planners/policy makers of a country or a region), it is preferable that the transmission system is designed with sufficient capacity and operated so that such restrictions would not apply.
A low degree of interconnectivity among neighbouring power systems can also force nuclear units to provide the required flexibility to the power system grid when faced with fluctuations in energy demand. Where neighbouring power systems have weak interconnections, it may be necessary for a nuclear power plant to operate flexibly to allow the required power flows across the interconnection. In this case, flexible operation of the nuclear unit may reduce transmission constraints.
3.2.5. Constraints on non-nuclear generating units
Increasingly strict environmental legislation in some Member States is starting to affect the operation of power plants that burn fossil fuels (e.g. coal or lignite). This may reduce or limit the ability of such plants to operate at much less than full RTP, or to change output rapidly or frequently if they are to remain within the environmental limits.8 In addition, as in the cases of North America and Europe, the legislation may cause older fossil fuel generating units that previously operated flexibly to be retired, as a result of being economically unfeasible, in order to comply with the new rules. A reduction in the capacity of such generating units that previously operated flexibly will increase the need for other generating units, such as nuclear power plants, to operate flexibly. For example, in the United States of America, proposed greenhouse gas limits under ‘clean coal’ initiatives have brought forward the need for flexible operation of nuclear power plants. This means that operators with mixed generation assets may prefer to utilize nuclear generating units, for example, before clean coal generating units, in order to ensure they are operating catalytic conversion systems efficiently, because these conversion systems are not efficient at low power.
Environmental legislation may also restrict the flexible operation of some hydroelectric units because of the need to maintain water flow rates to conserve fish stocks, for irrigation or for flood control. Run of the river hydroelectric plants (i.e. those without significant water storage) are generally treated as being inflexible.
3.2.6. Changes in electricity market rules
Some Member States have deregulated their public electricity supply systems, and more Member States are considering deregulation. The technical or commercial rules in the deregulated market may require all generating units to be treated similarly, and hence, require all generating units to have at least a defined minimum capability to operate flexibly. This requirement for capability may apply to nuclear units, even if there is rarely a need for them to operate flexibly.
In some Member States that have deregulated their public electricity supply system, there is a payment to generating units when they operate flexibly at the request of the grid system operator to change output or to support grid frequency control. Such incentives provide a potential commercial opportunity, as well as compensation, to nuclear units that are operated flexibly.
In other Member States, electricity trading is based around a spot market; therefore, the electricity price can become negative if a time of low demand coincides with high output from renewable generating units such as wind turbines and solar photovoltaic units. With such market arrangements, there would be a commercial incentive for nuclear units to reduce output at such times, to avoid the financial debit of negative prices.
— On an electricity network, where periods of lowest electrical demand are confined to a particular time of year, it is sensible to schedule the planned shutdown of the nuclear units for maintenance and refuelling for this time. This could avoid the need for flexible operation of nuclear generating units that would be necessary if they were in operation during these periods. A pumped storage power station will purposely increase electrical demand on the electricity system at low demand times, typically during the night, when it is in pumping (charging) mode. At other times, it provides additional generating capacity, and can operate flexibly to provide load following and frequency control. Hence, a pumped storage power plant may allow nuclear units to continue to operate at baseload, as it provides the capability for flexible operation that would otherwise have to be performed by nuclear units. This alternative has been implemented in some Member States where the specific geography is suitable. For example, it was utilized for the first nuclear units built in China [3]. Other forms of energy storage that could provide a similar function are under development (e.g.
flywheels or flow batteries). Figures 14 and 15 show the utilization of pumped storage for managing demand and maintaining baseload generation, respectively, in the Czech Republic on 5 March 2016.
— Where the electricity network in a country or region has suitable electrical interconnections with neighbouring countries or regions, it may be possible to develop commercial arrangements for the import/export of surplus generation during low demand periods (e.g. on weekends or at night), to reduce or avoid the need for flexible operation of nuclear units. The differences in the price of electricity among neighbouring countries or regions may provide a commercial incentive for building or increasing the capacity of such interconnections.
— It may also be possible to operate a nuclear unit at steady full thermal power while varying the electrical output, if other technologies could make use of the excess thermal power from the reactor (e.g. in forms of heat) that would otherwise be wasted [16]. This could include district heating, desalination of sea water or process heating (e.g. industrial steam, coal liquefaction/gasification or hydrogen production). As discussed in Ref. [17]9, a proper assessment of the technical and economic feasibility has to be performed to justify the alternatives being considered.
Some nuclear power plants in Member States are currently using part of their thermal power to provide heat for industrial or district usages. In this principle, as illustrated in Fig. 16, the heat transfer can be adjusted, allowing the electrical power to be varied while leaving the thermal power extracted from the nuclear core
9 Reference [17] discusses SMRs as a representative case; however, the basic economic principles apply to any technology.
FIG. 14. Increased demand by pumped storage in the Czech Republic on 5 March 2016 (reproduced from Ref. [15]).
unchanged. The provision of sufficient energy storage via hot water storage tanks would allow the varying heat demand to be met, independent of changes in the power supplied to the heating system by the reactor.
In this application, the economic models of heat and electricity generation have to be compatible. Use of the derived heat from reactors for district heating has been practised in several Member States, particularly in Northern, Central and Eastern Europe. In these plants, 5–15% of the thermal power is used for district heating [18, 19].
— Demand side management and load management, as well as other commercial arrangements, can be used to encourage electricity consumers to alter their electrical usage from periods of high electrical demand to periods of low electrical demand, including arrangements to increase night-time electricity demand. For
FIG. 15. Maintaining baseload generation with use of pumped storage in the Czech Republic on 5 March 2016 (reproduced from Ref. [15]). CCGT — combined cycle gas turbine, PP — power plant.
FIG. 16. District heating with constant reactor power and variable electrical output.
example, with the emergence of smart grid technology and electric vehicles, there may be more opportunities in the future for innovations in demand side management.
Commercial arrangements can also be used to persuade large industrial users of electricity to reduce demand on request when reserves are low, or to automatically disconnect demand in response to a low system frequency.
That is, large demand users may be willing to control their demand to provide ancillary services. One specific instance is in Indiana, United States of America, where services have been provided by an aluminium smelting plant [20, 21]. This would reduce the amount of frequency response and reserve that must be provided by generating units.