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Renewable-Energy Technologies. Agenda. Attractions & liabilities of renewables Resource potential vs current use Framework for analysis of renewable energy technologies (RETs) Sunlight solar-thermal technologies photovoltaics Wind Biomass Cross-cutting issues & comparisons.
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Agenda • Attractions & liabilities of renewables • Resource potential vs current use • Framework for analysis of renewable energy technologies (RETs) • Sunlight • solar-thermal technologies • photovoltaics • Wind • Biomass • Cross-cutting issues & comparisons
Attractions of renewables • The primary resources are free and widely distributed geographically • Tropics have biomass, ocean thermal; subtropics & temperate zones have sunlight, biomass; high latitudes have wind; all have hydro depending on topography. • Diversity of resources & conversion pathways facilitates matching a wide variety of E needs. • Many renewables can be harnessed with low-technology as well as high-technology methods. • Most renewables have zero to modest GHG emissions compared to fossil energy technologies. • Other environmental impacts & risks are often (but not always) smaller than for fossil options.
Liabilities of renewables • Renewable energy sources are dilute compared to fossil sources, thus requiring large equipment (with large capital costs) and/or large land area per unit of useful output. • Many renewables are intermittent and/or highly variable seasonally or annually, thus requiring (costly) energy storage or back-up. • The most economical sites for harvesting renewables are often highly valued for competing or conflicting uses.
Renewable energy resource assessment SUNLIGHT: 88,000 TW reaches Earth’s surface •26,000 TWy/y on land. Conversion of insolation on 1% of land area at 20% efficiency would yield ~50 TWy/y. HYDRO CYCLE: 40,000 TW in evaporation of water •From this, potential energy in runoff from precipitation that has fallen on land above sea level is 4-5 TWy/y; harvestable hydropower worldwide is 2-3 TW peak power, 1-1.5 TWy/y annual energy. WIND:1,000-2,000 TW from solar flow drives winds • Wind potential on the windiest 30x106 km2 of the world’s land area = 55 TWy/yr of electricity; exploiting only most attractive terrestrial sites would yield 2‑3 TWy/y of electricity. BIOMASS:Net primary productivity ≈ 100 TW • 65 TWy/y on land. Terrestrial NPP currently harvested by humans: food and feed = 4.5 TWy/y; fiber = 0.5 TWy/y; fuel = 2 TWy/y.
Framework for analysis of RETs • Basic energetics • Resource availability & distribution • Technology characteristics • conversion options & efficiencies • how the technologies work • materials requirements • Economics • construction costs • cost of energy • Environmental & sociopolitical issues • Rate of expansion, future prospects
Sunlight energetics & availability • SOLAR FLUX • At top of atmosphere perpendicular to beam, 1400 W/m2 • At Earth’s surface, at solar noon on a clear day, 1000 W/m2 • At surface, 24-hr, 365-day global average, 175 W/m2; best sites 250 W/m2 Seasonal varia-tion is larger at high latitudes; year-round avg on high-latitude side of temperate zones typically <150 W/m2.
Solar energetics(continued) DIRECT-BEAM & DIFFUSE RADIATION Direct-beam radiation consists of parallel rays that penetrate to Earth’s surface without scattering in atmosphere. It can be focused by concentrating collectors & heliostats (steerable mirrors). Diffuse radiation has been scattered before reaching surface, hence comes from all directions and cannot be focused. Typical locations have 50/50 direct/diffuse split; deserts have more direct, cloudy regions more diffuse radiation. HOW MUCH CAN A SOLAR COLLECTOR COLLECT? Average-to-peak power ratio for a fixed horizontal flat-plate collector ranges from 1/7 (140 W/m2 avg vs 1000 peak) to 1/4 (250 W/m2 avg vs 1000 peak). This ratio gives upper limit on capacity factor. Tilting a fixed collector toward equator at an angle equal to latitude increases average energy capture by 10-20%. Collectors that track sun on 1 axis gain 20-30%, those that track on 2 axes gain 30-40%; but tracking greatly increases collector cost.
Solar-collector efficiencies SOLAR-THERMAL COLLECTORS • 1st-step efficiency is ratio of thermal energy captured by collector divided by solar energy incident on it. • For simple collectors, this efficiency goes down as operating T goes up, because hotter collector transfers energy to its environment more rapidly than a less-hot one. • A typical efficiency for a composite flat-plate collector for domestic water heating is 50%. • If the collected thermal energy is used in a heat engine to produce mechanical work or electricity, the efficiency of this 2nd step = work or electricity output ÷ captured thermal energy. • This efficiency cannot exceed the Carnot value, 1 – TLOW/THIGH, where THIGH is that of the collected thermal energy. • The overall efficiency (e.g., electricity output ÷ incident solar energy) is then the product of the 1st- and 2nd-step efficiencies.
Solar-collector efficiencies (continued) PHOTOVOLTAIC COLLECTORS • The efficiency of a PV cell is defined as the electrical energy output divided by the solar energy incident on the cell. • The simplest and least costly PV cell types tend to have rather low efficiencies (5-10%). • More complex and costly PV cell types make use of a larger portion of the solar spectrum and can reach efficiencies of 30% or more. • Clearly there is a cost-efficiency trade-off that relates to the cost per m2 of higher- vs lower-efficiency cells: Is it better to cover more area with less efficient but also less costly (per m2) cells, or a smaller area with more efficient but more costly cells? • PV cells can use both direct-beam and diffuse solar radiation. But if focusing collectors are used to concentrate sunlight to save on cell costs, the diffuse solar input is mostly lost – another trade-off.
Economics of solar water heating AN EXAMPLE A well-designed flat-plate collector for solar water heating, costing about $20/m2 with associated piping and water tank, has an efficiency of ~50%. In an average climate, it receives 175 W/m2 if flat, ~200 W/m2 if tilted toward the equator at an angle = latitude. So in a year it receives 200 J/sec x 31.5x106 sec = 6.3 GJ/m2, and delivers half of this in heated water, hence 3.2 GJ/m2-yr. If the capital recover factor is 0.10/yr, the annualized cost of the collector is $200/m2 x 0.10/yr = $20/m2-yr. Thus the cost of this energy is $20/m2-yr ÷ 3.2 GJ/m2-yr = $6.25/GJ. At what cost of natural gas does this system become competitive as a supplement to a natural-gas-fired water heater, if the gas water heater has an efficiency of 85%?
Terminology of PV energy systems • PV cell: a bare semiconductor component, typically 10 cm in diameter, 1 W peak output • PV module: many cells connected in parallel and in series, sealed in a weatherproof unit • PV array: many modules electrically connected and structurally supported • PV system: one to many arrays, with “power conditioning” (e.g. conversion from DC to AC) and provision for tracking the sun (possibly)
PV: relation between capital cost & COE Estimated capital cost in 2000: modules $3-4 / peak watt balance of system $2-6 / peak watt TOTAL $5-10 / peak watt If site has average insolation of 200 W, then ratio of average to peak power is 200 W / 1000 W = 0.20, which means maximum capacity factor is 0.20. If the capital recovery factor is 0.10 per year and we use the upper end of range of system costs, hence $10/Wp, the contribution of the capital cost to the cost of electricity is $10 / W x 0.10/yr / (8760 hr/yr x 0.2) = $0.57/kWh If we used $5/Wp, we’d get $0.29/kWh.
Two additional solar collector types Still far from commercial application but under intensive investigation are: • Solar-thermochemical H2-producing collectors • would exploit thermochemical decomposition of H2O using solar heat & appropriate catalysts • might use standard high-T collector technology providing heat to a separate thermochemical reactor or an integrated collector/thermochemical system • Abiotic photochemical H2-producing collectors • would exploit photochemical processes mimicking photosynthesis to generate H2 directly • experimental versions have existed for more than 15 years, but have efficiencies <0.1%
Environmental & sociopolitical issues with solar technologies • All approaches • Materials requirements & emissions in connection with manufacturing the collectors (especially toxics with PVs) • Central-station solar-thermal & PV electricity generation • land use • aesthetic intrusion of power plants & transmission lines in desert regions • Rooftop collector systems • leakage of toxic working fluids; release of toxic constituents in fires • injuries from falls in connection with installing, repairing, cleaning rooftop collectors.
Wind energetics • The energy flux (power per m2) in a 20 mph wind = 440 W/m². • The energy flux across a square meter goes up with the cube of the wind speed: P ~ v3 • A modern wind turbine-generator system can convert the power in the wind to electricity at an efficiency of about 0.3; capacity factors at the better sites are 0.25-0.35. • Area requirement per average or peak megawatt can be estimated from windmill spacing requirements… • Tower centers at 2 to 5 blade diameters apart in rows perpendicular to prevailing wind direction; rows 10 blade diameters apart. • Some uses of land (or water) surface not precluded by windmill operation: cultivation, grazing, fishing
Environmental & sociopolitical issues with wind technologies • Best wind sites are on mountain ridges, coastlines, or a few miles offshore; these tend to raise objections on aesthetic grounds. • viz. Cape Wind site in Nantucket Sound • but not everybody thinks windmills are ugly! • Bird collisions with blades can be a significant concern, especially on migratory flyways. • but compare bird kills by transmission lines, no matter what the electricity source. • Offshore sites also may be navigation hazards, disrupt ocean bottom.
Biomass energetics From the “Key energy units & conversion factors” handout: E content of wood: 20.0 GJ / tonne dry organic matter (HHV) 16.0 GJ / tonne air-dried wood (20% moisture) (HHV) 14.3 GJ / tonne air-dried wood (20% moisture) (LHV) E content of grasses, leaves, crop wastes: 18.0 GJ / tonne dry organic matter (HHV) 14.4 GJ / tonne air-dried mat’l (20% moisture) (HHV) 12.7 GJ / tonne air-dried mat’l (20% moisture) (LHV) Conversion efficiency of high‑yield cultivation: energy in plant material ÷ insolation = 0.5‑2% (annual avg) = 1-4 watt / m2 = 1-4 MW / km2 = 1-4 TW / 106 km2 (TWy/106 km2-yr); harvestable yield is typically 0.5 to 0.7 of this. 10 dry t (harvestable) / hectare-yr = 1000 dry t / km2-yr = 20,000 GJ / km2-yr = 20x106 MJ / km2-31x106 sec = 0.63 MW / km2
Chemistry of biomass conversion • Pyrolysis (heating in the absence of oxygen) CH2O C + H2, CO heat • Bioconversion • anaerobic digestion: C6H12O6 3 CH4 + 3 CO2 (biogas) bacteria – fermentation: C6H12O6 2 C2H5OH + 2 CO2 (ethanol) yeast • Abiotic gasification, liquefaction, synthesis C6H12O6 H2, CH4, CO, CH3OH (methanol) heat, steam, pressure
Environmental & sociopolitical issues with biomass technologies BIOMASS GROWTH & HARVESTING • Competition with other valued uses of fertile land and biomass materials – food, feed, fiber, feedstock, fertilizer, ecosystem function • Impacts of intensive cultivation – irrigation impacts, fertilizer & pesticide impacts on- and off-site, erosion, access roads, ecosystem effects of monoculture BIOMASS CONVERSION & COMBUSTION • Air & water effluents from conversion processes • Acute indoor air pollution from biomass use in inefficient, dirty stoves in dwellings; outdoor air pollution • Risk of dwelling fires from unsafe or badly maintained biomass-burning stoves
Why biomass energy is not a CO2 problem if sustainably used The reaction when biomass material is burned is CH2O + O2 CO2 + H2O which adds 1 mole of CO2 to the atmosphere for each mole of CH2O burned. But exactly the same amount of CO2 was removed from the atmosphere when the biomass was formed by photosynthesis CO2 + H2O CH2O + O2 So, if a new mole of CH2O is grown for every mole burned, the net effect on atmosphere CO2 is zero.
