Solid State compression project
H2 has a very low volumetric density. For many applications that use H2, the H2 needs to be compressed. Currently, H2 produced from electrolysis is typically around 10-20 bar. For applications such as use in vehicles, this needs to be compressed to 250-700 bar.
Why we do this
Most compression is currently achieved using mechanical compressors, which can be large and expensive. A recent US DOE report found around 65% of the cost of supplying H2 at refuelling stations was from the cost of compression. Novel compression technologies offer the potential to achieve this compression at lower cost.
How we do this
Metal hydrides are metal alloys or interstitial metal compounds that react reversibly with H2. Absorption of H2 into the metal lattice (changing from a to b phase) can occur at nearly constant pressure. This equilibrium pressure increases exponentially with temperature. Thus, low pressure H2 can be absorbed at a low temperature. Heating the hydride then releases the H2 at a higher pressure, achieving compression. This method of compression is particularly favourable where waste heat is available.
The absorption of H2 is exothermic. Thus, the metal alloys require cooling as they absorb H2. Later, heat needs to be supplied at a higher temperature to desorb H2 at the higher pressure. The further apart the cooling and heat reservoirs, the higher the compression ratio that can be achieved. For large compression ratios, more than one compression stage is often required, with tailored metal alloys selected for each stage.
Heat transfer into and out of the metal-hydride containing compressor is often the rate limiting step. Care needs to be taken with the design of the compressor to ensure effective and efficient heat transfer.
In collaboration with Griffith University, CSIRO is evaluating various metal hydrides for use in a 2-stage compressor. The aim is to find hydrides suitable for compressing H2 from 10 to 350 bar using residential waste heat.
For more information, contact Ashleigh Cousins