Gas turbine unite inlet air cooling by using an excessive refrigeration capacity of absorption-ejector chiller in booster air cooler

. Two-stage Gas turbine unite (GTU) inlet air cooling by absorption lithium-bromide chiller (ACh) to the temperature 15 °C and by refrigerant ejector chiller (ECh) to 10 °C through utilizing the turbine exhaust gas heat for changeable ambient air temperatures and corresponding heat loads on the air coolers for the south Ukraine climatic conditions is analysed. An excessive refrigeration capacity of combined absorption-ejector chiller (AECh) exceeding the current heat loads and generated at decreased heat loads on the air coolers at the inlet of GTU can be used for covering increased heat loads to reduce the refrigeration capacity of AECh. The GTU inlet air cooling system with an ambient air precooling booster stage and a base two-stage cooling air to the temperature 10 °C by AECh is proposed. The AECh excessive cooling capacity generated during decreased heat loads on the GTU inlet air coolers is conserved in the thermal accumulator and used for GTU inlet air precooling in a booster stage of air cooler during increased heat loads. There is AECh cooling capacity reduction by 50% due to the use of a booster stage for precooling GTU inlet ambient air at the expense of an excessive cooling capacity accumulated in the thermal storage.


Introduction
Gas turbine unit (GTU) inlet air cooling by waste heat recovery refrigeration machine (HRRM) which transforms the exhaust gas heat into the cold is one of the main trends in increasing GTU efficiency. Gas turbine inlet air can be cooled to the temperature t a2 ≈ 15 ºС by transformation of waste heat into the cold with a high efficiency in the most widespread absorption lithium-bromide chiller (ACh). Its coefficient of performance is ζ = 0.7...0.8 [1,2]. Deeper air cooling to the temperature t a2 = 10 ºС and below is possible in refrigerant ejector chiller (ECh). The efficiency of transformation of waste heat into the cold by ECh is much less: ζ = 0.2...0.3 [3]. Therefore it is rational to use two-stage air cooling at the GTU inlet: to t a2 = 15...20 ºС in the ACh and to t a2 = 7...10 ºC in the ECh, that is in the combined absorption-ejector chiller (AECh) [4,5]. The heat load on the air cooler (AC) at the GTU inlet changes significantly according to the current ambient air temperature t amb and relative humidity φ amb that leads to adequate change of AECh cooling capacity. A cooling capacity reserve (AECh installed excessive heat load value compared with current heat loads) is conserved during reduced heat loads. It is expedient to cover the deficit of cooling capacity at high loads, thereby reducing the ACh installed cooling capacity and it cost.
Objective: reduction of the installed absorption chiller cooling capacity by conserving an excessive cooling capacity at low current heat loads on the air cooler at the inlet of GTU and using the reserved cooling capacity for ambient air precooling at increased current heat loads.

Results of investigation
The GTU inlet air cooling efficiency can be conveniently estimated by the annual temperature-hour potential, ∏ Σ ,°С·h, which represents the air temperature drop Δt a at the GTU inlet and the duration τ of GTU operation at reduced temperatures [4] To determine the specific cooling capacity of twostage AECh as the total cooling capacity related to the unit of air flow (G a = 1 kg/s) it is necessary to analyze the annual temperature-hour potential ∏ Σ° dependence on specific cooling capacity q 0 . The specific cooling capacity is calculated as q 0 = ξ·с ma ·(t amb -t a2 ), kW/(kg/s), or kJ/kg, where ξ is the moisture coefficient; t amb -ambient air temperature, °С; t a2 -air temperature at the air cooler outlet, °C; с ma -moist air specific heat, kJ/(kg·K).
The project (installed) specific cooling capacity q 0 , on the one hand, should cover the GTU inlet air cooling need for as long time as possible during the year, which would have the greatest effect in the fuel saving. On the other hand, it should not be overestimated so that for most of the year the AECh was operated at thermal loads close to the project value, otherwise there would be an AECh operation at partial loads far from the project value. And on the contrary, with the lowered project q 0 there will be undercooling of the GTU inlet air at high ambient temperatures t amb . Fig. 1 shows the value of annual specific fuel saving b e depending on the specific cooling capacity values q 0 (with G a = 1 kg/s) for ambient air cooled to the temperatures t a2 = 15 ºC in ACh and to t a2 = 10 ºC in two-stage AECh for 2017 year, Yuzhnoukrainsk city, Mykolaiv region, Ukraine.  As can be seen from Fig. 1, for the climatic conditions of the GTU operation in the Yuzhnoukrainsk city, Mykolaiv region, 2017, a specific cooling capacity q 0 = 34 kW/(kg/s) (for G a = 1 kg/s) can be considered as rational project AECh specific cooling capacity for cooling ambient air at the inlet of GTU to the temperature t a2 = 10 ºС at which the annual specific fuel saving b e is kept closed to the maximum value at high enough rate of its increment. When air is cooled to t a2 º=15 ºС in ACh the rational project AECh specific cooling capacity is respectively q 0 = 24 kW/(kg/s). Proceeding from the rational specific cooling capacity q 0 the total installed cooling capacity of AECh in accordance with the air flow G a through the GTU is chosen as Since a decrease in the rate of the annual specific fuel saving increment b e at high cooling capacity indicates that there is an excess of cold, it is advisable to determine the cooling capacity, which ensures its maximum build-up rate, which will be less than q 0 = 24 kW/(kg/s) for t a2 = 15 ºС for ACh, as shown in Fig. 1.
To determine the cooling capacity, which provides a maximum build-up rate, it is necessary to analyze the annual specific fuel saving b e related to the specific cooling capacity q 0 , b e /q 0 , in dependence on the specific cooling capacity values q 0 (Fig. 2) for the cooled air temperatures t a2 = 10 and 15 ºC. The values of annual specific fuel saving related to the specific cooling capacity b e /q 0 in dependence on the project specific cooling capacity q 0 for the cooled air temperature t a2 = 10 and 15 ºС.
As can be seen from Fig. 2, for the climatic conditions of the GTU operation, the maximum rate of increment in the annual specific fuel saving b e /q 0 due to cooling (maximum of the graph) for the temperature of air t a2 = 15 ºС cooled in the ACh takes place at the project specific cooling capacity values q 0 = =12…16 kW/(kg/s), and when air is cooled to t a2 = 10 ºС in two-stage AECh q 0°=°2 2…26°kW/(kg/s). The corresponding total project (installed) cooling capacities, provide the maximum rate of the annual specific fuel saving increment.
Since the project (installed) specific cooling capacity values q 0 , which ensure the maximum rate of increment in the annual specific fuel saving b e /q 0 (Fig. 2) are less than their values determined in accordance with the maximum annual specific fuel saving b e in Fig. 1, then at an increased ambient air temperature t amb there will be a deficit of cooling capacity, while at low ambient air temperatures t amb , on the contrary, its excess. The excess of cooling capacity, generated during reduced heat load periods, is expedient to accumulate in the cold storage and use during increased heat loads in the pre-cooling air booster stage AC B of air cooler.
With this the reserve of ACh cooling capacity, generated during current reduced heat loads, is defined as its excess compared with the current heat load on the basic high-temperature stage AC HT and the pre-cooling air booster stage AC B , that is, on the heat exchanger "AC HT + AC B ". The ECh cooling capacity reserve is also defined as the project ECh heat load excess over the current heat loads on the low-temperature stage AC LT of air cooler and the total cooling capacity reserve of AECh is the sum of ACh and ECh reserves.
The total project heat load on the AC HT is defined as and is based on the AC HT project specific heat load q 0 . HT16pr = 16 kW/(kg/s). With this the project heat load on the AC HT with pre-cooling air booster stage AC B : where q 0.HT24pr = 24 kW/(kg/s) -the total specific heat load on both AC HT and AC B , that provides the maximum annual fuel saving due to GTU inlet air cooling to the temperature t HT2 ≈ 15 ºС in ACh according to Fig. 1 [3]. The air consumption is G a = 40 kg/s. The project heat load on the AC LT is defined as and based on the project specific heat load on the AC LT q 0.LT10pr = 10 kW/(kg/s) according to the maximum annual effect (Fig. 1).
In determining the ACh cooling capacity reserve, the current heat load on the AC HT with booster AC B is compared with the project heat load where q 0.HT24pr = 24 kW(kg/s).
Partial replacement of high-temperature stage AC HT by pre-cooling air booster stage AC B with corresponding reduction of the project cooling capacity of ACh is possible if there is a reserve of the ACh cooling capacity, accumulated in the cold storage, which exceeds its spending for the ambient air precooling in the AC B . In accordance with this, the ACh excessive cooling capacity (reserve) is defined as Q 0.HT16exc = Q 0.HT16pr -Q 0.HT24 = 640 -Q 0.HT24 , kW, and its deficit is respectively: The ECh excessive cooling capacity (its reserve): and its deficit: The total excessive cooling capacity (total reserve) is Q 0.ACexc = Q 0.HT16exc + Q 0.LT10exc , kW, and the total deficit: These balances coincide for properly selected the sum of project heat load on the basic high-temperature stage AC HT and the booster stage AC B for precooling air at the expense of the ACh cooling capacity reserve, as well as on the low-temperature stage AC LT at the expense of the ECh cooling capacity reserve. Of course, the cooling capacity reserve depends on the climatic conditions. The same can be said about its rational choice, taking into account the excess (reserve) of cooling capacity accumulated during a certain period. The convergence of cooling capacity excess with its deficit proves rational choice of project (installed) cooling capacity.
An example of implementing the approach to determine the booster stage AC B project heat load and a value of fuel saving due to pre-cooling air in the booster stage AC B with decreased basic high-temperature stage ACh project (installed) cooling capacity during 3 days (10-12.07.2017) is shown in Fig. 3.
As can be seen from Fig. 3, with the project heat loads 320 kW for pre-cooling air booster stage AC B , 640 kW for high-temperature stage AC HT and 400 kW for low-temperature stage AC LT , the spending of excessive cooling capacity accumulated over 3 days eliminates the ACh cooling capacity deficit, as it is proved by the change in the summarized total value of cooling capacity deficit: from ΣQ 0.ACdef = 2500 kW·h to ΣQ 0.ACAcdef = 0. The presence of cooling capacity significant excess after covering the deficit ΣQ 0.ACAcexc confirms the correct approach to choose a reduced project heat load for the basic high-temperature stage AC HT , and respectively the total heat load 1040 kW on the high-temperature stage AC HT and low-temperature stage AC LT (cooling capacity of ACh and ECh) with the use of a cooling capacity reserve for the booster stage AC B in comparison with the initial project cooling capacity 1360 kW of ACh and ECh (without use of the booster stage AC B ), as well as the availability of a cooling capacity reserve for further reduction of the project ACh cooling capacity.
In order to increase the efficiency of using the cooling capacity reserve, an option with a larger reduction of the project load on the high-temperature stage AC HT to 480 kW is calculated, which is half as much as the output of 960 kW, with an increase in the load on the pre-cooling air booster stage AC B to 480 kW while maintaining its value on the low-temperature stage AC LT (respectively on the ECh) 400 kW.
As can be seen from Fig. 3, with the project heat loads 320 kW for pre-cooling air booster stage AC B , 640 kW for high-temperature stage AC HT and 400 kW for low-temperature stage AC LT , the spending of excessive cooling capacity accumulated over 3 days eliminates the ACh cooling capacity deficit, as it is proved by the change in the summarized total value of cooling capacity deficit: from ΣQ 0.ACdef = 2500 kW·h to ΣQ 0.ACAcdef = 0. The presence of cooling capacity significant excess after covering the deficit ΣQ 0.ACAcexc confirms the correct approach to choose a reduced project heat load for the basic high-temperature stage AC HT , and respectively the total heat load 1040 kW on the high-temperature stage AC HT and low-temperature stage AC LT (cooling capacity of ACh and ECh) with the τ, day ϕ amb t amb ΣQ 0HT16def , ΣQ 0HT16exc , ΣQ 0HTAc16def , ΣQ 0HTАc16exc , ΣQ 0LT10def , ΣQ 0LT10exc , ΣQ 0LTAc10def , ΣQ 0LTAc10exc , ΣQ 0ACdef , ΣQ 0ACexc , ΣQ 0ACAcdef , ΣQ 0ACAcexc , 10 kW⋅h t amb , t HT2 , t a2 , 10 -1°С ; ϕ amb , % 0 3 6 9 12 15 18 21 0 3 6 9 12 15 18 21 0 3 6 9 12 15 18 21 0 τ, h Fig. 3. The current values of the ambient air temperature t amb , the temperature after AC HT t HT2 , after AC LT t a2 , relative humidity of the ambient air at the inlet φ amb , the total deficit ΣQ 0HT16def and the excess ΣQ 0HT16exc of the ACh cooling capacity (without using the cooling capacity reserve in the booster stage AC B of air cooler), the total deficit ΣQ 0HTAc16def and the excess ΣQ 0HTAc16exc of the cooling capacity of the ACh (with using the cooling capacity reserve in the booster stage AC B ), the total deficit ΣQ 0LT10def and the excess ΣQ 0LT10exc of the cooling capacity of the ECh (without using the cooling capacity reserve in the booster stage AC B ), a total deficit ΣQ 0LTAc10def and the excess ΣQ 0LTAc10exc of the cooling capacity of the ECh (with using the cooling capacity reserve in the booster stage AC B ), the total deficit ΣQ 0ACdef , and the excess ΣQ 0ACexc of the cooling capacity of two-stage air cooler (without using the cooling capacity reserve in the booster stage AC B ), and the total deficit ΣQ 0ACAcdef , and the excess ΣQ 0ACAcexc of the proposed air cooler cooling capacity (with using the cooling capacity reserve in the booster stage AC B ) within 3 days (10-12.07.2017).
use of a cooling capacity reserve for the booster stage AC B in comparison with the initial project cooling capacity 1360 kW of ACh and ECh (without use of the booster stage AC B ), as well as the availability of a cooling capacity reserve for further reduction of the project ACh cooling capacity. In order to increase the efficiency of using the cooling capacity reserve, an option with a larger reduction of the project load on the high-temperature stage AC HT to 480 kW is calculated, which is half as much as the output of 960 kW, with an increase in the load on the pre-cooling air booster stage AC B to 480 kW while maintaining its value on the low-temperature stage AC LT (respectively on the ECh) 400 kW.
The results of air cooling with project loads 480 kW for booster stage AC B , 480 kW for high-temperature stage AC HT and 400 kW for low-temperature stage AC LT 400 kW with using excessive cooling capacity accumulated at low heat loads for pre-cooling air in the booster stage AC B in the times of increased heat loads are presented in Fig. 4.
As can be seen from Fig. 4, with the project heat load 480 kW on the booster stage AC B , 480 kW for hightemperature stage AC HT and 400 kW for lowtemperature stage AC LT , the spending of cooling capacity reserve, accumulated over 3 days, eliminates the ACh cooling capacity deficit, as evidenced by a change in its value: from ΣQ 0.ACdef = 9500 kW·h to ΣQ 0.ACdef = 0. Accordingly, there is a reduction in the cooling capacity reserve after eliminating the deficit ΣQ 0.ACAcexc to about 15% of the initial version without the use of booster stage AC B , which confirms the possibility of reducing the half of the initial ACh project cooling capacity through the accumulation of excessive cooling capacity during reduced heat loads on the hightemperature stage AC HT with its subsequent use for booster stage AC B at high heat loads with practically the constant heat load on the low-temperature stage AC LT and the reduced total cooling capacity 880 kW of the ACh and ECh compared to the initial value 1360 kW (without the using the booster stage AC B of air cooler at the GTU inlet). Fig. 5 shows the current В е and the total ΣВ е fuel saving calculation results depending on the climatic conditions during 3 days for such a combined ambient air cooling at the inlet of GTU.
As can be seen from Fig. 5, the high-temperature stage AC HT provides a significantly higher amount of saved fuel, compared with the low-temperature stage AC LT that proves the correct choice of decreased project heat loads of the ACh. So, the results of simulation have shown that an excessive cooling capacity of combined absorption-ejector chiller above current heat loads, generated at the decreased heat loads on the air cooler at the inlet of GTU, can be used for covering the increased heat loads on the air cooler to reduce the cooling capacity of the absorption-ejector chiller installed by about 50%.
ΣQ 0ACAcexc =ΣQ 0LTAc10exc ΣQ 0HT12def , ΣQ 0HT12exc , ΣQ 0HTAc12def , ΣQ 0HTАc12exc , ΣQ 0LT10def , ΣQ 0LT10exc , ΣQ 0LTAc10def , ΣQ 0LTAc10exc , ΣQ 0ACdef , ΣQ 0ACexc , ΣQ 0ACAcdef , ΣQ 0ACAcexc , 10 kW⋅h t amb , t HT2 , t a2 , 10 -1°С ; ϕ amb , % Fig. 4. The current values of the ambient air temperature t amb , after AC HT t HT2 , after AC LT t a2 , relative humidity of the ambient air at the inlet φ amb , the total deficit ΣQ 0HT12def and the excess ΣQ 0HT12exc of the ACh cooling capacity (without using the cooling capacity reserve in the booster stage AC B ), the total deficit ΣQ 0HTAc12def and the excess ΣQ 0HTAc12exc of the cooling capacity of the ACh (with using the cooling capacity reserve in the booster stage AC B ), the total deficit ΣQ 0LT10def and the excess ΣQ 0LT10exc of the cooling capacity of the ECh (without using the cooling capacity reserve in the booster stage AC B ), a total deficit ΣQ 0LTAc10def and the excess ΣQ 0LTAc10exc of the cooling capacity of the ECh (with using the cooling capacity reserve in the booster stage AC B ), the total deficit ΣQ 0ACdef , and the excess ΣQ 0ACexc of the cooling capacity for two-stage air cooler (without using the cooling capacity reserve in the booster stage AC B ), and the total deficit ΣQ 0ACAcdef , and the excess ΣQ 0ACAcexc of the cooling capacity for the proposed air cooler (with using the cooling capacity reserve in the booster stage AC B ) within 3 days (10-12.07.2017).  Fig. 5. Current values of the ambient air temperature t amb , the temperature after AC HT t HT2 , after AC LT t a2 , relative humidity of the ambient air at the inlet φ amb , current fuel saving В е.HT for AC HT , В е.LT -for AC LT and В е.AC -for the whole AC, the total fuel saving for the high-temperature cooling stage ΣВ е.HT , ΣВ е.LT -for low-temperature cooling stage and the total fuel saving ΣВ е.AC -for the whole air cooler.

Conclusions
An improved two-stage GTU inlet air cooling to the temperature 15 °C in the high-temperature stage AC HT of air cooler by absorption lithium-bromide chiller (ACh) and to the temperature 10 °C in the low-temperature stage AC HT by refrigerant ejector chiller (ECh) through utilizing the turbine exhaust gas heat has been proposed. It is shown the possibility of decreasing the project (installed) cooling capacity of the ACh by using the excessive cooling capacity conserved during reduced current heat loads to cover increased heat loads through precooling ambient air in the booster stage AC B for twostage air cooler at the inlet of GTU.
For the south Ukraine climatic conditions the application of ambient air precooling booster stage for two-stage air cooler at the GTU inlet provides a reduction of the installed cooling capacity of ACh by about 50% due to the use of cooling capacity reserve to match the changeable heat loads caused by current climatic conditions.