## Chemistry

EQ2.1.1 MANUFACTURING OF MONOCRYSTALLINE SILICON

(1 point possible)

Which of the following processes is used in order to fabricate monocrystalline silicon?

EQ2.2.1 EFFECT OF BYPASS DIODES

(1 point possible)

A given solar cell has the following specifications:

Isc=4A

Voc=0.7V

36 identical cells with the above specifications are to be interconnected to create a PV module.

What is the open-circuit voltage (in V) of the PV module if all the solar cells are connected in a series configuration?

What is the short-circuit current (in A) of the PV module if all the solar cells are connected in a series configuration?

(1 point possible)

Now suppose that the PV module mentioned above is set up using a series connection of solar cells with the above mentioned specifications. Two of the solar cells have gone faulty (completely stopped generating power), but fortunately you have bypass diodes connected across the faulty solar cells. Assume that the bypass diodes are ideal (they have a zero voltage drop when conducting).

What is the measured open-circuit voltage (in V) of the above PV module with the faulty solar cells?

The figure below presents the EQE of a triple junction solar cell with junctions A, B and C under short-circuited (V = 0 V) condition.

What is the band gap (in eV) of the absorber layer of the junction A?

What is the band gap (in eV) of the absorber layer of the junction B?

EQ2.3.3 TRIPLE JUNCTION SOLAR CELL

What is the band gap (in eV) of the absorber layer of the junction C?

EQ2.3.4 TRIPLE JUNCTION SOLAR CELL

Which of the following statements is true?

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Junction C acts as the top cell, junction B as the middle cell, and junction A as the bottom cell. Junction B acts as the top cell, junction C as the middle cell, and junction A as the bottom cell. Junction A acts as the top cell, junction B as the middle cell, and junction C as the bottom cell.

EQ2.3.5 TRIPLE JUNCTION SOLAR CELL

Each junction is illuminated under standard test conditions. Given the photon fluxes below, calculate the short-circuit current density (in mA/cm2) of each (separate) junction (A, B and C):

The Voc of each junction in V can be roughly estimated by the equation

Voc=Egap(J)2q=Egap(eV)2

where q is the elementary charge, Egap(J) is the band gap energy expressed in Joules, and Egap(eV) is the band gap energy expressed in eV. Assume a fill factor of FF=0.75. What is then the efficiency (in %) of the triple junction solar cell?

(1 point possible)

Figure 1 shows a simplication of the AM1.5 solar spectrum at 1000W/m2. The spectrum is divided in three spectral ranges:

A for 0nm<λ<620nm

B for 620nm<λ<1240nm

C for 1240nm<λ<1860nm

The photon flux in each spectral range is also shown in the figure.

The hydrogenated silicon carbide material (a-SiC:H) is a new type of amorphous semiconductor material which has been recently studied for PV applications. This material has a relative large band gap of 2.0 eV. Imagine we integrate this material in a single junction p-i-n solar cell as shown in Figure 2a below. In which spectral range does this solar cell convert light into charge carriers?

What is the Jsc (in mA/cm2) of the solar cell if only 65% of the absorbed photons result in a current?

The Voc in V of the a-SiC:H solar cell can be roughly estimated by the equation:

Voc=Egap(J)2q=Egap(eV)2

where q is the elementary charge, Egap(J) is the band gap energy expressed in Joules, and Egap(eV) is the band gap energy expressed in eV. The fill factor of the solar cell is FF=0.80. What is the efficiency of the solar cell (in %) ?

An up converter is a material which can convert two low-energy photons into a higher energy photon. Placing an up converter in our solar cell can help to reduce the spectral mismatch, since it can convert some photons with energy lower than 2 eV, which are not absorbed by the a-SiC:H cell, into a photon with an energy higher than 2 eV. Figure 2b depicts this possibility.

In the up converter 1, two photons are converted into one photon with 100% conversion efficiency. If all photons with energy above that of the band gap of a-SiC:H are absorbed in the a-SiC:H layer, in which spectral range can the photons be up-converted so that they contribute to the current in the cell as well?

In that case what would be the short-circuit current density and the efficiency of the solar cell illustrated in Figure 2b? Assume again that 65% of the absorbed photons result in a current.

In up converter 2 (see Figure 2c), three photons are converted into one photon with 100% conversion efficiency. If all photons with energy above that of the band gap of a-SiC:H are absorbed in the p-i-n cell, and converter 1 absorbs only the photons in the spectral range as considered in EQ2.4.4 to EQ2.4.6, in which spectral part can the photons be up-converted by converter 2 so that they contribute to the current in the cell as well?

In that case what would be the short-circuit current density and the efficiency of the solar cell illustrated in Figure 2c? Assume that 65% of the absorbed photons result in a current.

Family Smith has already installed PV in their house. Now, they also want to cover their needs for warm water with solar energy. For this, they consider having a solar thermal water heating system. Considering that the need for warm water is 100 L/day, the water has to be heated from 10 to 60 ºC and the specific heat capacity of liquid water is 4.18 J/gK.

What is the total energy, in Wh/day, that the system will need to supply to cover the warm water demand?

Considering an efficiency of 70% and an irradiance of 1000W/m2 for 3 equivalent sun hours, how much collector area, in m2, will be needed to cover the demand?

If only half of the hot water needed has to be stored, at least how big, in L, should the storage tank be?

The cost per m2 of collector is estimated as €120, and the extra costs for water tank and piping are €6 per L of storage. How much will the whole system cost?

However, Mr Smith has read an article in the newspaper in which it was claimed that at the moment it is more expensive to install a solar thermal system for water heating than directly use the PV electricity to heat up water. Therefore, he wants to make some calculations to check such assessment. If the price for Wp of a PV panel is €1 and the external costs are €400, and considering an efficiency of electricity to heat conversion of 85%, how much (in €) will it cost to cover the same amount of energy with PV technology?

Assume that the lifetime of both systems is 20 years and the maintenance costs are also equal for both cases. Which will be the better choice for water heating?

In this section, we talked about combining a solar cell with a photoelectrode to form a photoelectrochemical device. Why do we add a solar cell in the system?

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Because the photoelectrode is not able to produce any voltage difference to drive the reaction Because the photoelectrode is not able to produce any current to drive the reaction Because the voltage difference created in the photoelectrode is not able to produce enough voltage to drive the reaction Because the current created in the photoelectrode is not enough to drive the reaction

What is the approximate voltage that the solar cell needs to deliver for the photoelectrochemical device to work?