Solar-energy systems

An alternate and renewable source of power generation has become inevitable to meet the ever-increasing power demand. In this situation, solar-power generation has become the need of the day

K. Bharathan

Wednesday, July 02, 2014: ‘Solar’ is the Latin word for ‘sun.’ Solar energy from sun gives life to earth and its creation. Besides this, solar energy can also be successfully utilised for various applications such as cooking, heating, drying, lighting, power generation, communication, space technology, etc. Our country is blessed with abundant sunlight, which is a very large resource for development of solar photovoltaics (PV) power. There is no negative impact on the environment with utilisation of this power and it is freely available. Also, it is the safest renewable energy resource available in the world.

Solar cells, commonly called as PV cells, convert sunlight directly into electricity. The name PV is derived from the process of converting light (photons) into electricity (voltage). This is called the ‘PV effect.’ The PV effect was discovered in 1954 by the scientists at Bell Telephone Company. They discovered that silicon (an element found in sand) creates an electric charge when it is exposed to sunlight. Since then, these solar cells have been used to operate smaller devices like calculators, watches, lights, etc and also large aircraft and space satellites. In the present day, thousands and thousands of people power their houses, business establishments, manufacturing units, etc with solar PV systems.

In a solar-energy system, the most important component is a solar or PV cell. This is a solid-state device with a large light-sensitive area that converts the light energy from the sun directly into electricity by the PV effect. To increase the power, a number of individual PV cells are interconnected in a weatherproof sealed package called a PV module or a solar panel. Solar cells are very brittle and require great care and protection whilst handling. To protect them from damage, they are hermetically sealed between top layer of glass or transparent plastic and the bottom layer is protected with plastic sheet or a combination of metal and plastic. To increase the strength of the solar panel, the outer edge assembly is kept in a metal frame all the time.

A junction box is fixed at the bottom side of solar panel to interconnect and take the output voltage. A mounting frame made in metal is used to assemble these solar panels which are connected in parallel or a series combination as per requirement of the voltage and current.

Solar-home system
A solar-home system consists of a solar PV module, a battery bank and a power-conditioning unit (PCU). The output voltage from the PCU can operate light, fan, TV, computer, home appliances, etc depending on the capacity of the system.

Solar PV module generates DC voltage in proportion to solar panel size and the available sunlight. The power generated in the PV module is sent to the charge control circuit in the PCU. The charge control circuit controls the voltage and current and sends these to the battery bank to charge the battery. The inverter in the PCU processes the 12-24V DC voltage into the required output voltage of 220V AC at 50Hz, which is connected to the load.

The system has a microcontroller-based pulse width modulation (PWM) charge controller which can control the charging current. When the solar energy from the solar panel is available, the mains power will not be used for battery charging or for the load. The load takes power from the solar panel. Power from the mains will be used only when the battery voltage is below the preset voltage (this voltage can be preset). When mains power is on, mains supply and power from the solar unit does not load each other. The charging is carried out with the intelligently-shared current.

An important fact is that if the PCU gets overloaded whilst working on solar, the load gets shifted to mains supply automatically and again when the battery charge reaches the preset level, mains supply is cut off and the load gets shifted to solar/battery again. Battery deep discharge, overcharge protections are incorporated inside the PCU. This ensures health of battery thereby extending its life.

Solar water-pumping system
Soalr water-pumping system is specifically designed to pump water in areas where there is no reliable supply of electricity. The system is used to pump water for domestic use, agricultural irrigation, aquaculture industry, water treatment, etc.

Solar water-pumping system comprises a set of solar PV modules. The PV modules absorb sunlight energy and generate required level of DC voltage and current in proportion to the solar PV modules and available sunlight. The DC output voltage produced by the PV modules is sent to the pump control unit. The pump control unit processes the DC voltage into required levels of three-phase/one-phase AC voltage for the operation of the pumping system. Drive circuits are also integrated in the pump control unit, so no other additional devices are required either to start or run the motor. The system is also equipped with protection circuits to work against low voltage, overvoltage, overcurrent, overheat, etc.

In the pump control unit, the most important part is solar-pump inverter which has the maximum power point tracking (MPPT) function. This inverter adjusts the output frequency automatically to achieve MPPT according to the intensity of the sunlight.

With solar water-pumping system, the electricity bills and operating and maintenance costs can be minimised or eliminated.

Solar water-heating system
Solar water-heating systems are now widely used. Two types of solar water heating systems are popular. Evacuated tube collector (ETC) and flat plate collector (FPC). The collector assembly is installed at an angle facing the south direction. A storage tank is installed at a height behind the collector assembly and it is connected with a connecting pipe to the collector assembly. The water heating system is installed outdoors in an open space and it is connected to a continuous water supply from the overhead tank. Water flows in the collector assembly through tubes, absorbs heat from the solar panel and hot water is produced. This water is stored in the storage tank installed behind the collector assembly. The water thus stored in the tank can remain hot until the morning of next day.

Cold water from the overhead tank by gravity flow enters in the tank through point A (see Fig. 7) and fills it up. The cold water from the solar water tank flows through outlet pipe B in the bottom header pipe of the solar tube collector and goes into all the copper tubes through bottom header C, until all the collector tubes and solar tank at the top are filled with water.

When the sun rises to a level, its energy falls on the absorber fins and collector tubes. Energy from the sun begins to heat the water stored in the collector tubes. As the heated water is lighter than the cold water, it rises through the top header pipe D of the tube collector and flows into the top of the solar tank through pipe E. At the same time, cold water from the solar water tank flows into the tube collector through tube B. The cold water gets heated and rises to the top and this process continues until the temperature of the water in the solar tank and collector tubes equalises. This process is called ‘thermosiphon process.’ The solar water tank is now filled with hot water.

Fig. 7: Solar FPC (flat plate collector) water heater
Thermosiphon, also known as thermosyphon, is a process used to exchange heat from liquids without a pump. This system simplifies the heat-transferring process and eliminates the cost. This technique is commercially used in solar water heaters. Whenever the hot water is drawn from the solar tank through outlet F, cold water enters into the solar tank from the overhead tank through inlet pipe A. This lowers the overall temperature in the solar water tank and collector tube. In this condition, the thermosiphon process starts functioning again. At the end of 7-8 hours of operation in the bright sunlight, the average temperature of the water is around 60°C.

To collect maximum energy from the sun, the collector assembly should face south direction with proper angle, depending upon the latitude.

According to Indian conditions, 25 to 30 litres per day (LPD) per person is the average usage of hot water. Accordingly, a solar water heater with 100 to 125 LPD system is suitable for a family of three to four members.

Water heaters are available in 100, 125, 200, 300, 500 and 1000 LPD sizes for domestic use. Solar water heaters are also manufactured for use in large hotels, factories, hospitals, hostels, laundries, canteens, etc. Swimming-pool heating with a capacity of 2000, 3000, 4000, 5000 LPD is also done by solar water heaters.

Modern systems such as solar water pumping, solar cooking, solar air-conditioning and solar street lighting have now become popular in India. At present, the demand for electricity is growing along with increase in power costs. So an alternate and renewable source of power generation has become inevitable to meet the ever-increasing power demand. In this situation, solar power generation has become the need of the day.

The author is an ex-manager (R&D), UMS Radio Factory, Coimbatore, ex-general manager, Sulax Corporation, Bangalore and advisor, Electronics and Controls Corporation, Coimbatore