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Saturday, May 2, 2015

Solar farming and electricity generation

Here's a challenge in financial optimisation while combining available technologies:
  1. Algae Tec has commercialised a method of growing algae for nutrients, edible oil and biofuels. It includes solar concentrators - but green algae (like all green plants) can only use the visible light photons with wavelengths of 400 nanometres (nm) up to 700 nanometres.
  2. Solar photovoltaic panels can convert concentrated solar energy into electricity - but silicon photovoltaic cells only use the photons with wavelengths up to 1100 nm. The infrared energy with photons having longer wavelengths only heat up the solar cells and reduce their performance.
  3. Sundrop Farms has commercialised solar concentrators that use solar energy of any wavelength but only convert it to heat that is then used to convert seawater and saline ground water to fresh water for high-value food crops in arid regions.

An interesting possibility is to concentrate sunlight then split it into 3 beams - one with wavelengths that algae and other green plants use, one with wavelengths that silicon cells can convert efficiently to electricity, and the third that heats a transfer liquid to be used for, say, converting seawater to fresh water...

Calculating the return on investment is the challenging part of the puzzle. In particular, can the investment in concentrating sunlight that Algae Tec and Sundrop Farms exploit be made even more profitable by splitting the sunlight and using each of 3 beams in the most profitable application available for each?

From "A guide to solar energy"

The energy required to move an electron from the semiconductor atom to a conducting state is a fixed amount. The energy of a photon of light is determined by its wavelength, with shorter wavelength photons having higher energy than those with longer wavelengths.
energy spectrum of sunlight and how it affects photovoltaic efficiency
Energy spectrum of sunlight and how it affects photovoltaic efficiency

A photon with wavelength 1,100 nanometres (nm), corresponding to short wave infra-red light has just enough energy to promote an electron in a silicon atom, the most commonly used semiconductor material.

All photons with a longer wavelength than this have insufficient energy to promote the electron and either pass straight through the PV cell or are absorbed as heat. This part of the solar spectrum cannot be used by the PV cell.

Photons with a shorter wavelength than 1,100nm have more energy than is required to promote the electron. The excess energy above that needed to move it into a conducting state is lost as heat.

These two factors combine to produce a theoretical upper limit to PV efficiency of around 31%.

From "Starlight is the solar power of the earth"

All biological energy comes from sunlight and this energy encompasses the range of the electromagnetic spectrum known as light. The solar spectrum is shown below.

The green pigment, chlorophyll, plays a central role in photosynthesis. The fact that it is green means that it absorbs blue and red light and reflects green when it is illuminated by white (all wavelengths) light.

Light with  wavelength longer than 700nm has insufficient energy to drive photosynthesis.

From "A beam-splitting photovoltaic thermal receiver for solar concentrators"

Ahmad Mojiri, Cameron Stanley and Gary Rosengarten
Royal Melbourne Institute of Technology Melbourne, Australia

A photovoltaic thermal receiver that separates incoming light energy by wavelength can produce electricity and thermal output of 150° simultaneously.

8 January 2015, SPIE Newsroom. DOI: 10.1117/2.1201501.005704

Sunlight is an abundant source of energy that can be converted into heat and electricity using photothermal and photovoltaic technologies, respectively. Usually these devices are separate from each other, and occupy significant space on a rooftop or in a solar park. Combining thermal and electrical output in a single package would achieve several advantages, such as more efficient use of available space and light collection.
Spectral splitting mechanisms
Figure 1. Spectral splitting mechanisms using (a) wave interference effect and (b) selective volumetric absorption. HRI and LRI correspond to high and low refractive index materials such as titanium dioxide and silicon dioxide, respectively. A specific number (n) of these layers are required to achieve suitable spectral splitting.

The solar spectrum consists of wavelengths in the range 400–2500nm, while silicon solar cells function most efficiently for the range 700–1200nm. Our beam-splitting mechanism separates light in the 700–1200nm range and directs it to the PV cells, sending the rest of the solar spectrum to a thermal absorber. The thermal and PV receivers are then being fed by two separate beams of light. The silicon cells remain cool at ambient temperatures.

We designed our system for a commercial parabolic trough: a partially curved, mirror-lined solar collector. We constructed a detailed ray tracing model (used to calculate the path of light waves through a system) for the proposed integration of our device in the solar concentrator, and optimized the dimensions of the receiver to maximize the energy yield of the system.


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