Scaling Solar-Powered Electricity

A Production and Distribution Perspective
By Natasha Léger | Published January 23, 2012

Artist rendering of the Crescent Dunes Solar Energy Project that employs SolarReserve’s utility-scale molten salt power tower technology. The Crescent Dunes Solar Energy Project will provide reliable, zero-emission electricity on demand, day or night. Graphic courtesy of SolarReserve.

NV Energy has 46 separate renewable energy projects in the production or development phase or in the process of securing necessary permits, including wind, hydro, geothermal, and solar. By 2025, utilities in the state of Nevada will be required to generate 25% of their energy from renewable sources. Currently, NV Energy is on track to reach a 15% renewable energy level by year’s end, most of which is currently generated by geothermal resources. However, with 12 solar projects in operation or under development in the state, NV Energy is seeing a large amount of new capacity from this resource, especially as the price of photovoltaic solar (PV) continues to fall. In addition, solar has become more attractive because PV provides greater siting flexibility, including interconnection at lower voltage transmission levels – where the complexity is not as great, where there are smaller site requirements, and where siting is closer to customer loads.

While NV Energy has explored development of its own facilities, it primarily relies on 3rd party developers to develop, construct and sell electricity to the utility under long-term power purchase agreements, or “PPAs.” SolarReserve is one of the 3rd party developers that NV Energy will depend on for energy production and is responsible for developing what will be NV Energy’s largest solar energy source, a 110-megawatt (MW) solar thermal facility with storage capability.

Solar 101

A variety of technologies exist for converting solar energy to electricity. Photovoltaic, or PV, converts photons from sunlight directly into electricity. Other technologies utilize reflected sunlight to generate heat, which is used to boil water or some other heat transfer fluid to drive a turbine generator. These technologies are typically referred to as “solar thermal,” or concentrating solar power (CSP).

While sunlight may appear to provide an unlimited source of energy, it runs afoul of such things as clouds and dust storms which can interrupt energy production. The reason why the SolarReserve facility will be the largest producer of solar energy is because SolarReserve’s plants do more than just collect solar energy through concentrating mirror units called heliostats. They also store the energy in molten salt so that it can be dispatched on demand, and into evening hours in the summer when the sun is down but demand is still very high. The SolarReserve molten salt concentrating solar thermal power technology provides the reliable solar power needed for a complex electric grid distribution system.

NV Energy and SolarReserve are both in the business of generating electricity, and NV Energy in transmitting and delivering electricity to its customers. SolarReserve relies solely on solar resources. In the case of NV Energy, it relies on a mix of traditional energy sources as well as renewable sources to ensure reliable and consistent supply to its cust-omers. In both instances a number of location-based variables enter the equation that determine a site’s feasibility.

Solar Site Selection

The Role of Renewal Credits

Nevada, with its excellent solar resources, provides excellent opportunities for stand-alone solar facilities in the 20-30 MW range and above. The renewable energy statutes in Nevada also provide some unique opportunities for PV. In Nevada, by statute, if a PV facility is developed on a customer premise and the customer consumes at least 50% of the power from PV facility, their eligible renewable credits are multiplied by 2.4. For example, if one kilowatt per hour of electricity generated earns one renewable credit for a typical renewable energy source, for PV it generates 2.4. This renewable credits incentive has resulted in the development of smaller facilities at some of NV Energy’s customers’ sites, interconnected at the lower voltage distribution level. A typical customer site profile would include a manufacturing facility, a military base, or corporate campus, including both ground-mounted and rooftop installations. Sims noted, however, that the development of such facilities is dependent on the resulting price of the energy and the renewable credits, and the utility’s need for such credits.

Map of NV Energy Renewable Energy Projects. Courtesy of NV Energy.

Producing Reliable Solar Energy

To engineers, collecting photons and moving the electrons to create electricity is pretty straightforward. Collecting solar energy through photovoltaic panels is one thing; generating consistent reliable energy to the grid is a whole other complex matter. “While storage of solar power for effective grid management is the key technical component of a utility-scale solar energy facility, there are many location-based variables that determine the efficiency of any particular facility,” said Charles Diep.

Once a site has been identified using requirements such as regional market demand, access to transmission lines (which includes distance to an interconnection point or substation), regulatory requirements and water availability, a performance analysis based on other location-based parameters ultimately determines the feasibility of a particular SolarReserve project. These parameters include: solar resource pattern, other atmospheric conditions, as well as site topography. Weather and atmospheric conditions include temperature, water, wind, humidity, air density, and visibility patterns.

Did you know that Spain has the highest number of solar facilities in the world? Why? Not because it has the greatest solar resources (lots of sunny days) but because it receives the most government subsidies for solar energy production. With the ability to store solar energy, even regions with mediocre solar resources can produce reliable electricity.

For solar thermal technologies, temperature, for example, has an impact on the steam cycle efficiency. The higher the temperature, the more cooling needed resulting in the increased loss of efficiency. Lower temperatures will cause additional heat loss and require additional insulation. Temp-erature also impacts other solar technologies such as the PV panel; as temperature rises, the output of the panel declines.

Temperature issues create a bit of a challenge as areas consisting of high solar resources are generally warmer. Every project deserves its own treatment and attention to these variables, which requires a temperature distribution profile to analyze time of day and year in order to design a system based on the site’s specific temperature range. For each proposed facility, a projection of the average solar resources over the next 20-25 years is required to determine impact on yield and to guarantee production over the life of a 25-year purchase agreement by the contracting utility.

The siting process starts with knowing where... 

 

The complete article is available in the Fall 2011 Digital Edition of LBx Journal.