• Cooling India
  • Oct 15, 2015

Solar Refrigeration A Success Model




Energetic and Exergetic techniques help in evaluating the performance of the SPV refrigerator with a view to getting better information about useful work and lost work, and design some remedial techniques in future to overcome on these losses...

Er. Kapil Kumar Samar
Dr. Surendra Kothari




In current situations, energy demand is increasing with the increase in the population and improvement in the living standard. Energy is the crucial input to the social, economical, industrial and technological development of any country. A rational use of energy brings both economic and environmental benefits by reducing consumption of fossil fuels, electricity and pollutant emissions. The International Institute of Refrigeration in Paris (IIF/IIR) has estimated that approximately 15% of all the electricity produced in the whole world is employed for refrigeration and air-conditioning processes. In a tropical country, like India, refrigeration is most widely used and generally, the most energy consuming process. In general, refrigeration is defined as any process of heat removal from a place for preserving foods and medicines by enhancing their shelf life. Immunization through vaccine prevents illness, disability and death from preventable diseases like diphtheria, measles, pertussis, pneumonia, polio, rotavirus diarrhoea, rubella and tetanus. Immunization currently averts an estimated 2 to 3 million deaths every year, but an estimated 22 million people from remote area of developing country worldwide are still missing out their routine vaccination programs due to the lack in availability of the safe vaccine. According to WHO guidelines, vaccine should be kept in the temperature range of 0-8oC.

     For the storage of life saving drugs or vaccines in the innumerable area of the developing country – where the power supply is still irregular, renewable energy has to be a central part of energy solution. Out of the various renewable sources of energy, solar energy proves to be the best candidate for cooling – because of the coincidence of the maximum cooling load with the period of greatest solar radiation input. Cooling from solar energy has great potential for lower running costs, greater reliability and a longer working life than other conventional cooling systems, whereas it may also contribute in the reduction of global warming.

     Hwang and Redermacher (2011), Kim and Ferreira (2008) broadly classified different technologies that are available to use solar energy for refrigeration. The review covers solar electric cooling, solar thermal cooling and solar combined power cooling. A comparison between these different technologies is also described with individual COP value. M.M. Salah (2006) briefly discussed on application of solar power for producing refrigeration effect.
    
     Possible solar power refrigeration system as discussed are - absorption cycle, adsorption cycle, desiccant cycle, ejector cycle, solar mechanical and solar PV operated refrigeration system. Cooling systems based on solar thermal technologies are having less thermodynamic efficiency as compare to vapour compression refrigeration system because it is very difficult to keep the solar thermal system operating at steady condition throughout the day. Solar thermal based cooling systems are commercially available but mostly have the capacity of more than 20TR because a solar collector can’t scale down in size. Further because of the small capacity of cooling system, solar photovoltaic vapour compression refrigeration system is deemed to be the most viable route.
    
     Therefore, an attempt has been made in designing and develop solar vapour compression refrigeration systems at the Department of Renewable Energy Engineering, Udaipur. The principle objective of this article is to describe the result of thermodynamic tests conducted on the developed solar vapour compression refrigeration system.

System Description:

    The solar photovoltaic based refrigeration system was designed, developed and evaluated by Department of Renewable Energy Engineering, Udaipur under ‘no load’ and ‘full load’ conditions. A PV panel consisting of three modules (125 Watt peak each) connected in series was used to obtain the desired voltage and current, respectively. Three 12 V, 7 Ah sealed lead acid battery were used to supply the power at starting time and ensure for the smooth operation. The refrigerator operates on an alternative current based compressor, a compressor used in the common domestic refrigerators. Technical specifications of the solar refrigerator and Balance Of System (BOS) for the power supply are given in Table 1 and Table 2.


 
System Performance:

     Coefficient of performance: The coefficient of performance is an index of performance of a thermodynamic cycle or a refrigeration system. COP is used instead of thermal efficiency. For the vapour compression refrigeration cycle, COP is defined as the amount of cooling produced per unit work supplied on the refrigerant. For a reversible or Carnot refrigeration cycle it is expressed as:

Te = Evaporator temperature (°C)
TO = Ambient/room temperature (°C)
     But all the real processes are irreversible process. The actual COP of the refrigeration system was calculated with the help of pressure enthalpy curve produced by Hansen and Artu (Rathore et al). The COP can be evaluated by using the formula-

Fig.1 Pressure enthalpy diagram of operating system

Photovoltaic Efficiency:

     The efficiency of the solar panels, defined as the ratio of the electrical power produced to the incident radiation.  

where  
npv= efficiency of photovoltaic system
Pmax: Maximum power from photovoltaic system (W)
S = Solar irradiance (W/m2)
Apv = Area of the photovoltaic system (m2)


Exergy Analysis

     Exergy is defined as the maximum amount of work that can be done by a system. Unlike energy, exergy is not subject to a conservation law; exergy is consumed or destroyed, due to the irreversibility's present in every real process.



Photovoltaic Exergy

     The energy of a PV module depends on two major components – electrical and thermal. While electricity is generated by the PV effect, the PV cells are also heated due to the thermal energy present in the solar radiation. The electricity (electrical energy), generated by a photovoltaic system, is also termed ‘electrical exergy’ as it is the available energy that can completely be utilsed in useful purpose. Since the thermal energy available on the photovoltaic surface was not utilised for a useful purpose, it is considered to be a heat loss to the ambient. Therefore, due to heat loss, it becomes exergy destruction. The exergy output of the photovoltaic system can be calculated as:

     where Vm, Im hc, A, Tcell and To are the maximum voltage and current of the photovoltaic system, convective heat transfer coefficient from the photovoltaic cell to ambient, area of the photovoltaic surface, cell temperature and ambient temperature (dead state temperature), respectively.

     Exergy input of the photovoltaic system – which is the exergy of solar energy – can be calculated approximately as below

where TSUN = temperature of the sun taken as
5760 K
     Exergy efficiency of the photovoltaic system is defined as the ratio of total output exergy (recovered) to total input exergy (supplied). It can be expressed as


Conclusions

     Energetic and Exergetic techniques help inevaluating the performance of the SPV refrigerator with a view to getting better information about useful work and lost work, and design some remedial techniques in future to overcome on these losses. The installed system of solar photovoltaic refrigerator system is capable for cooling the vaccine for 7 hour in a day. The pull down test indicates that 375Wp photovoltaic capacity and 21Ah battery bank is the least possible configuration required for this converted system. The average COP during ‘no load’ and ‘full load’ tests were found high as 3.37. Second law efficiency of the refrigerator system remains close to 55% at no load full load conditions. The photovoltaic conversion efficiency and exergy efficiency found nearer to 10% and 8.5% respectively in both ‘no load’ and ‘full load’ condition. This indicates that the product load condition does not affect the PV system. The reason for low overall efficiencies is because both, the energy conversion efficiency and exergy efficiency, of the PV system are low – so that it can be said that exergy is destroyed highly in PV. The payback period of the proposed system was found six months. It is suggested that the design procedure may be improved by a variable speed compressor to cope with the variation of the refrigeration load due to different modes of operation. The performance curves are shown in Fig. 1 to 5. 


Er. Kapil Kumar Samar
Dr. Surendra Kothari

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