Hydro and pumped storage, CO2 free power from man made lakes.
Hydro generating systems consist of river water captured behind a dam at a high level, which flows to lower level driving a water turbine which drives an electrical generator.
The energy which can be extracted by electrical power in any scheme depends on the quantity of water flow and the height it can fall (the Head). Considerable land space is required for collection of water. In a pump storage scheme, which should work well with the intermittent green sources of power, solar, wind etc, the power potential is considerably more for the same area of land used.
Examples of large hydro power stations worth visiting are at the Hoover Dam in the USA (2080 MW) capturing the water from the Colorado river; the Aswan High Dam in Egypt (2100 MW), using the Nile river water caught in lake Nasser; the three Gorges Dam in China see below (22,500 MW)(ref 125); Itaipu on the Paraguay/Brazil border (14,000 MW) or Yacyreta Argentinia (4050 MW)(ref130).
Three Gorges Dam in China (22,500 MW) two miles across river 150m high (ref125).
Hydro power supplies 16.6 % of world energy (ref 173 April 08) and most of the renewable energy. Small hydro stations (SHP) ie up to 10 MW size, using rivers canals and streams, account for 37,000 MW worldwide today which should rise to 55,000 MW by 2010.
The lakes for hydro schemes often result in CO2 and methane release due to vegetation drowning and decay at the bottom of the lake. Peat bogs.
Aswan High Dam Egypt showing spillage in foreground, switchgear and connections to transmission lines and Dam retaining wall above.
Large concrete dams are massive civil engineering projects and give rise to several large problems which include:-
1) To build the dam in a dry area the river has to be diverted which involves construction of tunnels from upstream to downstream. To keep the area dry, cofferdams (temporary) are built either side of the construction area. These may remain after flooding the area behind the dam or be blown up when the dam is complete.
2) Concrete heats up and expands when setting. The internal concrete will loose heat more slowly than the surface concrete leading to stress, cracking and failure. To overcome this the concrete is poured in vertical square sections and cemented together and/or cooling pipes run through concrete during setting.
3) Spillways are built in to convey (shoot out) excess or flood water some way downstream and away from the dam foundations. Such spillways were not included in the first Grand Coolie dam which then overflowed, washed away the foundations and collapsed.
4) To enable boat traffic to travel up or downstream, tiers of locks or a lift to raise the vessel (and water around it) or a sloping track to raise individual vessels out of the water are built at the side of the dam.
Pump storage schemes.
A pumped storage power station evens out the load on the grid taking power when demand is low and generating when demand is high. There are two levels of dam. In peak hours, water flows from high to low providing power. At times of low power demand water is pumped back up to the high level. The overall efficiency from electrical power in to electrical power out is around 65%(ref 6).
Dinorwig North Wales is a 1320 MW pump storage scheme (zero to max generation can be achieved in 16 secs.)
Energy Island Scheme
An 'energy island' idea is a development of pump storage. A Dutch feasibility study will propose a man made island. Water is pumped out using surplus (eg wind) power and runs back when power demand is high via turbine generators. This will enable the Netherlands to reduce carbon emissions by 30% by 2020.
Dual Lagoon hydro plan taken from ref 9.
A large area of shallow sea area could provide a constant or flexible power on a large scale. As the tide rises, sluice gates allow the height of the high level lagoon to rise. Continuous or on demand generation is from the high to low level lagoon. The height of the high level lagoon can be boosted by further pumping at high tide (thus at low head -red in diag.) using surplus grid power if available. As the tide falls, water is let out of the low level lagoon and can be further lowered by pumping at low tide. The power density should be 4.5 watts/sqm is 50% higher than for a single lagoon (3 watts/sqm.)
Note: The width would be several km whereas height several tens of metres.
An alternative is to arrange to generate from the high level lagoon to sea at lower tide and from the sea to low level lagoon at higher tides or use both methods:-
The Dual Lagoon plan can be used to augment the Pump Storage facility.
Osmosis used for power production
Statkraft, Norway, hopes that osmotic power will provide up to 13 times the amount of Norwegian hydro power today. Sea water and fresh river water are led into different chambers separated by a membrane. The salt molecules draw the fresh water across causing pressure to rise on the saline side, which can be equivalent to 120 metres depth and therefore used to drive a turbo alternator.
.JPG)
.JPG)
