Addressing the effects of global warming and climate change require that we better understand energy use. The world economy uses and produces many different types and sources of energy, which can be grouped into the general categories of primary and secondary.[1]

Primary energy includes fossil fuels (petroleum, natural gas, and coal), nuclear energy, renewable energy (wind, solar, hydro, and geothermal) and biomass (wood and recently living plants). The Sun is the creative source of all primary energy. Virtually all greenhouse gas (GHG) emissions result from the use of fossil fuels and biomass. Renewable energy and nuclear energy are carbon-free, although there are concerns other than carbon associated with nuclear energy.

Secondary energy is generated (produced) from primary energy through a synthetic process. Electricity is the most dominant example of secondary energy. Electric power plants use primary energy to produce electricity. Other forms of secondary energy are hydrogen, compressed air, chemical energy from charged batteries, and the potential energy from water or other mass raised to a higher elevation (pumped storage). Secondary energy forms are man-made and most do not normally exist in nature (at least in a usable form); they are created from primary energy.

The diagram below shows primary energy forms at the top and secondary energy forms at the bottom. The diagram is used in this blog to illustrate the various energy flows in our economy. Processes convert one form of energy to another and from one location to another. The arrows represent power lines, pipes, etc. used to move energy from one form to another. Natural gas can be reformed to produce hydrogen (with CO₂ as a byproduct). District energy plants use primary energy to deliver chilled water, hot water or steam to buildings and other uses. GHG emissions occur in the production of secondary energy, but once created, using secondary energy produces little or no GHG (with the exception of burning biofuels).

 

Fossil fuel extraction, refinement, delivery and combustion is responsible for practically all GHG emissions. Some of this is the result of direct combustion in vehicles and buildings; some is associated with the combustion of fossil fuels at electric generators; and some is related to extraction, refinement and delivery of fossil fuels. Nuclear and renewables are carbon-free and are used almost exclusively to generate electricity, the principal form of secondary energy.

Fossil Fuels

The GHG emissions from directly burning fossil fuels are obvious and well understood. The main GHG is carbon dioxide, but small amounts of methane and nitrous oxide are also released to the atmosphere through combustion.  In addition to combustion emissions, significant GHG emissions also occur from extracting fossil fuels from the earth, refining and processing them, pressurizing gases, and delivering the products to buildings, power plants or gas stations. Some of the fuel is burned upstream to support these processes, but more significantly, a considerable amount of methane is leaked directly to the atmosphere between the well head and the point of combustion and methane is far more damaging than CO₂, especially in the short term. Emissions are increased even more when liquifying natural gas and shipping it to other countries. Opportunities to capture GHG from direct combustion of fossil fuels exist, but are emerging technologies that are far from being cost effective at scale.

Direct Use of Fossil Fuels

Nuclear

Nuclear energy accounts for about 20% of electricity generation in the United States, but only one new plant has become operational since the early 1990s.[2] US construction of new plants stalled after the Three-Mile-Island incident in 1979, and in the last three decades more plants have been decommissioned than have become operational. France has had more success, where about 70% of electricity is generated by nuclear energy. Research is continuing on new technologies,[3] but nuclear power generation is not expected to grow in the US, at least for the next decade or so.

Renewable Energy

Renewable energy includes hydro, wind, solar and geothermal. The heyday for hydroelectric generators in the United States was during the late 1930s when Hoover, Grand Coulee and other major projects were completed. Construction of major hydroelectric plants has been stalled for decades because of environmental impact and a lack of suitable sites. Hydropower is responsible for about 7% of electric power generation in the US. Wind and solar (coupled with batteries) are the emerging renewable energy technologies and are growing at an exponential rate. In 2020, electric power from wind surpassed hydro as the largest source of renewable electricity generation in the United States. Hydro, wind, solar and geothermal are used almost exclusively to generate electricity. Operational carbon emissions from all renewable energy is essentially zero.

Biomass

Biomass consists of wood, switchgrass, and other materials from recently living plants. Fossil fuels derive from plants and animals that lived 150 to 400 million years ago (thus the “fossil” adjective). Biomass is sometimes labeled as renewable energy, but in terms of carbon emissions, it is very different from wind, solar and hydro. Carbon emissions from the use of biomass are complicated. While living, plants absorb CO₂ and produce oxygen. When plants are harvested or die, they no longer absorb CO₂, instead they release CO₂ in to the atmosphere. Carbon is emitted slowly if the biomass is allowed to decompose naturally, but very quickly if the biomass is collected, processed and burned.

Timing is a key issue with biomass. The use of biomass usually results in a burst of GHG emissions when the biomass is burned. The case for biomass being considered renewable is that the biomass will grow back and the carbon absorbed as it grows would offset the emissions from combustion. There is uncertainty about whether the biomass will grow back in the future (with pressures of development, etc.), but even if it does we are trading off massive emissions at the present against the doubtful possibility of carbon capture decades in the future.

To assess the impact of using biomass two sets of emissions need to be calculated. First are the emissions that would occur as result of the proposed action. Second are the emissions that would occur if no action were taken, i.e., the counterfactual. The impact is the difference between the emissions of the proposed action and the emissions of the counterfactual. The following are some examples of how biomass may be used, ordered from somewhat beneficial to worst:

Possible Biomass Actions	Counterfactual	Impact
Methane is captured from feed lots or landfills and mixed with natural gas supplies or used to generate electricity. CO2 is released to the atmosphere as the methane is burned..	Methane would be released to the atmosphere but at a slower rate. Methane has a global warming potential much greater than CO2.so the climate damage could be greater	Somewhat beneficial. The global warming from the CO2 release would generally be less than the slower release of methane.
Switchgrass, corn or sugar is grown and processed into ethanol. GHG would be released through farming, processing, delivery and finally from combustion of the ethanol.	The land would be used (or continue to be used) to grow, process and deliver food to consumers, a process which would also generate GHG.	Depends. The main difference between the action and the counterfactual is the combustion of the ethanol. This represents a burst of GHG as opposed to a slow release from digestion and food waste in landfills. Also, land that would otherwise be used to grow food would be repurposed to generate a biofuel.
Wood or waste is burned to make electricity. CO2 and other pollutants are released to the atmosphere. This release would be near instantaneous in geologic time.	The wood or waste would release CO2 and other GHG if it is left in the forest or land fill to decay. This process would be slow and last for decades	Bad. Wood/waste power plants are extremely dirty, producing more pollutants per MWh than even coal plants. While similar amounts of GHG would result from the counterfactual, the release would be spread over decades.
Timber is harvested to produce pellets that are processed and shipped overseas to make electricity. GHG is released from harvesting, transportation, processing the pellets, shipping them to the power plant and from combustion at the power plant.	The unharvested forest would continue absorb GHG from the atmosphere.	Really Bad. Significant GHG emissions would be released from the activities of making pellets, shipping them to a remote location, and burning them. The impact would be amplified as a carbon sink is eliminated.
Rain forests are burned to clear land for agriculture or grazing.	The rain forest would be maintained and nurtured as a major carbon sink.	Worst. A significant carbon sink is destroyed and massive carbon emissions result from the fire.

Electricity

The secondary energy form of most significance is electricity. The GHG emissions from electricity use depends on how the electricity is generated and delivered buildings, industry and transportation. GHG emissions from electricity are small or negligible at the point of use. Generation emissions are extremely high for coal power plants and near zero for nuclear and renewables (wind, solar and hydro). Coal and petroleum plants emit over 500 kg of CO₂e per MWh generated. Gas plants are better because of combined cycle technology, but still emit almost 300 kg/MWh on average. There are opportunities to moderately improve the efficiency of these fossil-fired generators, but the only way to completely eliminate GHG emissions is through carbon capture and storage (CCS) or carbon capture and use (CCU). This is an expensive process that is currently used only on an experimental basis.[4] Electricity storage is minimal with conventional electricity, since gas-fired plants can easily modulate output to meet the instantaneous load.

Conventional Electricity

The carbon emissions from nuclear, wind, solar and hydro are near zero per unit of electric power produced. The EIA and others predict significant growth in renewable electricity generation and the current United States executive policy is to have a carbon-free electric grid by 2035, just 10 years away. California, New York and other states have similar goals and have already achieved carbon emissions less than half the national average. The levelized cost of electricity (LCOE)[5] from renewable sources is already competitive with the most efficient combined cycle natural gas plants and this is without considering the social cost of GHG emissions. Consideration of the social cost of GHG would vastly increase the cost of operating fossil-fired power plants making renewables even more cost effective, in comparison to fossil-fired plants. There is a clear pathway to a carbon-free economy by replacing fossil fuel with renewable energy and nuclear electricity, coupled with various forms of storage, e.g., batteries, pumped storage, etc.

 

Clean Electricity

Hydrogen

Hydrogen gets a lot of attention, but like electricity, it is a secondary form of energy. Hydrogen can be used in a fuel cell to produce electricity to power buildings or cars. Whether used in a fuel cell or burned for heat, the byproduct is water. Producing hydrogen is currently a very carbon intensive process. About 95% of global hydrogen is currently generated by reforming natural gas, which produces just as much GHG as burning the natural gas. This form of hydrogen is called “gray”.

Gray Hydrogen

About 5% of global hydrogen is produced through electrolysis, but most of the electricity used in the process is not from renewable sources.[6] “Green hydrogen”, produced through electrolysis and renewable energy, represents less than 1% of total global hydrogen production.

Since green hydrogen can be made from electricity by splitting water that can later be converted back to electricity through a fuel cell, it is constructive to view green hydrogen as a form of electric storage. An advantage over batteries is that hydrogen could possibly provide seasonal storage. However, the round-trip efficiency of starting with renewable electricity, converting it to hydrogen and ending up with electricity again is less than 50%.[7] For comparison, the estimated RTE of modern batteries is well above 90% and pumped storage is in the range of 75%.

Green Hydrogen

 

Thermal Energy

There are many forms of thermal energy. The most common are district heating systems, whereby coal, gas or petroleum is burned in a boiler to produce hot water or steam that is delivered to buildings or industry for heat. The entirety of Moscow and many other Russian cities are heated this way. A dozen or more US cities have district heating. City-scale systems are often inefficient with old boilers and  considerable pipe leaks. Smaller campus-level systems are generally more efficient. A common assumption for district heating is 70% efficient boilers with 10% distribution losses for hot water and 15% losses for steam.[8]

District Heating

Central multi-building thermal plants can also provide chilled water to cool buildings or industrial processes in addition to hot water or steam. Such plants are powered by electricity which can either be conventional or from renewable sources. The Stanford University and Google’s Bay View campuses are examples of modern all-electric systems powered by renewable energy. The diagram below is a simplification of these advanced systems, which also include hot and chilled water storage tanks and ground heat exchangers to substitute for or replace cooling towers. When powered by 100% renewable energy like the Stanford and Google case studies, the carbon emissions are zero.

District Heating and/or Cooling

The concepts of primary and secondary energy are well established. The goal of this blog is to introduce a common visual diagram to illustrate the differences and to provide nuances on particular technologies like electricity, hydrogen and thermal energy.