6.9: Energy Sources for Our Lives

Learning Objectives

• Know the diverse energy sources used in the United States.
• Apply the laws of thermodynamics in understanding how these energy sources function.

Ready access to energy is perhaps the greatest luxury of the modern world. It is also a limited resource which we have the necessary knowledge of the laws of nature to understand in context. Let us first consider where our energy comes from and where it might continue to come from going forward. Then we will consider how both the first and second laws of thermodynamics relate to our energy consumption.

Figure \(\PageIndex{1}\): The energy sources that were used for US primary energy consumption from 2022.

More than 90% of the energy use in the United States ultimately came from the Sun. (The two exceptions are nuclear electric power and geothermal energy. Nuclear electric power will be discussed more in a later chapter.) The three leading sources of energy (petroleum, natural gas, and coal, which are collectively referred to as fossil fuels) are all stored energy that initially came from the Sun. This energy was first stored via photosynthesis of living organisms, and remained stored in chemical bonds for hundreds of millions of years after those organisms died. When these resources are extracted from the Earth, it is an extraction of hundreds of millions of years of sunlight that will take an additional hundreds of millions of years to replenish.

Almost all of the renewable energy is also energy that ultimately comes from the Sun. The largest chunk of this energy is biomass, which is simply the renewable form of fossil fuels: energy stored in chemical bonds due to photosynthesis. In this case, that stored energy has not been buried within the earth for hundreds of millions of years. We could continue to rely on it without worrying that it will run out some day (although there are other considerations for this kind of energy that we will explore in a later chapter). Wind and hydroelectric power are also based on changes to our environment brought about by the Sun: there would be no wind or rain if it was not for the Sun.

One of the challenges with renewable energy sources is that the Sun does not always shine and the wind does not always blow. Because of this inconsistency, these energy sources have largely been used to supplement the existing fossil fuel-based energy economy. So long as we want to flip a switch to power a device at any time of day, we will need a consistent supply of energy.

One solution to this problem of inconsistent access to energy is finding renewable ways to store this energy when it is available for capture as either sunlight or wind and then use it at a later time when it is needed. There are a variety of technologies that have been created to aid in this process, but they are more expensive than the cheap fossil fuels we have become accustomed to. Such technologies include hydrogen electrolysis, several varieties of grid-scale batteries, pumped-storage hydropower, and solid mass gravity batteries. Some of these technologies are currently being used at small scale, while others have been proposed or are under construction.

The largest challenge to cheaply available public power, however, is not the limited times when renewable energy is available, but in the high demand for energy throughout the residential sector at certain times of day. You may be familiar with time of use variations in charges on your energy bills. These charges are meant to incentivize people to space out their energy consumption where possible so as to avoid using more expensive energy sources during times with higher demand. If the demand becomes too high, such as when temperatures are high and people are running their air conditioners, there could be intermittent power outages throughout some of the area where this power is being supplied.

The First Law and Energy Sources in Our Lives

The conversation to this point has been about applying the First Law of Thermodynamics to our use of energy. We can think of the internal energy of the Earth changing as we extract resources from it and as the Sun provides additional energy. We have a net Q coming to us from the Sun. Some of this energy was stored within the Earth to increase the internal energy for hundreds of millions of years, but now those energy storage reservoirs are being depleted. Currently, this internal energy of stored sunlight is decreasing as we tap into it to do all the work we want whenever we power on a device.

Likewise, all of the ways in which we have found to harness renewable energy sources are based on an understanding of the First Law. Each of these methods is centered around the idea that all energy can be changed to other forms of energy, and in the case of humans we prefer electrical energy. Realizing that the gravitational potential energy of water behind a dam or the chemical potential energy within a biomass could be converted to electrical energy is necessary to create a functioning electrical grid. Likewise, conservation of energy principles are at play in determining electricity rates at different times of day based on the energy sources that are always available and that could be made available but at a higher cost. The energy demanded must be supplied, but the cost of the supply energy can vary quite a lot.

The Second Law and Energy Sources in our Lives

It turns out that the Second Law of Thermodynamics also plays a role in supplying us with the electricity we have become accustomed to. As was mentioned earlier, the Second Law was discovered during the Industrial Revolution as researchers tried to maximize the amount of work that could be done due to heating an object. From this scientific research we know that there is a limit to the amount of useful work that can be obtained by heat flowing between objects at different temperatures. In fact, the best way to maximize the efficiency is by making the highest possible temperature for the hot reservoir and the lowest possible temperature for the cold reservoir.

If we apply this idea to the ways in which our power is generated and used, we can see why some options are preferable to others. One application is the massive energy needs of data servers. There are more and more people using the internet, and the growing demand for AI creates a need for increased server capacity. Although the increasing demands on servers have been massive, rapid improvements on energy efficiency within these servers has limited the impact. One of these improvements has been the recent increase in servers housed within very cold regions of the world. Keeping the servers cold maximizes the efficiency of the energy transfer.

Another application is in where we create energy by burning fossil fuels (or any other fuel, for that matter). Most power plants attempt to maximize the temperature of the burning substance in order to maximize the efficiency of the power production process. These power plants often operate at well in excess of 1000°C. Some of the energy generated is also stored by increasing the pressure of the steam into which this energy flows before it flows on to the electrical generator. But in the internal combustion engine of an automobile, the maximum temperature must be less than 900°C due to the limits of the engine materials. Additionally, the engine coolant system transfers much of this energy away from the engine in a way that does not result in any increase in the work being done by the car. (In contrast to the coolant system within a power plant, which is directly linked to the power generation process.) Because of these differences, a power plant is more than twice as efficient as an internal combustion engine in generating useful work.

The value of electric vehicles can be seen as relates to both the first and second law discussions. Electric vehicles can be charged at times of day that help to stabilize the load on the electric grid. Additionally, the electricity created to power an electric vehicle was done so much more efficiently at a power plant than it would be inside of an internal combustion engine. So although the battery of an electric vehicle is not itself a source of energy, the energy that led to the powering of the electric vehicle was created more efficiently than it would have been within an internal combustion engine. One of the benefits of electric vehicles is that they will decrease the depletion of stored energy from the Sun, even if they ultimately derive their energy from a power plant that burns fossil fuels.

Long term we want to find ways to become more reliant on renewable energy sources and less so on the limited supply of fossil fuels. In later chapters we will learn about additional challenges related to the use of fossil fuels.

Section Summary

• Fossil fuels consist of petroleum, coal, and natural gas, and consist of energy in chemical bonds which has been stored for hundreds of millions of years.
• The most prevalent sources of renewable energy include biomass, solar, wind, and hydroelectric.
• All energy sources except nuclear power and geothermal energy ultimately derive from the Sun.
• The United States relies mostly on fossil fuels as an energy source, but there are also a limited number of renewable energy sources.
• More efficient systems can be designed by having bigger differences in temperature between the hot and cold reservoirs and by lowering the temperature of the cold reservoir.