Coal, gas and oil are becoming frowned upon as being environmentally unfriendly energy sources. The alternatives now being proposed and implemented include tidal, wave, wind, solar, geothermal and biomass. Jon Severn provides an overview of the renewable energy industry.

Debates continue to rage over the cause of global warming, but one thing is for certain: the Earth's resources are finite. Twenty years ago some commentators were predicting that crude oil supplies would be virtually exhausted by now, but that has proved to be incorrect. Nonetheless, the cost of extraction is rising as the easily accessible reserves are pumped dry, and natural gas is being transported over ever-increasing distances, with associated costs plus risks from accidental damage and sabotage.
While the USA continues to follow its own agenda on energy policy, the rest of the world is taking a greater interest in renewable energy sources. The UK government, for example, has stated that it wants to see 10percent of the UK energy supply provided by renewable energy sources by 2010. Meeting this target will undoubtedly be a challenge but, even if it is missed, there will have been considerable movement in the right direction.
Because Great Britain is an island and therefore surrounded by a coastline, tidal and wave power are often suggested as two of the most suitable sources of renewable energy. In addition, several coastal wind farm projects are already underway, and numerous inland wind turbine generators are operating. The UK is not renowned for its sunny climate, but solar power is still viable here; elsewhere in Europe the advantages to be gained from solar power are even greater.

Tidal power

France was one of the first countries to install a tidal power station with the 240MW barrage at La Rance in the 1960s. The Netherlands and Great Britain also have potential to take advantage of tidal power systems, but environmental concerns have tended to restrict their development. Canada and China have both installed tidal systems but their capacities are smaller than the French scheme.
To overcome the environmental concerns, modern tidal projects tend to be smaller and avoid the construction of estuary-wide barrages or the creation of large lagoons. The Stingray, for example, is a pilot installation situated just off the Shetland coast, capable of generating 150kW. Stingray consists of a hydroplane that uses a simple mechanism to vary the angle of attack relative to the approaching water stream. The passage of water over the hydroplane causes the supporting arm to oscillate up and down, which, in turn, forces hydraulic cylinders to extend and retract. This produces high pressure oil that drives a generator. Trials in 2002 proceeded well and data is now being used to refine the design. Further tests are scheduled for 2003 and, in parallel, planning is underway for the construction of a 5MW power station comprised of a cluster of Stingray machines. Installation and connection to the electricity grid could commence as early as 2004.
Another alternative for extracting energy from the tides is being employed in Norway to supply electricity to the world's most northerly town, Hammerfest, which has a population of 11000. Mounted on the seabed, and therefore causing as little environmental damage as possible, the windmill-like turbine has a capacity of 300kW, though the plan is to expand the powerplant to 20 turbines and increase the output correspondingly in 2004.
One of the drawbacks with fully-submerged systems such as those in Shetland and Hammerfest is that maintenance is difficult. One answer to this problem is proposed by Marine Current Turbines (MCT), which has installed a 300kW tidal turbine 3km northeast of Lynmouth, UK. The turbine is the culmination of the Seaflow project that has been conducted by a consortium of UK and German companies with funding from the UK Government's Department of Trade and Industry, the German Government and the European Commission's Joule Programme. Built as a technology demonstrator, the turbine paves the way for MCT to design and construct a larger version that would be commercially viable. Unlike the alternatives, the MCT system features a rotor mounted on a steel column that is set in a socket bored into the seabed. For maintenance, the rotor, gearbox and generator can be raised above the surface, with no need for divers or slack currents (Fig. 1).

Wave power

Tidal-based systems have an advantage over wave-based systems in that tides are more predictable, though there are slack periods as the tide turns. Nevertheless, one wave energy converter has now been installed and is supplying electricity to the Danish grid. Known as the Wave Dragon, the system uses two curved reflectors to help concentrate the wave energy towards the specially-developed Wave Dragon Kaplan turbine installed in a siphon arrangement; the turbine's output during the initial trials matched that predicted by engineers from Aalborg University and the Technical University of Munch. When the system is fully developed it is anticipated that power will be generated at a relatively low cost of E0.04/kWh in 2010 - 2016.

Wind farms

Despite the attractions of tidal and wave power, there is currently a great deal of interest in coastal wind farms. Two of the main reasons for this are that, compared with inland wind farms, coastal winds are more predictable and there are fewer objections to coastal installations. The UK government has recently announced that it intends to make Britain the biggest generator of electricity from offshore wind, taking advantage of the shallow waters in the Thames estuary, the Wash and the Northwest. Some 3000 wind turbines could be installed to generate around 6GW of power.

To install such coastal generators, Mayflower Energy has commissioned a unique ship, the Resolution. This vessel is 130m long and measures 38m across the beam, yet she has been designed to be highly manoeuvrable. As well as dynamic positioning, she features a high-speed jacking system that elevates her out of the water during the construction and installation phases. By carrying the towers and turbines onboard, the Resolution is able to operate without any other support vessels and, compared with conventional construction and installation techniques, she should enable work to be completed far more quickly. The first wind farm on which The Resolution will be deployed is the National Wind Power offshore wind farm at North Hoyle, a few kilometres off the North Wales coast. This farm is one of 18 offshore wind farms planned for UK waters and it will consist of 30 turbines, each rated at 2MW.
Meanwhile, inland wind farms are still popular where local communities are prepared to accept the large structures in their midst. Swaffham, UK, now has two wind turbines, with the second being built as a direct result of demand from local people (Fig.2). Capable of supplying the power requirements for 1200 homes, the second turbine allows Swaffham to generate 75percent of its home electricity requirements. Both the Swaffham turbines are operated by Ecotricity, which has a UK-wide strategy to supply one million homes with environmentally-friendly electricity.

Solar power

Having very little environmental impact, solar power is popular with individuals and organisations wishing to use renewable energy and, in the long run, cut their energy bills. Developments continue in this field, with greater efficiency and power density being two of the main goals. Sunpower Corporation of the USA, for example, has recently announced what it claims to be the world's most efficient, low-cost silicon solar cell. The A-300 uses a rear-contact design that maximises the working cell area, hides obtrusive wiring and simplifies automated production. Compared with a typical efficiency of 12to15percent for conventional cells, the A-300 achieves in excess of 20percent. Each cell, measuring 125mm square, generates 3W of electrical power, leading to a cost-per-Watt that is claimed to be the most efficient in the photovoltaic industry.
One of the world's largest solar power installations was completed in 2002 at WL Delft Hydraulics, the independent research institute dealing with water-related issues. Located in Delft, the Dutch institute now benefits from a rooftop system that comprises 7272 modules mounted on an 11000m2 aluminium framework. At peak output the system delivers in excess of 350kW, relating to approximately 230000kWh annually.

Geothermal energy

For central and Eastern Europe, another form of energy that is under investigation is geothermal energy, where heat is extracted from underground sources of hot water or rocks. Denmark is taking a leading role in this field, with a demonstration plant that has been operating in Thisted since 1984. One of the most promising sources of geothermal energy lies several kilometres directly beneath Denmark's largest city, Copenhagen.
While the Danes are looking at utilising hot springs, the French have been exploiting hot dry rocks (HDR) at Soultz-sous-Forets since 1987. Today there is the European Deep Geothermal Energy Programme that is carrying out research into the wider application of HDR technology. Considering the fact that temperatures are in the region of 180to240ûC at a depth of 5000m below parts of Europe, the potential for extracting some of this energy is certainly worthy of further investigation.

Biomass

Geothermal energy has been appreciated since the Roman era and before. Another energy source that has been employed for even longer is biomass. Wood was once the main source of fuel, and it was only the industrial revolution that led to wood's replacement by coal. Biomass is now being rediscovered as part of the drive towards renewable energy sources (Fig. 3).
A typical modern biomass operation is the Elean power station, near Ely in the UK. Commissioned in 2000, this plant consumes around 200000 tonnes of straw every year, burning it on a specially developed two-stage grate and using advanced flue gas treatments to ensure that the combustion gases remain well within the UK Environment Agency limits when they are released to the atmosphere. Indeed, the emissions are claimed to be some 50percent lower than those that would be expected from a conventional fossil-fuelled power station. Heat from the burning straw is used to fire a boiler that superheats steam to 540ûC and 92bar pressure; the steam then flows to a high-efficiency two-stage condensing turbine. Around 270GWh of electricity is produced annually, which is sufficient to meet the needs of 80000 homes.
Whether or not you believe that the burning of fossil fuels causes global warming, it is hard to argue against the environmental benefits offered by renewable energy sources. For engineers, this also represents one of the most exciting areas in which to work, with massive investments being made and many of the technologies being at a stage where significant advances can stillbe made.