**1. Introduction**

Large-scale energy storage will likely play a significant role in any future energy system that wishes to implement high levels of renewable penetration [1]. This is due to its ability to provide much-needed flexibility in grids whose generation portfolio will become increasingly inflexible as a result of the natural variability of renewable sources. This widely accepted notion has led to a surge in the number and scope of energy storage technologies, both proposed and implemented.

Most of the work done in the energy storage sector has focused on standalone storage systems, which absorb electricity and convert to and from some other storable form of

**Citation:** Swinfen-Styles, L.; Garvey, S.D.; Giddings, D.; Cárdenas, B.; Rouse, J.P. Analysis of a Wind-Driven Air Compression System Utilising Underwater Compressed Air Energy Storage. *Energies* **2022**, *15*, 2142. https://doi.org/10.3390/en15062142

Academic Editors: Alon Kuperman and Alessandro Lampasi

Received: 12 January 2022 Accepted: 7 March 2022 Published: 15 March 2022

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exergy before re-supplying (a proportion of) the electricity at a later time. Standalone storage can be developed and implemented entirely separately from the generation, enabling traditional renewable energy technologies to be used for generation. There are also non-grid applications for energy storage, such as the use of Li-ion batteries in electric vehicles, that are, by definition, standalone.

In contrast to standalone storage, generation-integrated energy storage (GIES) technologies store energy before it is converted to electricity [2]. Figure 1 contrasts the energy conversions seen in GIES systems with those in standalone storage.

**Figure 1.** Comparison of energy conversions between (**A**) standalone and (**B**) generation-integrated energy storage configurations. From [2].

GIES systems minimise the conversions between the primary energy collected and the consumed output electricity for any energy that passes through storage. A familiar example of a GIES system is natural hydro, where the energy from flowing water is converted into a storable form (gravitational potential via damming) before it is ever passed through turbines to generate electricity. Although pure transmission efficiencies (that is, the output exergy divided by the input exergy for exergy that does not pass through storage) may be slightly lower than in conventional generation systems, GIES systems have the potential to achieve significantly higher efficiencies for energy that has passed through storage. Such GIES systems may therefore achieve higher overall throughput efficiencies than systems with independent generation and storage, if a sufficient proportion of the total output energy is required to pass through storage, as represented in Figure 2 (from [2]). GIES systems therefore have the potential to be relatively cheap forms of energy storage. Additional examples of well-established GIES technologies include many concentrated solar plants [3,4]. GIES systems have also been proposed for wind [5–8] and nuclear power [9,10].

**Figure 2.** Overall throughput efficiency against fraction of output energy that passed through storage for standalone and GIES stores. From [2].

An offshore wind-driven GIES system that exercises multiple stages of air compression, with integrated compressed air energy storage, is proposed in [11]. This paper clarifies and expands upon that idea. In Section 2, the technologies that constitute the system are explained and examples of other systems utilising them are given. In Section 3, the system from [11] is described, with particular note to possible alternative configurations. Section 4 describes simulations of the system that were performed in order to understand the relationships between parameters such as storage size and pressure ratio. The economic benefits of such a system are then discussed, along with the barriers impeding it.

#### **2. Constituent Technologies**

#### *2.1. Isothermal and Adiabatic Compressed Air Energy Storage*

Compressed air energy storage (CAES) is a set of thermomechanical energy storage technologies. In CAES, work is done on ambient air in order to compress it to a higher pressure, at which point it is stored (at near-ambient temperature) for some period of time before being expanded back to ambient conditions to generate electricity. Two grid-scale CAES plants in Huntorf, Germany, and Alabama, USA, are diabatic (D-CAES), meaning that the heat from compression is discarded and the air is stored cool, before being reheated prior to expansion. In these cases, the pre-heating of air is done through the burning of natural gas. For several reasons, not least of which is the desire not to use fossil fuels in energy storage, modern CAES proposals tend towards Isothermal (I-CAES) or Adiabatic (A-CAES) configurations, which do not consume any combustible fuel, although CAES systems that use some carbon-neutral fuel (such as green hydrogen) have also been posited [12–14].

In I-CAES systems, air is compressed and expanded near-isothermally with the use of effective heat transfer. In a successful implementation, almost no exergy is lost, because any heat taken from the air is close to ambient temperature and therefore has negligible exergetic content. Assuming ideal gas behaviours, the work done on a volume of gas compressed isothermally is given by:

$$W\_{ISO} = p\_1 V\_1 \ln \left(\frac{p\_2}{p\_1}\right) \tag{1}$$

$$\text{where}$$

where *p* is the gas pressure, *V* is the gas volume and subscripts 1 and 2 denote inlet and outlet, respectively.

Conversely, A-CAES systems purposely minimise the heat transfer between the air and its surroundings. In A-CAES, the heat is stripped from the air after compression and is then stored in thermal energy stores. For adiabatic compression, the work done on a gas is:

$$W\_{ADIA} = p\_1 V\_1 \frac{\left(r^{\left(\left(\gamma - 1\right)/\gamma\right)} - 1\right)}{\left(\left(\gamma - 1\right)/\gamma\right)} =: p\_1 V\_1 \frac{\left(r^\chi - 1\right)}{\chi},\tag{2}$$

where *γ* = 1.4 for diatomic gases. As before, *r* represents the pressure ratio *p*2/*p*1.

#### *2.2. Wind-Driven Energy Storage*

Integrating energy storage into wind power generation has been proposed in various formats. The simplest form of this is the co-location of a conventional energy store with wind turbines, used to smooth the intermittency of renewable generation [15–17]. If sufficiently inexpensive, this has some economic advantages for the operator, as electricity can be supplied to the grid during periods of higher value. There is also a benefit to the grid by reducing transmission losses. However, such systems cannot be described as GIES. Wind farms providing energy in the form of electricity to some co-located energy store could be described as "wind-powered".

By contrast, "wind-driven" systems are defined here as those that initially convert the energy provided by the wind into a form of energy other than electricity. The majority of wind-driven GIES systems in the literature involve some form of gas compression, although the use of induction or other forms of direct heating have also been suggested [18].

Of the systems utilising gas compression, D-CAES [5], adiabatic compression [6] and offshore I-CAES [19] configurations have been proposed. In all cases, the gearbox of a conventional geared wind turbine—a source of considerable capital and maintenance cost—is removed, and the generator is relocated close to the energy stores at ground- or sea-level [20–22]. This leaves the nacelle of the wind turbine relatively free for compression machinery. In [19], a liquid piston compressor is used, with seawater performing the duty of the liquid and also acting as an excellent heat sink. Liquid piston technology is also used in [6], but is designed to perform near-adiabatically, with a radial piston configuration employing multiple stages to minimise the temperature gradients and therefore heat transfer.

#### *2.3. Direct Drive Air Compression and the Problem of Intake Swept Volumes*

The wind-driven CAES systems mentioned are direct drive, meaning that the compressor runs synchronous with the wind turbine itself. For most modern turbines, the tip-speed ratio (*TSR*) relates the tangential speed of the blade tips (*uTIP*) to the oncoming wind speed (*u*):

$$TSR = \frac{\mu\_{TIP}}{\mu} = \frac{\omega D}{2\mu},\tag{3}$$

where *ω* is the rotational speed and *D* is the diameter of the turbine's swept area. The rotational frequency of the rotor, *f*, is:

$$f = \frac{\omega}{2\pi} = \frac{\mu TSR}{\pi D}.\tag{4}$$

The parameters *ω* and *f* have rated values for any wind speed at or above the turbine's rated wind speed *uR* (and below its maximum wind speed). In this paper, the MHI Vestas V164-8.0 MW turbine will be used as the reference turbine. Data for this turbine is summarised in Table 1.


**Table 1.** Data for reference turbine used in this paper. From [23].

For wind-driven compression, it is useful to find the intake swept volume of air, *Vswept*, that must be absorbed by the compressor in each rotation. This can be done by rearranging Equations (1) and (2), using the fact that the work done per rotation is equal to the rated power of the turbine *Pturbine* divided by its rotational frequency. For isothermal compression, this gives:

$$V\_{susp,ISO} = \frac{P\_{turbine}}{fp\_1 \ln(r)},\tag{5}$$

and for adiabatic compression:

$$V\_{\text{super},ADIA} = \frac{P\_{turbine} \chi}{fp\_1(r^\chi - 1)}.\tag{6}$$

For a set upper pressure *p*<sup>2</sup> = *p*1*r*, a minimum swept volume is found in an isothermal compressor when *<sup>r</sup>* <sup>=</sup> *<sup>e</sup>* <sup>≈</sup> 2.7182 and in an adiabatic compressor when *<sup>r</sup>* = 1.41/*<sup>χ</sup>* <sup>≈</sup> 3.2467. For pressure ratios above these optima, swept volumes increase much more slowly than for pressure ratios below these values (with *Vswept* tending to infinity as *r* → 1).

For illustration, if air were to be compressed by the reference turbine from ambient pressure to 74 bar, the upper pressure of the D-CAES plant at Huntorf, an adiabatic compressor would need to intake 47.2 m<sup>3</sup> of air per revolution and an isothermal compressor would need to intake 92.9 m3. It should be noted that the swept volume is relatively insensitive to the upper pressure. It has also been assumed here that the adiabatic compression would take place over a single stage (with regard to heat transfer). A multi-stage compressor might be used in a scenario where the upper temperature would be too high for conventional materials.

To estimate the acceptable swept volume for a compressor housed in a wind turbine, consider that the volume of both the gearbox and generator is freed up in a wind-driven system.

Assuming the use of a radial piston compressor design similar to that described in [6], the machine can be split into two distinct parts: here defined as the "displacer" and the "converter". The displacer houses a camshaft and radial pistons and the converter contains the compression cylinders. It is therefore reasonable to make the approximation that the displacer replaces the gearbox and the converter replaces the generator in the turbine nacelle. The nacelle dimensions of the reference turbine are 24 × 12 × 7.5 m and the generator takes up approximately 1/8th of the total volume, or ~270 m3. A prototype compressor utilising the technology described in [6], in development at the University of Nottingham, has a ratio between the inlet swept volume and overall converter volume of roughly 0.004. Scaling up to the reference nacelle size, this relates to an intake swept volume of ~1.1 m3. Accounting for slightly larger allowable nacelles, as well as more optimised compressor geometries and nacelle layouts, it is likely that intake swept volumes of ~5–15 m<sup>3</sup> might be achievable in the reference turbine. Even at the upper end of this

estimate, a turbine compressing to 74 bar would have a power of 2.54 MW (adiabatic) or 1.29 MW (isothermal)—only a fraction of the 8 MW reference turbine power.

The problem of large intake swept volumes is therefore significant for all direct drive systems. The I-CAES system in [19] solves this problem by moving the liquid piston to sea-level and instead utilising a variable displacement hydraulic pump in the nacelle. This combines well with the use of seawater as the liquid piston (thereby not requiring seawater to be pumped up to the nacelle). By contrast, Ref. [6] solves the problem of intake swept volumes by increasing the inlet air pressure *p*1. Rather than a CAES system, Ref. [6] is a pumped thermal energy storage system, wherein all exergy stored is in the form of high-grade heat (or coolth) that is stripped from the compressed gas [24]. This involves a closed gas loop, meaning that the inlet and outlet pressures for the compressor can be freely set.

#### *2.4. Underwater Compressed Air Energy Storage*

Underground caverns are often considered the default method of air storage when discussing CAES plants. The CAES plants in Huntorf and Alabama both use this method of storage, as do many other CAES proposals.

Such storage is generally isochoric (constant volume), meaning that emptying a proportion of the air in the store reduces the pressure of the remaining air. Expanders used in an isochoric CAES system must therefore run over a series of pressures, limiting their efficiency, as they cannot be optimised for a single pressure. As there will be a pressure below which an expander cannot run at an acceptable efficiency, this also significantly limits the proportion of the store that can be flexed. Cavern stability also limits operational pressure ranges, as a certain volume of "cushion gas" is required to ensure that the caverns do not collapse.

In contrast, isobaric (constant pressure) storage allows for the flexing of the entire storage volume, at a single pressure for which the expander can be optimised. Isobaric storage can be achieved in combined hydro-CAES plants where a water reservoir is used to keep constant pressure [25]. However, most isobaric storage designs revolve around the store being placed deep underwater, in a lake or ocean. This is known as underwater compressed air energy storage (UWCAES). In UWCAES, each 10 metres of water depth provides approximately 1 bar of additional pressure on the store. By filling the store with air of equivalent pressure to the hydrostatic pressure supplied by the water, the store itself does not need to provide any confining force. A UWCAES system utilising flexible canvas bags (energy bags) is described in [26,27] and expects storage capacity costs in the region of 25 \$/kWh (dependent on the storage depth). A representation of this design can be seen in Figure 3.

The components comprising an energy bag system are anchoring ballast (AB), tension cables (TC) and the canvas surface area itself (SU). The role of the tension cables is to counteract the buoyant force created by the air volume. The cables are therefore the sole structural element of the energy bag. It is explained in [26] that the cost of the anchoring ballast is proportional to the buoyancy force created by the energy bag volume and is therefore proportional to the cube of the characteristic dimension (the diameter, *D*) of the energy bag:

$$
\mathbb{C}\_{AB} \ll D^3. \tag{7}
$$

The tension cable cost is proportional to the product of its length *L* and tension force *FT*, which is also proportional to the buoyancy force:

$$C\_{TC} \propto LF\_T \propto D^4. \tag{8}$$

The cost of the canvas is clearly proportional *D*<sup>2</sup> and so the total energy bag cost can be written as:

$$\mathcal{C}\_{BAG} = \mathcal{C}\_{AB} + \mathcal{C}\_{TC} + \mathcal{C}\_{SII} = aD^4 + bD^2 + cD^3,\tag{9}$$

where *a*, *b* and *c* are constants. By noting that the energy stored in the bag, *E*, is equal to another constant, *k*, multiplied by the energy bag volume,

**Figure 3.** Energy bag underwater compressed air energy storage (UWCAES) system. Shown without pipework for simplicity.

Differentiating this to find the optimum and substituting into Equation (9) then shows that the optimum diameter is achieved when *CSU* is equal to *CTC*. Multiple energy bags of this optimal diameter would therefore be better value for money than a single energy bag of equivalent volume. The use of multiple smaller bags also has the benefit of reducing the proportion of storage that is out of service if damage to a single bag occurs. Assuming that any storage system employs multiple optimally sized energy bags (with the volumetric capacity of *Vopt*), the total cost of the system is the number of energy bags used multiplied by the cost of a single bag. Therefore, for a system with a volumetric capacity of *nVopt*, where *n* is an integer, the total cost of the storage system is directly proportional to its volume.

The technologies discussed in this section have a clear synergy. The use of UWCAES specifically for the balancing of wind energy has been investigated previously [28,29]. However, iterations of this concept assume a conventional wind turbine with standalone UWCAES. Thus far, no literature has discussed the combination of wind-driven air compression with UWCAES. The possibility of utilising UWCAES in a GIES-type system is unexplored. The concept of wind-integrated energy storage itself (beyond the co-location of standalone energy storage) is underexplored.

Furthermore, no wind-driven air compression system attempts to solve the problem of large swept volumes with a prior stage of electrically driven air compression. It may initially seem counterintuitive to add electrically driven air compression to a wind-driven system. However, there is significant advantage to be had by removing the wind turbine gearbox and allowing direct drive air compression. There are also potential grid-balancing advantages to such a system, discussed later in this paper.

The system described in the remainder of this paper is not intended as a catch-all solution to the problem of inflexible wind generation. Indeed, it is a complimentary technology that might be used in tandem with a grid utilising any subset of systems previously mentioned, conventional wind turbines, and large-scale standalone storage (for example, in the form of CAES or green hydrogen).
