The costs of clean and renewable energy like solar and wind have dramatically fallen in the last decade. An important benchmark is grid parity, meaning that the cost of electricity from renewable sources equals the cost from conventional sources like coal or natural gas. Wind energy has already reached grid parity in areas with strong wind, as has solar energy in sunny states like California. After reaching grid parity, the cost of electricity from renewable energy will be lower than electricity from fossil fuels. Based on these projections, there appear to be no advantages for fossil fuels.

Renewable energy, however, has two primary problems: immobility and intermittency. While an immovable building can be constantly connected to the grid and to solar cells, a car or plane needs to carry its energy (such as gasoline or diesel) on board. Thus, solar and wind cannot be used to directly power automobiles. Secondly, renewable energy is intermittent: the sun does not always shine, and the wind does not always blow. Yet there is always demand for electricity. Currently, utilities use natural gas turbines with fast startup times (several minutes) to generate electricity on demand. However, increased renewable penetration will overwhelm the ability of utilities to stabilize the grid. In Hawaii—where sunlight is abundant but electricity is expensive—solar energy adoption has stalled because increased solar energy is destabilizing the grid. For significant adoption of renewable energy, we need to be able to store the energy so that it can be used in cars and when the sun does not shine or the wind does not blow.

Batteries are devices that store electricity (or electrical work) in chemical bonds. When there is excess electricity, the battery is charged by converting electrical work into chemical bonds; when electricity is needed, the battery is discharged and the work is recovered. By storing and releasing energy on demand, batteries solve the problem of intermittency associated with renewable energy. Moreover, because batteries are portable, they can be used in transportation (for example, in the Tesla Model S or in modern electric buses).

In this module we describe the thermodynamics of batteries. In particular, we explain the origins of the open-circuit voltage in batteries and show how a battery is analogous to a fuel cell. (Later sections discuss how the voltage varies with the state of charge, and how entropy and phase transformations come into play.)