Articles
Optimal H2 production from photovoltaic with batteries
Special Focus: Electrolyzer Technologies
J. WILSON and G. HUTCHISON, PSC Consulting, England, United Kingdom
The European Commission (EC) has approved several measures to support the development of hydrogen (H2) infrastructure, further affirming H2's use as an important fuel to decarbonize the region's various industrial sectors. With a mix of public and private investments, this latest raft of measures is expected to provide around €12 B of investment into green H2 projects. In contrast to the bulk of the H2 available in today’s markets, which is derived from fossil fuels (known as blue or gray H2), green H2 is produced via water electrolysis using renewable electricity. Such H2 offers numerous well-known benefits as a clean, drop-in alternative to fossil fuels that can also be deployed in fuel cells to produce heat and power. Furthermore, there is an increased interest in methods to convert renewable energy from resources like wind and solar into storable and transportable forms of fuel, and H2 is perceived as an attractive solution. Indeed, among the plans set out in the EC’s IPCEI Hy2Infra project is the deployment of 3.2 gigawatts (GW) of large-scale electrolyzers to produce green H2.
While electrolysis is relatively straightforward in principle, the performance of electrolysis technologies requires a large amount of energy and relatively pure water. Fortunately, one of the apparent nuances of renewable energy, in particular photovoltaic (PV), can be a substantial benefit regarding H2 production because PV can generate excess energy when there is no or low demand from the grid. Rather than curtailing this power and effectively wasting this energy, it can instead be used to generate H2.
In fact, H2 is a solution to the variability challenge that renewable energy presents—e.g., on the weekend when demand from business, commerce and industry is at a minimum.
Rather than curtail supply whenever power demand falls below that supplied from the renewable capacity, it becomes more practical to use any excess capacity to desalinate seawater (where potable water is not available) and use it to produce H2. Similarly, it may also be the case that a particular PV plant is ideally located in terms of annual energy production but has limited opportunities to export this power to consumers. Plants with attractive resource profiles could even be dedicated to H2 and water production.
Once produced, this H2 can be compressed and stored for export and subsequent use in transportation, industry or any other application. However, a new analysis from the author's company indicates that H2 storage is a more costly approach relative to electrical storage in batteries and thus a more cost-effective investment could be found in electricity storage capacity.
Modeling a PV and battery hybrid for optimal H2 production. The author’s company has developed a linear optimization model, setting out a typical green H2 production scenario using a proton exchange membrane (PEM) electrolyzer coupled to a solar PV installation, with a reverse osmosis desalination plant for water supply. The model uses actual solar data at an existing project to determine how much H2 could be produced in a day. However, given a scenario in which a certain quantity of H2 is required per day or over a specific period, the renewable energy intermittency may mean that H2 targets may not be met.
Keeping H2 on tap is expensive, given the requirement for compression and high pressure and/or cryogenic storage. Assessing this premise, the model also considered a comparison between indirect energy storage through H2 against a more direct form of energy storage using an onsite lithium battery. Taking this one step further, the model considered a typical scenario in which a freshwater supply is also constrained, requiring the use of desalination to produce water of the necessary quality.
The energy optimization model explored the effects of adjusting the availability and cost of a battery energy storage system (BESS) in this process, where electrical energy from PV is used to desalinate seawater and the PV system is subsequently deployed to turn the purified water into H2. A previous analysis of the benefits of BESSs regarding H2 production from a PV plant found that the inclusion of a BESS could increase system profitability by 10%, considering total costs and production. However, while this shows that there is a definite benefit to including a BESS in such a system, earlier research has not considered optimized sizing for the complete system.
The linear optimization model developed by the author’s company was established with an objective function to minimize the cost to produce H2, subject to several constraints. The total cost was calculated as a sum of the capital cost of each piece of equipment in the system as these are the dominant costs. This cost was then minimized considering the efficiencies of each piece of technology and the fact that the daily H2 production goal must be met. In addition, the PV irradiation and generation profile was chosen to correspond to a random point in the interior of Singapore (but could be based on any PV or other renewable generation profile).
Based on a single-axis tracking of a 1-megawatt (MW) solar system, an hourly generation profile for 1 yr was computed using the system advisor model (SAM). The battery system was included to support H2 production during extended periods when sufficient sunlight was not available in the context of achieving a minimum daily H2 production target. All capital costs were based on established industry figures; however, the battery system’s capital cost per kWhr of storage was varied to determine the effect on H2 production. These different scenarios based on varying the cost of the battery system were then solved using an open-source solver.
The benefits of batteries explained. Having completed the analysis, the results are emphatic. If a specific volume of H2 is required over a fixed period, the addition of a battery system significantly enhances the plant’s operational characteristics, and thus, its economic performance. Indeed, because a battery system enables H2 production to continue for longer periods, a smaller electrolyzer can be deployed to meet the daily H2 requirements that may otherwise be required with a PV system alone. The lower electrolyzer investment effectively offsets the additional cost of a battery system, and thus, the further capital investment needed does not drastically impact the overall capital cost of the system. Furthermore, the addition of a battery allows sustained H2 production when PV output is reduced, even in areas typically associated with high solar irradiation, such as deserts.
Some of the reasons for this outcome are the relative efficiencies of a chemical battery when compared with the conversion of electricity into H2, its storage and subsequent use. Using current generation technologies, good quality batteries may have a round-trip efficiency of 90% or 95%. This compares favorably with the H2 cycle, which may have a round-trip efficiency as low as 30%, especially if the H2 is used in a gas turbine to produce electricity. The additional costs of H2 storage equipment tip this balance further in favor of the BESS approach.
According to this analysis, given the requirement that a daily H2 target is met, the addition of a BESS is an economic and operational necessity. Even when considering extreme variations in the cost of a battery system, the model still optimizes H2 with a contribution from a battery storage system when the BESS has a very high capital cost. Although electrolyzer ramp rates and constraints were not considered in this model, there is clear evidence that, depending on the type of electrolyzer used, the addition of a battery may also have an impact on the ability of the system to handle varying PV output. Desalination is another consideration in this model and is generally known as an energy-intensive process. However, this model shows that the cost and energy use of the desalination component is a small percentage compared to the rest of the H2 production system.
With the energy density that H2 offers, there is little doubt that H2 produced from low-carbon energy will form a major plank of the future energy system. It will likely be widely deployed in transportation to fuel ships, aircraft and other vehicles. These will likely have daily H2 requirements as shown in this model, as a specific number of ships and/or aircrafts may need to be refueled each day. Many nations worldwide are already looking to develop their H2 infrastructure, and the latest developments in the EC are just one more example of this global megatrend.
However, given that H2 storage is one of the most expensive components of a H2 energy system, any opportunity to deliver alternatives should be considered. As this analysis shows, the addition of a battery storage solution enables H2 production targets to be met more reliably with smaller electrolyzers and PV installations. It all boils down to a lower cost of production from a more efficient system to pave the way for the fuel of the future. H2T
About the authors
JAMES WILSON is an electrical engineer and specializes in power systems. He has worked on renewable power plant analysis and protection schemes and has been performing larger scale analysis for interconnecting power systems throughout Africa. Wilson’s skills also include high voltage systems and software development and control. He earned a BS degree in electrical engineering from the University of the Witwatersrand (Wits) and an MS degree in space studies from the University of Cape Town (UCT), South Africa. Wilson is registered as a candidate engineer with the Engineering Council of South Africa (ECSA).
GRAEME HUTCHISON is a chartered electrical engineer and technical director within the strategic advisory team for PSC in Europe. He has been involved in numerous projects relating to power system planning, investment appraisal, privatization and regulatory studies, system operations and security and rural electrification. Hutchison is an experienced project manager and lead having delivered large long-term assignments in Europe and the Middle East. He was the project lead in a series of long-term studies for ENTSO-E and the Nordic TSOs, investigating the impact of high renewable penetration and low inertia to the emergency operating regimes of the network. Hutchison has significant experience in conventional and renewable generation integration and has supported investors in the purchase of varying asset bases in Europe, the Middle East and West Africa.
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