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Replace coal with hybrid natural gas and solar electricity

Several locations across the United States are replacing coal fired power plants with hybrid natural gas and solar. Although burning natural gas is not ideal, it is preferable to coal in that it provides low emissions for cleaner air, decreases dependence on foreign oil and eliminates the coal ash disposal dilemma.

Increasingly strict emission controls have increased pressure on coal plant operators with the Obama administration recently adding more inducements. Unfortunately, most plant operators do not know how hybrid plants work having no experience with them. The solar community must stay abreast of conversion projects and begin educating utility operators and regulators on solar options as soon as possible so projects can be grandfathered before expiration of state solar incentives.

The U.S. Energy Information Agency says at least half of the 15 gigawatts (GW) of power generation retiring in the next two years are in locales where a solar hybrid plant could be used to produce half of the generating capacity. The potential new solar installation capacity through 2015 is almost 4 GW, about 2 GW annually, or a 37 percent increase in annual solar capacity growth.

Hybrid gas plants use parabolic troughs or power towers where the heat-transfer fluid goes through a heat-exchange boiler to produce steam for the turbine that drives the generator. Thermal storage or burning natural gas can also heat the boiler. The solar system is the primary source of heat with the fossil fuel as backup for insufficient solar heat.

A second hybrid method is to pair a large PV field with one or more gas-turbine combustion engines. But gas turbine systems are not cost effective in producing lots of electricity for long periods. Cost of the PV field versus the combustion turbine size. Governing factors are economics, available solar resource, geography and climate with clear sky conditions necessary.

In Florida, FPL reports that by using solar energy and natural gas they have reduced their dependence on foreign oil by over 98 percent with the equivalent of 750,000 barrels of oil just from solar energy. It has helped make the air cleaner since emissions-free solar plants prevent millions of tons of greenhouse gases.

FPL's Martin Next Generation Clean Energy Center is the first-of-its-kind "hybrid" solar facility in the world. In the Sunshine State, it uses American-produced natural gas all the time plus over 190,000 mirrors on about 500 acres to direct solar rays to heat fluid-filled tubes that produce steam to generate electricity when there is sunshine.

A second Space Coast Solar Center in conjunction with NASA at Kennedy Space Center uses PV solar panels to convert light into about 18,500 MWh of electricity feeding Florida's electric grid yearly. At a third DeSoto Solar Center 90,500 solar photovoltaic panels change position to catch solar rays producing about 52,000 MWh of electricity annually.

Solar power is clean but currently is costly and does not work when there is no sun. Pacific Northwest National Laboratory researchers in 2013 began testing a hybrid solar/gas system that harnesses solar heat, increasing a natural-gas power plant’s efficiency by 25 percent, and reduces the carbon footprint of natural gas power plants. Testing done on the Richland, Washington campus is part of a program to increase the system’s efficiency and lower the cost to a projected six US cents per kilowatt-hour by 2020, making it competitive with conventional fossil-fuel plants.

South Carolina environmental groups in 2014 asked regulators to delay construction of Duke Energy Carolinas' William States Lee Steam Station 750MW natural gas plant in Anderson County until 2018 to require Duke to install a solar farm as part of the project. Duke is currently procuring equipment and contracts to begin construction in summer 2015 with the unit expected to be online in November 2017.

The new plant on the Saluda River has no effect on Duke’s conversion of one of Lee's retiring coal units to natural gas by 2015. Converting the coal boiler to a natural gas boiler will produce 170 megawatts of electricity with a turbine. Natural gas plant sulfur dioxide and nitrogen oxide emissions are very low and about half the carbon dioxide emissions of coal.

A combined cycle unit uses combustion turbine generators, boilers and a steam turbine generator to produce electricity by burning natural gas in the combustion turbines, producing mechanical power converted into electrical power by the generators. Hot exhaust gases from the combustion turbines create steam in the boilers that spin a steam turbine generator for additional power.

On June 2, 2014, the United States Environmental Protection Agency unveiled the Clean Power Plan proposal to reduce power plant carbon emissions by 30 percent by 2030. Coal fired power plants which generated 39 percent of electricity in the U.S. will decrease by 9 percent by 2030.

The average life of coal plants is 42 years and some have been operating for over 60 years. They cannot compete with low natural gas rates. The most inefficient coal plants are retiring between now and into 2016. No good solution has been found for safe cost-effective coal ash disposal. In February 2014 Duke Energy experienced in North Carolina the horrific coal ash spill into the Dan River which has projected cleanup costs of between $2 billion to $10 billion, with customers facing a possible 20 percent increase in electricity rates as a result.

More business and residential customers are taking advantage of Smart Grid drivers to defect from the grid and cleanly generate their own power, particularly with solar photovoltaics (PV) as solar costs decrease and capital sources increase in PV technology and energy storage.

The time is short and plants need to consider hybrid solar natural gas solutions as coal-to-gas plant conversions occur. Integrating solar power at the same time is more carbon-efficient and cost-effective.