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Lecture 20

 

Assessment of Biomass

•         Advantages

–        Large potential energy source

–        Part of natural C-cycle ΰ no addition to greenhouse problem

•         Disadvantages:

–        Low efficiency

–        Competition with food production

–        Large installations needed for combustion; hydrogenation; pyrolysis ΰ transport of low energy material needed

•         ΰ Viable as alternative if waste can be used, not very promising if ‘energy crops’ are needed

 

Hydropower

•         Extraction of power from flowing water

•         Energy equation: E = mgh

–        m mass; g gravitational attraction; h height

•         Three types of approaches

–        Impoundment (Reservoir)

–        Run-of-river

•         Diversion

•         River dams

–        Pumped Storage

Systematics of a power plant (Fig. 1)

Example: Niagara Falls (Fig. 2)

Large Power stations in the World (Fig. 3)

 

Issues with Hydropower

•         Advantages

–        Continuous low cost power

–        Low maintenance costs

–        No consumption of exhaustible fuels

–        Little air pollution

–        Reservoir lakes ΰ recreation

–        Flood protection – regulation of downstream flow

–        Groundwater recharge

–        High efficiency 80 ΰ 90 %

•         Disadvantages:

–        High initial cost

–        Strong variations in water levels affect recreation

–        Conflict between flood protection and power production

–        Loss of land for agriculture

–        Stream valley inundated

–        Evaporation losses

–        Seismic instability

–        Fish migration

–        Change in ecology ΰ River ecology to lake ecology

 

 

Windpower

•         Extraction of energy from moving air

•         Energy equation

–        Ekin = ½ mv2

–        M = rAv

–        Ekin = ½ rAv3

•         r density of air (1.25 kg/m3); A area swept out by rotor; v wind velocity

•         Efficiency (Fig. 4):

–        Theoretical: 63 % 

–        Practical: 40 %

 

Wind potential in the USA (Fig. 5)

 

Installed wind capacity (Fig. 6)

 

Issues with wind energy

•         Advantages:

–        Low maintenance cost

–        No exhaustible fuel needs

–        No green house gas release

–        Many areas are suitable for wind energy

–        Compatible with other land use

•         Disadvantages:

–        Relatively small energy output ΰ many structures needed

–        Unreliable energy production

–        Problems with birds

–        Interference with radiosignals (?)

–        Best places along coast or off shore

 

Ocean Thermal Energy Conversion
OTEC

 

•         Production of Electricity using temperature difference in ocean waters

•         Origin of T differences: Variation of solar flux with latitude (Fig. 7)

•         Deep water is cold due to thermohaline convection (Fig. 8)

•         Efficiency: Carnot Cycle

–       h = (Tin – Tout)/Tout  (in deg K)

 

An OTEC system works like a heat-based power system, using the deep water to cool surface water (Fig. 9)

 

OTEC - Assessment

 

•         Collection of energy from solar heat

•         Energy production based on Carnot cycle

•         Small temperature differences ΰ low efficiency ΰ large systems needed

•         Small environmental impact in open system

•         Closed system: Use of fluids with low evaporation T ΰ some problematic fluids under consideration

•         Not in current use

•         Future: Open ocean systems ΰ problem of energy transfer ΰ combination of electricity and aluminum production

 

Other solar schemes

•         Waves

•         Ocean currents

•         Solar Tower (Fig. 10)