Solar Energy

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2007 August - A solar energy farm, in Qianwei village, on Chongming Island,near Shanghai.

Solar power converts sunlight into thermal or electricity energy. It is already producing electricity and heating homes around the world, and it is truly a power source of almost unlimited potential. Properly exploited, it could meet the world's energy demands many times over.

In the past, solar had a reputation for being uneconomic and impractical, but this is no longer the case. Rapid advances in manufacturing techniques mean that solar is now a mature technology. Solar can be used to directly produce electricity or for heating and even for cooling. Future potential of solar is mostly limited by our willingness to seize the opportunity.

Solar Power in China

Solar energy has transformed the lives of millions of people in China, through solar-powered water heating. For many villagers in rural China, these heating systems are often the first water heating system they purchase, and supply the first hot showers they can take at home. As of 2009, China had over 1,000 solar thermal system manufacturers with revenues of over 20 billion yuan ($3 billion), and 600,000 employees [1].

By contrast, China's installed solar power for electricity generation capacity is low. Currently, the country only has 1 GW of solar energy installed, with a target to expand this to 5GW by 2015. Though the country leads the world in manufacture of photovoltaic panels, over 90% of its production is exported. One reason for this is the high price of photovoltaic panels.

However, the untapped potential of solar energy in China is vast. According to the China Solar PV Report, China's installed capacity of solar PV can reach 100GW by 2030, generating 130 terawatt-hours of electricity annually. This is equal to the capacity of more than 30 large-scaled coal power stations.

Types of Solar Power

Three main types of solar power are in regular use today: photovoltaic, solar thermal, and concentrated solar plants.

Concentrated Solar Plants

The most exciting recent developments have been in concentrated solar plants, which have been described as "the technology that will save humanity".

A form of solar thermal power generation, a CSP uses massed banks of parabolic mirrors to focus the sun's energy on a single point and generate intense heat. This is used to super-heat water, creating hot, pressurized steam, which in turn drives a turbine and generates electricity. Importantly, CSPs don’t only work when the sun shines – once built, they can operate well into the night.

Concentrated solar plants are rapidly growing in popularity. Spain has pioneered their development, and has over 50 plants either built or in construction, plus hundreds more export orders from all over the world.

Because the energy density (or amount of power generated) per square mile of land for solar plants is low, at the moment they are still best suited to desert regions where land is cheap and the sun is strong. But the potential is enormous. It's estimated that plants covering just 0.3% of the surface area of North Africa's Sahara Desert could meet all of Europe's electricity needs.

This is starting to happen. One of the most exciting projects now in development is a massive solar energy field planned for the Sahara. The Desertec Indus trial Initiative aims to provide Europe with 15% of its power by 2050. Financed by a European consortium of leading companies including Deutche Bank and Siemens, construction is due to start within the next five years.

Photovoltaic cells

This is what many people think of when they think of solar power. Photovoltaic power uses semi-conductor cells – usually made of silicon – to convert sunlight directly into electricity. Photovoltaic cells (PVs) are most commonly used as roof tiles to take advantage of the light coming from the sun. This is trapped by the cell and turned into electricity.

Solar panels can either operate as a stand-alone or feed energy to a grid. They work best when in direct sunlight, but they continue to produce electricity on cloudy days. Due to the reflection of sunlight from clouds, days with a few clouds can even result in higher energy yields than completely sunny days.

Small devices, like calculators are often powered by very small solar cells. PV is also used to provide electricity in areas with no power grid. Greenpeace has developed a refrigerator, called Solar Chill, which can run on solar energy. After testing, it will be used by humanitarian organizations to help supply vaccines in areas without electricity.

Architects are also using photovoltaic cells increasingly as a design feature. For instance, solar roof tiles or slates can replace conventional roofing materials. Flexible thin film modules can even be integrated into vaulted roofs, while semi-transparent modules allow for an interesting mixture of shading and daylight. PV can also be used to supply peak power to the building on hot summer days when air-conditioning systems need the most energy, thus helping to reduce the maximum electricity load.

Solar Thermal (Heating)

Solar thermal uses the heat of the sun directly. A solar thermal collector on your roof can provide hot water for you home and help heat your house.

This system is based on the ancient principle that water in a dark container will warm up when the sun shines on it.

Solar thermal technologies on the market now are efficient and highly reliable and have a range of applications, such as domestic hot water heating, space heating in residential and commercial buildings, swimming pool heating, solar-assisted cooling, industrial process heat and desalination of drinking water.

Domestic hot water production is the most common application for solar thermal today. It has many advantages, especially in China, where they are many families’ first water heater in the home. Not only do solar water heaters emit no CO2 and reduce the consumption of fossil fuels, they also help lower utility bills – sunlight is free after all. In China, solar-powered water heaters are only slightly more expensive than conventional systems, and pay for themselves in five years or less through savings on electricity or natural gas bills.

There are two main types of solar thermal technology:

Vacuum tubes: Sunlight is absorbed by vacuum tubes to heat up the fluid inside. Additional heat is picked up by the reflector behind the tubes. Whatever the sun's angle, the round shape of the vacuum tube allows the sun to be absorbed directly. Even on a cloudy day, when the light is coming from many angles at once, the vacuum tube collector can still be very effective.

Flat panel solar collector: This is basically a box with a glass cover that sits on the roof like a skylight. Inside the box is a series of copper tubes with copper fins attached. The entire structure is coated in a black substance designed to capture the sun's rays. These rays heat up a mixture of water and anti-freeze that circulates from the collector down to the water heater.

Solar Cooling

Solar chillers use thermal energy to produce cold and/or dehumidify the air in a similar way to a refrigerator or conventional air-conditioning. This application is well suited to solar thermal energy, as the demand for cooling is often greatest when there is the most sunshine. Solar cooling has been successfully demonstrated. Large-scale use can be expected in the future, as the cost of the technology is reduced, especially for small-scale systems.

Reference

[1] http://www.treehugger.com/files/2009/05/china-solar-city-dezhou-video.php

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