With 1255 two-axis sun-tracking heliostats and mirrors, PS20 has double the capacity of PS10. The PS20 tower is 165 m high and also uses steam, but it employs second-generation technology in the receiver, in its control system, and in energy storage.
DISH/ENGINE SYSTEMS
Dish/engine systems (Fig. 4) are another photovoltaic alternative. Their optimum size, however, seems to be more on the scale of a distributed power generator (1 to 40 kW) than a bulk-power plant, though they can be scaled up in arrays. They are expected to find applications in emerging global markets.
In a dish/engine system, a single concentrator (which may actually be a mirror array) continuously tracks the sun, reflecting its solar energy onto a receiver where it is absorbed, converted to heat, and applied to an engine/generator. This is often a Stirling-cycle engine.
Recall that in the Stirling cycle, the working gas is alternately heated and cooled by constant-temperature and constant-volume processes. Stirling engines usually incorporate an efficiency-enhancing regenerator that captures heat during constant-volume cooling and replaces it when the gas is heated at constant volume. The best of the Stirling engines achieve thermal-to-electric conversion efficiencies of about 40%. Stirling engines are a leading candidate for dish/engine systems because their external heating makes them adaptable to concentrated solar flux and because of their high efficiency.
A great deal of insightful information about dish/engine design is available from SolarPACES (Solar Power and Chemical Energy Systems), a program of the International Energy Agency. According to SolarPACES, the thermal receiver cooling fluid, usually hydrogen or helium, may be both the heat transfer medium and the working fluid for the engine.
Directly illuminated Stirling receivers adapt the heater tubes of the Stirling engine itself. Because of the high heat transfer capability of high-velocity, high-pressure helium or hydrogen, direct-illumination receivers can absorb approximately 75 W/cm². One design challenge is balancing the temperatures and heat addition between the cylinders of a multiple-cylinder Stirling engine. Liquid-metal, heat-pipe solar receivers are used to achieve that balance.
In a heat-pipe receiver, liquid sodium metal is vaporized on the absorber surface of the receiver and condensed on the Stirling engine’s heater tubes, evening out the temperature and permitting a higher engine working temperature. It is also possible to make a heat-pipe receiver that isothermally transfers heat by the evaporation of sodium on the receiver/absorber and condensing it on the engine heater tubes.
Stirling cycle engines used in solar dish systems use a hydrogen or helium working gas at temperatures of over 700°C and pressures as high as 20 MPa. Several mechanical configurations implement these constant-temperature and constant-volume processes. Most involve the use of pistons and cylinders. Some use a displacer (a piston that displaces the working gas without changing its volume) to shuttle the working gas back and forth from the hot region to the cold region of the engine.
For most engine designs, power is extracted through a rotating crankshaft. An exception is the free-piston configuration, where the pistons are not constrained by crankshafts or other mechanisms. They bounce back and forth on springs. A linear alternator or pump extracts the power from the power piston. Several excellent available references describe the principles of Stirling machines.
Some future dish/engine designs may use gas turbines, in which efficiency is over 80%, instead of reciprocating Stirling engines. In the most successful of these designs to date, the receivers have used “volumetric absorption,” in which the concentrated solar radiation passes through a fused silica “quartz” window and is absorbed by a porous matrix. Still, the Stirling engine is today’s norm.