Power tower systems
In power tower systems, heliostats reflect and concentrate sunlight onto a central tower-mounted receiver where the energy is transferred to a heat transfer fluid. This energy is then passed either to storage or to power-conversion systems which convert the thermal energy into electricity and supply it to the grid.
The major components of the system are the heliostat field, the heliostat controls, the receiver, the storage system, and the heat engine which drives the generator. The heliostat design must ensure that radiation is delivered to the receiver at the desired flux density at minimum cost. Various receiver shapes have been considered, including cavity receivers and cylindrical receivers. The optimum shape is a function of the radiation intercepted and absorbed, thermal losses, receiver cost and design of the heliostat field. For a large heliostat field a cylindrical receiver has advantages when used with Rankine cycle engines, particularly for radiation from heliostats at the far edges of the field. Cavity receivers with larger tower height to heliostat field area ratios are used for higher temperatures required for the operation of Brayton cycle turbines.
As the collector represents the largest cost in the system an efficient engine is justified to obtain maximum useful conversion of the collected energy. Several possible thermodynamic cycles can be considered. Brayton or
Stirling gas cycle engines operated at inlet temperatures of 800-1000 °C provide high engine efficiencies, but are limited by low gas heat transfer coefficients and by practical constrains on collector design (i. e. the need for cavity receivers) imposed by the requirements of very high temperatures. Rankine cycle engines employing turbines driven from steam generated in the receiver at 500-550 °C and have several advantages over the Brayton cycle. Heat transfer coefficients in the steam generator are high, allowing the use of high energy densities and smaller receivers. Cavity receivers are not needed and cylindrical receivers that are usually employed permit larger heliostat fields to be used. The use of reheat cycles improves steam turbine performance, but entail mechanical design problems. Additionally, it is also possible to use steam turbines with steam generated from an intermediate heat transfer fluid circulated through the collector or boiler. With such systems the fluids could be molten salts or liquid metals, and cylindrical receivers could be operated at around 600 °C. In fact, these indirect systems are the only ones that can be combined with thermal storage.
Power tower plants are defined by the options chosen for a heat transfer fluid, for the thermal storage medium and for the power-conversion cycle. The heat transfer fluid may be water/steam, molten nitrate salt, liquid metals or air. Thermal storage may be provided by phase change materials or ceramic bricks. Power tower systems usually achieve concentration ratios of 300-1500, can operate at temperatures up to 1500 °C, and are quite large, generally 10 MWe or more.
Power tower systems currently under development use either nitrate salt or air as the heat transfer medium. In the USA, the Solar One plant in Barstow, CA was originally a water/steam plant and is now converted to Solar Two, a nitrate salt system. The use of nitrate salt for storage allow the plant to avoid tripping off line during cloudy periods and also allow the delivery of power after sunset. The heliostat system consists of 1818 individually oriented reflectors, each consisting of 12 concave panels with a total area of 39.13 m2, for a total array of 71 100 m2. The reflective material is back-silvered glass. The receiver is a single pass superheated boiler, generally cylindrical in shape, 13.7 m high, 7 m in diameter, with the top 90 m above the ground. It is an assembly of 24 panels, each 0.9 m wide and 13.7 m long. Six of the panels on the south side, which receives the least radiation, are used as feedwater preheaters and the balance are used as boilers. The panels are coated with a non-selective flat black paint which was heat cured in place with solar radiation. The receiver was designed to produce 50 900 kg/h of steam at 516 °C with absorbing surface operating at a maximum temperature of 620 °C .
Meanwhile the PHOEBUS consortium, a European industry group, is leading the way with air-based systems. Gaseous heat transfer media allow for significantly higher receiver outlet temperatures, but require higher operating pressures. Pressure-tolerant gas-cooled ceramic-tube receivers have, however, relatively high heat losses compared to water/steam or advance receivers. The PHOEBUS consortium is developing a novel Technology Solar Air (TSA) receiver, a volumetric air receiver which distributes the heat-exchanging surface over a threedimensional volume and operates at ambient pressures. Because of its relative simplicity and safety, these plants can be used for applications in developing countries .
Future work will concentrate on the scaling up of the nitrate salt and TSA/PHOEBUS systems. The target size for nitrate salt plants in south-west USA is 100-200 MWe, while a 30 MWe plant is the aim for the PHOEBUS consortium. In addition to these two systems, a 20 MW Solgas plant, using a combined cycle plant with a solar power tower back-up, is planned for southern Spain .
Recent research and development efforts have focused on polymer reflectors and stretched-membrane heliostats. A stretched-membrane heliostat consists of a metal ring, across which two thin metal membranes are stretched. A focus control system adjusts the curvature of the front membrane, which is laminated with a silvered-polymer reflector, usually by adjusting the pressure (a very slight vacuum) in the plenum between the two membranes. Stretched-membrane heliostats are potentially much cheaper than glass/metal heliostats because they weigh less and have fewer parts.