Massive Scale Solar Projects Needed to Harness the Sun’s Energy Potential

Greenpeace calculates that the exploitation of less than 1% of the total solar thermal potential of the sun would be enough to stabilise the world climate through massive carbon dioxide reductions. Some large scale technologies must become viable to even approach this level.

One’s first thought on solar energy is often the photo voltaic systems (PV) that convert sunlight directly into electricity. These small systems seem to have real potential, especially in areas that lack grid based electricity. However, PV material is expensive and has, to date, not found application as a mainstream alternate energy source.

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The amount of solar energy that reaches the earth’s surface day after day is enormous. Greenpeace notes that in many regions of the world, one square kilometre of land is enough to generate as much as 100-200 Gigawatt hours (GWh) of electricity per year using solar thermal technology. This is equivalent to the annual production of a 50 MW conventional coal or gas-fired power plant. It is therefore important to be implementing technologies able to tap this enormous potential.

Concentrated Solar Power

The best known large scale technology is Concentrating Solar Power (CSP), where focussing mirrors are used to concentrate solar radiation to provide medium to high temperature heat. This heat is used to operate a conventional power cycle, for example through a steam or gas turbine or a Stirling engine. Solar heat collected during the day can also be stored in liquid, solid or phase changing media to be extracted to run the steam turbine at night.

CSP is the most mature, with 354 MW of plants connected to the Southern California grid since the 1980s which required more than two square kilometres of parabolic trough collectors. These plants supply 800 million kWh a year, enough for more than 200,000 households. The electricity cost of around 12 cents/kWh is more than double the fossil power based cost. However, advanced technologies, mass production, economies of scale and improved operation are expected to eliminate this difference within the next 10 to 15 years.

Solar Updraft Tower

In the Updraft Tower air heated by the sun flows trough electricity generating turbines as it flows up a large tower.

The concept is related to the solar chimney, a way of improving the natural ventilation of buildings by using convection of air heated by passive solar energy, that has been in use for centuries and was used in Roman architecture.

In the modern version a one and a half kilometer high tower with a quarter kilometer diameter is surrounded at its base with 37 km2 of greenhouses. The greenhouse effect heats the air, decreasing its density. The density difference between this hot air and dense cold air at the top of the tower sets up an upflow in the tower. The large flow of air drives turbines located around the base of the tower generating 400 MW of electricity.

Using the greenhouse section of the tower for horticulture, adds the benefits of countering desertification, job creation and food production which are of great benefit to the arid and hot area where towers can operate efficiently.

A small, 200 m high tower has been successfully installed and tested in Spain.

Super Chimney

As if a 1 km high tower is not enough, the Super Chimney is a five kilometers tall and one kilometer in diameter It operates on the same principle although the increased height means that the greenhouses are not required. In theory warm air escaping out of the top of the chimney could cause precipitation when it cools, generating rain that could transform desert into arable land.

The concept has, as is maybe obvious, not yet been tested!

Seawater Greenhouse Integrated with CSP

The prevailing wind, supplemented by electric fans, drives air into the Seawater Greenhouse where it is humidified by contact with sea water flowing over a contact medium. The greenhouse uses a specially designed roof that allows only visible light to pass into the greenhouse, trapping the infra red light in an air channel in the roof. The low air temperature, high humidity and intense visible light in the greenhouse promotes very fast plant growth but limits transpiration. The cool air from the greenhouse and the hot air from the roof are blended and again humidified at the exit from the greenhouse. This warm moist air is passed through a sea water cooled condenser where the vapor is condensed creating distilled water. The air from the condenser passes into a large shaded area where other crops are grown.

In the Sahara Tree Project, the electricity required to run the greenhouse is provided by a Concentrating Solar Power Plant which utilises more solar energy. The CSP plant is sized to provide extra electricity for local use and export adding to the local economic development benefit of the greenhouse.

The current design which would cost 80 million Euros would export electricity and speciality crops while providing electricity, fresh water and food for local consumption and as inputs to business. Although the current electricity generating capacity of Europe is 50,000 times larger than the 10 MW produced by this project there seems to be no reason why it can’t be scaled up massively. To produce 20% of Europe’s electricity would require 200,000 ha of greenhouse which is equal to the total area currently installed in the Mediterranean. Electricity could be exported to the € 45 billion European supergrid that is under consideration.

Both the Seawater Greenhouse and the CSP Power Plant are proven technologies, their combination and integration into a major economic development scheme is the challenge.

Overall

What is interesting about these ideas is that they are based on known technology and can be installed without major uncertainties. The technological challenge lies in reducing the cost to overcome the challenges of sourcing financing which is currently a major constraint.

Other more radical ideas such as the collection of solar energy in space where radiation levels are not effected by the earths atmosphere and at sea where potentially valuable land is not being used pose larger technological challenge.

Image Credit: By fr:Utilisateur:Kilohn limahn at Wikimedia Commons under a GNU Free Documentation License.

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