## 4 Methodology
### Analysis of an Upgraded Method of Operation of Various Building-Energy Systems
#### 4.3.2 Controlling the Heat-Recovery Air Handling Unit's Operation  
The principle of indoor air temperature control is shown in Figure 12. The air supplied to the heat recovery unit is preheated or cooled in a ground heat exchanger of the pipe-in-pipe type located at a non-freezing depth. The supply air temperature (sensor \"1\"), which is fed into the rooms after treatment, is controlled centrally for all rooms. The control actuators are the three-way control valves of the heater \"3\" and the cooler \"4\". The controller \"5\" calculates the actual supply air temperature depending on the set indoor air temperature setpoint (sensor \"2\") in the reference room, depending on whether it is a winter or summer operation. The controller controls the flow of the heating medium to the heater by comparing the setpoint of the supply air temperature and the temperature of the heating water at the outlet of the heat storage tanks (sensors \"5\", \"6\") by controlling the actuator of the three-way change-over valve \"9\". The chiller is supplied with refrigerant  
Figure 11: Block diagram of thermal barrier temperature control for active thermal protection function mode. \(\theta_{TB}\)—thermal barrier temperature; \(\theta_{sp,h}\)—thermal barrier temperature setpoint—heating mode; \(\theta_{sp,c}\)—thermal barrier temperature setpoint—cooling mode; \(\theta_{SA}\)—output solar absorber temperature; \(\theta_{LT}\)—output long-term heat store temperature; \(\theta_{S\text{-}THS}\)—output short-term heat store temperature; \(\theta_{S\text{-}TCS}\)—output short-term cold store temperature; \(\theta_{L\text{-}TCS}\)—output long-term cold store temperature; \(\theta_{s}\)—heating/cooling water supply temperature behind the three-way mixing valve before the controller; \(\theta_{s}\)’—heating/cooling water supply temperature behind the three-way mixing valve valve after the controller. from long-term cold storage. The temperature at the outlet of this reservoir is sensed by a sensor \"8\". Figure 13 shows a block diagram of the indoor air temperature control system. Its desired value is set separately for winter and summer operations. In heating operation, heating water is fed to the circuits from the long-term heat storage tank and the short-term heat storage tank. The indoor air temperature \(\theta_{i}\) is controlled to the desired value \(\theta\)sp by changing the supply air temperature \(\theta_{s}\) by controlling the heater's actuator's three-way mixing valve. The outlet temperature of the long-term heat storage tank \(\theta_{L\text{-}THS}\) and the outlet temperature of the short-term heat storage tank \(\theta_{S\text{-}THS}\) is also sensed. If \(\theta_{i}=\theta_{sp}\), then no control intervention is required. If this condition is not met, the procedure is as follows:  
1. If \(\theta_{i}<\theta_{sp}\), the supply air temperature \(\theta\)s must be increased and at the same time the heating water supplied upstream of the three-way valve must have a higher temperature than the calculated supply air temperature \(\theta_{s}\). The controller compares the outlet temperatures \(\theta_{L\text{-}THS}\) and \(\theta_{S\text{-}THS}\) in a fixed order and redirects the flow of heating water from the device that satisfies the condition. If \(\theta_{S\text{-}THS}<\theta_{s}\), it will commission the peak heat source. It then controls the actuation of the three-way mixing valve of the heater, thus opening the direct path;
2. If \(\theta_{i}>\theta_{sp}\), the supply air temperature \(\theta_{s}\) must be reduced. The controller controls the heater's three-way mixing valve's actuator, closing the valve's direct path;  
Figure 12: Block diagram of indoor air temperature control during the cooperative air handling unit operation.