Can solar play nicely with coal and natural gas?

Cara Libby, Project Manager at the Electric Power Research Institute (EPRI), spoke on Wednesday evening at the Silicon Valley Photovoltaic Society (SVPVS) on solar thermal hybrid applications.  Libby leads EPRI's solar research within the Renewable Energy Generation Program.  EPRI is funded, to a great extent, by the existing electric utility companies.

As fossil fuel prices rise and emission allowances are mandated, solar-augmented steam cycles might be a pragmatic option for energy companies.

When an energy producer wants (or has) to add solar, the utility can add photovoltaics, solar thermal or perhaps solar thermal hybrids, combining solar thermal with coal, gas, oil, geothermal, biomass, etc.  Libby suggested that thermal hybrids are potentially the lowest cost option.  Any of the three solar thermal technologies -- parabolic troughs, power towers or compact linear fresnel reflectors (CLFR) as used by Ausra (now Areva) can be employed.

A typical system would use steam generated by a solar field coupled to a conventional coal or natural gas-powered steam cycle, offsetting some of the fossil fuel required to generate power. Many energy companies are interested in adding solar power to their generating mix, but most solar applications are not cost-competitive with other power generating options.  The highest intensity solar energy typically is within a few hours of peak summer loads, making it an attractive renewable option.

Why build a hybrid plant instead of a stand alone PV or solar thermal plant?

  • Reduces the amount of fuel being burnt
  • Produces less emissions
  • Potentially "greens" existing assets
  • Addresses regulatory pressures and potentially helps meet Renewable Portfolio Standards   
  • Most of these projects could earn RECs
  • Transmission and Balance of Plant (like the power block) are already in place, as well as existing plant staff along with permits and a water supply

Combined cycle power plants working with a solar trough system added can integrate up to 20 percent into the steam cycle, according to EPRI's calculations.

EPRI recently completed a study to evaluate the viability of solar augmentation for coal and natural gas steam cycle plant designs. Detailed heat and mass balance models were developed to evaluate various solar steam integration architectures. Development guidelines were prepared for two coal plants located in New Mexico and North Carolina and two natural gas combined cycle plants in Nevada and Arizona. The guidelines include a detailed analysis of siting, plant cycling limitations, plant controls, reliability considerations, operations and maintenance requirements, regulatory and environmental permits, and construction and commissioning schedules. 

The insights gained in the EPRI study provide options to electricity producers who wish to leverage existing plant assets, reduce their carbon footprint, and gain experience with solar thermal electric systems. Compared to standalone solar thermal plants, the economics of the integrated solar steam cycle applications were favorable.

Hybrid solar thermal is gaining popularity.  There are fourteen projects in development (5 operational) with a total solar component of about 450 megawatts including Xcel Energy's Cameo project in Colorado.  The largest is the FPL project with 75 megawatts of solar in Florida.

It's not economical to add solar in EPRI's current cost analysis.   Case studies show a cost range of $3000 to $4600 per kilowatt and EPRI calculates that the hybrid reduces CO2 emissions by about 1000 tons per year per megawatt installed.  The hybrid improves the solar efficiency from 35 percent to 43 percent compared to 33 percent for stand-alone solar.  Of course, it also lowers the efficiency of the power plant to some extent.  

There are two demonstration projects currently supported by EPRI, both expected to start up in 2014:

  • NV Energy at the Chuck Lenzie Station in Nevada is an 1100-megawatt natural gas plant with 95 megawatts of proposed solar.
  • Tri State G&T in Escalante, New Mexico is a 245-megawatt coal plant with a proposed 36 megawatts of proposed solar.

The best-case scenario for the hybrid plant yields a 30-year levelized cost of energy (LCOE) of 11 cents per kilowatt-hour.