Following the requirement to reduce CO2 emissions from power generation, concentrating solar thermal power (CSP) systems are currently entering the market due to their promising perspective for cost competitive renewable power supply. Commercial CSP plants with a total capacity of almost 1760 MWe are in operation, CSP projects capable of producing hundreds of MWs are under construction and CSP projects with a total capacity of several tens of GWe are under development worldwide. Cost reduction of solar thermal electricity is one of the biggest challenges in order to become competitive with conventional power plants. The greatest cost reduction is expected to come from economies of scale, emergence of new suppliers, mass production of components, but technological development will also play an important role.

Today’s commercial CSP plants generate steam that is converted into electrical power using a steam turbine and generator. The most common technology so far is based on a series of parabolic troughs, which consistent of linear-parabolic shaped mirrors that track the sun to collect the sun’s energy. The sun’s energy is directed onto a receiver pipe to heat synthetic oil, which is later used in a steam generator to transfer the solar heat to steam.

Solar parabolic trough

Solar parabolic trough power plant

The solar steam generated by these state of the art commercial solar thermal power plants does not exceed 380ºC, which limits the conversion efficiency of the steam turbine connected downstream. This performance limitation of CSP plants can be overcome by raising the operability temperature of the receiver and thereby increasing the conversion efficiencies of the thermodynamic cycle. In “point-focussing” CSP systems, precise solar concentrations on the receiver can be achieved. Therefore, for the same output power, the receiver size can be reduced to lower thermal losses and thus enables a highly efficient operation of solar high temperature receivers. The associated increase in overall system efficiency together with the cost reduction of components are the main reasons why solar power towers, in particular, are expected to have a remarkable cost reduction potential.

Solar power tower technology

In solar power towers, two axis tracking mirrors known as heliostats are used to concentrate the solar energy to a receiver that is placed on the top of a tower. Concentrations of several thousands of suns can be achieved with this point focusing solar concentrators. In the last decade various test plants with this technology have been built and operated. Since early 2007 the first commercial solar tower plant named PS10 with a nominal power of 11 MWe began operation near Seville, Spain, and a second one with a total installed power of 20 MWe is currently being commissioned at the same site. At PS10, 624 heliostats with the size of 120 square meters each, concentrate the solar radiation to the top of a 115-meter tower where a tubular receiver transfers the heat directly to saturated steam, which is then used in a steam turbine.

First commercial solar power tower - PS10

Solar hybrid gas turbine systems

One of the most promising concepts for the cost reduction of solar thermal electricity is the introduction of the solar energy into the Brayton topping cycle of a combined cycle (CC) power plant. In this way the solar heat can be converted to renewable electric energy with the highest currently available conversion efficiency of modern combined cycle plants.

Scheme of solar-hybrid gas turbine system (Solugas demonstration system in dotted box)

Power towers with heliostat fields are used to concentrate the solar radiation to a receiver placed on the top of the tower. The receiver absorbs the concentrated solar irradiation and transfers the solar heat to pressurized air, which can be heated to temperatures up to 1000ºC. The hot, pressurized air from the solar receiver is directly fed into the combustion chamber of a gas turbine where natural gas is added to further heat the air to the turbine firing temperature design point.

The serial connection of pressurized solar air receivers with the combustion chamber of a gas turbine allows a solar-hybrid operation that can compensate for any deficiency in solar radiation. Thus, the power output of the solar-hybrid power plant can be guaranteed independent of the sun’s position or meteorological conditions, and still meet the utility power demand requirements. Depending on the system configuration and operation strategy, the power output from solar energy can be between 40% and 90% depending on the design conditions. In terms of thermodynamic efficiency, the concept of a central solar high temperature receiver, in which pressurized air is heated up to 1000ºC and converted into electrical energy by means of a combined cycle, is superior to any currently available solar thermal application for electricity generation.

The concept of solar-hybrid gas turbine systems leads to the following advantages:

• High cost reduction potential due to the high conversion efficiency
• Low environmental impact due to low water consumption
  - gas turbine Brayton cycle: no cooling water required
  - combined cycle configuration: up to 70% less cooling water required
• Reduced land usage due to high conversion efficiency which reduces collector area and land use
• Guaranteed dispatchable power, independent on meteorological conditions
  

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