23-CS-3 Sustainability, Engineering and the Environment · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2017 — 11-CS-3 Sustainability, Engineering and the Environment. Open book; non-communicating calculator permitted. Any four questions constitute a complete paper; all questions are of equal value (25 marks each).
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
| Technology | Land | Fuel requirement | GHG emissions | Local health risk | Initial cost |
|---|---|---|---|---|---|
| Wind | H (spacing) | L (none) | L | L | M |
| Nuclear | L (+ mining) | L–M (uranium) | L | M (accident/waste) | H |
| Solar PV farm | H | L (none) | L | L | M |
| Wood-pellet plant | H (forestry) | H (biomass) | M (combustion; partial regrowth offset) | M (particulates) | M |
| Natural gas | L | H (gas) | M | L–M | L |
Basis: each option delivers 200 MW of rated capacity. Wind and solar have capacity factors of only about 35% and 15–25% in Canada, against about 90% for nuclear and 50–85% for thermal plants, so they deliver less energy per rated MW. That lowers their true cost and land advantage per MWh.
Land requirement. Wind, H: turbines must be spaced several rotor diameters apart, so 200 MW spreads over roughly 20–60 km². Only about 2–5% of that area is physically occupied, however, and farming can continue between the towers. Nuclear, L: the plant itself is compact (well under 1 km²); uranium mining and the waste-management sites upstream add a little. Solar PV, H: about 2–4 ha per MW, so roughly 4–8 km² of land fully covered by panels. Wood pellets, H: the plant site is small, but a continuous fuel supply needs hundreds of km² of managed forest. Natural gas, L: a compact plant, plus well pads and pipeline corridors upstream.
Fuel requirement. Wind and solar, L: no fuel, because the wind and sun are free and inexhaustible. Nuclear, L–M: only a few tonnes of enriched fuel per year, although it comes from an energy-intensive mining–milling–enrichment chain. Wood pellets, H: several hundred thousand tonnes of pellets per year to harvest, dry, pelletize and truck in. Natural gas, H: a continuous pipeline supply of a finite fossil fuel, with price volatility.
GHG emissions (life cycle). Wind, nuclear and solar, L: about 10–50 g CO₂e/kWh, all from manufacturing, construction and fuel processing. Wood pellets, M: the stack CO₂ is high, but it is partly offset by forest regrowth, and harvest, drying and transport emissions add to it. Natural gas, M (towards H): about 400–500 g CO₂e/kWh from combustion, plus upstream methane leakage (GWP 25).
Health risks to local populations. Wind, L: noise, shadow flicker and ice throw are nuisances rather than hazards. Nuclear, M: routine emissions are negligible, but a severe accident is low in probability and high in consequence, and mining and waste must be managed. Solar, L: there are no emissions in operation; the upstream silicon and metal processing hazards fall on workers elsewhere. Wood pellets, M: fine particulates, NOₓ and CO come from combustion, and truck traffic adds dust. Natural gas, L–M: NOₓ is the main pollutant; the upstream wells and pipelines carry leak, explosion and flaring risks.
Initial cost. Wind, M: roughly 1.5–2 million dollars per MW. Nuclear, H: the highest capital cost (well over 6 million dollars per MW), with long licensing and construction periods; a single 200 MW unit is also below the usual economic size. Solar, M: roughly 1–1.5 million dollars per MW, and falling. Wood pellets, M: a boiler–turbine plant plus fuel handling. Natural gas, L: about 1 million dollars per MW or less, the cheapest to build, although fuel is its main lifetime cost.
Wind: wide turbine spacing (high land, ground usable); no fuel; negligible GHG and low local health risk; medium capital. Nuclear: compact plant but upstream uranium mining/enrichment; near-zero GHG; a low-probability, high-consequence accident and waste-management risk; very high capital. Solar PV farm: large panel area; no fuel; negligible GHG; low health risk; medium capital. Wood-pellet plant: needs large forested land and continuous biomass fuel; combustion emits CO₂ and particulates (medium GHG, nominally offset by regrowth) plus local air pollution (medium health risk); medium capital. Natural gas: compact; needs gas fuel; emits CO₂ (medium GHG) with low–medium local health risk; low capital. Considering upstream processes, the true low-carbon options are wind, solar, and nuclear; biomass is only conditionally low-carbon (depends on sustainable regrowth) and adds local air pollution; gas is the lowest-capital but a fossil source.
