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The Design of Adsorption Refrigerator with Energy
Storage Function
1Peike LI Shouyuan HUANG Changzhi LIU School of Mechanical Engineering
Abstract
The adsorption refrigerator is a prospective conditioner for the engine because it can be driven by the waste heat directly, it has high coefficient of performance (COP) for refrigeration. The system presented in this paper is a new technology that car air-conditioning system is driven by waste heat using the principle of adsorption refrigerator and with energy storage function. It is aimed to fulfill the request that the cars will not be hot for passengers to get in comfortably after a long time parking directly under the sunlight. Considering that the heat source from the exhaust gas of the engine is unstable but the requirement of the refrigeration is continuous, an adsorption refrigerator with the energy storage function is analyzed. As the engine is stopped, results showed that the proper choice of the working pair and the energy storage materials could keep the refrigeration output stable at around 20-25oC for at least 4 hours when the environmental temperature is 32-35oC. It is a good example of utilizing waste heat and developing the first-step adsorption cooling system for cars in replacement of conventional air-conditioner for automobiles
KEY WORDS: Waste heat, Adsorption refrigeration, Phase-change material, Intermittent heat source
1
INTRODUCTION
With the development of the economy and the energy consumption, the problems relating with the energy and the environment are paid more and more attention in recent years [1]. Nowadays automobiles’ air conditioning system use car engine and battery to supply compression-type refrigeration system, which has many problems. Firstly a great part of the power generated by engine is to supply the air conditioning system, which will cause average consumption of fuel increase about one fifth, meanwhile it will cause the output of automobile decrease about one eighth. This will notably lead to a lack of power, especially for small gasoline-powered cars under climbing situations. Moreover the working medium will cause climate change and the depleted ozone layer, causing contamination and damage to the environment and destroying natural resources or harming the health of human beings
It is acknowledged that only around 30% of the energy released by burning fuel is
1 This research was supported by Participate in Research Program(PRP), Shanghai Jiaotong University,
actually used for running the wheels, however, burning fuels is actually not the best way to use them, as much of the energy is wasted and disappears into the air and cause an environmental problem sequentially.
Table 1 composition of the engine heat
effective work cooling water taken exhaust gas taken other loss gasoline engine 20~30% 25~30% 30~45% 5~10% disel engine 30~45% 10~35% 30~40% 5~15%
Adsorption refrigeration as a type of energy saving and environmental benign technology, has been an significant way for utilizing the low grade energy such as the waste heat and the solar energy. Adsorption cooling system is an energy-saving and environmental friendly technology, with the benefits of zero ODP (ozone depletion potential), zero GWP (global warming potential). In the waste heat utilization fields, adsorption cooling system has broad development prospect, which accords with the human development demands on economy, environmental protection, and sustainable development. Among numerous adsorption cooling systems, silica-gel water and adsorption chillers plays a greater role at residual and waste heat utilization fields, for its superior low driven temperature performance. The silica-gel water adsorption chiller develops rapidly recently and there are already some successful applications in Japan and China.
The earliest country that made the adsorption refrigerators commercialized is the academics in Japan. They successfully recovered the low grade heat with the temperature of 55-80°Cby the silica gel-water adsorption chillers [2]. On the basis of that Shanghai Jiao Tong University utilized the heat pipe technology, and developed the small type adsorption chillers, which had already been commercialized [3,4]. The low grade heat also can be utilized for the freezing conditions if the ammonia is chosen as the refrigerant. Such as the two-stage adsorption refrigerator that can generate the ice when the heat source temperature ranged from 70°Cto 90°C[5].
But there are two common disadvantages for the adsorption refrigerators with the ammonia as the refrigerant. Firstly, the liquid ammonia exists in the system, which will cause the safety problem because of the high pressure. Secondly the refrigeration performance of the refrigerator will decrease notably when the ambient cooling temperature increases. The refrigeration performance will decrease significantly when the cooling temperature of the adsorbent bed is higher than 35-40°C. Compared with the adsorption refrigeration technology, the adsorption refrigeration with silica gel-water could overcome these drawbacks and also could improve the refrigeration efficiency effectively [6].
2
ESTABLISHMENT OF THE CYCLE
The working process of the adsorption cooling system is as follows,
the adsorbent bed, thus the working medium desorbs from the adsorbent bed i.e. silicone gel. The vapor flows and inflates to evaporator, cooled by the atmosphere and then condenses in to water. After the desorption process and during the condensing process, the exhaust gas stops to heat the adsorbent bed and the fresh air intake cools it. After the condensing process, the cooled silicone gel starts to adsorb, which decreases the pressure of the water in the condenser (and now works as an evaporator). Then, the water evaporates and the latent heat required produces the cooling capacity. As demonstrated, the heating process is not consistent. But with the energy storage function it could continuously output the cooling capacity.
The cycle is established and the principle is shown in Figure 1. The adsorption refrigeration cycle with the energy storage function mainly includes one reactor for the adsorption bed, one PCM energy storage tank, and the co-build compressor and evaporator. The refrigeration and energy storage processes of the cycle are as follows:
(1) The desorption process of the adsorption bed
In this process, valve 1, 2, 6 and pump 1 open, the reactor of the adsorption bed in Figure 1 is heated by the high temperature heat source(exhausted gas), and the refrigerant is desorbed from the reactor.
(2) After the desorption process
The valve 1, 2, 6 and pump 1 shut down, meanwhile the valve7, 8 and pump 2 open. Adsorption bed is cooled by the low temperature heat sink(environment).
(3) The adsorption refrigeration process
In this process, valve 7, 8 and pump 2 open, the reactor in the Figure 1 is cooled by the heat sink with the environmental temperature, the pressure as well as the temperature decreases. It will adsorb the refrigerant from the reactor of the evaporator. Such a process will lead to an evaporating effect of the evaporator (with the valve for refrigerant opens). The evaporate heat will provide the refrigeration power, which stored in the energy storage tank of the low temperature PCM by opening the valve 3, 4 and pump 3.
(4) The refrigeration process by the low temperature phase change materials
PCM energy storage tank. In this process the phase change of PCM will storage the cooling power, which is transported to the outside by the thermal fluid. The refrigeration can be transported under a constant level because the phase change process happens under a constant temperature.
Figure 1 The adsorption refrigeration cycle with energy storage function
3
CALCULATIONS OF THE ADSORPTION COOLING
SYSTEM
3.1The calculation of the cooling load
The cooling load in the car is similar to the cooling load in the building. It contains serial parts, for example, cooling load caused by automobile heat transfer, cooling load caused by the solar radiation through glass, cooling load caused by the fresh air, the cooling load caused by the passengers’ body heat, the cooling load caused by the heat dissipation of the equipment, the cooling load caused by lighting heat dissipation and the cooling load caused by the heat transfer of the engine, etc[10]. To simplify the calculation, since the good sealing performance of small cars, we neglect the cooling load caused by automobile heat transfer. We neglect the heat of the lighting source since it is relatively low. We also neglect the heat produced by the engine, since it is taken away by the cooling water and exhaust gas mostly.
(1) The cooling load caused by automobile heat transfer
The envelope structure of the automobile is quite complex, what’s more, it is always moving around. So the calculation of this part has many uncertainties. Affected by the temperature of the air outside the automobile, solar radiation intensity and the speed of the automobile, the cooling load in this part is always changing. To simplify the calculation, we assume that the speed of the automobile is 40km/s, and the relevant parameters are constants to do the theoretical calculation.
The formula is
(
)
1 c n Q =KF t − t
The K is heat transfer coefficient of the structure, the unit is W/m2. Because of the coefficient varies with the speed of the automobile, it is determined that the K is 3.5W/m2
when the speed of the automobile is 40km/h. F is the total surface area of the automobile. Assume that the length is 5m, the width is 1.7m and the height is 1.6m, we find that the surface area of the automobile is approximately 36m2. tn is the temperature in the car, considered as 30°C. tcis the temperature outside the car, considered as 35°C.
Then result is
1 3.5 36 35 30 630
Q = × ×（ − ）= W
To simplify the calculation we didn’t neglect area of the glass because it can also transfer the heat.
(2) The cooling load caused by the solar radiation through glass
Since the automobile is continuously moving forward, the cooling load varies with the direction of the automobile. To simplify the calculation, the coefficient is identified by the four directions, that is, we take the average. What’s more, we ignore the situation that the sun is hidden in cloudy days.
The formula is
max 2 2 Z J
Q =FC D CQ
Where the F is the area of the windows, according to 12m2. Cz is the Glass shielding coefficient, Cz=CsCn, which is calculated according to standard glass, Cs=1; Generally with sunscreen, checking the data, Cn=0.12. DJmax is sunshine heat coefficient which equals to
380 which is the average of the data. CQ2 is the coefficient of cold loading, here we take the average value 0.49.
The result is
2 12 380 0.12 0.49 268 Q = × × × = W
(3) The cooling load caused by the passengers’ heat.
The designed for parking condition and no one is in the car. So we could ignore this part of cooling load.
(4) The calculation of the gross cooling load.
From the results above, we know the cooling load of ordinary automobile is
268 630 898+ = W. And our lowest requirement is to cover the radiation heating power, that
is 268W
3.2waste heat power of the engine
different working situation i.e. various kinds of automobiles. Low power, low displacement cars were selected to calculate.
After the query of some 4S automobile stores, the output power of some typical and widely sold car were listed.
Table 2 Output power of some typical cars
Type Displacement(L) Output Power(kw)
Hyundai Accent 1.4 70 Swift Sport 1.3 63 Peugeot 206 1.4 56 VW Polo 1.4 55 Honda Fit 1.3 60 Chevrolet Lova 1.4 69
After comparing, the type which has the lowest output engine power was selected i.e. VW Polo.
The efficient waste heat of VW Polo 1.4 engine Qy was ( / )
y e w
Q = N η ⋅η ⋅COP
Where N is the output power of the engine,
η
e is the efficiency of the engine ,η
w is the proportion of heat taken away by cooling water and exhaust gas in the total fuel combustion energy, COP is the coefficient of performance, for the adsorption cooling system here we take 0.60 supposedly.55 0.3 50% 0.6 55 y
Q = ÷ × × = kw
The heating load of the cooling system:
0/
g
Q =Q COP
Where Q0 is the cooling capacity of the car.
0.898 0.60 1.50
g
Q = ÷ = kW
Apparently, it is quite capable to provide the cooling capacity for the car with the utilization of waste heat.
3.3Design of the PCM (Phase-Change Material)
We can simplify the material that makes up the roof of car as steel and polyurethane, For the transfer rate of cold energy must satisfy the heat come into the car, we have
(
)
) / ( • / i/ i p p
P= A ΔT δ λ δ+ λ
P is the power of cooling energy, from the calculation above 268W
λ is thermal conductivity, refers to the heat transfer through 1 meter thick material on 1 square meter within 1 second with temperature difference of 1 celsius degree under stable heat transfer conditions, unit W/ (m2• C)
°
A is the area of the car roof, which we consider as 2 square meters.
T
Δ is temperature difference of PCM and air in car.
c
T is the temperature of air in car, Tpis the temperature of PCM
δ is the thickness of material that build up the roof of car, we can simplify as iron and polyurethane, the thickness of iron is 7 mm and the thickness of polyurethane is 6mm.
c
T is 35℃.
Then we solve the equation and have ΔT is equal to 4℃, thus Tp is equal to 31℃. Table 3 Characteristics of phase-transition HAM for accumulation of
medium-potential (100< m.p.<500oC)thermal energy
System (mass%) M.p./oC ρ/kgm-3 ΔHm
kJ kg-1 MJ m-3
29LiNO3-17NaNO3- 49.4KNO3-4.6 Sr(NO3)2 105 2087 110 229.6
17.5LiCl-82.5AlCl3 114 2376 251 596
21.6KCl-78.4AlCl3 128 2343 254 595
31.7LiNO3-68.3KNO3 135 1780 135.6 241.4
55.4LiNO3-4.5NaNO3-40.1KCl 160 1905 266 507
58.1LiNO3-41.9 KCl 166 1918 272 522
47.9LiNO3-1.4LiCl- 50.7NaNO3 180 1986 267 530
Table 4 The characteristics of phase-transition HAM from metal alloys
System (mass%) M.p./oC ρ/kgm-3 ΔHm kJ kg-1 GJ m-3 78.55 Ga-21.45In 15.7 6197 69.7 0.4429 86.5 Ga-13.5Sn 20.55 5885 81.9 0.4942 96.5Ga-3.5 Zn 25.0 5946 88.5 0.5415 67Ga-20.5In-12.5 Zn 10.7 6170 67.2 0.4185 82Ga-12Sn-6Zn 18.8 5961 86.5 0.5255 74Ga-22Sn-4Cd 20.2 5983 75.2 0.4571 93Ga-5Zn-2Cd 24.6 6020 85.03 0.5246
Above there are characteristics for some typical phase-transition materials [12,13]. After a comprehensive consideration, we choose a kind of PCM that a kind of compound of hydrate salts (CaCl2·6H2O 32.5%; SrCl2·6H20 12%; NaCl 6.5%; acrylic acid 3%; propanetriol 5%;
HEMA 3.5%; cis-butenedioic anhydride 3.5% Carboxymethylcellulose sodium 4%) and based with water, which has already been used in industry with a phase transition temperature of near 10.6℃, with the specific latent heat ν = 226kJ kg/
Then we can get the mass we need for the PCM mpcm =17.1kg 3.4Heat pipe
A heat pipe is a heat-transfer device that combines the principles of both thermal conductivity and phase transition to efficiently manage the transfer of heat between two solid interfaces.
At the hot interface within a heat pipe, which is typically at a very low pressure, a liquid in contact with a thermally conductive solid surface turns into a vapor by absorbing heat
from that surface. The vapor then travels along the heat pipe to the cold interface, condenses back into liquid, releasing the latent heat. The liquid then returns to the hot interface through either capillary action or gravity action where it evaporates once more and repeats the cycle. In addition, the internal pressure of the heat pipe can be set or adjusted to facilitate the phase change depending on the demands of the working conditions of the thermally managed system.
What’s more, the process in the heat pipe is reversible, so the entropy production Sg=0,
and the destruction of exergy also approximately equal to zero. According to the first law, when the low temperature side of the heat pipe gets the cooling capacity, it can almost transfer to high temperature side of the heat pipe totally.
According to the temperature Teva and Tpcm, we should choose the normal temperature
heat pipe. So we consider water or methanol(CH3OH) as the working medium. But
considering the pressure, when we choose the methanol as the working medium, the material of the heat pipe should be more pressurized[14]. At last, we choose water as the working medium.
3.5Design and Analysis of the adsorption bed and co-built evaporator and condenser.
The mass of water we use depends on the required cooling power and the cycling time. It is very convenient of us to take the cycling time asτ =900s
During adsorbing process, we take following postulation and simplifications:
For there being a set of heat pipes, the heat transfer with the PCM is well-optimized. So we can take the evaporating temperature as 30℃. By chart of vapor characteristics, the saturated pressure is p=4.2kPathe evaporating latent heat ν = 2429.6kJ kg/
Knowing that, the temperature of heat source of the adsorption bed is high enough, the desorption process can be finished in rather short time.
Consequently, we take the whole adsorbing time as 1h. By calculation of the maximum coldness quantity required during a day, Q=3859.2kJ the required power is
P Q t= / u =1072W
By equation of 1st Law of Thermodynamics:
m m v
P q= υ−q c TΔ
Solving the equation and we can getqm=0.43 /g s
Total mass of water is: mw =qmτ =0.389kg. And in real manufacturing, we should take
a little more for balance.
The saturate adsorbing ratio of silicone gel is 0.3, under the cooling condition of 40℃[2]. However, when we chose the cycling time as 15min, the silicone gel is far from saturated. By the result of isothermal and isohumid experiment of adsorbing speed, it covers a proportion of the saturated adsorbing ratio, which can be approximately taken as 0.1. So the
上海交通大学 SJTU 9
mass of silicone gel required is msilicone =0.1mw =3.89kg
Easily calculated that the evaporator needs no larger than 1 L, and when made flat at
2
2m , it can be very thin.
In the desorption process,t=80°C,
Where, p=47.7kPa h, '' 2643 /= kJ kg; ν =2308.2kJ kg/ .
We evaluate the performance of evaporator by simplify the process in two extreme ones: completely isothermal, `and adiabatically heat transfer, which means the mass flowing process is much faster than heat transfer, under such postulation, the vapor first undergoes a adiabatic inflating process and then the cooling and condensing process is in a closed system.
Need not calculate, the latter process required a much higher pressure to condense and made the process period much longer (in this case, the situation can be modified to a flowing gas inflating an adiabatic vessel, and calculate by d mu
(
)
cv =h minδ in, so we know that awell-optimized heat exchange system is essential for the condenser.
The temperature range between that in the condenser Tcon=80°C and atmosphere 35
at
T = °C is quite wide, which is convenient for us to choose a type of heat pipe of mass
production rather than redesign it.
In the situation above, the waste heat and the cooling air is generally considered to be inexhaustible heat sources, so we do not need to consider its efficiency when designing the adsorbent bed. While it is a key point to improve the heat exchanging performance for the adsorbent bed in order to save time for the effective working process.
Viewed as a whole, the exergy efficiencies are as follows,
In the heating and cooling process of the adsorbent bed, the heat capacity is rather small in comparison with the desorption heat and the structure of the base is undefined, we neglect it and modify is as a heat transfer between 150℃ (constant because of steady flow) and 80℃. And for the similar reason, the cooling process can also be modified as heat transfer between 40℃ and 35℃
Thus, assume the unit heat Q is transferred:
1
1
0
, ,
e
(1
)
0.1473
H
x Q T
H
T
Q
Q
T
=
−
=
2 1 (1 H ) 0.1133 net c L T W Q Q T η = = − = 2 , , 0.65 H net ad x Q T W E η = =For the co-built evaporator/condenser:
The heat transfer through the heat pipe is fast enough and is considered to be reversible heat transfer. So the refrigeration cycle of can be simplified as a changing heat source from 80℃ to 35℃ with the specific heat of water 4.2kJ/kg, a reversible heat transfer at 35℃, and
a heat transfer process between 35℃ and 30℃.
Figure 2 Thermo diagram for the cycle
As shown in the figure above, check the EES software and find (apply linear postulation in the irreversible part):
37. 6225
net w
W = m
240.5 w Qc= m
So the efficiency of the cycle is c c 6.38 net Q W η = =
4
DESCRIPTION OF THE ADSORPTION COOLING SYSTEM
The adsorption cooling system is placed in a car as the figure shown:Figure 3 Schematic plot of the cooling system
1. exhaust gas regulating valve 2. adsorbent bad 3. refrigerant valve 4. heating coil 5. adsorbent 6. refrigerant reservoir 7. refrigerant 8. phase-change materials and heat pipe 9. cooling capacity regulating valve 10. Compartment 11.pump
The adsorbent bed is placed along the exhaust gas pipe of a car and heated when the exhaust passes through its radiator pipe in the desorption process. A wind gate is designed to control the flow of high temperature exhaust gas and allow the lower temperature wind cool down the adsorbent bed in the adsorption process.
The working mass, say, water is condensed in the co-built vessel in the desorption process and release the heat directly to the environment, and in the adsorbing process, it evaporates to provide the cooling capacity.
The designed refrigerating power is far larger than that required in the parking period, and cannot provide right during that period. The energy storage PCM is designed to solidify to store the cooling capacity with its latent heat when the system is working and melt to release it to the inner environment of car.
To guarantee the single-directional heat transfer, we utilize gravitational heat pipe between the co-built vessel and the PCM, for its working mass can only condense on the low temperature side and flow downward.
The working process of the adsorption cooling system is as follows:
During the working time of the car engine, the exhaust gas in high temperature heats the adsorbent bed, and the working mass desorbs from the adsorbent bed silicone gel. The vapor flows and inflates the evaporator, cooled by the atmosphere, condenses in to water. After the desorption process and during the condensing process, the exhaust gas stops heating the adsorbent bed and the air intake cools it. And after the condensing process, the cooled silicone gel starts to adsorb, which lows the pressure on the water in the condenser (and now works as an evaporator). Then, the water evaporates and the latent heat required produces the cooling capacity.
Below there is the 3D model of the adsorption refridgertor system,
Figure 4 The 3D model of the system
5
CONCLUSION
The shown system provides a realization of adsorption cooling system for cars, fulfilling the customers’ need that the cars will not be too hot to get in comfortably after long parking directly under the sun. The principle working materials are: silicone gel 3.9kg, water 0.4kg, PCM (of a certain type) 17.1kg
According to the former analysis, the main constraint of design of the system is the efficiency of the adsorption cooling system and the mass of PCM.
The adsorption cooling system is well-simplified and enlightened to be feasible for cars. The evaporator and condenser share the same vessel, and the refrigerating process is not consistent, regardless of the adsorbent bed, which indicates that it is not convenient to raise the efficiency by heat recovery and mass recovery optimization.
The PCM makes the adversary constraint of the system, for which we had to sacrifice some performance to reach the criterion. Nano-capsule may offer a better choice in the future, and we may turn to the chemical energy storage to ameliorate the performance.
There are also some optimizations, for instant, with a sensor and addendum programmed controlling unit, the system can perform an extra cycle after parking, which would amplify the total cooling capacity for about a quarter.
The waste heat from cars’ engine is a large heat reservoir that has not been well utilized even today and adsorption refrigerating system is considered to be an elixir for low quality waste heat. This kind of air-conditioner for cars is growing intriguing especially in the modern trend of energy saving and environmental friendliness. Though theoretical and technical problem exist for a long time, it surely owns a promising outlook under the
co-development of multi-areas.
6
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