In this article we will discuss about:- 1. Introduction to Domestic Solar Refrigerator 2. Conventional Domestic Refrigerator 3. Selection of Solar Refrigeration Process 4. Use of Zeolite 5. System Design 6. Specifications.
Contents:
- Introduction to Domestic Solar Refrigerator
- Conventional Domestic Refrigerator
- Selection of Solar Refrigeration Process
- Use of Zeolite for Solar Refrigeration
- System Design of Solar Domestic Refrigerator
- Specifications of Solar Refrigerator
1. Introduction to Domestic Solar Refrigerator:
Solar cooling and refrigeration are ideally adaptable in India. India is bestowed with high solar insolation and long sunshine hours. Refrigeration units are most needed in rural areas for food preservation where it is difficult to reach with electric power. Therefore, development of small solar-driven refrigeration units is urgently needed for India.
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Solar energy is better suited to space cooling and refrigeration than to space heating, but until recently this application of solar energy has received little attention. However, energy for air conditioning and refrigeration has been the faster growing segment of energy consumption due to increasing standard of living and population in India and other developing countries.
At the 1975 UNESCO conference on Solar Energy several developing countries stated that small solar driven refrigeration units are urgently needed for preservation of food in rural areas where little electric power is available. Consequently, accelerated effort is needed to effect progress in the application of solar energy to space cooling and refrigeration.
The seasonal variations of solar energy are extremely well suited to the space cooling and refrigeration requirements. Since warmest seasons of the year correspond to periods of high incidence of solar energy, it is most available when comfort cooling and refrigeration are most needed. Moreover, the efficiency of solar collector increases with increase in incidence in solar energy and increasing environmental temperature. Consequently, in summer the amount of energy delivered per unit surface area of collector can be larger than that in winter.
In the light of the above situation, a project was undertaken for the design and development of a small domestic solar refrigerator. The capability of the existing conventional refrigerators was studied and a survey of literature was made to find out a suitable technology for a solar refrigerator to meet the requirements of conventional domestic refrigerator.
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It was found that a solid absorption-Zeolite refrigerator is most suitable because of absence of moving parts, toxic and corrosive chemicals and expectation of long trouble free operation life. The system is ideally suited to local environment. Heat load and temperature requirements can be met by a convenient and compact sized solar system at competitive price.
There is limited operating experience with solar cooling system. Therefore, the system and components have been designed from the fundamental principles taking into account the operational experience of a conventional refrigeration cycle and special features of the components in a solar system. The project experience can be used to develop small cold storages for preservation of food in rural areas and commercialization of domestic solar refrigerators.
2. Conventional Domestic Refrigerator:
It has been found that the present 165L capacity refrigerator working on vapour compression cycle using Freon-12 as refrigerant is most suitable and popular size in India. The design of the solar domestic refrigerator has therefore, been based on the main specifications of 165L capacity refrigerator. A suitable solar refrigeration system has been sized for fitting onto the cabinet of the present domestic refrigerator of this capacity.
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The present conventional refrigerator consists of a well-insulated cabinet to maintain a temperature of 8-10°C in the refrigerated space and -4 °C in the freezer.
The main dimensions of the cabinet are as follows:
An air-cooled extended surface type condenser of sufficient surface area is mounted on the back of the refrigerator. Plate type or wire finned tubular condensers are normally used with 6.35/4.8 mm copper or steel tubes of total length of about 7m.
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The evaporators are housed in the upper space of the cabinet and consist of 7.9/6.35 mm copper tubing of about 5m length soldered to the plate.
An hermetically sealed compressor of 1/8hp is spring mounted to isolate vibrations. A filter and capillary tube acts as division point between high pressure and low pressure sides of the cycle and flow of liquid refrigerant is regulated into the evaporator by the capillary tube.
A maximum cooling load of 317W can be produced by circulation of 125.5 g/min of refrigerant at 1.2 kgf/cm2 and – 25°C in the evaporator and 12.25kgf 1 cm2 and 50°C in the condenser. The maximum coefficient of performance achieved is 4.3.
An experiment was conducted to verify the actual cooling load and temperatures obtained in a conventional domestic refrigerator under normal working conditions. A 165L Kelvinator refrigerator was switched off from the power supply and was loaded with usual eatables. The freezer box was filled with 8kg of ice. The temperature of the cooled space and amount of ice melted were periodically measured.
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It was found that similar temperature of 8-10°C was obtained as produced by the refrigerator working on compression cycle. The total ice required for 24 hours operation was 7.5 kg for extreme environmental conditions. In order to take care of a non-sunny day, the solar refrigerator was designed for production of 15kg of ice by the solar system.
3. Selection of Solar Refrigeration Process:
The main requirements of a solar powered domestic refrigerator can be summarized as follows:
i. An ability to keep refrigerated space at 8 to 10°C in an external temperature of + 43 °C during day and + 32°C at night. The system should be compact and suitable for attachment to the cabinet of an existing 165L domestic refrigerator.
ii. The system should be free of corrosive, toxic and irritating chemicals.
iii. The system should have few components, no moving parts, operational simplicity and long trouble-free life.
iv. The physical size, weight should be small for ease of transportation and installation.
v. It should be possible to manufacture the system and components from local materials and skills.
The following processes can be used for the solar domestic refrigerator:
1. Photovoltaic-Compression System:
These systems use the photovoltaic cells which produce electricity when exposed to light. The electricity that is generated is used to charge a battery that operates a 12 or 24 volt d.c. compressor. It provides cooling through an ordinary Freon cycle as in conventional domestic refrigerators.
Cooling can be provided during period without sun by a store of ice in the refrigerator or by stored surplus power in batteries. The electric power is controlled automatically by a regulator. The system is the most advanced solar cooling technology and many coolers have been marketed as such.
The photovoltaic panels and batteries are very costly; hence initial cost will be very high. Batteries have to be replaced every two to three years making high operating cost. Reliability of photovoltaic panels is unknown at this stage. The major weakness is the complexity and a number of electrical components.
2. Photovoltaic Thermoelectric Refrigeration System:
This method uses the Peltier Effect according to which one end of the element is kept at a high temperature and other at low temperature then emf is produced. In this case electricity is produced using photovoltaic panels exposed to sun. The electricity is used to produce a heat flow through a bimetallic (peltier) solid state element.
The cold side of this element is exposed to the inside of the refrigerator and the hot side to outside air. A single element can produce a temperature difference of 27°C between inside and outside faces. The elements can be cascaded together to obtain higher performance. A fan is usually required to remove the accumulated heat from the outside surface. This system has same limitations as photovoltaic-compression system.
3. Ammonia-Water System:
Ammonia gas is boiled out of a solution of water and liquid ammonia in a solar collector and is condensed and stored until the collector cools at night and the pressure in the system falls. The liquid ammonia passes into the evaporator and boils at – 12°C. The frozen surface of evaporator can make 5 kg of ice in 8 hours. The ammonia gas returns to the collector where it is reabsorbed into a solution with water ready for the next day. The ice can be removed for use or left to cool the refrigerator in times of no sun.
Ammonia has pungent and irritating smell and is not used in conventional domestic refrigerator. It has leakage problem. The complexity of system and welding of pressure parts require special skills. The system development at present is at university level.
4. Lithium Bromide-Water System:
This is a similar system as ammonia-water system with the difference that steam is boiled out of the solution of lithium bromide and water. Steam is condensed and its evaporation at low pressure produces cooling.
In this system the temperature required in the solar collector is in the range of 120°C which cannot be obtained in a flat plate collector. The concentrating type collectors are needed to get the necessary temperature which makes economy unfavourable. It is difficult to obtain adequate circulation expecially during regeneration cycle without a pump. The risk exists that liquid lithium bromide will also vaporise.
5. Calclium Chloride-Ammonia System:
Calcium chloride is solid at room temperature and it works as solid absorption system. During day, ammonia gas is driven out of solar collector filled with calcium chloride and cement. The gas is condensed by cooling in a water tank to liquid ammonia and passes into the evaporator. At night, as the collector cools and pressure falls, the liquid ammonia passes into the evaporator and boils at – 12°C.
The frozen surface of evaporator can make 9 kg of ice in 8 hours. The ammonia gas then returns to the collector to be reabsorbed into calcium chloride for the next day. The ice can be removed for use or left to keep the refrigerator cool at times of no sun.
Same objections to the use of ammonia as refrigerant are applicable in Water-ammonia system. This system also works at high temperatures of 120°C which cannot be easily obtained in a flat-plate collector. It is difficult to keep solid absorbent as porous and to prevent its clogging.
6. Solid Absorption Zeolite Refrigeration System:
The solar collector is filled with zeolite which is a sandy material available in nature as lava of volcanoes or manufactured as molecular sieve. Water vapour is driven out of the collector by solar heating during the day. The vapours are condensed and stored in a tank until evening. As the collector cools at night due to sky radiation, a vacuum in the system causes the water in the evaporator to boil constantly at a temperature of – 2°C at a pressure of 4mm of Hg.
The frozen surface of evaporator can make 9 kg of ice in 8 hours. The water vapours return to the collector for reabsorption into zeolite and the system is ready for the next day .The ice can be removed for use or left to cool the refrigerator at times of no sunshine.
This system has no moving parts; it is quite compact and has a long operational life. No chemicals are used. Water being the refrigerant, there is no leakage problem. Due to high heat of vaporization of water, the amount of refrigerant circulated is very small as compared to other refrigerants.
It can run at collector temperatures of 50°C which are easily obtained in ordinary flat-plate solar collectors. Some manufacturing experience is available in USA for zeolite solar refrigeration. This system meets most of the requirements for a domestic refrigerator as enumerated above and hence has been selected for the project.
4. Use of Zeolite for Solar Refrigeration:
It is possible to use a number of liquid or solid desicants for solar refrigeration. Any material that has high affinity for water vapour can be used. It is regenerated by heating with solar energy It has been found that zeolites have an unusual absorption property that they can absorb about 30% by weight of water at all relative humidities and have been successfully tried in ice-making and solar refrigerators.
It is an aluminum-silica complex which has a unique property of absorbing water at low temperatures and giving out at higher temperatures on heating. It can provide refrigeration with excellent engineering efficiency due to their extremely nonlinear absorption isothermals. Since the heat of vaporization of water is largest of any common refrigerant, being ten times larger than that of Freon’s, the zeolite-water vapour combination can provide the most efficient system requiring the smallest quantity of zeolite for its operation.
The adsorption of water zeolite depends exponentially on at lest the second to the fifth power of ΔH/RT, i.e., exp (ΔH/RT)2 to exp (ΔH/RT)5. The adsorption of ammonia in water or water vapour in LiBr or in Silica Gel or activated alumina is directly proportional to exp (δH/RT) where δH = Heat of Adsorption and T is the absolute temperature. It is this extreme non-linearity of thermal activation that makes zeolite so well suited for cooling.
The zeolite is sealed in an air-tight container that is irradiated by sun. During the day cycle, the zeolite and its container is heated to high temperature. At about 40°C water vapour starts desorbing from the zeolite and partial pressure begins to rise. When the pressure reaches the value determined by the condenser temperature, i.e., 55 mm of Hg at 40°C, the vapours begin to liquify. Heat is rejected to the outside and liquid water is stored in a storage tank.
During night cycle, the zeolite is cooled by sky radiation and is ready to adsorb water vapour even at low pressure; liquid water absorbs heat from the space to be cooled and is converted into water vapours. If the partial pressure can be maintained at 4 mm of Hg, the water in the evaporator will boil at – 2°C. The function of zeolite is to absorb the water vapour produced by the evaporator maintaining the partial pressure below 4mm of Hg.
Zeolite system should be free of air; therefore the zeolite is placed in hermetically sealed metal panel. The panel is painted black for maximum solar absorption. It is connected through ordinary plumbing to the condenser, water storage tank and evaporator. After the zeolite is charged with water vapour, the whole system is outgassed, evacuated and sealed off.
The zeolite solar panel consists of a flat-plate conventional collector. For the extreme condensation and evaporation pressures of 55mm and 4mm of Hg respectively for most zeolites the differential water loading is about 55% by weight between ambient temperature and 120°C which is the maximum temperature attainable with flat-plate solar collectors. In most of the practical uses, it is closer to 3% by weight. After considerable experimentation and computer analysis, it is found that optimum results are obtained with zeolite capacity of 50 kg/m2 with a depth of 5cm.
The upper surface of the panel is painted black and pipes are provided at the bottom to permit the water vapours to enter and leave. Each panel has to be leak tested prior to use.
Zeolites can provide refrigeration from the sun without any external power and without any moving parts. The solar refrigerator with an efficiency of 15% produces about 900Wh of cooling per square meter of collector area for a solar input of 6 kWh. This corresponds to about 9Kg of ice manufactured per day per square meter of collector area.
5. System Design of Solar Domestic Refrigerator:
The design data for the solar domestic refrigerator has been assumed as follows:
6. Specifications of Solar Refrigerator:
The main specifications of the solar refrigeration are worked out as follows:
