Long-Term Energy Storage
1. WHY DONN’T WE USE MORE SOLAR ENERGY?
Solar energy is an abundant and potentially valuable natural resource that is shared to some degree by every country on earth. However, if we exclude the solar-driven photosynthesis processes occurring in plants, the sun provides at most only a few percent of the primary energy our societies use. Why? It isn’t because of accessibility. On a yearly averaged basis, immense quantities of solar energy are deposited more or less uniformly across all populated regions of our planet. Also, it isn’t because of technological limitations. We know how to efficiently convert solar radiation into the types of energy that keep our modern societies functioning. And it certainly isn’t because of cost. In many instances, energy derived from solar radiation is cheaper than energy from conventional resources—and the cost of solar is falling every day. So what’s the problem?
The answer is this. Solar has experienced only limited acceptance in our energy-hungry societies because the solar resource is variable. This variability means that stand-alone solar is unreliable. Solar can be considered a viable primary energy resource only in conjunction with efficient storage technologies that are capable of bridging the hourly, daily, and seasonal variations of the solar resource. With effective seasonal storage, solar becomes a replacement for, rather than a just a part-time supplement to, existing reserves of fossil fuels. Without seasonal storage, solar will always be relegated—as it is today—to small, specialized energy markets, and it will, in nearly all instances, require complete duplication by equivalent fossil-fuel infrastructure.
2. FORMS OF ENERGY DERIVED FROM THE SOLAR RESOURCE
Solar energy conversion systems usually transform solar radiation into either electrical energy or low-grade thermal energy, or in the case of cogeneration systems, into both electricity and low-grade thermal energy. The term “low-grade thermal energy” is used here to refer to the thermal energy associated with physical masses of solid, liquid, or gaseous materials that have temperatures less than roughly 50 to 75°C.
Because of the variable nature of the solar resource, energy derived from sunlight must be stored or converted to other forms of energy which can be stored. The required storage involves seasonal time scales and massive quantities of energy. Dealing with low-grade thermal energy is not a problem as long as it can be used near the point where it’s generated. This is because it’s very difficult to transfer low-grade thermal energy over distances of even a few miles without incurring substantial losses. Electrical energy cannot be stored directly (super capacitors, superconducting inductors, etc.) in any commercially significant quantity. That’s why storage of electrical energy always involves first converting it to an intermediate energy form which can be easily stored. Then, at some later time, when electrical energy is needed, it can be regenerated (with some energy loss) from the intermediate energy form.
3. STORING LOW-GRADE THERMAL ENERGY—BOREHOLE SEASONAL THERMAL ENERGY STORAGE
Technologies for cost-effectively storing low-grade thermal energy have been successfully implemented at several sites in Europe and Canada. A good example is the Drake Landing Solar Community in Alberta, Canada, the details of which can be found at (DLSC). Drake Landing and other similar projects use flat-plate absorbers to convert unfocused sunlight into low-grade thermal energy. That energy can be easily and efficiently transferred to underground storage reservoirs (undisturbed layers of soil, gravel, and rock) through borehole heat exchangers. This proven technology is generally referred to as Borehole Seasonal Thermal Energy Storage (BSTES). The energy stored by using this process can be reclaimed as needed, even after months of storage, with measured Coefficient of Performance (COP) values reported to be above 30. This easily stored form of energy, which is usually discarded from power generating facilities as waste heat, could completely displace the large quantities of electricity and high-value fossil fuels that are now used for low-grade thermal applications in our homes, businesses, and industries.
4. STORING ELECTRICAL ENERGY—PUMPED HYDRO
Pumped-hydro storage involves using solar generated electricity, or electricity generated from other renewable energy resources, to drive pumps that move water from a location of lower gravitational potential energy to a location of higher gravitational potential energy. As the water falls back to the lower elevation, its gravitational potential energy is converted into kinetic energy which can drive an electric generator. The process is efficient, with round-trip efficiencies as high 85 to 90% previously reported, but water and land area requirements, as well as the need for relatively large elevation differences, prohibit use of this technology in most locations. In some areas, however, this technology can provides effective short-term (day-to-day) storage of electrical energy.
5. STORING ELECTRICAL ENERGY—BATTERIES
Electrical energy may be stored as chemical potential energy in batteries. At the present time, and into the foreseeable future, batteries are far too expensive to consider for long-term storage of electrical energy. For example, according to the most optimistic predictions, the cost of lithium ion batteries may drop to $100 per kilowatt-hour by 2025. The capital expense (battery cost) involved in storing enough electrical energy for a typical home for one month would be more than $125,000 and that cost would be more than $625,000 over the first 50-years of a home’s lifetime because the batteries would have to be replaced 5 times. Seasonal storage (summer to winter) would involve nearly $4,000,000 in battery costs over a 50 year time period, and remember, that’s for just one home. So batteries are prohibitively expensive for long-term storage of electricity. However, the fact that batteries have a high charge/discharge efficiency means that they can provide a convenient, although still rather expensive, storage option for day-to-day storage. In the near future, batteries will replace internal combustion engines in most transportation applications, but the electricity needed to recharge the batteries every day or two will still have to be provided from a long-term storage technology that is much less expensive than batteries.
6. STORING ELECTRICAL ENERGY—MOLTEN SALT
Solar energy can be stored as thermal energy at high-temperatures (350 to 700°C) in molten salt. Steam generated from the thermal energy stored in the salt can be used to drive a conventional steam-cycle electrical generating system. In effect, the molten salt stores electricity (actually delays its generation) for later use. Molten salt storage has been used at a few locations in the US and in other countries. It is still an active area of exploration for the US Department of Energy (DOE). In fact, the ability to provide temperatures compatible with molten salt storage has been a big part of the justification for continuing DOE development of Solar Power Tower (SPT) systems. However, there are several fundamental problems associated with SPT systems, and additional problems arise when long-term energy storage requirements are considered.
As an example, consider the Crescent Dunes SPT plant near Tonopah, Nevada, which has a nameplate capacity of 110 MW. It uses 32,000 metric tons of salt to store thermal energy which can provide up to 10 hours of full power operation without the sun. The cost of the salt is difficult to ascertain directly, but data in other Department of Energy (DOE) reports, such as NREL/SR-5200-58595, would imply that the cost was roughly 45 million dollars, or roughly $40 per kilowatt-hour—and that’s just for the salt. (The cost of specialized storage tanks and transfer plumbing add significantly to this cost.) $40 or $50 or $60 per kilowatt-hour is less than the projected cost for battery storage, but molten salt is still prohibitively expensive for season-to-season storage of electricity. And there are other problems with molten salt storage, such as the large volume occupied by the molten salt storage tanks, the complexity of the technology for moving and handling the salt, and the cost of an associated steam-cycle generating plant.
The table below shows the actual production of the Crescent Dunes Plant from its startup in November of 2015 until April of 2019 when the plant was mothballed because its Power Purchase Agreement (PPA) with the state of Nevada was cancelled due to lower than expected electricity production. The plant never met its expected production of 40,000 MW-hours per month and it was plagued with technical and operational problems. (The plant was shut down from November of 2016 until July of 2017 because of leakage problems in a salt storage tank.)
Crescent Dunes, kW-Hrs per Month, 40,000 kW-Hrs Design Goal
It is true, as SPT advocates claim, that molten salt storage could possibly be a less costly option for short term storage than batteries, but molten salt technology is far more complex and certainly less convenient than battery storage, and several additional costs must be considered in evaluating molten salt storage, not the least of which is the additional cost of the steam-cycle generating plant. Molten salt is not a realistic option for either short-term or long-term storage of electrical energy. During the last decade, performance improvements and cost reductions have made batteries a less expensive and more convenient option for short-term storage of electricity than molten salt. Add to this the fact that solar thermal power plants convert only roughly 15 percent of incident sunlight into electricity (single-crystal back-contact solar cells have efficiencies above 25 per cent) and it would seem that (1) battery technology has displaced molten salt as a technology for short-term storage of electrical energy, (molten salt was never considered a reasonable technology for long-term storage), and (2) cogeneration plants utilizing solar cells have displaced solar thermal plants as an electrical generation technology.
7. STORING ELECTRICAL ENERGY—FUELS
Solar-generated electricity can be stored as chemical potential energy in chemical compounds. The process involves electrolysis of a chemical compound to create a combustible fuel. The most common example of this process is the electrolysis of water to produce hydrogen. Hydrogen can be stored directly as a high pressure gas or used as a feed stock to produce other fuels, such as liquid ammonia, which has a higher energy density than gaseous hydrogen. In either case, whether produced directly or indirectly, the fuel will have a very high energy density and large quantities of energy can be stored in relatively small volumes. High energy density is a necessity for storing the large quantities of energy involved in seasonal energy storage of electricity generated from the solar resource.
Electrolysis is just the first process in an Electrolysis-Storage-Reconversion cycle (ESR Cycles). ESR cycles consists of three main steps: (1) electrolysis of a chemical compound to produce chemical potential energy in the form of a combustible fuel, (2) storage of the fuel for period of time until its energy is needed, and (3) burning the fuel in a heat engine to produce mechanical energy which can drive an electric generator. The greatest barrier to practical implementation of solar-driven ESR cycles on a global scale has always been the overall inefficiency of previously proposed cycles. But this picture has changed dramatically in the last few years because of the development of water electrolysis technology that is more than 80% efficient; the deployment of scalable, high-efficiency ammonia synthesis plants (Proton Ventures), and SEA’s work to provide a high-efficiency two-stroke internal combustion engine which could have efficiencies in the range of 60 per cent or more.
8. COMPARISON OF ELECTRICAL ENERGY STORAGE TECHNOLOGIES
In terms of converting electrical energy to an intermediate form of energy for long term storage, pumped storage hydro requires large volumes of water and suitable geography. These two requirements can only be met at certain specific locations, but the technology does provide an option for short-term energy storage in some areas. Battery storage, even when low projected costs are assumed, is far too expensive for storing the large quantities of energy required for seasonal storage. However, batteries are a convenient and energy-efficient option for many short-term storage situations, with automobile batteries being an important end-use application. Molten salt storage is too expensive for long-term storage and it has only limited utility for short-term storage because of the complexity and cost of the associated infrastructure and because of the inefficiency of the steam-cycle reconversion process. Conversion of electrical energy to the chemical energy of fuels is a practical and cost-effective technology, but it can only be considered as a viable long-term storage option if used in conjunction with a high-efficiency heat engine. SEA can now meet this need with its two-stroke internal combustion engine. The SEA engine makes the end-to-end efficiency of ESR cycles a cost-effective storage technology for electrical energy. Our engine also offers additional benefits in terms of its mechanical simplicity, its scalability, and its ability to integrate effectively with currently available water electrolysis units and ammonia storage technoogy.