In my last post, I introduced the hydrogen energy storage system architecture being developed by Isotherm Energy. But why choose a hydrogen-based approach? What are the compelling reasons to consider such a system? Let's start with some of the key advantages:
Energy Density and System Life - The amount of energy per unit mass for hydrogen storage technology is much higher than batteries or ultracapacitors. It also has long storage times and equipment life, resulting in a high energy density system with low lifecycle replacement and disposal requirements.
Ratio of Energy Stored to Energy Invested - A net energy analysis of grid storage options conducted by authors from Stanford University, Imperial College of London, and Western Washington University* found that a regenerative hydrogen fuel cell (RHFC) configuration provides higher lifetime energy returned, relative to the energy inputs required to build it, than the best battery technology available (lithium-ion). The analysis study also reported that the reference RHFC could provide the same overall energy benefit as batteries for over-generation from wind farms, even at a round trip efficiency of 30% for the RHFC.
Carbon-Free Energy Carrier - In addition to storing energy for reuse in electrical systems, hydrogen is an energy carrier that produces no carbon emissions when used as a fuel. It can be transported and used for a variety of purposes that are currently the domain of fossil fuels. This permits a multitude of options for stored hydrogen beyond the energy storage needs of a system. Hybridized hydrogen systems of this type can be optimized to meet combined requirements of energy storage and fuel production.
Water Production - A potentially valuable byproduct of a hydrogen energy storage system is the production of water when it is used in a fuel cell or combustion process. With appropriate material selection and design, potable water can be extracted in significant quantities from the system. Furthermore, use of a variety of water sources (e.g. saltwater, wastewater), biomass, and other hydrogen feedstocks can produce a water processing function that operates in parallel with energy storage.
However, there are some inherent considerations to take into account for a hydrogen energy storage system:
Roundtrip efficiency - The primary drawback of hydrogen energy storage systems is the relatively low roundtrip efficiency of the "charge-discharge" cycle. For example, an electrolyzer can split water to produce hydrogen with an efficiency in the range of 70%. When the hydrogen is used to produce electrical energy in a fuel cell or by heat of combustion, additional energy is lost in the conversion process. For a fuel cell operating at 50% efficiency, the resulting roundtrip efficiency is roughly 35%.
Safe handling - Despite some misperceptions, hydrogen systems are routinely used in a variety of industries(e.g. aerospace, processing, and manufacturing). These systems require certain precautions, material selection, design features and operational practices to insure safe operation. Hydrogen has a wide flammability range in the presence of oxygen, and a relatively low ignition energy. However, it's lighter than air and dissipates much more readily than gasoline and other fuels in an open environment. It also has no lasting environmental impact once dissipated. When properly designed and operated, a hydrogen system poses no more risk than many commonly used fuel and gas systems.
Looking forward, Isotherm Energy is focused on several trends and opportunities that will drive the wider adoption of hydrogen energy storage systems:
1. Technology advances in electrolyzer and fuel cell performance that continue to improve the round trip efficiency.
2. Hybrid systems that produce potable water and/or hydrogen fuel thereby increasing the overall performance by adding functionality that would normally require separate systems and processes.
3. Recovery of waste heat produced by system inefficiences to increase the overall performance.
4. Creation of system analysis and modeling tools that allow optimization of hydrogen energy storage systems tailored to specific application requirements.
*Pellow, M.A., et al., "Hydrogen or Batteries for Grid Storage?", Energy Environ. Sci., 2015