In the present study, to limit the analysis to high temperature levels and large scale plants, the attention is only focused on the central receiver technology.
Thermodynamics and Possible Process Configurations
The basic idea for a TCS system is to use a reversible chemical reaction, typically involving solid species, with a high reaction enthalpy:
During the charging phase, the solar energy is used for the endothermic reaction to convert the solid
R in solid
C and gaseous product
G with higher enthalpy, which are then stored separately. In the discharging phase,
C and
G are contacted again under suitable temperature and pressure conditions, so that the reverse reaction occurs with the release of heat. From a thermodynamic point of view, according to the phase rule, the reactive system at the equilibrium has one degree of freedom, i.e., it is possible to identify on the T-P plane a single equilibrium curve, as reported in
Figure 1a,b. If the point corresponding to the temperature and partial pressure of
G lies below the equilibrium curve, the endothermic decomposition of
R occurs, with a driving force given by the difference
and a positive heat of reaction; on the other hand, if the operating point lies above the equilibrium curve,
R will be produced with a driving force given by
and the release of heat of reaction. Therefore, it is possible to realize a cyclic process, alternating endothermic charging and exothermic discharging phases, either in thermal swing (
Figure 1c) or pressure swing mode (
Figure 1d).
It is evident that a reaction is suitable for application in a thermochemical storage process if it is strongly endothermic and the equilibrium constant is high at the temperature of the solar technologies employed. When operating in the thermal swing mode, thermal energy is released at a temperature lower than that of the charging phase. It is advisable that this temperature gap be as low as possible, in order to have a high global exergetic efficiency; indeed, it is possible to modulate the partial pressure of the gaseous component to reduce the thermal gap between the charging and discharging phases and to adjust the reaction temperature according to the process specifications.
The reactive material may be confined in an adiabatic reactor (fixed bed or fluidized bed) or can circulate within the plant to be directly irradiated into the solar receiver (
Figure 2). In the first case, the heat required by the endothermic reaction is transferred to the chemical system by the process fluid in an adiabatic reactor, whereas in the second case the concentrated solar radiation directly invests the reacting material. Clearly, the directly irradiated system virtually enables to store solar heat with high global exergetic efficiency, while the use of an adiabatic reactor, which degrades the quality of the solar energy stored due to the inevitable reduction in the operating temperature levels, leads to a simple and robust plant configuration.
The chemical plant may have a closed or open configuration: in the first case the reacting materials are cyclically produced, stored, and consumed, while in the second case, the reaction products are removed from the plant. The open loop option is particularly suitable for oxi-reduction thermochemical systems, especially if air represents the heat transfer fluid: the oxygen produced by the reduction step can be released into the atmosphere, with undeniable advantages for plant simplicity.
With regards to the integration between the thermochemical storage unit, the CSP plant, and the power block, two main configurations are possible, if the solar energy is captured, stored, and used in the same site: series (
Figure 3) and parallel (
Figure 4) schemes.
The option of connection in series requires, in the charging phase (
Figure 3a), different and complementary temperature ranges for the storage unit (
T3 −
T2) and the power block (
T4 −
T3): the TCS unit is powered by high-temperature heat while the power unit is powered at a lower temperature level. Therefore, in the heat releasing step, the operating conditions of the power block may remain virtually unchanged; in fact, maintaining the same operating pressure, the thermal level of the exothermic reaction (discharging) is always lower than the temperature of the endothermic reaction (charging).
In the parallel scheme (
Figure 4) the temperature ranges of the thermochemical storage and the power block have to be quite aligned (
T3 −
T2 ≈
T5 −
T4) to keep an overall high exergetic efficiency. Compared to the series integration scheme, this process option provides a higher flexibility in the repartition of the stored power
Pst and the consumed power
Put, defined by the following equations:
where
T1 is the temperature at the outlet of the solar receiver,
T2 and
T3 are the temperatures of the streams entering and exiting the storage unit, respectively, and
T4 and
T5 are the temperatures of the streams entering and exiting the power block, respectively;
cg is the heat capacity of the heat transfer fluid,
mst and
mut the inlet mass flow to the TCS and the power block, respectively,
mtot the mass flow entering the receiver, corresponding to the sum of
mst and
mut, and
Ptot the power absorbed by the receiver (
Pst +
Put). The ratio between
Pst and
Put can be modulated through the regulation of the mass flow
mst (=m2) and
mut (=m4).
In practice, the two temperature ranges,
T3 −
T2 and
T5 −
T4, cannot be homogeneous since
T3 is usually greater than
T5. This means that the temperature of the stream entering the solar field (
T6) is an intermediate value between
T3 and
T4: neglecting the variability of heat capacities with temperature and the possible change in composition of the
mst e
mut streams, the temperature
T6 can be calculated through the following relation:
From the previous equations it is possible, having fixed the target Pst/Put ratio, to identify the corresponding mst/mu ratio at various temperature differences ΔTst (T3 − T2) and ΔTu (T5 − T4).
In the discharging phase (
Figure 4b) the power block can elaborate the same temperature differences as the charging step only through the increase of the operating pressure, if feasible.
An “optimized” variant of this scheme can use different reactive materials, arranged in series and operating in complementary thermal levels, in order to make the
T4 −
T2 and
T5 −
T6 temperature ranges homogeneous. The stratification of the materials could take place in the same reactor or in separate systems, depending on the operating pressure and system requirements. Another possibility is the introduction of a latent or sensible heat storage downstream of the thermochemical accumulation to cover the whole temperature range available. Finally, an alternative integration solution consists in a combination of schemes in series and parallel, as shown in
Figure 5: the alignment in series of the chemical reactor and power block can achieve similar enthalpy changes between streams 2 and 4 and streams 5 and 6, while the parallel connection between the main power block and the TCS unit can introduce a margin of flexibility in the plant operation. This implies, however, a different conversion efficiency of the two power blocks, which operate at different temperature levels.
The selection of the best interface between the CSP plant and the suitable reacting systems clearly requires, in addition to the in-depth thermodynamic analysis here presented, an accurate kinetic characterization of the reacting media, a detailed process design, and a realistic cost estimation, which go beyond the objective of this work. In the following, therefore, some potential integration options are considered, with the identification of possible reacting materials and the definition of preliminary process schemes, together with the identification of the operating conditions of the TCS charging and discharging steps, mainly on the basis of thermodynamic considerations.
The proposed integration schemes refer to steady state conditions and do not take into account transients operations (start-up, shut-down and off-design conditions). Therefore, in the discharging phase, the initial transient heating step, most probably carried out through the use of an external back-up heater, is not taken into account.
For the identification of reactive systems compatible with the central tower technology [
27], a thermodynamic analysis is reported in the following paragraph. Regarding the preliminary definition of the operating conditions of the charging and discharging steps, in each considered case the process simulation of the entire system was carried out using the simulation tool Aspen Plus. In particular, from the process analysis, suitable values of the following operation parameters related to the TCS unit were identified:
- -
Inlet pressure Ptot,in;
- -
Inlet temperature Tin;
- -
Outlet temperature Tout;
- -
Ratio of the thermal energy stored/released to the molar inlet flow Qst/mst; and
- -
Ratio of the molar inlet flow to the solid reaction rate mst/vrx.
The latter parameter can be taken as a reference, in the future, for calculating the duration of the TCS unit operation knowing the mass flow circulating in the solar system and the associated absorbed/released power. Indeed, the objective of this exploratory phase of the work is limited to the identification of the operating characteristics of the TCS system regardless of the solar plant dynamics and control logic, this topic being the aim of a more focused and detailed future analysis.