Year 9 Science · Unit 3 · Lesson 15
Challenge Worksheet
Learning Goals
1. Describe the energy changes when green hydrogen is made from surplus solar electricity and later used to make electricity again.
2. Complete the comparison table using the values in the lesson.
| Criterion | Lithium-ion battery | Pumped hydro | Green hydrogen | Compressed air |
|---|---|---|---|---|
| Approximate cost per kWh stored (dollars) | 400 | 500 | 150 | |
| How long it can supply power | 6 to 100 hours | 2 to 24 hours | ||
| Round-trip efficiency (%) | 85 to 95 | not given in the lesson | ||
| What the site needs | Almost any site | Tanks, at almost any site | An underground cavern |
Find the mistakes: a student’s calculations
(a) Snowy 2.0 stores 350,000 MWh and can generate 2,200 MW. Hours at full power = 2,200 ÷ 350,000 = 0.0063 hours.
(b) To store as much as Snowy 2.0, you would need 350,000 ÷ 13.5 = 25,926 home batteries of 13.5 kWh each.
(c) A pumped hydro scheme is 80% efficient. Put in 100 MWh, and you get back 100 − 80 = 20 MWh.
(d) Raising 1,000 tonnes of water by 300 m stores Eₚ = 1,000 × 9.8 × 300 = 2,940,000 J.
Source for the Snowy 2.0 values: Snowy Hydro (snowyhydro.com.au).
3. Find the single slip in each of the student’s four storage calculations. Explain the slip and give the right result, with its unit.
| Part | What went wrong | Correct answer |
|---|---|---|
| (a) | ||
| (b) | ||
| (c) | ||
| (d) |
Design brief: Sunvale
Sunvale is a town of 5,000 people with a large solar farm. The town uses 100 MWh of electricity each day, and 60 MWh of that is used between 6 pm and 6 am. There are no rivers or hills nearby. On a sunny day, the solar farm makes more than enough energy for the whole day. A few times each winter, the town has three cloudy days in a row when the solar farm makes almost nothing. Use the lesson’s approximate costs in your comparison table (1 MWh = 1,000 kWh).
Source: sample values for this worksheet.
4. Calculate the cost of lithium-ion batteries big enough to store (a) one night’s electricity and (b) enough for three cloudy days in a row.
(a) One night dollars
(b) Three cloudy days dollars
5. Green hydrogen could store energy for the three cloudy days instead. If hydrogen storage returns 40% of the electricity put into it, how much solar electricity must be stored to get back three days of electricity? What does this mean for the size of the solar farm?
Input to hydrogen storage MWh
6. Design a storage plan for Sunvale that uses at least two technologies. Justify each choice using your comparison table and calculations, describe how the technologies work together across a normal day and a cloudy spell, and state the biggest trade-off the council must accept.