the lower return temperature might allow the collectors to exchange more heat into the fluid (lower Ti-Ta).  These trim valves also came in handy for tuning the system for reasons having to do with viscosity change in Cryotek/water mixtures noted below.  The valves, combined with a variable speed pump, allow me to “throttle” the flow of fluid through the system up or down depending on seasonal collector performance.

The balance of the system is a collection of pumps, valves, meters and controls.  These will be discussed below.

How it was Designed

Our initial intention was to provide as much of our household’s domestic hot water as possible.  We heat our home primarily with a reasonably high efficiency wood burning insert that we adore.  We do have an oil boiler in the basement and hydronic baseboard through-out the house. But they only get used on the coldest days when we are at our laziest or on the occasional weekend morning when we are allowed to sleep in.  Thus, the hot water coil on the boiler was the only reason the boiler was running most days and it aggravated me to hear it turn on when we could be getting hot water from the sun.

We are a household of 2 adults and 1 child, which I estimated amounted to 20 gallons of hot water per day using ASHRAE guidance and assessing our habits.  It should be noted that the US Consumer Product Safety Commission Protection has established 120 deg F as the maximum limit for hot water heater setpoints (which should be confirmed with a thermometer at the tap.)  In addition to following this guidance, I also installed a thermal mixing valve with a setpoint adjusted to provide 120 deg F water  measured at the tap.  I selected a SuperStore Contender 40 gallon hot water tank made by Heat Transfer Products. This tank is well insulated, has both a solar hot water coil and an electric heating element, and was the perfect height to fit in my basement.

To heat our daily 20 gallons of hot water from 55 deg F to 120 deg F requires 10,790 BTU.  But because we tend to use our hot water at night and in the morning, we had to install sufficient capacity to carry us through the night.

The SRCC rating for the solar collectors I was considering reported the daily duty on a mildly cloudy day with Ti-Ta of 90 deg F as 16,000 BTU/day.  Considering my less than optimum installtion of due east facing collectors, I assumed 50% of the performance of a due south installation.  So I figured I could expect 8,000 BTU/day from each 20 tube collector array.  I estimated, therefore, I should get two 20 tube arrays for a total of a 40 tube system which would produce 16,000 BTU/day on an average winter day (mildly cloudy, 90 deg F Ti-Ta).

Lessons Learned

1. Digging. Digging a 5′ deep x 1′ wide x 30′ long  trench is back breaking and tedious, a despite my own stubbornness, I concede that renting a backhoe would have been money well spent.
   

   Chris and Henry (2 months) inspecting the trench in progress.

2. Viscosity Swings. Viscosity  of Cryotek / water mixtures vary significantly with temperature and if you have any substantial outdoor runs in your system that will soak to low ambient temperatures overnight, you need to account for cold fluid operation in your pump sizing (and pipe pressure drop).
3. Trim Valves. God invented multi-turn trim valves for a reason.  Use them.  See #2 above.  The changes in viscosity over the course of the year require a pump that will be over-sized for warm weather operation.  In order to reduce the flow in this case (and thus have the collectors operate at their most efficient) a trim valve can be used to impose an artificial pressure drop and, in a sense, throttle the pump to a lower flow rate.
4. Sweated Joints. It is impossible to over-clean a copper fitting.  Clean, well-sanded fittings make for leak-free joints the first time. This is another area where hired professional help would have been worth the money.
5. Air Bleeds. God inspired someone, one day to invent automatic air bleeds and hinted that they be installed high in the system.  Use them and place them as directed.
6. Heat Dump. Despite the fact that things never seem to over-perform, you should include the ability to either dump heat, direct it to another use or shade your collectors on days when you have more heat than you can use.

Performance

Check back regularly for updates on performance measurement.  It is early days in this system’s operation and the first month involved a fair bit of tuning and adjustment.  I will say that with only half of the tubes installed we’ve seen a 25% reduction in our electric bill (year on year).