Breakthrough in Energy Storage: Isentropic Energy

Where does Carnot enter all of this? Isn’t this device a heat engine, and subject to Carnot efficiencies?  So, plugging in the numbers you get….. 72% to 85%.  Cool…..

Great idea. What’s the cost of an average house installation, in which all energy needs are met for a few months with only the PHES sourcing the energy? What’s the payback period? Is this a breakthrough for off-the-grid homeowner wannabes, or simply for the future ‘smartgrid’, in which corporate America still holds the reins on our ability to survive? Cheaply meeting peak demands is a great improvement, but a vastly more distributed array of independent energy sources will provide much higher economic security, and, with it, national security. Solar with long term storage is cheap, totally clean, available on every acre of Planet Earth, and can never be significantly disrupted by the next evil group of malcontents.

Eric, nice find.  This is a v e r y intriguing, scalable concept.

processinventor,
their website states a key innovation is the record setting 99% isentropic efficiency.

http://www.isentropic.co.uk/index.php?page=technology.

I going to try again….

If you take as an example, a perfect heat pump with a COP of say 4, that you want to supply heat at a temperature X. If you input one unit of electricity to the system then you end up with 4 units of heat at temperature X. If this was a perfectly reversible process then, when reversed, the heat engine cycle would have an efficiency of 25% ie it would take your 4 units of heat at this temperature X and generate 1 unit of energy. The COP is the inverse of the engine efficiency.

In this example I have just given I could have a perfect heat engine with a Carnot efficiency of 25% (limited by temperature range) that as part of a storage cycle has a 100% charge/discharge efficiency. In the perfect machine it would not matter if the Carnot efficiency was 10% or 50% as the heat pump will simply be the inverse of this ie 10 or 2.

I am hoping that this will have shown why the Carnot efficiency does not effect the charge/discharge efficiency of our storage system. The things that do effect it are the irreversible losses, such as, friction, electrical losses, pressure drops, heat transfer etc.. that occur during the cycle.

I don’t understand why the system wouldn’t work with external heat, say from solar thermal.  If the energy is stored in the form of a difference in temperature x mass, which operates the heat engine, why does it care how the temperature got there?  Maybe the balance point would shift both sides to a higher temperature, but wouldn’t it still work?  (I do see that there are materials limits, though—500C is already really hot.)

If you are interested in what Isentropic is doing, check out http://www.cleanandcoolmission.com. 19 of the leading UK clean technology companies are currently on a trade mission to California. Meeting peers, partners and investors. and attending the CleanTech Forum.

I love this Carnot obsession. The Carnot ratio is an availability ratio, not really an efficiency, ie, heat pump COP of an ideal engine is the inverse of its engine efficiency.
What is important here is that the engine cycle is reversible, not that it is a Carnot-equivalent cycle. A Brayton cycle, in its ideal form, is reversible (isentropic compression and expansion coupled with isobaric heat transfer) but not Carnot equivalent. This cycle in its purest form is actually the first Ericsson cycle and Ericsson did it well before Brayton or Joule.
The reason that we can show high reversibility is that isobaric heat transfer in the constant pressure stores can take place slowly under drifting flow conditions. Adiabatic compression and expansion takes place rapidly in very carefully designed cylinders. The transfer valves are also an example of extremely careful design with exceptionally low pressure losses. Parasitic heat transfer within the cylinders is also very carefully controlled resulting in very high thermodynamic reversibility, dead volume and its effects must also be minimised since it thermally pollutes the gas charge at the end of the stroke. Greater losses are actually electrical and mechanical.
Of course irreversinilities manifest themselves as waste heat, just not very much of it and we dump it in one limb of the circuit via heat exchange. This is a very non-critical part of the operation since we are creating a thermal split around a datum temperature. If this datum temperature is a bit above ambient (as it has to be to dump the heat of irreversibility) then it has an inconsequential effect on system performance.
This absolutely cannot work as a prime mover heat engine at 80% efficiency as James tried to explain (I thought very clearly). It is a thermodynamic battery in which the ONLY element of importance is process reversibility.
Jonathan Howes.
CTO Isentropic Ltd

James…

You should not open your page with condition: “99% isentropic efficiency is, we believe, a record.”

You need to know that you know you have the record… It comes off really dumb when you come out with a revolutionary technology that you aren’t quite sure is revolutionary. You tell me - Am I right?

All an all I love the idea of your company. I wish you all the best of non-sarcastic luck.

The current strength of the Brayton cycle in engines is that you can use rotary equipment to get to very high temperatures eg 1200 deg C, which compensates for it’s failings. It is not a great engine cycle as the exhaust temperature is always well above ambient - Hence why you can run a steam turbine off of it in a CCGT.

You are quite right that you could run a solar thermal plant off of this engine, but in that application it is just acting as a heat engine and the cycle efficiency, at say 500 deg C, is unlikely to be as good as a large steam turbine over the same temperature range. In smaller sizes (sub 30MW) and applications where water is a problem this may not be the case.

Let me be specific:

A typical adiabatic compressor has an isentropic efficiency of 81%. A typical expander has an isentropic efficiency of 85%. A typical generator has an efficiency of 97%, as is a motor. If you ignore the losses in the heat exchangers and heat leaks, you get a round trip efficiency of .81 * .97 * .85 * .97 = 65%. In reality, you have thermal exergy degradation and pressure drops in regenerators and there will be heat leaks. Assume your losses in the regenerators are 5% each, you get 55% efficiency. Adding the 5% heat leak loss, you get 50% round trip efficiency.

This has nothing to do with Carnot efficiency. The whole thing is bases on exergy (or availability if you like).

If the company insists their efficiency claims, they should list all the numbers and assumptions and let the technology community comment on them.

The claim “The current strength of the Brayton cycle in engines is that you can use rotary equipment to get to very high temperatures eg 1200 deg C” is either naiive or deceptive. As far as I know, there is no compressor that is capable of going to 1200 C - not even 700 C. At 1200 C, all the known materials for making compressor rotors become plastic.

In gas turbine area, certain gas turbines can tolerate such a high temperture because the blades are cooled by air so they do not see this temperature. That cannot be done for gas compression, or such machines are not invented yet, to say the least.

If the company insists the claim of 1200 C, they should name the specific compressor they are using.

Not that it matters: the 72% + efficiency claim is a gross overestimate, as many of the other claims.

If Isentropic has a compressor with 99% isentroi effiency and the machines are not extremely costly, then I think their first step should be to get into heat pump/air conditioner business and they will dominate. They should then get into power generation business and they will take off the power generation business. Why bother with the energy storage business in which it is difficult to be cost effective?

First I have already offered to share additional information with you but you have not taken me up on my offer.

Secondly my comments about the Brayton cycle were referring to gas turbines and I assumed that the 1200 deg C was post combustion and pre-turbine stage. This was made in reference to a comment asking why could we not use our storage engine with solar thermal ie 500 deg C temperatures to generate power. I was attempting to explain that the Brayton cycle (that our machine runs on) was not particularly useful in the application of solar thermal at 500 deg C and that it’s strength in GT’s was that you can go to 1200 deg C. Consequently my final comment was that you would be better to use a steam turbine for solar thermal power generation.

Thirdly our system operates using reciprocating machinery. In this context you need to be very careful how you use Isentropic Efficiency. In large turbo machinery I would expect to see state of the art isentropic efficiencies for compressors around 93% and for turbines around 94%. These will be large machines that are operating in an almost adiabatic regime. Consequently the Isentropic efficiency is a measure of the irreversibility of the gas process - principally kinetic heating of the gas by the blades. However, in reciprocating machinery we can tolerate much lower flow rates than turbo machinery and consequently these kinetic losses are not the issue, but instead the problem is valve pressure losses and in cylinder heat transfer during compression/expansion. We have solved these problems and not surprisingly we don’t broadcast the nuts and bolts of how we have done it as this is commercially valuable/sensitive.

The pressure losses in the stores are nothing like 5%, we control the cross section of the stores and consequently have slow drifting flows. This is not the same situation as turbo machinery where you would have to use diffusers to slow the gas down or suffer high pressure losses. I am not sure what heat leakage you mean, but we would expect less than 1% per day, although this varies with store size. Multiplying the isentropic efficiency of a compressor by an expander does not give an answer for round trip efficiency. In our cycle we see the compressor and the expander on both the input and output cycle. You also need to add mechanical losses of piston rings, bearings etc..

To answer your last post - in terms of air conditioning we have developed an air cycle machine - when you do the analysis you find that the cycle is very sensitive to parasitic losses for low duty regimes (ie low temperature range) and the benefits of high isentropic efficiency are negated by these parasitic losses. In power generation we are limited by our top temperature in our expanders of 500 deg C and therefore are not going to compete with GT’s, which are a proven technology and can burn gas at 1200 deg C.

Very interesting article and comments. A company called Elcal Research has developed a new energy storage technology using a (phase change material). They store the energy as heat , they have used glycol filled solar panels off peak electrical, wind at there test site. A heat pump heats or cools the building as needed. The phase change material they use freezes at 78 degrees. The energy stored during the phase change is tremendous. They say a 250 gallon tank will store 216000 BTU’s of energy. There systems payback maybe as low as 4 years. I’ve seen it work I’m amazed.

James,

Your explanation on 1200 C is appropriate.

The 93-94% efficiency for large turbomachinery are too high. The highest expander efficiency I have seen is 92%, but it was obtained not under the conventional power generation conditions. Most power generation use turbo-expanders have efficiencies in the range of 83-87%, while the adiabatic compressors for a large pressure ratio is in the 81-82% range. Isentropic efficiency is an extremely important parameter for a power plant. If 93-94% machines are available at reasonable prices, there would be no market left for the 81-85% machines that are currently in the market place. As for reciprocating machines, they may be more efficient, but they are very costly, especially if you reduce the linear velocity. They cannot be economical for power applications where the low cost is achieved by economy of scale. That is the reason why no power of scale would use reciprocating machines.

My calculation for the round trip efficiency is simplified. In Isentropic’s process, a gas goes through compression and cold expansion during the energy storage step and cold compression and hot expansion during the heat-to-electricity conversion step, so my 81%*85% calculation does not include the inefficiencies of the cold expander and cold compressor. I also did not mention the losses in the aftercooler, either. The heat leak loss I meant is the sum of heat exchange between envronment and the cold regenreator and hot regenerator, as well as the heat transfer within the regenerators in the flow direction during the 24 hour cycle - even during the shoulder hours when the unit is not working, heat is being transferred from the high temperature side to the lower temperature side in the same regenerator. The losses inside the regenerators are not just the dp losses. They also include the heat transfer dT losses, and the losses due to the heat transfer in the axial direction due to the temperature gradient inside the regenerator.

Given the information you gave here, I honestly think you should focus your efforts on the heat pump/air conditioning market. The electrical energy market needs very cheap machines/equipment in order to compete. For energy storage, situation is even worse, since the utilization rate of the machines are so small. On the other hand, if your machines are indeed as efficient as you claim, then the air conditioners using your machines will be much more efficient that the incumbent. As a consequence, the summer peak demand will decrease since air conditioners are the main cause of the summer peak power demand. You can help solving the need for energy storage that way.

Having watched the gladitorial efforts of James Jonathon and Process Inventor, I’m glad a concensus has been reached, as the system and elcal sytem mentioned by Greg R. could counter many of my reservations on solar and wind not being base load. The question for me is whether they are at a commercial stage where we could couple wind or solar voltaic into a base load supply with an aceptable price tag per MWe. My work is in Pakistan where currently biomass and geothermal seem the most appropriate base load technologies. Peak load is indeed for A/C and since heating is almost never required, this is as hig as 70% of peak load and 50% of offpeak. Your thoughts would be appreciated gentlemen