Solar thermal electricity

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Solar energy is more evenly distributed across the sun belt of the planet than either wind or biomass. The downside is that deserts do not attract centres of population. However, as the world gradually switches to becoming a hydrogen-based energy economy, solar thermal electricity could be the key to substantial hydrogen production by electrolysis. African countries bordering the Mediterranean could greatly boost their economies by exporting solar hydrogen to Europe by tanker or pipeline. This may also be the future for the Gulf States (see Chapter 12).

In response to a challenge by the European Union in 1999 the Soltherm Europe Initiative was launched. Its aim was to install 15 million square metres of thermal solar collectors in Europe by 2004. The European Commission's White Paper, 'Energy for the Future', set a target of 100 million square metres across Europe by 2010.

district heating network

Figure 2.15 Diagram of the Ravensburg central solar heating plant diurnal storage project (courtesy of Renewable Energy World)

district heating network

Figure 2.15 Diagram of the Ravensburg central solar heating plant diurnal storage project (courtesy of Renewable Energy World)

There are four key elements to solar thermal power technologies:

• concentrator - which captures and focuses solar radiation

• receiver - this absorbs the concentrated sunlight transferring the heat energy to a working fluid

• transporter-storage - which passes the fluid from the receiver to the power conversion system; in some systems a proportion of the thermal energy is stored for later use such as night-time

• power conversion - this is the generation phase via a heat engine such as a Stirling engine or steam engine.

There are two basic types of concentrator.

• Parabolic trough collector: This system comprises long parallel rows of concentrator modules using trough-shaped mirrors. It tracks the sun from east to west by rotating on a linear axis. The trough collector focuses sunlight onto an absorber pipe located along the focal line of the trough. A heat transfer fluid, typically oil-heated to 400°C or water to 520°C, is transported through the pipes to drive a conventional steam power generator (see Fig. 2.16).

• Solar central receiver or 'power tower': In this instance an array of mirrors or 'heliostats' is arranged around a central axis, focusing solar radiation onto the focal point of the array. A receiver situated on a tower at the focal point transfers the solar heat to a power block in the form of a steam generator (see Fig. 2.17).

Research is being conducted into ultra-high-energy towers which can heat pressurized air to over 1000°C then to be fed into the gas turbines of combined cycle systems.

A version produced in the US by STM Power of Michigan links their 'SunDish' tower system to a Stirling engine to produce electricity.

The solar collector or 'SunDish' consists of an array of mirrors which tracks the path of the sun, focusing its rays on a thermal concentrator. The solar energy is focused onto a hemispherical absorber in the engine's heat pipe receiver. The heat pipe receiver transfers heat at between 300 and 800°C to the Stirling engine, which is hermetically sealed, producing daytime electricity with zero emissions. At night the engine is heated by a range of possible fuels

Figure 2.16 Solar thermal parabolic trough collector (courtesy of Caddet)
Figure 2.17 Solar thermal generator (courtesy of Caddet)

including landfill gas, wood chippings and biogas from an anaerobic digester. An operating cost of about 3.2 cents per kWh is claimed.

Where there is a high level and rate of solar radiation, multiple dish mirrors focusing on a Stirling engine have been producing grid-quality electricity for over 25,000 hours in the west of the USA (see Fig. 2.18).

A criticism of solar thermal plants is that they are only effective in the day-time. The Australian National University has met this challenge head-on by developing a sun dish

Dish Solar Thermal
Figure 2.18 Solar Energy Systems Solar Dish Stirling technology (courtesy of Solar Energy Systems Inc.)

which focuses radiation onto a thermochemical reactor containing ammonia. Under the intense heat the ammonia is broken down into hydrogen and nitrogen which is then stored at ambient temperature. It is stored in lengths of former natural gas pipelines. When needed the gases are recombined using an adapted industrial ammonia synthesis reactor. The 500°C of heat generated by the recombination is used to generate steam for a conventional power plant. The beauty of the system is that the gases are constantly recirculated within a closed loop. The leaders of the research team, Dr Keith Lovegrove and Dr Andreas Luzzi, claim that a solar field the size of a suburb would power a city the size of Canberra.

A major advantage of solar thermal systems is that they can be integrated into existing conventional power technologies. This will ease the transition to a fully renewable electricity supply system in the long-term future. In large measure this will be achieved by the re-powering of existing fossil fuel plants.

There is confidence in the industry that future developments of this technology will deliver much higher efficiencies whilst driving down costs of electricity to a level that will compare favourably with conventional power plants.

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Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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