Any type of power generation system that relies on wind for energy generation requires oversizing to achieve a high level of demand satisfaction. This is owed to the stochastic nature of wind energy. In order to avoid expensive oversized Wind-CAES configurations, an alternative solution is proposed. More precisely, to counterbalance the need for extreme wind power and energy storage capacity, a dual-mode CAES plant is adopted. It has the ability to switch its operation from the CAES mode to the traditional gas-turbine cycle with the addition of a second compression system and the help of a clutch that allows connection between the gas-turbine and the compressor. In this way, the system can appreciate increased levels of energy autonomy, although of course relying on natural gas, without having to oversize wind capacity and storage volume. The same configuration also makes sense in the case of smaller-scale systems, not necessarily destined to support 100% energy autonomy of such an island system.
Instead, they can be used as a private-owned power producing units, following the dispatch strategy developed in section 3.3 of the thesis. In such a case, the dual-mode CAES system operator could cover guaranteed output requirements on the basis of wind energy and natural gas, relying also on the ability to switch to the classical Brayton cycle, opposite to the scenario of thematic section 3.3 where the system, in the absence of sufficient wind energy surplus, draws conventional power directly from the grid. This normally implies exposure to higher costs since even if off-peak, oil-based grid power is used to charge the system, the use of primary natural gas fuel to operate the gas turbine would normally be more cost-effective, especially if also considering storage losses introduced in the first case.
The proposed system (see also Figure 4.1) comprises of the following components:
Figure 4.1: The proposed dual-mode Wind-CAES system
A wind farm that includes a number of wind turbines with total capacity "NWP".
A CAES motor of rated power "Nm", used to exploit any wind energy surplus and
feed the compressor under an efficiency of "ηm".
A multi-stage compressor, used in the CAES cycle to compress ambient air into the air cavern/tank, under a given pressure ratio "rc". Similar to the case of the motor,
the compressor power "Ncr-CAES" is determined in relation to the maximum wind
energy surplus appearing, i.e. "NW-Nd", taking also into account any energy losses
induced by the motor. "NW" represents the mean hourly wind farm power output
and "Nd" the mean hourly load demand.
A second compression system, operated in the case of the dual-mode cycle execution, i.e. when energy deficit appears and the combined Wind-CAES system is not able to cover it. Its rated power is "Ncr-dual" and its pressure ratio is "rc΄".
A storage cavern or tank of maximum volume storage "Vss" and maximum depth of
discharge "DODL", determined by the ratio of [(rc-rt)∙rc-1], where "rt" is the pressure
ratio of the gas-turbine. The approach currently adopted concerns constant storage volume and sliding pressure, with the latter allowed to reduce up to the minimum permitted level determined by the expansion ratio of the gas turbine. To this end, the pressurized air outlet is controlled by the introduction of constant pressure valves that allow supply of pressurized air under constant pressure that is set to align with the expansion ratio of the gas turbine. Given the gradual reduction of the
pressurized air mass inside the storage cavern/tank, the pressure reduces proportionally, not allowed to violate the maximum "DODL" condition.
A combustion chamber, where the required amount of compressed air and natural gas are mixed for the production of gases that will operate the gas-turbine under a maximum permitted temperature of "Tcc".
A natural gas tank, used for fuel storage and the fuel’s calorific value (CV) "Hu".
A gas-turbine of power output "Ngt-f", determined after considering the maximum
appearing deficit in the case of both the CAES "Ndef" and the dual-mode "Ndef΄"
cycle, that is connected to an electrical generator responsible for the delivery of electrical energy to the demand side.
The main variables taken into account are the wind farm capacity and storage volume, while detailed wind speed and ambient temperature-pressure data alongside hourly electricity load are required. At the same time, the technical characteristics of the main system components are also required (Table 4.1). Finally, to simulate operation of similar systems, a sizing algorithm was developed in C# (Figures 4.2 and 4.3).
The operation scenarios of the proposed configuration are the following (Figure 4.2): When wind energy production is sufficient to meet demand, wind energy is fed
directly to the local consumption. Any potential energy surplus is used to compress air inside the cavern/tank, provided that the latter is not full. If the latter is full, then a second-level wind energy surplus appears, which if possible, can be exploited to operate secondary loads, electric vehicles, desalination plants, etc. During this stage, and given also the maximum appearing wind energy surplus exploited for compression, the nominal compression power required is estimated. This of course could suggest oversizing of the compression side in many cases, which could be avoided if treating compression power as an additional problem variable.
When wind energy production is not sufficient to meet demand, the required amount of compressed air and fuel are used in order to operate the gas-turbine. In that case, the CAES cycle is operated, exploiting wind energy stores together with natural gas, under a reduced heat rate, provided of course that the maximum depth of discharge condition is not violated. Given also the maximum appearing energy deficit or residual load to be covered, the nominal gas turbine power is determined for the case of the CAES cycle, while, simultaneously, the fuel consumption required to operate the gas turbine is also recorded.
When the combined operation of wind energy and CAES is not able to meet demand, the energy deficit is covered by the dual-mode system operation of CAES, i.e. the gas-turbine is clutched to the dual-mode compressor, under a different heat rate in comparison to the CAES cycle. In that case, fuel consumption of natural gas increases, while the size of the gas turbine is challenged by the need to also operate the compression side. To this end, the gas turbine power required to operate the typical Brayton cycle is also determined and is then compared with the respective size required for the CAES operation in order to define the final power required for the gas turbine side. At the same time, the fuel consumption under the operation of the dual-mode cycle is also recorded.
Figure 4.2: The Wind-CAES-DM-2 algorithm
In this context, for a fixed wind farm capacity and storage volume, the annual hours of load rejection are recorded and to minimize load rejection the storage capacity is gradually increased within a predefined range. Furthermore, when energy autonomy is not achieved, the wind park capacity is increased, until 100% energy autonomy is made possible relying only on the Wind-CAES solution. The obtained results include the complementary energy (fuel consumption) required by the dual-mode CAES cycle in case that 100% energy autonomy is not achieved by the original Wind-CAES system, emphasizing the fuel savings achieved by the system operation.
Figure 4.3: Screen-shots of the Wind-CAES-DM-2 algorithm
Table 4.1: Energy-related problem inputs (Cavallo, 2007; Jubeh and Najjar 2012; Kim
et al., 2011; Lund and Salgi, 2009; Lund et al., 2009; Zafirakis and Kaldellis, 2010)
Parameter Symbol / Unit Assigned Value
Compressor isentropic efficiency ηisc 0.85
Gas turbine isentropic efficiency ηisT 0.88
Compressor mechanical efficiency ηmc 0.98
Gas turbine mechanical efficiency ηmc 0.98
Motor efficiency ηM 0.98
Electrical generator efficiency ηgen 0.98
Storage temperature Tcav (K) 300
CAES compressor pressure ratio rc 75
Dual-mode compressor pressure ratio rc΄ 32
Gas turbine pressure ratio rt 30
Specific heat capacity of air CpA (J/kg/K) 1004.5
Specific heat capacity of gases CpR (J/kg/K) 1105
Air ratio λa 4
Mass of air for stoichiometric combustion ma (kg/kgNG) 15
Combustion chamber max operational temperature Tcc (K) 1200
Air constant Rg (J/kg/K) 287