Planning January 2016

Planning — January 2016

The Sun Industry

A look at utility-scale solar energy in central Arizona.

By Robert H. Kuhfuss, AICP

The Valley of the Sun is, not so surprisingly, at the top of the list for solar energy potential in the U.S. That's due to the rugged, wild topography and famously sunny climate of Maricopa County. Add in the fact that the county is largely undeveloped outside the Phoenix metro area, with much of its flat western portion consisting of active and fallow farmland or sparsely vegetated desert, and the region is ideal for utility-scale energy development.

In fact, were it not for the inefficiencies of converting solar energy into electrical energy, just a fraction of the land mass of the county — which is 9,000 square miles — could conceivably meet the residential electrical demand of the entire nation.

Lying at roughly 33.5 degrees north latitude, the region receives about 6.5 kilowatt-hours of solar energy per square meter per day, according to the National Renewable Energy Laboratory, based in Golden, Colorado. According to the U.S. Energy Information Agency, the average American residential customer consumes about 10,932 kilowatt-hours of electricity per year. The math works out: If solar energy could generate electricity more efficienly, this county alone could supply electricity to the nation's nearly 128 million residential customers. 

In response to the demand for solar power, a number of solar energy providers sought to tap into the abundant sunshine of central Arizona. From roughly 2007 to 2012, the Maricopa County Board of Supervisors approved 30 comprehensive plan amendments geared toward utility-scale solar energy, covering a cumulative total of about 27,000 acres of land. (The county makes a distinction between systems that constitute the primary use of a property — utility-scale solar energy — and those that are an accessory use.)

To date, there are six utility-scale solar power plants operating in unincorporated Maricopa County, collectively generating about 620 megawatts of clean solar electricity. Much of the utility-scale solar installed recently in western Maricopa County occurred on fallow farmland. In the late 1990s and early 2000s, merchant electrical energy providers purchased large tracts of farmland in order to acquire the groundwater rights needed to support the cooling demands of the combined-cycle natural gas generating stations they were building in the region. 

Many of these retired farm fields were later converted to photovoltaic energy production, as was the case with Mesquite Solar 1, a 150-megawatt photovoltaic facility, and Arlington Valley Solar Energy II, a 125-megawatt photovoltaic facility. Three smaller photovoltaic facilities, Badger1 at 15 MW, RE Gillespie at 20 MW, and Saddle Mountain at 20 MW, as well as one concentrating solar power facility, the 280 MW Solana Generating Station, converted active farmland or raw desert to solar energy production.

Aerial view of the 150 megawatt Mesquite Solar 1 facility as seen from  the southeast. Photo courtesy Sempra U.S. Gas and Power.

Aerial view of the 150 megawatt Mesquite Solar 1 facility as seen from the southeast. Photo courtesy Sempra U.S. Gas and Power.

Pluses and minuses

While photovoltaic solar has many benefits, including minimal water consumption, there are certain inherent limitations. One is that the peak production curve does not necessarily correlate with the peak demand curve. Residential consumption is typically at its highest in the late afternoon and early evening, when workers return home, turning their air conditioners down to cool the house, and heating up their ovens to cook dinner.

In order to meet that spike in electrical demand, utility companies must deliver more power, often generated from antiquated "peaking plants," which are expensive to maintain and operate. A similar condition exists on cloudy days when electrical output is reduced.

Concentrating solar power, on the other hand, uses the sun's heat to generate steam. Plant operators can adjust electrical output to match demand by storing energy. That is what happens at the 280 MW Solana Solar Electric Generating Station in western Maricopa County, where a portion of the heat collected during the day is diverted to several tanks filled with molten salt, a material that retains thermal energy well. This allows the facility to not only compensate for cloudy days, but most amazingly, to generate electricity from solar energy well into the night.

While CSP facilities have certain advantages, they do have constraints. They require a lot of water for cooling to maintain the Rankine cycle, in which a heat source is used to generate stream that drives a turbine connected to an electrical generator, which produces electricity. The steam circulates back to the heat source to begin the cycle over, but along the way, a certain amount of the heat must be "rejected" through a cooling process. 

Often, this is accomplished through the use of cooling towers, which bathe the steam pipes in liquid water, much like an evaporative cooler. The cooling water is recirculated until its dissolved solids reach a concentration level that it is too high for reuse, at which time the cooling water is routed to large evaporation ponds and allowed to dry.   

In a Rankine cycle, the source of the heat used to create the steam does not matter. It can come from the burning of fossil fuel, nuclear energy, or the sun, but it turns out that parabolic trough technology, a form of CSP, ranks among the highest in water consumption.  

The reasons for this are related to thermodynamics and beyond the scope of this article, but according to the U.S. Environmental Protection Agency, parabolic trough technology using a recirculating wet cooling technology (as opposed to pass-through wet, dry, or hybrid) consumes 800 gallons of water per megawatt-hour of electricity produced, compared to 200 gallons per megawatt-hour for combined-cycle natural gas and about 500 gallons for coal and nuclear.

In a desert, where does that water come from? In the case of the Solana Generating Station, groundwater is used, which seems counterintuitive given that groundwater is generally considered a nonrenewable resource. As it turns out, however, the Solana facility was constructed on more than 3,000 acres of former agricultural land and it consumes just a fraction of the water that the productive farmland did. Considering the facility's ability to generate power at night, this seems to be a win-win scenario.

Map showing Utility-Scale Generation in Unincorporated Maricopa County

Map showing Utility-Scale Generation in Unincorporated Maricopa County

Timing is everything

Interestingly, the increase in solar energy demand came at about the time the economy failed and the housing market collapsed.

In order to meet the demands for utility-scale solar energy development in Maricopa County, the Board of Supervisors in 2011 approved a new Utility-Scale Solar Energy Program, which sought to find ways to bring efficiency to the entitlement and permitting processes. Program elements include top-of-stack prioritization for all applications involving utility-scale projects, along with concurrent processing of comprehensive plan amendments, special use permits, and construction permits. When demand was especially high, regular meetings with a single county point person kept things moving along.

Another provision allows utility-scale solar power plants to "bridge" construction code cycles in order to maintain design continuity in the event that codes are updated during facility construction. The effort has been a success, garnering an Achievement Award in 2013 from the National Association of Counties.

Streamlining the regulatory process certainly helps, but many other variables come into play in determining the feasibility of a large-scale solar project. Seeking to understand these factors, former Gov. Jan Brewer created the Governor's Office of Energy Policy. Its 2013 report, emPOWER ARIZONA, listed the proximity of transmission infrastructure to suitable land, financial constraints, and redundant and overlapping jurisdictional regulations as factors influencing the deployment of solar energy.

All of these elements affect how the cost of solar power compares to other sources of energy. If solar energy producers are to compete toe-to-toe with other energy resources, either the cost to produce it must come down or the government must step in to help offset the cost disparity. Solar-friendly policies and regulations are a part of the latter, and as evidenced by the Database of State Incentives for Renewables and Efficiency, solar policies abound; however, such policies only account for a small portion of the overall cost to deliver power.

Clearly, there is momentum in the greater Southwest toward the implementation of solar energy at a utility scale, and the potential is profound. We have the land. We have the sunshine. We have the interest. What we need are ways to bring the cost of developing these facilities down to make them competitive with other energy resources.

Solar energy cannot, nor should it, totally replace other forms of energy, but it should be part of a robust energy portfolio that reduces greenhouse gas emissions and leads to energy independence.

Robert H. Kuhfuss is the former program manager for Maricopa County's Utility Scale Solar Energy program. He has extensive experience in the planning, zoning, and permitting aspects of both combined-cycle and solar energy production. 


Arizona energy facts, Wind Energy Industry Association:

Solar resources of the U.S., National Renewable Energy Laboratory:

FAQs on residential energy use, U.S. Energy Information Agency:

"Concentrating Solar Power Commercial Application Study: Reducing Water Consumption of  Concentrating Solar Power Electricity":

emPOWER Arizona, Governor's Office of Energy Policy: