Project Profile: Improving a SHW System’s Efficiency

Intermediate

Inside this Article

Solar hot water collectors on Andrew Goldbaum's roof.
Solar hot water collectors on Andrew Goldbaum's roof.
The guts of Andrew’s solar hot water system
The guts of Andrew’s solar hot water system, now running better with a little automatic—and manual—intervention.
An X-10 transmitter
An X-10 transmitter recognizes 125°F water in the solar storage tank and transmits a signal.
The X-10 appliance module
The X-10 appliance module receives the signal and energizes the instant hot water circulator.
Instant hot water circulator
Only when the instant hot water circulator is powered by the X-10 appliance module and when it senses water temperature below 100°F at the faucet will hot water circulate.
Solar hot water collectors on Andrew Goldbaum's roof.
The guts of Andrew’s solar hot water system
An X-10 transmitter
The X-10 appliance module
Instant hot water circulator

I considered installing a solar hot water system for many years, but delayed for financial reasons. Then, heating oil approached $4 per gallon, and it finally made economic sense. After considering my family’s hot water usage, the federal and state tax credits, and a system payback of just five to six years, it seemed financially irresponsible not to install one. I contacted an experienced local solar contractor and had him install a three-collector, 120-gallon antifreeze/drainback system that would preheat water for an existing 50-gallon hot water tank, which is indirectly heated by an oil-fueled hot water boiler.

Even before the installation, I knew my system would have two efficiency issues. The first was that even though the solar hot water tank might be filled with 150°F water, if no hot water was being used, the existing water heater tank would slowly cool as it had always done. This meant that occasionally the boiler would fire just to reheat it a few degrees, despite having a tank of “free” solar-heated water right next to it. When I witnessed my boiler firing up on a hot, sunny, summer day, I became determined to find a solution.

The second inefficiency has to do with heat energy that becomes “trapped” in the drainback tank: At the end of a sunny day when the collector circulator pump shuts down and the solar tank temperature equalizes, the tank water might be 135°F. After filling the bathtub and running a load or two of laundry, the temperature at the bottom of the storage tank might be reduced to 75°F, while the top of it might be 130°F. However, the glycol, just sitting in the drainback tank, might be 140°F. If during the evening, that heat could be transferred into the solar storage tank, it would heat the water in the bottom of the storage tank to about 105°F. This would:

  • Lessen or even stop the use of the boiler in the evening during high hot water use.
  • Delay the need for backup heating if the following day is not sunny enough to activate the circulator.
  • Reduce heat loss, since the solar storage tank is better insulated than the drainback tank.
  • Result in higher system efficiency, since collectors operate more efficiently when heating glycol from a lower temperature to a higher one (100°F to 120°F, for example, as opposed to 130°F to 150°F). The next day, more hot water will be available sooner.

System Description

Three SunEarth EC32, 8- by 4-foot collectors. Despite their slightly higher cost, I chose SunEarth’s Empire series black chrome collectors for their higher efficiency compared to the more standard, selective-black-painted collectors. The collectors are SRCC approved, which is important for tax credit purposes.

Two Grundfos UP 26 circulator pumps. In this system, one circulator alone couldn’t pump the glycol the almost-40 feet from the basement to the rooftop collector array, so a second circulator was added in series. This nearly doubles the pressure and head, costing less and using less electricity than a single larger circulator pump.

One Rheem 120-gallon Solaraide tank. The Rheem tank with integral heat exchanger is an industry standard for solar hot water storage. The Whirlpool 19-gallon electric water heater, which serves only as the drainback tank—not as a heated backup—was chosen because of its convenient connections. It’s fairly well-insulated and holds 9 more gallons of glycol than a typical drainback tank, but costs much less. The extra glycol provides additional heat storage.

One SunEarth AECA differential controller. The controller has three temperature inputs from sensors: T1, at the solar collector output; T2, at the bottom of the storage tank; and T3, at the top of the storage tank. The controller activates the circulator based on the difference between T1 and T2, so that when the collector fluid is hotter than the storage water, the pump comes on. When T1 or T2 is too high, the controller shuts down the pump and the system drains. T3 temperature is for monitoring purposes only, and does not affect pump operation.

Comments (0)

Advertisement

X
You may login with either your assigned username or your e-mail address.
The password field is case sensitive.
Loading