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Directory:Isentropic Gravel Batteries for Renewable Grid Storage
Page first featured April 28, 2010
- " In thermodynamics, an isentropic process is one in which for purposes of engineering analysis and calculation, one may assume that the process takes place from initiation to completion without an increase or decrease in the entropy of the system, i.e., the entropy of the system remains constant." 
This directory page is about a British company called Isentropic.
Pumped Heat Electricity Storage (PHES)
Isentropic has designed a system that uses the Isentropic heat pump to store electricity in thermal form ("Pumped Heat"). The storage comprises two large containers of gravel, one hot (500C) and one cold (-150C). Electrical power is input to the machine, which compresses/expands air (argon gas) to (+500C) on the hot side and (-150C) on the cold side. The air is passed through the two piles of gravel where it gives up its heat/cold to the gravel. In order to regenerate the electricity, the cycle is simply reversed. The temperature difference is used to run the Isentropic machine as a heat engine. 
Isentropic's heat engine/heat pump has a 99% isentropic efficiency, compared to hydro turbines, which have an isentropic efficiency of ~90%. Their system is highly reversible, the machine works both as an engine that turns a temperature difference into work, and as a heat pump, which turns work into a temperature difference - every fridge has one. High reversibility means that if it first turns electricity into a temperature difference, it can then regenerate most of the electricity from the temperature difference it has created. Additionally, it uses no damaging refrigerants, chemicals or water; just gravel and air.
Isentropic's machine is a reciprocating device but is quite unlike existing machinery. The machine re-imagines the first Ericsson cycle of 1833, while implementing new aircraft techniques that massively reduced piston weight and cost. The also are using a completely new valving approach that virtually eliminated pressure losses while employing a new sealing concept that gives ultra-low friction and very long life.
The company claims that the energy in the hot silo can easily be stored for extended periods of time; calculating that a silo 50m tall and 50m in diameter would lose only half of its energy through its walls if left alone for three years.
The company points out that these features provide radical new possibilities in electricity storage, energy production, heating, cooling, compression and expansion. So grid energy storage is not the only application. As a high efficiency air-cycle heat pump for heating and cooling applications, it delivers significant improvements in efficiency and uses no environmentally damaging refrigerants. It is low cost and straightforward to manufacture.
Another application is in solar generation. It is a high efficiency external heat engine that allows for the efficient and low cost generation of solar thermal power via a concentrating system. It can work with conventional solar collectors to deliver efficiencies as good as large steam turbines but in sizes down to 100kW. This will enable efficient, low cost and small scale CSP close to demand (rooftops) for the first time. It also does not suffer significant performance losses as the temperature drops.
Benefits of PHES, according to http://www.isentropic.co.uk/index.php?page=storage
- Low Cost
- High Efficiency
- Not geographically constrained
- Safe and environmentally inert
- Modular and scalable
- Rapid response to load variation
Quoting from http://www.isentropic.co.uk/index.php?page=about-us
Isentropic is a developer of a groundbreaking engine/heat pump of unprecedented efficiency and low cost. A private technology company based outside Cambridge, UK, Isentropic has spent five years developing the technologies that underlie its high efficiency, low cost devices. It has now demonstrated a reversible engine/heat pump with a record breaking 99% isentropic efficiency.
In the News
- Giant gravel batteries could make renewable energy more reliable – (Guardian; UK; Apr. 26, 2010)
- Top 50 VC-Funded Greentech Startups - Isentropic is named (Greentech Media; Mar 15, 2010 )
On Apr. 26, 2010, Kupfernigk wrote:
If the argon has cooled to ambient by the time it exits the hot chamber, the temperature of the gravel at the top is no more than ambient (see the Laws of Thermodynamics.) There will be a temperature gradient through both silos, so during the energy release cycle the argon will be passing from the bottom of the hot silo to the bottom of the cold one, to get the maximum output in accordance with Carnot's equation. As the extremes of temperature drop, the efficiency will also drop, which suggests to me that to maintain efficiency the silos will need to be bigger than is being suggested. Otherwise you will get a vicious circle - the longer the wind doesn't blow, the faster the silos will lose or gain heat, and the longer it will take to regenerate when the wind starts again.
The other issue is what to use for the gravel. You need something in one side that will withstand cycling from -160 to probably around 250C, and on the other side something that can cycle from around 0C to 550C. I doubt that this will come straight out of the quarry.
So how long will this take to develop and productionise? I'm going to stick my neck out and guess 20 years.
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