1.5     Scaling & Integration

Chapters 6–8 of this study deal with issues that would arise if solar energy were to play a major role in electric power systems — specifically, issues of scaling and integration. Chapter 6 provides a quantitative analysis of the materials-use and land- area requirements that would follow if solar energy were to account for a large share of global electricity production by mid-century. As the IEA scenario discussed above indicates, this would require a dramatic increase in solar generating capacity. Nonetheless, Chapter 6 suggests that the availability of commodity materials such as glass, concrete, and steel is unlikely to prove an important hindrance to PV expansion on this scale if today’s commercial technologies are employed. And, provided reliance on silver for electrical contacts can be decreased, there seem to be no significant materials-related barriers to a dramatic increase in the deployment of crystalline silicon-based PV, today’s dominant solar technology. It is important to note, however, that some thin-film PV technologies currently in use or under development rely on rare materials such as tellurium and indium.

Increasing the usage of these materials far above current levels would increase their costs dramatically and perhaps prohibitively. This makes the corresponding technologies poor candidates for large-scale deployment — and thus relatively unattractive as targets for government research and development spending. On the other hand, as Chapter 2 indicates, there are emerging technologies with the considerable promise that use Earth-abundant materials and that could be deployed at a large scale if their efficiency and stability could be dramatically improved.

Chapter 7 analyzes the impact of connecting distributed PV generation to existing low-voltage electricity distribution systems. Having generation near demand reduces the use of the high-voltage transmission network and thus cuts the associated (resistive) losses of electric energy; proximity to load also reduces such losses in the distribution network (except at very high levels of penetration). But, as Chapter 7 demonstrates, when distributed generation accounts for a large share of the overall power mix, any savings from associated reductions in network losses are generally swamped by the cost of the distribution-system investments needed to accommodate power flows from facilities connected at the distribution level out to the rest of the grid. The magnitude of these investments depends on features of the local distribution system (e.g., population and load density) and the characteristics of the local solar resource and its location in the network.

Chapter 7 reports on simulations that explore the impact of large-scale solar integration at the level of the wholesale power system, considering operations, planning, and wholesale electricity market prices. Our analysis focuses on the variability of solar output, not its imperfect predictability. An important finding is that incremental solar capacity, without storage, may have little or no impact on total requirements for non-solar capacity, because system peak demand may occur during the late afternoon or early evening hours when there is low or no insolation, or even at night in the case of systems where annual peak load is not driven by air- conditioning.

Because solar PV has zero marginal cost, a substantial increase in solar PV penetration will tend to make existing plants with high marginal costs non-competitive in the wholesale electricity market. Also, because solar PV is intermittent, substantially increasing solar PV penetration will tend to increase the need for thermal plants to vary their output. This cycling of thermal plants can involve substantial cost increases. All else equal, a more flexible generation mix— in particular, one with more hydroelectric plants with reservoirs — will incur a smaller increase in cycling costs. At higher levels of PV penetration, it will be increasingly desirable to curtail solar production (and/or other zero-variable cost production) to avoid costly variation of thermal power plants’ outputs and, in the long run, to shift the fleet of thermal generators toward more flexible technologies. The coordination of solar energy production and storage, through thermal storage at CSP facilities or through other means, can also help reduce the need for thermal-plant cycling and thereby increase the value of solar generation.