EVEN 2909: Introduction to Sustainability Engineering — Week 7
Evan A. Thomas, PhD, PE, MPH — University of Colorado Boulder — Fall 2026
The grid is a network that delivers electricity from producers to consumers. It must balance supply and demand in real time — every second of every day.
Power plants convert energy (fossil fuels, nuclear, wind, solar) into electricity. In the US, most power plants produce AC at 60 Hz.
High-voltage lines (115–765 kV) carry power long distances. Higher voltage = lower current = less energy lost as heat (P = I²R).
Transformers step voltage down (4–35 kV) for local delivery to homes and businesses. The poles and wires on your street.
Final step-down to 120/240V. Homes, commercial buildings, and industry consume electricity for lighting, heating, motors, and computing.
The US generates about 4,100 TWh of electricity per year. Natural gas is now the dominant source, having surpassed coal around 2016.
Source: EIA Electric Power Monthly, 2025. Coal was 52% of US generation in 2000 — now under 15%.
Key trend: Renewables (wind + solar) surpassed coal for the first time in 2022 and now generate more than 17% of US electricity.
A power plant's nameplate capacity (in MW) is its maximum output. But no plant runs at full power 24/7. The capacity factor tells you how much energy it actually produces relative to its theoretical maximum.
Capacity Factor = Actual Energy Output / (Nameplate Capacity x Time)
This is why installed solar capacity (GW) can be large but generation (TWh) can be modest. Wind and solar have lower capacity factors because the sun does not always shine and the wind does not always blow — not because the technology is flawed.
Source: EIA, 2024 capacity factor averages for US fleet.
Electricity demand fluctuates throughout the day. Grid operators must match supply to demand in real time — overproduction wastes energy, underproduction causes blackouts.
The minimum level of demand over 24 hours. Traditionally served by plants that run continuously: nuclear, coal, large hydro. These are expensive to start and stop.
The maximum demand, typically late afternoon on hot summer days (air conditioning). Served by "peaker" plants — usually natural gas turbines that can ramp up in minutes.
Think about it: Your lights do not dim when you turn on the microwave because grid operators are constantly adjusting generation — second by second.
Xcel Energy is the largest electric utility in Colorado, serving about 1.5 million customers along the Front Range. Colorado is in the middle of one of the most aggressive coal-to-renewables transitions in the country.
Local connection: CU Boulder's campus electricity comes from Xcel Energy. The university has committed to carbon neutrality.
Solar module costs have fallen 99% since 1976. This is the most dramatic cost decline of any energy technology in history.
Solar module price per watt ($/W), inflation-adjusted.
Power in wind = ½ ρ A v³
Power scales with the cube of wind speed. Double the wind speed → 8x the power. This is why turbines are getting taller (to reach stronger, steadier winds).
Betz Limit: Maximum theoretical efficiency is 59.3% — no turbine can capture more than that.
Onshore Wind
Mature, cheap ($20–40/MWh). Turbines up to 7 MW. Capacity factors 30–45%. Eastern Colorado has excellent wind resources.
Offshore Wind
Stronger, steadier winds. Turbines up to 15 MW. Higher capacity factors (40–55%) but 2–3x the cost. Growing rapidly in Europe; early stage in the US (Vineyard Wind, MA).
Modern onshore turbines: 100m+ hub height, 170m rotor diameter. A single turbine can power 2,500 homes. Eastern Colorado wind farms are some of the largest in the country.
Storage is the key to integrating variable renewables. Without it, we need fossil fuel backup for when the sun is not shining and the wind is not blowing.
Flow Batteries
Liquid electrolytes in tanks. Scale storage by adding more electrolyte. Good for long-duration (10+ hours). Iron-air and vanadium redox types.
Compressed Air (CAES)
Compress air into underground caverns when power is cheap. Release through a turbine to generate electricity. Two plants operating worldwide.
Thermal Storage
Store energy as heat in molten salt, sand, or bricks. Can store energy for days. Being piloted for industrial heat applications.
As solar capacity grows, midday electricity supply exceeds demand. The resulting net load curve (demand minus solar) looks like a duck. This creates two problems:
Nuclear power provides ~19% of US electricity with zero direct carbon emissions. It is the largest source of clean electricity in the country. But it is deeply controversial.
The carbon budget is the total amount of CO2 humanity can still emit while keeping warming below a specific target. It is a finite quantity — like a bank account we are rapidly draining.
At current emission rates, the 1.5°C carbon budget will be exhausted around 2030. The 2°C budget (~1,150 Gt remaining) gives us roughly until 2050 — but only if emissions begin declining now.
Source: IPCC AR6, 2023. Carbon budget estimates from the start of 2023.
Decarbonization requires tackling every sector of the economy. Each has different challenges and solutions.
Source: EPA Greenhouse Gas Inventory, 2024. Percentages of total US GHG emissions.
Key insight: Cleaning up the electricity sector is the foundation of all other decarbonization — because electrifying transport, buildings, and industry only reduces emissions if the grid itself is clean.
The core strategy: replace fossil fuel combustion with electric alternatives powered by a clean grid. Electrification is more efficient because electric motors and heat pumps waste far less energy than combustion.
A natural gas furnace is 80–95% efficient: burns gas, makes heat.
A heat pump achieves 200–400% efficiency: uses 1 kWh of electricity to move 2–4 kWh of heat from outside air into your home. It is not creating heat — it is moving it.
Some sectors cannot easily electrify. These "hard-to-abate" sectors account for ~30% of global emissions and need alternative solutions.
Cement
8% of global CO2. Heating limestone to 1,450°C releases CO2 chemically (not just from fuel). No easy substitute for the chemical reaction.
Steel
7% of global CO2. Blast furnaces use coke (from coal) to remove oxygen from iron ore. Green hydrogen can replace coke — but at higher cost.
Aviation & Shipping
~5% of global CO2. Batteries are too heavy for long-haul flights. Sustainable aviation fuels (SAFs) and green ammonia/methanol for ships are early-stage solutions.
Point-Source Capture
Capture CO2 from smokestacks at power plants or cement factories. Proven technology but expensive ($50–120/ton). CO2 is compressed and stored underground.
Direct Air Capture (DAC)
Giant fans pull air through chemical filters that bind CO2. Much more expensive ($400–1,000/ton) because atmospheric CO2 is only 0.04%. Climeworks (Iceland) operates the largest DAC plant.
Geological Storage
Captured CO2 is injected into deep saline aquifers or depleted oil reservoirs. Must be monitored for leakage over centuries. Capacity is vast — trillions of tons globally.
Hydrogen is not an energy source — it is an energy carrier. It takes energy to produce hydrogen, and how you make it determines its climate impact.
Where hydrogen makes sense: Steelmaking, long-haul trucking, ammonia production (fertilizer), shipping fuel, seasonal energy storage. It does not make sense for passenger cars or home heating — direct electrification is far more efficient for those applications.
Energy is foundational to human development. Without it, you cannot refrigerate vaccines, run a hospital, study after dark, or pump clean water.
The people with the least access to energy have contributed the least to climate change. Sub-Saharan Africa is responsible for ~3% of cumulative global CO2 emissions but is disproportionately harmed by climate impacts.
Energy access and climate mitigation are not in conflict: providing electricity to all 770 million unelectrified people would add less than 1% to global emissions.
The US coal industry employed ~40,000 workers in 2024 (down from 90,000 in 2012). These are real people in real communities — Craig, CO; Gillette, WY; Appalachian towns.
Rooftop Solar
Generates power at the point of use. Reduces transmission losses. But upfront cost excludes renters and low-income households.
Community Solar
Shared solar arrays where anyone can subscribe for a share of the output. Colorado has one of the strongest community solar programs in the US.
Microgrids
Self-contained local grids that can operate independently. Critical for resilience during outages. Used in hospitals, military bases, island communities, and rural developing regions.
Justice question: If rooftop solar lets wealthy homeowners reduce their grid payments, who pays to maintain the shared grid that everyone depends on?
Clean energy is the fastest-growing job sector in the US. Colorado is a national hub.
Engineering Roles
Solar/wind project design, grid modernization, battery system integration, building energy modeling, EV charging infrastructure.
Policy & Finance
Utility regulation, carbon markets, ESG analysis, climate policy, energy access programs.
Research & Innovation
NREL, universities, startups. Next-gen solar, green hydrogen, long-duration storage, grid AI, carbon capture.
Colorado Companies
Xcel Energy, Vestas (wind, in Brighton), Solid Power (batteries, Louisville), Crusoe Energy, Sundrop Fuels, NREL.
Is this a false choice? What would a plan that does both look like? What role should CU Boulder graduates play?