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Solar-Assisted Cooling And Air-Conditioning

Solar energy can be used to heat, dry and/or cook things, and to generate steam power, but solar cool technologies harvest the sun’s heat to accomplish just the opposite i.e., cooling. Since the changing lifestyle is full of comfort, sunny summer days are beautiful when we have access to cooling; if that cooling comes from the heat of the sun converted directly or indirectly into cooling, then it is like killing two birds with one stone. Sun’s heat can be converted into electricity using photovoltaic cells, and this energy can supplement production of cooling or sun’s heat can be directly used to generate cooling effects with solar refrigeration techniques based on thermoelectric adsorption or absorption. Solar cooling technologies specialize in developing portable power and refrigeration systems that run off of clean and renewable energy solar energy.

Scientists, engineers and industries are constantly modifying and improving these technologies to bring to the consumers the high quality, reliable and environmentally friendly products they (consumers) need. Recognising and harnessing the ‘free’ solar energy that surrounds us every day, they promise to provide customers with quality and environmentally-friendly products that power their everyday needs of cooling. This article presents some innovations in the use of solar heat producing cooling directly or indirectly and becoming popular among masses.

Solar Cooler

The Solar Cooler is the world’s first portable, solar-powered refrigeration cooler. Plugging into the sun can keep food and drinks cool. Not only does the Solar-Cooler provide the convenience of off-grid power and refrigeration, one also has the ability to reduce carbon footprint and to join the movement toward a greener, cleaner and healthier tomorrow. The Solar Cooler keeps its contents cold using a compact refrigeration system connected to solar panels. According to the designers, the Solar Cooler can hold a steady 42°F (5.5°C) for over 24 hours, depending on how often the lid is opened, but can also go as low as 14°F (-10°C) if needed. The temperature is set precisely using a digital display on the side. Users can also get more power by attaching additional solar panels or pre-charging the batteries through an electrical outlet before going out. The cooler itself measures 16 x 14 x 17 in (41 x 36 x 43 cm) and weighs 55 lb (25 kg). Even with the refrigeration system, this still leaves an interior volume of 40 L (10.5 gal), or enough for 60 12 oz (355 ml) soda cans. And naturally, since it doesn’t require ice or cooling packs, that entire space can be completely filled with cold food and drinks. The cooler comes with handles so it’s easier to carry, and the whole case features a rugged design – so it can survive numerous trips and parties unscathed. Some models also sport an optional pair of beach wheels that can traverse sandy terrain during a trip to the ocean. As an added bonus, both USB and 12 V outlets are located on the side, which can be used to charge a mobile phone or plug in a blender. The inside lid even contains a small light, so to find what is needed in the dark. The Solar Cooler goes wherever you do with no need to stop for ice – just pack up and go. Plug right in and let the sun provide a little juice; each cooler offsets five thousand pounds of CO2.

It may look like a simple concept, but getting the right balance of size and power management required some thorough research and testing. The Solar Cooler incorporates some advanced circuitry to collect solar energy from the photovoltaic cells on the lid, store it in the internal batteries, and then distribute it to the refrigeration system – all while taking up a relatively small amount of space. Keeping food and drinks chilled with solar power is handy enough on its own, but the developers have bigger plans for the solar cooler’s technology.

If the current recreational version is successful enough, the company plans to manufacture a similar cooler specifically for vaccines. Naturally, a rugged, solar-powered cooler would help preserve any medicine traveling to remote areas of the world. Unfortunately, all that advanced technology may come with quite a hefty price tag. Even with the added convenience and eco-friendliness though, many people may balk at paying over ten times as much as a similar-sized cooler filled with ice. If all goes as planned however, the first batch of solar coolers are expected soon.

Solar Air Conditioning

A hot day can be altogether stressful because productivity can suffer under such conditions. Therefore, more and more buildings are being fitted with air-conditioning systems. This is where solar air conditioning comes in: the summer sun- which heats up offices- can also deliver the energy to cool them. Days that have the greatest need for cooling are also the very same days that offer the maximum possible solar energy gain. The demand for air conditioning in offices, hotels, laboratories or public buildings such as museums is considerable. Under adequate conditions, solar and solar-assisted air conditioning systems can be reasonable alternatives to conventional air conditioning systems. Such systems have advantages over those that use problematic coolants (CFCs), not to mention the incidental CO2 emissions that are taking on increasingly critical values. Should buildings be cooled with the help of solar energy, then water-assisted air conditioning systems or ventilation systems can be powered with heat that is made available by solar collectors. No long-term intermediate storage is necessary in months of high solar energy gain or in southern lands. The sun can, at least seasonally at our latitudes, provide a substantial part of the energy needed for air conditioning. Combination water-assisted systems and ventilation systems are also possibilities.

The basic principle behind (solar-) thermal driven cooling is the thermo-chemical process of adsorption or absorption: a liquid or gaseous substance is either attached to a solid, porous material (adsorption) or is taken in by a liquid or solid material (absorption). The sorbent (i.e. silica gel, a substance with a large inner surface area) is provided with heat (i.e. from a solar heater) and is de-humidified. After this ‘drying’, or desorption, the process can be repeated in the opposite direction. When providing water vapour or steam, it is stored in the porous storage medium (adsorption) and simultaneously, heat is released. Processes are differentiated between closed refrigerant circulation systems (for producing cold water) and open systems according to the way in which the process is carried out. That is, whether or not the refrigerant comes into contact with the atmosphere. The latter is used for dehumidification and evaporative cooling. Both processes can further be classified according to either liquid or solid sorbents. In addition to the available refrigerating capacity, the relationship between drive heat and realised cold energy (coefficient of performance; CoP) is also an essential performance figure of such systems.

Absorption refrigeration: Closed absorption refrigeration machines with liquid sorbent (water-lithium bromide) are most often operated in combination with heat and power generation (cogeneration) (i.e., with block unit heating power plants, district heating), but can also be assisted by vacuum tube solar collectors (operating temperature above 80°C). With a single-step process, the CoP is 0.6-0.75, or up to 1.2 for a two-step process.
Adsorption refrigeration: Closed processes with solid sorbents work with so-called adsorption refrigeration machines (operating temperatures 60° – 95°; COP = 0.3 – 0.7). Solar energy can easily be used in the form of vacuum tube or flat plate collectors. The refrigerating machine is composed of two adsorbers, one an evaporator and the other a condenser. An adsorber chamber takes up the water vapour, which is transformed into the gas phase under low pressure and low temperatures (about 9°C) within the evaporator. Granulated silicate gel, well known as an environmentally friendly drying agent, then accumulates it (adsorbs the water vapour). In the other sorption chamber, the water vapour is set free again (the chamber is regenerated or ‘charged’) by the hot water from the solar collector (about 85°C). The pressure increases and at the temperature of the surroundings (30°C) the water vapour can be transformed once again into a fluid within a cooling tower (condensed). The water is led back into the evaporator through a butterfly valve and the cycle begins from the beginning. Both the condensed water (low temperature) and the sorption heat (high temperature) are discharged.

Sorption refrigeration: Although the process of sorption-assisted air conditioning has been known for a long time, it has only been used for about 15 years. In principle, sorption-assisted air conditioning systems can be operated everywhere an air conditioner is wanted, for example in ventilation control centers. Their economical operation is then possible, if cost-effective heat energy is available, i.e., from co-generation plants, rather than from over loaded district heating systems. Solar thermal systems are new heat systems that offer much promise. Open sorption-assisted air conditioning systems are fresh air systems that dry the outside air through sorption, pre-cool it with a heat reclamation rotor, and finally cool it to room temperature through evaporation-humidification. At present, systems with rotating sorption wheels (sorption rotors) are mostly in use. The sorption wheel has small air channels that create a very large surface contact area, which has been treated with a material that easily takes up moisture, such as silica gel. The inflow air is dehumidified in one of the two sectors of the rotor and heated through the adsorption process (the exhaust air serves to dry the rotor). Finally, the inflowing air is cooled down in a heat reclamation rotor. The heat transfer here is made possible through the contact between the air and the rotor material. The last step in cooling the inflowing air is with conventional evaporation humidification.

Cost Effectiveness

A number of systems that use thermal solar energy to air condition buildings, and that can be technically and economically assessed, have been installed all over the world – but there are still a number of obstacles to be overcome when it comes to the general implementation of solar-assisted air conditioning. Pilot and demonstration programmes are still necessary – so that cost reductions become possible, and so that relevant energy savings can be assured. Standardised programmes, matured concepts and the development of components are starting points that can contribute to improved cost effectiveness and wide applicability of solar-assisted air conditioning. Because solar cooling is based on thermally driven processes instead of the normal electrical cold production, the costs for the used heat plays a central role: a fundamental problem arises from the inherently higher costs of solar heat compared to heat energy produced by fossil fuel systems or waste heat. Their use becomes interesting, if favourable requirements for a high output of solar heat is present, and if the system also delivers energy for heating. The cost of electricity could also pose an argument for solar cooling: The thermally powered cooling process requires only a fourth (absorption/adsorption) or half (sorption-assisted air conditioning) of the electrical power required by the conventional reference system.

Conclusion

Normal cooling systems use chlorofluorocarbon chemicals that destroy the ozone layer and contribute to greenhouse gases. Maintaining food in places where high temperatures prevail, using little energy at a low cost, is now possible with solar cooling technology. The design is inexpensive, easy to manufacture and environmentally beneficial.


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