The SolarDish® System is an integration of six different subsystems/technologies:
The Dish / Concentrator is a Quasi Parabolic dual-axis reflector that accurately follows the Sun trajectory throughout the day and focuses solar beam radiation at the focal point / receiver. The diameter of the dish comes into three sizes: 7m, 9m, and 11m, depending on the system configuration.
The Solar Receiver located at the reflector focal point collects / absorbs the concentrated solar energy from the dish concentrator. The high absorptance (>90%) solar receiver is a cavity design based on a heat-pipe system with sodium based working fluid that transfers large quantities of heat (60 to 100 kW) at relatively high input flux levels (100W/cm2) at high temperatures (800°C). In dish Stirling systems, a heat-pipe solar receiver has been demonstrated to improve system performance by 20%. Heat-pipe receivers have also been identified as an ideal exergetic coupling to latent heat thermal energy storage. An alternative pool-boiler solar receiver design will also be evaluated due to its simplicity and cost effectiveness.
The Heat Transfer System (HTS) is a closed loop “pumped” two-phase heat transfer design which moves energy collected at the solar receiver to the thermal energy storage (TES) system mounted behind the concentrator. The HTS is a high temperature sodium-based heat-pipe system where the vaporized sodium at the solar receiver moves to the TES where it condenses on the TES exterior surface transferring its energy into the TES container. The condensed liquid sodium is returned to the solar receiver by an external electromagnetic (EM) pump.
The Thermal Energy Storage (TES) system is based on Phase Change Material (PCM) offering up to twelve (12) hours of storage. The TES system design is a “pool boiler system” where a eutectic salt that melts at 680oC is contained within a high-temperature, hermetically sealed, stainless steel container. Enough salt is contained in the TES so that when melted, it could sustain full-power operation of the Stirling engine which is also integrated and sealed into the TES system. The heat is removed from the melted salt by the “heat-pipe” action of a small amount of sodium in the TES. The liquid sodium vaporizes from the heat of the melted salt and condenses on the engine heater head releasing its energy to the Stirling engine for conversion into electricity output. The TES comes in three versions, with 170 KWh, 300 KWh, and 600 KWh thermal energy capacity depending on the configuration, providing up to 12 hours thermal storage for a 4.0 KW, 7.1 KW, and a 14.2 KW Power Blocks, respectively.
The Free-Piston Stirling Engine: The thermal energy-to-electricity converter is a free-piston, Stirling engine system. The hermetically sealed engine system absorbs energy from the sodium vapor condensing on its dome shaped receiver. The Stirling engine takes that heat energy and converts it into a variable frequency, variable voltage AC output with a high efficiency conversion of 30% or more. The variable AC output is rectified to a stable direct current (DC) output. The Stirling engine system includes an engine controller, a power management package of capacitors and resistors, and a closed-loop heat removal / radiator system. The uniqueness and innovation of Free-Piston Stirling Engines enables very low engine maintenance and has been used successfully for many years in US space and defense industry. Free-piston Stirling engines have been demonstrated in critical power applications that require quiet operation, high reliability, and very long, low-maintenance service life. The free-piston Stirling engine has demonstrated a record of more than 120,000 hours (13+ years), of continuous, maintenance free operation.
The SolarDish® will use two-versions of the free-piston Stirling engine: a 4 KW or 7.1 KW power, depending in the configuration.
The ITPCS would provide comprehensive control and monitoring of all SolarDish® subsystems [dish tracking, Heat Transfer System (HTS), Thermal Energy Storage (TES), and power block] to accommodate the varying power input from the dish concentrator with the HTS/TES operation and the power block output. The supervisory control involves solar tracking and calibration using open- and closed-loop (sensor) two-axis motor control and solar tracking algorithms, engine start-stop, fault handling, and temperature monitoring of all subsystems. The engine control unit provides power modulation for load following, monitor and control of heater head temperature, and other engine safety features such as coolant flow. The ITPCS would also monitor and control the engine cooling system (radiator fan and coolant pump) and any Uninterrupted Power System (UPS) operational requirements for SolarDish® unit start-up and shutdown procedures as well as emergency system shutdown due to wind gusts, tracking system failures, etc. A separate system, the Field Supervisory Control and Communications System, would monitor and/or control Field inverters (voltage level and integration with utility grid), wind speed sensors, GPS, etc. and provide inputs to the ITPCS as necessary.