5. Advanced control strategies and simulations

#### 5.1. System disturbance

4.3. Actual model of the DHS

1.4, 1.5, 1.35, 1.4], respectively.

design value simultaneously.

In practice, the affluent factors of both heat transfer area of each substation and terminal are greater than 1 because of the safety consideration from designers. The circulation water flow rate could be adjusted rather than design values. With these situations, the ideal dynamic model should be modified to simulate the real DHS, which is entitled as actual dynamic model. Regarding the experience and operational data of typical DHSs in China, the affluent factors of each heat transfer area and terminal in Substations #1–#3 are provided as [1.4, 1.4,

100 Sustainable Buildings - Interaction Between a Holistic Conceptual Act and Materials Properties

While outside and indoor air temperature and water mass flow rate in primary and secondary system are identical to their design values, no solar radiation and internal gains exist, the water leakage and heat losses from pipe segments are considered, the control signal of fuel equals to 0.854 and the dynamic responses of the temperatures from actual model are shown in Figure 4. In this figure, the steady-state values of the supply and return water temperatures from the heat source and Substations #1–#3 are 97.5, 33.2, 57.6, 31.6, 58.6, 33.6, 38.4 and 28.1C, while the zone air temperatures equal to 20.8, 19.6 and 18.8C, respectively. From the values, the supply water temperature from the heat source is not necessary to satisfy its design value (120C), while outside air temperature is 16.9C. Meanwhile, the zone air temperatures are not same as the design values. The reason behind is that the affluent factors of the heat transfer area affect the operation very much in the DHS. It is also hinted that the zone air temperature should be controlled separately because they cannot approach its

(a) (b)

(c) (d)

Figure 4. Dynamic responses of actual model (a) Time(h), (b) Time(h), (c) Time(h), (d) Time(h).

Usually, the disturbances taking place in DHSs include outdoor air temperature, solar radiation and internal heat gains, while outdoor air temperature plays the biggest rule in system operation. On the other hand, when the comprehensive heat transfer coefficient (Uen value) of the buildings is getting smaller and smaller, the additional heat gains (solar radiation and internal gains) should be considered in the simulation and in real system operation. In this chapter, outdoor air temperature, solar radiation and internal heat gains are drawn into actual model with the range from 8.2 to 13.1C, from 0 to 45 W/m<sup>2</sup> and from 0.9 to 6.8 W/m<sup>2</sup> , respectively, for all simulations of the cases.

#### 5.2. Control signals

In many circumstances, DHSs are operated with experience; likely, the supply water temperature from the heat source has been controlled depending on the experience of operators. Nevertheless, the disturbances described above change based on time. It means that the heating supply from the heat source and the heat consumption (heating load) should be tracked and balanced. Thus, the DHS must be regulated accordingly. Otherwise, the zone air temperature could fluctuate in larger range, which influences thermal comfort of end-user. By simulating the dynamic responses of actual model with different conditions (change outdoor air temperature, indoor air temperature as similar as design value, design water mass flow rate in the pipe network, constant water leakage rate, considered pipe insulation heat loss, no solar radiation and internal gains), the simulated stable results from OLTs are listed in Table 2 as set points for related parameters used in control strategies.

#### 5.3. Control strategies

Five cases are selected for dynamic simulations (given in Table 3) to study the system responses, the energy consumption (heat consumed in the cases) and the thermal comfort of the end-user [14–17]. Note that typical PI algorithm is used to all controllers to gather output signals [18].

#### 5.4. Case study based on dynamic simulation

#### 5.4.1. Case 1

Many operators run DHSs according to their experience if they cannot realize the set points of supply water temperature from the boiler. In this case with 5 days consciously, the dynamic


Table 2. Set points used in control strategies.

responses of the DHS are presented in Figure 5. From this figure, the supply water temperature from the boiler changes depending on the outside air temperature (Figure 5(a)). The average water temperature responses of the secondary side in Substations #1 and #2 are almost similar and higher than that in Substation #3 due to the difference between the radiator and the floor heating terminals (Figure 5(b)). The difference of indoor air temperature dynamic responses shown in Figure 5(c) is mainly resulted from the structure of terminals. By considering the responses of the last 2 days rather than the influences of the initial parameter settings, the average and the range of the zone air temperatures in Substations #1–#3 are 20.2, 17.4–24.1, 20.9, 18.1–25.2, 21.1 and 20.7–22.2C, respectively. The control signal of fuel in the heat source is changed based on the heating load (Figure 5(d)). The water mass flow rate in the

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From the simulation by using experience of the supply water temperature from the heat source, the average zone air temperatures excess their design values with bigger fluctuation. This situation should be improved by utilizing tuned supply water temperature set points (Ts1sp). With Ts1sp given in Table 2, the simulation results are shown in Figure 6. With the tuned setting value, the average zone air temperatures in Substations #1–#3 are given as 19.7, 20.2 and 20.1C, meaning that average zone air temperatures are reduced comparing with

From Cases 1 and 2, the average and the fluctuation of zone air temperatures are still larger than the expected results. Because the heat properties of the substations are different, indoor air temperatures should be controlled separately to balance their heat supply and requirement. In this situation, the simulation is made and shown in Figure 7 for zone air temperature responses only, and considering the time-consuming simulation, the time span is decreased to

(a) (b)

(c) (d)

Figure 6. Dynamic responses in case 2 (a) Time(h), (b) Time(h), (c) Time(h), (d) Time(h).

pipe network is set to be the design values.

5.4.2. Case 2

those in Case 1.

5.4.3. Case 3


Figure 5. Dynamic responses in case 1 (a) Time(h), (b) Time(h), (c) Time(h), (d) Time(h).

considering the responses of the last 2 days rather than the influences of the initial parameter settings, the average and the range of the zone air temperatures in Substations #1–#3 are 20.2, 17.4–24.1, 20.9, 18.1–25.2, 21.1 and 20.7–22.2C, respectively. The control signal of fuel in the heat source is changed based on the heating load (Figure 5(d)). The water mass flow rate in the pipe network is set to be the design values.

#### 5.4.2. Case 2

controller

Cf, C1, C2, C3

Cf, C1, C2, C3

Cf, C1, C2, C3

Cf

Cf

responses of the DHS are presented in Figure 5. From this figure, the supply water temperature from the boiler changes depending on the outside air temperature (Figure 5(a)). The average water temperature responses of the secondary side in Substations #1 and #2 are almost similar and higher than that in Substation #3 due to the difference between the radiator and the floor heating terminals (Figure 5(b)). The difference of indoor air temperature dynamic responses shown in Figure 5(c) is mainly resulted from the structure of terminals. By

102 Sustainable Buildings - Interaction Between a Holistic Conceptual Act and Materials Properties

Case Control strategy Description Used

based on tuned set points

equivalent To (Toe)

controlled based on Tz

(a) (b)

(c) (d)

Figure 5. Dynamic responses in case 1 (a) Time(h), (b) Time(h), (c) Time(h), (d) Time(h).

average water temperature in the secondary system of each substation controlled based on tuned set points

Supply water temperature from the heat source controlled based on To and average water temperature in the secondary system of each substation controlled based on

Supply water temperature from the heat source controlled based on To and zone air temperature in each substation

1 Experienced Ts1 control Supply water temperature from the heat source controlled based on experience

2 Tuned Ts1 control Supply water temperature from the heat source controlled

3 Tuned Ts1 and Tw2arg control Supply water temperature from the heat source and

4 Tuned Ts1 controlled based on

on Tz

To and Tw2arg controlled based on Toe

5 Tuned Ts1 controlled based on To and zone air temperature controlled based

Table 3. Control strategies used in the cases.

From the simulation by using experience of the supply water temperature from the heat source, the average zone air temperatures excess their design values with bigger fluctuation. This situation should be improved by utilizing tuned supply water temperature set points (Ts1sp). With Ts1sp given in Table 2, the simulation results are shown in Figure 6. With the tuned setting value, the average zone air temperatures in Substations #1–#3 are given as 19.7, 20.2 and 20.1C, meaning that average zone air temperatures are reduced comparing with those in Case 1.

#### 5.4.3. Case 3

From Cases 1 and 2, the average and the fluctuation of zone air temperatures are still larger than the expected results. Because the heat properties of the substations are different, indoor air temperatures should be controlled separately to balance their heat supply and requirement. In this situation, the simulation is made and shown in Figure 7 for zone air temperature responses only, and considering the time-consuming simulation, the time span is decreased to

Figure 6. Dynamic responses in case 2 (a) Time(h), (b) Time(h), (c) Time(h), (d) Time(h).

2 days continuously. The results illustrate that the fluctuation of zone air temperatures is reduced significantly, but average zone air temperatures are still high compared with their design values.

equivalent outside air temperature is introduced to reset the original average water temperature set points and improve the stability and decrease zone air temperature swing. The equivalent outside air temperature is calculated in Eq. (16). This case is simulated and addressed in Figure 8. Compared with Figure 7, the purpose of decreasing zone air tempera-

Advanced Control Strategies with Simulations for a Typical District Heating System to Approaching Energy…

Toei ¼ To þ qsolsFsi þ qintFi

temperature control to improve the dynamic response of zone air temperature.

As known that, average water temperature in a terminal is related to the zone air temperature indirectly, and the return water temperature from the terminal is delayed due to the thermal capacity of the terminal. If the zone air temperature is measured and applied for the control strategy, it would be better to elevate the thermal comfort of indoor environment. To this end, the simulation is made and shown in Figure 9. The dynamic responses of zone air temperatures in Substations #1–#2 are improved very much. Because of the huge thermal capacity of the radiant floor heating structure, the zone air temperature in Substation #3 although approaches 18�C still needs more advanced control strategy such as predictive control or two-

Due to relevant smaller parts of electricity and water consumption in DHSs, the heat consuming is considered only for energy comparison. The simulated results in the fuel control signal responses are presented in Figure 10. From this figure, the fuel consumption in Case 2 has the lowest value but with larger zone air temperature fluctuation. By observation with all cases,

U�<sup>1</sup>

eni (16)

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ture fluctuation is realized perfectly:

5.5. Comparison with energy consumption

Figure 9. Dynamic responses in case 5.

5.4.5. Case 5

#### 5.4.4. Case 4

The meaning behind average zone air temperatures excessed the deign values is that the disturbances are never considered in the control algorithm. Consequently, a concept of an

Figure 7. Dynamic responses in case 3.

Figure 8. Dynamic responses in case 4.

equivalent outside air temperature is introduced to reset the original average water temperature set points and improve the stability and decrease zone air temperature swing. The equivalent outside air temperature is calculated in Eq. (16). This case is simulated and addressed in Figure 8. Compared with Figure 7, the purpose of decreasing zone air temperature fluctuation is realized perfectly:

$$T\_{o\text{ei}} = T\_o + \left(\eta\_{s\text{ds}} F\_{si} + \eta\_{\text{int}} F\_i\right) \mathcal{U}\_{eni}^{-1} \tag{16}$$

5.4.5. Case 5

2 days continuously. The results illustrate that the fluctuation of zone air temperatures is reduced significantly, but average zone air temperatures are still high compared with their

104 Sustainable Buildings - Interaction Between a Holistic Conceptual Act and Materials Properties

The meaning behind average zone air temperatures excessed the deign values is that the disturbances are never considered in the control algorithm. Consequently, a concept of an

design values.

5.4.4. Case 4

Figure 7. Dynamic responses in case 3.

Figure 8. Dynamic responses in case 4.

As known that, average water temperature in a terminal is related to the zone air temperature indirectly, and the return water temperature from the terminal is delayed due to the thermal capacity of the terminal. If the zone air temperature is measured and applied for the control strategy, it would be better to elevate the thermal comfort of indoor environment. To this end, the simulation is made and shown in Figure 9. The dynamic responses of zone air temperatures in Substations #1–#2 are improved very much. Because of the huge thermal capacity of the radiant floor heating structure, the zone air temperature in Substation #3 although approaches 18�C still needs more advanced control strategy such as predictive control or twotemperature control to improve the dynamic response of zone air temperature.

#### 5.5. Comparison with energy consumption

Due to relevant smaller parts of electricity and water consumption in DHSs, the heat consuming is considered only for energy comparison. The simulated results in the fuel control signal responses are presented in Figure 10. From this figure, the fuel consumption in Case 2 has the lowest value but with larger zone air temperature fluctuation. By observation with all cases,

Figure 9. Dynamic responses in case 5.

Nomenclature

f factor

c specific heat (J/kgC)

F heated floor area (m<sup>2</sup>

G water mass flow rate (kg/s) HV heating valve of fuel (J/kg)

q heating load per m2 (W/m<sup>2</sup>

Q heating load (W)

T temperature (C)

U heat transfer rate (W/C)

3 number of substation

en enclosure of building

hl heat loss from pipe segment

ex heat exchanger

ht heater-radiator

j refer to pipe segment j

f fuel

i 1–3

in inlet

int internal

u control signal

t time (s)

arg average b boiler d design

Subscripts

C thermal capacity (J/C) or controller

)

)

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LMTD logarithmic mean temperature difference (C)

1, 2 number of substation or primary/secondary system

Figure 10. Comparison with fuel consumption.

Case 5 is the best control strategy based on both dynamic responses of zone air temperature and the fuel consumption. Without measuring zone air temperature for compensation, Case 4 is the best one for optimal operation of the DHS.

#### 6. Conclusions

