Year 9 Science · Unit 3 · Lesson 15
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Learning Goals
1. Why does a grid need storage that responds in milliseconds AND storage that can run for days? Give one example of each.
Real-world context: a pumped hydro scheme in the Flinders Ranges
South Australia sometimes produces more solar electricity at midday than it needs. Engineers have proposed a pumped hydro scheme in the Flinders Ranges. When there is surplus electricity, pumps push water from a lower reservoir up to a higher one. In the evening peak, the water flows back down through turbines.
Energy chains: charging and discharging
Fill in the missing forms of energy.
2. The scheme pumps 2,000 tonnes (2,000,000 kg) of water up a height of 150 m. Use Eₚ = mgh with g = 9.8 m/s² to calculate the energy stored. Give your answer in J, MJ and kWh (1 kWh = 3.6 MJ).
Held by the raised water J
Held by the raised water MJ
Held by the raised water kWh
3. The lesson says pumped hydro gets back 70–85% of the energy used to pump the water. Taking 80%, how much electrical energy does the scheme return from this water? Where does the rest go?
Energy returned kWh
Data: a home battery on a sunny day
A house in Perth has rooftop solar and a 13.5 kWh home battery. Table 1 shows how much energy was stored in the battery during one day. The household buys grid electricity at 30 cents per kWh (sample tariff).
| Time | Energy stored (kWh) |
|---|---|
| 6 am | 2.0 |
| 9 am | 3.5 |
| 12 noon | 8.5 |
| 3 pm | 13.5 |
| 6 pm | 13.0 |
| 9 pm | 6.0 |
| 12 midnight | 3.0 |
Source: sample data.
4. Plot the energy stored against time.
5. (a) Use P = E ÷ t to find the average charging power from 9 am to 12 noon and the average discharging power from 6 pm to 9 pm. (b) How much energy did the battery supply from 6 pm to midnight, and how much money did that save?
(a) Charging power kW
(a) Discharging power kW
(b) Energy supplied kWh
(b) Money saved dollars
6. A regional grid has two jobs for storage. First: when a generator trips off, the frequency starts to fall within seconds. Second: each winter brings three or four calm, cloudy days. Recommend pumped hydro or lithium-ion batteries for each job, and justify your choices using response time, how long each can supply power, efficiency and lifespan.