# Harnessing direct solar energy – a progress report

## Activity 2

Calculating the amount of solar radiation that can be converted to electricity

The following table lists the daily average hours of sunshine for a number of Australian cities, together with their latitudes. Use the information in the table to help you answer the following questions.

 City Daily average sunshine (hours) Geographical latitude (degrees south) Darwin 8.4 12.2 Brisbane 7.5 27.3 Perth 7.8 31.6 Sydney 6.7 33.5 Adelaide 7.0 34.5 Melbourne 5.7 37.5 Canberra 7.2 35.2 Hobart 5.6 42.5
1. Use the information in the table to plot a graph of daily average sunshine against geographical latitude. What relationship does your graph show? Which cities do not fit the relationship perfectly? Suggest reasons for these variations.

2. In Australia, an average of 400 joules of energy are received by each square metre of the Earth’s surface each second of daylight.

Photovoltaic cells convert solar radiation to electricity and are about 25 per cent efficient. What area of photovoltaic cells is needed to generate enough power to run:

• a desk-top computer using 300 watts
• an electric frying-pan using 1350 watts
• a 2-slice toaster using 600 watts? (Power is the rate at which energy is transferred and is measured in watts. Bear in mind that 400 joules of energy per second are equivalent to 400 watts.)
What area of photovoltaic cells is needed to run all of these appliances simultaneously?

3. Briefly outline the main problems associated with using photovoltaic cells for domestic use.

Teachers notes

Sunshine is not the same as daylight. Calculations in this activity are only very approximate because we are not taking into account the fact that photovoltaic cells can work in daylight without sunshine, albeit at a reduced rate.

1. Plotting the data given in the table shows an inverse relationship between geographical latitude and daily hours of sunshine – as latitude increases, the average hours of sunshine decrease. Perth and Canberra (and perhaps Adelaide) are the cities that do not fit the relationship as well, because they have fewer cloudy days.

• a desk-top computer needs 3 square metres of photovoltaic cells
• an electric frying-pan needs 13.5 square metres
• a 2-slice toaster needs 6 square metres
• 22.5 square metres is needed to run all of the appliances simultaneously.
Note that the precise figures vary according to the elevation of the sun in the sky and local factors such as cloud cover.

2. The main problems associated with photovoltaic cells for domestic use are:

• the sun is not available when domestic energy needs are the highest;
• the energy has to be stored until it is needed and this requires expensive, bulky batteries;
• a large area of photovoltaic cells is required;
• the photovoltaic cells need direct exposure to the sun – northerly aspect without shade.
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Posted February 1997.