There is little flexibility in the current electrical grids to accommodate high penetration of variable renewables. Three significant challenges currently exist:

1. The geographical location of renewable energy resources are often removed from the major population centers.

2. High variability and, hence, low reliability exist for renewable resources, such as wind and solar.

3. The dispatchability of renewable resources is limited (meaning it cannot be throttled up or down on demand).

For illustrative purposes, projected wind turbine generation rates for one week are shown in Figure 13.3 for a wind farm in Wyoming where a 40 percent annual capacity factor has been realized. There are periods of high or low output that last for days, but significant changes in the generation rate can occur over a very short time (an hour or two). There are also periods of intermediate levels of generation.

There are several strategies for operating a hybridized power generation system to smooth the variability of wind generation. In the first and conceptually simplest, the primary heat source in the hybrid system (e. g. an iPWR or advanced SMR) and the wind farm are operated together to produce a combined electrical output that is constant, essentially mimicking base load generation. For example the wind farm

shown in Figure 13.3, would correspond to making 300 MWe at all times, or as close to this objective as possible. This type of operation would allow the integrated hybrid system to replace an existing baseload 300 MWe coal-fired power plant that was shut down either by obsolescence or to avoid the cost of future CO2 capture requirements or emissions penalties.

A difficulty with this approach is that because of wind’s low availability, on the order of 30-45 percent for regions considered attractive, the primary heat source in the hybrid system must generate power for the balance of the operation, or 55-70 percent of the total energy generated. This, in turn, means that the primary heat source of the hybrid system delivers heat to the coupled process application only when the wind blows, 30-45 percent of the expected annual total, and during some of that time only reduced heat delivery is available. This level of operation would not be economical for the process plant.

An alternative operating basis is to supply a specified minimum amount of heat to the process plant so its operating rate might vary between, for example, 70 and 100 percent of its nominal capacity as needed to offset wind fluctuations. This leads to exceptionally large process plants coupled to relatively small wind farms, both outside desirable ranges. Alternatively, an auxiliary fossil-fired steam generator (a third input system) could provide heat to the process plant when nuclear heat is not available. However, it would have to run 55-70 percent of the time, largely countering the CO2 emission advantages of a nuclear-based hybrid system.

Another strategy is to address only the problematic high frequency components of wind variability. Power grids are well adapted for handling the diurnal variations in demand, so wind variability on that timescale can be handled similarly using the same equipment as long as the wind capacity is no more than approximately the amount of diurnal cycling, typically about 30 percent of the daily peak. In this operating mode, by switching heat output between the power system and the process plant, the hybrid energy system would generate electricity such that the total of the wind-derived and the nuclear-derived electricity does not vary faster than a specified rate. The ability of an SMR hybrid system to compensate for rapid changes in wind generation depends on how fast steam can be switched between the electrical generation and process plants, not on how fast the reactor itself can respond to a transient. In the case of a rapid drop in wind generation, the electric output from the nuclear plant would rise rapidly to moderate the drop in wind generation.

The following section addresses possible applications that could be coupled to a hybrid energy system to maximize use of the thermal energy generated while still meeting the electricity demand. While some the industrial processes would utilize only thermal energy, others require both thermal and electrical energy input from the integrated system.