Keywords : Absorption machine; Solar flux; Numerical simulation. 1. Introduction: .... Transient heat balance equations of the other heat exchangers: For the first ...
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Energy Procedia 6 (2011) 130–135
MEDGREEN 2011-LB
Numerical Simulation of Solar Absorption Machine Sahar Fayada, Rafic Younesb, Said Abboudia a
Technology University of Belfort Monbeliard,Belfort,France b Lebanese University, hadath, Beirut, Lebanon
Abstract An analytical study is to performed on solar energy utilization in space cooling using a solar lithium bromide absorption system. The introduction presents the importance, the availability and the applications of solar energy, followed by a description for solar absorption machine [1-2]. Matlab numerical simulation of this system illustrates the change of the temperatures in the main parts starting from the collector since the temperatures change with the variation of the heat flux density provided from the sun. The temperature changes only in the part of heat exchanger collector and the idle heat exchanger where as in all other part. An electric source is required to ensure the necessary heat that when the solar heat flux does not supply the total air conditioning load. Finally the paper is concluded by studying the variation of solar flux in three cities, the collector length and the auxiliary source required in each city. Keywords : Absorption machine; Solar flux; Numerical simulation
Nomenclature : C : specific heat [kg/(kcal.ºC)] h: convection coefficient [W/(m2 .°C)] m : flow rate [kg/s] L: length [m] R: radius [m] T: temperature [C] ȡ: density [kg/m3@Ȝ &RQGXFWLRQ&RHIILFLHQW>: P& @ ij VRODUIOX[>:P2]
1. Introduction: Solar energy is one of the most available forms of energy on WKH (DUWK¶V VXUIDFH. its availability depends on several factors such as latitude and sky clearness. At the same time, its system requires high initial cost. But on the other hand, it has some attractive features such as its system requiring minimum
1876–6102 © 2011 Published by Elsevier Ltd. doi:10.1016/j.egypro.2011.05.015
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maintenance and operation cost, in addition solar energy is clean. It does not have negative effects on the environment. One of the methods to achieve cooling by solar energy is by use of an absorption cycle. A water lithium bromide absorption system operates moderately well with delivery temperatures of 65-95ºC to the generator. Flat plate solar collectors can generally heat fluids up to those temperatures which stimulated a considerable amount of research and development into adaptation and use of absorption systems for solar air conditioning.
2. Description for Solar absorption machine: Solar heat is used to displace electricity used for cooling. This solar air conditioning system consists of a closed absorption machine driven by an array of high performance flat plate collectors, a cooling tower and an auxiliary heater [3]. The auxiliary heater should ensure the necessary power when the solar KHDWIOX[GRHVQ¶WVXSSO\WKHUHTXLUHGKHDWThis machine dissociates, by boiling point, a solution of water and bromide of lithium. After cooling, the recombination of the two components produces the cold air which is distributed then into the zones like classic air-conditioning. The water and bromide of lithium is a clean, efficient and silent solution. It reduces the CO2 emissions, the use of liquids refrigerants and the urban noise, but its technique is still in development phase [5].
Fig. 1.Schematic Diagram
Figure 1 shows the schematic diagram of a single effect solar absorption air-conditioning system. This system has been the basis of most of the experience to date with solar air- conditioning. Here, the solar energy is gained through the collector, and is accumulated in the storage tank. Then, the hot water in the storage tank is supplied to the generator to boil off water vapour from a solution of Lithium Bromide and water. The water vapour is cooled down in the condenser and then passed to the evaporator where it again is evaporated at low pressure, thereby providing cooling to the required space. Meanwhile, the strong solution (solution of water and bromide of lithium rich in water) leaving the generator to the absorber passes through a heat exchanger in order to preheat the weak solution (solution of water and bromide of lithium poor in water) entering the generator. In the absorber, the strong solution absorbs the water vapour leaving the evaporator. Cooling water from the cooling tower removes the heat by mixing and condensation. Since the temperature of the absorber has a higher influence on the efficiency of the system than the condensing temperature, the heat rejection (cooling water) fluid, is allowed to flow through the absorber first, and then to the condenser [4]. An auxiliary energy source is provided, so that hot water is
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supplied to the generator when solar energy is not sufficient to heat the water to the required temperature level needed by the generator.
3. Transient heat balance equations of the collector: The dynamic behavior of the collector, the glycol and rich solution heat exchanger and the tube in tube heat exchanger are directly submitted to the solar heat flux and his variation. The study of these heat exchanger systems are realized in the unsteady regime. All the above heat exchangers have a cylindrical form constituted of a thick tube and traversed by a fluid flow. The external surface of the tube is submitted to convective heat transfer with the ambient medium, figure 2
wTp
For the wall:
( UCS )w
For the glycol:
( UCS )f
PeKtM s Pi hi ( Tp Tg ) OS
wt
wT f wt
( m C )f
wT f wx
w 2Tp
(1)
wx 2
Pi hi ( Tp T f )
(2)
4. Transient heat balance equations of the other heat exchangers: For the first heat exchanger, the internal fluid is the glycol and the external fluid is a rich solution (solution of water and bromide of lithium rich in water) while for the second heat exchanger, the internal fluid is a poor solution and the external fluid is a rich solution. r
Internal wall Re1
Tfe Ri
Re
Tfi x External fluid
Tfe L Internal fluid
Fig.2. Physical model of the other components of the system.
( UCS )fi
wT fi
( m C )fi
wT fi
Pi hi ( Tp T fi ) wt wx w 2Tp wTp For the internal wall: ( UCS )w Pi hi ( T fi Tp ) Pe he ( Tp T fe ) OSi wx 2 wt wT fe wT ( m C )fe fe Pe he ( Tp T fe ) For the external fluid: ( UCS )fe wt wx For the internal fluid:
(3) (4) (5)
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5. Solar Flux Variation in some countries and results: In this part we study the variation of solar flux in some cities (Abu Dhabi (United Arab Emirates), Learmouth (Australia), Beirut (Lebanon)) and its effect on the load of absorption machine. The matlab simulation of the six above equations is presented in figures 3, 4, 5. These results nicely illustrate the evolutions of the distributions temperature of the wall, the glycol and the rich solution inside the collector and the first heat exchanger in the hottest day during the year corresponding to each city with g = 0.05 kg/s; First heat exchanger length = the following conditions: Solar sensor efficiency = 0.8; m (rich solution) = 0.2kg/s; Collector length (Beirut, Learmouth)= 240m; Collector length (Abu 14m, m Dhabi)= 300m . The solar radiation is calculated using meteonorm software, it computes the hourly value for solar fux for each day of the year. We obtain 24 values for each day. glycol temperature variation for the solar sensor
5
10
15
100
5
10
15
time in hour
time in hour
glycol temperature variation for the first heat exchanger
wall temperature variation for the first heat exchanger
60
5
10
15
20
time in hour
5
10
15
5
80
40
20
5
temperature in C
temperature in C
65
50
15 time in hour
Fig. 3 Beirut( 1 july)
5
10
15
20
L=3.5m L=7m L=10.5m L=14m
60
50
40
5
10
15 time in hour
20
L=3.5m L=7m L=10.5m L=14m
60 55 50 45
5
10
20
wall temperature variation for the first heat exchanger 70
10
15
rich solution temperature variation for the first heat exchanger
L=3.5 L=7 L=10.5 L=14
10
60
time in hour
time in hour
60
5
80
40
20
60
rich solution temperature variation for the first heat exchanger
45
15
L=3.5m L=7m L=10.5m L=14m
100
time in hour
55
10
100
glycol temperature variation for the first heat exchanger 120
50
40
60
L=75m L=150m L=225m L=300m
time in hour
L=3.5 L=7 L=10.5 L=14
60
80
40
temperature in C
80
100
20
70 L=3.5 L=7 L=10.5 L=14
temperature in C
temperature in C
60 40
20
100
40
80
glycol temperature variation for the solar sensor 120
L=75m L=150m L=225m L=300m
temperature in C
60
120
L=60 L=120 L=180 L=240
temperature in C
80
temperature in C
temperature in C
100
40
wall temperature variation for the solar sensor
120 L=60 L=120 L=180 L=240
temperature in C
wall temperature variation for the solar sensor 120
15
20
time in hour
Fig. 4 Abu Dhabi ( 15 august)
20
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wall temperature variation for the solar sensor
glycol temperature variation for the solar sensor 120
L=60 L=120 L=180 L=240
100 80
temperature in C
temperature in C
120
60 40
5
10
15
L=60 L=120 L=180 L=240
100 80 60 40
20
5
10
time in hour
glycol temperature variation for the first heat exchanger temperature in C
temperature in C
70 L=3.5 L=7 L=10.5 L=14
60
40
5
10
20
wall temperature variation for the first heat exchanger
100 80
15 time in hour
15
20
time in hour
L=3.5 L=7 L=10.5 L=14
60
50
40
5
10
15
20
time in hour
rich solution temperature variation for the first heat exchanger temperature in C
60
L=3.5 L=7 L=10.5 L=14
55
50
45
5
10
15
20
time in hour
Fig.5 Learmouth (1 March)
6. Discussion: The collector length is calculated with matlab simulation for maximum glycol temperature 120ºC (boiling temperature) inside the collector tubes. The collector is double-glazed to minimize collector losses. The finned tubes collector have black chrome plated selective absorptive surfaces to minimize infrared radiation losses. The mix ratio with water and the cap pressure determine collector boiling temperature. Higher- pressure operation is desirable to prevent boiling, thereby allowing higher temperature and more efficient heat transfer. The length of the first heat exchanger is small regarding the collector length. ,W¶V the same in these three cities. So we study only the collector length variation and its influence on water temperature required in the second heat exchanger. We can after that compute the auxiliary energy with the formula:
Auxiliary power =
C ( Z1 Z ) m , temps
Z1 is the surface below the cyan curve (L=14m) in the graph which represents the evolution of temperature for the rich solution ,Z is the global surface for this graph. When we increase the collector length, the glycol temperature inside the tube increases, than the temperature of water entering in the generator increases and the load of auxiliary energy decreases. The temperature of rich solution entering in the generator should be 83 ºC to let the absorption cycle work normally and to obtain the desired comfort. Cooling load calculations for a certain small residence vary from city to city and from day to day. We use Elite CHVAC software for these calculations. The power assured by the absorption machine (green curve), the air conditioning load (blue curve) and the auxiliary power (red curve) are presented in the graph below:
Sahar Fayad et al. / Energy Procedia 6 (2011) 130–135
Ac load and auxiliary power variations in Beirut
Ac load and auxiliary power variations in learmouth
26
25
auxiliary power AC load Passur
24 20
22
auxiliary power AC load Passur
Load (kw)
20
Load (kw)
135
18 16
15
10
14 12
5
10 8
5
5.5
6
6.5
7 7.5 time or month
8
8.5
0
9
0
2
4
6 time or month
8
10
12
Ac load and auxiliary power variations in AUH 28 26 24
auxiliary power AC load Passur
Load (kw)
22 20 18 16 14 12 10 8
0
2
4
6 time or month
8
10
12
Fig.6 Auxiliary Power in Beirut, Abu Dhabi, Learmouth
7. Conclusion Numerical simulation of solar absorption machine under daily variable solar flux is presented in three different cities. This simulation illustrates the temperature variation through all parts of the machine and computes the maximum collector length relative to each city. For example if the collector length is bigger than 240 meters in Beirut, the glycol inside will evaporate. All parts in solar absorption machine are optimized mainly heat exchangers (diameter, material, convection coefficient). Rich solution temperature is the most important parameter to compute the auxiliary power needed to assure the desired comfort. The perspective is to continue the study with other solar flux in other different cities. Index: a: absorber p: surface
c: condenser s: sun
e: exterior v: vapor
f: fluid
g: glycol
i: interior
References [1] F.Meunier, Solid sorption heat powered cycles for cooling and heat pumping applications, Applied Thermal Engineering 18 ,1998 [2] G.Cacciola,G. Restuccia,Progress on absorption heat pumps, Heat Recovery Systems & CHP 14(4) ,1994. [3] Meza J.I, A.Y. Khan, and J.E. Gonzalez, Experimental assessment of a solar-assisted air cinditionning system for applications in the Caribbean, New mexico, 1998. [4] Li ZF & Sumathy K. , Technology development in the solar absorption air conditioning systems, Renewable and sustainable energy Reviews, vol(4),2000. [5] ALEFELD & al : Advanced absorption cycles and systems for environmental protection, Japan, 1991.
