In Hamburg-Altenwerder, the Siemens subsidiary Siemens Gamesa has put into operation a new energy storage system that uses simple technology to store electricity as heat and later regain electricity via a steam turbine. The facility, which was built on the site of an aluminum smelter, uses hot air to heat a storage facility with 1,000 tons of volcanic rock to around 750 degrees Celsius. This is an unspecified volcanic rock.
The system has a heat capacity of 130 megawatt hours, comparable to about 10 tons of heating oil. To recuperate the energy as electricity, hot air is used to heat up cold air and produce water vapor at 600 degrees Celsius, which then generates electricity via conventional power plant turbines. This amount of heat should be stored for about a week.
The pilot plant in Hamburg has a 1.5 MW steam turbine that can be operated with a full tank for around 24 hours. After that, the capacity of 120 megawatt hours is by no means exhausted, but the temperature of the extracted hot air drops below 600 degrees Celsius. Thus, the turbine can not be operated with its full efficiency of about 45 percent.
The store can also use combined heat and power
The operators assume that 30 megawatt hours of electricity can be generated with steam at 600 degrees Celsius. This retrieves 67 of 120 megawatt hours. Subsequently, however, the storage could be brought back to full temperature with 67 megawatt hours of electricity. The rest of the energy is not lost. For heating, hot exhaust air from the aluminum smelter could also be used.
The remaining 53 megawatt hours could also be accessed at lower temperatures and used differently. In addition, the plant could be operated in combined heat and power. The otherwise unused waste heat of power generation would be provided as hot water. Depending on the temperature of the water but that would be associated with certain losses in the production of electricity.
Much larger facilities are planned
The concept was first tested in 2014 with a first prototype and 40 tons of rock. It had a capacity of 5 megawatt hours and powered a 700 kW turbine. The Hamburg plant with 1,000 tons is the demonstration plant. According to the company's plans, a pilot plant with 10,000 tons of rock and a capacity of more than one gigawatt hour is to follow in 2020, before the technology is to be commercially sold from 2022 onwards. The turbine of the pilot plant should be able to generate 30 megawatts of electricity.
The construction of the largest possible systems has great physical advantages in the technology. The most important advantage is the minimization of heat loss. The storage rock is stored in a concrete building and surrounded by thermal insulation. The heat is lost but only on the outer surface. A building that is twice as big in all directions has four times the outer surface but eight times the volume. Compared to the content, the losses would be halved.
Hydrogen has much greater losses
How large the bearing losses are exactly, Siemens did not announce. Energy Nest, which builds similar concrete storage tanks, claims the losses of larger plants at less than 2 percent per day. At exactly 2 percent loss per day, half of the initially stored energy would be lost after 35 days. At Energy Nest, the losses will decrease according to the size of the plants. Energy Nest's largest planned plants are similar to Siemens' demonstration plant in Hamburg.
In practice, the technology is currently intended primarily to compensate for short-term peaks in power generation and consumption. But Siemens wants to build storage facilities that even go far beyond the planned 2020 pilot plant with 30 MW and 10,000 tons of rock. The storage technology could also operate the turbines of coal power plants, where powers of several 100 MW are common.
Even in such large plants, even storage times of weeks or a few months with low losses could be realistic. However, other factors could make the enlargement more difficult. Thus, the air resistance while blowing the memory should not be too large. In addition, the repeated thermal expansion of the rock could cause it to collapse over the long term, reducing the gaps in it. At the moment the effect has probably not been observed yet.
Technology does not need scarce resources
In terms of cost and resources, the simple storage medium and conventional power generation make even large plants seem at least realistic, unlike molten salt storage tanks or Energy Nest's embedded concrete pipes. In stationary storage of electricity, over days and weeks, this technology should already be clearly superior to hydrogen storage.
In current hydrogen storage systems, around 40 percent of the energy is already lost in the electrolysis, as 2017 spokesman of ITM Power Golem.de said at the Hannover Messe. When stored in the accumulator, another 10 to 15 percent of the energy is lost. In methanation and similar processes for hydrogen storage in oil, these losses are about 25 percent. When generating electricity, the fuel cell then loses around 40 to 50 percent of the energy, which makes the overall system less than 30 percent efficient.
A Cheops pyramid as memory is not enough
Of course, the thermal storage is significantly less efficient than a pumped storage power plant with an efficiency of around 80 percent. However, there is needed for the storage of a kilowatt hour of electricity over a ton of water, with a difference in altitude of 360 m. In the thermal storage of the demonstration plant suffice for 30 kg of rock.
For comparison, a store of the volume of the Pyramid of Cheops would consist of 3.1 million tons of rock and would thus have a storage capacity of about 105 gigawatt hours. Enough to run a 1 Gigawatt power plant over 4 days. Germany-wide, about 1,600 gigawatt hours of electricity are needed per day. In order to save this amount, 47 million tons of rock would be needed. Coal production in Germany in 2017 was around 175 million tonnes.
