Technology

The gravitational force of the moon causes the waters of the earths oceans and seas to bulge along the axis directly pointed at the moon (Tidal Power 2010). These forces, coupled with centripetal and centrifugal forces resulting the earths rotation cause the rise and fall of oceanic tides. Tides are highest (spring tides) when the moon and the sun are in line thus pulling earths oceanic waters to one direction and lower (neap tides) when the moon and the sun describe a perpendicular axis centred on the earth (Tidal Power 2010).

Figure 1 Tidal range as affected by the moon and the sun (Currie et al 2002)
One lunar cycle takes approximately 4 weeks and the earth rotates about its axis once every 24 hours (Tidal Power, 2010). This causes a tidal cycle approximately every 12.5 hours. At a time when the world is striving to undergo a green revolution where renewable sources of energy that cause minimal damage to the environment are the future, the predictability of the tidal cycle makes this natural phenomenon a very potential source of renewable energy.

Potential Tidal Power Sites in the UK
The United Kingdom has many potential sites for the generation of tidal power. River Severn between Wales and England is very suitable for a barrage as is the Sound of Islay and Pentlands Firth in Scotland and Pembrokeshire (Sustainable Development Commission 2007). The UK, according to DUKES, had a total electricity generation of 385 Terra-Watt hours (TWh) equivalent to 43.9 Giga watts of electrical output (GWe). The total tidal power generation capacity of all tidal barrages, tidal streams and estuaries having at least a bank on English shores 5.57 GWe, meaning that the UK can source up to 13 percent of its total electricity requirements from the harnessing tidal energy (Smith 2010).

The table below is a summary of the potential tidal power sites and their projected capacity.
SiteProjected Electrical Output (TWh)Severn 25.00Solway   9.66Morecambe Bay   5.98Wash   3.70Humber   1.65Thames  1.37Dee  0.89Mersey  0.57Total48.82
Table 1 Tidal site capacities in the UK (Smith 2010)

Design and Technology Considerations
To harness tidal power, current technologies allow either the construction of a tidal barrage or utilisation of tidal streams. A barrage is an installation at a bay or an estuary that lets water flow through it as the tide rises (Tidal Power 2010). When the tide stops, gates are closed, effectively damming water in the basin behind the barrage bearing a hydrostatic head. This water can then be delivered through these gates to drive turbines, generating electricity.

The diagram below is an illustration of a simplified tidal barrage.

 Figure 2 Simplified tidal barrage (Currie et al 2002)
There are various turbine designs available for use in barrage power generation. In a bulb turbine, water flow is around the turbine. The disadvantage of this design is that maintenance requires water flow to be stopped, causing time delays and loss in generation. In a rim turbine, the alternator is connected perpendicular to the waterway foe easier access and maintenance (Tidal Power 2010). The disadvantage with this arrangement is difficulty in power generation regulation. Tubular turbines are most recommended for the UKs greatest potential tidal site, the Severn Estuary (Sustainable Development Commission 2007). In a tubular turbine, the blades are coupled through an elongated shaft and oriented at an angle in such a way that the generator is at the top of the barrage.

(a)

(b)

(c)
Figure 3 (a) a bulb turbine, (b) a rim turbine, and (c) a tubular turbine. (Currie et al 2002)

The implementation of tidal generation plants in the UK has been slow because of the high initial costs involved and lack of technologies that do less harm to the flourishing marine ecosystems around estuaries and lagoons (Smith 2010). Feasibility studies and further research of employing tidal streams in the deeper seas should be carried out to tap this source of green energy.

Power Available From a Barrage
Figure 4 Diagrammatic representation of a barrage (Currie et al 2002)
If  is density of seawater (kgm3), g the constant of gravity, Cd the barrages discharge coefficient and A is the approximate area of the basin (m2), then at any instant the power derivable from the turbine is given by

Z1 and Z2 are the levels of the water in the sea and the basin respectively.

Economic Factors
Barrage construction requires large investment capital. Private investors are reluctant to take up such projects since the payback period is long. The UK government and should therefore either directly invest or attract able long-term investors to tap this energy source (Sustainable Development Commission 2010). After initial installation, maintenance is minimal and a turbine may function for over 30 years.

The cost-effectiveness of tidal power stations is determined by size of barrage and height difference between the low and high tide (Tidal Power 2010). The feasibility of tidal generators is directly proportional to the ratio of the barrage length to its annual generation in Kilo-Watt hours. This factor is behind the initiation of the Swansea, Fifoots Point and the North Wales tidal generation stations in Wales since tidal ranges are high.

Environmental and Social Impacts of Tidal Power Generation
Tidal energy is renewable, providing electricity without emitting greenhouse gases or any toxic by-products (Sustainable Development Commission 2007). If it were efficiently harnessed, tidal energy would reduce reliance on nuclear generators which cause thermal pollution and radioactive radiation. However, it there is a risk of disrupting the marine and shoreline ecosystems.

Damming bays or estuaries could also affect the geography of the shoreline, affecting recreational activities, fishing and shipping (Sustainable Development Commission 2007). However, construction of tidal power generating station should take the example of the La Rance barrage that has been operational in France since 1966 causing negligible disruption to the ecosystem and recreation activities.

Potential for Reducing Carbon Emissions
The Severn Barrage, if completed, is estimated to save 18 million tons of coal every year. If other feasible projects are built, then the levels of carbon emissions could be reduced significantly (Sustainable Development Commission 2007). At a time when climate change and global warming are posing a serious threat to world ecosystems and the survival of humanity, tidal energy should be appreciated as a way of powering homes and industries while decreasing carbon emissions to the atmosphere.

Conclusion
An increasing demand for energy has brought about environmental degradation due to the use of fossil fuels that release carbon into the atmosphere. Mining in itself pollutes the environment and non-renewable sources of energy are on their way to exhaustion. This is a call to the UK and the world to invest in renewable energy. The UK is endowed with enough natural resources to develop tidal energy into a major contributor of its electricity requirements (Smith 2010). Capital should therefore be raised to realise this potential as it offers the opportunity of a more sustainable future of the energy sector that conserves the environment.