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Date May 2017 Marks available 2 Reference code 17M.2.SL.TZ1.3
Level Standard level Paper Paper 2 Time zone 1
Command term Explain Question number 3 Adapted from N/A

Question

Two renewable energy sources are solar and wind.

An alternative generation method is the use of wind turbines.

The following data are available:

Length of turbine blade = 17 m
Density of air = 1.3 kg m–3
Average wind speed = 7.5 m s–1

Describe the difference between photovoltaic cells and solar heating panels.

[1]
a.

A solar farm is made up of photovoltaic cells of area 25 000 m2. The average solar intensity falling on the farm is 240 W m–2 and the average power output of the farm is 1.6 MW. Calculate the efficiency of the photovoltaic cells.

[2]
b.

Determine the minimum number of turbines needed to generate the same power as the solar farm.

[3]
c.i.

Explain two reasons why the number of turbines required is likely to be greater than your answer to (c)(i).

[2]
c.ii.

Markscheme

solar heating panel converts solar/radiation/photon/light energy into thermal energy AND photovoltaic cell converts solar/radiation/photon/light energy into electrical energy

 

Accept internal energy of water.

a.

power received = 240 × 25000 = «6.0 MW»

efficiency « = 1.6 6.0  = 0.27 / 27%

b.

area = π × 172 «= 908m2»

power =  1 2 × 908 × 1.3 × 7.5 3  «= 0.249 MW»

number of turbines « = 1.6 0.249 = 6.4 » = 7

 

Only allow integer value for MP3.

Award [2 max] for 25 turbines (ECF from incorrect power)

Award [2 max] for 26 turbines (ECF from incorrect radius)

c.i.

«efficiency is less than 100% as»

not all KE of air can be converted to KE of blades

OR

air needs to retain KE to escape

thermal energy is lost due to friction in turbine/dynamo/generator

 

Allow velocity of air after turbine is not zero.

c.ii.

Examiners report

[N/A]
a.
[N/A]
b.
[N/A]
c.i.
[N/A]
c.ii.

Syllabus sections

Core » Topic 8: Energy production » 8.1 – Energy sources
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