Solar Water Collectors: FPC or ETC?

Many wonder what is the difference between Flat plate solar water heaters (FPC) and Evacuated tube solar water heaters (ETC)? Well, one way of looking at the difference in performance is by comparing their efficiency.

Efficiency is defined as the useful heat produced divided by the solar energy received per unit area. Efficiency can be written in a form:   , where A and B are constants that depend on the detailed design of a collector and G is the incident solar radiation flux.

So efficiency is greatest when the collector fluid is at or close to the ambient temperature and therefore there are no heat losses. Efficiency generally drops with high temperatures. No two collectors have the same efficiency curve. In fact, FPC’s and ETC’s have curves that have different slopes and intercepts. ETC’s start with a low efficiency but have a shallow slope thus retaining efficiency at higher temperatures.

It is clear that the best practice, most economical USAGE depends on the usage pattern anticipated. Thus, if a large temperature difference is needed such as in colder regions (or in some industrial usages), then an ETC may be better, On the other hand for normal home, water heating usages in relatively milder area, both ETC and FPC are nearly comparable. For swimming pool heating where only a small heating is needed, unglazed collectors suffice.

Characteristic FPC ETC

Well proven and supported technology

x

x

Typically less expensive

x

 
Collector orientation less important  

x

More efficient at high temperatures  

x

Shed snow more easily

 

x

 
More efficient in cloudy weather  

x

Amenable to drain back systems

x

 

However, other consideration may need to be considered as well. Table shows a comparison of FPC and ETC.

Confused, and not able to decide, LSES can help you make the decision.

By Dr. Rida Nuwayhid