Simulation and experimental investigation of solar absorption cooling system in Reunion Island

Applied Energy 88 (2011) 831–839

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Simulation and experimental investigation of solar absorption cooling system in Reunion Island Jean Philippe Praene ⇑, Olivier Marc, Franck Lucas, Frédéric Miranville Physique et Ingénierie Mathématique appliquées à l’Energie et à l’Environnement Laboratory (PIMENT), Université de la Réunion, Département Sciences du Bâtiment et de l’Environnement, 117 Rue du Général Ailleret, 97430 Tampon, France

a r t i c l e

i n f o

Article history: Received 10 March 2010 Received in revised form 26 August 2010 Accepted 19 September 2010 Available online 20 October 2010 Keywords: Solar energy Absorption cooling TRNSYS Evacuated-tube collector Experimental plant

a b s t r a c t With the development of technologies and the fast increase of our population we will need to adjust the conventional electrical source to meet the continuous increasing demand. Since the energy cost as well as the environmental awareness is growing fast, technologies using renewable energies appear as an interesting alternative. The aim of this research is to present a solar-driven 30 kW LiBr/H2O single-effect absorption cooling system which has been designed and installed at Institut Universitaire Technologique of Saint Pierre. The first part of this article deals with the simulation of the solar thermal plant. A pilot plant has been setup as part of RAFSOL which is a research program managed by the national research agency (ANR). Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Reunion Island is a French overseas department located in the southern hemisphere characterised by a tropical humid climate. Conventional energy will not be sufficient to meet our continuously increasing need for energy in the future. Political development has defined through a global program the energetic independency of the island as an objective for 2030. Thus, the use of renewable energies appears as an inescapable alternative to our insularity so as to diminish our dependency to petroleum products, cf. Fig. 1. Solar energy is in plentiful on the island (insulation >5 kW h/m2/day), which explains that the development of solar thermal system has grown up very fast since the 90s. At present solar water heating is the most important application with 20,000 new installations per year for 800,000 people. Solar cooling plants are very limited (two industrial plants and one experimental pilot at the university which cool down four classrooms). For the time being, building remains the most consuming sector in electricity. Under our latitudes, one of the most important demands for electricity is related to the air-conditioning of the buildings during the summer (active period from November 1st to April 30th), which represents approximately 50% of the total energy consumption. The use of solar energy in solar cooling system is an attractive application in which the cooling demand closely ⇑ Corresponding author. Tel.: +33 262 57 94 45. E-mail address: [email protected] (J.P. Praene). 0306-2619/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2010.09.016

matches the solar energy availability, both in seasonal and daily variations. Moreover, the use of chlorofluorocarbons (CFCs) as working fluids during the last 60 years has contributed to global warming [1,2]. The possibility of a cooling system using solar energy was first initiated by the technological developments in the solar field, reported by Tabor [3]. Commercial applications of solar energy for air-conditioning systems are fairly recent. Florides et al. [4] and Ziegler [5] give an overview of the latest developments and available technologies. Solar cooling systems can be basically classified in three categories, such as described by Zhai and Wang [6], namely solar sorption cooling, solar-mechanical systems and solar-related systems. Clito [7] offered another classification based on the final energy used to operate the system: (i) Electrically operated, (ii) Thermally operated, (iii) Hybrid. Actually, lithium bromide–water [LiBr–H2O] absorption chillers are most commonly used in solar cooling system, because of its larger tonnages in process applications and its readily available commercial equipment [8,9]. Absorption air-conditioning systems are similar to vapor compression air-conditioning systems, with a difference in the pressurization stages. In general an absorbent in the low pressure side absorbs an evaporating refrigerant (H2O). The most usual combinations of the chemical fluids used include lithium bromide–water (LiBr–H2O), where water is the refrigerant and ammonia–water (NH3–H2O) system where ammonia is the refrigerant [10]. The electric quantity of power consumed by the pump is almost negligible. Fig. 2 presents the basic single-effect absorption cycle.

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Nomenclature Cf Cg Cp G? hfp hga hsky _ m Q_ S Ta Tf

fluid heat capacity (J/kg K) heat capacity of glass (J/kg K) heat capacity of absorber (J/kg K) global solar irradiance of the collector (W/m2) heat transfer coefficient fluid – absorber (W/m2 K) heat transfer coefficient glass – ambient (W/m2 K) heat transfer coefficient glass – sky (W/m2 K) mass flow rate (kg/s) useful thermal power (W) surface (m2) ambient temperature (°C) fluid temperature (°C)

The lithium bromide–water solution works as an absorbent. The absorption also includes heat exchangers, pumps, valves and piping. The single-effect absorption chillers require for their operation hot water temperature on the level of the generator of 70–95 °C. Moreover, the temperature of cooling water must lie between 7 °C and 43 °C [11]. The higher limit is founded in order to limit the differences in pressure between the generator and the absorber but also between the condenser and the evaporator. The lower limit enables to avoid the crystallization of lithium bromide, which is carried out at a low temperature. The performance of absorption chiller is usually rated in terms of coefficient of performance (COP). The COP of the system is defined as the ratio of the cooling output and the net heat input. The typical value for single-effect absorption chiller is between 0.6 and 0.8 which is higher than that the one for NH3–H2O systems [9]. The principle of a solar absorption cooling system is based on five steps: (i) The solar collector plant receives energy from the sun, the latter is transferred through high temperature energy storage tank from the cooling system; (ii) The generator uses the heat produced by solar collectors in order to separate water to the weak solution;

Tfi Tg Tin Tout Tp Tsky u

inlet fluid temperature (°C) temperature of glass cover (°C) collectors field inlet temperature (°C) collectors field outlet temperature (°C) absorber temperature (°C) sky temperature (°C) fluid velocity (m/s)

Subscripts f fluid g glass p absorber plate

(iii) The refrigerant vapor leaves the generator at a high temperature and pressure. Water is condensed in the condenser where heat is removed, then it is directed to the evaporator through an expansion valve; (iv) The weak solution (rich in LiBr) flows back to the absorber as a spray and absorbs the water vapor which will form the LiBr/H2O solution; (v) The LiBr/H2O solution is finally pumped through a liquid– liquid heat exchanger and goes to the generator to complete the cycle. Due to exothermic reaction, which occurs in the absorption process, cooling water both remove heat from the absorber and the condenser unit to reject it to the environment. The generator temperature depends on the heat production of solar collectors. However, a minimum temperature of 80 °C must be maintained to provide efficient operation. Many authors published works both on simulation and experimental investigations. Ghaddar et al. [12] performed simulation for a typical house in Beirut and defined a ratio of 23.3 m2 per ton of refrigeration. Others authors carried out the optimisation of absorption cooling system [13–19]. The different studies consist in the comparison between collector type performance and the

Fig. 1. Repartition of energy production sources in Reunion Island since 2000.

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833

Fig. 2. Single effect absorption cycle.

definition of optimum storage tank capacity. These works have in common the fact that simulation and optimisation were performed using TRNSYS program. A European research program, IEA Task 25, has been initiated in 1998, in order to study experimental pilot plants and define a design method to help planners in the dimensioning of cooling systems components. The purpose of this work is to present simulations in which the design project step has occurred as well as to show the results obtained with the pilot plant installed at University of Reunion Island.

2. Solar absorption system modelling TRNSYS program is used to model the system. Solar collectors represent the heart of the performances and the investments (57%) of the installation. Thus, a good prediction of the performances of the solar loop is particularly judicious. The first part of this study deals with the dynamic modelling of the solar collectors. Then a coupling between the various components under TRNSYS is used to model the global system. At the project study phase, evacuated-tube collectors were chosen for the heat production. Many types of solar thermal collectors can be used to supply heat to the generator such as flat-plate, parabolic or vacuum collectors. Fig. 3 shows that for a single-effect absorption system, evacuated-tube collector is particularly interesting. The choice of solar

collector type both depends on economical aspects and available roof area. The literature contains numerous researches on the modelling of solar collectors. These developed models have different levels of complexity. Usually, stationary models describe solar collectors, considering that the collector is working under steady-state conditions. These approaches are generally based on the work of Klein et al. [20]. A dynamic approach is more interesting in several cases: control strategies, dynamic testing procedures, coupling with others elements, in particular the prediction of collectors behaviour for a short time step. With the intention of using TRNSYS program for a short time step, the first part of this work deals with the modelling of an advanced evacuated-tube collector. 2.1. Evacuated-tube collector This collector is composed of 3  3 series evacuated tubes. It is a direct fluid flow solar collector. The glass tube is shown on Fig. 4. The fluid, in which most of the tile water flows in a copper U-tube, welded to a narrow flat selective absorber. Thus, the inlet and outlet are at the same side of the glass tube. The mathematical model developed to simulate vacuum tube collector is based on the description proposed by Kamminga [21]. This model includes a three nodes description presented on Fig. 5. These nodes represent (from left to right) the transparent glass cover, the absorber plate and the fluid itself. We have chosen to consider the fluid temperature as a function of x. The fluid is

Fig. 3. Comparison of three types of solar collector for different cooling systems production. ETC (evacuated tube) – CPC (parabolic) – FPC (flat plate).

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Fig. 4. Evacuated-tube collector type (CORTEC 2C).

Glass cover:

dT g ¼ eg rSg ðT 4SKY  T 4g Þ þ hga Sg ðT a  T g Þ þ ep rSp ðT 4p  T 4g Þ dt

Cg

ð1Þ

Absorber plate:

Cp

dT p ¼ sg ap G? þ hf p Sf ðT f  T p Þ þ ep rSp ðT 4g  T 4p Þ dt

ð2Þ

Fluid:

Cf

Fig. 5. Thermal networks of the evacuated-tube collector.

moving in a single channel with the velocity u, along the x-axis that is parallel to the length of the tube collector and perpendicular to the section of the tube. When applying the heat balances to different parts of the collector, the temperatures in the nodal points can be described by a set of three differential linear equations.

  @T f @T f þu ¼ hf p Sf p ðT p  T f Þ @t @x

ð3Þ

The detailed elements and experimental investigation on the validation of the model have previously been presented by Praene et al. [22]. The developed model was also adapted to a heat pipe vacuum tube modelling, in desiccant cooling processes [23]. A comparison between a model forecast and measurements is presented on Fig. 6. The modelling results showed a fairly goof coincidence with measurements. A test bench under natural conditions was set up to test the evacuated-tube collector according to EN 12975-2 standard and also to have a database for dynamic modelling.

Fig. 6. Results of simulation of the outlet collector temperature – measurements/model comparison.

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Fig. 7. Reference classrooms used for design of the solar cooling plant.

The simulation is carried out at the minute time step. A new type has been introduced under TRNSYS program, to make a link with the rest of the solar cooling system. 2.2. Building modelling and cooling load Thanks to the final stage of simulation of the plant, the cooling load of the Department classrooms has been achieved. It consists in four classrooms of 54 m2 each. Fig. 7 and Table 1 provide some details of a typical classroom. The thermal behaviour of the classroom has been simulated using CODYRUN code [25]. The choice of this software is justified by the fact that CODYRUN precisely matches the objectives of a dyTable 1 Description of the reference classroom. Walls

Roofs Floors Windows

20 cm white concrete. Solar protection using overhangs canopies on eastern and western walls Door: thickness 52 mm, k = 0.11(W/mK) External: white sheet metal roof, 55 cm air layers, 23 cm concrete Interior: 13 mm plasterboard 20 cm concrete floors West: 2 windows 1.5  1.0 m East: 3 windows 1.5  1.0 m k = 1.50 (W/mK)

namic forecast in tropical humid conditions. Both experiments and theoretical tests have been investigated to validate this program [24,25,26]. CODYRUN has been translated under TRNSYS16 instead of TYPE56 to finally evaluate the cooling load of a classroom. The assessment of the cooling load of a building is the crucial point in the design of the cooling system. Comfort in the classrooms under natural ventilation has been evaluated. Simulations were done over one year. The comfort level is assumed to be at a temperature lower than 25 °C. As shown in Fig. 8, the period from May to October, which corresponds to southern winter, presents natural conditions of comfort. To define the required cooling load of the classroom, simulations were conducted on the period of January. The maximum cooling load value is 9 kW. This peak occurs after midday due to the building inertia. Thus for all the classrooms of the Civil Engineering Department, a 30 kW absorption chiller is needed. Fig. 9 shows the cooling rate for a typical classroom. As the chiller power is defined the last step consists in the simulation of the solar cooling plant and the building. 2.3. Solar cooling system simulation The simulation has been performed with TRNSYS16 (acronym for ‘‘transient simulation program”) [27]. Four main components have been used, namely the evacuated-tube collector, absorption

Fig. 8. Yearly evolution of air temperature in classroom – cooling period (red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).

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Fig. 9. Daily average cooling rate for a classroom.

chiller, hot-water storage tank and building. Each component of the system is represented by a subroutine. The LiBr–water absorption chiller used is the single effect unit, based on ARKLA model WF-36 (Type 7). No auxiliary heater is used. The storage tank is defined by Type 38, which is a vertical cylinder thermally insulated

with polyurethane. The meteorological datas required for the simulation are the hourly global insolation, ambient temperature, relative humidity, wind speed and direction, position of the sun during the cooling period (a sequence of January is used). The final configuration used for the project simulation is 60 m2 of ETC., 1 m3 of hot storage tank and a 35 kW single-effect absorption chiller. The results displayed on Fig. 10 clearly show that the average temperature inside a classroom is about 27 °C, while the solar cooling system is working. The difference between inside and outside temperature is around 4 °C. The retained configuration is sufficient to achieve a comfort temperature level. As the solar plant is simulated without any auxiliary boiler or heat unit, when the temperature of 80 °C is not obtained in the generator, the absorption chiller does not work. Water from the hot storage tank flows through the solar loop to be heated again if the global insolation is sufficient. Fig. 11 illustrates the cooling power production according to hot storage tank temperature. On the second day of simulation the temperature is sufficient to meet the set point of 80 °C at the generator. However, to obtain optimal conditions of cooling production (30 kW), the hot storage tank

Fig. 10. Difference between inside (Tres) and outside (Tair) air temperature.

Fig. 11. Cooling power from absorption chiller and outlet temperature of hot-water storage tank.

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temperature must be around 100 °C. When the temperature is near 80 °C, the cooling rate drops to 20 kW. 3. Experimental setup 3.1. Description of the plant The system under consideration (RAFSOL) is an absorption cooling plant located at the University. It was installed in 2007 and supplied cooling for classrooms of the Civil Engineering Department. The experimental setup is one of the international plants retained in ‘‘Task 38 – Solar Cooling and Refrigeration” supported by the International Energy Agency (IEA). Table 2 presents the different plants where a monitoring procedure for SHC plants is actually carried out. The main objective of this task is to define a way of collecting data and evaluating the performance of SHC systems. The solar loop is indeed a major point in both the economic field (budget dedicated to the project) and the space needed which depends on the available area for the solar field. This explains why some changes have been realized on the solar collectors between the project and the final plant. The major change is the use of double-glazed solar flat-plate collector instead of ETC. In terms of economic aspects, using ETC. represents approximately 60% of the total investment of the solar cooling plant. When using the FP collector the investment drops down to 28% from a total of 300 k€. Fig. 12 presents a photograph of the solar absorption cooling system. The roof mounted solar field and cooling tower are both located above the classrooms, whereas the absorption chiller and the different storage tanks are on the ground floor in a technical office. The solar cooling plant, which has been set up, consists in six major elements:  90 m2 of double-glazed flat-plate collectors, arranged in three series of 10 units and a serie of 6 units.  30 kW LiBr/H2O single-effect absorption chiller (EAW LB30), which has a nominal operating temperature that ranges from 70 to 95 °C.  80 kW cooling tower.  1500 L hot-water storage tank to ensure a stable supply of hot water.  1000 L cold-water storage buffer.  13 fans coil units mounted in the classrooms (room 1 and 2: four fans – room 3: three fans – room 4: two fans). We also use other components for the control system and monitoring. The detail of the monitoring procedure has been previously developed in [28]. 3.2. Cost of the SAC plant and economical analysis Some financial data are presented in Table 3. The double-glazed solar collector field represents 24% of the total investment. Using

Fig. 12. Photographs of the solar absorption cooling system: (1) solar collector field; (2) cooling tower; (3) absorption chiller; (4) hot-water storage tank; (5) coldwater storage tank.

Table 3 Financial aspects of the cooling plant. Cost Solar collectors field Hot/cold storage tank Absorption chiller Cooling tower Hydraulic/technical works/setup

54 k€ 6 k€ 60 k€ 11.7 k€ 92.8 k€

evacuated-tube collector would imply an additional cost of 46 k€, and would represent 37% of the global investment. Thus, because of available roof area and the cost of ETC., double-glazed solar collector was finally preferred. Considering, for example a cooling effect of 100 kW h, an absorption chiller COP of 0.8 and a compression chiller COP of 3.5; the input energy necessary is respectively 125 kW h and 28.5 kW h.

Table 2 List of monitored SHC plants analyzed in detail (FP: flat-plate collectors; ET: evacuated-tube collectors; Abs: adsorption chillers; DEC: dessicant evaporative cooling systems). Name of installation

Location

Destination

Solar collectors

Heat driven cooling

Solar loop efficiency (%)

Solar fraction (%)

Primary energy saving (%)

RAFSOL

La Reunion (F) Palermo (I) Valladolid (E) Gleisdorf (A)

University

90 m2 FP

30 KW Abs

39

100

18.5

University Offices

24 kW DEC 35 kW Abs

40 24

100 18.5

7.2 –

Offices

25 m2 FP 37.5 m2 FP plus 40 m2 ETC 134 m2 FP

34

31

–

Freiburg (G)

Canteen

21.9 m2 FP

35 kW Abs + 35 kW DEC 5.5 kW Abs

–

58

–

Dream CARTIF 1 Town hall Canteen Fraunhofer ISE

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The SAC plant uses no hot or cold backup energy system. The solar collector field bring all the 125 kW h required by the absorption chiller. The current electricity cost in Reunion Island is 10.40 c€/kW h. Therefore the associated cost is 3€ for the compression cooling. This evaluation of electricity consumption concerns only the cooling production (distribution not included). The carbon dioxide emissions associated with the Reunion grid electricity production system is 815 g of CO2/kW h. This high value is due to the use of thermal powerhouse for electricity production. The environmental impact of the SAC plant instead a compression system is 23 kg of avoided CO2 emissions to the atmosphere for a 100 kW h cooling effect.

80

°C

60

40

20

3.3. Results of the first field test TECA TSCA

0

G Tair

6

12

18

0

6 12 18 Time (Hours)

0

6

18

85

25

80

20

75

15

10

kW

70 65 5

60

0

55 PDIST THBC

50

0

6

12

18

0

6 12 18 Time (Hours)

0

6

12

-5

18

Fig. 15. Cooling power produced (PDIST) and hot storage tank temperature (THBC).

40

25 TATD4 TEXT PDIST

35

800

12

Fig. 14. Inlet (TECA) and outlet (TSCA) temperature of the solar collector field.

40

1000

0

°C

In 2008, the first test field of the described solar cooling system under study was carried out in Saint Pierre. The third field test started in January 2010. The detailed initial results of the system are presented in [26]. Our aim in this article is to analyze the real performance of plant in comparison to the forecasting made for the project. Fig. 13 shows weather data measure for Saint Pierre corresponding to the three days results as presented. The figure represents the total solar irradiation and ambient air temperature, at the time step of 10 min. The instantaneous temperature of the solar collector field is shown in Fig. 14. The difference in temperature between the inlet and outlet collector field is around 10 °C, for a mean outlet value of 75 °C. The solar loop produces hot water to fire the absorption chiller from 8:00 AM to 5:00 PM. Thus, at 11:00 AM the absorption process begins when the temperature of the hot storage tank is higher than 70 °C. The cold production stops at 4:30 PM. As the hot water temperature to the generator is approaching 75 °C, nominal conditions for the chiller are not obtained, cf. Fig. 15. For this reason that, the maximum cooling power available after midday on this period is 17 kW. Theses conditions are sufficient to obtain thermal comfort. The classrooms are used from 8:00 to 12:00 AM and from 1:00 to 5:00 PM. As we can see on Fig. 16, from 8:00 to 11:00 AM the mean air temperature inside the classrooms is 26.5 °C. At 11:00 AM when the cooling process starts this temperature drops below 25 °C until 4:30 PM, which corresponds to the chiller’s shutdown.

20

35

30

15

400

°C

30

10

15 10

200

kW

G

20

Tair

25

600

5 25 0

5 0

0

6

12

18

0

6 12 18 0 Time (hours)

6

12

18

0

0

Fig. 13. Variation of total solar irradiation and ambient temperature for three days.

20

0

4

8

12 Time (Hours)

16

20

0

-5

Fig. 16. Temperature inside the classroom (TATD4) and ambient air temperature (TEXT) – cooling power (PDIST).

J.P. Praene et al. / Applied Energy 88 (2011) 831–839

The French thermal legislation adjusted to our tropical humid climate (RTDOM) [29] recommends to set the temperature for cooling systems at 25 °C. Thus, if the cooling production could reach 30 kW, it clearly appears that it will be more effective to cool at least four classrooms instead of having a lower temperature in the present classrooms. The optimisation of this system is actually the objective for the coming years. 3.4. Measurement uncertainty evaluation The evaluation of measurement uncertainty is a crucial point for the validation of forecast models. Thus, a special attention was paid during the setup of the SAC plant. The absorption chiller has is own temperature measurement. PT100 (Class A) sensors are used for the temperature in the solar collector field. The accuracy of this sensor is, as indicated by manufacturer. Considering that the maximum deviation occurred when temperature in solar field reaches 100 °C, the associated accuracy is 0.35 °C. The pyranometers used for global and diffuse solar radiation measurement have accuracy of 2%. These pyranometers have been calibrated with those used on the local meteorological weather station. The measured deviation is 25 W m2. Concerning the flow meters accuracy is 3%. The useful thermal power extracted from the solar field is expressed as:

_  cf  ðT out  T in Þ Q_ ¼ m Considering an uncertainty DT on the temperature and Dm on the flow rate, the induced uncertainty on the useful thermal power is given by:

_ DT out þ DT in DQ_ Dm ¼ þ _ T out  T in m Q_ For example, for an input temperature of 65 °C, the relative error on the useful thermal power is estimated to be 9%. 4. Conclusion In this paper we presented a solar absorption cooling plant, from the modelling and simulation of the project to the first results obtained from the experimental setup. The first test on the classroom clearly shows that 20 kW is enough to reach thermal comfort conditions, as there are hot and cold-water storage tanks. An optimisation of the solar loop is necessary to be able to produce 30 kW. The first measurements campaign gives rise to several problems due to the lack of values on some components of the absorption chiller. The optimisation of the global plant is actually the main objective. A new french national research program called MEGAPICS has started on 1st January 2010 to investigate all French experimental setup in solar cooling field in order to define a single optimisation chart. Our plant has been retained by MEGAPICS. Our first step will be to fully qualify and guaranty the performance of an installation. Several tools have been investigated such as a sensitivity analysis or genetic algorithm, to define which parameters have significant influence on the cooling power. Secondly, the program will determine the value that will most likely influence the parameters of the solar cooling plant, in terms of efficiency and economical aspects.

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