The most important issue facing wind and solar energy as replacements for fossil fuel power plants is intermittency. The sun doesn’t always shine and the wind doesn’t blow steadily everywhere, so storage of the energy produced is crucial to wide, grid scale implementation of these technologies.
Lithium, cobalt and vanadium-based battery technologies have been developed to store electricity, but a low-cost alternative storage system is now on the ground: heat.
Two developments in energy storage using heat are deployed in Europe right now and an MIT innovation in heat to electricity conversion promises to revolutionize energy storage at scale.
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The move to clean energy has traditionally focused on energy production, especially the switch from the combustion of fossil fuels to alternate sources like solar photovoltaic, wind, geothermal and nuclear. Some types, such as nuclear and geothermal, are steady-state, and can produce consistent power over extended periods of time. But the easiest to deploy sources such as solar and wind turbines, are highly intermittent. And neither source is effectively throttleable, making a practical energy accumulator essential if these are ever going to provide a substantial amount of reliable energy for power utilities.
A major solution to this problem has been grid scale electric battery storage, mainly using technology such as lithium iron phosphate, nickel manganese cobalt and vanadium flow batteries, but there is another form of energy: heat. But storing heat, although conceptually simple, is very difficult from an engineering perspective. Conduction and convection are serious loss pathways at below incandescent temperatures, so storing energy as heat is primarily a problem of thermal insulation.
Swedish alternate energy developer Vattenfall has announced a unique thermal storage battery in Berlin, Germany that uses an insulation medium seen in every lunchbox: vacuum. Essentially a giant Thermos bottle, the heat storage tank looks more like something out of an oil refinery than a battery, standing 45 metres tall, about 150 feet, and holding 56,000,000 litres, or 15,000,000 gallons of hot water. Space heat is a considerable energy need in Berlin, especially in winter, and the $52M system has a thermal capacity of 200 megawatts, approximately enough to supply 10 percent of Berlin’s hot water needs in the cold season.
According to the company, the water can stay hot for up to 13 hours, and can be warmed by excess power from solar and wind generation operations as well as from wastewater.
A similar concept is now operating in another Scandinavian country, Finland, also using a large storage tank. The storage medium however is sand, which system developer, Polar Night Energy has designed into an 8 megawatt hour capacity unit to provide district heating for Finnish utility Vatajankoski. Polar Night Energy’s systems are designed based on simulations using COMSOL Multiphysics software.
As engineers know, heat transfer efficiency is directly proportional to delta t, and sand can be heated to a much higher temperature than water, from 600 to 1000°C in the Polar systems. The company claims a storage cycle with timelines from hours to months.
Compared to water-based systems however, sand-based storage will require a secondary loop for district heating and most process heat applications. From an energy quality perspective, converting electricity into heat is the conversion of a very high-quality form of energy into a much lower quality form.
But imagine if there was a simple, solid-state device that could effectively convert the heat back into electricity. Engineers at MIT and the National Renewable Energy Laboratory have developed a thermophotovoltaic cell or TPV, similar in concept to the familiar light to electricity photovoltaic cell. The researchers anticipate that the technology could be used to convert heat stored in a thermal battery into electricity, but with one catch: the TPV cells convert very high energy photons, the kind that are emitted from sources at temperatures of 2000°C or higher. For a thermal battery, the team anticipates a storage medium that can tolerate these extreme temperatures, initially graphite.
The MIT/NREL team are talking about grid scale systems, which could allow solar energy to be banked, then transmitted as current from locations too distant to pipe a hot working fluid. Batteries are a hot technology right now, but hot water, sand and graphite may be the energy storage media of choice in the future.