In fact there are geometrical and optical concentrations and from their names you can know the definition of each one.

Basically, the answer of your second question can guide you to the answer of the 1st one.

When your main aim to benefit from the light part (photons) of the irradiance, then you are using the optical part of the irradiance flux and in most cases expel the heat or the thermal part of the flux. See the PV panels for example.

The other important application is the thermal one, when one tries to maximize the received heat of the irradiance in order to maximize the temperature of the working fluid (instead of using fuel). For example Concentrated Solar Power CSP which used in cycles like Brayton or Rankine etc. Here the optical performance had a direct influence on the thermal one. 

With a silver-coated parabolic mirror, you may achieve both optical and thermal concentrations.

Ravita:

Be careful with your words now. Based on your question let me be sure to remind you of the following definitions (because I think it might help). First "optical" is referring to electromagnetic radiation. That is, light that spans the range of radio waves, through microwaves, infra-red, visible, ultraviolet, X-ray and finally out to gamma ray. All these are electromagnetic, hence may be controlled through 'optical' means.

Heat is a mechanical thing. It represents the amount of energy stored within a material by molecular vibrations. People sometimes confuse "thermal" with "heat", when in fact they are different things.

What doesn't help at all is that things that are hot emit 'thermal radiation'. That is to say, they emit radiation (electromagnetic waves, light waves) that are in the infra-red range of wavelengths. The reverse is also true. If you irradiate something with infra-red radiation, you warm it up (change the vibrations within the body).

So, if your interested in increasing the temperature of something use an optical device (e.g., a lens or a parabolic mirror) that happens to be optimized for the infra-red.

That's not to say that you cannot heat anything up unless it's IR radiation. That's not true. As any kid on a playground will tell you it's easy to fry ants with a hand lens.

The expression "thermal concentration" is just a rather loose way of saying the point of the optical component is to heat the objective. Whereas with "optical concentration" the point of the optical component is just to increase the flux density, in a general way.

I hope this helps!

Hi Ravita,

Optical concentration is when the power flux (W/m2) on a receiver is increased by means of optical concentrators (lenses, reflectors, etc.).

Thermal concentration is when the power flux (W/m2) on a receiver is increased by means of heat localization from a large highly-conductive absorber to a small receiver area.

An illuminating example where both concepts have been successfully used is thermoelectric generators.

Both techniques have their advantages and drawbacks. Optical concentration is much more flexible in terms of design, while for thermal concentration to work, the receiver area must not be solar-interactive (e.g., a solar cell requires interaction with solar radiation not conducted thermal energy, whereas for a thermoelectric generator you only need a heat source for the temperature gradient).

All the best,

Omar

Optical and heat concentration refeers to the increase of the power density (W/cm2) of radiation and heat, respectively, through space. For optical concentration you can use lenses. For thermal concentration you might use a "tapered" and thermally insulated solid, through which heat flows. As the cross-sectional area of the solid decreases in the direction of the heat flow, the heat density (in W/cm2) increases. You can see examples in the following references:

Article Steady state analysis of a storage integrated solar thermoph...

https://aip.scitation.org/doi/abs/10.1063/1.49686?journalCode=apc