Panel Analyses, Part 1 — Lab Note 1.04

Last week I set up the four test panels in the commons across from the institute and let them run for the day. I posted the raw data of the results of that test, and today I am following up with some preliminary analyses.

The first graph shows the raw temperature and flow data for the panels, reservoir, and ambient air. Panel temperatures (solid lines) in this graph appear to show a similar trend throughout the day, but this is misleading, as the water flowing into each panel was an equal temperature as it was drawn from a common reservoir. The flow rates (dotted lines) show a greater range. We cannot compare raw temperatures because they are heating different amounts of water. This data must be broken down a little more carefully to show the panel-by-panel comparison.


The first step was converting the raw temperatures and flow rates into a standardized measurement of heat produced. In this case, I’ll be using British thermal units or BTUs defined as the amount of energy necessary to raise one pound of water one degree Fahrenheit. This is necessary to compare panels with different flow rates. For example, if one panel was moving a gallon of water (a little over 8 lb) through it each minute and the temperature was raised 1°F, that would amount to about 8 BTU/min. A different panel that only moved a half a gallon per minute that raised the temperature by 1°F would only be producing 4 BTUs even though the thermometers would both show a rise of 1°F.

The formula to convert the raw temperatures and flow rates to BTUs per minute per square foot of collector is as follows:

BTU/min/ft² = ((P – R) × (F × 8.344 lb)) ÷ A

= Panel Fluid Temperature at Exit
R = Reservoir Fluid Temperature
F = Flow Rate in gal/min
A = Area of panel in ft² (in this case, all panels were 8 ft²)
8.344 lb is the weight of one gallon of water

This formula calculates the temperature change from the water flowing into the panel (reservoir temperature) to the water exiting the panel after being warmed (panel fluid temperature). This is multiplied by the amount of water heated (gal/min) and divided by the total number of square feet. For example, at noon, the manifold collector was putting out water at 138.0°F, a rise of 3.8°F over the reservoir water coming in at 134.2°F. That difference times the flow rate of 0.25 gal/min, or 2.086 lb/min, meant the panel was putting out 7.9268 BTUs/min. Most outputs are given per square foot of collector, so we divide this result by 8 ft², to arrive at 0.99085 BTUs/min/ft² (this shows up at 0.98 BTUs/min/ft² in the table below due to rounding).

The temperature change of each type of panel is shown below (the panel designs are described in Lab Note 1.02). The temperature change is the difference between the temperature of the water in the reservoir and the water exiting the panel, that is, how much each pass through the panel raises the temperature of the water. As you can see, the panels track together for much of the morning, but in the afternoon, when a few clouds covered the sun, some panels suffered more temperature loss than others, notably the trickle-down design. The manifold design was consistently the top performer.


The next graph shows similar results as BTUs/min/ft². The trickle-down design started the morning with high BTU output, but its efficiency suffered as water evaporated in the panel, reducing light transmission and overall performance. By the afternoon, the trickle-down design was a drag on the reservoir temperature, giving negative BTUs. This means that the top temperature of this design is below the temperature of the reservoir (i.e., the water was coming into the panel and cooling down instead of heating up because this design couldn’t produce a high enough temperature).



In tomorrow’s post, I’ll outline each panel’s strengths and weaknesses.

“Lab Notes” are a series of posts chronicling the daily progress our research projects. Research Project No. 1 is the testing and installation of a solar heating system for domestic water and space heating. These notes may be useful for anyone interested in building such a system at home. Others might prefer the more succinct guide to solar heating, videos, and other formal publications that will result from this research project and be posted to the website as they are available.

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