Was wondering if trapping light in a reflective chamber could heat rods sufficiently enough to heat water? Trying to think of new ways in which we can utilise solar energy.

Ref pic A.


2 Answers 2


Yes, harnessing light can be a good way to heat water to usable hot temperatures, i.e. 50 to 80 degrees Celsius.

The two factors that are important are the surface area of your collector (which looks quite small in your design), and the efficiency of the system.

Typically, we use either flat plates (high area) or (partially-)evacuated tubes (high efficiency).

Sunlight is around 1kW per square metre at the surface in full sun, and thermal collector efficiencies might be in the range 50-90% or so. Depending on where on earth you are, you might get 1-7 full sun hours per day. External air temperatures, collector tilt, alignment, and your latitude, will all affect the result.

For a home in the UK, solar thermal energy should be sufficient for a small installation to meet half the home's annual hot water demand. If you can tap into larger-scale seasonal storage, or you're closer to the equator, you can increase that proportion.

Here's an Evacuated-tube diagram from wikipedia's article on solar thermal collectors:

enter image description here

  • $\begingroup$ Nice answer. I do think it's worth emphasizing, though, that "trapping" the light doesn't do anything; it's just a matter of absorption. $\endgroup$
    – David Z
    Jul 9, 2011 at 21:02
  • $\begingroup$ Evacuated tubes are great for efficiency, they minimize conductive loses, which can be important in a cold climate. A big expense for solar hot water heating is protecting against the possibility of freeze damage. One can also use single axis concentrators, such as parabolic troughs, or linear fresnel mirrors to concentrate sunlight onto a pipelike collector. Concentration allows much higher working temperature, but requires some mechanism to tracking the sun. $\endgroup$ Jul 10, 2011 at 1:41

Any mirror can reflect light onto an absorbing rod, and increase the heat transfer to that rod. A 'reflecting chamber', however, sounds like a black body made with reflecting surfaces and a few absorbers, and THAT is less effective in proportion to the number of reflections before light hits the target. Reflecting surfaces may be 95% reflective, but all will absorb at least a LITTLE light, which will therefore not produce heat in the absorbing rods. There's also the problem of the light that inevitably gets scattered out of the aperture, which might just hit your mirrors and exit without ever encountering any target.
That's why careful geometry and aiming of reflectors is a feature of a good solar furnace; generally incoming sunlight makes one reflection then it's on the target. The absorptivity of the rod (if it does 95% absorption, some light is lost) can be enhanced by using some optical tricks, like making the rod black at visible-light wavelengths but white at infrared (this mimics the greenhouse effect).


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