2012-01-09 - Generating power from the sun even at night
According to NASA, the US space agency, the average intensity of solar energy reaching Earth’s outer atmosphere, directly in-line with the sun, is about 1,360 watts (W) per square meter (m2). Since most of that energy is either reflected or absorbed by the atmosphere, only about 48 percent reaches the ground, which means at best around 648 W/m2 at the equator at midday.
It doesn’t sound much, but if we were to capture just 0.3 percent of the light that falls on the Sahara and Middle Eastern deserts, we would harness enough energy to satisfy the needs of Europe, North Africa and the Middle East, a vision that drives the Desertec Industrial Initiative in North Africa.
The benefit of the sun’s energy is that it radiates constantly and does not contribute to carbon emissions, but since the planet is a sphere much of that energy hits the atmosphere at an oblique angle or is hidden by the planet’s daily rotation, reducing the average energy levels that reach the ground still further.
The most efficient use of solar energy is to convert it directly into heat using a solar thermal collector. Flat plate solar thermal collectors operate at relatively low temperatures and can provide hot water, heating, cooling, and ventilation to offset a portion of the energy used in residential and commercial buildings across the globe.
To produce electricity, however, less efficient photovoltaics can be used, which convert solar energy directly into electricity; or alternatively concentrated solar power (CSP) collectors that focus the sun’s energy using mirrors or lenses to raise the temperature of a fluid and generate steam to drive a turbine.
Such centralized CSP plants operate most effectively at or near the equator where the solar energy is most intense, helping reduce the total land use per unit of power generated, which lowers the plant’s environmental impact and its cost.
Parabolic mirrors focus the sun's energy on the collector tube
Heat transfer fluid, at the focal point of the mirrors, reaches temperatures high enough to generate steam. The steam drives turbines in adjacent electric generators providing enough power to satisfy 200,000 Spanish households, circumventing the addition of 500,000 tons of CO2 into the atmosphere every year.
To maintain reliable supplies even when the sun goes down, excess energy collected at the plant is stored in liquid salt tanks absorbing part of the heat produced in the solar field during the day. This heat, stored as molten salt, heats water to produce steam, which drive the turbines long into the evening, providing about 7.5 hours of additional operating time at full-load every day after dark.
Linear Fresnel CSP technology
Recent investments in Novatec Solar, a leading provider of Linear Fresnel CSP technology, and GreenVolts, which supplies turnkey concentrating photovoltaic systems including proprietary optics and tracking technology, has strengthened ABB’s presence in the utility-scale solar sector. These new capabilities in solar have enabled ABB to successfully deliver 14 turnkey photovoltaic power plants in Italy alone in the space of just nine months (between December 2010 and August 2011). Twelve plants were completed ahead of schedule, and one 24 MW plant built and commissioned within just five months.