What is the best type of solar thermal panel in terms of performance?

There are three main types of solar hot water panels: Evacuated tube, glazed flat panel, and unglazed flat panel. Each of these types of panels have different operating conditions where they have better efficiency than the others.
Our analysis shows that on average, in Vancouver’s environmental conditions for residential systems, unglazed panels work best for pool heating and glazed for domestic hot water heating. People commonly think that evacuated tubes are more efficient but that it only true when the inlet temperature is sufficiently higher than ambient temperature, or in some conditions when there is little solar energy available. To elucidate the different solar hot water panel efficiencies we will explain the metrics a bit and provide some examples. One should note however that the most important metric is not commonly efficiency but installed cost per energy savings collected.

Typical panel efficiency curves (based on gross panel area) are usually presented for one particular case of insolation (i.e. incoming radiation,) which is usually the case of full sun (1000W/m^2.) However the efficiencies strongly depend on the level of the incoming radiation:

Solar hot water panel efficiencies at full sun (1000W/m^2) and cloudy (200W/m^2)

Solar hot water panel efficiencies at full sun (1000W/m^2) and cloudy (200W/m^2)

One can look up the efficiency of most solar hot water panel manufacturers at http://www.solar-rating.org. As the efficiency is a function of inlet and ambient temperatures and incident radiation, it’s better if we compare the performance using a contour plot. The contour plots of the efficiencies for the three types of panels are shown below:

Efficiencies contours for the three main types of solar hot water collectors

Efficiencies contours for the three main types of solar hot water collectors

In order to compare two types of collectors it is better to consider the difference between their efficiencies, i.e. Collector A efficiency minus Collector B efficiency. Let us first compare Evacuated Tube and Glazed Flat Panel collectors.

Comparison of efficiencies between glazed flat-panel and evacuated tube collectors.

Comparison of efficiencies between glazed flat-panel and evacuated tube collectors. Solid line represents the efficiency cross-over. Positive values indicate percent efficiency that glazed flat-panel is better than evacuated tubes, and negative values vice versa.

In full sun conditions we see that glazed flat panel perform more efficiently than evacuated tubes. This is true up until the Inlet-Ambient temperature reaches 70C. In the summer an average ambient temperature is roughly 22C. This would mean the bottom of the solar storage tank would have to be greater 92C for evacuated tubes to perform better than glazed flat-panel collectors; a very unlikely scenario. In the winter an average ambient temperature is roughly 6C. This would mean the bottom of the solar storage tank would have to be greater 76C for evacuated tubes to perform better than glazed flat-panel collectors; again an unlikely scenario. One can clearly see that when the solar energy is highest and most abundant glazed flat-panel collector perform more efficiently than evacuated tubes.

Glazed flat-panel and evacuated tube efficiencies in full sun and cloudy conditions.

Glazed flat-panel and evacuated tube efficiencies in full sun and cloudy conditions.

In cloudy conditions we see that glazed flat-panel is more efficient than evacuated tubes up until the Inlet-Ambient Temperature is 20C. Using the same summer average ambient temperatures of 22C would mean the bottom of the solar storage tank would have to be greater 42C for evacuated tubes to perform better than glazed flat-panel collectors. With a winter an average ambient temperature of roughly 6C, the bottom of the solar storage tank would have to be greater 26C for evacuated tubes to perform better than glazed flat-panel collectors. One should also keep in mind that there is typically 1/5th the amount of solar energy available when it’s cloudy and that the winter days are shorter than the summer days. If one compares on a annual basis, flat panel collectors perform better than evacuated tubes for typical residential applications.

Let us now compare Glazed vs Unglazed flat-panel collectors.

Glazed versus unglazed flat panel efficiencies.

Glazed versus unglazed flat panel efficiencies. The solid line represents the efficiency cross-over. Positive values indicate higher efficiencies for glazed panels and negative values indicate higher efficiencies for unglazed panels.

We can clearly see that the glazed flat panel performance better than unglazed once the Inlet-Ambient temperature is greater than roughly 8C. However if we consider heating a swimming pool in the summer the inlet temperature from the pool might be 15-25C and the ambient temperature might also be 20C. In that particular case we see that the unglazed panel would be the most efficient panel. This might also be the case if for large commercial systems with very large water storage tanks. Once the ambient temperature drops to below roughly 8C from the pool temperature the glazed flat panel performs better; common in the off-summer seasons.

Finally, let’s consider evacuated tubes versus unglazed flat-panel collectors (“high-tech” versus the “low-tech”.)

Evacuated tube versus unglazed flat panel efficiencies.

Evacuated tube versus unglazed flat panel efficiencies. The solid line represents the efficiency cross-over. Positive values indicate higher efficiencies for evacuated tube panels and negative values indicate higher efficiencies for unglazed panels.

The surprising result shown is that in full sun conditions the “low-tech” unglazed panel performs more efficiently than evacuated tubes until the Inlet-Ambient temperature reaches roughly 25C. So for example, in summer if one considers a ambient temperature of 22C, the temperature of the bottom of the tank would have to reach 47C before evacuated tube panels performed more efficiently in full sun!

Posted in: Frequently Asked Questions - Solar Domestic Hot Water, Frequently Asked Questions - Solar Pool Heating

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