solar thermal

solar thermal collectors at top left corner of roof.

solar thermal collectors at top left corner of roof.  read below for system description.  let the sun shine!

Major System Components Based on an evaluation of anticipated domestic hot water demand, the proposed closed loop antifreeze solar hot water system consists of:

  • (2) Wagner EURO C20 AR flat plate solar thermal collectors with Sunarc solar glass.
  • (1) Caleffi 119G solar storage tank with electric element backup
  • (1) Flowstar solar pump station by Stiebel Eltron; includes temp. gauge, flow meter, and PRV
  • (1) Stiebel Eltron SOM 6 plus Delta T controller with variable speed pump control

The system is designed for primary solar domestic water heating with seamless automatic backup from the electric element in the tank.

System Operation Whenever the rooftop collectors are warmer than the water in the bottom of the storage tank, a differential temperature sensor automatically activates a solar circulating pump. Sun-heated antifreeze pumps up to the collectors, then down through a heat exchange coil located in the bottom of the storage tank. In the spring, summer and fall, the system will provide a significant fraction of the household’s domestic hot water for showering, laundering, dishwashing, etc.

This is a closed-loop system, meaning that the non-toxic propylene glycol antifreeze never comes in contact with the domestic water supply. In case of unusually high hot water demand, or an extended period of cloudy weather when there isn’t enough sun to heat the top of the tank to its set point, the back up electric element will take over. The integration of the solar hot water system and the backup element will ensure that you always have an ample supply of hot water.

System Overheat Protection Under certain conditions solar hot water collectors can reach temperatures of 350 degrees F or more. Glass, copper and aluminum live comfortably at these high temps, but the propylene glycol heat transfer fluid in the system needs to be kept below 250 degrees to prevent acidification.

ReVision Energy incorporates ‘steamback overheat protection’ into every solar hot water system to protect the heat transfer fluid from a ‘stagnation event’ which can occur during power outages or during long stretches when hot water is not being used (i.e. you are away on vacation) and the solar circulating pump shuts down to prevent overheating the storage tank. Steamback is a robust, well-proven strategy developed in western Europe and widely accepted as the best industry standard (rather than building a separate heat dump zone).

Steamback is simple and elegant: when the solar pump has stopped and the collectors reach approximately 250 degrees F, a small amount of the water in the water/glycol mix begins to boil. As the water boils and turns to steam it expands to several thousand times its liquid volume, thereby forcing all of the propylene glycol out of the collector and into a specially sized expansion tank where the fluid stays cool and happy until the collectors fall back below the boiling point. At this point the water condenses, contracts and the system will self-recover as soon as the solar pump starts running again.

You can learn more about this important system design feature at

System Performance This solar hot water system is expected to:

  • Produce roughly 11,592,000 Btu’s of clean, renewable heat energy annually
  • Offset roughly 4,906 lbs of C02 emissions from fossil-based energy sources

Incentives This system qualifies for the following state and federal rebates:

  • The solar system is eligible for a 30% federal tax credit. This credit (not deduction) is subtracted directly from an existing tax liability. Please consult with your accountant or tax professional to ensure that you will be eligible for the tax credit.
  • This system is eligible for an Efficiency Maine Solar Thermal Rebate. Incentives are based on projected annual energy production and must be cost effective to be eligible.

One thought on “solar thermal

c'mon, tell us what you're a thinkin'!