Saguaro Solar Power Plant, Red Rock, Arizona

Familiar features

The north end of the site contains the OEC power block, cooling towers, and control building. The Solargenix solar collector loops encompass the larger south end of the site, including six rows (three loops) of collectors, each 1,300 ft long and spanning 50 ft row to row. Each loop consists of eight SCAs, oriented north-south. The site allows for future solar field expansion to a total of six loops. The east side of the site includes a 1.5-acre evaporation pond.

The Ormat power plant is a fairly standard organic Rankine cycle adapted from geothermal applications. Its working fluid is n- pentane. An organic bottoming plant makes sense for a small plant like this, but for larger plants a steam bottoming cycle probably would be needed to achieve higher conversion efficiency. The Ormat OEC is a complete engineered system comprising a turbine, a preheater, a vaporizer/superheater, a recuperator, and a water-cooled condenser. The power block's footprint is approximately 8 ft by 40 ft.

The OEC control cabinet houses the control system, which uses programmable logic controllers interfaced to local controllers at each of the SCAs. The main control system interface PC is located in the main Saguaro Plant remote control room. The control system, when started, automatically accelerates the turbine to synchronous speed (conditioned on solar field fluid supply) and then switches the generator to the mains. Once linked, the control system monitors and controls the operation of the generating unit.

The plant was designed to operate unattended, to minimize O&M costs. During the design phase, generating efficiency was sacrificed to achieve those goals. The plant operates in very straightforward fashion. In the morning, it starts up as the sun rises. The solar field's heat-transfer fluid (HTF) is circulated in bypass mode until it reaches a minimum temperature (currently, about 300F); simultaneously, the OEC goes into its start-up and warm-up phases. Once threshold temperatures are met, valves modulate to send the HTF to the OEC.

From this point forward, the power block's output is a function of solar insolation and HTF temperature. The OEC then is operated until several operating parameters fall below certain thresholds—for example, as a result of the sky becoming cloudy. At that point, operation of the solar field returns to bypass mode, with the HTF either rising to the operating-temperature window or cooling for standby or shutdown conditions. The system automatically shuts down just before sundown each day.