Solar Water Heaters
Solar water heaters are used to heat water to 40-80 degree Celsius. They are made up of three main components: a collector, a heat transfer curcit to take the heat from the collector to the water and a water reservoir where the hot water is stored. The collector is made up of an absorber (usually a black metal plate with metal tubing) in an insulated box with a glass cover. The collector heats up the water in the storage tank either directly or through a heat exchanger.
A solar water heater will provide about 50-100 liters of warm water (60 degree Celsius) per square metre of collector area on an average sunny day in sunny countries. When water reaches higher temperatures, the efficiency of the heater becomes much lower. Solar water heaters usually cease to be economic when water is raised above 60 or 70 degree Celsius.
- Reliable and simple technology.
- Can be incorporated in the hot water system of the enterprise without much difficulty.
- Low maintenance and repair cost.
- Delivers warm water for low price.
- Relatively high investment
- If not properly installed, leaks and malfunctioning often occur
- The efficiency of the heater becomes much lower when water is raised to higher temperature, and it usually ceases to be economic when water is raised above 60 or 70 degree Celsius
Drying is traditionally done by exposing what has to be dried to the sun and wind. Solar driers work on the same principle, but they take up less space, require less time, and produce less waste. Moreover, they protect the food from dust, flies, birds and other animals, resulting in a more hygienic way of drying. Solar drying increases the time that food can be stored without spoiling. In general, solar drying becomes more appropriate when:
- Products have high value per tonne;
- A high proportion is being spoiled in the open air;
- A drier can be used for long periods during the year (more than one month)
The design and operation of solar driers are very product specific (as all drying processes are) as product characteristics and climate conditions strongly influence the process.In general, solar driers require some extra skills. Cases have been reported where the product was over-dried (too low moisture content) or even burnt. The driers must be specifically designed for the kind of product and the climatic conditions.
- Simple technology.
- Faster drying process.
- Low running and maintenance costs.
- Less attention is needed for the drying process compared with open air drying.
- Higher product quality (lower moisture content, less spoilage, increased storage time).
- Low investment compared with oil fired or electrically powered driers.
- Low lifetime of the drier.
- Costs for plastic/glass sheets.
- Not commercially available in most countries.
- Needs special skills (to prevent burnt or over-dried product).
- Technology is very product specific and needs to be specially designed.
Solar photovolatics (PV)
Solar PV systems convert sunlight directly to electricity. This electricity can be used to meet a number of energy demands. The more important ones are lighting, refrigeration and water pumping. We shall deal with the first two.
A simple PV system consists of:
- Solar panels which generates the electricity.
- A regulator or charge controller which regulates the amount of electricity going to either the battery or the end use (the appliances).
- If necessary, an inverter which converts electricity generated by the panels, which is direct current (DC), into alternating current (AC), the type that can be used by most electrical appliances.
- One or more appliances (light bulb, radio, refrigerator or pump).
- A battery which stores electricity for use when the sun is no longer shining.
The electricity generated with a PV system varies with its size. One square metre of a PV panel yields about 100 watts at full sunshine. In sunny areas which cannot be reached by the grid, PV can be economic for needs up to 20 kWh/day, which is usually enough to run refrigerators and light bulbs. The electricity required for most production processes is much higher; for this, PV is usually not a realistic option. Costs of installing PV should also be compared to costs of extending the grid. When the distance the grid is large, PV may be more economic option.
As a rule PV systems can be useful when the following conditions are met:
- There is enough sunlight.
- Low power supply is required for production.
- Grid electricity is not available, or the SME needs an extremely reliable electricity supply that the grid does not give
This is a mature technology which is used in many countries. PV household systems have been successfully used on a large scale in Germany, China, Kenya, India, Sri Lanka, Indonesia, Polynesia and Zimbabwe. PV systems have a long lifetime (20 years) and low maintenance costs. Now a days the technology is commercially available in India.
- Clean technology (no fumes, no noise, no moving parts).
- Low maintenance costs.
- Can be set up in modules.
- Available in most countries.
- The high investment makes it initially more expensive than grid electricity.
- Only economically feasible for low energy demand.
- Batteries must be replaced every 3-5 years.
- Requires skill to install and repair.