Conversion of gas engine waste heat into cold using absorption chillers

A possibility of gas engine waste heat conversion into cold for air conditioning in mines using lithium bromide absorption chillers is investigated. Dependencies of parameters of a thermodynamic cycle and energy indicators of chillers on temperatures of a heating medium and a coolant are obtained using mathematical modelling. It is shown that it is rational to use two chillers with sequential movement of a heating medium and a coolant through them in opposite directions for a full conversion of gas engine waste heat. COP of such a system is 0.733. This allows obtaining 2140 kW of cooling capacity with a coolant temperature of 7 °C when using a gas engine JMS-620 by Jenbacher.


Introduction
Non-traditional energy-saving technologies [1 -15] start to be applied more widely in coal mines of Ukraine at the present time. These technologies include heat pumps based on lowpotential heat and technologies of burning coal mine methane using gas engines (GE). GEs are cogeneration plants (modules) that generate electrical energy and waste heat for heating systems [1]. In some cases, large amounts of generated waste heat are "dumped" into the environment due to the lack of heat consumers. A possibility of converting this heat into additional electric energy is studied in the papers [13 -15]. Excess waste heat of GEs can be converted into cold for air conditioning using lithium bromide absorption chillers (LBAC) in deep coal mines that require air conditioning of mine workings. The usage of LBAC instead of the traditionally used vapor-compression refrigeration systems significantly reduces consumption of electric energy and operating costs of generation of cold. There are known cases of generation of cold using LBAC in the world practice of air conditioning in mines, including those when waste heat from GEs is converted into cold [16].
The goal of this research is to determine thermal modes and energy efficiency of LBAC during utilization of GE waste heat and generation of cold for air conditioning in coal mines under the conditions of a specific scheme of LBAC and temperature modes of heating and cooled media.

Methods
GE JMS-620 by Jenbacher is currently used in Ukrainian mines. This module generates 3035 kW of electric energy and 2920 kW of thermal energy in a form of hot water from the engine cooling system. The water temperature varies from 110 to 70 °C as a result of heat loss. Consider a single-stage LBAC as a consumer of heat, the scheme of which is shown in Figure 1.  1. Scheme of LBAC, where А -absorber, E -evaporator, G -generator, C -condenser, HEheat exchanger of solutions, P1, P2, P3, P4 -pumps, TD1 and TD2 -throttling devices; t s , t h , t w ,temperatures of a coolant, heating and cooling media, °С; q 0 , q cd , q a , q h -specific refrigerating capacity and specific thermal loads of a condenser, absorber, generator, kJ/kg. The numbers indicate characteristic points of flows of working media.
A refrigerant is water vapor and an absorbent is a water solution of lithium bromide LiBr.
The liquid refrigerant is converted into steam in the evaporator E due to the specific refrigerating capacity q 0 supplied from an external source (coolant of the air conditioning system). The steam enters the absorber A and is absorbed by the water solution of LiBr. The specific thermal load of absorber q a is removed by cooling water. A weak solution (with a low concentration of LiBr) is supplied to the generator G by the pump P1 through the heat exchanger of solutions HE. The weak solution is heated in the heat exchanger of solutions HE by the heat received from the strong solution (with a high concentration of LiBr), which comes from the generator G. The refrigerant is evaporated from the solution in the generator G due to the specific thermal load q h obtained from the heating medium (hot water from the GE cooling system) and is directed to the condenser C. Almost pure water vapor is evaporated in the generator G since the boiling points of water and lithium bromide are very different. The refrigerant vapor gives specific thermal load q cd to the cooling water and condenses in the condenser C. The condensate is discharged into the evaporator E through the throttling device TD1. The strong solution of LiBr is discharged into the absorber A through the throttling device TD2 after passing through the heat exchanger of solutions HE.
The absorber A and the evaporator E are located in the lower drum, and the generator G and the condenser C are located in the upper drum. The absorber A and the generator G are film type devices. Their usage allows reducing thermodynamic losses from incomplete solution evaporation during the steam generation and incomplete solution saturation during the absorption compared to the flooded devices. The scheme provides recirculation of solutions and a refrigerant by using pumps P2, P3 and P4 in order to intensify the heat and mass transfer processes.
The operating process of LBAC is graphically represented in Figure 2 in h, ξcoordinates (h is enthalpy, kJ/kg, ξ is mass concentration of lithium bromide in a solution, %). The cycle of solution state change 2 -7 -6 -5 -4 -8 -9 -10 -2 is shown on the background of isobars p a and p h of a saturated liquid. Isobars of the superheated water vapor, which is in equilibrium with the saturated solution of LiBr, are shown in the upper part of the figure. The numbers of characteristic points of the process correspond to the numbers of points shown in the scheme of LBAC (Fig. 1). (2) -concentration of the weak solution at the absorber A outlet (at point 2) ξ a differs from the equilibrium one ξ * a for a temperature t 2 at point 2 by the amount of incompleteness of saturation * a a a -concentration of the strong solution at the generator G outlet (at point 4) ξ r differs from the equilibrium one ξ * r for a temperature t 4 at point 4 by the amount of incompleteness of evaporation r r r (4) Lines 2 -7 and 4 -8 in Figure 2 correspond to processes of heating the weak and cooling the strong solution in the heat exchanger of solutions HE. Point 6, which determines the state of solution before the generator G nozzles, is located on a line 7 -4 of mixing of solutions with parameters of states at points 7 and 4. Similarly, point 9 is located on a line 8 -2 of mixing of solutions with parameters of states at points 8 and 2. The solution is superheated at point 6 compared to a saturation state at temperature t 6 and pressure p h in the generator G. The solution is supercooled at point 9 compared to a saturation state at temperature t 9 and pressure p a in the absorber A. Processes 6 -5 and 9 -10 are considered isobaric-adiabatic. The refrigerant is evaporated during process 6 -5 due to reduction of the solution enthalpy. Further evaporation of the refrigerant occurs during process 5 -4 due to the supply of heat from the heating medium. The solution superheats in this case and the resulting refrigerant vapor is in a state of equilibrium with a saturated solution. The state of saturated solution changes along line 5 -16. Assume that the temperature of formed refrigerant vapor is equal to the average solution temperature t m on line 5 -16 The refrigerant vapor state at the generator G outlet (condenser inlet) is determined by point 3′, at the condenser C outlet -by point 3, at the evaporator E outlet -by point 1′ in the scheme. All these points are located on the vertical ξ = 0.
The initial data for calculating the parameters and constructing the cycle are: t s2 is final temperature of cooled coolant, °С; t h2 is final temperature of heating medium, °С; t w is final temperature of cooling water, °C; Δt 2-8 is temperature difference at a cold end of a heat exchanger of solutions HE, °С; Δt a , Δt g , Δt ev and Δt cd are minimum temperature differences in absorber A, generator G, evaporator E and condenser C, °С; Δp a are refrigerant vapor pressure losses during movement from evaporator E to absorber A, Pa; Δξ a is incompleteness of saturation of solution in absorber A, %; Δξ r is incompleteness of evaporation of refrigerant in generator G, %; α g is coefficient of recirculation of solution in generator G, which is a ratio of mass flow rates of strong solution m at points 11 and 4 ( Fig. 1): 4 11 m m g = α .
Calculation of parameters at the characteristic points of the cycle is performed in the following way.
Temperature of the weak t 2 and strong t 4 solutions at the absorber and generator outlet (at points 2 and 4, respectively) is determined. The boiling t 0 and condensation t cd points of the refrigerant at the evaporator E and the condenser C outlet (at points 1 and 3, respectively), and the temperature t 8 of the strong solution at the outlet of heat exchanger of solutions (at point 8) are determined: The saturated water vapor pressure in the evaporator E p 0 and in the condenser C p cd are determined using the temperatures t 0 and t cd . The pressure in the generator G p h and the absorber A p a . is determined using the formulas (1) and (2).
Concentration of the weak solution ξ a * is determined from the pressure p 0 and the temperature t 2 , concentration of the strong solution ξ r *from the pressure p h and the temperature t 4 (for the theoretical cyclewhen losses from incomplete saturation of the solution in the absorber A and incomplete evaporation in the generator G are absent). Concentration of the weak ξ a and strong ξ r solution for the real cycle is determined using the formulas (3) and (4). After that, the location of points 2, 4, 8 and the solution enthalpy in these points are determined using temperatures and concentrations.
Enthalpy of the heated weak solution (at point 7) is calculated using the heat balance equation of the heat exchanger of solutions HE where q he is heat flow in heat exchanger, kJ/kg; h 4 , h 8 where m 6 and m 11 are mass flow rates of solution at points 6 and 11 of scheme of LBAC, kg/s; h 11 is enthalpy of solution at point 11, kJ/kg; ξ 4 , ξ 7 , ξ 11 are mass concentrations of LiBr in solution at points 4, 7, 11, %. Balance equations are characteristic for the isobaric-adiabatic process 6 -5 of separation of the superheated solution into the saturated solution with parameters of state at point 5 and the superheated water vapor, which is in equilibrium with a saturated solution: where h v5 and m v5 are enthalpy of water vapor (kJ/kg) formed in process 6 -5, and water vapor mass flow rate (kg/s); m 5 is mass flow rate of solution at point 5 of scheme of LBAC, kg/s; h 5 and h 6 are enthalpies of solution at points 5 and 6, kJ/kg; ξ 5 and ξ 6 are mass concentration of LiBr in solution at points 5 and 6, %. Then the concentration of LiBr, the saturated solution enthalpy at point 5 of the cycle, the parameters of the water vapor, which is in equilibrium with a saturated solution, and the mass flow rates m 5 and m v5 are calculated using equations (16) - (18). This is performed considering the dependencies of enthalpy of saturated solution and of superheated water vapor, which is in equilibrium with a saturated solution, depending on the temperature and concentration of LiBr.
The water vapor enthalpy h 3′ , which is formed as a result of the solution boiling during process 5 -4 is determined using the concentration ξ m and the pressure p h . The enthalpy of saturated water vapor h 1′ at the evaporator E outlet (at point 1 ′) is determined using the temperature t 0 .
Calculation of thermodynamic parameters of the solution of LiBr and water vapor are calculated using the formulas given in papers [17,18].
Energy characteristics of the cycle are calculated after determining the parameters at the characteristic points of the cycle: specific refrigerating capacity q 0 , specific thermal loads of condenser C q cd , absorber A q a , generator G q h (kJ/kg) and coefficient of performance (COP): COP indicates how much cold can be obtained in the evaporator E during consumption of a unit of heat in the generator G.
The following limitations are applied to the parameters of cycles of LBACs. The mass concentration ξ r of the strong solution of LiBr must not exceed 64 % due to the danger of crystallization [19], and the solution degassing zone Δξ should be in a range of 3.5 to 4.5 % [20], according to the rational hydraulic conditions of the absorber A and the generator G.

Results and discussion
The specific refrigerating capacity q 0 produced by LBAC depends on the amount of specific thermal load q h and the chiller COP as shown in the formula (23). These values depend on external conditions, i.e., on the temperatures of heating medium t h and the coolant t s . The temperature t h of heating medium (water from the GE cooling system) can vary from t h1 = 110 °С to t h2 = 70 °С. The coolant of air conditioning systems is cooled in chillers by 10 ... 12 °C [21 -23]. Therefore, the boundaries of changes of temperature range can be assumed equal to t s1 = 17 °С and t s2 = 7 °С.
Calculations are performed according to the method described above in order to establish the influence of temperatures of heating and cooled media on a thermal mode and LBAC efficiency.
The following parameters are assumed for calculations: -final temperature of the water cooling the condenser C and the absorber A, t w = 26 °С; -temperature difference at the cold end of the heat exchanger of solutions HE   Figure 3, and  Table 1). A decrease of t h leads to narrowing of the degassing zone, which at 81 °С (mode 3) becomes equal to the lower boundary of the 3.5 ... 4.5 % range recommended in the paper [20].  Thus, the results indicate that the operating range of final temperatures of the heating medium (from 80 °C to 84 °C) is significantly limited. It is impossible to utilize all the GE waste heat in one LBAC, while reducing its potential to 70 °C and cooling the coolant to 7 °C. The analysis also shows that the increase of final coolant temperature allows reducing the heating medium temperature (modes 10 -15), and the solution degassing zone widens as well.
The scheme of simultaneous operation of two LBACs with the sequential movement of the heating and cooled media through them is suggested (Fig. 5) considering the character of change of parameters of the LBAC cycle. This is aimed at increasing the completeness of GE waste heat conversion and, accordingly, the completeness of cooling the coolant for air conditioning systems.
The heating medium first enters LBAC number 1 with a high-temperature generator and a low-temperature evaporator, and then flows to LBAC number 2 with a low-temperature generator and a high-temperature evaporator, as shown in Figure 5. The coolant is first cooled in the evaporator of LBAC number 2, and then the final cooling occurs in the evaporator of LBAC number 1. The cooling capacity of GE JMS-620 is 2920 kW [1]. In this case, the cooling capacity generated by a system of two LBACs at COP of 0.733 is 2140 kW.

Conclusions
Dependencies of COP of heat into cold conversion and solution degassing zones on temperatures of heating and cooled media are established on a basis of mathematical modelling of thermal modes of a single-stage LBAC with a film generator, absorber and recirculation of solution of LiBr. A scheme of a system of two LBACs operating under different temperature modes is suggested. This allows fully utilizing GE JMS-620 waste heat, reducing its heat potential from 110 to 70 °C, and providing necessary temperature reduction of a coolant for an air conditioning systems from 17 to 7 °C. COP of heat into cold conversion of a system is 0.733. Research results evidence a potential of practical implementation of a suggested technical solution.