Top Plants: El Dorado Energy’s Solar Facility, Boulder City, Nevada

Dr. Robert Peltier, PE

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Modular Design Simplifies Development

One of Sempra’s goals in constructing El Dorado was to gain experience with the design and operation of a "small" PV project before moving ahead with plans for larger systems (see sidebar). Sempra’s market plan, however, is much different from those of merchant plant developers chasing opportunities who, more often than not, end up with little to show for the effort. Instead, Sempra envisions that PV projects can be successful and economic to operate if they are constructed adjacent to existing power facilities.

El Dorado’s Modular Design Supports Future Expansion

The design of the 10-MW El Dorado Energy PV project is based on a single 1-MW building block that is replicated 10 times. A future 50-MW expansion project will continue to use the same 1-MW block. Here’s how 167,400 individual solar modules, or panels, spread over 80 acres are connected together to form ten 1-MW power blocks.

Each panel is mounted on a structural steel platform that faces south at a 30 degree tilt, the best angle in this locale for capturing the most energy from the sun during a day. Each panel is rated at approximately 75 W and features an open circuit voltage of 90 VDC and is rated at about 0.8 A current production capability. Each module acts much like a battery system, where the voltage and current supplied is a function of how many batteries are connected in series and parallel; the PV panels are also connected in series, which determines the desired voltage, and in parallel to produce the desired current output. A series of 10 panels are connected in series to produce 900 VDC. This series of panels is called a "string." Fourteen strings are then connected in parallel at a "combiner box." Four combiner boxes are then connected, again in parallel, to produce the current production that is connected to a single inverter. The inverter converts direct current (DC) to alternating current (AC).

Each power control station (PCS) is housed in a one of the small white buildings shown in Figure 1, which house two 500-kW inverters that convert direct current to produce 1 MW at 315 VAC. This configuration is the basic 1-MW building block for the plant that is replicated 10 times at El Dorado.

Behind each PCS building there is a 315-VAC to 4,160-VAC transformer that connects the PV energy to the plant’s switchgear. Two sets of 5-MW switchgear reside in cubicles in the combined-cycle plant’s switchgear lineup, where the energy enters the main power system. The PV power is then stepped up to 230 kV, as is the power produced by the El Dorado combined-cycle plant, which leaves the plant via the 230-kV switchyard.

In anticipation of building the future 50-MW expansion, Sempra has already completed the switchyard expansion and a separate 135-kV overhead interconnection that will go to a transformer in a new switchyard that feeds into the existing 230-kV switchyard.

Many synergies can be enjoyed by colocating PV projects with existing plants. El Dorado Energy Plant Manager Kevin Gillespie pointed out that by leveraging their highly qualified and experienced combined-cycle plant operating and maintenance staff, they have reduced the PV plant’s operating costs and improved reliability. Leveraging the plant’s transmission interconnection also gives the PV plant a significant economic advantage over other merchant plant developers in time and cost.

Gallagher also pointed out that by modularizing the plant’s design into 1-MW blocks, Sempra is able to quickly capture and transfer lessons learned to the design and construction of future projects.

In my opinion, Sempra has quickly established perhaps the most rational development program for PV technology development in the U.S. and is on the road to positioning itself as preeminent PV system developer. Gallagher was quick to note that when El Dorado was conceived, there were another 13 PV projects in the NV Energy queue looking for an interconnection agreement. (El Dorado is in the NV Energy service territory, so the plant’s physical interconnection is to NV Energy’s grid.) A year later, all 13 of those projects had been delayed or abandoned; only El Dorado was actually built.

At El Dorado, all the energy produced by the PV field connects directly to the main 4,160-kW bus of the adjacent combined-cycle plant and the plant switchyard. Additionally, the combined-cycle plant’s distributed control system is completely integrated with the PV plant (Figure 3). A single technician manages all aspects of the PV plant, while First Solar provides maintenance and remote monitoring as part of its ongoing support contract. In addition, other technicians have been cross-trained to operate the PV plant.

3. Tightly knit system. The combined-cycle plant control room staff can monitor the performance of the entire PV plant on a single monitor. This monitor reports that the PV system was producing 9,585 kW when the photo was taken. The remainder of the screens report on the performance and control the 2 x 1 combined-cycle plant. Source: POWER