In the following, simulation studies are conducted in order to identify cost reduction potentials and losses in round trip efficiency. Firstly, results are presented on the basis of the reference concept in order to evaluate the impact of thermal storage efficiencies on total round trip efficiency. Subsequently, the influence of the electrical heating power is investigated depending on its locations and the pressure level in the first stage, for the extended and alternative concepts. Finally, a comparison of the investigated concepts is presented.
4.1. Reference Concept
On the basis of the compact formulated thermal models in
Section 3, variation studies regarding the thermal storage efficiencies (η
LP-TES, η
HP-TES) are conducted for the reference concept, allowing to determine the total round trip efficiency, volumetric thermal storage densities, HP-TES heat exchanger size, charging–discharging fluid masses, and compressor powers.
For this purpose, the pressure in the first stage (p
LP) is set to 8.1 bar, leading to equal pressure ratios in all stages during compression. The resulting total round trip efficiencies as well as the thermal storage densities are illustrated in
Figure 3 for variations simulations relating to the thermal storage efficiencies.
The results show an increase in total round trip efficiency (η0) with increasing thermal storage efficiencies. Assuming a highly efficient HP-TES with ηHP-TES = 90%, up to 74.2% total round trip efficiency is achievable with LP-TES efficiencies of ηLP-TES = 95%. Simultaneously, a dramatic decrease in thermal storage density is visible for the regenerator-based LP-TES starting at ηLP-TES efficiencies of more than 90%. This characteristic is based on increasing requirements for high temporal averaged discharging fluid outlet temperatures, leading to a significant increased ratio of the iteratively calculated dimensionless parameters Π and Λ defined in Equations (1) and (2).
As illustrated in
Figure 3, higher LP-TES thermal storage densities are reached with lower HP-TES efficiencies. This system-related effect results from decreasing LP-TES inlet temperatures during discharging—thus increasing the total temperature spread—caused by lower HP-TES and HP-turbine outlet temperatures, respectively. At the same time, lower HP-TES efficiencies lead to decreasing HP-TES heat exchanger sizes (kO
TES-HTX,0) from 3100 kW/K at η
HP-TES = 90% to 860 kW/K at η
HP-TES = 70% and to decreasing HP-TES thermal storage densities from 63 kWh/m
3 at η
HP-TES = 90% to 49 kWh/m
3 at η
HP-TES = 70%, due to reduced temperature spread inside the liquid storage medium. Further effects are associated with TES efficiencies, for example, the resulting mass flow rates—and, thus, the compressor and cavern sizes—and need to be optimized in a techno-economic way.
In order to investigate the influence of the additionally implemented P2H option on total round trip efficiency and cost reduction potential, thermal storage efficiencies of 90% are chosen for the reference case. Central results regarding the round trip efficiency (η), volumetric thermal storage densities (Q/V), HP-TES heat exchanger size (kO
TES-HTX), charging–discharging fluid masses (m
F), and compressor powers (ΣP
Compressor) are summarized in
Table 2.
4.2. Extended Concept
The extended concept pointed out in
Figure 1 provides an additional electrical heating in the low- or in the high- pressure stages, as well as depending, proportionally, on both pressure stages. To identify suitable configurations with low losses in round trip efficiency and high potentials in component size reduction, simulation studies are performed related to the pressure in the first stage (p
LP) and the heating power and its distribution (x
LP,P2H) in the LP and HP ranges, respectively. Here, the LP pressure variations are limited by the maximum compressor outlet temperatures of 350 °C for both stages, leading to LP compression pressures between 5.3 bar and 12.4 bar and pressure ratios π between 0.7 and 1.5 bar compared to an equal pressure ratio in all stages of p
LP = 8.1 bar. Additionally, inside the simulation studies with varying electric heating powers, the maximum operational temperature for the HP-TES liquid storage medium of 400 °C is considered.
Due to highly linear behaviors with increasing electrical heating power, P2H-specific results are illustrated. These results—representing the effect of decreasing total round trip efficiencies and the potentials of cost reduction through lower component sizes—are compared directly with those of the reference case (index 0) without an additional electrical heating. Therefore, three potential heating power locations are investigated: the first, with uniform power distribution in both pressure stages (xLP,P2H = 50%), the second, only in the low-pressure (xLP,P2H = 100%) stage, and the third, only in the high-pressure stage (xLP,P2H = 0%).
Firstly, the influence on total round trip efficiency is shown in
Figure 4 related to the pressure level in the first stage (p
LP).
As illustrated by the thick line representing the reference case without an additional electrical heating, a nearly constant total round trip efficiency of 73.7% is visible, and only negligible effects are observed with varying low pressures. Those effects results from temperature- and pressure-dependent material properties in combination with heat losses inside the safety coolers.
Considering the influence of the investigated electrical heat locations, significant effects with varying low pressures are obviously. The results show the lowest reductions in specific total round trip efficiency of −0.34%/MWP2H for the integration location only in the high-pressure stage (xLP,P2H = 0%) at low pressures of 5.3 bar and with an opposed behavior of the integration location only in the low-pressure stage (xLP,P2H = 100%) at high pressures of 12.4 bar. These characteristics are based on increasing pressure spreads inside the HP and LP turbine during the discharging period, respectively, resulting in a higher exergetic utilization of the electrical heat. In contrast, a different behavior is visible for the integration option in both pressure stages with uniform power distribution (xLP,P2H = 50%). Here, the benefits regarding higher exergetic utilization with adapted pressures in the first stage are equalized through electrical heating power distribution, leading to a nearly constant loss of specific round trip efficiency of −0.38%/MWP2H.
With respect to the lowest specific losses in round trip efficiency with additional electrical heating, the results indicate, as integration location, only the low- or high-pressure range with adapted pressure levels in the first stage. To investigate the potential of cost reduction, specific results related to the cyclic stored air mass—as a relevant parameter for cavern and compressor size—as well as the thermal storage density for both options are illustrated.
Figure 5 shows the resulting cyclic stored air mass as well as its specific reduction with varying pressure in the first stage for the investigated three integration options.
Comparable with the mentioned results regarding the total round trip efficiency, a comparatively constant required air mass of 3450 t is needed for the reference case. Similar to
Figure 4, the highest specific reductions in cyclic stored air mass of −15.3 t/MW will be achieved with integration locations only in the low- (x
LP,P2H = 100%) and in the high-pressure range (x
LP,P2H = 0%), respectively, with simultaneous pressure adaptions in the first stage. According to the defined boundary conditions and specifications, the specific reductions in cyclic storage mass correspond to smaller cavern and compressors sizes of up to −0.5 MW
Compressor/MW
P2H.
In addition, further potentials in cost reduction are linked to electrical heating. These are shown in
Figure 6 with varying low pressures related to both thermal energy storage options: left for the TES option in the first stage, and right for the TES option in the second stage.
Concerning the reference case without additional electrical heating, indicated by the thick lines, higher thermal storage densities are visible with increasing low pressure levels for the LP-TES (
Figure 6 left) and with decreasing low pressure levels for the HP-TES (
Figure 6 right). Both result from higher compression outlet temperatures leading to a higher temperature spread inside the sensible storage media.
With a further temperature elevation due to electrical heating, different effects on specific thermal storage density for both TES options are visible, mainly influenced by the P2H integration locations. For electrical heating location only in the low-pressure stage (x
LP,P2H = 100%), an additional increased specific thermal storage density of 2.3 kWh/m
3/MW
P2H is reached inside the LP-TES (
Figure 6 left), whereas no effects occur inside the HP-TES (
Figure 6 right) due to the integrated safety cooler. In contrast, with increasing share of electrical heating inside the high-pressure stage, higher specific thermal storage densities of up to 1.65 kWh/m
3/MW
P2H are achieved in the HP-TES, but with simultaneous lower increasing specific thermal storage densities in the LP-TES. Actually, regarding the limit case of pure electrical heating in the high-pressure stage (x
LP,P2H = 0%), negative specific thermal storage densities of –1.3 kWh/m
3/MW
P2H are visible inside the LP-TES (
Figure 6 left). This behavior is caused by electrical heating inside the HP-TES during the charging period, leading consequenly to increased HP-turbine outlet temperatures during the discharging period and, thus, to decreasing temperature spreads inside the LP-TES. This drawback prohibits cost reductions for all components, in spite of the additional achieved specific reductions of the HP-TES heat exchanger size of up to −18 kW/K/MW
P2H.
Additionally, restrictions occur for the permitted electrical heating power in the high-pressure location to fulfil the requirements of maximum operation temperature of 400 °C for the liquid storage medium. These operational limitations are illustrated in
Figure 7 for the investigated pressure range inside the first stage.
As can be seen, higher maximum electrical heating powers (PP2H,HP-TES,max) are feasible in the HP-TES with increasing low pressure levels. This results from decreasing pressure ratios between the first and second stage, accompanied by lower HP compressor outlet temperatures.
On the basis of the presented effects, the results strongly suggest that the electrical heating system must be located in the first pressure stage to allow significant cost reduction potentials for all components without drawbacks inside the cyclic operation. Additionally, in order to minimize the specific losses in total round trip efficiency through the P2H option, the pressure level in the first stage must be chosen as high as possible. With this background, an alternative concept is elaborated, offering an efficient P2H utilization and further cost reduction potentials.
4.3. Alternative Concept
In contrast to the previous system configuration, the alternative concept is based only on one thermal energy storage system (
Figure 1c). Here, the heat during compression in the low-pressure stage—additionally elevated by the P2H element—is stored in the regenerator-based heat storage and cooled down during compression in the second stage. Differing from the previous concept, the temperature of the pressurized air leaving the cavern during the discharging mode is elevated by the stored heat via a heat exchanger and expanded over the whole pressure range to ambient conditions.
Here, to prevent freezing problems during the discharging period inside the turbine, a minimum of electrical heating power (PP2H,min) is required. Despite the associated losses in round trip efficiencies caused by the exiting compression heat in the second stage, high exergetic utilizations are feasible due to the large discharging pressure ratio, as well as cost reductions due to the missing thermal energy storage system in the high-pressure range.
According to the defined boundary conditions and specifications in
Section 4.2, variation studies related to the low-pressure levels (p
LP) are conducted and evaluated with regard to total round trip efficiency and P2H specific results (
Figure 8). To prevent freezing problems, the pressure-dependent total round trip efficiencies η are calculated iteratively to determine the minimum electrical heating power leading to turbine outlet temperatures at ambient condition. The simulation results are based on the regenerator model described in
Section 3.1, which is extended by the NTU model for the heat exchanger. Similar to the previous concept, the thermal storage efficiencies concerning the LP-TES and the heat exchanger are set to a value of 90 %.
As illustrated in
Figure 8, increasing total round trip efficiencies results in higher pressure levels in the first stage due to the associated decreasing high-pressure compression ratio and, thus, lower heat losses via the aftercooler. Considering the defined specifications and the maximum compressor outlet temperatures of 350 °C, maximum total round trip efficiencies of 58% are achieved. Comparing these characteristics with the results in
Section 4.2, significant lower efficiencies are reached. Although improvements in total round trip efficiency are feasible up to 5% through a two-stage high pressure compression with internal cooling, distinct differences with respect to the adiabatic CAES case remain.
In spite of this characteristic, advantages of the electrical heating are visible. Compared with the previously mentioned concept, significant lower P2H specific losses in total round trip efficiency (Δη/ΔPP2H) occur, showing an opposed behavior with increasing low-pressure levels. This effect is based on a pressure-dependent thermodynamic change in the state of the heat-transfer fluid: with a fixed elevation of enthalpy through the P2H option, higher temperature differences are achieved at low pressure levels, leading to higher turbine outlet powers during discharging at the defined high pressure. This behavior shapes the characteristics of the P2H-specific losses in total round trip efficiency.
In spite of this behavior, highly efficient exergetic P2H utilization is achieved with the alternative concept compared with the previously mentioned concept. Further results regarding the component sizes and the minimum electrical heating power to prevent freezing problems are summarized in
Table 3 and
Table 4 for a first-stage compression pressure of 12.4 bar.
Concerning the significant higher cost reduction potentials, the low complexity, and the efficient P2H utilization, the alternative system can be treated in spite of the lower total round trip efficiencies as a base for upgrading the existing CAES power plants and for modifying operational concepts.