Validation of single effect AC model

A LiBr–H2O single-effect AC energized by the hot distillate water recovered from the

MSF stages provides a cooling effect that can be used to reduce GT inlet temperature and thus augment the cogeneration plant power produced. The single-effect AC was modelled using the IPSEpro refrigeration library and validated against the existing unit and its physical possibility confirmed on a Dühring chart. The model was then modified to fit with Oman climate parameters, and the Dühring chart used again to confirm its continuing thermal applicability in these new conditions.

The model-simulated data were used to perform detailed energy and exergy analyses of this system with distillate extracted up to various MSF power stages to investigate the effect of change of powering source parameters on unit performance indicators. In addition, the single-effect AC exergy analysis takes into account the chemical exergy that most authors in this field have neglected [100, 107]. A parametric study was carried out to find the impact of variations of the chilled water temperature and cooling water temperature on a single-effect AC performance.

While cooling, the GT inlet temperature is an established technology, and the single-effect AC powered from MSF stage hot distillate water is considered to be original. The effect of this hybridization on cogeneration plant performance was investigated for the three actual operation scenarios studied in Chapter 3.

To ensure the model reflects a realistic AC performance, working data for an existing unit were obtained from the literature [165] and accordingly, a model for this case was built. Table 8.1 presents the specifications uploaded into the IPSEpro model described in Figure 8.1. The labelled components are linked by the numbered streams used to indicate the thermodynamic properties of the fluid transferring from one component to another. Some of these properties were fixed in the model (called set values) as they were obtained from the unit specifications and the model (represented as calculated values) calculated the others. Appendix 8-A identifies the stream properties and whether set or calculated.

M. A. Al-Washahi 156 Newcastle University

Description Unit Specifications

Manufacturer – Carrier Sanyo

Model – LJ

Heat source – Hot water

Cooling type – Series (absorber/condenser)

Working fluid – LiBr–H2O

Capacity kW 2213

Hot water temperature °C 90

Hot water flow l/s 151

Cooling water temperature

(inlet/outlet) °C 28.5/34.5

Chilled water temperature

(inlet/outlet) °C 16/6.0

Pressure (high/low) bar (abs) 0.067/0.0084

solution concentration

(strong/weak) % 59.5/55.0

Table 8.1: Single-effect AC specifications [165]

Figure 8.1: Schematic drawing of LiBr–H2O AC IPSEpro model

High Pressure

M. A. Al-Washahi 157 Newcastle University For model validation comparisons between model results and existing unit, the performance data [165] are shown in Table 8.2. The compared values have a maximum difference of 4.1% for COP, while other parameters have acceptable matching. Moreover, Sanyo AC suggests an energy balance requires that the heat amount coming into the chiller cycle (heat transfer to the generator and evaporator) should be almost equal to the heat rejected from the cycle (heat rejected to cooling water of absorber and condenser) [166]. The model energy balance results reflected exact matching between both these heat amounts but a 7 kW difference can be observed for the existing unit. This small difference could be due to the assumption of specific heat values for the heat calculations or minor heat losses to atmosphere (which are neglected in the model).

Parameter Unit Existing

unit data Model Result Differences (%) Coefficient of performance (COP) – 0.74 0.77 4.1 Refrigeration capacity kW 2213 2283 3.2

Generator heat transfer kW 2987 2957 1.0

Generator outlet temperature ºC 85.0 85.2 0.24

Generator and evaporator heat

transfer kW 5200 5240 0.77

Absorber and condenser heat

transfer kW 5193 5240 0.91

Cooling water flow kg/s 211 209 0.95

Chilled water flow kg/s 52.6 54.3 3.2

Table 8.2: Single-effect AC validation

Any proposed AC cycle should be checked for thermal practicality by superimposing thermodynamic state points on a Dühring chart [92, 167], as this helps avoid a number of pitfalls, such as avoiding the crystallization [167]. On Figure 8.1, points (1, 4, and 8) are saturated liquid; (10) is saturated vapour; (2, 3, and 5) are sub-cooled liquid; (7) is superheated vapour; and the other two (6, 9) are the two-phase vapour-liquid phase. These thermodynamic state points plotted on the Dühring chart (Figure 8.2) show a practical thermodynamic working AC cycle with working parameters away from the crystallization line.

M. A. Al-Washahi 158 Newcastle University Figure 8.2: Single-effect AC (Table 8.1 and 8.2) on a Dühring chart 8.1.2 AC powered by MSF hot distillate

As mentioned in Chapter 7, the hot distillate water from the different MSF desalination stages (i.e., the powering source for the single-effect AC) varies in temperature and mass flow rate. The hot distillate water temperature drops as stage number increases, while the accumulative distillate mass flow rate increases (Figure 7.1), so it was concluded that using only the first eight stages both guarantees a higher exergy efficiency of the MSF desalination and a suitable temperature for powering heat recovery technologies.

The previous model was updated with the MSF minimum hot distillate water temperature, and with the average site recorded cooling temperature (27ºC) while maintaining the 2°C pinch point between the cooling water temperature and absorber outlet temperature (T1).

From this updated model, it was found that the minimum working temperature that could be used to power the single-effect AC (at 1.4 kPa low pressure cycle and 4.5% solution concentration difference between strong and weak LiBr solutions) is 72ºC. This matches with the stage 7 MSF hot distillate parameters. The impact on the low pressure side caused an increase in saturated vapour evaporator temperature (T10) to 12.0ºC, leading to an

increase in chilled water outlet temperature. The chilled water inlet temperature was chosen to be 18ºC, the minimum ambient temperature recorded for this site, whereas the outlet was kept at 14ºC to maintain the 2ºC pinch point temperature between the temperatures of the evaporator (T10) and the chilled water (T17) [104]. In addition, the

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M. A. Al-Washahi 159 Newcastle University temperature difference (T11–T12) was kept constant at 4.85ºC to ensure no impact on the

original MSF desalination unit (Figure 7.15).

To ensure realistic thermodynamic representation, the modified single-effect AC model state points were plotted for all power stages on a Dühring chart. Figure 8.3 shows the working envelope for the AC powered by the first and last (7th) hot distillate water stage, with the other stages located between these (Appendix 8-B lists the details for each heat recovery stage).

Figure 8.3: Single-effect AC powered by MSF hot distillate water (Stages 1 and 7) on Dühring chart

8.2 Analysis of single effect AC powered by MSF stages