CONIFER SOLAR CONSULTING

Jon Klima, Owner
360 Trout Lane, PO Box 23, Guffey, CO 80820
Phone: 719 479-2281,  Email: jon.klima@gmail.com
In business for 37 years

 

• Article 22 - April, 1997

 

SENSORS 1©

 

This month, I am going to take a break from the technical series dealing with the programming capabilities of the solar controllers used in the Colorado area. Instead, I am going back to July, 1984 and will review, in this and the next CoSEIA newsletter, a SERI (now NREL) technical report entitled An Analysis of the Effects of Active Solar Energy System Control Sensor Degradation on System Performance by R. B. Farrington and W. Short (SERI/TR-253-2185). I believe what was studied and published almost 17 years ago ought to be re-visited for its applicability today. The goal of the 1984 study was to discuss the implications of sensor failures in typical solar DHW systems. The most important of these failures result in (1) operating the pump(s) when the value of the collected energy is less than the cost to collect it, (2) operating when the collectors are cooler than the storage and reject heat from the storage tank, and (3) loss of over-heating and freeze protection. More specifically, the study was to determine the effect of sensor degradation on the annual performance of active solar energy systems. Prior work by Farrington and Myers at SERI demonstrated that exposure to high temperatures reached under extreme stagnation conditions could seriously degrade thermistor sensors. Additionally, it was found that thermistors from the same controller manufacturer could produce different responses at the same temperature, i.e., differences of up to 4° F, which is close to the same, and in some cases, greater than the turn-off temperature differential set point of the controller.

 

The selection of the controller turn-on temperature differential set-point is not critical as long as the set point is within reasonable limits, i.e., as long as short cycling is minimized under typical morning start up conditions. The most critical system parameter for energy collection is the turn-off temperature differential set point. Two obvious reasons for this are to not turn the pump(s) off too early or when usable solar energy is still available and to not operate the pump(s) when the value of the collected energy is less than the cost of collecting it. The optimum turn-off differential temperature is when the value of the collected energy is equal to the cost of collecting it. This can easily be calculated. Typical values, which depend primarily upon the cost of electricity, the displaced fuel, flow-rate through the collectors, efficiency at which the displaced fuel is consumed, the pump(s) power, and specific heat of the heat transfer fluid, will usually range from 0.5° to 1.5° F. This figure does not account for sensor inaccuracies/drift. Controllers, therefore, are usually set to turn the pump(s) off well above the 1.5° F figure. Three to five degrees is typical, but is this adequate?

 

Even though the controller set points may be properly set, the system may operate inefficiently because of sensor drift. Sensor drift results in the sensor having a different resistance-temperature response than its original calibration curve. Since the controller specifies pump operation based upon the difference between resistances of the storage tank and collector mounted sensors and not the temperatures of the sensors, any change from the sensor calibration curve will result in control errors.

 

What are the effects of these control errors? One is that the pump(s) operates longer than it should, i.e., the pump(s) may start too early or stop too late. The resulting additional yearly operating costs could range from a few dollars to in excess of $100 depending upon pump .sizes, time of excess operation, and electricity costs. It is also possible that the pump could operate less than it should thereby resulting in lower than possible savings.

 

In the next CoSEIA newsletter, I will discuss the costs of excess pump(s) operation, impact of the pump(s) operating at less than optimum times, and suggest possible options available to you when troubleshooting a controls problem.

 

Tech-Tip #1: Quick-Check Users - Make sure the Yellow, White, Green, and Brown loose wires are capped (at least three of the four) when using the spade lug part of the tester. If the wires are uncapped and touch, you can inadvertently short out the storage and/or collector sensor input to the Quick-Check and get a false indication for the controller you are testing.

 

Tech-Tip #2: Carry a can of TV tuner cleaner (available from Radio Shack) and clean ON-OFF-AUTO and sensor display selector switches (like those on the Heliotrope DTT-80T controllers). Squirting a bit of cleaner into these switches and cycling them several times will often clear an intermittent display and or operating problem.

 

If I've made a mistake or have left out some important information, or you otherwise take exception to what I have written, please write to me (c/o CoSEIA or to my address listed in the CoSEIA membership directory) and we will include your comments in the next newsletter.

 

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