## 3 Solar Field Model
### Solar Field Control Structures
#### 3.1.2 HTF Main Control Valve at the Inlet of the SF (HTF MCV)  
The HTF temperature at the outlet of the SF is regulated using HTF MCV at the inlet of the SF. This control process is performed by regulating the thermal oil mass flow passing through the SF. Two processes are achieved via this control valve: firstly, after sunrise the thermal oil mass flow in the SF is regulated at a constant value of 2.5 kg/s per loop. The thermal oil flow is kept constant at this value until achieving the design temperature of the HTF (295 \({}^{\circ}\)C) at the inlet of the SF. Secondly, the thermal oil temperature at the outlet of the SF continues to rise until it reaches its nominal value of (393 \({}^{\circ}\)C). Thereafter, this design outlet temperature of the HTF is kept, using this controller (HTF MCV), by increasing or reducing the thermal oil mass flow passing through the solar field. It is worth mentioning that there is a control valve before the inlet of each loop; these valves are controlled by the same procedure that applies to the HTF MCV. The advantage of using these valves is to maintain the design temperature of thermal oil (393 \({}^{\circ}\)C) at the loop outlet, when some clouds prevent the solar radiation from falling on a certain loop. Unfortunately, in this study, the location of loops adversely affected by the existing clouds was not available from the supplier. Therefore, the decrease in the heat collected within the solar field was evenly distributed among all loops. It should be mentioned here that each selector includes two functions. These functions are chosen based on one, two, or more boundary conditions for these selectors. The HTF MCV is modelled with four selectors, as demonstrated in Figures 3 and 4. The boundary condition of selector 1 is when the DNI has reached a value more than 25 W/m\({}^{2}\), when selector 1 will change its function from a certain orifice (which keeps the thermal oil mass flow at 156 kg/s) to the second function (increase HTF mass flow from 156 kg/s to 390 kg/s). The boundary condition of selector 2 is when the thermal oil temperature reaches the design inlet value of 295 \({}^{\circ}\)C. After achieving this condition, selector 2 will choose the second function, which regulates the thermal oil mass flow rate to reach the design temperature of the thermal oil at the outlet of the SF (393 \({}^{\circ}\)C). Selector 3 consists of two functions; the first function is applied during normal cases (clear periods), and the second function is activated during the cloudy periods. The second function of selector 3 is enabled when two boundary conditions are achieved: the first condition is that the thermal oil mass flow continues decreasing down to a minimum condition value of 312 kg/s, and the second condition is that the HTF temperature decreases to below 393 \({}^{\circ}\)C. After achieving both conditions, the thermal oil mass flow is still unaltered at a minimum value of 312 kg/s, despite the decline in the design outlet temperature of the HTF. Two functions are passed through selector 4, the first of which comes from selector 3. When two boundary conditions are achieved (312 kg/s and 377 \({}^{\circ}\)C), the second function starts maintaining the thermal oil temperature of 377 \({}^{\circ}\)C at the outlet of the SF by closing the HTF MCV gradually until it is totally closed.  
Figure 2: DNI control circuit (Simplified).