Solar Energy System Descriptions
Utility-scale solar energy developments typically involve a large commitment of land, and consist of primary and ancillary facilities, including a grid connection.
Solar Energy System Types
Utility-scale solar energy facilities generate electricity using either concentrating solar power (CSP) systems (linear, power tower, or dish engine) or photovoltaic (PV) systems.
Concentrating Solar Power (CSP) Systems
CSP systems include linear, power tower, or dish engine systems.
PV systems use solar cells that are made of layers of semiconducting materials. When sunlight is absorbed by these materials, the solar energy knocks electrons loose from their atoms, allowing the electrons to flow through the material to produce electricity.
The three main types of materials used for solar cells are:
An individual solar cell may only be capable of generating a few watts of electricity. They are normally combined into modules of about 40 cells; the modules are in turn assembled into PV arrays of up to several meters on a side. For utility-scale applications, hundreds of arrays are interconnected to form a single, large system.
PV technologies include flat plate and concentrating systems. Flat plate systems use solar panels, designed to allow sunlight to strike the solar cells directly, without the benefit of any light-concentrating or focusing device. As the conversion efficiencies of most solar cells increases with increasing intensity of incident light, a concentrating device such as a Fresnel lens can greatly increase the amount of electricity generated by the solar cells.
The equipment required for PV facilities can be tolerant of slope change and engineered to accommodate slope change across a site. Construction will be more complex, particularly on slopes greater than five percent.
A concentrating PV system can produce as much as 30% more power than a flat plate system having the same total solar cell area. The drawback is that more expensive high-performance solar cells and precise dual-axis tracking is needed. Also, both mirrors and concentrating lenses must be kept clean to perform optimally. The rest of the solar energy facility would be similar for both PV systems.
Each block of PV arrays would have a inverter and transformer. The inverter changes the direct current electricity generated by the solar cells to alternating current that is used in the transmission grid. The transformer steps up the voltage of the PV panel output to a medium-voltage collection system voltage (e.g., 34.5 kilovolts [kV]). The medium-voltage collection system lines that transmit power from each PV bock would be buried underground and connected to the project substation. At the substation, the output would be stepped up to the existing transmission system's voltage (e.g., 230 kV).
Solar Energy Facility Size
Due partly to low efficiencies of solar systems, solar energy developments typically involve a large commitment of land. Power tower, dish engine, and PV systems require about 9 acres/megawatt (MW) and linear systems require about 5 acres/MW. More land would be needed for facilities that have energy storage or hybrid systems.
Solar Energy Facility Components
Major components of a solar collector include the mirrors or reflectors, the concentrator structure and foundation, and the heat collection element. For most solar energy technologies, the mirrors can be mounted at a fixed angle facing south or can be mounted on a tracking device that follows the sun. Duel-axis tracking devices allow tracking of the sun on a daily basis and throughout the year. The solar collector field would be unpaved and ungraveled in order to prevent rock damage from vehicles involved in mirror washing. Space is required between collector rows, dishes, or heliostats sufficient to allow for maintenance access, and to prevent collectors from shading each other.
The concentrator structure supports the mirrors, maintains them in optical alignment, withstands external forces such as wind, and, on tracking devices, allows the collector to rotate so that the mirrors can track the sun.
Each solar collector assembly has its own local controller that controls its operation (e.g., tracking of the sun). It also monitors for any alarm conditions, such as a high or low fluid temperature in the receiver. The local controller communicates with a supervisory computer in the power plant control building. The supervisory computer controls when a collector assembly should start or stop tracking the sun. Both power and signal cables will likely be buried.
Power Block (Power Plant) Facility
Linear and power tower technologies require a large power block, whereas dish engine and PV systems are comprised of a large number of modular units that do not use a main power station. The power block is the power plant system at which electrical power is generated. It includes the steam heat exchanger where the steam is produced, the steam-turbine generator that produces electricity, and the electrical equipment contained in a substation.
Availability of cooling water for the power block is a potential barrier to flexibility in siting linear and power tower systems, although this would be less of a concern when using dry cooling. Molten salt storage tanks (for linear and, more likely, power tower systems) or battery arrays (for other systems) could be needed for energy storage and facility operation during periods when the sun is not shining.
Components and ancillary components of a power block could include:
An electrical substation that receives the power from all solar energy systems may also be necessary unless an existing substation is available in close proximity to the facility. The substation allows the solar energy facility to be safely connected to a local power distribution grid or a long-distance power transmission grid. As PV systems generate DC electricity, it must be converted to AC using an inverter before being added to the electric grid.
The substation can range in size from 10 to 15 acres, and contains various electrical devices including current inverters, switches, transformers, capacitors, and other devices needed to meet transmission or distribution grid power compatibility requirements. For fire safety, central electrical substations are cleared and maintained free of vegetation throughout the operating period of the facility. Gravel is added to the land surface for drainage. Electrical equipment is positioned on concrete pads; a metallic grounding grid is buried beneath the substation. A 6- to 10-foot fence surrounds the substation to prevent unauthorized access by individuals or wildlife.
Supervisory Control and Data Acquisition (SCADA) System
The power block or central control facility would typically contain a supervisory control and data acquisition (SCADA) system to monitor and control plant operations (including solar collector operations), the various pumps and piping systems by which HTF and steam water are circulated, air compressors, steam condenser, chemical treatment equipment, and cooling system.
On-Site Ancillary Facilities
Solar energy plants would require a number of ancillary facilities and components.
Fencing: All facilities would require fencing. It would be about 6 to 10 feet high and have electric, barbed, or razor wire on top. Some fences are modified to exclude some wildlife (e.g., desert tortoises) or allow access for other wildlife (e.g., kit foxes).
Buildings: Other than the power plant, other buildings that could be present at a solar energy plant include an administration/control building and a warehouse/shop building.
Boilers, pumps, and generators: CSP facilities would probably have auxiliary boilers fueled by natural gas to reduce startup time and for HTF freeze protection. Diesel-fueled pumps may also be required for fire protection. Facilities would also have emergency diesel generators.
Water use, storage, and discharge facilities: Solar energy plants would require water to meet cooling, mirror washing, or potable needs. Dish engine and PV systems do not require water for cooling. Water needs could be met by on-site wells, if water rights can be obtained. On-site circulating water treatment would be required to minimize corrosion and scale formation; while demineralization would be required for process and mirror cleaning water. On-site tanks would be used to store raw water, fire water, and demineralized water. On-site evaporation ponds may be required to contain circulating water, blowdown, or other discharges. An on-site wastewater treatment system (e.g., septic system and leach field to treat sanitary wastes) would also be required. These could be sized to retain all solids that would be generated during the lifetime of the project.
The CSP systems that produce steam (linear and power tower facilities) must be able to condense and recycle that steam. This can be done by once-through wet cooling, recirculating wet cooling, dry cooling, and hybrid wet/dry cooling. Once-through wet cooling systems use the greatest amount of water, which could be an issue in arid climates where the most prospective solar resources are located. Dry cooling reduces water consumption by about 90% over conventional wet cooling systems, since both evaporation and drift are eliminated. However, power loss can be over 17% during the hottest days and average about 5% annually. The area required for cooling equipment would be about 2.5 times greater for dry cooling vs. wet cooling. Hybrid wet/dry systems (that have a conventional wet cooling tower and a dry-cooling surface condenser) would provide a water savings of 50 to 85% with only a 1 to 3% drop in annual power output.
The heat concentrated on PV systems does not create electricity, but must be controlled to prevent solar cell deterioration or loss of solar cell performance, reliability, and longevity. There is about a 0.5% decrease in voltage for every degree above 77°F of the solar cell. Cooling system designs can include the use of cooling fans, the use of forced air flow across the solar cell assembly, water circulation between the solar cell assembly and a remote heat exchanger, or HTFs. Cooling systems introduce additional maintenance costs as well as the potential for release of coolants in the event of system failure.
Thermal and electric storage: Linear and power tower systems can incorporate thermal storage so that the plant can operate when the sun is not shining. Adding thermal energy storage (TES) to a CSP facility greatly increases the reliability of a plant as a source of power for connection to the grid. Adding TES would require adding both the TES components and additional solar collectors to meet the amount of heat expected to be added to the TES. The heat stored for later use could increase the plant capacity from 20% to as high as 60%. Adding thermal storage could allow a plant to operate as long as six hours without solar insolation. The collector field is oversized to heat a storage system during the day that can be used in the evening or during cloudy weather to generate additional steam to produce electricity.
TES designs can be direct (the conventional synthetic-oil HTF is replaced with molten salt and is circulated between the solar field and a storage facility consisting of one or two tanks and subsequently circulated between the storage location and the conventional steam heat exchanger) or indirect (the conventional HTF is used to transfer heat from the solar field to either the molten salt in storage or the conventional steam heat exchanger). Dish engine and PV facilities can have batteries that store energy for use when the sun is not shining.
Solar energy systems can also be designed as hybrid plants, meaning they use fossil fuel to supplement the solar output during periods of low solar radiation. In such designs, a natural gas-fired heater or gas steam boiler/reheater is used. Future solar systems may be integrated with existing or new combined-cycle natural gas or coal-fired plants.
Off-site facilities: Off-site facilities that would be required for a solar energy plant would include access roads and a transmission line. A gas pipeline and water pipeline may also be required (i.e., if gas boilers are required for the heat transfer fluid, if the solar plant would be part of a hybrid facility, or if on-site wells are not used to meet water needs). Environmental impacts from construction and operation of transmission lines and gas pipelines are discussed separately in the Energy Transmission section. Activities associated with a water pipeline would be similar to those for a gas pipeline.