13.3: 100% Renewable Grid

This section sets out two pathways toward a 100% renewable electricity grid, using the four new paradigms discussed in Section 13.1 and the fuel cell technologies discussed in Section 13.2. These pathways, illustrated in Figure 13.3.1, are being implemented today in many regions of the world, particularly Europe and California. The two pathways differ in the management of (1) load balancing, reliability, and dynamics associated with diurnal and seasonal variation, intermittency, and the limited capacity factors that accompany a high penetration of solar and wind power generation; and (2) the uncertainty in forecasting intermittent solar and wind resources. Both scenarios hold in common that energy storage is required, but they differ in (1) the amount and types of energy storage and (2) the need for a clean, firm, 24/7 power generator in addition to solar and wind power generation.

Figure 13.3.1 100% renewable grid.

Pathway 1

Pathway 1, depicted in Figure 13.3.1a, assumes that electric battery technology and pumped hydro storage alone will be sufficient to manage the diurnally varying and intermittent solar and wind resources. To this end, Li-ion batteries are being deployed at the transmission⓵ and distribution⓶ levels of the utility grid, at industry⓷, at hotels/hospitals/ universities⓸, and at homes⓹. The basic strategy is for the batteries and pumped hydro to absorb the excess electricity generated by solar and wind resources when loads on the grid are below the renewable generating capacity, and then to recover the energy as electricity from the electric batteries and hydro reservoirs when the utility loads exceed the renewable generating capacity (particularly at times solar and wind resources are not generating during their diurnal cycles).

Pathway 2

Pathway 2, depicted in Figure 13.3.1b, argues that electric battery and pumped hydro storage alone, while providing a cornerstone to storing energy available from otherwise curtailed wind and solar resources, are insufficient to provide a reliable electricity supply. To systematically and rigorously evaluate the requirements, energy systems analyses tools have been developed to explore the technologies required to enable and manage the solar and wind resources associated with a 100% renewable grid. Under the auspices of the California Energy Commission, for example, a systems analysis tool, the Holistic Grid Resource Integration and Deployment (HiGRID) code, was developed to guide planning for a modern electric grid. From evaluation of a myriad of scenarios to determine the resources needed to manage the intermittency, diurnal variation, and constrained capacity factor associated with solar and wind, two key resources emerged as being required: (1) a “hydrogen battery” resource and (2) a 24/7, clean, load-following renewable power-generating resource.

“Hydrogen battery” resource

Due to the massive amounts of energy that are projected to be (1) available from otherwise curtailed solar and wind resources, (2) required to support the grid when loads exceed the available wind and solar, and (3) required to overcome the limitations of electric batteries (degradation, cost, self-discharging, and inability to accommodate seasonal shifts in energy demand), systems analyses such as HiGRID are consistently demonstrating that hydrogen in general, and renewable hydrogen in particular, is required as a major cornerstone in achieving a 100% renewable grid. To this end, a number of sources of renewable hydrogen are emerging. Here are some examples:

• Electrolytic renewable hydrogen: The generation of renewable hydrogen through electrolysis (Figure 13.3.1b ⓵) is expected to be the largest source that can absorb the levels of projected curtailed energy, store the energy by injection into the natural gas or dedicated hydrogen pipeline (Figure 13.3.1b ⓶), and convey the energy to the points of use (Figure 13.3.1b ⓷).
• Tri-generation: A smaller-scale source is the generation of carbon-neutral hydrogen from a stationary fuel cell operating on biogas produced, for example, at waste water recovery facilities that process human sewage and food waste, landfills that store biodegrading human waste, and dairies that deal with large volumes of cow manure (Figure 13.3.1b ⓸). These facilities typically produce biogas rich in methane, which, if emitted, is significantly more climate change intensive than CO₂. Tri-generation captures and uses the biogas to produce carbon-neutral electricity and heat. By operating the fuel cell with more biogas than required for the electricity and heat alone, excess carbon-neutral bio-hydrogen is made available at the stack and can be extracted and injected into the natural gas or dedicated renewable hydrogen pipeline. At waste water recovery facilities and dairies, the heat can be used to support the digesters and thereby displace fossil fuel boilers, further reducing CO₂ emissions. Tri-generation is the epitome of sustainability, namely recovering and converting the energy from human and animal waste to renewable electricity, renewable heat, and renewable hydrogen.

24/7, clean, load-following renewable power-generating resource

Stationary fuel cell systems, of the designs discussed in Section 13.2, are emerging as a technology to generate the required clean, 24/7, load-following, renewable power with the added attribute of virtually zero emission of pollutants. Already meeting initial market demand for base load power generation, more than 30% of the fuel cells operating today in California are generating renewable power by operating on locally derived and directed biogas. To meet the challenge of the next-generation 100% renewable grid, stationary fuel cell systems are being deployed today with the requisite load-following attributes and also the ability to operate on hydrogen as well as natural gas and biogas. Simply stated, stationary fuel cell systems are:

• A resource, along with energy storage, to enable and manage a 100% renewable grid.
• A match for the utilization of the renewable hydrogen generated from otherwise curtailed wind and solar resources (Figure 13.3.1b⓹).