generation: geothermal, biomass, & nuclear + side by side bottlenecks comparison (part 2)

part 2 of a series on the overview of renewables + clean generation sources

This is part 2 of a series exploring how we generate electricity through clean sources in the US. Part 1 is here.

The 6 sources of energy we explore in this series:

1. Solar

2. Wind

3. Hydropower

4. Geothermal

5. Biomass

6. Nuclear

For each, I cover:

1. How it works

2. Key bottlenecks

3. Some companies in the space

4. A comparison of bottlenecks and upfront costs across all sources (at the end of part 2)

This post covers the final three: geothermal, biomass, and nuclear + a summary of bottlenecks facing all 6 generation sources.

geothermal

Geothermal energy is generated by harnessing heat that is continuously produced by the earth's interior (core and mantle) as a result of radioactive decay. It can be used for heating, generating electricity and cooling.

Deep underground there are hot rocks, fluid, and permeability (the ability for fluid to move between rocks). These are the three key components of geothermal energy generation - each must be present. They can exist either by natural causes or using human-created techniques.

To create energy, fluid must flow through the hot rocks, absorbing their heat. That fluid then is drawn up through wells to the earth’s surface. This heat energy is converted to steam, which drives turbines that produce electricity before it is re-injected into the wells.¹

There are 3 main types of geothermal power plant technologies:²

1. Dry steam. This process uses hydrothermal fluids that are already mostly steam - which is a rare natural occurrence. These are the oldest types of plants, first used in Italy in 1904.

2. Flash steam. This is most common. Fluids at temp greater than 360F are pumped from underground and travel under high pressure to a low pressure tank at the earth's surface. This sudden change in pressure causes fluid to transform, or "flash,” into vapor. That vapor drives a turbine, which drives a generator to create electricity.

3. Binary cycle. These use lower temp geothermal resources, and the geothermal reservoir fluids never come into contact with the power plant's turbine units. Low-temp fluids (*below 360F*) pass through a heat exchanger with a secondary (binary) fluid which causes it to flash to vapor, which then drives turbines to create electricity.

As of 2023, geothermal makes up about 0.4% of US utility-scale electricity generation.³

bottlenecks for geothermal

• Interconnection delays + costs. Geothermal resources are often far away from existing transmission infra, making connection costly and slow.

• Permitting, siting, & regulatory constraints. Geothermal faces lengthy and complex permitting processes.

• Supply chain & materials. Drilling deep wells and construction of geothermal plants requires advanced technology.

• Skills gap. There’s a dearth of skilled labor that can build and maintain geothermal plants.

• High upfront costs. Building new geothermal plants is very capital-intensive, ranking third most capital-intensive on this list, behind only nuclear.

• Environmental concerns. There is potential for induced seismic activity like earthquakes. Over-extraction without proper re-injection of fluids can deplete resources and cause sinkholes.

companies doing geothermal

• Ormat Technologies has over 1100MW of geothermal capacity

• Calpine does both NG + geothermal

• Fervo Energy is a startup using fracking methods to create artificial reservoirs in hot rocks, pumping in water

biomass

Biomass is generated from the energy stored in organic matter like plants, trees, and agricultural waste (woody biomass, crop straw, corn, waste from households and businesses, and landfill gas). This energy is released through three processes, including combustion, bacterial decomposition (fermentation or anaerobic digestion), or conversion to gas or liquid fuel.⁴

1. Combustion: burning biomass generates heat or steam which turns turbines to produce electricity

2. Bacterial decomposition: anaerobic digestion happens when microorganisms break down biomass in absence of oxygen, which creates biogas, similar to natural gas. fermentation can create biofuels like ethanol.

3. Conversion to gas or liquid fuel. Two main ways:

   1. Gasification exposes solid biomass to high temperatures with very little oxygen present to produce synthesis gas (syngas), which is mostly CO and H. This can be burned in a conventional boiler to produce electricity or replace NG in a combined-cycle gas turbine.

   2. Pyrolysis heats biomass at a lower temp range in complete absence of oxygen to produce a crude bio-oil, which can be substituted for fuel oil or diesel in furnaces, turbines, and engines for electricity production.

Biomass was a huge source of US energy consumption in the 1800s. Today it is 5% of total primary energy consumption.⁵ As of 2023, biomass makes up 1.1% of total US utility-scale electricity generation.⁶

bottlenecks for biomass

• Permitting, siting, & regulatory constraints. Biomass faces lengthy and complex permitting processes because it competes with food crops for land and requires significant water, fertilizer, and energy for cultivation + processing.

• Supply chain & materials. Biomass resources are unevenly distributed or seasonable. Low energy density of wet biomass means transportation and storage is inefficient and costly.

• High upfront costs. Building new geothermal plants is very capital-intensive, ranking third most capital-intensive on this list, behind only nuclear.

• Environmental concerns. Biomass is considered carbon neutral but can emit CO2.

companies doing biomass

• Greenleaf has 154 MW of biomass generation capacity

• Gevo is a renewable chemicals and biofuel company

• Montauk Renewables specializes in recovery & conversion of biogas from landfill methane

• Rex specializes in ethanol production

nuclear

(Note: I dive deeper into nuclear in this post, part 3 [TBU])

Nuclear power generates electricity through fission. This happens when uranium atoms (isotope U-235 specifically) are split to release heat. This heat boils water into steam, which turns a turbine to generate electricity.

In the U.S., all 94 commercial reactors are light water reactors (LWRs), which use regular water to cool and moderate the reaction. There are two types of LWRs:

• Pressurized water reactors (PWRs): keep water under high pressure to prevent boiling.

• Boiling water reactors (BWRs): boil water directly in the reactor vessel to produce steam.

Nuclear provides ~50% of U.S. carbon-free electricity. Historically, the space has been dominated by hulking, large-scale plants that the public largely associates with nuclear catastrophes of the past. However, there has been renewed interest in developing alternative forms of nuclear generation, including through advanced reactors.

Advanced reactors are defined by aims to reduce cost, improve safety, and support grid flexibility. Small modular reactors (SMRs) are included in this category, as well as molten salt reactors, and high-temperature gas reactors. More on each of these in my deep dive on nuclear here.

bottlenecks for nuclear

• Permitting, siting, & regulatory constraints. Nuclear faces perhaps the most lengthy and complex permitting processes because of past catastrophes and faces social perception barriers.

• Supply chain & materials. Nuclear requires advanced components.

• High upfront costs. Building new nuclear plants is extremely capital-intensive, the most capital-intensive on this list.

companies doing nuclear

• Westinghouse, GE Vernova (has an SMR business), and Framatome are legacy players that have built traditional LWRs, but are also exploring the advanced reactor space. Nuscale is also an incumbent developing SMRs.

• Newer advanced reactor companies include:

  • TerraPower (founded by Bill Gates) which works on liquid metal + MSR

  • X-Energy which works on high temperature gas reactors HTGR

  • Oklo which works on microreactors

  • Kairos Power which works on salt-cooled / molten salt reactors

There’s been a lot of renewed interest in nuclear recently following the hyperscalers’ interest in getting firm power ASAP to fuel their AI datacenter arms race. I cover this more in depth in my part 3 on just nuclear. Read it here. [TBU]

bottleneck comparison

Below is a side-by-side comparison of the key bottlenecks facing each of these sources. Exact challenges vary by project and geography, but many themes, especially interconnection delays, permitting, and high upfront costs, and supply chain and materials struggles, are common appear across the board.

upfront cost comparison

Bottlenecks also translate into differences in capital requirements. Below is a rough comparison of how expensive it is to build new utility-scale projects in each category, based on average capital intensity. Data from Statista.

1. https://www.energy.gov/eere/geothermal/electricity-generation

2. https://www.energy.gov/eere/geothermal/electricity-generation

3. https://www.eia.gov/tools/faqs/faq.php?id=427&t=3

4. https://www.energy.gov/eere/bioenergy/biopower-basics#:~:text=Biopower%20technologies%20convert%20renewable%20biomass,conversion%20to%20gas/liquid%20fuel

5. https://www.eia.gov/energyexplained/biomass/

6. https://www.eia.gov/tools/faqs/faq.php?id=427&t=3