The paper justifies the application scope of biogas as a fuel for traditional vapour absorption refrigeration systems. Both environment and economics benefits can be achieved when biogas is used as a source of energy for refrigeration...
- Er. Kapil Samar,
Dr. Deepak Sharma
In the current situation the energy demand is increasing with burgeoning population. Energy is the crucial input to the development of any country. 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 the 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 its shelf life.
Farmers mostly dairy farmers who sell their products to export markets, refrigeration could play an important role to increase their annual income. Without cooling capabilities the dairy products have to be sold immediately after taking from animals. This reduces the chance of negotiating good prices, because the buyer is in a better bargaining position. Particularly in these sectors, farmers have the potential to produce a lot of biogas through available cattle dung. Biogas based refrigeration technology would be a good opportunity for such farmers to take maximum benefits.
Cooling effect is produced by the evaporation of a refrigerant. Heat is used in different ways to operate a refrigerator system for evaporating the refrigerant in the cycle. Biomass energy is a good source especially for agro based rural areas where a lot of organic materials are being wasted. Extraction of bio-energy with carbon neutral process is possible. As regular hike in the conventional fuel prices like LPG and CNG, biogas serves a good source of fuel for refrigerators. Bio-chilling denotes that heat is produced through any conversion process of biomass such as biogas, producer gas etc to operate a refrigeration cycle. Biogas refrigeration technology can be classified into mainly two categories: electrical refrigeration and thermal refrigeration.
Kim et. al.(2008) & Hwang et. al. (2011) have provided with a broad overview of the various technologies available to use non-conventional energy for refrigeration purposes which includes electric, thermo-mechanical, sorption and some newly emerging technologies. They have also compared the potential of these different technologies in delivering competitive sustainable solutions.
A bio-electric refrigeration system consists mainly of electric generator and a compressor based refrigeration unit. Biogas is used as fuel to generate electricity. The biggest advantage of using bio-generator for refrigeration is the ease of operation and high overall efficiency when combined with a conventional vapour compression system. Winrock International, Pakistan installed a biogas based vapour compressor milk chilling unit during the year 2012-13. They installed 4 biogas plants: two plants of 50m3 and two plants of 100m3. The milk chillers run on electricity with capacity of 500 litres and 1,000 litres for eight hour. But high initial investment is the major issue for the development of this technology.
Bio-thermal refrigeration system uses heat produced from burning of biogas. Based on sorption principle, this type of system uses physical or chemical attraction between a pair of substances to produce refrigeration effect. A sorption system has a unique capability of transforming thermal energy directly into cooling power. Among the pair of substances, the substance with lower boiling temperature is called sorbate and the other is called sorbent. The sorbate plays the role of refrigerant. This category is further classified into two streams as absorption systems and adsorption systems.
Room for Research
A few designs of absorption refrigeration system are commercially available which operates on conventional fuels. However, there appears to be a lack of products specifically designed to operate on biogas. A simple way of using biogas for refrigeration is by adapting commercial absorption refrigerators. In this situation, the burner in the refrigerator needs to be modified in order to deal with the safe and controlled combustion of biogas with its impurities and the varying levels of methane content. Without modification, chances of components failure are more. Remote ignition via a piezoelectric element substantially increases the ease of operation.
Apart from the physical modification, energy analysis will also play an important role in the further modification towards energetic optimization. In this regard, an evaluation based on first law and second law of thermodynamic gives result in point energy loss and identify the reversibility that lead to energy destruction. The second law analysis recognises that heat energy has a lower availability than work energy.
Fig.1 Schematic view of biogas based absorption refrigeration system.
Biogas based Absorption Refrigeration System
Vapour Absorption Refrigeration Systems (VARS) belongs to the class of vapour cycles. The absorption refrigeration cycle consist of a generator, condenser, evaporator, absorber, expansion valve and pump as shown in Figure 1. During one cycle the refrigerant passes through four main stages:
• In the evaporator, the fluid refrigerant evaporates by extracting heat from the product or room being refrigerated.
• The evaporated refrigerant flows into the absorber where it mixes with the secondary fluid.
• The resulting solution is then driven into the generator, where it is heated. This heat causes the refrigerant to vaporise.
• The resulting vapour passes into the condenser, where it returns to liquid state and is ready to start a new cycle.
Srikhirin et.al discussed various designs of vapour absorption refrigeration system. Rao et.al. studied the thermodynamic simulation and analysis the biogas operated double effect ammonia water based GAX absorption refrigeration system. A computer code was developed for computing the effect of temperature and pressure of the high temperature generator and the pressure of the evaporator over the COP for a constant condenser and absorber temperature. It was suggested that biogas can be used to operate the absorption cycle.
It is estimated that one kerosene refrigerator emits between 900 kg CO2 per year. Biogas based refrigeration can replace conventional refrigerators and offers a sustainable solution for cooling using renewable energy.
There are some commercial products are available based on heat driven refrigeration processes. The basic principles of heat driven refrigeration have long been known but the market for heat driven cooling is still small. Technologies for generating cold from biogas seem to be an innovation field with significant potential. Harnessing this potential would require serious effort in terms of research, development and bringing the technology to market.
Key areas to focus in order to improve the overall performance of heat driven refrigerators are efficiency improvements of the different systems and the possibility of the joint operation of various refrigeration cycles. Modification ensures user-friendly operation and maintenance. In the case of users who can produce their own biogas, switching to biogas can result in economic benefits in the medium term, due to the savings made in fuel costs. However, there are currently too few biogas refrigerators in use to provide concrete figures.
The gas demand for refrigeration varies depending on the outside temperature. A 100 litres volume refrigeration system needs about 2000 litres of biogas per day to down the temperature from ambient to five degree Celsius. A large household refrigerator consumes about 3000 litres of biogas per day.
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