There is a great deal of public and government regulatory support for significant growth in renewables for electricity generation at the expense of fossil fuels, primarily coal. Unlike fossil fuels whose output is controlled by a utility dispatcher, solar and wind depend on exposure to sunlight and wind speed for determining their output. This chapter deals with the use of simulation to match uncertain supply of solar and wind energy with indeterminable demand using a pumped storage facility to store excess electricity generation and to supply electricity to cover shortfalls. A modeling template using @RISK simulation analysis is proposed. The challenge of fossil fuel plants coupled with solar and wind farms with a strong seasonal demand for electricity is addressed by determining the size of an upper reservoir of a pumped storage facility that would ensure reliable delivery of electricity throughout the year. Scoping of the project to identify courses of action that would reduce the size and the investment in a pumped storage plant are covered.
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A primary challenge facing public utilities is the role of renewables in the mix of energy sources. There is widespread popular and regulatory support for substituting solar and wind for fossil fuel; particularly coal. But there is a major difference between supplying power with fossil fuels, nuclear, and hydro than with renewables. Conventional sources of power can with varying degrees of flexibility be ramped up and down in response to ever changing electricity demand. Hydro plants can be ramped up to a power level dependent on water depth in the reservoir, and, of course, ramped down. Hydropower and natural gas are most amenable to addressing rapid changes in power levels. The problem with solar and wind is that they cannot be ramped up to their full capacity as can fossil fuel and nuclear plants in response to an upsurge in electricity consumption. Actually they can’t be ramped down – solar and wind energy not consumed is “wasted”. Solar operates at nameplate capacity for about 3-4 hours per day depending on geographic location and the season of the year. Cloud cover significantly degrades solar output. Wind patterns drive wind turbine output. Wind speeds too high and too low reduce power output to nil. Consider a utility dispatcher with solar and wind farms under his or her control when an eclipse of the sun occurred in Europe in 2015. Suppose that was accompanied by calm winds.
If nearly 100 percent fossil fuel and nuclear backup are needed for solar and wind to be considered reliable, what then is the economic justification for solar and wind? For solar and wind to be a true substitute for fossil fuel, these ultimately unreliable sources must be transformed into reliable sources. This can be done to some extent with currently available electricity storage batteries. Some home solar installations have an associated battery for storage of excess day solar energy to be tapped at night. If a solar installation and battery capacity are sized correctly, it is possible for a home to be removed from the grid. There are neighborhoods, and in a few cases, communities where individual homeowners have entered into cooperative agreements to install solar panels, not necessarily on the roofs of homes, and share solar power in a distributive fashion that considerably reduces the role of the grid in supplying electricity. Night time wind power is often “thrown away” because coal and nuclear power plants were built for continuous full-power operations. They generally respond poorly to power level fluctuations necessary to compensate for swiftly and unpredictably changing wind patterns and must provide 100% backup coverage when wind speed is too low or too high. The economic benefit of wind power is clearly overstated if night time output cannot be used. Storage batteries can transform unneeded night to needed day time power. But then the cost of storage batteries would have to be included along with the greater output of wind turbines in shifting night to day time generation when performing an economic analysis. Even if night time generation of wind electricity can be consumed by backing out low cost base load generators, it may still pay to store electricity for day time consumption to obtain a higher rate. The normal practice in performing an economic analysis of wind and solar is generally without consideration of electricity storage and, for that matter, added transmission costs for wind and solar installations in remote locations.
While conventional storage batteries can handle small scale solar and wind turbine output, they do not have near the capacity necessary the ensure reliability of utility-scaled solar and wind farms. This is the job of a pumped storage plant or gravity battery, which consists of lower and upper reservoirs. Water is pumped from lower to upper reservoir when wind and solar power exceeds demand. During times when wind and solar are not generating sufficient electricity to supply a utility, reverse gravity flow of water from upper to lower reservoir drives the same turbine to generate electricity that pumped water to the upper reservoir. It now becomes a matter of scale: