A company called Keuka Energy is developing a new type of wind energy system which uses wind energy to directly drive a mechanical refrigerator/compressor which produce liquid air. Frankly the concept seem a little flaky to me, but it something completely new in he world of wind energy. Keuka’s wind turbine design is many bladed rather than three bladed like current commercial power generating turbines. They look more like a traditional water pumping windmill used on American farms. However, the blades are attached at their outer edge to a circular metal framework. The rim of this framework contacts a drive mechanism at the low point of its revolution which turn drive belts and attached machinery. Hence Keuka calls the technology “Rimdrive”.
Interestingly enough the machinery that Keuka is planning to attach to these drives is not electrical generators, but instead compressors and turbo-expanders which are part of a refrigeration system designed to produce liquid air. Keuka’s business concept is that they will produce liquid air at cost far below those than can be achieved using electricity from the grid. They will then sell the liquid air as a fuel to electrical generating plants. Actually liquid air is not a fuel, but it can be used to enhance the efficiency of electrical generation from existing fuels such a natural gas and it could allow efficient production of electrical energy from waste heat.
It would even be possible to generate electricity from heat sources at ambient temperature with a cold reservoir at the temperature of liquid air (-196C). However, the sequence fuel to electricity / electricity to liquid air / liquid air to electricity has a very low round trip efficiency. A British company called Highview Power is actually developing an energy storage system based on this sequence. They are using waste heat in the last step to enhance the round trip efficiency, but I am skeptical about the economics of this energy storage system.
Keuka is trying to improve the first two steps of this sequence by producing liquid air directly from wind power without the production of electricity. What the chances are that Keuka will really be able liquid air at a low enough cost for this technology to be economically practical I cannot really evaluate, but I am pretty skeptical. They emphasize the fact that their wind turbine design does not have a gear box (according this Windpowerengineering.com article the gear box typically represents 11% of a wind turbines CAPEX) and that their blades are made of marine grade aluminum the cost of which is 10% of the composite blades using in current mainstream wind turbines (blades represent 20% of CAPEX). However, the Keuka turbine design appears to have many blades and much less empty areas than traditional three bladed designs so that any the cost advantage of aluminum will at least partially offset by more total blade material per unit of swept area.
They are specifically planning to install their plants offshore with a series a turbines towers attached along the length of a hollow v-shaped floating structure. This structure will contain the compression/refrigeration machinery and will also act as storage for the liquid air. Liquid air tankers are supposed to slip into the calm water inside the V and off load liquid air. The conceptual picture posted a Keuka’s website shows the lower rim of the turbine as being close (i.e. less than a radius away) from the floating support structure.). This physical closeness is probably desirable for the mechanical drive system, and it avoids more severe stability problem associated with tall wind turbine towers. However turbines closer to the surface of the water will harvest less wind energy than turbines on taller towers.
Three bladed composite turbines completely dominate the commercial wind turbine market in spite of their high expense because their high efficiency at harvesting energy from the wind flow more than makes up for their costs. The Keuka design may be superior for the direct mechanical drive of machinery, but I have to believe that the efficiency of energy extraction per square meter of turbine surface (I am counting the empty spaces between the rotors as part of his surface.) will be substantially reduced compared to state of the art three bladed composite turbine on tall towers. All things considered I think that I am not going to hold my breath waiting for long lines of liquid air tankers to come steaming into port.
One other aspect of this proposed energy of ‘sustainable’ energy production deserves comment. Keuka’s proposed methods of electricity production require the use of fossil fuels. They propose using liquid air to replace the compressed air required in operation of natural gas turbines. This compressed air is currently produced by the turbine, thus representing a parasitic consumption of energy. The use of liquid air would allow the turbine to produce more electricity from a given amount of natural gas, but would not eliminate the use of natural gas. Keuka also propose using waste heat in conjunction with the liquid air to run a heat engine/generator. In order for this scheme to work a source of waste heat is required, most of which in the current of energy production comes from the combustion of fossil fuels. Again, this electricity production scheme would produce more electricity from a given quantity of fossil fuel. However, sustainability requires that we entirely eliminate the use of coal an natural gas.
When we are no longer burning coal and natural gas where is the heat going to come from that needs to be combined with the liquid air to efficiently produce electricity? If one believes in a nuclear powered future then the long term source of waste heat is obvious. Using the nuclear waste heat to produce electricity is more environmentally friendly than discharging it into the acquatic system as is done at present. Furthermore the energy storage aspect of this system would help in running the nuclear generators as high capacity factor base load plant which is the most economical use of this energy source.
On the other hand, if one is trying to imagine a future powered by renewable energy then the heat source for the the liquid air generators is not so easy to imagine. We could get heat from the combustion of biomass, but the amount of biomass that we can sustainably harvest will probably not support a population of seven billion plus people people in high energy consumption lifestyles even if it is supplemented by stored wind energy.
So thermal energy could conceivably be combined with liquid air to produce electricity. For example a heat engine operating between -196C and +100C would have a limiting carnot efficiency of 80%. Of course one never achieves the carnot efficiency, but the potential for a very effective heat engine would exist with this temperature range. 100C is a very modest temperature compared to the temperatures required to run the steam turbines of a CSP plant. The solar field and heat storage system required for a 100C thermal plant might be relatively cheap. If cheap wind energy in the form of liquid air could be combined with cheap solar thermal energy then one might have an economically interesting proposition. However, the first if is a very big one, and i am not holding my breath waiting for it to become a reality.