Solar Hot Water (aka Solar Thermal)

Solar Evaculated Tubes on Roof (Jesse Pyles Photo)

The domestic hot water needs of TerraHaus residents—showering, washing—will largely be met through a solar hot water system designed and installed by ReVision Energy, a local leader in solar energy employing several Unity College alums.

TerraHaus will use a closed loop system consisting of four 30-tube collector arrays that circulate food-grade propylene glycol from the collectors to coils in two water storage tanks and back up to the collectors. As the glycol passes through the coils located in the bottom half of the tanks, the heat is transferred to the water. An electric resistance coil in the top of each of the tanks serves as a back-up that kicks in automatically if the water in the tank drops below 110 degrees F. Circulation of glycol stops if the temperature in the collector is less than 20 degrees higher than the temperature in the bottom of the tanks to prevent the circulating glycol from cooling the tanks.

Hot Water Storage Tanks

Circulation Pump

I have the same system on my home, except that I have only 60 tubes and one tank. I pick up some shade to the east and west in the fall, winter, and spring. Still, I have saved as much as 270 gallons of oil per year  (oil is my backup rather than electric). I did a net present value calculation on my system, using very conservative estimates for the future cost of oil (I used a ridiculously conservative  assumption that oil would never get above $4/gallon) and 20-year treasury bonds as my alternative investment, and I estimated that I’ll easily make $8,000 over the next 20 years, with a 7-8 year payback. (To be clear, that is $8,000 in profit over what I would have made from treasury bonds, and after paying off the system.)

In an era of global change and uncertainty about future oil costs, thinking about resilience in our home and work systems is prudent. The cost of heating water with solar is independent of the price of fossil fuels, hence adding resiliency to our residential systems while also mitigating climate change. Leaving aside the alternate investment calculations that a financial advisor might want me to make, it gives me comfort to think that in my home I have, in a sense, pre-paid for 250 gallons or so of heating oil per year for the next 20 years or more at $1.40/gallon.

(250 gallons per year savings x 20 years = 5000 gallons @ $7,000 installed cost (after incentives and rebates) = $7000/5000gallons = $1.40 per gallon.)

I’m proud to have several Unity alums involved in this project: Matt Wagner on the installation, John Luft as General Manager in the Liberty branch which did the design and installation, and Brett Irving back at the garage as support. And I look forward to alum Brian Byrne’s contribution when we contract with ReVision to install a PV system to make TerraHaus net zero!

Doug Fox, Director, Center for Sustainability and Global Change, Unity College


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