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Why store energy?

Energy and power · level 7, lesson 1 of 6

A question to start

Solar panels give power only by day. A Mars base needs power at three in the morning too, for air, heat and light. How much energy must it keep for the night, and what could it keep it in?

The idea

Supply and demand rarely match. Panels make most at noon; people want light after dark. An energy store fills up when there is spare power and empties when there is not. Its size is

\[\text{energy stored} = \text{average power needed} \times \text{time it must last}.\]
from the storefrom the store chargingdemandsolarnoon

There are many ways to keep energy:

Store Energy kept as Example
Battery Chemical Phones, cars, every rover
Fuel and oxygen Chemical Hydrogen in tanks, used in a fuel cell
Raised water or weights Gravitational Pumped storage power stations
Spinning wheel Kinetic Flywheels
Hot material Thermal Tanks of hot water, molten salt or hot sand

Engineers compare stores by energy per kilogram (specific energy), by how much of the energy comes back out (round-trip efficiency), and by how long they can hold it. Some numbers that matter on Mars:

Storing energy Mars Earth
Night at the equator, about 12 h 20 min (half a sol) 12 h
1 kg raised 100 m stores 371 J 981 J
Oxygen in the air, to burn a fuel 0.16% 20.95%

That last row matters: on Earth, a fuel cell or engine takes its oxygen from the air for free. On Mars the oxygen must be made and stored too.

Worked examples

1. A night's store. A base uses 10 kW on average. Through a 12 h 20 min Mars night it needs \(10 \times 12.33 = 123\) kWh. On Earth, through a 12 h night, 120 kWh.

2. In batteries. Lithium-ion cells store about 200 Wh per kg. 123,000 Wh ÷ 200 \(\approx 620\) kg of cells.

3. In raised water. 123 kWh \(= 4.4\times10^8\) J. Raised 100 m on Mars, that needs \(m = \dfrac{4.4\times10^8}{3.71 \times 100} = 1.2\times10^6\) kg: 1,200 tonnes of water. Gravity storage is bulky.

Try it

  1. Why does a solar-powered base need an energy store?
  2. A base uses 4 kW through a 12 h 20 min Mars night. How many kWh must it store?
  3. How many kilograms of 200 Wh/kg cells is that?
  4. Why do energy-per-kilogram figures for hydrogen flatter it on Mars?
Answers
  1. Its panels give nothing at night, and little in a dust storm, but it needs power all the time.
  2. \(4 \times 12.33 \approx 49\) kWh.
  3. \(49{,}000/200 \approx 250\) kg.
  4. They count only the hydrogen. On Mars the oxygen it needs must be stored as well, and there is eight times as much oxygen as hydrogen by mass.

Common mistake

Mixing up kW and kWh. The power (kW) says how fast energy is used; the store must hold energy (kWh). A 10 kW base needs a 10 kW supply and a store of 123 kWh for the night.

Where it leads