Solar Energy Perspectives - IEA

Chapter 7: Solar heat
Chapter 7
Solar heat
Capturing the sun’s energy as heat is relatively easy, and can be done with considerable
variety of devices, stationary or concentrating. The choice typically depends on the
temperature levels required for end uses: water heating, space heating, space cooling,
process heat, electricity generation or manufacturing of fuels. Storing heat is significantly less
costly than storing electricity, but entirely offsetting seasonal variations of the solar heat at
affordable costs remains a challenge.
Background
The use of solar heat is sometimes tracked back to Archimedes, who is said to have set fire
to attacking Roman vessels with a giant mirror concentrating sunrays in 214 BCE, but there
is no contemporary account of the siege of Syracuse to confirm the story. René Descartes
thought the feat was impossible – but in April 1747 Georges Buffon set fire to a fir plank,
besmeared with pitch, with 128 glasses concentrating sunrays.
In 1767, the Swiss scientist Horace de Saussure built the world’s first solar collector, or
hotbox. Astronomer John Hershel was inspired in the 1830s to hold little family cookouts
with just such a box. William Adams, a British colonist, developed solar cooking in India to
combat fuel wood depletion. Félix Trombe built a concentrating solar oven at Mont-Louis
(French Pyrenees) in 1952, then a more powerful one at Odeillo in 1968.
On an industrial scale, at the end of the 19th century solar water heaters were developed in
California, but the discovery of natural gas in the 1920s killed their expansion. In the 1960s,
solar water heaters were installed by the million on Japanese and Israeli roofs. The boom
expanded to other countries after the 1974 oil shocks but the 1986 counter-shock (oil glut)
killed the nascent industry apart from in a few countries such as Israel, Germany and Austria.
More recently, China dominated the global market for heating with domestic installations,
mostly “thermo-siphon” solar water heaters based on evacuated tube collectors.
In 2010 solar heat collectors covered a global surface area of 28 000 hectares (ha), of which
16 500 ha was in China alone.
Solar heat today, despite the recent boom of PV, represents the largest solar contribution to
our energy needs, with more than 196 gigawatt thermal (GW th
) of capacity and 162
terawatthour thermal (TWh th
) produced in 2010 (see Figure 4.1). It is second only to wind
among the “new” renewable energy technologies (i.e. apart from hydro power and bioenergy).
This comparison takes no account of passive solar energy in buildings.
Collecting heat
Devices to capture solar energy as heat essentially offer a receptive surface to the sunlight,
whether direct or diffuse. Absorption of solar rays heats those surfaces. To absorb as much
incoming radiation as possible, black is the preferred colour.
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© OECD/IEA, 2011