Global concern about the environmental effect of greenhouse gas emissions from the continued use of fossil fuels for power generation has led to an increased interest in clean, green and non-polluting sources of renewable energy, such as solar, hydropower, geothermal, biomass and wind. Integration of renewables into the grid does, however, pose a number of challenges such as intermittency of the resources, connection to grid interconnects from remote generation locations, and comparative cost vs. fossil fuel generation. High temperature superconductivity (HTS) solutions offer a number of advantages that are expected to address some of these concerns.
Renewables don’t answer all our energy needs, but they do safeguard our environment while generating a significant amount of useful energy. Renewables today account for some 25% of our energy usage worldwide and are expected to continue to gain more of a foothold, as concern about the environment and interest in renewable energy increases.
In fact, in order to address the steadily increasing demand for more clean and non-polluting power, a number of countries in Europe, Asia and South America plus Australia and 30 of the United States and the District of Columbia have established Renewable Portfolio Standards (RPS), regulations that require the increased production of energy from renewable energy sources such as solar, wind, hydropower, geothermal and biomass. New York, for example, has set a goal of generating 30% of its electricity from renewable energy sources by 2015.
Germany, setting perhaps the world’s most aggressive goal, is aiming for 100% renewables by 2050. The goal is the steady elimination of greenhouse gases that come from fossil fuel generation of electricity. It is felt that market implementation of renewables will result in competition, efficiency and innovation that will deliver renewable energy at the lowest possible cost, allowing it to compete with cheaper fossil fuel energy sources.
Although wind and solar are attractive sources of renewable energy because they do not produce greenhouse gases, they also pose difficulties because of their inherent variability. Wind is not a steady resource available everywhere, and the sun rises and sets and is frequently shaded by clouds.
Wind is a clean source of renewable energy that produces no air or water pollution. Today it represents the most mature and fastest growing source of renewable energy production. Currently wind accounts for ca. 1-2% of the total electricity produced worldwide, and this contribution continues to increase steadily. Germany has the most installed wind energy capacity, followed by Spain, the United States, India and Denmark, with fast growing development in France and China. According to the U.S. Department of Energy, offshore wind farms could provide enough energy to power our entire nation. To date we have barely touched the amazing capabilities of wind power and can expect to see it become a massive source of renewable energy in the U.S. and around the globe.
Wind energy is costly to set up, so it requires significant amounts of capital to establish wind farms. After the initial investment and start-up costs, however, wind is one of the cheapest forms of electricity generation to maintain. The extraordinary electrical efficiency and power density characteristics of HTS offer clear benefits for wind energy generation. HTS generators can be more powerful and much smaller than conventional devices. This is expected to contribute to the increase in offshore wind energy generation, particularly units of up to 10 MW.
Use of superconducting wire in the windings allows for very slow speed generators, and high currents without losses, and precludes the need for a gearbox, one of the turbine’s heaviest components, thereby enabling smaller turbines – one third the size and a quarter of the weight to generate as much power as larger units. Eliminating the gearbox also reduces the number of bearings and other major failure-prone components, thereby reducing wind turbine maintenance needs and operating costs. Incorporating zero-resistance, HTS wire will boost efficiency and lead to smaller, lighter turbines which are also easier to transport, install and maintain.
Today’s electricity grid has insufficient storage capability. Power must be generated when it is needed, making renewable energy an often unreliable source due to the unpredictability of sources for wind and solar power.
Superconducting Magnetic Energy Storage (SMES) is a solution for storage of electrical energy in a powerful magnetic field. SMES systems have been in development for about three decades. Past devices that used low temperature superconductors however, were designed to supply power only for short durations, generally less than a few minutes. The recent development of HTS wire that enables enhanced performance at high magnetic fields is expected to reduce the cost of storing energy in a SMES device and thereby extend the duration during which power is available.
In order to help manage electricity supply load variability, SMES technology for longer term (hours) storage with quick charge and discharge capability is being explored.
Storing electricity generated during periods of low demand, such as when the wind blows at night, allows that energy to be released for use during periods of high demand, such as when production plants are in full operation during the daytime. The energy stored in the magnetic field of the SMES coil is charged or discharged by increasing or decreasing the current in the coil. Scaling up the size of the coil, or adding additional coils to an array of coils, increases the storage capacity. SMES systems can be cycled indefinitely and provide instantaneous power, making them an attractive solution for load leveling in power plants.
With a life expectancy of 20+ years, SMES systems are expected to have a substantially longer life than batteries (1-10 years) and flywheels (8-12 years). Further, unlike pumped hydro and compressed air, other forms of renewable energy storage, SMES can be deployed almost anywhere.
Renewable energy sources such as wind farms and large solar arrays are most often located inland or offshore, remote from population centers where the power is used, thus requiring transmission of this green power to the distant users. Long distance transmission of power by conventional ac or dc transmission lines can be difficult to install because of right of way requirements and community objections over aesthetics or perceived safety. Placing conventional conductors underground has limited applicability due to resistive or reactive power losses in the cables.
Superconductor cables have 5-10 times the current carrying capacity of conventional copper cables of the same size. They have no dc losses and only small ac losses and power can, therefore, be transmitted at lower voltage and higher currents, offering the possibility of creating a fully green grid that connects the remote renewable generation sources with the consumers via these “green power superhighways.” A dc HTS transmission cable will have losses (including refrigeration) that are less than half that of conventional ac overhead transmission lines. Also, with distributed generation, and many more locations of generation with solar and wind farms, superconductors solve congestion problems with high reliability through the use of superconductor based Fault Current Limiters and current limiting cables.
A number of demonstrations of HTS cable technology have been completed and are ongoing in the U.S., including the AEP HTS cable in Bixby, Ohio; the National Grid HTS cable in Albany, NY; the LIPA HTS Cable in Holbrook, NY; and project “Hydra” in NYC. Other demonstrations have been completed or are underway in other areas of the world as well, including Japan, South Korea, China and Germany. Though successfully demonstrating the technology in each case, these cables have not yet been generally adopted to due the huge initial capital costs and the lack of a broad track record of performance. If HTS superconductor cables can live up to their promise of cutting grid transmission losses at acceptable expense, this will help the viability of wind and solar farms that must transmit their power over long distances to established distribution networks.
In order for the renewable energy industry to take full advantage of the earth’s resources, it is essential that superconductivity solutions such as wind turbine generators, SMES, current limiters and long distance transmission lines be fully developed, demonstrated and deployed into the grid. This will require further development of HTS wire capabilities, cryogenic systems, and power electronics. Much of this work is underway, but would be significantly accelerated with a period of additional government support for these activities and demonstration of the devices.