Advanced Method of Variable Refrigerant Flow (VRF) System Design to Forecast on Site Operation—Part 3: Optimal Solutions to Minimize Sizes

: Outdoor air conditioning systems (ACS) are used as autonomic systems as well as in combined outdoor and indoor ACS of the variable refrigerant ﬂow (VRF) type, with variable speed compressors (VSC) as their advanced version. Methods for determining the optimal value of refrigeration capacity and providing the maximum rate of the summarized annual refrigeration energy generation increment, according to its needs at minimum compressor sizes and rational values, are applied to reveal the reserves for reducing the designed (installed) refrigeration capacity, thus enabling us to practically achieve maximum annual refrigeration energy generation as the primary criterion at the second stage of the general design methodology previously developed by the authors. The principle of sharing the total thermal load on the ACS between the ranges of changeable loads for outdoor air precooling, and a relatively stable load range for further processing air are used as its basis. According to this principle, the changeable thermal load range is chosen as the object for energy saving by recuperating the excessive refrigeration generated at lowered loading in order to compensate for the increased loads, thereby matching actual duties at a reduced designed refrigeration capacity. The method allows us to determine the corresponding level of regulated loads (LRL) of SRC and the load range of compressor operation to minimize sizes.


Introduction
Ambient air conditioning systems (ACS) are desired to provide comfortable environments in buildings [1,2] and other stationary objects [3,4].They are widespread in transport application, in particular in railways [5,6] and ships [7,8].As the air is a working fluid (cyclic air) for combustion, ACS are used for cooling air sucked out of combustion engines, these being internal combustion engines [9,10], gas turbines [11,12] and gas engines [13,14].The latter are effective in the development of engine intake air conditioning systems as subsystems of trigeneration (in-cycle trigeneration) [15,16] and integrated power plants [17,18] for combined cooling, heat and power (CCHP) generation [19,20].In these cases, the ACS function as waste heat recovery systems [21,22] and their cooling potential depends on a depth of engine exhaust heat utilization [23,24]; the deeper the exhaust heat utilization, the higher the thermal (refrigeration) potential of the ACS.
Such mutual penetration of ACS into power plants and, conversely, the use of energy heat exhausts as a thermal source for ACS reveal the potential for application of the principal findings gained in energetic application to ACS.Therefore, evaporative cooling [25,26], two-stage cooling air [27,28] by chilled water and refrigerants coolants [29,30] in hybrid coolers [31,32] and combined chillers of different types [33,34], including absorption [35,36] and refrigerant [37,38] chillers as cascade chillers [39,40], and excessive heat recuperation to cover peak loads [41,42] were implemented into ACS as two-stage outdoor air conditioning systems that use refrigeration recuperation [43,44]; they are especially efficient for combined outdoor and indoor air-processing unites [45,46].The outdoor unit is desired for conditioning the ambient air in order to avoid fluctuated heat loads and overloading the indoor unit [47,48].The VRF systems save more than 20% energy compared to the variable air volume ACS [49,50].
A performance of ACS is characterized by off-design modes which are especially evident in temperate climatic conditions and off-season operation.Therefore, the VRF systems are the most well adapted to cover efficient off-season operation [51,52].
The varying heat loads on ACS and heat exchangers accordingly are accompanied by heat flux drops which require application of efficient heat exchangers and working fluid circulation circuits.The application of high-efficiency heat exchangers [53,54], in particular compact evaporators [55,56] and condensers [57,58], accelerates research focused on intensifying heat transfer [59,60] and hydrodynamics [61,62] to mitigate the instabilities of two-phase refrigerant flows [63,64] and uneven refrigerant [65,66] and air [67,68] distribution.Advanced air conditioning, refrigerant feeding and exhaust heat recovery circuits, in particular with the application of ejectors [69,70] and thermopressors [71,72], as circulation devices which use potential energy and exhaust heat [73,74], were developed.
Methods for determining the rational value of the design refrigeration capacity enable us to achieve practically maximum annual fuel saving [81,82] as well as refrigeration energy generation according to current need, as the primary criterion and its optimal value, providing the maximum rate of the summarized annual refrigeration energy generation increment at minimum compressor sizes, was previously developed by the authors [83,84].The general methodology of rational designing also includes the rational distribution of the overall current thermal loads in the ranges of changeable loads for ambient air preconditioning, and a relatively stable load range for further air subcooling from a threshold temperature to the set value.
The method for estimating the SRC compressors performance efficiency by comparing the load ranges of regulated and unregulated by SRC with the ranges of changeable and unchangeable loads was developed by the authors earlier.With this, the efficiency of SCR operation is estimated by the loading rate of the unregulated (stable) range assumed as the object of investigation [84].
It is quite evident that the range of unstable (regulated) loads can be accepted as the object for the recuperation of excessive refrigeration to cover the current increased loads that result in the reduction of the range needed for load regulation.
In reality, the SRC is the oversized (underloaded) compressor that operates efficiently at part loads.With a higher level of regulated (changeable) loads (LRL) of the SRC, there is more oversizing of the compressor.It is quite preferable to reduce the SCR sizes through narrowing the range of changeable thermal loads, leading to a decrease in the LRL of the SRC applied.In its turn, in concrete site climatic conditions, the magnitude of the changeable thermal load range depends on the exceedance of the design refrigeration capacity over current loads.
The lesser the exceedance of the design refrigeration capacity (the higher the level of loading LL), the narrower the changeable (booster) thermal load range.
Thus, reduction in the SCR size may be possible due to reduction in the exceedance (excess) of the design refrigeration capacity through its use to cover the pick loads that leads to shortening of the changeable load range due to its part-stabilization [85].The level of loading LL is a criterion for shearing the total range of loading q 0 to the ranges of changeable (1 − LL) q 0 , and unchangeable LL q 0 loads, and is calculated as the ratio of unchangeable load LL q 0 to the total load q 0 .The enhancement of the ACS operation efficiency is focused on raising the level of load LL followed by reducing the range of changeable load (1 − LL)q 0 while correspondingly growing the range of unchangeable load LL q 0 .
The maximum rate of the summarized annual refrigeration energy generation increment according to its consumption (as the maximum level of loading LL) is associated with a threshold (optimal) value of refrigeration capacity as the minimum permissible value of the design refrigeration capacity [84].
The further reduction of the design refrigeration capacity to less than its threshold (optimal) value at the maximum rate of the annual refrigeration increment is unreasonable, because it propagates an unchangeable (stable) load range.The operation in the unchangeable load range is characterized by full loading of the compressor and the ACS as a whole (LL = 1).
The rational value that enables us to achieve close to maximum annual refrigeration energy generation as a primary criterion is determined at the second stage of the general design methodology developed by the authors [84].
It is quite evident that an increase in the design refrigeration capacity from its optimal value as a minimum to the rational value, providing maximum refrigeration energy generation in response to current need, is accompanied by widening the changeable (requiring the regulation by SCR) load range and an increase in the exceedance (excess) of the design refrigeration capacity available for recuperation leading to part-stabilization of initially changeable load range for outdoor air preconditioning [85].
Thus, it is quite reasonable to assess the application of both methods to reveal the reserves for reducing the design (installed) refrigeration capacity of ACS and the level of regulated loads (LRL) of SRC, as well as the load range of compressor operation.These reserves can be evaluated by comparing the exceedance of the installed (design) refrigeration energy determined according to both methods and conserved at lowered actual loads with current need.Their realization is made possible by recuperating the excess refrigeration energy to enhance the operation efficiency of the advanced VRF system with modern SRC to minimize the oversizing.
The object of the research is the range of unstable (regulated) loads within the overall range of actual loading as the source of exceedance (excess) of refrigeration to cover the current increased loads, resulting in a reduction in the range of necessary load regulation.
The aim of the research is to reveal the reserves for reducing the design (installed) refrigeration capacity of ACS, determined by different methods, to provide maximum annual refrigeration energy generation or a maximum rate of its increment (rational and optimal values), and to realize them through recuperation of the excessive refrigeration to cover the current increased loads, resulting in a reduction in the range of unstable (regulated) loads, the level of regulated loads (LRL) of SRC and the load range of compressor operation.
The following tasks are to be solved to reach these aims: -Determine the ranges of changeable thermal loads for optimal and rational refrigeration capacities of ACS, calculated according two methods of providing the maximum rate of the summarized annual refrigeration energy generation increment, or providing close to maximum refrigeration energy generation; -Develop a method to determine the range of artificially stabilized loads due to recuperation of excessive refrigeration energy, reserved at lowered current loads, to cover peak loads and the rest of the range of unstable loads regulated by SRC, thereby defining the level of regulated loads (LRL) of SRC and the load range of compressor operation.
These reserves are evaluated by comparing the exceedance of the installed (design) refrigeration energy conserved at lowered actual loads, according to both methods with current need.

Methods
In order to generalize the results and adopt their application for ACS of any sizes (refrigeration capacity Q 0 according to air mass flow rate G a ), they are presented in relative values as specific refrigeration capacity q 0 , id est, referred to in the unit of air mass flow rate (G a = 1 kg/s): and calculated as where t a -initial or ambient t amb air temperature, K or • C; t a2 -a set air temperature; ξ-relative heat ratio as the total heat, removed from the air, related to its sensible heat; c a -air specific heat, kJ/(kg•K).
The summarized annual refrigeration energy generation in response to consumption is accepted as a primary criterion.The corresponding specific annual energy generation is calculated as In order to avoid the errors of about 20% caused by approximation of the actual changeable thermal loads and corresponding required refrigeration capacities, their fluctuations are considered by the rate of their summarized annual value increment versus the refrigeration capacity q 0 used as a cumulative annual refrigeration energy characteristic: Such an approach allows us to use the rate of their summarized annual values increment in response to refrigeration capacity q 0 as the indicative criterion Σ(q 0 •τ)/q 0 to choose the optimal value of refrigeration capacity q 0.opt corresponding to its maximum.
The same indicative criterion is applied to determine the precise value of rational refrigeration capacity q 0.rat within the range of cumulative annual refrigeration energy characteristics above the optimal value q 0.opt to avoid the overestimation of refrigeration capacity accompanied by a negligible increment of annual output.
The level of load (LL) on ACS proceeding from the summarized annual refrigeration energy can be applied as a modified criterion in contrast to the current level of load LL cur used in conventional practice: LL 10rat = (q 0.10rat − q 0.15rat )/q 0.10rat ≈ 0.3, where q 0.10rat − q 0.15rat is the range of stable thermal load and q 0.15rat is the range of changeable thermal loads.
The maximum value of the indicative criterion Σ(q 0 •τ)/q 0 reflects the maximum rate of the summarized annual refrigeration energy increment, and naturally, the minimum deviation of the optimal value of refrigeration capacity q 0.opt from the current loads q 0 , followed by minimum exceedances of q 0.opt over q 0 .Therefore, the rational value of refrigeration capacity q 0.rat , being higher than q 0.opt , is characterized by larger deviation from the current loads q 0 and exceedances (excesses) of q 0.rat over q 0 .The latter are considered the reserves for refrigeration exceedances (excesses)' recuperation to cover peak loads and reduce a design refrigeration capacity less than q 0.rat .The refrigeration exceedances (excesses) are associated with changeable load range of the total one.Therefore, the range of changeable loads is considered the object for partly stabilizing due to refrigeration exceedances (excesses) recuperation that leads to a reduction in its value and the total design refrigeration capacity q 0.rat as result.
The residual part of the range with initially changeable loads becomes considerably narrower than the primary one that leads to reducing the ratio of the changeable load range, covered by the RSC, to the overall load range, id est. the required level of regulated loads (LRL).

Results and Discussion
According to the aim of the research, the range of changeable loads is considered as the object for partly stabilizing due to refrigeration exceedances (excesses)' recuperation in order to reduce its value and the total design refrigeration capacity q 0.rat as result.Such an approach, substantiated by the results of calculation of the summarized annual refrigeration energy generation Σ(q 0 •τ) and optimal q 0.opt and rational q 0.rat refrigeration capacities (Figure 1), should be proven by the exceedances (excesses) of design refrigeration capacities q 0.opt and q 0.opt over current loads q 0 , and corresponding monthly summarized values of the refrigeration energy exceedances Σ(q 0 •τ) within initial changeable load range q 0.15 reduced by its partially stabilization due to refrigeration energy exceedance's recuperation in the range to q 0.20 .
The total values of specific refrigeration capacities q 0.10 needed for conditioning outdoor air to 10 • C can be sheared into the range of changeable values q 0.15 , which are needed for preconditioning outdoor air to 15 • C. Practically unchangeable refrigeration capacities q 0.10-15 are needed for subsequent conditioning of air from 15 • C to 10 • C. The calculation results for July 2017 in climatic conditions in southern Ukraine, Mykolayiv region, as example of temperate climate, are presented in Figure 2.
As Figure 2 shows, the current total changeable heat load q 0.10 for conditioning outdoor air to 10 • C can be shared in the range of changeable load for preconditioning outdoor air to 15 • C, and in the range of practically unchangeable heat load q 0.10-15 for subsequent conditioning air from 15 • C to 10 • C. Accordingly, the latter is accepted as the basic practically unchangeable part, q 0.10-15 ≈ q 0.10rat − q 0.15rat , of the total rational design value q 0.10rat , whereas the rest, as a remainder of the rational design value q 0.10rat , might be used as the residual booster one q 0.b10-15 = q 0.10rat − q 0.10-15 , available for preconditioning outdoor air to 15 • C. It is chosen as the object for reduction through refrigeration exceedances (excesses)' recuperation.
Based on the above, the intermediate temperature 15 • C is accepted as a threshold one t thr , stabilizing the heat loads for further conditioning outdoor air below t thr = 15 • C, and as an indicator to share the overall range of design heat load q 0.10rat (Figure 1) in two ranges according to different character of the loading.q0.15 reduced by its partially stabilization due to refrigeration energy exceedance's recuperation in the range to q0. 20.
The total values of specific refrigeration capacities q0.10 needed for conditioning outdoor air to 10 °C can be sheared into the range of changeable values q0.15, which are needed for preconditioning outdoor air to 15 °C.Practically unchangeable refrigeration capacities q0.10-15 are needed for subsequent conditioning of air from 15 °C to 10 °C.The calculation results for July 2017 in climatic conditions in southern Ukraine, Mykolayiv region, as example of temperate climate, are presented in Figure 2. As Figure 2 shows, the current total changeable heat load q0.10 for conditioning outdoor air to 10 °C can be shared in the range of changeable load for preconditioning outdoor air to 15 °C, and in the range of practically unchangeable heat load q0.10-15 for subsequent conditioning air from 15 °C to 10 °C.Accordingly, the latter is accepted as the basic practically unchangeable part, q0.10-15 ≈ q0.10rat − q0.15rat, of the total rational design value q0.10rat, whereas the rest, as a remainder of the rational design value q0.10rat, might be used as the residual booster one q0.b10-15= q0.10rat-q0.10-15, available for preconditioning outdoor air to 15 °C.It is chosen as the object for reduction through refrigeration exceedances (excesses)' recuperation.
Based on the above, the intermediate temperature 15 °C is accepted as a threshold one tthr, stabilizing the heat loads for further conditioning outdoor air below tthr = 15 °C , and as an indicator to share the overall range of design heat load q0.10rat (Figure 1) in two ranges according to different character of the loading.
Issuing from a changeable character of loading and accompanied by inevitable excesses of design refrigeration capacity q0.10rat over actual loads q0.10, reflected in the booster refrigeration capacity q0.b10-15 = q0.10rat-q0.10-15 and available for preconditioning outdoor air to 15 °C, the latter is accepted as the object for analyses in order to use the excess refrigeration capacity for covering the peak loads.Therefore, the next step of the analyses aims to partly stabilize the initially changeable heat loads q0.15, which would lead to reduce the booster load range from q0.b10-15 to q0.b10-20, as the regulated load range and the LRL of SRC compressor recuperate the refrigeration energy exceedance.
Proceeding from the approach to partly offset the current heat load fluctuations due to a reduction in the refrigeration capacity q0.15 by using the value q0.20rat to condition air to 20 °C, the latter might be accepted as the artificial threshold temperature tthr = 20 °C, and The current values of specific refrigeration capacities q 0.10 required for outdoor air conditioning to 10 • C; refrigeration capacities q 0.10-15 for subsequent air conditioning from 15 • C to 10 • C; available booster values q 0.b10-15 remained for outdoor air conditioning to 15 • C: q 0.10-15 = q 0.10 − q 0.15 ; q 0.b10-15 = q 0.10 − q 0.10-15 .
Issuing from a changeable character of loading and accompanied by inevitable excesses of design refrigeration capacity q 0.10rat over actual loads q 0.10 , reflected in the booster refrigeration capacity q 0.b10-15 = q 0.10rat − q 0.10-15 and available for preconditioning outdoor air to 15 • C, the latter is accepted as the object for analyses in order to use the excess refrigeration capacity for covering the peak loads.Therefore, the next step of the analyses aims to partly stabilize the initially changeable heat loads q 0.15 , which would lead to reduce the booster load range from q 0.b10-15 to q 0.b10-20 , as the regulated load range and the LRL of SRC compressor recuperate the refrigeration energy exceedance.
Proceeding from the approach to partly offset the current heat load fluctuations due to a reduction in the refrigeration capacity q 0.15 by using the value q 0.20rat to condition air to 20 • C, the latter might be accepted as the artificial threshold temperature t thr = 20 • C, and the range of heat loads q 0.10-20 as the artificially stable range in the initial approximation (Figure 3).
The results of the refrigeration energy exceedance recuperation for covering the booster preconditioning load q 0.15 using the reduced rational refrigeration capacity q 0.20rat are presented in Figure 3.
As can be seen, the actual values of available booster refrigeration capacities q 0.b10-20rat are mostly higher than the current requirement of q 0.15 for preconditioning outdoor air to 15 • C (Figure 3a).Accordingly, the current exceedances of the booster refrigeration capacities, q 0.b10-20rat.ex, in the majority offset the current deficit, q 0.b10-20rat.def, which is proven by the dominant rise in the summarized exceedance of booster refrigeration energy values ∑q 0.b10-20rat.exτ, except on a couple of days at the end of July (Figure 3b).
The practically constant summarized exceedance of the booster refrigeration energy values ∑q 0.b10-20rat.exτ between the 10-13th and 20-26th July justifies that the daily values of deficit ∑q 0.b10-20rat.defτ are compensated by their values of reserved refrigeration energy ∑q 0.b10-20rat.exτ.However, the briefly lowering values ∑q 0.b10-20rat.exτ within the 27-28th July indicate the presence of small daily refrigeration capacity deficit of q 0.20rat .
the range of heat loads q0.10-20 as the artificially stable range in the initial approximation (Figure 3).
As can be seen, the actual values of available booster refrigeration capacities q0.b10-20rat are mostly higher than the current requirement of q0.15 for preconditioning outdoor air to 15 °C (Figure 3a).Accordingly, the current exceedances of the booster refrigeration capacities, q0.b10-20rat.ex, in the majority offset the current deficit, q0.b10-20rat.def,which is proven by the dominant rise in the summarized exceedance of booster refrigeration energy values ∑q0.b10-20rat.exτ,except on a couple of days at the end of July (Figure 3b).
The continuously rising character of the available summarized exceedance of the booster refrigeration energy curve ∑q 0.b10-20rat.exconfirms that the booster refrigeration energy enables it to cover the current need q 0.15 for preconditioning outdoor air to 15 • C instead of 20 • C, and to offset the actual deficit q 0.b10-20rat.defby recuperating the daily excess of refrigeration energy ∑q 0.b10-20rat.exreserved at lowered current heat loads q 0.15 , with a significant monthly exceedance of 6600 kWh/(kg/s) (Figure 3).The latter indicates the refrigeration energy reserve for reducing the installed booster refrigeration capacity from q 0.15rat to q 0.20rat , and the total q 0.10rat by the value of their difference q 0.15rat − q 0.20rat = 10 kW/(kg/s) according to Figure 1.
Meanwhile, the booster refrigeration capacities q 0.b10-20opt , based on the optimal design value q 0.10opt , are not able to offset the current need q 0.15 for preconditioning outdoor air to 15 • C (Figure 4).nificant monthly exceedance of 6600 kWh/(kg/s) (Figure 3).The latter indicates the refrigeration energy reserve for reducing the installed booster refrigeration capacity from q0.15rat to q0.20rat, and the total q0.10rat by the value of their difference q0.15rat -q0.20rat = 10 kW/(kg/s) according to Figure 1.
As can be seen, the actual values of available booster refrigeration capacities q 0.b10-20opt are lower than the current need q 0.15 within 10-13th and later 20th of July, which leads to considerable values of the current deficit q 0.b10-20opt.def(Figure 4a).Accordingly, the current deficits q 0.b10-20opt.defare comparable with the current exceedance of booster refrigeration capacities q 0.b10-20opt.exthat is proven by alternating the rising and falling of the summarized exceedance of booster refrigeration energy values ∑q 0.b10-20opt.exτ during July (Figure 4b).
Thus, in contrast to the rational value q 0.20rat , the optimal value q 0.20opt is lower than the current need q 0.15 to be covered by daily reserved refrigeration energy ∑q 0.b10-20opt.exτ.
As may be seen, the actual values of available booster refrigeration capacities q 0.b10-20opt are lower than the current need q 0.15 from 10-13th and later 20th of July, which leads to considerable current deficit values of q 0.b10-20opt.def(Figure 7a).Accordingly, the current deficits q 0.b10-20opt.defare comparable with the current exceedances of the booster refrigeration capacities q 0.b10-20opt.exthat are proven by alternating the rising and falling of the summarized exceedance of booster refrigeration energy values ∑q 0.b10-20opt.exτ during July (Figure 7b).
Thus, in contrast to the rational value q 0.20rat , the optimal value q 0.20opt is lower than the current need q 0.15 to be covered by the daily reserved refrigeration energy ∑q 0.b10-20opt.exτ.
As Figure 8 shows, the current values of required level of regulated load (LRL) of SRC in the ratio q 0.b10-20opt /q 0.10-20rat fluctuate within the range of the required design nominal value LRL nom = q 0.15rat /q 0.10rat , of about 0.7 to 0.3-0.2.The range of load, regulated by an SCR compressor, is characterized by the level of regulated load LRL as a ratio of the regulated load to the overall load q 0.10 , including the unregulated load.As may be seen, the actual values of available booster refrigeration capacities q0.b10-20opt are lower than the current need q0.15 from 10-13th and later 20th of July, which leads to considerable current deficit values of q0.b10-20opt.def(Figure 7a).Accordingly, the current deficits q0.b10-20opt.def are comparable with the current exceedances of the booster refrigeration capacities q0.b10-20opt.ex that are proven by alternating the rising and falling of the summarized exceedance of booster refrigeration energy values ∑q0.b10-20opt.exτduring July (Figure 7b).
Thus, in contrast to the rational value q0.20rat, the optimal value q0.20opt is lower than the current need q0.15 to be covered by the daily reserved refrigeration energy ∑q0.b10-20opt.exτ.
As Figure 9 shows, the booster optimal refrigeration capacities q 0.b10-20opt of q 0.10opt enough to cover current need q 0.20 for cooling air to 20 • C with considerable exceedance q 0.b10-20opt.ex20, but sometimes less than the current need q 0.15 for cooling air to 15 • C (Figure 8), when corresponding exceedance q 0.b10-20opt.ex15drops to zero (Figure 9).The latter is also proven by the alternating the rising and falling of the summarized available exceedance of the booster optimal refrigeration energy values ∑q 0.b10-20opt.exτ = Σ(q 0.b10-20opt − q 0.15 )τ during July (Figure 9).There are opposite results for the summarized available exceedance of the booster rational refrigeration energy values ∑q 0.b10-20rat.exτ = Σ(q 0.b10-20rat − q 0.15 )τ, characterized by a continuous rise that demonstrates that the booster rational refrigeration energy is able to cover the current need q 0.15 for preconditioning outdoor air to 15 • C instead of 20 • C; this is achieved by recuperating the daily excess of refrigeration energy ∑q 0.b10-20rat.exreserved at lower current heat loads q 0.15 .
The similar character of the summarized exceedance of the booster optimal refriger ation energy values ∑q0.b10-20opt.ex20τ= Σ(q0.b10-20opt− q0.20)τ indicates that operation of the com pressor at optimal loads also requires refrigeration energy exceedance recuperation fo booster air to be preconditioned to 20 °C.
The similar character of the summarized exceedance of the booster optimal refrigeration energy values ∑q 0.b10-20opt.ex20τ = Σ(q 0.b10-20opt − q 0.20 )τ indicates that operation of the compressor at optimal loads also requires refrigeration energy exceedance recuperation for booster air to be preconditioned to 20 • C.
The results of reduction in installed (design) refrigeration capacities through refrigeration energy exceedance recuperation, proceeding from optimal q 0.10opt and rational q 0.10orat values calculated for temperate climatic conditions in southern Ukraine, 2017, are presented in Figure 10.
As may be seen, the values of reduction in initial rational design specific refrigeration capacity due to optimal design ∆q 0.10opt and by refrigeration energy exceedance recuperation in booster air preconditioning to 15 • C ∆q 0.15-20rat , resulting in reducing the rational value of design refrigeration capacity from q 0.15rat to q 0.20rat , are nearly the same.However, the annual refrigeration energy generation according to current consumption ∑(q 0 •τ) 10opt at the optimal refrigeration capacity q 0.10opt is considerably lower than ∑(q 0 •τ) 10rat at the rational refrigeration capacity q 0.10rat .In order to increase the annual refrigeration energy generation, as the primary criterion for the effect gained at optimal refrigeration capacity q 0.10opt , the refrigeration energy exceedance Σq 0.b10-20opt τ = Σ(q 0.10opt -q 0.10-20 )τ (Figure 8) has to be recuperated.As may be seen, the values of reduction in initial rational design specific refri capacity due to optimal design Δq0.10opt and by refrigeration energy exceedance re tion in booster air preconditioning to 15 °C Δq0.15-20rat, resulting in reducing the value of design refrigeration capacity from q0.15rat to q0.20rat, are nearly the same.H the annual refrigeration energy generation according to current consumption ∑ at the optimal refrigeration capacity q0.10opt is considerably lower than ∑(q0•τ)10rat a tional refrigeration capacity q0.10rat.In order to increase the annual refrigeration generation, as the primary criterion for the effect gained at optimal refrigeration q0.10opt, the refrigeration energy exceedance Σq0.b10-20optτ = Σ(q0.10opt-q0.10-20)τ (Figure 8) h recuperated.

Conclusions
The methods earlier developed by authors and aimed at determining the refrigeration capacity of ACS, providing the maximum annual refrigeration ener eration according to its consumption and its optimal value at the maximum rate o refrigeration energy increment, are adopted to reveal reserves for reducing the de frigeration capacity of ACS through recuperation of the excessive refrigeration r at lowered loads to cover the current increased loads.
The modified methods of rational and optimal design of the ACS allow us t mine the initial ranges of changeable and unchangeable thermal loads, subsequ lowing us to partly stabilize the changeable load range through covering it with r ated excessive refrigeration.
The artificial threshold air temperature, limiting the range of initially cha loads stabilized through excessive refrigeration recuperation, is determined and p from the rising character of the monthly summarized available exceedance of th eration energy beyond its needs.

Conclusions
The methods earlier developed by authors and aimed at determining the rational refrigeration capacity of ACS, providing the maximum annual refrigeration energy generation according to its consumption and its optimal value at the maximum rate of annual refrigeration energy increment, are adopted to reveal reserves for reducing the design refrigeration capacity of ACS through recuperation of the excessive refrigeration reserved at lowered loads to cover the current increased loads.
The modified methods of rational and optimal design of the ACS allow us to determine the initial ranges of changeable and unchangeable thermal loads, subsequently allowing us to partly stabilize the changeable load range through covering it with recuperated excessive refrigeration.
The artificial threshold air temperature, limiting the range of initially changeable loads stabilized through excessive refrigeration recuperation, is determined and proceeds from the rising character of the monthly summarized available exceedance of the refrigeration energy beyond its needs.
Such artificial thermal load stabilization through excessive refrigeration recuperation leads to a narrowed range of changeable loads and a reduction in the level of the regulated loads (LRL) of the SRC compressor.
Thus, the modified methods of rational and optimal design of the ACS allow us to determine the level of regulated loads (LRL) of the SRC, thus the load range of the compressor's operation was reduced more than 1.5 times.It has been shown that for temperate climatic conditions, the required LRL is about 0.5 with excessive refrigeration recuperation, versus 0.7 without refrigeration recuperation.
The further investigation is focused on the application of the approaches to designing two-stage outdoor ACS and the methods of defying the rational and optimal refrigeration capacities, developed initially for outdoor air conditioning, to realize two-stage principal in indoor subsystem with using the refrigeration excess gained in outdoor subsystem to reduce installed refrigeration capacity of indoor one.