Efficiency of Duda Solar Vacuum Tube Collectors

Duda Solar vacuum tubes have an absorption efficiency of about 93~96%. Since the tubes barely lose any heat, this makes their efficiency impossible to beat. However, they do not cover 100% of the surface area which the collector takes up.

There are 3 types of areas to consider with vacuum tube collectors. There is the gross area, aperture area and the absorber area. The gross area is the area which the entire collector covers. The aperture area is the area which the tubes and manifold cover. The absorber area is the area of the inner tubes which actually absorb the sunlight.

SRCC Certifications

The Solar Rating & Certification Corporation, often abbreviated as SRCC, certifies solar water heaters for durability and performance. They use accredited labs which test the collectors thoroughly. If a collector passes SRCC testing, it is proven reliable and efficient.SRCC also gives an idea of how well some collectors perform against others.

SRCC produces an empirical equation based on testing data with different temperature differentials between the collector and the ambient air. Some collectors perform better in low temperature differentials (high absorption efficiencies) but can suffer in high temperature differentials (bad insulation). It is important to choose a collector which not only collects sunlight efficiently, but can also prevent loss of that collected heat.

SRCC provides two formulas. There is a 1st order and 2nd order equation. The 1st order equation works fine for lower temperature differential applications such as pool heating. However, as the difference in temperature between the collector and the ambient increases, the less accurate the 1st order equation is. Therefore, it is much better to use the 2nd order equation when calculating efficiency for a collector when the temperature differential may be higher.

An example of a 2nd order equation is with our Duda Solar 14mm Heat Pipe 15 Tube Solar water collector.

Duda Solar 15 Tube Collector Efficiency (ISO Units)

η = 0.420 - 0.6544(P/G) - 0.00310(P²/G)

η represents the overall efficiency of the collector, dependant on the temperature differential and insolation from the sun. The 0.420 is the efficiency intercept of the equation. This is the base efficiency of the collector when there is no temperature differential between the collector and the ambient air. G is equal to the insolation given in units of kw-hr/m². P = Collector Temperature - Ambient Temperature, also often noted as T_i - T_a.

As can be seen, when temperature differentials are low, the P²/G coefficient has little effect on the efficiency. However, as P increases, the negative value of P²/G increases exponentially, and thus the hotter your collector gets against the ambient temperature, the even more heat it loses.

Once you plug in the corresponding units into the equation, η gives the overall efficiency. In order to use this, multiply η by the gross area of the collector and the amount of sun power applied to the collector, which can be in units of kw/m² for a real-time calculation resulting in kw. To convert kw to btu/hour, simply multiple by 3412. For calculations of overall heat gain over a certain period of time, multiple η by KW-hr/m²to get KW-hr which can be converted into KJ by multiplying the number by 3600, which then can be further converted to Btu's by multiplying the KJ by 0.95.

When you start comparing the SRCC efficiency equation between different collectors, it is important to not only look at the intercept but also the temperature differential coefficients. You may find collectors which have a slightly higher intercept than others, but then suffer from more heat loss with a higher ΔT which means after a few degrees change in weather, it's not as efficient as the others.