‘Other’ renewable energy sources
Other = Hydro; Pump Storage; Tidal; Wave; Geothermal; Biomass
(Solar and Wind covered separately)
Hydro-electricity
Francis turbine – optimised for liquid (water) flow rather than gas flow. Vertical axis. Can be huge!
Low head ~ 10 m (river)
Medium head ~ 50 m (reservoir)
High head ~ 500 m (mountain reservoir).
Power = flow x g x head (g = 10 m/s2)
No emissions.
Not a heat engine, hence high efficiency possible (> 90 %).
Very quick start/stop, hence good for fluctuating demand (e.g. World Cup Final!).
Pump storage – run water turbines backwards using surplus energy from conventional power station. I.e. a giant battery. E.g. ‘Electric Mountain’.
Tidal Energy
Gravitational energy – not weather dependent.
No emissions.
Eco-system disturbance.
Single barrage – large power fluctuations
Multiple barrages – near steady power.
Proposed Severn estuary scheme ~ 2GW ~ 5 % UK demand Wave energy
Waves produced by wind, hence not constant! Power in Atlantic waves ~ 40 kW/m
UK coastline ~ 1000 km , pop ~ 60x106 hence, max. tidal power = 2/3 kW per person (small!)
Various ingenious devices for extracting power (Pelamis ‘Sea Snake’), but none yet in large scale production…
Marine environment very difficult and expensive!
1
Geothermal
Energy flux from rock: Surface area: Available energy: Drilling depth
Iceland
~ 0.3 W/m2
~ 100,000 km2 ~ 30 GW
~ 0.5 km
UK
~ 0.05 W/m2
~ 250,000 km2 ~ 12 GW
~ 15 km
Low grade heat – direct use, not electricity generation
Biomass
Biomass = recently dead organic material = Wood; Straw; Sugar cane; Maize; Animal Dung; Refuse; Landfill gas
Regarded as ‘carbon neutral’.
Global picture: 10 % energy from biomass; >25% in developing countries.
Direct combustion: [CH2O] + O2 à CO2 + H2O + Energy Gasification (anaerobic digestion)àgas turbine Fermentationàbiofuel (alcohol)
Oil extractionàbiodiesel
Energy densities Refuse 9 GJ/tonne Straw 14
Wood 15
Animal Dung 16
Coal 28 Bioethanol 30
Oil 42
Natural gas 55
Example:
Sugar cane yield ~50 tonnes/hectare; Bioethanol yield ~ 3500 litres/hectare;
Car fuel tank ~ 50 litres; 1 hectare ~ 1 football pitch; Hence:
1 football pitch ~ 70 fill-ups… … but land is not available to grow food… football.
UK Energy Target
20 % energy generation form renewable sources by 2020. A lot to do!