Solar Success: Hot Water at SeaCat’s Rest

The solar hot water experiment continues. So far, the project’s outcome has been very positive with a few bumps along the way.  Here in Maine, solar makes sense believe it or not. We have a need for heating during winter’s sunny days as well as an increased need for domestic hot water during the longest days of summer when we have guests in our SeaCat’s Rest apartment.

Several years ago I started assembling small solar test panels using various materials to find the sweet spot on the cost vs. performance matrix. I considered copper sheet soldered to copper pipes (too expensive but the best performing), copper pipe laced to aluminum sheet with copper wire (not very good conduction), and finally, aluminum sheet (flashing, really) bent into a “U” to surround the pipe, with silicone bedding the copper pipe, and painted black of course. Then I had to figure out the best size for the pipe (1/2″, anything smaller and it would be too difficult to bend the aluminum) and the optimum spacing of the pipe in the aluminum backing. Then there was my brief foray into a trough type parabolic reflector, with the copper pipe in the center or “focus”–too directional and in need of constant cleaning.  I also toyed with the idea of copying the new evacuated glass tube type collector. These are expensive to build, require a vacuum pump and produce water so hot that you need stainless steel pipes to carry the heated water to the storage tank.  Also, even though they are the most efficient, they really cover a small area and are thus limited in how much hot water they produce, unless you have megabucks.

I wanted a low tech local hardware store type of system and thought I could compensate for lower efficiency by just building it bigger. So I went with the aluminum flashing and copper pipe combo encased in a wooden box, but I spent  a little extra on solar absorbing black paint ($18 for a 13 oz spray can), dedicated solar collector fiberglass glazing (“Sun-Lite”) and non-outgassing “angel hair” insulation for under the assembly. This stuff came from Solar Components Corp., a mom and pop solar store in Manchester, NH. They also sold me another essential device, a differential thermostat. This starts the circulator pump when the difference between your solar storage water and the collector water is whatever you set it at.

Diff. thermostat, expansion tank, heat exchanger and pumps

I made my boxes 24″X96″ so I could take advantage of stock materials and end up with a box I could handle by myself. I decided to build 7 so I could end up with about 100 square feet of collectors. I ran this area through my climate data to come up with an optimum angle to the sky and  a matched storage tank size. I was aiming at a cheap electric water heater tank (again, off the shelf stuff), which max out at 80 gallons. You can imagine that first summer I was very worried I would generate too much hot water, but it all worked out. Now for the heat exchanger. I decided to not have a “drain down” type of system, in which collector water is completely drained whenever the sun is not shining. This would have taken a large pump, capable of pumping water from the basement to the roof, about 32 feet. The plus side is that no heat exchanger would be required because there would be no antifreeze involved. With a closed system filled with antifreeze and a small circulator pump I needed a heat exchanger, or a way for the hot antifreeze solution to give up heat to the domestic hot water. I made this out of concentric sizes of copper pipe with the hot solution in the inside and the storage water flowing in the opposite direction on the outside. I really winged it on this, but it turned out OK. This required yet another pump to go from the tank to the heat exchanger, but I used the differential thermostat to turn on both pumps at once.  Closed loop pumps are cheap, made of cast iron. Open loop pumps are expensive and made of bronze or stainless steel. The antifreeze side pump is iron but the tank pump is bronze.

Now I needed a way to get the antifreeze into the collector loop. Additionally, there had to be some positive pressure (maintained by an expansion tank) applied to this closed loop to keep the fluid all the way to the top of the system. This pressure turned out to be about 20 pounds when measured from the basement.  Here’s where I made a big mistake. I decided to use water pressure to supply the extra pressure after pouring in 6 gallons of (rather expensive) solar system antifreeze. I reasoned that the small amount of water would be overwhelmed by the 6 gallons and not freeze. Everything was fine until March of 2009 when I saw a vast amount of water cascading off my roof. EVERY ONE of my 7 collectors had a burst pipe! Every one had to come off the roof, down to the shop, remove the glazing, solder in a new pipe section, etc. I decided I would never let devil water anywhere near my collectors again. Now the antifreeze (another $66 dollar’s worth) is pumped into the system with a dedicated pump in the boiler room, with trips to the roof to bleed out the air.

Other than that the system has performed fine, with summer hot water needs taken care of. The solar tank has been used as a pre-heater so the water which goes into our oil-fired water heater is much warmer than well water, sometimes needing no further heating. Our oil-fired water heater also serves as our heating source for our radiant heating system. Our oil usage has dropped from an 8 year average of $1450 gallons per year to around $1055, so that 395 gallons not used had saved us  $2187 in 2 years based on the average price of oil. This is good since the whole system cost (not including cheap labor) somewhere in excess of $2000—I have yet to muster the courage to total up all the bills. Any alternative energy system which pays back in a year or two is a winner.  In the years since we have rented the apartment, our oil usage has gone up to closer to $1700 gallons per year, so the drop to 1055 gallons is even more significant.

Drain valves removed and circulator installed. Solar tank on right

My latest improvement was inspired by the frustration I felt whenever I heard the oil burner come on just because the oil-fired heater cooled off, whether from circulating radiant heat or just sitting idle, while I knew there was 80 gallons of solar hot water in the nearby tank. Now I have added another bronze circulator to come on instead of the burner whenever the solar tank is hot. The pump will run until the water heater has met its temperature goal just as if it were running the burner. Should the temperature in the solar tank fall to below a useful temperature, the pump will stop and the burner will come on. I was able to do this with the built in controls around the (disconnected) electric elements. The only problem I have had so far is the tendency for thermosiphoning  to occur between the two tanks. This was corrected with a $15 check  valve. Thermosiphoning is a real bugaboo for people making heating systems out of circulating fluids.  Hot water wants to rise, cold to sink. Since I made this last improvement we have not had a single sunny day, so I’m not absolutely sure this works, but I see no reason why it won’t. This is a great time of year for solar; longer days and still a need for heating. A week ago I had a tank of 140° water. This new pump will effectively increase my solar storage capacity to 80+35 gallons or 115 gallons. Maybe I should add a few more collectors….

Update: the extra circulator pump has worked out fine–just don’t ask me to calculate the payback time!

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