2.1.3. Auxiliary Boiler

To operate the absorption chiller when the captured radiation is insu fficient and the solar tank is depleted, an auxiliary system is employed to maintain the thermal energy at the desired level to drive the thermal chiller, the thermal energy . *Qaux* supplied by the auxiliary boiler can be calculated by Equation (3):

$$\dot{Q}\_{\text{max}} = \frac{\dot{m} \, \mathbb{C}\_P (T\_{\text{set}} - T\_{\text{in}}) + lLA\_{\text{max}} (T\_{\text{aux}} - T\_o)}{\eta\_{\text{max}}} \tag{3}$$

where: *Tin* is the fluid inlet temperature, *Tset* is the thermostat set temperature, *UAaux* refers to the overall coe fficient of loss to the environment, η*aux* is the auxiliary heater e fficiency, and *Taux* is the average temperature can be calculated by Equation (4):

$$T\_{\text{aux}} = \frac{(T\_{\text{set}} - T\_{\text{in}})}{2} \tag{4}$$

In this work, a boiler with a nominal power of 60 kW ( *Qe*/*COP* = 35/0.70 = 50 *KW*) has been selected, and a performance of 90%, which will be assumed constant. For the auxiliary boiler, the parallel arrangemen<sup>t</sup> is preferred to prevent its operation from contributing to the heating of the water in the storage tank.

#### 2.1.4. Heat Rejection System: Cooling Tower

A heat rejection system is attached to the thermal absorption chiller in order to evacuate the heat from the absorber and the condenser of the chiller and eject it to the ambient air. In this paper, the counterflow mechanical wet cooling tower was selected with the Baltimore Aircoil Company (BAC, Madrid, Spain) [32], The selected tower is the FXT-26 model, which has the nominal operating conditions listed in Table 2 and it is capable of dissipating all the heat evacuated by the absorption chiller under any environmental conditions in the location of installation (in our case, Baghdad, Iraq). The counterflow, the forced draft-cooling tower can be modeled in TRNSYS, based on the number of transfer units (NTU) [33]:

$$NTUI = c \left[\frac{\dot{m}\_a}{\dot{m}\_{a^p}}\right]^{(n+1)} \tag{5}$$

where: . *mw* and . *ma* are the mass flow rates of water and air, respectively, *c* and *n* are the coe fficients of mass transfer constant and exponent; their values are given by the manufacturer's curves in this paper, the selected values of *c* and *n* are 0.5 and −0.856 respectively.


**Table 2.** Technical characteristics of the FXT-26 cooling tower [32].

#### 2.1.5. Cooling Cycle: Absorption

.

.

The proposed chiller simulated here is the single-e ffect LiBr–H2O absorption chiller YAZAKI WFC-SC10 (Yazaki Energy Systems Inc., Plano, TX, USA) with a nominal coe fficient of performance (COP) of 0.70 and nominal cooling capacity . *Qe* of 35 kW. The technical specifications of the chiller are listed in Table 3 [34]. For the analysis of facilities, we will always assume a maximum demand capable of being satisfied by this chiller to cover the cooling load (in our case the maximum demand will be 25 kW). The simulation program required data from the chiller catalog that describes the chiller's operating map in order to determine the operating variables. The absorption machines are usually characterized by two basic parameters:

→ COP nominal. COPnom. (0.7 for Yazaki WFC-10, Yazaki Energy Systems Inc., Plano, TX, USA)

.

→ *Qe* Nominal evaporator power *Qe*,*nom*. (35 kW for Yazaki WFC-10, Yazaki Energy Systems Inc., Plano, TX, USA)


**Table 3.** Specifications of the YAZAKI WFC-SC10 absorption chiller [34].

From these, the nominal generator power *Qg*,*nom* is immediately available by simply dividing the nominal cooling power . *Qe*,*nom* by the nominal coe fficient of performance COPnom. The TRNSYS model also requires entering the target temperature to be obtained at the outlet of the evaporator *Te,set*, as well as the temperatures and flows entering the three external circuits: evaporator *Tei*, condenser *Tci* and generator *Tgi*. In this way, the model can determine the load regime in which the chiller works. Under these conditions, two situations can occur: if there is su fficient output power available on the evaporator, the set temperature will be reached. If not, the lowest possible value will be reached with the available power.

.

The instant heat *Qremove* that should be removed from the incoming flow of the child as well as the load fraction *fLoad* are determined by Equations (6) and (7).

.

$$Q\_{\text{recmov}\prime} = \dot{m}\_{\text{c}} C\_{p,\text{c}} (T\_{c\dot{i}} - T\_{\text{c},\text{sct}}) \tag{6}$$

.

.

$$f\_{Load} = \frac{\dot{Q}\_{\text{rerawve}}}{\dot{Q}\_{\text{e,now}}} \tag{7}$$

With the load fraction and the temperatures indicated above (set, evaporator, condenser, and generator) it is possible to access the configuration file, whose structure will be commented on later, and which has been made from the chiller operation curves offered by the manufacturer for a set of operation points, establishing two basic parameters:

→ Fraction capacity *fcapacity*: is the ratio of the evaporator's output power to the nominal power of the chiller. With the manufacturer's data for each of the established operating points, the quotient between the output power it has in each of these conditions and the nominal power of the evaporator is evaluated.

.

$$f\_{\text{capacity}} = \frac{\dot{Q}\_{\varepsilon}}{\dot{Q}\_{\varepsilon, \text{nom}}} \tag{8}$$

where *Qe* is the output power under the particular conditions;

→Energy input fraction *fEnergyinput*: is the ratio of the generator power to the nominal generator power necessary to satisfy the evaporator power. Similarly, it is obtained from the operation curves as: .

$$f\_{Eur gy input} = \frac{Q\_{\mathcal{S}}}{\dot{Q}\_{\mathcal{S},nom}} = \frac{\dot{Q}\_{\mathcal{e}}}{\dot{Q}\_{\mathcal{e},nom}} \cdot \frac{COP\_{nom}}{COP} \tag{9}$$

where *Qg* and COP are the values for the particular evaluation conditions obtained from the manufacture's curves. The maximum output power . *Qe*,*max* that the chiller will be able to offer on the evaporator for each of the conditions evaluated is calculated from Equation (10).

$$
\dot{Q}\_{\varepsilon, \text{max}} = f\_{\text{capacity}} \ast f\_{\text{Energy input}} \dot{Q}\_{\varepsilon, \text{nom}} \tag{10}
$$

On the other hand, the output power of the evaporator will be the minimum between the maximum power it is capable of offering in each of the conditions, and the demand is given by Equation (11).

$$
\dot{Q}\_{\varepsilon} = \text{Min} \, of \, \left( \dot{Q}\_{\varepsilon'} \dot{Q}\_{\text{remnv}} \right) \tag{11}
$$

With this evaporator power value, the flow rate, and the inlet temperature, the outlet temperature of the evaporator *Teo* can be determined. Logically, at partial loads.

.

$$T\_{\rm av} = T\_{\rm ci} - \frac{\dot{Q}\_{\rm e}}{\dot{m}\_{\rm e} \cdot \mathcal{C}\_{\rm pc}} \tag{12}$$

The generator demand is taken from the energy input fraction *fEnergyinput* (whose value has been given by the operating curve file for the operating conditions), multiplied by the standardized generator power.

$$
\dot{Q}\_{\mathcal{S}} = f\_{\text{Energiyinput}} \cdot \dot{Q}\_{\mathcal{S},nom} = f\_{\text{Energiyinput}} \cdot \frac{Q\_{\mathcal{L},nom}}{\text{COP}\_{nom}} \tag{13}
$$

.

The output temperature is an immediate value, the input temperature, and the generator flow rate are known as shown in Equation (14):

.

$$T\_{\mathcal{S}^p} = T\_{\mathcal{S}^i} - \frac{\mathcal{Q}\_{\mathcal{S}}}{\dot{m}\_{\mathcal{S}} \cdot \mathcal{C}\_{\mathcal{P}\mathcal{X}}} \tag{14}$$

If it is assumed that the machine is adiabatic and, therefore, has no heat loss or gain; the power in the condenser is equal to the sum of the generator plus the evaporator:

$$
\dot{Q}\_{\mathcal{L}} = \dot{Q}\_{\mathcal{L}} + \dot{Q}\_{\mathcal{S}} \tag{15}
$$

The output temperature of the condenser *Tco* is calculated in the same way as for the evaporator and the generator:

$$T\_{c0} = T\_{c\dot{l}} - \frac{\dot{Q}\_c}{\dot{m}\_c \cdot \mathcal{C}\_{pc}} \tag{16}$$

Finally, The COP of the chiller is determined by Equation (17) [35]:

$$\text{COP} = \frac{\dot{Q}\_{\text{c}}}{\dot{Q}\_{\text{g}} + \dot{Q}\_{\text{max}}} \tag{17}$$
