The ERoEI of High Altitude Wind Power

For several weeks I have been researching and writing a review post on high altitude wind power. It has grown into a 6000 word monster that should hopefully fly on Monday. While doing this it has been difficult to find time to write other posts. Hence this is a preview of one section on Energy Return on Energy Invested (ERoEI) which makes a nice post in its own right.

KiteGen have presented a back of the envelope style ERoEI calculation for their 3 MW stem indicating a value of 562 which is incredibly high. I have done my own calculation using a variant of their methodology and my own input variables. The idea is to try and estimate the energy intensity of a wind turbine structure and to interpolate that into a KiteGen stem. This involves making many weak assumptions but should be good for arriving at a ball park number.

I will begin with estimating the energy intensity of a wind turbine based on the mass of the superstructure. According to Vestas, their V112 3 MW turbine contains 372 tonnes of metal in the tower and nacelle (I will ignore the 947 tonnes of steel and concrete in the foundations for the time being).

I am going to make the following assumptions:

ERoEI = 18 [1]
Capacity factor = 0.3
Lifespan = 20 years
Power = 3 MW
Mass of superstructure = 372 tonnes

Energy produced during life time = 3*24*365.25*20*0.3 = 157,788 MWh

Energy required to create and maintain machine = 157,788 / 18 = 8,766 MWh for an ERoEI of 18.

Energy intensity = 8,766 MWh / 372 tonnes = 23.6 MWh / tonne

In his PhD thesis, Lorenzo Fagiano provides the following table for the theoretical capacity factors for a KiteGen [2]:

The average is 0.54 which is used in the calculation below. The capacity factor is higher than a turbine because high altitude winds (500 to 2000 m) blow more steadily than at the surface. Assumptions for a 3 MW KiteGen stem:

Capacity factor = 0.54
Lifespan = 20 years
Power = 3 MW
Mass of superstructure = 20 tonnes

Energy produced during lifetime = 3*24*365.25*20*0.54 = 284,018 MWh

Energy required to create and to maintain machine = 20 tonnes * 23.6 MWh / tonne = 472 MWh

ERoEI = 284,018 MWh / 472 MWh = 602

This is an astonishingly high and difficult to believe number but it is born out of the much lighter weight and higher capacity factor for the KiteGen. In his calculation, Massimo Ippolito got a number of 562 using an energy intensity of 40 MWh / tonne. Using that figure, my ERoEI estimate falls to 355 which is perhaps more realitsic.

It is this aspect of the KiteGen and high altitude wind that really caught my attention. Many years ago when I first began looking into global energy issues I believed the problem may be easily solved by a combination of wind power and partial conversion of surplus power to an energy store such as hydrogen. Unfortunately there are many who still believe this is a solution. The problem with this approach and conventional turbines is the low ERoEI of wind turbine electricity, that makes it expensive combined with round trip energy losses in going to storage such as hydrogen that are typically of the order 70%. With 70% losses, the ERoEI of a wind turbine – hydrogen system falls to 5.4 (ERoEI of 18 * 0.3) and we drop off the net energy cliff (Figure1). In other words, with a wind turbine – hydrogen system you take expensive electricity and waste 70% of it to mitigate for intermittency. Consumers and economies don’t like this!

Figure 1 The estimated ERoEI for a 3 MW KiteGen plotted on the Net Energy Cliff. The electrical output from a KiteGen is not smooth. This is partly mitigated by on-board super-capacitors that can store and discharge power to smooth out the supply. To convert the output to dispatchable power a very conservative approach would be to convert all the electricity to an energy carrier like hydrogen and then combust the hydrogen in a gas turbine to generate electricity. This will consume about 70% of the available energy, but even doing this leaves an ERoEI > 100. In reality some of the output power can be sent direct to the grid while some can be stored to mitigate for intermittency. For explanation of the net energy cliff see ERoEI for Beginners.

The KiteGen stem is a complex machine, but it is light weight and cheap to build and to install. IF it works according to expectation then it may produce large quantities of cheap, unsubsidised electricity. A KiteGen – hydrogen generator would still have ERoEI of 355*0.3 = 107 which is still huge compared to most other forms of electricity generation today. A KiteGen may also be used to make synthetic fuels. An important point is that a KiteGen may be able to make the liquid fuel to mine materials and make the electricity to manufacture more KiteGens with ample energy left over for the rest of society to use. But all this depends on the assumptions made above holding true and the machines actually working to specification.

energy matters