# Design of a 35 kW Solar Cooling Demonstration Facility for a Hotel in Spain

^{1}

^{2}

^{3}

^{4}

^{*}

## Abstract

**:**

^{2}). From an economic point of view, the saving of 1515 euros per year when compared with an electric compression chiller does not compensate the investment of 3000 euros per kW of cooling capacity that cost the thermal solar cooling facility.

## 1. Introduction

_{2}emissions.

_{2}O absorption chillers and the corresponding water consumption [5]. These are the main reasons why most thermal solar cooling facilities currently in operation are part of demonstration projects.

_{2}O single-effect absorption chiller is important to simulate a thermal solar air-conditioning system. TRNSYS includes an absorption chiller model in its library that needs a specific input file based on performance data provided by the manufacturer. This fact limits its use when those data are not available [8].

## 2. Materials and Methods

_{0}= 0.84; a

_{1}= 3.36 (W/m

^{2}K) and a

_{2}= 0.013 (W/m

^{2}K

^{2}). This efficiency depends on the temperature of the fluid circulating through the flat-plate collector (T) according to Equation (1).

^{2}°C. The tank has been divided into a total of 10 nodes in order to adequately model their degree of stratification. The tank interacts with two water flows, one that connects it to the heat exchanger (where the energy from the solar collector field comes from) and the other that connects it to the load (absorption machine) and through which the nominal drive power (50 kW) is extracted when the temperature of the water in the tank is higher than the set-point temperature.

#### 2.1. Design Variables Levels

^{2}/kW

_{cool}, 3 m

^{2}/kW

_{cool}, 3.5 m

^{2}/kW

_{cool}, and 4m

^{2}/kW

_{cool}. The specific collector area is defined as the ratio between the surface of the collector field and the nominal power of the absorption chiller (35 kW). In the present study, the specific surfaces analyzed correspond to collector surfaces of 87.5 m

^{2}(35 collectors), 105 m

^{2}(42 collectors), 122.5 m

^{2}(49 collectors), and 140 m

^{2}(56 collectors), respectively. The values tested are common in the design of this type of facilities [15].

^{2}, 32.5 L/m

^{2}and 50 L/m

^{2}. The volume of the hot water storage tank (chiller driving medium) is specified per unit of solar collector surface [16]. The volume of the storage tank will therefore vary from 1.3 m

^{3}corresponding to the collector surface and specific storage values of 87.5 m

^{2}and 15 L/m

^{2}, to 7 m

^{3}corresponding to the collector surface and specific storage values of 140 m

^{2}and 50 L/m

^{2}.

#### 2.2. Validation of the TRNSYS Model

^{2}of laboratories and offices. It has a collector field of 38.4 m

^{2}composed of 16 flat-plate collectors oriented toward the south with a 30 degrees slope. The hot water storage tank has a volume of 1 m

^{3}.

^{2}), the efficiency curve (a

_{0}= 0.818; a

_{1}= 3.47 (W/m

^{2}K) and a

_{2}= 0.0101 (W/m

^{2}K

^{2})) and the inclination (30°) of the solar collectors and the storage volume (1 m

^{3}). Once the model for the Elche facility has been validated, it is applied for the design analysis of the demonstration facility in Madrid.

_{2}O as working fluid in the environment of TRNSYS. A first way could be using the Type 107 of the TRNSYS library. However, the manufacturer does not provide operating data for cold water temperatures other than 7 °C for the Yazaki WFC-SC5 installed at the Miguel Hernandez University as well as for the Yazaki WFC-SC10 planned in this work, and therefore Type 107 cannot be used.

## 3. Results

^{2}/kW

_{cool}the difference between the energies delivered corresponding to the specific storage volumes of 15 and 50 L/m

^{2}is 26 kWh/day (208−182), while for the surface area of 4 m

^{2}/kW

_{cool}and the same specific storage volumes, the difference is 7 kWh/day (278−271).

^{2}/kW and storage volumes of 15 and 50 L/m

^{2}. In Figure 6b the same variables are represented for the specific storage volume of 15 L/m

^{2}and collector surfaces of 2.5 and 4 m

^{2}/kW.

^{2}(continuous line) since it takes longer to bring the 4375 L to 77.5 °C with the energy provided by the 87.5 m

^{2}of solar collectors.

^{2}(discontinuous line). Figure 6b shows that for the specific storage volume of 15 L/m

^{2}, the collector surface of 4 m

^{2}/kW (140 m

^{2}) is sufficient to achieve an increase in the temperature of the water in the storage tank simultaneously with the removal of energy for driving the absorption machine. This combination yields the highest energy delivered per day.

^{2}. Lower storage volumes were analyzed and represented in Figure 7.

^{2}. The increase in accumulation above 30 L/m

^{2}has a negligible effect on the range studied. An additional reduction below 10–15 L/m

^{2}also does not seem to have a positive effect and generates transients that should be analyzed from an experimental point of view. Theoretically, the direct coupling idea (no storage) was evaluated in [18].

^{2}for the specific storage volume.

## 4. Discussion

^{2}(49 solar collectors), a driving temperature of 75 °C and a specific storage volume of 15 L/m

^{2}, 31,466 kWh of thermal energy are sent to the absorption chiller from the hot water storage tank during the summer season (June–September). Considering an EER of 0.722 corresponding to the 75 °C driving temperature (Table 2), the absorption chiller would provide a cooling capacity of 22,718 kWh.

- The collector field performance is defined by: orientation (south), tilt angle (15° less than the site latitude), collectors area A
_{col}the optical factor FR_{τα}and the thermal losses FRU_{L}(0.825 and 1.1 W/m^{2}K, respectively). - The absorption machine efficiency is determined using the Felix Ziegler characteristic equation [19]. It has three parameters: thermal size s (kW/K) (i.e., machine size), the heat exchange capacity allocation α and the internal losses ΔT
_{min}. As discussed in [18], a good design will approach α = 0.5 and ΔT_{min}≈ 0. As practical values we assumed: ΔT_{min}= 3.5 K and α = 0.2, α = 0.5 for a bad and good design respectively.

_{e}(5 °C or 14 °C). The generator temperature T

_{g}is variable as the result of an energy balance between the machine and the solar field. The machine is allowed to produce cooling only if T

_{g}> 75 °C. The absorber temperature T

_{a}equals the wet-bulb temperature and the condenser one is T

_{c}= T

_{a}+ 2.5 (°C). Finally, the Ψ = A

_{col}/s parameter represents how big is the collector field with respect to the machine size. In our case, for a 35 kW cooling capacity machine its “size” would be s ≈ 1.2 kW/K, thus for A

_{col}= 122.5 m

^{2}, Ψ = 100 m

^{2}K/kW. The relative size of the driving solar power to the cooling machine is very important and determines the overall cooling driving potential DDGH (K·h) (alike to the degree days concept). The annual cooling effect is computed as:

_{e}(kWh) = s·DDGH

_{e}= 27,626 kWh while our results provide Q

_{e}= 22,718 kWh. A machine designed badly, in the same conditions, would yield Q

_{e}= 14,072 kWh. Moreover, the overall performance computed as the ratio between the cooling energy effect and solar radiation impinging onto the collector field in our case is 0.25. Roughly, for Spanish climates, this ratio has a peak value of 0.33 achieved at around Ψ = 200 m

^{2}K/kW. Our conclusion is that our design is close to be optimal.

## 5. Conclusions

^{2}for the specific storage volume.

_{2}O absorption machines is another objective on which research should be focused.

## Author Contributions

## Funding

## Conflicts of Interest

## References

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**Figure 4.**Weather conditions on an hourly basis for the summer season (June–September) in Madrid (red dots) and Elche (green triangles).

**Figure 5.**Energy delivered per day to the absorption chiller as a function of the specific collector surface.

**Figure 6.**Effect of the specific collector field surface and storage volume on the energy supplied to the absorption chiller (

**a**) 87.5 m

^{2}-1313 L and 87.5 m

^{2}-4375 L (

**b**) 87.5 m

^{2}-1313 L and 140 m

^{2}-2100 L. (Driving temperature of 75 °C).

**Figure 7.**Effect of the driving temperature and the specific storage volume on the fraction of solar radiation usable to drive the absorption machine.

**Table 1.**Registered and simulated fractions of the incident solar radiation used to drive the absorption chiller for driving temperatures of 75 and 80 °C.

Driving Temperature (°C) | Useful Fraction Registered | Useful Fraction TRNSYS |
---|---|---|

75 | 0.348 | 0.4 |

80 | 0.293 | 0.365 |

Collectors Slope (°) | Energy (kWh/m^{2}-day) Elche | Energy (kWh/m^{2}-day) Madrid |
---|---|---|

10 | 7.19 | 6.90 |

20 | 7.24 | 6.96 |

30 | 7.10 | 6.86 |

40 | 6.80 | 6.59 |

60 | 5.72 | 5.60 |

Tg, in (°C) | Cap (kW) | Qgen (kW) | EER |
---|---|---|---|

75 | 16.6 | 23.0 | 0.722 |

80 | 24.3 | 32.5 | 0.748 |

85 | 31.0 | 43.0 | 0.721 |

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**MDPI and ACS Style**

Martínez, P.J.; Martínez, P.; Soto, V.M.; Bujedo, L.A.; Rodriguez, J.
Design of a 35 kW Solar Cooling Demonstration Facility for a Hotel in Spain. *Appl. Sci.* **2020**, *10*, 496.
https://doi.org/10.3390/app10020496

**AMA Style**

Martínez PJ, Martínez P, Soto VM, Bujedo LA, Rodriguez J.
Design of a 35 kW Solar Cooling Demonstration Facility for a Hotel in Spain. *Applied Sciences*. 2020; 10(2):496.
https://doi.org/10.3390/app10020496

**Chicago/Turabian Style**

Martínez, Pedro J., Pedro Martínez, Victor M. Soto, Luis A. Bujedo, and Juan Rodriguez.
2020. "Design of a 35 kW Solar Cooling Demonstration Facility for a Hotel in Spain" *Applied Sciences* 10, no. 2: 496.
https://doi.org/10.3390/app10020496