Thermal Stabilization as a Key to Sustainable Operation of Combustion Engines and Power Plants—Part 2: Thermal Stabilization of IES Due to Advanced Intake Air Cooling and Heat Recovery Assessed by Appropriate Criteria

Abstract

The sustainable performance of combustion engines in integrated energy systems (IESs) at a safe thermal level and with high fuel effectiveness is possible if intake air is cooled and the released heat is fully utilized by a heat recovery chiller (HRCh). Herein, the temperature of return hot water, used as a coolant, at the engine inlet is not to be higher than 70 °C to ensure safe, thermally stabilized engine performance. This cannot be ensured by only an absorption lithium-bromide chiller (LBCh) as the most efficient HRCh, which has a COP of about 0.7 and provides a return hot-water temperature of about 75 °C against the required 70 °C, which reduces the engine lifetime by about 10%. To ensure the thermally stabilized performance of the engine in conventional IESs, the residual heat left from the LBCh is ejected into the atmosphere by an emergency radiator with a heat loss of about 25% as the “cost” for safe engine performance. The concept of IES thermal stabilization in terms of both aspects—intake air cooling and the full utilization of the released heat by a combined LBCh boosted by an ejector chiller (ECh), easily implemented into an existing IES—was developed. The appropriate new unified criterion indicators for estimating the rate of thermal stabilization (RS) were adopted to ensure sustainable performance and increased the lifetime of the engine by about 10%, providing an innovative heat recovery intake air cooling system and boosting the conventional IES as a prosperous trend. Thus, for the first time in the practices of IES design and operation, the aggregated effect on engine thermal stabilization, followed by maximizing its efficiency, has been gained due to intake air cooling and full utilization of the heat released as mutually affecting aspects.

1. Introduction

Integrated energy systems (IESs) [1,2] are aimed at the combined production of electrical/mechanical power, heat and cold, in addition to the heat and power inherent in cogeneration or combined heat and power (CHP) plants [3,4]. They are generally manufactured with heat exchangers for extracting heat from lubricating oil, the engine jacket, charge air and exhaust gas to provide a hot-water temperature of about 90 °C or steam [5,6]. The hot water or steam meets thermal demands in CHP and is converted to refrigeration by heat recovery chillers (HRChs) in CCHP plants [7,8].

The cold produced in trigeneration plants is generally applied for outside consumption: food industries and other industrial needs [9,10] and space air conditioning [11,12]. They are widespread in buildings [13,14] and stationary and transport applications [15,16]. The yearly performance of these plants is associated with varying loads [17,18], which need different coolants and the application of efficient heat exchangers [19,20] to ensure their operation with a high heat flux within a wide range of load changes.

Internal combustion engines (ICEs) [21,22], gas engines (GEs) [23,24] and gas turbines [25,26] have a widened application as prime movers in IES of the trigeneration type.

Their effectiveness is strongly affected by the intake air temperature and drops if it increases [27,28]. Therefore, its cooling provides engine performance with high fuel efficiency due to reduced specific fuel consumption (SFC) [29,30].

If cold is spent on cooling engine cyclic air to increase fuel and overall efficiency, so-called in-cycle trigeneration takes place [31,32]. Engine cyclic air cooling makes it possible to nearly double the duration of IES efficient operation, when outer cooling needs are decreased or absent.

CCHP plants with cooling cyclic air have high flexibility because they match the actual loads upon driving engines [33,34] according to electrical, heat and cold demands [35,36] under variable parameters of intake air according to the site’s climate.

Engine intake air cooling can be provided by transforming the heat removed into cooling capacity by heat recovery chillers (HRChs) [37,38].

The reserve for improving engine thermal stabilization through increasing the efficiency of cooling intake air depends on the heat removed [39,40] and the effectiveness of its transformation into cooling capacity, estimated by the COP as the cooling capacity gained related to the heat spent.

In the case of ICEs as the prime movers combusting fossil fuels, the heat released might be increased due to the application of exhaust boilers with condensing surfaces [41,42], which allow for lowering the temperatures of exhaust gas, i.e., to increase the heat utilized, and provide the easy extraction of contaminations, enabling a decrease in corrosion [43,44] and pollution into the environment. Furthermore, the increased heat being transformed into cold enables deep intake air cooling and provides thermally stable engine operation while applying the HRCh, even with a lower COP.

The heat released from engines can be transformed into cooling intake air by different HRChs [45,46]. Lithium-bromide-type absorption chillers (AChs) are the most widespread and produce chilled water of about 7 °C by converting the heat of hot water at about 90 °C with a COP of about 0.7 [47,48]. Herein, the temperatures of cooled intake air are limited to 15 °C.

Jet apparatuses such as thermopressors [49] and ejector chillers (EChs) [50] generally include heat exchangers, easily integrated into existing power plants. Herein, the EChs are able to cool air to 10 °C and lower to produce an enhanced effect but at a decreased COP [51,52]. Moreover, the EChs are very sensitive to changeable thermal loads [53,54], which causes a drop in the COP. They require pre-cooling air, for instance, by vapor compression machines (VCMs) [55,56] or AChs [57,58], to compensate for load fluctuations, which would ensure a stabilized load on EChs.

A wide range of exhaust temperature differences might be covered by stage [59,60] or cascade [61] transformation. The stage combination of AChs and EChs [62,63] provides deeper cooling air due to the application of hybrid coolants in air coolers: water and refrigerant [64].

The cascade configuration [65] enables the involvement of the heat potential over a wider range through hybrid cooling [66], inaccessible for one-stage heat transformation by AChs.

An enhancement in cogeneration [67,68] and trigeneration [69,70] efficiency might be realized by optimizing a design load to provide the maximum output by adapting to actual loads [71,72] without oversizing at the same time.

Most approaches and concepts focus on matching outer cooling needs: building comfort climate or various technological demands [73,74].

Respectively, they are not desired to enhance the prime engine efficiency through intake air cooling and to prolong trigeneration plant (TGP) performance at reduced outer cooling needs [75,76].

The sizing and operation strategies of TGP matching electric [77,78] and heat needs [79,80] were analyzed.

Multi-criteria optimization of TGP connected to a district heating and building cooling network under legal constraints [81,82] according to various energy management strategies enables the choice of the optimal configuration [83,84].

Practically, all criteria for assessing the efficiency of cooling intake air of combustion engines are based on an intake air temperature drop and its duration [85] as the factors most affecting their thermal stabilization through cooling air evaluated by the cooling degree-hour (CDH) number [86,87].

The criteria [88] and strategies of engine intake air pre-cooling [89,90], however, did not take into consideration the effectiveness of trigeneration plants from the point of view of their prime engine thermal stabilization.

The approaches, hypotheses, criteria and methods to highlight the gap in existing principal solutions were investigated, and the concept of sustainable fuel-efficient operation of IES in the aspect of intake air cooling was substantiated in Part 1 of the present complex research [91].

Herewith, the rate of intake air cooling is assumed as the rate of engine thermal stabilization (RS), which is evaluated as actual temperature drops of air related to their designed specified magnitudes.

As the fuel efficiency of combustion engines is strictly affected by a drop in intake air temperature, the RS as a universal criterion might be applied to build a bridge from sustainable to fuel and overall efficient performance due to cooling intake air [91], whereas the influence of the utilization of the hot-water heat released from the engine upon the engine thermal stabilization is still not highlighted.

The point is that the return hot water from LBCh is used as a coolant to reject the heat from the engine jacket and lubricating oil. Herewith, its temperature at the engine inlet must not be higher than 70 °C to ensure safe and long-lifetime performance.

Exceeding 70 °C has a crucial impact on the engine efficiency and lifetime: each 1 °C in temperature exceeding the return hot water above the limited magnitude 70 °C at the entrance of GE leads to shortening the engine lifetime by 2% [28], whereas the aspect of enhancing the sustainable performance of IES through full recovery of the heat released to ensure GE safe running at 70 °C at the entrance has not been investigated, and appropriate criteria were not proposed.

Therefore, the concept of sustainable IES operation through intake air cooling (Part 1, [91]) should be extended to include the aspect of utilization of the heat released to ensure the long-term performance of prime engines.

Herewith, the additional factors affecting the thermal stabilization of the IES, involved through the incomplete utilization of the heat released from GE, need further investigation to modify the RS criterion for excluding engine lifespan shortening and heat losses of about 25…30% inherent in the typical IES.

This research aims to develop the concept of sustainable, fuel-efficient and long-lifetime operation of IES due to both intake air cooling and full utilization of the heat released and substantiate general approaches, hypotheses, and criteria in its core.

2. Materials and Methods

The manufacturer guidelines and empirical data underline the importance of maintaining the specified temperature of return hot water as a coolant at the engine inlet no higher than 70 °C to ensure the efficiency and durability of Jenbacher gas engines [28]. Otherwise, each 1 °C in temperature, exceeding the return hot water above the limited magnitude of 70 °C at the entrance of GE, leads to shortening the engine lifetime by 2%, and by 10% at 75 °C accordingly.

Safe engine operation due to satisfying the above manufacturer requirements might be realized through full utilization of the return hot-water heat, provided at the design stage. Therefore, the present research is focused on developing methodological supervision accompanied by appropriate criterion indicators and a system solution to ensure thermally stabilized and long-lifetime engine performance, issuing from the aspect of fully utilizing the heat released from GE, not considered in Part 1 of this research [31]. Thus, the problem of GE and the overall IES thermally stabilized performance and lifespan might be solved from both aspects: effective intake air cooling (Part 1 [31]) and full exhaust heat utilization, considered in the present investigation. Moreover, converting the rest of the heat released from GE for cooling intake air means both aspects are mutually affected.

The authors applied a method of phenomenological analysis, similar to fundamental approaches [92,93] and based on the fundamental heat balances and correlations, supplemented by the simplest numerical simulation, which enables its easy reproduction in designing and operating practice. Moreover, it ensures a high level of generalization of the results in the most widened fields of application.

The following hypotheses are provided to substantiate new approaches:

• For the first time in IES and energetics as a whole, the sustainable operation of a combustion engine is treated through compiling both aspects: intake air cooling (Part 1) [91] and full recovery of the released heat to provide the temperature of return cooled hot water as a coolant for engine jacket and lubricating oil at the safe level of about 70 °C at the entrance of GE;
• Issuing from the above, the temperature drop in intake air due to cooling and its drop in hot water released from GE due to utilization of its heat are accepted as the motive parameters and their relative magnitudes, as a universal criterion for estimating the rate of GE thermal stabilization (RS) from both aspects;
• The limitation for recovery of the heat released from GE, caused by a drop in hot-water temperature in LBCh of about 15 °C against the available 20 °C, which leads to heat loss of about 25%, might be overcome by converting the heat loss in the boost chiller, where, as an example, ECh was easily implemented into the existing IES with ACh in a joint chain through recovering the heat loss for cooling engine intake air and thereby enhancing the engine’s efficient and thermally stabilized operation.

The RS as a quantitative indicator is defined as the ratio of the actual drop in air (Part 1, [91]) and hot-water (Part 2) temperatures to their target values. Additionally, it makes it possible to consider the engine’s operational lifetime in relation to the RS provided by converting the heat loss, inherent in typical IES with LBCh, in ECh for cooling engine intake air in a joint chain “heat utilization-intake air cooling” to increase the electrical and total efficiency of GE (Part 2). Thus, the RS as a quantitative indicator, proposed in [91], has gained further development due to its extension over the utilization of the heat released to enhance engine efficiency within both aspects: intake air cooling and heat utilization.

The general correlations and heat balances used for the RS, caused by intake air cooling as the first aspect, considered in Part 1 [91], are the following.

The actual values of an hour fuel reduction due to intake air cooling, for instance, to 15 °C:

B = Δt_in15 (Δb_e/Δt_in)·P_e,

(1)

where Δb_e/Δt_in—a reduction of SFC per 1 °C drop in temperature of air at the inlet of engine, g/kWh°C; Δt_in15—air temperature drop due to cooling air to 15 °C at GE inlet, Δt_in15 = t_amb − 15 °C; P_e—engine power, kW.

All the above parameters are received by treating monitoring data [90].

The annual fuel saving:

ΣB = ΣΔt_in τ (Δb_e/Δt_in)·P_e,

(2)

where τ—duration, hours.

The rate of thermal stabilization RS is calculated as a ratio of the actual temperature drop of air at the inlet of GE Δt_in to its design value Δt_in15 when cooling air at the GE inlet to 15 °C (10 °C at AC outlet with AECh):

RS = Δt_in/Δt_in15.

(3)

The average weighted RS_avr over a definite period (month, season or year) is determined as a ratio of the sum of actual drops in temperatures at the engine inlet due to cooling over the corresponding period of engine operation at lowered temperatures to the sum of their target values, for instance:

RS_20/15avr = ΣΔt_in20 τ₂₀/ΣΔt_in15 τ₁₅,

(4)

where ΣΔt_in20 τ₂₀ and target ΣΔt_in15 τ₁₅, °C·h—sum of actual drops in temperatures at the engine inlet due to cooling to 20 °C and target 15 °C over the corresponding period of engine operation (cooling degree hours CDH); removed τ₁₅ and τ₂₀—duration of engine operation at lowered temperatures 15 °C and 20 °C.

The second aspect of the engine RS is associated with the efficiency of utilization of the heat released and its conversion to refrigeration for cooling intake air by ECh, as an example of boosting chiller. Thus, both aspects are mutually affected.

Input data: t_hwGEout, t_hwAout, t_hwGEin—monitoring data on temperatures of hot water at the GE outlet (ACh inlet), ACh outlet, GE inlet; G_w—water flow.

Output data: Q_hwGE, Q_hwA, Q_hw.loss—total hot-water heat from GE, heat converted by ACh, heat loss;

η_tot.hwAE—total electrical and heat efficiency; Δη_tot.hwE.loss—increment in heat efficiency due to converting heat loss by ECh; RS_hwA,

RS_hw.loss—rate of GE thermal stabilization due to utilization of the rest of the heat loss by ACh;

LT_hwGE, LT_hwA, LT_hw.loss—level of engine lifetime due to full utilization of the heat from GE, LT_hwA—due to converting heat by ACh, LT_hw.loss—due to utilizing heat loss by ECh.

The measured and manufacturer data, assumed parameters and calculated results are provided in

Table 1. Data and their sources.

Data	Symbol	Unit	Source
Temperature of hot water from GE	t	°C	Monitoring, Part 2
Volume flow of hot water from GE	G	m3/h	Monitoring, Part 2
Lifetime affected by hot-water temperature	LThw	–	Calculation, Manufacturer [5,28], Part 2
Total efficiency affected by hot-water temperature	ηtot.hw	–	Calculation, Manufacturer [5,28], Part 2
Coefficient of performance of ACh and ECh	COP	–	[94,95], Part 1,2

.

The cooling capacity Q₀, kW, required to provide a drop Δt_a in intake air temperature:

Q₀ = c_a ξ · Δt_a · G_a,

(5)

where c_a—specific heat of air, kJ/(kg·K); ξ—specific heat ratio as the overall sensible and latent heat rejected from air related to sensible heat; Δt_a = t_iamb − t_a2, where t_amb and t_a2—temperatures of ambient air and air cooled by ACh to 15 °C at the outlet of air cooler and subcooled further by ECh to 10 °C.

Temperatures t_amb and relative humidity φ_amb of ambient air are taken according to the program “METEOMANZ” [96].

Cooling capacity of ECh Q_0.E, kW, gained by converting heat loss Q_loss:

Q_0.E = Q_loss COP_E,

(6)

where COP_E = 0.2 [95].

The COP of ECh about 0.2 is accepted for the following temperatures of the refrigeration cycle: boiling refrigerant at high pressure in generator 80…90 °C, boiling refrigerant at low pressure in evaporator 5…7 °C, and condensing temperature 25…30 °C in condenser, cooled by cooling water from wet tower of about 20 °C, which is close to the wet bulb temperatures of ambient air.

Temperature drop Δt_E provided by ECh:

Δt_E = Q_0.E/(c_a ξ · G_a).

(7)

Similar to the rate of engine thermal stabilization due to cooling intake air RS_in, the rate RS_hw due to its operation at the safe temperature of return cooled hot water (as a coolant for the engine jacket and lubricating oil) at the engine entrance might be applied from the point of exhaust heat utilization.

The overall temperature drop of hot water Δt_hwGE = t_hwGEout − t_hwGEin of about 20 °C is equivalent to utilizing all the heat released from the GE. It involves both heat converted to refrigeration in ACh with corresponding temperature drop Δt_hwA and heat loss extracted into the atmosphere in the emergency radiator Δt_hw.loss. Accordingly, the overall temperature drop Δt_hwGE is assumed as the magnitude providing the maximum rate RS_hwGE = 1.

Herein, the rate RS_hwA in a typical IES with ACh is calculated as a hot-water temperature drop in ACh related to the overall temperature drop to the specified 70 °C:

RS_hwA = Δt_hwA/Δt_hw70°C or RS_hwA = Δt_hwA/Δt_hwGE.

(8)

The additional increment as RS_hw.loss might be achieved due to utilization of the rest of the heat loss by ECh, as an example:

RS_hw.loss = Δt_hw.loss/Δt_hwGE.

(9)

Herein, the level of engine lifetime in a typical IES with ACh:

LT_hwA = 1 − (Δt_hwGE − Δt_hwA)0.02, or LT_hwA = 1 − Δt_hw.loss 0.02,

(10)

where Δt_hw.loss = Δt_hwGE − Δt_hwA ≈ 5 °C (monitoring data);

0.02 or 2%—a degradation factor as a drop in the maximum lifetime of GE LT_hwGE = 1.0 for each 1 °C of the return cooled hot water above specified 70 °C at the inlet of GE [28].

It should be underlined that return cooled hot water is used as a coolant for the engine jacket and lubricating oil.

According to the guidelines for Jenbacher engines, each 1 °C increase in return cooled hot water at the inlet of GE above the specified 70 °C leads to a decrease in engine lifetime by about 2%, or, for a 5 °C increase, by about 10% [28].

Herewith, the engine’s normal operational lifetime at the specified return cooling water temperature of 70 °C is assumed as 20,000 h.

The calculation results on the lifetime degradation are shown in

Table 2. Lifetime degradation.

№	Temperature of Return Cooling Water at the Engine Inlet, °C	Lifetime, %	Lifetime, Hours
1	70	100	20,000
2	71	98	19,600
3	72	96	19,208
4	73	94	18,824
5	74	92	18,448
6	75	90	18,079

.

According to the guidelines for Jenbacher engines, there might be a reduction in overall engine efficiency by 0.5% for each 1 °C increase in return cooled hot water at the inlet of GE above the specified 70 °C, or by 2.5% per 5 °C increase over 70 °C, i.e., at 75 °C [28]. This is caused by applying return water as a coolant for cooling the engine jacket and lubricating oil.

Herewith, the overall temperature drop of hot water Δt_hwGE of about 20 °C is equivalent to utilizing all the heat released from GE and involving both heat converted to refrigeration in ACh with corresponding temperature drop Δt_hwA and heat loss extracted to the atmosphere in the emergency radiator Δt_hw.loss. This temperature drop Δt_hwGE is assumed as the magnitude providing the maximum ratio RS_hwGE = 1.

Respectively, in typical IES with ACh the RS_hwA = Δt_hwA/Δt_hwGE. The additional increment as RS_hw.loss might be achieved due to utilization of the rest of the heat loss: RS_hw.loss = Δt_hw.loss/Δt_hwGE by ECh as example.

3. Results and Discussion

3.1. Engine Thermal Stabilization in the Aspect of Utilizing the Heat Released

The efficiency of utilization of the heat released from gas engines for intake air cooling was investigated for the IES of the enterprise “Sandora”–“PepsiCo Ukraine” (Figure 1a). The monitoring data on the hot-water temperatures and flow rate were measured according to the heat utilization circuit (Figure 1b).

The return cooled hot water from the ACh is used as a coolant to extract heat from lubricating oil and the engine jacket. The IES includes two cogeneration modules based on Jenbacher gas engines JMS 420 GS-N.LC (INNIO Group, Tyrol, Austria; rated electric power P_eISO = 1400 kW, heat power Q_h = 1500 kW). The main rated characteristics of the Jenbacher gas engine JMS 420 GS-N.LC, applied as a driving one, at the ISO parameters (air temperature T_in.ISO = 298.15 K and relative humidity φ_ISO = 30%) are the following: electrical power P_el.ISO = 1420 kW, heat power Q_h = 1500 kW, specific gas fuel consumption b_e.ISO = 167.76 g/kWh [5].

The heat rejected from lubricating oil, engine jacket cooling water, scavenge air–gas mixture and exhaust gas is converted in the AR-D500L2 Century (Century Corporation, Seoul, Republic of Korea) [94] absorption Li-Br chiller (cooling capacity 2000 kW) to produce refrigeration in the form of chilled water with a temperature of 7 °C. The refrigeration capacity is used to cover the technological cooling needs and to feed the central air conditioner intended for cooling the ambient air entering the machine room, from where the air is sucked by the engine turbocharger (Figure 2).

The central conditioner CIC Jan HREBEC 1LG4223-8AB60 has a cooling capacity of 350 kW and a volume air flow of 60,000 m³/h [97].

During monitoring, temperatures of hot water were measured at key points in the heat utilization system: at the outlet of the exhaust boiler (outlet of GE, inlet of ACh), t_hwGEout, at the outlet of ACh, t_hwAout, and return hot water, cooled in the emergency radiator, at the GE inlet t_hwGEin, chilled water from ACh t_cwAout and return chilled water at the inlet of ACh t_cwAin. Water temperatures were measured every 10 min throughout the day.

The volumetric flow rates of hot and chilled water were measured using Vzlet ERSV-410(510)L electromagnetic flowmeters installed in the corresponding water circuits, as shown in Figure 1. The hot-water flow rate is denoted as G_hw and the chilled water flow rate as G_cw. The maximum permissible relative error of the flow measurement was evaluated as δ_G = ±(0.9 + 0.15/v)%, where v is the water velocity in the measuring section, m/s. The relationship between water velocity, volumetric flow rate and nominal flowmeter diameter is provided in the note to

Table 3. Measurement devices and their metrological characteristics.

No.	Parameter	Measurement Device	Measurement Range	Accuracy Class/Maximum Permissible Error
1	Relative humidity RH of ambient air	DVT-01 humidity and temperature sensor (RegMik)	0–100%RH	±2%RH
2	Ambient-air temperature	DVT-01 humidity and temperature sensor (RegMik)	−40 to +120 °C	±0.5 °C
3	Hot-water temperature	TSPU 1–3 Pt100 temperature sensor	0 to +150 °C	±(0.10 + 0.0017·t) °C
4	Chilled-water temperature	TSPU 1–3 Pt100 temperature sensor	0 to +150 °C	±(0.10 + 0.0017·t) °C
5	Water pressure	MP3-Uf pressure gauge	0–0.6 MPa	Accuracy class 1.0; ±1.0% FS (±0.006 MPa)
6	Hot-water volumetric flow rate, Ghw	Vzlet ERSV-410(510)L electromagnetic flowmeter	0–340 m3/h	δG = ±(0.9 + 0.15/v)%
7	Chilled-water volumetric flow rate, Gcw	Vzlet ERSV-410(510)L electromagnetic flowmeter	0–340 m3/h	δG = ±(0.9 + 0.15/v)%
8	Fuel-gas volumetric flow rate	KURS-01 G250 A1 ultrasonic gas meter	1.6–400 m3/h	±1%
9	Electric power	SL 7000 Smart (SL761) electricity meter	1–120% of rated value	±0.5%

Note: t—the measured water temperature, °C; v—the water velocity in the flowmeter measuring section, m/s; δ_G—the maximum permissible relative error of the volumetric flow measurement, %. For the Vzlet ERSV-410(510)L flowmeter, v = 353.68G/D_y², where G_w—is the water volumetric flow rate, m³/h, and D_y—the nominal diameter, mm.

.

The measurement ranges and metrological characteristics of all instruments are summarized in Table 3.

In Figure 1b, FT-HW—electromagnetic flowmeter for hot-water volumetric flow rate; FT-CW—electromagnetic flowmeter for chilled-water volumetric flow rate; TT—temperature sensor; PI—pressure gauge.

As an example, the relative errors in the heat flows calculated according to monitoring data on the heat of the hot water removed from the engine are indicated in Appendix A,

Table A1. The relative errors in determining the heat of hot water released from GE.

№	Heat Released from GE	Relative Errors
		Systematic Errors	Confidence Probability
			p = 0.9	p = 0.68
	QhwGE	$\frac{\mathbf{\Delta }{\mathit{Q}}_{\mathbf{hwGE}}}{{\mathit{Q}}_{\mathbf{hwGE}}}$	$\frac{\mathbf{\Delta }{\mathit{Q}}_{\mathbf{hwGE}}}{{\mathit{Q}}_{\mathbf{hwGE}}}$	$\frac{\mathbf{\Delta }{\mathit{Q}}_{\mathbf{hwGE}}}{{\mathit{Q}}_{\mathbf{hwGE}}}$
1	2806	0.004289	0.00511	0.002311
2	2826	0.004287	0.005108	0.00231
3	2785	0.004288	0.005109	0.00231
4	2868	0.004289	0.005109	0.002311
5	2847	0.004288	0.005109	0.00231
6	2868	0.004288	0.005109	0.00231
7	2847	0.004292	0.005114	0.002313
8	2888	0.00429	0.005111	0.002312
9	2847	0.00428568	0.005106	0.002309
10	2888	0.004281	0.005101	0.002307

, with the heat of the chilled water (coolant) generated in ACh (

Table A2. The relative errors in determining heat of hot water consumed by ACh.

№	Heat Consumed by ACh	Relative Errors
		Systematic Errors	Confidence Probability
			p = 0.9	p = 0.68
	QhwA	$\frac{\mathbf{\Delta }{\mathit{Q}}_{\mathbf{hwA}}}{{\mathit{Q}}_{\mathbf{hwA}}}$	$\frac{\mathbf{\Delta }{\mathit{Q}}_{\mathbf{hwA}}}{{\mathit{Q}}_{\mathbf{hwA}}}$	$\frac{\mathbf{\Delta }{\mathit{Q}}_{\mathbf{hwA}}}{{\mathit{Q}}_{\mathbf{hwA}}}$
1	1964	0.004214	0.005021	0.002271
2	1978	0.004212	0.005018	0.002269
3	1949	0.004211	0.005017	0.002269
4	2007	0.004214	0.005021	0.002271
5	1993	0.004215	0.005022	0.002271
6	2007	0.004216	0.005023	0.002272
7	1993	0.004219	0.005026	0.002273
8	2022	0.004219	0.005027	0.002273
9	1993	0.004213	0.005019	0.00227
10	2022	0.004211	0.005017	0.002269

), the cooling capacity produced by ACh (

Table A3. The relative errors in determining cooling capacities of ACh.

№	Cooling Capacity of ACh, kW	Relative Errors
		Systematic Errors, %	Confidence Probability, %
			p = 0.9	p = 0.68
	Q0.A	$\frac{\mathbf{\Delta }{\mathit{Q}}_{0.\mathbf{A}}}{{\mathit{Q}}_{0.\mathbf{A}}}$	$\frac{\mathbf{\Delta }{\mathit{Q}}_{0.\mathbf{A}}}{{\mathit{Q}}_{0.\mathbf{A}}}$	$\frac{\mathbf{\Delta }{\mathit{Q}}_{0.\mathbf{A}}}{{\mathit{Q}}_{0.\mathbf{A}}}$
1	1572	1.846	0.02199	0.009945
2	1586	1.848	0.022019	0.009958
3	1561	1.841	0.021933	0.009919
4	1605	1.852	0.022064	0.009978
5	1596	1.857	0.022125	0.010006
6	1603	1.848	0.022021	0.009959
7	1595	1.851	0.022058	0.009975
8	1619	1.869	0.022267	0.01007
9	1591	1.856	0.02211	0.009999
10	1613	1.855	0.022103	0.009996

).

The relative errors in the heat flows calculated according to monitoring data were determined by taking into consideration the error of measurement devices, methodological and systematic errors and were about 5–6%.

The monitoring data on daily changes of electrical power output P_el and volumetric gas fuel B_gas consumed by gas engine JMS 420 were measured using an ultrasonic gas meter KURS-01 G250 A1 and electricity meter SL 7000 Smart (SL761) accordingly (Table 3).

The volumetric gas fuel B_gas was used for the calculation of gas fuel heat Q_gas and electrical efficiency η_el = P_el/Q_gas.

The heat efficiency η_h = Q_h/Q_gas and total electrical and heat efficiency η_el+h = (P_el + Q_h)/Q_gas of IES were calculated by using the heat Q_h released from GE according to the measured volume flow G_hw and temperatures of hot water at the outlet t_hwGEout and inlet t_hwGEin of GE.

The available cooling capacities Q_0.ACh of ACh are limited by the heat of hot water Q_h.ACh converted into refrigeration, which is limited by the hot-water temperature drop of about 15 °C optimal for ACh.

The monitoring data on the hot-water temperatures in the heat utilization system of conventional IES based on GE JMS 420 GS with ACh are shown in Figure 3 for the time interval τ = 12:00–24:00 (28 July 2017).

The current temperature drops of hot water due to recovery of its heat in ACh Δt_hwA, caused by extracting heat into atmosphere in the emergency radiator Δt_hw.loss and overall temperature drops Δt_hwGE in hot water from GE are presented in Figure 4.

The overall heat Q_hw2GE rejected from two GE, heat converted by ACh Q_hw2A to refrigeration and the heat loss Q_hw2loss left from ACh and extracted into the atmosphere by the radiator were calculated based on the monitoring data on water flow G_w and changes in temperature of water in the engine heat utilization circuit (Figure 3 and Figure 4).

Herewith, the following heat balances for two GE were conducted:

• The overall heat released from two GE Q_hw2GE = G_w c_w (t_hwGEout − t_hwGEin);
• Heat converted by ACh into cooling capacity Q_hw2A = G_w c_w (t_hwGEout − t_hwAout);
• The heat loss Q_hw2loss = G_w c_w (t_hwAout − t_hwGEin).

The corresponding heat released from two GE Q_hw2GE, converted by ACh into cooling capacity Q_hw2A, and the remaining heat loss Q_hw2loss removed by the radiator to the atmosphere, are depicted in Figure 5.

Herewith, the heat of hot water at 90 °C released from two GE Q_hw2GE is about 2800 kW, i.e., one GE–Q_hwGE = 1400 kW, whereas heat converted by ACh to refrigeration for two GE Q_hw2A is about 2000 kW, and for one GE–Q_hwA = 1000 kW; heat loss rejected into the atmosphere from two GE Q_hw2loss is about 800 kW and from one GE–Q_hw.loss = 400 kW, respectively.

Thus, the heat losses Q_hw.loss in the heat-converting circuit of a typical IES are within 25 to 30% of the heat Q_hw.GE produced by GE.

These heat losses Q_hw.loss are caused by reducing the temperature of return cooled hot water at the GE inlet from about 75 °C to 70 °C, i.e., by a temperature difference Δt_hw.loss of about 5 °C as the issuing point for quantitative assessment of the deepness (level) of the released hot-water heat utilization by the rate of engine thermal stabilization (RS) and lifespan in the aspect of utilization (Figure 6).

The guidelines for Jenbacher engines provide detailed maintenance schedules and recommendations to mitigate the impacts of return cooled hot-water temperature at the GE inlet, exceeding the specified value of 70 °C on engine performance [5,28].

According to the guidelines for Jenbacher engines, for each 1 °C increase, the engine’s lifespan could decrease by about 2%, or for a 5 °C increase, by about 10%.

Considering engine intake air cooling and the deep utilization of released hot-water heat, required to provide thermally safe GE operation at the temperature of return cooled hot water as a coolant for engine jacket and lubricating oil at the inlet of engine not higher than 70 °C, as both aspects of engine sustainability due to thermal stabilization, the analogies in the criterion indicators for estimating the efficiency of solutions to the problem in both aspects can be drawn.

So far, with deeper cooling of intake air, estimated by its temperature drops Δt_in, as well as deeper utilization of the heat released with hot water, evaluated by its temperature drops Δt_hw, the more thermally stabilized GE operation, the hot-water temperature drops Δt_hw are considered general factors affecting engine thermal stabilization and fuel-efficient operation. From the above, the corresponding cooling capacities Q_0.in (first aspect) and heat Q_hw converted to Q₀ (second aspect), which are needed for sustainable operation and an adequate thermally stabilized level compared to the target magnitude assumed as a maximum of 100%, can be determined.

Thus, issuing from the generalized approach above, likewise, with the approach for estimating the RS due to intake air cooling in the first aspect [91], the RS of GE can be evaluated in the aspect of heat utilization by return cooled hot-water temperature deviation from the specified 70 °C (Figure 6).

Herein, the overall temperature drop of hot water, Δt_hwGE = t_hwGEout − t_hwGEin, of about 20 °C, is equivalent to utilizing the overall heat released from GE involving both heat converted to refrigeration in ACh Q_hwGE = 1400 kW (with corresponding Δt_hwA) and heat loss Q_hw.loss = 400 kW extracted into the atmosphere in the emergency radiator (Δt_hw.loss). This temperature drop Δt_hwGE is assumed to be the magnitude providing the maximum ratio RS_hwGE = 1.

Respectively, in the typical IES with ACh, the RS_hwA = Δt_hwA/Δt_hwGE ≈ 0.7…0.73. The additional increment as RS_hw.loss might be achieved due to utilization of the rest of the heat loss: RS_hw.loss = Δt_hw.loss/Δt_hwGE ≈ 0.25…0.30. Herein, RS_hwA = Δt_hwA/Δt_hwGE ≈ Q_hwA/Q_hwGE and RS_hw.loss = Δt_hw.loss/Δt_hwGE ≈ Q_hw.loss/Q_hwGE.

The calculation results on the engine stabilization rates RS_hw due to the utilization of the heat released from the GE are presented in Figure 6. Herewith, its full conversion provides a maximum magnitude of RS_hwGE of about 1.0 (100%).

As seen, in a typical IES with ACh the RS_hwA = Δt_hwA/Δt_hwGE ≈ 0.7…0.73, as well as the additional increment RS_hw.loss due to utilization of the rest heat loss RS_hw.loss = Δt_hw.loss/Δt_hwGE ≈ 0.25…0.30 (Figure 6).

Here,

• RS_hw.A—due to converting heat by ACh in a typical IES with ACh;
• RS_hw.loss—additional increment due to utilization of the rest of the heat loss in the modified system.

According to the guidelines for Jenbacher engines, for each 1 °C increase in return cooled hot water at the GE inlet above the specified limit of 70 °C, the engine’s lifetime LT decreases by about 2%, or for a 5 °C increase, by about 10%.

For example, if the engine’s normal lifetime at 70 °C is 20,000 h (100%), operation at 71 °C could reduce this to around 19,600 h (2%); likewise, at 75 °C, it will be reduced to 18,079 h (90%), i.e., by about 10% (Table 2) [5,28].

The calculation results on the level of engine long-term lifetime LT_hw due to utilization of the heat released from GE are presented in Figure 7.

As seen, full utilization of the released heat provides a maximum magnitude LT_hwGE of about 1 or 100% against LT_hwA of about 0.9 or 90% in typical IES. Herewith, the increment in LT_hwGE of 10% is gained due to the utilization of heat loss, converted by ECh into refrigeration for cooling the intake air.

The following correlations are applied.

There is a decrease in the level of engine lifetime ΔLT_hw.A caused by the increased temperature of hot water from ACh (before extraction of its heat by the emergency radiator into the atmosphere):

ΔLT_hw.A = (t_hwAout − t_hwGEin)0.02 = LT_hw.loss,

where 0.02 or 2% is a decrease in the engine lifetime for each 1 °C increase above the specified limit of 70 °C [5,28].

Corresponding reduced level of engine lifetime:

LT_hwGE = 1; LT_hwGE = LT_hwA + LT_hw.loss;

LT_hwA = LT_hwGE − (t_hwAout − t_hwGEin) 0.02; LT_hwA = LT_hwGE − LT_hw.loss;

LT_hw.loss = (t_hwAout − t_hwGEin) 0.02 = ΔLT_hw.A; LT_hw.loss = LT_hwGE − LT_hwA.

Furthermore, according to the guidelines and empirical data of Jenbacher gas engines, each 1 °C increase in return hot coolant at the inlet of GE above the specified 70 °C causes a reduction in total engine efficiency η_tot by 0.5% [5,28].

So, according to the guidelines for Jenbacher engines, there might be a reduction in overall engine efficiency by 0.5% for each 1 °C increase in return cooled hot water at the inlet of GE above the specified 70 °C [5,28]. Respectively, a decrease in the temperature of the return water from 75 °C after the ACh to 70 °C at the inlet of the GE, i.e., by 5 °C, leads to an increase in the engine’s total efficiency by 2.5%. This is caused by using the return water as a coolant for cooling the engine jacket and lubricating oil.

The effect of full utilization of the heat, released from GE, in the increase in total efficiency and heat efficiency of a modified IES compared to the typical version, is evident from the results in Figure 8 and Figure 9.

The daily variation in total electrical and heat efficiency η_totA due to converting the heat of hot water by ACh in a typical IES with its temperature at the outlet of ACh of about 75 °C, its increment Δη_tot.hwE due to utilization of the heat loss by ECh and their total efficiency η_tot.hwAE due to converting all the heat of hot water in ACh and ECh with temperature of hot water at the inlet of GE of about 70 °C are depicted in Figure 8.

As Figure 8 shows, full utilization of the heat released from GE, for instance, by ECh, provides lower temperatures of return cooled hot water as a coolant at the inlet of GE ≈70 °C compared to about 75 °C when there is incomplete utilization with heat losses extracted to the atmosphere in an emergency radiator, which enables avoiding overheating return water as a coolant by 5 °C.

The latter leads to an increase in GE total efficiency by about Δη_hwE.loss ≈ 2.4% according to a reduction in total efficiency by 0.5% for each 1 °C increase in return coolant temperature over 70 °C [5,28]. Herewith, the total efficiency η_tot.hwAE is increased to about 85% compared to the efficiency η_totA ≈ 82.5% of basic typical IES with incomplete utilization of heat in ACh.

It should be noted that this effect is achieved solely through the full utilization of the heat released and assessed according to the guidelines for Jenbacher engines [5,28].

The following correlations are applied for calculation of the total efficiency of IES.

There is an increase in the total efficiency of GE due to reducing the temperatures of return cooled hot water as a coolant at the inlet of GE by Δt_hw.loss = 5 °C: from 75 °C, when incomplete utilization of the hot-water heat with heat losses, to 70 °C when converting the heat losses by ECh for cooling intake air: Δη_tot.hw.loss = Δη_tot.base Δt_hw.loss 0.005 = 0.024, or 2.4%, where 0.005 is the increase in GE total efficiency for each 1 °C drop in return hot coolant according to [5,28].

A typical IES with ACh without considering heat losses is as follows:

• Electrical efficiency η_el = P_el/Q_gas ≈ 0.45;
• Heat efficiency η_h = Q_h/Q_gas ≈·0.5;
• Total efficiency η_tot.base = η_el+h ≈ 0.95.

Typical IES with ACh with considering heat losses of about 75%:

• Heat efficiency η_h.hwA = 0.75 η_h ≈·0.375;
• Total efficiency η_el+h.hwA = η_el + η_h.hwA = 0.825.

Modified IES with ACh and ECh, converting heat losses for cooling intake air:

• Increase in GE electrical efficiency by about Δη_el.incr ≈ 2.0…2.5% due to cooling intake air in ECh by Δt_Ein;
• Increased total efficiency due to cooling intake air in ECh: η_{el.incr+h.hwA} = η_el.incr + η_h.hwA ≈ 0.82…0.84 (Figure 7);
• Increase in GE total efficiency by about Δη_tot.hw.loss ≈ 2.4% according to its increase by by 0.5% for each 1 °C drop in return hot coolant beyond 70 °C;
• Increased total efficiency η_el+h.hwAE = η_el+h.hwA + Δη_tot.hw.loss ≈ 0.849.

Herewith, the data on electrical power P_el and heat of gas fuel Q_gas for the calculation of heat efficiency η_h = Q_h/Q_gas and electrical efficiency η_el = P_el/Q_gas are taken from the monitoring results (Figure 9).

The results of monitoring data on daily changes in measured electrical power P_el, volumetric gas fuel B_gas consumed hourly by GE JMS 420, calculated heat of gas fuel Q_gas, heat of hot water used by ACh Q_hwA and all the heat released from GE Q_hwGE, heat efficiency of typical IES with ACh η_hwA and modified IES with ACh and ECh using all the heat released η_hwAE (1 June 2019, Mykolayiv, southern Ukraine), are presented in Figure 9.

As seen, the electrical efficiency η_el of gas engine JMS 420 is about 0.47.

The real heat efficiency η_hwA of a typical IES with ACh is about 0.37, whereas the heat efficiency of a modified IES with ACh and ECh using all the heat released η_hwAE is about 0.5.

Herewith, the monitoring data on daily changes in electrical power output P_el, kW, and volumetric gas fuel B_gas, m³/h, consumed by gas engine JMS 420, were measured using an electricity meter SL 7000 Smart (SL761) and ultrasonic gas meter KURS-01 G250 A1 accordingly (Table 3).

The volumetric gas fuel B_gas, m³/h, was used for the calculation of gas fuel heat Q_gas, heat efficiency η_h = Q_h/Q_gas and electrical efficiency η_el = P_el/Q_gas.

Gas fuel heat Q_gas is calculated according to the measured volumetric gas fuel B_gas: Q_gas = B_gas LHV, where LHV = 9500 kWh/Nm³.

Respectively, the heat efficiency of the modified system is calculated as η_hwAE = Q_h/Q_gas, where Q_h = Q_hwGE—heat of utilized hot water in the modified system; the heat efficiency of the typical system η_hwA = Q_hwA/Q_gas, with Q_hwA—heat of hot water utilized by ACh in the typical system.

The overall heat Q_hwGE rejected from GE, the heat converted by ACh Q_hwA to refrigeration and the heat loss Q_hwloss left from ACh and extracted into the atmosphere by the radiator were calculated based on the monitoring data on water flow G_w and changes in temperature of hot water in the engine heat utilization circuit (Figure 3 and Figure 4).

Herein, the following heat balances for GE were conducted based on 0.5G_w (G_w is the hot-water flow for two GE):

• The overall heat released from GE Q_hwGE = 0.5G_w c_w (t_hwGEout − t_hwGEin);
• Heat converted by ACh into cooling capacity Q_hwA = G_w c_w (t_hwGEout − t_hwAout).

The total efficiency of IES was calculated by using electrical power output P_el and the heat Q_h utilized: η_el+hwAE = (P_el + Q_hwAE)/Q_gas for modified system and η_el+hwA = (P_el + Q_hwA)/Q_gas for a typical system.

These findings substantiated the importance of maintaining the specified return cooling water temperature of 70 °C to ensure the efficiency and longevity of Jenbacher gas engines.

Therefore, the rational design of a heat recovery system providing conversion of all the heat released from GE is crucial to prevent deviations from 70 °C, which can lead to significant performance losses and reduced engine lifetime.

So, the next step in the analysis is aimed at assessing the efficiency of converting the heat losses, inherent in typical IES with ACh, into refrigeration for cooling engine intake air by combining ACh and ECh as an example of the simplest chiller in terms of design and ease of implementation into a typical heat recovery system.

3.2. A New Approach to Engine Thermal Stabilization in Both Aspects: Cooling Inlet Air and Full Utilization of the Heat Released

Regarding deeper cooling intake air, meaning more thermally stabilized and fuel-efficient GE operation, a further step in the investigation is focused on assessing the influence of cooling inlet air by ECh, converting the heat loss, on the engine stabilization rate RS_in as a general factor, affecting engine thermal and fuel stabilization [91].

As a thermally stabilized and fuel-efficient operation of GE takes place at reduced temperatures due to cooling intake air, the RS_in is determined as a drop in actual cooled air temperature related to its target magnitude, providing maximum allowable depth of air cooling and the RS_in accordingly.

Taking into account the temperature increase Δt_incr = 5 °C, caused by heat influx from MR through the wall of the air duct to cooled air of mass flow 2G (4 kg/s) in modified IES, the needed cooling capacities Q_0.15G2 provide 15 °C with Δt₁₅ at the central AC outlet and t_in = 20 °C with Δt_in20 at GE inlet accordingly.

The values of cooling capacities Q_0.in23G4 required in a typical system for cooling ambient air with 4G by ACh to 15 °C at the outlet of AC with ACh and to 23 °C at the GE inlet accordingly (increased by 8 °C) and Q_0.15G2 in the modified system when air temperature at the ACh outlet is 15 °C and at the GE inlet 20 °C (increased by 5 °C), available cooling capacities Q_0.E due to converting the heat loss by ECh and their summarized values ΣQ_0.in23G4 τ, ΣQ_0.15G2 τ and ΣQ_0.E τ are depicted in Figure 10.

As shown, the available cooling capacities of ECh Q_0.E are enough to cover the needs Q_0.20G2 and ΣQ_0.20G2 τ for cooling air of twice flow 2G to 20 °C at the GE inlet, which is increased by 5 °C due to heat influx to cooled air in the air duct, compared to its temperature of 15 °C at the AC outlet in the modified cooling system.

In turn, in a typical IES, when ambient air of 4G flow is cooled by ACh and mixed with the air in the machine room before being sucked by GE, the temperature of the mixed air is increased by 8 °C according to monitoring data [31]. This leads to considerably enlarged cooling capacities Q_0.in23G4 spent in the typical system for cooling air of flow 4G to temperatures of about 23 °C at the GE inlet (15 °C at the ACh outlet), which are even higher than t_in = 20 °C at the GE inlet with less cooling needs Q_0.20G2 and ΣQ_0.20G2 τ in the modified cooling system.

Regarding deeper cooling inlet air, the more thermally stabilized GE operation, the cooling capacities Q_0.in15G2 needed for cooling ambient air of twice flow 2G to 15 °C at the GE inlet (10 °C at the AC outlet) in the modified system are assessed (Figure 11).

Daily changes in available cooling capacities Q_0.ECh of ECh converting heat loss and cooling capacities Q_0.in20G2 and Q_0.in15G2 needed for cooling ambient air of twice flow 2G to 20 °C at the inlet of GE (15 °C at the outlet of AC with ACh) and to 15 °C at the inlet of GE (10 °C at the outlet of AC with AECh) in the modified system, and the corresponding sum of refrigeration energy ΣQ_0.in20G2 τ, ΣQ_0.in15G2 τ and ΣQ_0.E τ, are depicted in Figure 11.

As can be seen, the sum of available ECh cooling energy ΣQ_0.E τ is enough to cover the needs ΣQ_0.20G2 τ and ΣQ_0.15G2 τ for cooling air to 20 °C and 15 °C at the GE inlet (15 °C and 10 °C at AC outlet) in the modified cooling system, whereas, to cover the current cooling needs Q_0.15G2 by ECh cooling capacities Q_0.E, it is necessary to recoup the current excesses of Q_0.E at lower ambient air temperatures to compensate for the lack of Q_0.E at increased ambient air temperatures within an interval from 12 to 19 h when Q_0.E is less than Q_0.in15G2. This might be conducted by applying thermal energy storage (TES) technology or by involving the cooling capacity of ACh.

Based on the above, the temperature of 15°C for air at the GE inlet (10 °C at the AC outlet) might be assumed as a target magnitude ensuring the sustainable operation of the GE with thermal stabilization rate RS_in15 = 1. Such an assumption enables assessment of the thermal stabilization rate RSE provided by ECh, converting the heat loss for cooling ambient air at the GE inlet by the available temperature drops Δt_Ein related to cooling inlet air by Δt_in15 to 15°C at the GE inlet (by Δt_in10 to 10 °C at AC outlet with AECh) as the target maximum value.

The calculation results of engine stabilization rates RS_in23G4/15G2 when cooling sucked air to 23 °C at the GE inlet in a typical system, as well as RS_in20/15G2 in the developed system with an air duct when cooling sucked air by Δt_in20 to 20 °C at GE inlet, in both cases related to RSi_n15 when cooling inlet air by Δt_in15 to 15 °C at GE inlet as the target maximum magnitude, are presented in Figure 12.

As can be seen, the maximum daily rate of stabilization RS_20/15G2 is about 0.6 when the intake air of 2G cooling by Δt_in20 to 20 °C at GE inlet (by Δt₁₅ to 15 °C in AC with ACh) in the modified system against RS_in23G4/15G2 of about 0.4 when intake air of 4G cooling by Δt_in23G4 to t_in of about 23 °C in the typical variant (Figure 6).

The range of the lack of ECh cooling capacity, exposed by exceeding the available temperature drop Δt_Ein by the needed values Δt_in15 for cooling air to 15 °C at the GE inlet, is limited by the points of coincidence Δt_Ein = Δt_in15 (Figure 12) or t_Ein = 15 °C (Figure 13). Herein, the conditional lack (deficit) in the rate of thermal stabilization as 1 − RS_Ein/15 varies from 0 to 0.4 accordingly (Figure 13).

The findings on the rate of engine thermal stabilization RS analysis, based on daily monitoring data on intake air cooling processes, are further extended over the yearly operation taking into account the actual varying climatic parameters to calculate the current and annual average weighted RS of GE with different cooling systems.

The average weighted rate RS_20/15avr = ΣΔt_in20 τ₂₀/ΣΔt_in15 τ₁₅ of the modified system with ACh and cooled air sucked through a duct is calculated for actual temperature drops Δt_in20 due to cooling air at the inlet of the engine to 20 °C (15 °C in AC with ACh) related to Δt_in15 for 15 °C at the GE inlet (10 °C in AC with stage SAECh) accepted as a target minimum temperature (Figure 14b).

Similarly, the annual average weighted rate RS_Ein.avr = ΣΔt_Ein τ_Ein/ΣΔt_in15 τ₁₅ of the modified system with ECh using heat loss is calculated (Figure 14c) for the available temperature drop Δt_Ein (Figure 12).

The annual average weighted rate RS_in23G4avr for a typical case with ACh and cooled air sucked from machine room RS_Ein.avr is depicted in Figure 14a.

As can be seen, the annual average weighted rate RS_20/15avr of GE when cooling ambient air in ACh to 15 °C at AC outlet and further preheating cooled air in a duct to 20 °C at the GE inlet in the modified system (Figure 14b) is nearly double the annual rate RS_in23G4avr of the GE with a typical cooling system (Figure 14a): 44% against 24%. In the typical system, ambient air of 4G flow is cooled in ACh to 15 °C and injected into the machine room, from which the mixed air is sucked by the engine turbocharger at 23 °C. In turn, ECh converting heat loss enables RS_Ein.avr of 60%.

• RS_20/15G2—cooling inlet air of 2G by Δt_in20 to t_in = 20 °C at GE inlet (15 °C at AC outlet with ACh) related to cooling inlet air by Δt_in15 to t_in = 15 °C at GE inlet (10 °C at AC outlet with SAECh) as a target rated value for the modified system;
• RS_Ein/15—cooling inlet air of 2G by Δt_Ein reduced by 5 °C compared to Δt_E calculated according to heat balance (Equation (7)) to consider heat influx;
• RS_in23G4/15G2—cooling inlet air of 4G by Δt_in23 to t_in = 23 °C at GE inlet (15 °C at AC outlet with ACh) in a typical system related to cooling inlet air by Δt_in15 to t_in = 15 °C at GE inlet (10 °C at AC outlet with SAECh) as a target rated value in the modified system.

The developed modified IES, synthesized from the result of analysis, is depicted in Figure 15.

Daily changes in the overall electrical and heat efficiency due to heat transformed by ACh to refrigeration for the outer needs without intake air cooling η_el+hw.A and with engine intake air cooling by ECh η_el.inc+hw.A using heat loss, the overall efficiency due to converting heat to refrigeration for outer needs by both ACh and ECh without intake air cooling η_el+hw.AE, are presented in Figure 16 [31].

When the excess heat (heat loss) is transformed by ECh to refrigeration, used for lowering the GE-sucked air temperature to generate additional electricity and boost electrical η_el.inc and the overall efficiency η_el.inc+hw.A, the heat efficiency η_hw.A remains the same (Figure 16).

As Figure 16 shows, the increment η_tot.Eloss = η_el.inc+hw.A − η_el+hw.A ≈ 0.022, or 2.2%, due to cooling engine inlet air by ECh converting the heat loss, extracted into the atmosphere in a typical case, is quite close to its value Δη_hw.loss = 0.024, or 2.4%, (Figure 8) gained due to reducing the return hot-water temperature by 5 °C, issuing from the reduction in the overall engine efficiency by 0.5% for each 1 °C above 70 °C [5,28].

Thus, the hypothesis of converting the heat loss, usually extracted into the atmosphere, by ECh boosting the ACh to ensure the return hot water at specified 70 °C and to cool the GE inlet air has been satisfied quantitatively from both aspects of engine thermal stabilization: through full utilization of the heat released from GE, on the one hand, and by cooling engine inlet air, on the other hand. In both cases, the engine stabilization rates RS_hw (Figure 6) due to converting the hot-water heat and RS_in (Figure 12) due to cooling engine inlet air have been applied as indicators.

Moreover, in the case of utilizing the heat, the engine stabilization rates RS_hw are associated with the level of engine lifetime LT_hw as an additional indicator according to [5,28], which quantitatively estimates the reinforced impact of the released heat utilization on the engine’s sustainable operation.

Thus, the full utilization of the return hot-water heat ensures its temperature at the GE inlet of 70 °C and increases the engine stabilization rate from basic RS_hw.A = 0.7, or 70%, to RS_hwGE = 1.0, or 100, which leads to an increased level of engine lifetime from LT_hwA = 0.9, or 90%, at 75 °C for typical case to LT_hwGE = 1, or 100%, at 70 °C for modified IES.

The application of the developed combined two-stage engine inlet air cooling system with AECh enables the engine operation at a practically stabilized low sucked air temperature at varying climatic conditions, which results in monthly B and annual ΣB (Equation (2)) reductions in fuel consumption (Figure 17).

Thus, by minimizing the heat influx to cooled sucked air from the machine room environments, the two-stage air cooling system enables engine performance at stabilized low intake air temperatures under varying climatic conditions.

The considered method of design focuses on providing just the initial basic data as rational technical characteristics for further complicated, detailed economic analysis.

4. Conclusions

4.1. Challenges

Sustainable performance of combustion engines and IES as a whole at a safe thermal level with high fuel effectiveness is possible due to cooling intake air and full utilization of the heat released with hot water and converted into refrigeration, typically by the absorption of a lithium-bromide chiller (LBCh) as the most efficient and widespread. Herewith, the return cooled hot water from the LBCh is used as a coolant for removing the heat from the engine jacket and lubricating oil, for which the temperature at the engine inlet is not to be higher than 70 °C to ensure a safe thermally stabilized performance of the engine.

Incomplete utilization of the heat, caused by a hot-water temperature drop in LBCh of about 15 °C against the available 20 °C, leads to exceeding this value and shortening the engine lifetime by 10% at 75 °C.

To ensure thermally stable performance of the engine in a conventional IES, the residual heat left from LBCh is rejected into the atmosphere by the emergency radiator with inevitable heat losses of about 25% as the “cost” for safe engine performance.

4.2. Solutions

The aspect of enhancing the sustainable performance of an IES due to ensuring the long-term lifetime of operation through full recovery of the heat released from the GE to provide a temperature of return cooled hot water at the entrance of the GE no higher than 70 °C was investigated. Such a constraint is rigorous because the return cooled hot water is used as a coolant for the engine jacket and lubricating oil.

The universal methodological basis to satisfy the challenges from the aspects of cooling inlet air and full utilization of the heat from the GE was developed based on the rate of thermal stabilization (RS), calculated as a ratio of the actual temperature drop of air Δt_in (hot water Δt_hw) to its design value.

Herewith, the RS is considered the unified criterion to combine both aspects in a joint chain through the utilization of the heat loss of about 25%, inherent in a conventional IES, for cooling the engine intake air and thereby enhancing engine efficiency.

It is proven that the available ECh cooling capacities Q_0.E, gained due to the utilization of the heat loss, are enough to cover the current Q_0.in20G2 and sum needs ΣQ_0.in20G2τ for cooling air to 20 °C at the GE inlet.

This provides an enhancement in the annual average weighted rate RS_avr due to cooling intake air by converting heat loss in ECh to about 60% in a modified cooling system with cooled air transported by a special duct, against 24% for the typical case with LBCh and cooled air sucked from the machine room.

Moreover, cooling the intake air by converting the heat loss enables an absolute increase in the GE’s total efficiency by 2.0…2.5%.

The hypothesis of estimating the efficiency of the exhaust heat utilization by the rate of thermal stabilization RS has been further extended through its application for evaluating the level of engine lifetime LT from basic LT_hwA = 90% for a typical system with incomplete utilization of the heat in LBCh to LT_hwGE = 100% for a modified system with full utilization of the heat, including heat loss converted in boost ECh.

Thus, the bridge between the full utilization of the heat released and cooling intake air was built based on the unified criteria adapted to provide a sustainable long-term lifetime and fuel-efficient operation of GE as a prosperous trend in IES and energetics as a whole.

The further investigation aims to evaluate the input of intake and charge air cooling separately upon the GE thermal stabilization to develop a universal cooling system to cover actual cooling needs by redistributing available capacity between both objects of cooling.