Economical assessment of the scalability of the ground station in pumping mode

Analysis Industry and Market Analysis
1

Pumping = yo-yo = reeling.

The topic goal is an attempt to analyse the potential of scalability of the ground station in an economical way, considering pros and cons, and what is achieved.

Several topics of which this one mention the figure 15 from https://www.annualreviews.org/content/journals/10.1146/annurev-control-042820-124658. We can see an average of 92 kW with peaks at 400 kW. More recently a power curve was performed, presenting a comparable average power.

So some level of scalability was carried out.

Now, considering more precisely the ground station:

1. Ground Station (GS)

Converts the mechanical energy of the kite into electrical power and reels the kite in by using the generator as a motor.

  • Dimensions:2.44 m x 2.60 m x 6.06 m
  • Weight:9.6 t
  • IP Rating:IP64
  • Lifetime:25 years

The weight seems to be far higher than the weight of the nacelle of 2.2 tons, direct drive, for a wind turbine with equal rated power of 100 kW, or 4.6 tons, with a gear box. This difference of weight can be due to the “low speed of the tether extension” (see the text cited below). However the comparison should take into account of other positive or negative factors such like the ground station including the generator and which can be heavier as such, intermittent generation, energy consumption during reel-in phase by using the generator as a motor, the two last factors leading to the requirement of a more powerful generator in order to compensate them.

About the “low speed of the tether extension”:

Page 236:

While a kite wing, flying crosswind, is extremely efficient, the tether moves in
the tangential direction slowly. According to Loyd [21], the maximum power output
is provided at the tether extension speed, equal to one third of the wind speed, if
the tether is collinear with the wind (i.e., the speed is further reduced by the angle
with the horizon). The low speed of the tether extension requires high tether forces
to transmit the same amount of power. In a conventional design with the tether
unrolling from a drum, the drum has low RPM, a high-ratio gearbox is required to
increase the angular speed to 1,500–1,800 RPM, as required by conventional electric
generators. Thus a large part of the advantage that an AWECS has compared with
a conventional wind turbine is offset by the higher cost of the drive train. The drive
train constitutes 40% of the cost of the conventional wind turbine, according to [19,
p. 178].

This seems entirely relevant.

2

4 posts were split to a new topic: Networked Ground Stations

6

From the publication, in “Power Generation” section:

For the system with four groups

Control: at any time, the working umbrellas of three groups are traveling up, one is traveling down

The total mechanical power transferred to the transmission system is constant at any time

The goal is therefore to have constant generation at the price of a high complexity. But such an installation does not make it possible to reduce the cost of ground stations, whatever the configuration. A large generator or several small connected generators will always have the handicap of:

This would be valid both for a single large generator (with an even lower angular speed than smaller generators) and for several smaller generators that are connected or not.

If it were possible there would be no point. See just above and the quote in the initial post. Another excerpt:

You can take all possible combinations and networks, this problem will remain.

Add to this the intermittency of ground stations in pumping mode, leading to wear and stress on the material and electric installation, and you understand that, in spite of the relative success of large kites, there is no pumping mode AWES in the regular market. And I take the risk to state that it is unlikely a significant market occurs in the future. @dougselsam, as our expert in regular wind energy, knew it.

7

If you want to compare the costs of different approaches it’s probably a good idea to actually try to do that. What are the lifetime costs associated with 10 or 50 smaller ground stations that have one tether going in and out of them for example versus a single larger one that has 10 or 50 tethers going in and out of it? I’d start by trying to list the elements that could go into either option for example and go on from there.

The more thorough and exhaustive the analysis the more useful it would be. Now it’s jumping straight to the conclusion without doing the supporting work.