Process description

4.2.1 Process description of the CAES system integrated with ORC for waste heat recovery

The schematic diagram of the proposed CAES system integrated with the ORC is

shown in Figure 4-1. The organic working fluid of the ORC has two flow path

options for heat recovery in the integrated system because charging and discharging

operations do not occur simultaneously. During the charging operation of the CAES

94

95

the valve V1 to recover waste heat from the intercoolers and aftercooler of the

compressors to generate electricity to reduce the electrical energy taken from the

electricity grid for driving the compressors. During the discharging operation of the

CAES system in the peak period, the organic working fluid will flow through the

valve V2 to recover waste heat from the exhaust gas leaving the recuperator to

generate more electricity for the improvement of system performance. Therefore, an

ORC integrated with the CAES system can recover waste heat during both charging

and discharge operations. This can improve the efficiency as well as reduce the

operating cost of the system. Similar as Columbia Hills CAES system from the

PNNL technical report, 3 hours per day of the charging operation and 6 hours per

day of the discharging operation were specified in this study (McGrail et al., 2013).

4.2.2 Integration of the CAES system with ORC

With regards to the successful operation of a commercial size system, the design

and operating procedures are based on experimentation, process simulation and

demonstration with a small-scale operation (Sulaiman, 2018). The scale-up for the

large-scale chemical process system is very important and need to be considered. It

was noticed that the volume of the initial state of air is huge before compressed by

the compressors due to the large mass flowrate of the air (353 kg/s). The volume of

the initial state of the air exceeds the volume of the compressors from industrial

mechanism and manufacturer. Therefore, there will be three compressors for LPC,

two compressors for HPC1 in real applications, although the schematic diagram

(refer to Figure 4-1) shows only one compressor for LPC or HPC1. There is only

one compressor for HPC2-HPC6 respectively because the volume of compressed air

96

An ORC would have different operating points during charging and discharging

operations because of the different flow rates, operating pressures and temperatures.

This will affect the design of the ORC components, especially the ORC expander.

Hence, there will be two ORC expanders in real applications, although the process

diagram (refer to Figure 4-1) shows only one ORC expander. Two expanders will

be engaged during the charging operation period. However, one of the expanders

will be withdrawn during the discharging operation because the recovered waste

heat during the discharging period is less than that during the charging period.

All of the components of the CAES system integrated with ORC are also taken into

consideration for the economic evaluation of the integrated system in Chapter 6. The

APEA will be used to evaluate each of the components of the integrated system

depended on the database from the real industrial plants and manufacturers.

4.2.3 Saturation curves and operating point of ORC

In conventional steam Rankine cycle, water is used as the working fluid is used to

recover waste heat. However, the temperature of waste heat could not be high

enough to superheat water. When outlet stream of the steam expander contains more

than 15% saturated liquid, this could damage expander blades and reduce the

efficiency of the expander (Desai and Bandyopadhyay, 2009). The dry organic

working fluid (e.g. R123 and R134a) does not have this problem because the dry

organic working fluid does not need to be superheated and the outlet stream of the

ORC expander can be always saturated vapour. The wet organic working fluids (e.g.

R717 and water ) could require superheating (Desai and Bandyopadhyay, 2009;

97

the Temperature-Entropy (T-s) diagram of ORC which includes the following

processes:

1-2: pressure increase in the compression process by the pump;

2-3-4: heat absorption process in the vapour evaporator;

4-5: expansion process through ORC expander;

5-6-1: heat rejection process in the condenser.

Figure 4-2. The T-s diagram of ORC.

The thermo-physical properties of these refrigerants should be considered because

the ORC working fluids have a significant impact on the ORC performance (Wang

et al., 2013). Also, different working fluids have different operating points in the

ORC. For example, Figure 4-3 illustrates the Pressure-Enthalpy (P-h) diagram for

ORC working fluid R123. When using R123 as ORC working fluid to recover waste

heat from a CAES system, all of the pressure operating points should be at or within

the region of the curve (the red line) because the left side of the curve stands for the

98

vapour state of R123 (ASHRAE, 2014). Especially, the operating pressure of

working fluid should be significantly considered. The operating pressure can be

estimated and chosen by the temperature of working fluid heated by the evaporator.

It is noticed that the maximum operating pressure cannot exceed the pressure of the

highest operating point because the pressure at the highest operating point is the

critical pressure of the ORC working fluid.

Figure 4-3. The P-h diagram for ORC working fluid (e.g. R123) (ASHRAE, 2014).