A British company called Highview power is proposing to store electrical energy using liquid air as the storage medium. Electrical energy will be used to run an air liquefaction plant, and at a later time the air will be reheated, expanded, and use to power a turbine/generator. The discharging cycle can use ambient heat, but with higher temperature heat source better round trip efficiency can be obtained. Highview is planning to use waste heat from existing industrial processes in order to obtain better round trip storage efficiency. A prototype of the turbine generator powered by liquid air and waste heat is already in operation delivering power to the grid, but a fully integrated storage system with on site generation of liquid air is not yet operational.
Some details about the proposed cyrogenic energy storage systme taken from a briefing note published on Highview Power's web site are given below:25 Year+ Lifetime
Cost per kWh of energy delivered can be estimated from the numbers given above. I assume a lifetime of 30 years, and I use the middle value of the range of O&M costs quoted above.
Storage Cost/kWh = ($2300+30×.0225×$2300)/[1Kwh×4hr/day×(365-7)×30] = $0.09/kWh
If you want to do discounted cash flow analysis you have to add on the cost of interest. The total cost is the sum of the above cost plus the cost of generating the electricity in the first place:
Total Cost/kWh = G/0.7 +$0.09
G is the cost of generating a kWh of electrical energy. Since the round trip efficiency is only 70% more than 1 kWh must be generated to deliver a kWh to end users.
This technology is not very cheap by the standards we have become used to in our draw down of earth's one time bounty of fossil fuels. It should be noted that the cost of $0.09/kWh applies to a long term equilibrium situation in which we are mere replacing worn out storage facilities as required. During a period of growth the cost per unit of delivered energy will be substantially higher.
Highview tries to make a virtue out of the fact that they are converting waste heat to electricity at high efficiency, but this claim is misleading. From a whole systems perspective the efficiency that is important is the percentage of the electrical energy used to run the cryogenic is unit which is returned to the grid by generating turbine. Even with the use of waste heat at 115°C this efficiency is only 70%. The fact that waste heat is required to achieve this efficiency increases the system cost rather than being a benefit.
The requirement for an external heat source to achieve a reasonable efficiency also raises questions about how such a storage system would be used in the long term. In our current fossil fuel dependent society sources of waste heat are abundant. But in the long term fossil fuels are going away, and if believe that human GHG emission are driving climate change, then the sooner they go away the better. So where would the necessary heat come from in a post fossil fuel world?
If you believe in a long term future for nuclear energy then nuclear reactors are an obvious candidate. Such reactors are a source of both electricity and waste heat, and in fact they could probably supply heat at a much higher temperature than 115°C. This technology would allow nuclear reactors to run as base load generators and could store night time power for later use.
In a renewable energy future a possible source of heat is solar energy, although this would not longer be 'waste' heat since solar collectors would be required to produce temperatures of 115°C or higher. However, the technology for producing this temperature is less expensive that the technologies that produce high temperature steam or electricity from incoming solar radiation. One good thing about this storage technology used in conjunction with solar electricity productions is that it does not (I believe) require cooling water. The need for large quantities of cooling water is the Achilles heel of concentrating solar power (CSP) in my view.
July 15, 2011Energy Storage News
rogerkb [at] energystoragenews [dot] com