RE: Vanadium prices1 Jun 2020 08:29
good to see - those who may have been around over the weekend will have seen that a number of us have been forced into vigorously defending our investment in Invinity from a whole load of spurious tripe from some rather obvious trolls.
This process usefully encouraged me to revisit the efficiency numbers being put out for the Cryogenic Air Energy Storage people Highview and I have to say it is pretty misleading stuff, I quote what I wrote:
The thing is I actually know something about cryogenics having a PhD in the subject plus having received awards as founder of a cryogenics company - I have good reason to believe that they cannot get greater than 60% round trip efficiency on a daily cycle.
You see there is an awful lot of irreversibility in vortices in the turbines and this leads to an inevitable loss of energy when you try going around a real thermodynamic heat pump cycle. That's before you start factoring in the inevitable entropic costs of trying to get heat into and out of materials that have high specific heats but low thermal conductivities. Quite simply you cannot do it at temperatures greater than about 10Kelvin because the phononic specific heats typically rise with the cube of temperature (see for example Pobell: Matter and Methods at Low Temperature graph 3.7 or Jack Ekin's Experimental Techniques for Low-Temperature Measurements Tables A6.1, A6.2 and A6.3 )
Thermal losses into the cryogenic vessels can be reduced to c. 1% per day due to the fact that the cold vessel effectively cryopumps the dewar vacuum but thermal losses in the high temperature store are not so easily reduced because there is no cryopumping (unless you try and get the cryogenic vessel to also cryopump that vessel, but this then introduces an additional heat load on the cryogenic vessel) plus there is outgassing of the metals at high temperature which undermines the ability of multilayer foil insulation to reduce the T^4 radiative losses.
Furthermore Highview readily admit that they can only get over 50% efficiency if they can use 'waste heat' at 115 degrees C - that is more than boiling water. This is a replacement for the high temperature thermal store that I have picked apart above. This is a huge amount of energy that is being injected into the system but which is not being counted in the 'round-trip efficiency' calculations. So-called 'waste' heat at that temperature is nothing of the sort it is the sort of thing that you could heat hundreds of house with - to compare batteries that are not being provided with free central heating, with this CAES system that effectively is, is highly misleading and needs to be called out.
So, yeah I am going to call their efficiency figures out as wrong.
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