Keywords

1 Introduction

Over the last decade, the rise of the standard of living, the increased demand of comfort and the high outdoor summer temperatures have led to a strong development of the air-conditioning systems. Besides, the energy consumption in the buildings has increased with the development of the world economy, accounting for 30% of the total energy used [1, 2].

Today, the energy management of buildings is a major challenge, in order to optimize energy consumption while improving their comfort and quality of use and to meet national commitments to reduce CO2 emissions and greenhouse gases. Especially since air conditioning in the industrial, commercial and domestic sectors represents between 40 to 50% of the of global energy consumption [3], in which the residential sectors alone are responsible for more than 20% of CO2 emissions in the European Union [4].

In this context, the contribution of renewable energies is gradually becoming indispensable to achieve the reduction targets set by the various authorities to improve the energy efficiency of buildings. For that, the solar-powered cooling systems appear to be an attractive alternative to conventional electric compressor units, as they use a natural refrigerants that are not harmful to the ozone layer so virtually no environmental impact, besides the main advantage of solar cooling lies in the simultaneous demand for cold and sunshine. When the heat necessary for the operation of the refrigeration machine is provided by the sun, the cold provided is free (i.e. no cost, no pollution). Simulations of solar cooling systems especially LiBr/ H2O absorption systems have been studied, but a general model for all conditions has not yet been developed. Burckhart et al. [5] described the simulation of a solar cooling system using TRNSYS software with real data, consisting of a 250 m2 field of evacuated tube solar thermal collectors and a lithium bromide/water absorption chiller with a capacity of 95 kW, to cover the thermal loads of a 4000 m2 building. Mateus and Oliveira [6] carried out a comprehensive energy and economic analysis of the use of solar cooling for various buildings and climatic conditions. Based on their study, they concluded that the usage of evacuated tube collectors lead to a lower collector area of approximately 15% and 50% compared to flat plate collectors. Buonomano et al. [7] investigated the applicability of a solar-powered absorption cooling system based on a novel type of flat-plate evacuated tube collector (ETC) embedded in a double-effect LiBr-H2O absorption cooler. The simulation results show that the systems coupled with a flat plate ETC achieve a higher solar fraction of 77% versus 66.3% for PTC collectors. Wang et al. [8] analyzed the efficiency of a double effect absorption solar cooler combined with a parabolic trough collector (PTC) to cover the cooling load. They found a high COP near to 1.195 and a solar energy utilization efficiency of 61.98%. In a recent study, Al-Falahi et al. [9] evaluated the energy effectiveness of a solar absorption cooling system combined with ETC using weather Baghdad data (Iraq). The seasonal collector efficiencies and solar fractions were determined to be 54% and 58% respectively. The COP was found 0.44. Elahi et al. [10] conducted a computational investigation to examine the effect of using solar plasma to promote improved absorption cycles using solar power. In terms of thermo-economic analyses, the absorption energy efficiency can exceed 90%.

From the literature, a variety of solar collector types and configurations has been employed for this application. The most commonly used type is the evacuated tube collector (ETC). Besides, it was remarked that LiBr-H2O took part in most of the research activities in this field.

In this regard, the objective of this work is to master the utilization of solar energy in the field of air conditioning of buildings in order to evaluate the dimensions of the installation for the specific climate of Oujda city (eastern Morocco), which is characterized by a semi-arid climate. To this end, this paper presents simulations of the different factors that can affect the solar cooling system to meet the cooling demands of a building located in hot and dry semi-arid climates.

2 Brief System Description

The solar cooling system, in this investigation, is based on a single effect absorption-cooling machine coupled with an evacuated tube collector. The model was created in TRNSYS to satisfy the cooling needs of a building with a total ground area of 1022 m2 located in the Oujda city of eastern Morocco. The real view as well as the sketch model of the case study building are presented in Fig. 1. For more details about the case study building, please refer to [11].

Fig. 1.
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Case study building

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The solar absorption system under study consists of two main loops as shown in Fig. 2, the first loop is the solar loop in which in an evacuated tube solar collector, the heat transfer fluid (HTF) is heated and the heat energy collected is passed to a sensible heat storage tank. The second loop is a cooling loop for the absorption chiller. It contains the absorption chiller and the space to be cooled. It is pertinent to mention that through the second loop an auxiliary boiler has been added and controlled to supply hot fluid to the absorption unit at the required levels (i.e Tg,i = 108.89 °C) in the presence of insufficient solar energy.

Fig. 2.
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Schematic of the configuration adopted for solar absorption cooling system

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In this context, the required generator input temperature is 108.89 °C. This value is the minimum value for the operation of the absorption cycle. Therefore, it is very important to adjust the generator inlet temperature in order to control the primary energy consumption and to guarantee the safety of the system. In addition, due to the exchange between the temperature of the working fluid coming from the chiller and the temperature of the fluid in the storage tank, the generator outlet temperature (Tg,o) seems to decrease (see Fig. 3).

Fig. 3.
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Variation of Tg,i, Tg,o on a typical summer day 16 July

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2.1 Thermal Loads Calculation

In order to adequately establish the refrigeration needs of the building, it is crucial to take into account the load components in the particular area to be cooled. To achieve this, it is essential to consider the following parameters: the location, the dimension of the building and the construction materials, as well as the configuration of the doors and windows, mainly of the external building envelope. All these factors play a role in determining the amount of energy that flows in and out of the building. In the current study, the thermal load were computed over a Typical Meteorological Year (TMY) of Oujda city (34° 41′ 12″ N, 1° 54′ 41″ W) located in eastern Morocco. It was found that the maximum hourly cooling load reaches 18 kW.

2.2 System Modeling

To model and simulate the system, TRNSYS software is a good choice because of its ability to predict the dynamic system behavior and its suitability for dealing with solar cooling systems, since this application is not standardized and there are no usable experimental cooling plants with accurate data. In addition, the TRNSYS interface dialogues with the operator as a graphical programming tool. Moreover, no prior knowledge of the programming language is required to run a simulation. For all these advantages, TRNSYS has attracted the interest of researchers in recent years.

Besides, in the newest release of TRNSYS (version 18.1) [12], we can find the type 56 (multizone building) which gives the possibility to call a 3D model created by sketchup (Fig. 1). So from a 3d Google sketchup model we can create an executable TRNSYS simulation which can be run directly from Google Sketchup, hence joining the strength of the TRNSYS simulation environment with the ability and simplicity of using Google sketchup.

The system is made up of several items where the evacuated tube solar collectors employed were Sunstar Olymp 2470 [13], which were represented by “Type 71” in TRNSYS. The absorption system was a Yazaki WFC SC 50 with the specified default capacity and the COP of the chiller was 1494 kW and 0.53 respectively expressed as “Type 107”. The typical characteristics and description of the main components, which were used as input parameters in the simulator developed in TRNSYS, are specified in detail in Table 1. Finally, the climate database of Oujda city used was obtained from a high precision MHP station located at Mohamed Premier University [14] built as a TMY2 format (i.e. Typical Meteorological Year) and this data was read using “Type 109”. As the purpose of this study is to analyze the effect of the main design keys of the system configurations on the overall performance, the following simplifying assumptions are made in the TRNSYS modeling and simulation [14]:

Table 1. Typical operating parameters of the absorption chiller used in the TRNSYS modeling
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  • Power losses in valves and/or connected tubes are not taken into account.

  • The effects of boiling and/or freezing of the working fluid have not been assumed.

3 Results and Discussion

The study of the case envisaged in this work represents a system of refrigeration by solar absorption responding to a request for air-conditioning under the dry and hot climate of Oujda city (Eastern Morocco). Based on the simulation performed with TRNbuild (Type 56), we tend to identify the cooling demand in the studied building. The obtained results indicate that this demand begins from April to September, with critical periods for the months of June, July and August in which the maximum load of 18 kW occurs.

Using the TRNSYS model, a number of simulations considering the weather data for Oujda are performed to identify the key parameters that provide insight into the ideal size of the most important components of the solar absorption refrigeration system and to evaluate those critical variables on the system performance. Such factors include the effect of solar collector area, thermal tank volume on useful energy gain as well as auxiliary energy consumption are studied.

An auxiliary heater with natural gas as a back-up fuel, which has a maximum capacity of 106 kJ/hr was carried out. the effect of solar collector area on the useful collector energy and auxiliary energy required by the system is shown in Fig. 4 and Fig. 5. The domain of analysis for the collector area variable ranges from 5 m2 to 50 m2, with increments of 5 m2. It is observed that an increase in collector area results in an increase in useful collector energy as expected, which translates into a decrease in the auxiliary heating energy required by the system. Therefore, the optimum value needs to be decided by following an economic analysis.

Fig. 4.
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Effect of the collector area on the gain of useful energy

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Fig. 5.
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Effect of the collector area on the auxiliary heat of the system

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Another parameter that affects the performance of the system is the size of the storage tank volume on the auxiliary energy consumption of the backup boiler. The result in Fig. 6 shows that by increasing the storage volume, the auxiliary energy is maximized. Besides, the minimum value of the obtained auxiliary heat energy was for a tank volume of 2 m3 as can be seen from the results of this simulation.

In addition, Fig. 6 clearly shows that the oversized solar tank leads to an increase in auxiliary energy. Therefore, it is not surprising that the optimal capacity of the storage tank is in the lower values of the recommended range. The finding results are in a good agreement with those obtained by Djelloul et al. [15].

Fig. 6.
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Effect of tank volume on the consumption of auxiliary energy of the system

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The solar fraction is an important coefficient in solar installations; it represents the proportion of solar energy in the total energy consumption of an air-conditioning system. In our case, the solar fraction is equal to the ratio of the thermal energy gained by the collector to the sum of this energy and the energy supplied by the auxiliary heating.

In order to enrich our work, we have plotted the variation of the solar fraction as a function of the surface of the solar collector and the volume of the storage tank in Fig. 7 and Fig. 8, respectively. This solar fraction is estimated for the same cooling power during the studied year.

Fig. 7.
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Effect of total area on the solar fraction.

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Fig. 8.
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Effect of tank volume on the solar fraction

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Figure 7 shows the solar fraction for a collector with a surface area ranging from 5 to 50 m2. As was predicted, the larger the collector area, the greater the amount of heat collected and the less strain on the boiler. This observation is clearly shown in the Fig. 7 where the solar fraction gets to increase when using a higher collector area.

While Fig. 8 shows that by increasing the storage tank capacity, the solar fraction tends to decrease. Therefore, we can conclude that the oversized solar tank has a tendency to decrease the solar fraction due to the increase in thermal losses. However, a thermo-economic analysis it is required for the optimization process of the important components such as the storage tank capacity and the solar collector surface.

4 Conclusion

This paper proposes a detailed analysis of the performance of a solar driven absorption cooling system as an alternative technology for the air conditioning of buildings under semi-arid climates of Morocco. Different parameters influencing the solar cooling performance of the proposed system were discussed.

Firstly, the Sketchup software was used to model the building under study, taking into account the different factors that can affect the air conditioning demand. Secondly, the developed 3D model was called by the TRNSYS TRNbuild, which permits to provide realistic simulations of the building’s air conditioning needs.

The simulation of a solar absorption chiller with the thermal COP is 0.53 coupled with evacuated tube collector (ETC) was carried out on a real case building with a peak cooling load of 18 kW. The results generally indicate that the greater the area of the evacuated tube collector, the greater the solar fraction obtained. While, the larger the capacity of the storage tank, at a certain level, the solar fraction becomes less. The results show that an evacuated tube collector with a surface area of 50 m2 and a hot water tank of 2 m3 can successfully meet the air-conditioning demand of the building studied (total floor area of 1020 m2.) However, a techno-economic analysis must be done to decide the final optimum configuration that can be employed in any circumstances.

Finally, due to the global pollution problem from burning fossil fuels, switching to solar energy as an energy source for absorption chillers is an alternative that should not be overlooked.