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Collector flow rate and fluid temperature in and out are the data needed to calculate the amount of power being delivered by the collector loop. Adding those values together over a period of time will give us the energy collected. Ambient temperature and solar irradiance, with the previous data, will provide enough information to determine the efficiency of the collector loop and to compare performance to SRCC test results. Plus, you'll get a much better feel for how the system operates, and perhaps use the data to troubleshoot or improve the design.

Taking Measurements

Flow rate (gpm) in the loop you're testing is often hard to estimate—it really needs to be measured. If your system uses an AC pump, the flow rate will be fairly constant—once you know it, it won't need to be logged. With a PV-direct pump, this isn't the case. In my system, the voltage to the PV pump is regulated by a linear current booster so that, in full-sun conditions, the pump runs about the same speed most of the day. I'm able to measure that flow with an in-line flow meter, but as a result of not being able to constantly log the flow from my PV-powered pump, my flow rate information is only applicable to sunny days. Flow meters for data loggers are available, but are more expensive than the other data collection sensors in the system.

A temperature gauge well can also be used to place a temperature probe.

The ambient temperature probe hanging in the shade behind the collectors.

Collector fluid temperatures are relatively easy to obtain. The data logger I used has standard temperature probes and you can choose to download the information in either degrees Fahrenheit or degrees Celsius. I placed the probe tips into thermal wells that usually house temperature gauges in the collector loop. Insulation stuffed into the well holds the probes in place and blocks the ambient temperature from influencing the measurement. The probe measuring ambient temperature should be positioned out of direct sunlight, but near the collectors.

The most complicated data to collect is solar irradiance, which requires using a pyranometer. Finding an affordable model for home use can be challenging, but I chose to use an Apogee PYR-P Class 2 pyranometer ($170), which uses a silicon PV cell to produce electrical current proportional to irradiance. Because the PV

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