Solar Water Heating System

Solar water heaters are one of the common applications of conversion of solar energy to heat water. Some of the typical applications include domestic hot water, swimming pool heating and commercial and industrial hot water supplies.

 Generally a solar water heater consists of 3 parts – 

1. A heat collection system

A heat collector system commonly known as a collector converts the solar radiation to heat. This heat is transferred to the fluid contained within the collector unit. In a typical solar water heater, cold water is supplied at the lower side of the collector and as it is heated the warm water rises and exists from the upper side.

2. A heat storage system

The hot water from the collector is stored in the hot water tank. The tank is well insulated to prevent heat loss. Different kinds of insulation is available in the market some of the common types are Rockwool, poly urethane foam etc.

3. An auxiliary heating system 

During cloudy days an auxiliary heating system can heat water through electricity or other energy source. Normally such system is used only when required.

Type 

Typically there are 2 types of solar water heaters-

• Passive systems, and

• Active system

Passive system

In passive system, there are no pumps or fans used in the operation. The collector is close to the area where heat is stored or required. Natural process such as conduction, convection and radiation are used to transfer heat to the load. Passive systems are simpler and in general cheaper than active systems because they do not require the use of pumps or fans. A typical example is a thermosyphoning closed coupled of a passive solar water heater system.

Active system 

In active systems, the energy is used or stored in a location away from the surface being heated by the sun. Therefore, the pump or fans are used to transport the fluid heated by the collector to the storage unit and sometimes from the storage unit to the load. A common example is solar pool heating system.

Fuel from Charcoal

 More than 2 billion people use wood, charcoal, dung or agricultural residues as the primary fuel for their cooking and heating needs, leading to significant health, economic and environmental consequences. Burning wood or agricultural residues produces smoke with a variety of irritant pollutants, some of which are known carcinogens. More than 1.5 million deaths a year are caused by acute respiratory infections from breathing smoke from indoor cooking fires. Women and children are generally exposed to the greatest levels of pollutants and it is children who suffer the greatest health risk – respiratory infections are the leading cause of death of young children worldwide.

 Furthermore, many areas of the developing world face massive environmental problems due to deforestation as almost half the world’s timber harvest is burned for fuel.

Several attempts have been made to introduce environmentally-friendly, cleaner cooking alternatives such as solar cookers, fuel-efficient stoves and charcoal briquettes made from paper. Solar cookers can be slower than traditional stoves, may work only during limited hours on sunny days, and are often unable to fry foods, which precludes them from being used to cook many traditional dishes. Fuel-efficient stoves have started to be disseminated in regions ranging from India to Sudan, but while they reduce the consumption of wood-based or fossil fuels, they may not necessarily eliminate it. An effort to produce briquettes from waste paper was introduced to reduce wood use, but initially these briquettes can be difficult to use and they still produce a significant amount of smoke. Furthermore, waste paper is not readily available in many rural areas. For alternatives to traditional cooking to be met with better success, they must prove to be affordable, effective and culturally acceptable.

Corn Sheller

Maize (corn) is one of the most important staple crops in the world. In Asia, maize production is over 200 billion kilograms a year and it is expected that the total maize production in developing countries will eventually overtake production in industrialized countries.

In many rural areas of developing countries, the maize kernels are removed from the cob by hand in a process called shelling. Shelling the annual maize harvest by hand typically takes weeks and may pull children out of school, since processing food for survival takes priority over education in subsistence farming households. The hardened, dry maize can also be painful to shell and lead to hand injuries.

 Existing alternatives to shelling maize by hand are often unaffordable or difficult to obtain for subsistence farmers. An estimated 550 million small-holder farmers in the world lack access to mechanized agricultural technology. Industrial maize shellers are prohibitively expensive and small-scale hand-cranked or pedal-powered maize shellers cost more which is still more than many families can afford. While industrial shellers are highly productive, their energy infrastructure requirements can render them unusable in rural villages. Furthermore, mechanized equipment and stationary pedal-powered devices are difficult to transport to the users. As a consequence, farmers may be required to travel long distances to process their crops or the technology may not be able to reach the communities who need it most.

Another option is a simple tool that makes it possible to shell maize several times faster than by hand. The device has the additional advantages of being robust, portable, transparent to users and only a fraction of the cost of other alternatives commonly on the market. The maize sheller is currently mass-produced using injection-molded plastic or cast aluminum. In regions where the device is not currently being manufactured or distributed, the tool cannot be locally made using these infrastructure-intensive processes.

There has been development of several versions of the maize sheller that can be made using locally available materials and methods.

Solar Dryers

The food production especially fruits and vegetables are surplus during the harvesting season, resulting in low selling price. Towards the end of the season the produce which was not sold goes uneaten or rots. Similarly, in alpine climate the food production is limited to few months in a year. Hence food preservation is important, among the various techniques available sun drying is one of common. 

Basically, drying involves the extraction of moisture from the product by heating and the passage of air mass around it to carry away the released vapor. Under ambient conditions, these processes continue until the vapor pressure of the moisture held in the product equals that held in the atmosphere. Thus, the rate of moisture released from the product to the environment and absorption from the environment are in equilibrium, and the crop moisture content at this condition is known as the equilibrium moisture content. Under ambient conditions, the drying process is slow, and in environments of high relative humidity, the equilibrium moisture content is insufficiently low for safe storage. The objective of a dryer is to supply the product with more heat than is available under ambient conditions, thereby increasing sufficiently the vapour pressure of the moisture held within the crop and decreasing significantly the relative humidity of the drying air and thereby increasing its moisture carrying capacity and ensuring sufficiently low equilibrium moisture content.

In solar drying, solar-energy is used as either the sole source of the required heat or as a supplemental source. The air flow can be generated by either natural or forced-convection. The heating procedure could involve the passage of preheated air through the product or by directly exposing the product to solar radiation or a combination of both.

Solar energy is an obvious energy source to use for drying many products, particularly food crops. Many crops are harvested in the summer months and are usually dried at temperatures below 700C - a temperature which can be readily attained by solar technology. 

The importance of food drying is likely to increase. Nepal suffers from serious food crisis. Out of 75 districts 41 districts have food shortage and experts from the United Nations warn that the situation is bound to deteriorate. Annually huge relief funds from international and government budget is allocated for food supply in the regions with supply deficit. Further the Government figures say food deficit has tripled in recent years with most of the increase since 2009 drought.

A solar dryer uses the energy from the sun to dry food efficiently and hygienically with little capital investment. The dried food life may be extended to a year or more depending on the process.  In addition to foods for human consumption there are many other products we use that require drying. These include organic crops like timber and rubber and inorganic materials like paint. All of the above arguments emphasize the importance of drying in our lives. 

Drying is also an energy intensive process. The shortage of energy is an issue for many countries, particularly those in the developing world. Even where conventional energy is plentiful, there is pressure to reduce the amount of fossil fuels used. Concern over global warming is universal and this has focused our attention on energy intensive processes like drying where fossil fuels can often be replaced by renewable and non-polluting sources of energy. 

Principle

Drying involves the removal of the internal moisture to the surface and then to remove this moisture from the surface of the drying material. The sun has been used for drying as long as humans have inhabited the planet and laying a product out in the sun to remove its moisture is known sun drying. When sun drying, the temperature of the surrounding air remains at ambient temperature, while the temperature of the product is raised by the direct absorption of solar radiation. Although sun drying is still by far the most common method of drying it does have several inherent disadvantages. The unprotected crop can be damaged by rain, contaminated by dirt and animals and/or eaten by birds and insects. Since the temperatures attained during sun drying are usually lower than in a solar dryer, drying times are longer. This usually results in poorer final quality because of color discoloration caused by enzymic and non-enzymic browning, and often because of the formation of moulds.In a solar dryer however the temperature of the air surrounding the product is raised above the ambient air temperature. Depending on the type of solar dryer, the temperature of the product may also be raised by direct absorption of solar radiation. The temperatures in a solar dryer are higher than in sun drying and this reduces the drying time and usually improves the final product quality. Crop losses and spoilage from rain and animals are prevented because the crop is protected within the solar dryer. 

Types:

 

There are many different types of solar dryer but they can all be conveniently classified into three distinct categories depending on the mode of heat transfer from the sun to the product.