Technology

The Isentropic Thermal Stores work by direct heat exchange between high pressure gas and particles of crushed rock or gravel. The design of a 'packed bed' store is normally a compromise between pressure drop and heat transfer. Reducing the particle size improves heat transfer efficiency but smaller particles offer more resistance to gas flow through the store. (The reverse is true for larger particles). The particle size of the storage media must therefore be carefully selected to mediate between these loss mechanisms.
 

Layered Store

Isentropic Ltd's most important contribution to 'packed bed' storage is the layered store. Gas flow can be confined to the region where heat exchange is active, termed the 'thermal front', while inactive areas, above and below, are bypassed. This allows the use of small particles for efficient heat exchange whilst maintaining a low pressure drop through the store. Active control of the thermal front allows for much higher energy density by ensuring that the front is kept short.


 

 

 

Simple to Produce

The packed bed is contained within an insulated pressure vessel, such as the ones shown in the image below. The patented thermal store design is unique to Isentropic Ltd. Current designs envisage peak pressures of 25 bar and peak temperatures of 550°C. The external pressure vessel can be manufactured by a wide range of suppliers.
 

Two Thermal Stores installed at our site in Fareham

 

Extensively Tested

Following an extensive development programme, Isentropic Ltd has built two thermal stores with 12 tons and 26 tons of storage media, respectively. A range of successful tests have been conducted which confirm the principles of operation and the efficiency of energy recovery of the system. Energy recovery can exceed 95%.

The video below, generated from test data, shows the movement of a real thermal front through an Isentropic Thermal Store.

The Isentropic GTI-Storage system is based on the process of extraction, storage and subsequent reinjection of hot compressed air into a gas turbine. Air extraction and air injection has been used in gas turbines in the past and this system utilises this process, but with the addition of the Isentropic Thermal Store to enhance flexibility and efficiency.
                                                                                          

A conventional gas turbine
 

In a conventional gas turbine air is drawn through the compressor, fuel is added to the mixture and the products of combustion all pass through the turbine. Power is used to turn the compressor but a greater amount is produced by the turbine. The difference is the net shaft power, which is used to drive an electrical generator. (Typically the turbine power is around twice that of the compressor).                                            

The Isentropic  GTI-Storage system includes the following components:                                    
   - CCGT (new or existing)
   - Thermal stores (in particular, the patented Isentropic  Thermal Store)
   - An additional compressor for pumping up to the 250 bar storage pressure
   - Pressurised steel pipe designed to store compressed air at high pressure

With GTI-Storage, air is injected into or bled from the gas turbine just before the combustor. This allows hot, pressurised air from the compressor to be stored and fed back into the gas turbine at a later time. By controlling the bleed or injection rate the output power of the turbine can be varied significantly.

 

Charge Mode                                                          Discharge Mode

By bleeding some air to the Air &                             Injection of stored air supplements the
Heat Storage unit, less air is passed                           flow to the combustor, increasing fuel 
through the combustor and so the                           burn and leading to an increase in the  
power output of the turbine is reduced.                   power output of the system.           


Note that when charging or discharging the reduction or increase in the power output is greater than the storage power. The gas turbine has an amplifying effect on the stored power.


 

The Company is developing a stand-alone energy storage system called Pumped Heat Energy Storage (PHES). The cycle uses a proprietary reciprocating heat pump/engine (developed in-house by Isentropic Ltd). It will take electricity from the grid to charge the system.

The reciprocating heat pump/engine is a positive displacement machine based on an entirely new concept. It has been designed from first principles to achieve high efficiency at low cost. The result is flexible power machinery that is optimised for use in electricity storage, that can be sited anywhere. It is designed to operate between 500ºC and -160ºC without lubrication. There are a number of potential applications for this machine.

The video below describes the charging and discharging of a PHES system.