178Concentrating solar power in deserts delivers an average power per unit area
of roughly 15 W/m2. My sources for this number are two companies making
concentrating solar power for deserts.
www.stirlingenergy.com says one of its dishes with a 25 kW Stirling engine
at its focus can generate 60 000 kWh/y in a favourable desert location. They
could be packed at a concentration of one dish per 500 m2. That’s an average
power of 14 W/m2. They say that solar dish Stirling makes the best use of
land area, in terms of energy delivered.
www.ausra.com uses flat mirrors to heat water to 285 °C and drive a steam
turbine. The heated, pressurized water can be stored in deep metal-lined
caverns to allow power generation at night. Describing a “240 MW(e)” plant
proposed for Australia (Mills and LiÈvre, 2004), the designers claim that
3.5 km2 of mirrors would deliver 1.2 TWh(e); that’s 38 W/m2 of mirror. To
find the power per unit land area, we need to allow for the gaps between
the mirrors. Ausra say they need a 153 km by 153 km square in the desert to
supply all US electric power (Mills and Morgan, 2008). Total US electricity
is 3600 TWh/y, so they are claiming a power per unit land area of 18 W/m2.
This technology goes by the name compact linear fresnel reflector (Mills and
Morrison, 2000; Mills et al., 2004; Mills and Morgan, 2008). Incidentally,
rather than “concentrating solar power,” the company Ausra prefers to use
the term solar thermal electricity (STE); they emphasize the benefits of ther-
mal storage, in contrast to concentrating photovoltaics, which don’t come with
a natural storage option.
Trieb and Knies (2004), who are strong proponents of concentrating solar
power, project that the alternative concentrating solar power technologies
would have powers per unit land area in the following ranges: parabolic
troughs, 14–19 W/m2; linear fresnel collector, 19–28 W/m2; tower with helio-
stats, 9–14 W/m2; stirling dish, 9–14 W/m2.
There are three European demonstration plants for concentrating solar power.
Andasol – using parabolic troughs; Solúcar PS10, a tower near Seville; and
Solartres, a tower using molten salt for heat storage. The Andasol parabolic-
trough system shown in figure 25.4 is predicted to deliver 10 W/m2. Solúcar’s
“11 MW” solar tower has 624 mirrors, each 121 m2. The mirrors concentrate
sunlight to a radiation density of up to 650 kW/m2. The receiver receives
a peak power of 55 MW. The power station can store 20 MWh of thermal
energy, allowing it to keep going during 50 minutes of cloudiness. It
was expected to generate 24.2 GWh of electricity per year, and it occupies
55 hectares. That’s an average power per unit land area of 5 W/m2. (Source:
Abengoa Annual Report 2003.) Solartres will occupy 142 hectares and is
expected to produce 96.4 GWh per year; that’s a power density of 8 W/m2.
Andasol and Solartres will both use some natural gas in normal operation.
179HVDC is already used to transmit electricity over 1000-km distances in South
Africa, China, America, Canada, Brazil, and Congo. Sources: Asplund (2004),
Bahrman and Johnson (2007). Further reading on HVDC: Carlsson (2002).