Evacuated Tube Versus Flat Plate Solar Hot Water Panels – Who Has The Upper Hand?
I was recently discussing the different types of solar hot water panels with a business owner considering the purchase of a system. He had heard a lot about evacuated tube collectors but he knew the system I was offering him used flat plate collectors. (“Collectors” is just another name for panels because they collect solar energy.) So he asked me which type of panel was better. “The answer is simple,” I told him. “Both.”
A Vintage Astron Flat Plate Solar Hot Water Panel
Of course it’s not quite that simple. The real answer is found by plotting the performance curves of both types of collectors on the same graph. The graph below shows that a typical flat plate collector has a maximum efficiency of about 70% when the inlet water temperature is equal to the outside (ambient) air temperature. This is where the collector efficiency curve crosses the “0″ vertical line, which means none of the energy captured is lost to the surroundings. As the temperature difference between the water and the air increases, the efficiency of a flat plate collector falls because it loses energy to the colder surroundings. At some point, the collector is loosing as much energy as it is gaining and the efficiency is zero. Evacuated tube collectors are only about 45% efficient when the water and air temperatures are equal and there are no losses. But their efficiency does not fall off as fast at higher temperatures.
Plotted on the same graph, the two curves cross each other around the 103oF differential temperature mark. This is called the crossover point. So, before the crossover point (to the left on the graph) flat plate collectors are more efficient. To the right of the crossover point evacuated tube collectors are more efficient. Due to their higher price and lower durability, the economic crossover point can be moved to the right before evacuated tube collecctors should be selected.
For example, let’s say it is 40oF degrees outside. A flat plate collector will be more efficient as long as the water temperature is below 143oF. An evacuated tube collector will be more efficient as the water temperature rises above 143oF. On a day where the outside temperature is 0oF, the efficiency crossover point occurs when the water temperature hits 103oF. Note that we are only interested in the daytime temperature when the collectors are running, not the coldest possible temperature in that location. The average daytime temperature for a given month is a good number for this calculation.
A better way is to do a comparison over a whole year. After all, we are not going to swap out collectors monthly based on which is better. For example, for a locale whose average daily temperature ranges from 39ºF in the winter to 80ºF in summer (mid North Carolina), the annual efficiency of the flat plate collector is equal to the annual efficiency of the evacuated tube collector when the inlet temperature going into the collector is 187 ºF. The collector inlet temperature of 187ºF is way above any residential requirement and above most commercial requriements.
The bottom line is evacuated tube collectors are needed only when the calculations show they are needed.
Domestic hot water (DHW) and space heating (SH) applications occur before the crossover. Above the crossover are higher temperature applications like absorption chillers, with requirements of 230ºF water. In this range evacuated tube collectors perform better.
A partial list of commercial applications that are best served by flat plate panels include:
Military barracks and housing
Jails and Prisons
Hospitals, Nursing Homes
It certainly helps to be working with a solar professional who understands the differences between collectors and is not just trying to promote one over the other to make the sale. So, when it comes to choosing between evacuated tube and flat plate solar hot water collectors, the most critical factor is the temperature at which they need to operate for a given application.
If you’re looking for some assistance with a specific solar hot water project, let me know what I can do to help.