HOT SAND BATTERIES

“large silo at night using thermal imaging camera”

LONG DURATION ENERGY STORAGE
https://cleantechnica.com/hot-sand-to-store-energy
https://rechargenews.com/long-duration-system-energy-storage
https://energy-storage.news/worlds-first-sand-battery-goes-online
https://bbc.com/news/science-environment-61996520
World’s first sand battery stores power for months at a time
by Matt McGrath / 5 July 2022

“Finnish researchers have installed the world’s first fully working “sand battery” which can store green power for months at a time. The developers say this could solve the problem of year-round supply, a major issue for green energy. Using low-grade sand, the device is charged up with heat made from cheap electricity from solar or wind. The sand stores the heat at around 500C, which can then warm homes in winter when energy is more expensive. Finland gets most of its gas from Russia, so the war in Ukraine has drawn the issue of green power into sharp focus.

It has the longest Russian border in the EU and Moscow has now halted gas and electricity supplies in the wake of Finland’s decision to join NATO. Concerns over sources of heat and light, especially with the long, cold Finnish winter on the horizon are preoccupying politicians and citizens alike. But in a corner of a small power plant in western Finland stands a new piece of technology that has the potential to ease some of these worries.

The key element in this device? Around 100 tonnes of builder’s sand, piled high inside a dull grey silo. These rough and ready grains may well represent a simple, cost-effective way of storing power for when it’s needed most. Because of climate change and now thanks to the rapidly rising price of fossil fuels, there’s a surge of investment in new renewable energy production. But while new solar panels and wind turbines can be quickly added to national grids, these extra sources also present huge challenges.

The toughest question is about intermittency – how do you keep the lights on when the sun doesn’t shine and the wind doesn’t blow? Adding more renewables to the electricity grid also means you need to boost other energy sources to balance the network, as too much or too little power can cause it to collapse. The most obvious answer to these problems is large scale batteries which can store and balance energy demands as the grid becomes greener.

Right now, most batteries are made with lithium and are expensive with a large, physical footprint, and can only cope with a limited amount of excess power. But in the town of Kankaanpää, a team of young Finnish engineers have completed the first commercial installation of a battery made from sand that they believe can solve the storage problem in a low-cost, low impact way.

“Whenever there’s like this high surge of available green electricity, we want to be able to get it into the storage really quickly,” said Markku Ylönen, one of the two founders of Polar Night Energy who have developed the product. The device has been installed in the Vatajankoski power plant which runs the district heating system for the area. Low-cost electricity warms the sand up to 500C by resistive heating (the same process that makes electric fires work). This generates hot air which is circulated in the sand by means of a heat exchanger.

Sand is a very effective medium for storing heat and loses little over time. The developers say that their device could keep sand at 500C for several months. So when energy prices are higher, the battery discharges the hot air which warms water for the district heating system which is then pumped around homes, offices and even the local swimming pool.

The idea for the sand battery was first developed at a former pulp mill in the city of Tampere, with the council donating the work space and providing funding to get it off the ground. “If we have some power stations that are just working for a few hours in the wintertime, when it’s the coldest, it’s going to be extremely expensive,” said Elina Seppänen, an energy and climate specialist for the city. “But if we have this sort of solution that provides flexibility for the use, and storage of heat, that would help a lot in terms of expense, I think.”

One of the big challenges now is whether the technology can be scaled up to really make a difference – and will the developers be able to use it to get electricity out as well as heat? The efficiency falls dramatically when the sand is used to just return power to the electricity grid.

But storing green energy as heat for the longer term is also a huge opportunity for industry, where most of the process heat that’s used in food and drink, textiles or pharmaceuticals comes from the burning of fossil fuels. Other research groups, such as the US National Renewable Energy Laboratory are actively looking at sand as a viable form of battery for green power.

But the Finns are the first with a working, commercial system, that so far is performing well, according to the man who’s invested in the system. “It’s really simple, but we liked the idea of trying something new, to be the first in the world to do something like this,” said Pekka Passi, the managing director of the Vatajankoski power plant. “It’s a bit crazy, if you wish, but I think it’s going to be a success.”

DISTRICT HEATING
https://newscientist.com/world-without-sand
https://lemonde.fr/sand-is-part-of-our-daily-life
https://nrel.gov/modular-particle-thermal-energy-storage-technology
https://wiom3.org/rhot-sand-battery-charges-up
First commercial sand-based heat storage system is operating in Finland
by Alex Brinded / 7 September 2022

“Designed and built by Polar Night Energy, Finland, the battery uses 100t of sand to store and supply low-emission district heating in a town called Kankaanpää. It has 100kW heating power and 8MWh of capacity, and heats residential and commercial buildings, including a municipal swimming pool.

Energy utility firm Vatajankoski uses the storage to prime waste heat from their data servers. The 60°C waste heat is raised to 75-100°C before being fed into the district heating network. Beyond this application, the sand battery concept works as a high-power and high-capacity reservoir converting excess solar and wind energy to heat, where 70% is stored at 300-500°C and can be used to heat homes or provide process heat to industry.

As sand heat storage can store several times the amount of energy as a similar sized water tank, it is considered a more efficient method of heat transfer. The sand in this instance is stored in a 7x4m insulated steel silo with heat transfer pipes and charged when clean and cheap electricity is available – with electrical energy transferred to heat storage using a closed loop air-pipe arrangement. Air is heated using electrical resistors and circulated in the heat transfer piping.

Heat is extracted by blowing cool air through the transfer pipes, with the air heating up as it passes through and converts water to process steam or district heating in an air-to-water heat exchanger. Chief Technology Officer of Polar Night Energy, Markku Ylönen, says, ‘Our storage temperature is so high that we have no issues in providing heat for homes. We can provide 200°C steam for industries with the same principle, so temperature levels at residential systems are low in our perspective.

‘We use the same closed air loop that we use for charging. There’s an air-water heat exchanger connected to the system. We can also generate steam, or hot air to processes, if the heat exchanger is of that type. ‘We have resistive elements outside the sand that heat air to high temperature. This air passes to the storage and heats the sand. The energy comes from the electric grid, we charge it when clean and cheap electricity is available.’

‘We don’t keep it constantly hot – the sand goes through charge and discharge cycles constantly, similar to any energy storing device.’ Ylonen continues, ‘[At Kankaanpää,] we sized the storage according to the heating power needs to boost the waste heat from the data servers on the site…We also discussed with the metal workshop to find the cost-optimal size in regard to construction and on-road transportation of the storage.’

“Design concept for molten silicon thermal energy storage
in Australia which could store heat at above 1,000C”

Polar Night Energy says that the maximum temperature is about 600°C, but in practice this is limited by the heat resistance of the materials, the construction and control of the storage. Although Ylönen says, ‘Between the outer steel layer and the inner one we have an insulation all around the storage, so the conductive losses due to the steel structure is marginal.’ As the heat storage is not very sensitive to grain size, Polar Night Energy ideally uses high-density, low-cost sand that is unsuitable for the construction industry.

Closed-Brayton Combined Power Cycle
“In a new particle thermal energy storage system, silica particles are gravity-fed through electric resistive heating elements. The heated particles are stored in insulated concrete silos. When energy is needed, the heated particles are fed through a heat exchanger to create electricity for the grid. The system discharges during periods of high electricity demand and recharges when electricity is cheaper.”

They say that the sand can stay hot for months in winter if needed, but the sand heat storage in Kankaanpää is charged in approximately two-week cycles.They say that the sand can stay hot for months in winter if needed, but the sand heat storage in Kankaanpää is charged in approximately two-week cycles. They have found that the best range is with 20-200 times a year of charging depending on application. Ylönen says, ‘The more energy we put through the storage per year, the better the financial numbers look like. And, wind is more available in the winter than summer, so the low-cost and clean moments in electricity production are still there.’

“Particle thermal energy storage systems can be constructed
with existing infrastructure from retired coal and gas power plants”

The silo in Kankaanpää was built after a 3MWh pilot plant was connected to the district heating network of the City of Tampere in Finland and heated a couple of buildings. Polar Night Energy has plans for a larger sand heat-storage system to be built from 2023. Ylönen says, ‘We have ready plans for a storage with 10,000t of sand-like material. This is about 100 times larger in volume than the Vatajankoski storage. That size is aimed to be our standard-size for some years, as it is a good fit for many district heating networks and for industrial sites.’

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