Update example with variable flow rate with figure of COP over time and...

The time-profile of the flow rate used in the simulation is shown in Fig. 1.

![DU_variable_flow_rate_A_T_SRtimeplot_flowrate.png](images/DU_variable_flow_rate_A_T_SRtimeplot_flowrate.png)

![DU_variable_flow_rate_A_T_SRtimeplot_flowrate.png](images/DU_variable_flow_rate_A_T_SR_timeplot_flowrate.png)

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Fig 1: Daily-variable flow rate in each pipe of the double U-tube BHE over the simulated period.

The inlet temperature and calculated outlet and borehole wall temperature are shown in Fig. 2. 

![DU_variable_flow_rate_A_T_SRtimeplot_T_bave_T_fi_T_fo.png](images/DU_variable_flow_rate_A_T_SRtimeplot_T_bave_T_fi_T_fo.png)

![DU_variable_flow_rate_A_T_SRtimeplot_T_bave_T_fi_T_fo.png](images/DU_variable_flow_rate_A_T_SR_timeplot_T_bave_T_fi_T_fo.png)

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Fig. 2: Inlet, outlet and borehole wall temperature over the simulated period.

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The Coefficient of Performance (COP) of the fluid circulation pump is shown in Fig. 3.

![DU_variable_flow_rate_A_T_SR_timeplot_COP.png](images/DU_variable_flow_rate_A_T_SR_timeplot_COP.png)

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Fig. 3: COP of the fluid circulation pump over the simulated period.

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