Comparing numerical performance data has its place, just be sure to remember that the numbers don’t tell the whole story.

In the June Solar Design Notebook we looked at the construction of flat plate and evacuated tube collectors. The distinct differences between these types of collectors beg the question: Which is better?

There are both qualitative and quantitative ways to compare solar collectors. Qualitative comparisons would include collector construction, mounting options, life expectancy, ability to shed snow, and compatible methods of freeze protection. Quantitative comparisons would deal with thermal performance, installed cost, and return on investment for a given application. This month we’ll look at a quantitative method for assessing the thermal performance of solar collectors.

Formula 1
In some respects, solar collectors are like boilers. They convert one form of energy to another. The greater the ratio of output energy to input energy, the better the solar collector serves their intended purpose. In a general sense, this ratio of output energy to input energy is called thermal efficiency. However, the measurement of input and output energy must be precisely defined to provide a consistent means of comparison.

In the United States, the thermal efficiency of a solar collector is usually measured and expressed based on ASHRAE Standard 93-77, Methods of Testing to Determine the Thermal Performance of Solar Collectors. This standard defines thermal efficiency as the ratio of useful heat output rate divided by the rate of solar radiation incident upon the gross collector area (see Formula 1). Gross area is based on the overall dimensions of the top surface of the collector.

Formula 2
During the test procedure, the collector’s thermal efficiency is measured at several operating conditions. The efficiency data is then plotted as a function of a parameter called the inlet fluid parameter (see Formula 2).

Formula 3
It’s convenient to think of the inlet fluid parameter as an indicator of how adverse the conditions are under which the collector is operating. The greater the value of the inlet fluid parameter, the more difficult it is for the collector to provide useful heat output.

A straight line is fit to the thermal efficiency test data using least squares regression. This line has the general mathematical form:

Figure 1. Thermal Efficiency Vs. Inlet Fluid Parameter
Figure 1 shows two efficiency lines, one for a flat plate collector (green) and the other for an evacuated tube collector (red). These efficiency lines are representative of typical flat plate and evacuated tube collectors based on results reported by the SRCC (Solar Rating and Certification Corp.).

Notice that collector efficiency drops as the value of the inlet fluid parameter increases. Higher values of the inlet fluid parameter represent more adverse operating conditions for the collector. This graph shows that the collector with the highest efficiency depends on the value of the inlet fluid parameter under which the collectors are operating.

For example, assume a fin-tube baseboard system supplies water at 170°F to the inlet of both flat plate and evacuated tube collectors having the efficiency shown in Figure 1. The outdoor ambient temperature is 20°F, and the solar radiation incident on the collector is 250 Btu/hr/sq. ft. (at reasonably bright conditions). Under these conditions, the inlet fluid parameter (p) is:

Figure 1 shows the corresponding efficiency of the evacuated tube collector to be 0.40 (40%), while the efficiency of the flat plate collector under the same conditions is only 0.27 (27%). That’s a significant difference in performance. On a relative basis, the evacuated tube collector is yielding about 48% more useful heat output under these conditions.

However, don’t jump to the conclusion that evacuated tube collectors will always have the best efficiency. Instead, consider what happens when the two collectors operate within a low-temperature space heating system, such as one supplying slab-type floor heating. Assume the inlet temperature to the collectors is now 95°F and the solar radiation intensity and air temperature remain the same. The inlet fluid parameter (p) is now:

Figure 2. Flat Plate Efficiency Increases
Under these conditions the efficiency of the evacuated tube is 0.46 (46%), while that of the flat plate collector is 0.52 (52%) (see Figure 2). Although the difference is not as great as in the previous example, the flat plate is now clearly ahead in terms of efficiency.

The conditions under which a solar energy system operates are constantly changing, and, hence, the inlet fluid parameter “slides” back and forth along the horizontal axis of the efficiency graph. At any given instant, one collector may have a higher efficiency than the other. If the inlet fluid parameter happens to be at 0.4, both collectors represented in Figures 1 and 2 would have the same efficiency.

It’s also important to remember that collector efficiency and total solar energy collected are not the same thing. For example, an evacuated tube collector might have twice the efficiency of a flat plate collector on a cold, cloudy day, but the energy it collects will still be small because there is simply not much solar energy available on such a day.

So how does one know which is better on a seasonal basis? The only way to accurately answer this question is through the use of performance simulation software, which can run a proposed system configuration through its paces for a complete meteorological year and calculate the annual “solar harvest.” Overall, you’re likely to find that flat plate collectors tend to provide higher efficiencies in low-temperature applications, while evacuated tube collectors will yield higher efficiency conditions in higher-temperature applications.

Although it’s always interesting and informative to compare numerical performance data, the numbers don’t tell the whole story. In the next Solar Design Notebook, we’ll look at a qualitative comparison between flat plate and evacuated tube collectors. One or more of these factors may be what “seals the decision” to use flat plate or evacuate tube collectors in a given application.