Today, one of the main goals of the commercial, residential, industrial building and architecture is to use the solar source for the air conditioning of buildings. Over the recent years many activities have started to develop new buildings and plant technologies oriented to energy saving by improving indoor comfort air conditioning and reducing environmental pollution emission.
Worldwide, the growing demand for conventional air-conditioning has caused a significant increase in demand for electricity, resulting in a significant increase in peak power demand in summer, in many cases, the capacity limits of the network and causing the risk of blackouts due to increasing living standards, comfort expectations and global warming. In many countries, air-conditioning is one of the main consumers of electrical energy today. Solar air-conditioning is one of the technologies, which allow obtaining important energy savings compared to conventional air-conditioning plants by using the renewable solar source.
Problem in Conventional AC
The AC is a dominant energy consuming appliance and it consume approximately 40% of the total electricity consumption in comparison to lighting and other electrical appliances. Conventional cooling technologies which typically use an electrically driven compressor system that exhibits several clear disadvantages such as high energy consumption, high electricity peak loads demand. Because of high energy cost and the rise of environmental pollution, the utilization of low level renewable energy sources such as solar energy in refrigeration systems has become a way to address these problems.
Solar Assisted AC System
Selection of Working Fluids
The selection of the working fluid plays an important role in design of solar assisted air conditioning system. The size of high pressure generator unit, absorber unit, condenser and evaporator depends on the type of fluid selected.
Ammonia and Water as Working Fluids in SAAC System
NH3 water is commonly used as working fluid in SAAC system. As ammonia and water are volatility, the cycle requires a rectifier to strip away water that normally evaporates with NH3. Without a rectifier, the water would accumulate in the evaporator and affect the system performance. As ammonia toxic in nature causes health hazards and hence it is decided to replace ammonia and water with other working fluid.
Lithium Bromide and Water as Working Fluids in SAAC System
The outstanding features of LiBr/water are non-volatility absorbent of LiBr (no need of rectifier) and extremely high heat of vaporization of water (refrigerant). However, using water as a refrigerant limits the low temperature application to that above 0°C. As water is the refrigerant, the system must be operated under vacuum conditions. The driving temperature of system should be more because in this case water is a refrigerant and to vaporize water the temperature should be more than 100°C. As the absorber temperature increasing, the ability of the LiBr to absorb the refrigerant is reduced. Hence, the COP is decreased. It also presents risk of crystallization on concentrations over 68% and cannot work at refrigeration conditions.
New Fluids for SAAC System
Even if the most common working fluids used are LiBr/Water and Water/NH3. Although LiBr/water and water/NH3 have been widely used for many years and their properties are well known, much extensive research has been carried out to investigate new working fluids. Fluorocarbon refrigerant based working fluids have been studied. R22 and R21 have been widely suggested because of their favourable solubility with number of organic solvents. The two solvents, which have stood out are Diethyl Ether of Tetra ethylene Glycol (DMETEG) and Diethyl Form Amide (DMF).
R410A and DMF
Boiling point of this fluid is -52°C, higher COP can be obtained. R410A is wildly used in the present conventional air conditioning system, but operating pressure is 50% higher than R22. The system requires the components capable of working at higher pressure and hence not preferred for this SAAC.
R22 and DMF
R22 is often referred to by names such as Freon, hydro chlorofluro carbon [HCFC] or Freon-22 or Di fluro-chloromethane.
From the Table 1 the ODP and GWP of the selected refrigerant (R22) are better when compared to the conventionally used refrigerant R410A. Though R410A is theoretically superior to R22, practically the components designed for SAAC cannot handle the high pressure of the R410A refrigerant. After this detailed study on the properties of various refrigerants, the refrigerant R22 is the best suited for the proposed SAAC model.
Refrigerants & Absorbents Volume
The volume calculation based on the formula:
Volume= mass /density (1)
1. Volume of R22 required
For 1 kg of R22
Mass (m) = 1kg
Density of R22 = 3.66 kg/m3
From the eq: 1, we know that, density(ρ)= mass(m)/ volume(v)
Volume= mass/ density = 1/ 3.66 =0.273 m3
2. Volume of R22 required
Mass (m) = 1kg
Density of DMF = 931 kg/m3
From the eq: 1, we know that, density(ρ)= mass(m)/ volume(v)
Volume= 1 /931 =0.00107 m3
Total Volume required for working fluid=0.273 + 0.00107=0.2741 m3=0.28 m3
Therefore, from the above calculation we observe that,
Total Volume required for working fluid=0.28m3.
Solar Assisted AC System
SAAC system commonly uses vapour absorption techniques. This systems use solar heat rather than electricity to produce refrigeration effect. In this, solar energy is used for heating of water which is stored in a tank and later it is supplied to generator unit. Solar absorption system is quite similar to vapour compression (conventional air conditioning) system because except compressor unit all the units are same and working principle of those units are also similar.
The working principle and different components of SAAC system are discussed in this context. Block diagram of Solar Assisted Air Conditioning System is shown in figure 1. The working fluid is pumped from the absorber to the generator through the pump. In generator unit, the solution gets heated up by the hot water circulation from solar water heater. Since the temperature is high in generator unit, refrigerant is vaporized with high pressure. Refrigerant changes its state to gaseous form whereas absorbent remains in liquid form because absorbent has high boiling point. The absorbent which is in liquid form returns to absorber unit with the help throttle valve. The high pressure and temperature refrigerant vapour from generator is passed to condenser through copper tube. In condenser, as vapour boiling point is above to that of surrounding temperature, it loses the latent heat and becomes high pressure liquid. The high pressure liquid refrigerant is now passed to expansion valve. The expansion valve reduces the pressure. This low pressure refrigerant is passed to the evaporator. In evaporator as the pressure is low, the boiling point of the refrigerant is less than the surrounding temperature. Hence it absorbs the latent heat from the immediate surrounding and vaporization gives the cooling effect. When heat is absorbed from surrounding, the liquid refrigerant changes to gaseous form and then gas form moves to absorber with low pressure. In absorber, unit refrigerant is mixed with absorbent and this solution is cooled by supplying cold water through cooling coils
Figure 1: Block diagram of Solar Assisted Air Conditioning System
Design of High Pressure Generator
Depending on the volume required for working fluid the length, width and height of the tank is selected. The volume of working fluid calculated is:
V=0.288 m3
In this project, a pyramid shape tank is considered for designed.
Volume of base square tank
Length=0.6m
Height=0.6m
Width=0.6m
Volume of square tank=L × H × W
V= L3
V=0.216 m3
Volume of pyramid tank
Length=0.6m
Height=0.6m
Volume of pyramid tank= L2 × h3
V=0.072 m3
Verifying the total volume of the working fluid:
Total volume=volume of square + volume of pyramid
V=0.288 m3
Tank is fabricated using stainless steel with thickness 2mm as it has to with stand the pressure of 60-80psi.
Design of Absorber
The Absorber tank is also designed on the basis of the volume required for working fluids. A cylindrical shape tank is fabricated, made up of stainless steel with thickness 2mm. A copper coil is provided for cold water circulation.
The volume of working fluid calculated is:
V=0.288 m3
Volume of cylinder=π × r2+h
Where, r = radius of cylinder and h = height of cylinder
Radius = 0.3m
Height = 1m
Volume of cylinder=π × 0. 3 2+ 1
V=0.282 m3
A standard design for condenser, evaporator, tubes, fins and throttle valve are retained for the SAAC system.
Advantages
- The proposed model has less power consumption as compared to the conventional AC. Since the SAAC uses solar energy for its operation, the system uses less power from the utility grid. Thus, the conversion efficiency is high in the proposed system.
• No moving parts in SAAC system, hence the operating is essentially quite and subjected to a very little wear, so that the maintenance cost is low.
• The reduction in power consumption is achieved by SAAC system .
• Though the COP of the proposed model is less when compared the conventional AC, this model uses renewable energy source instead of the energy generated from non-renewable source of energy. This clean energy used in the SAAC helps in reducing the global carbon footprint.
• This system also reduces the peak load demand on the utility grid and hence reduces the peak load demand on the grid.
Conclusion
Solar cooling is possible and reliable. Exploitation of solar energy for cooling is an efficient way to use the renewable energy source to meet air conditioning needs. Solar assisted air conditioning system is an innovative, promising alternative to reduce the electrical energy consumption generated by excessive use of conventional air conditioning compressor, especially during summer months. The attractiveness of utilizing solar energy is mainly due to the demand and supply of energy coincides. In fact, cooling is required when the solar radiation is abundantly available. Furthermore, great majority of solar techniques employs harmless working fluids.
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