Solar Energy

Solar energy is abundantly available, but in diluted form.

Solar radiation incident on outer limit of earth's atmosphere:1367 ± 7 W/m2
At earth’s surface:240 W/m2
Total at cross-section of earth:1.77 × 1016 W (105 times more than the demand)

India is a tropical country, where sunshine in greater intensity is available for longer hours per day. Annually about 5000 TW⋅h energy is incident over India's land area, with most parts receiving 4–7 kW⋅h∕m² on a typical day, with 250–300 sunny days in a year or 2300–3200 sunshine hours per year. So, solar energy has great potential in India. The highest amount of solar energy is received in western part of the country, while NE region received lowest. The distribution of solar radiation is depicted in the map.

Advantages of solar energy: 

  1. No fuel cost.
  2. No disposal of waste.
  3. No major safety concerns.
  4. Environment friendly.
  5. Near load center: It permits decentralized distribution of energy.
  6. Less losses.
  7. Negligible maintenance and operational costs.
  8. Gradual decrease in cost.
  9. Saving in forex.

Performance of typical energy convertors
Energy Conversion DeviceType of I/PType of O/PConversion η %
Steam Power PlantChemicalElectric40
Electric MotorElectricMechanical90
Light BulbElectricRadiation07
Solar CellRadiationElectric15-16
Solar CollectorRadiationThermal40

Solar energy experienced by us as light and heat can be harnessed through two routes namely solar photovoltaic and solar thermal, by direct conversion to electricity and heat energy respectively.

Table of Contents

Solar Photovoltaic Technology

Solar Photovoltaic (SPV) is the process of converting solar radiation into electricity using a device popularly called solar cell or photovoltaic cell. A solar cell is a semi-conducting device made of silicon or other materials, which, when exposed to sunlight, generates electricity. The magnitude of the electric current generated depends on the intensity of solar radiation, exposed area of the solar cell, the type of material used in fabrication of solar cells and ambient temperature.

The photovoltaic technology is one of the most promising ways to generate electricity in a decentralized manner at the point of use for providing electricity for lighting and meeting needs especially in non-electrified households, remote, far flung and unmanned locations.

Photovoltaic cells are becoming cheaper with new technology. There are newer, reflector-based technologies that could enable setting up megawatt scale solar power plants across the country.

Crystalline Silicon Solar Cells: Most of the commercially available solar cells are made of a single crystal or multi-crystalline silicon material. Silicon ingots are made by the process of crystal growth, or by casting in specially designed furnaces. The ingots are then sliced into thin wafers. These wafers are then processed to fabricate solar cells. Solar cells can also be made from ribbons grown from silicon melt.

Thin Film Solar Cells: It is possible to deposit thin layers of certain conducting materials to make solar cells and integrated modules. The advantage of thin film solar cells is that they consume less material (fraction of a mm) and less energy. Therefore, these are expected to be less expensive. Some of the commonly used matertials to make thin film solar cells include amorphous silicon (a-Si), copper indium selenide/cadmium sulphide (CuInSe2/CdS) or cadmium telluride/cadmium sulphide (CdTe/CdS). However, the present level of conversion efficiency of commercial and large size thin film solar cells/modules are relatively lower compared to that of crystalline silicon solar cells. The other thin film solar cells structures include dye-sensitized, organic cells, organic-inorganic material structures etc.

Unit Cells: A typical cell develops a voltage of 0.5 - 1 V and a current density of 20-40 mA/cm². These will vary with semiconductor material and temperature. The current of a cell is proportional to the input light intensity and the area of the cell. A single cell is about 10.4 cm x 10.4 cm in area. Typically one cell produces about 1.5 W of power. Typical characteristic of unit cell is shown in fig below:

PV Modules: In order to obtain desired voltage which can be used in the system and also suitable current output from the PV cell, they are arranged in series and parallel combinations to form PV modules and modules are interconnected for form arrays.

The solar cell strings are sandwiched between layers of an insulating material called ethyl vinyl acetate (EVA) which is placed on both sides of the solar cells. To protect cells from mechanical and environmental damage, it is hermetically sealed between a highly transparent tempered glass and a layer of plastic material.

Availability of SPV modules: The PV modules are available from a fraction of watt to few hundred of watts for a variety of applications. The PV module is rated for its peak power, which is defined as the maximum power output that the module could deliver under STC (standard test conditions). The STC conditions used for controlled measurement are:

  • 1000 W/m² solar radiation intensity.
  • Air-mass 1.5 reference spectral distribution.
  • 25 °C ambient temperature.

As per the report of CEA, the average power delivered by SPV system is not more than 20 % of peak power. Thus SPV of peak power 1 kW corresponds to 1 kW × 24 h × 20 % = 4.8 kW⋅h. Typically SPV of peak power rating of 1 kW generates about 3.5 - 4.5 kW⋅h electricity per day. The two commonly available modules are:

Specifications36 W module (≈ ₹ 5000)53 W module
Cell Size100 mm dia Si100 mm pseudo square
No. of cells36 in series36 in series
Application12 V DC12 V DC
Dimension1012 × 403 × 40 mm980 × 471 × 41 mm
Weight5.2 kg5.7 kg
Humidity0 – 100 %0 – 100 %
Temperature-40 °C - +90 °C-40 °C - +90 °C
Windup to 200 km/hup to 200 km/h

When we combine modules or connect modules to batteries following precautions should be taken:

  1. Blocking diode:If the system includes a storage battery system, a reverse flow of current from the batteries through the photo-voltaic cell can occur at night and can drain the batteries. A blocking diode is used to stop this reversed flow of power.
  2. By-pass diode:If one module in a series string fails, it provides so much resistance that other modules in the string may not be able to operate either. Shading of a cell in a module also causes similar condition and as a result of high resistance is offered, heating of the cell and resultant damage takes place. Hence shading should be completely avoided. However to safeguard, a bypass path, around the disabled module will eliminate this problem. This bypass diode allows the current from other modules to flow through it in right direction. Many modules are supplied with bypass diode right at their electrical terminals.
  3. Isolation Diode: These are used to prevent the poqwer from rest of an array from flowing through a damaged series of modules.

Solar Photo-voltaic System:Solar PV modules in series / parallel combination, interface electronics, mechanical supports structure, cables, switches etc constitute a SPV system. Storage batteries are used to store solar energy produced during the day for use in non-sunshine hours. The period between 9 AM and 3 PM, when 80 % of the total radiation of the day falls on the collector is called 'Solar Window'. The collector should be free from shading, which mar arise from trees or buildings in the South, during Solar Window throught the year. In SPV system, the components other than PV modules are collectively known as "Balance of System". A diagram for power system is:

As positioning of solar panels or collectors greatly influence system output, tilting mechanism is provided to collectors, which is adjusted according to season to maximize incident radiation. PV tracking is an alternative to fixed PV system. PV tracking systems are provided with tracking mechanism to follow the sun. These run entirely on its own power and can increase output by 40 %.

Uses of SPV Systems

The SPV system can be used as a "standalone / off-grid" with adequate backup or as a "grid-connected" system, which does not require batteries. Some of the popular SPV systems are:

Solar Lanterns

Solar lantern is a portable lighting device. It is easy to carry around and therefore ideal for both indoor and outdoor usage. Solar lantern models with CFL/LED luminares are available. If it is used for 3 - 4 hours per day, it can function upto 3 days without sunshine. Typical rating: PV module of Peak Power 8-10 W, 7 A⋅h 12 V battery, 7 W CFL at a cost of ₹ 3 000 - 3 300.

Solar Home Lighting

A solar home system (SHS) privides a illumination in one or more rooms of a house through CFL/LED luminaire, but can also run a DC fan or a 12 V DC TV.

Solar Street Lighting

A solar street-lighting system (SLS) is an outdoor lighting unit used to illuminate a street or an open area usually in villages. SLS with CFL or LED luminares are available. PV module, facing south is placed at the top of the pole and battery is placed in the box at base of the pole. It automatically lights up when surroundings become dark and switches off in day time. Typical rating: 74 W peak power PV module, 75 A⋅h 12 V battery, 11W CFL at a cost of ₹ 19 000.

SPV Water Pumping

The electricity generated by an SPV array can be used to run the motor and pump up water. The water can be stored in tanks for use during non-sunny hours. No storage batteries are used. These are used to draw water for irrigation, drinking and other purposes. The normal heads are in the rage of 7-15 m for irrigation and 10-50 m for drinking.

Building Integrated PV

In a building-integrated photo voltaic (BIPV) system, PV panels are integrated into the roof or facade of a building. PV panels generate electricity during the daytime, which is used to meet a part of the electrical energy needs of the building. Although the initial costs of a BIPV system are high (₹ 1 to 3 lakh for panels, inverter, batteries as of 2010), long-term savings result from a reduction in electricity consumption. Still payback period is high at about 10 years.

In order to encourage this application and to prepare manufacturers and users, the Ministry supports BIPV projects by meeting 80 % of the cost of PV modules installed in the systems on government and semi government buildings.

SPV Information Display System

In SPV based information display system, LEDs replace traditional neon tubes, the latter are not only expensive, but also consume much more energy. In a typical LED information system of size about 1 × 2 m and consuming about 300 watts of energy, an SPV module of about 2 kW peak power capacity is required to enable it to operate all through the night. This system requiring 15 hours of operation, the approx cost can be about ₹ 7 lakh.

Stand-alone SPV Power Plants

Here electricity is centrally generated and stored in battery banks. Inverter converts it into AC for various applications. Stand-alone system is used where conventional grid supply is not available, or is erratic or irregular. The most common use is for electrification of remote villages, power for hospitals, telecom and police stations, hotels, railway stations, border outposts and battery charging stations.

Stand-alone SPV can also be used during daytime without using the batteries. This electricity is made available to users through local line. These are mainly meant to replace conventional generators used during load-shedding periods.

Case Study I: Rural exchange, Multi 64–E. 

Here, exchange DC load is 160 W, so total energy needed for exchange load will be 160 × 24 = 3 840 W⋅h; 3 nos tube lights of 40 W, operated 4 hours per day will need 480 W⋅h; A 0.5 HP pump operated 1 hour daily needs 350 W⋅h. So, total W⋅h needed is 3 840 + 480 + 350 = 4 670 W⋅h.

So daily current @ 230 V is ≈ 20 A⋅h.

Selection of Array:

Avg. peak Sun hours = 5 hours.

Amp required = Daily Current / Peak Sun hour × 1.2 = 20/5 × 1.2 = 4.8 A.

If one module is required to give 2.4 A at 230 V, then no. of modules required = 2.

Case Study II: USOF Sites.

For USOF sites, government is giving subsidy to support both passive (land, tower, power) and active (mobile eqipments, antenna) infrastructure. The land size is 400 m² and each site has 3 service providers. Each of the service provider is given 3-phase AC power, but is using only single phase.

Average power consumption of BTS as indicated by manufactures is:

Sl NoBTS configurationAvg power consumption
12+2+21.3 kW
24+4+42.0 kW
36+6+63.5 kW

The combined load is about 4 kW and load is unbalanced. Power plant and batteries has been individually installed by service providers. The battery provided by service providers is either 2 × 200 A⋅h or 1 × 400 A⋅h. 20 kVA EA set has been installed by IP.

For this arrangement, a single SPV will be more economical than 3 separate SPVs with a single EA backup. The SPV required for 1 day autonomy (i.e. 1 day without sunshine) at various loads is:

Total loadStand alone systemWith EA set
2 kW17 kWp9 kWp
3 kW25 kWp12 kWp
4 kW34 kWp17 kWp
6 kW50 kWp25 kWp

Space required for SPV is 12-13 m²/kW of peak power, so 17 kW peak power and 25 kW peak power system will need 220 m² and 320 m² space respectively, which is available at sites. Thus:

  • For 2 kW load, 17 kW peak power SPV will work as a stand alone system with no EA operation required.
  • For 4 kW load, the same 17 KW peak power solar system will work in hybrid combination with EA set operating 4 hours a day.
  • For 6 kW load, the solar panel needs to be augmented to 25 kW peak power to work in hybrid combination with EA set operating 7 hours a day.

CAPEX of the solar / hybrid solar-wind systems ranging from 10 kW to 17 kW comes to around 26 - 51 lakh after considering the 80 percent accelerated depreciation benefit given by the Government of India. There is saving in run of EA set. Also Individual SMPS, battery charger and battery banks provided by USPs are not required.

For other applications in telecom mobile BTS sites, SPV systems may be selected on the following criterion:

  • 3.75 kW peak power systems for smaller configurations of outdoor BTS sites and where the availability of power is more than 12 to 14 hours per day.
  • 5 kW peak power system for outdoor BTS sites where availability of power 6 - 12 hours a day.
  • 7.5 kW peak power system for outdoor BTS sites where power availability is less than 6 hours per day.
  • 10 kW peak power system for outdoor sites having more than one BTS.

Grid Connected SPV Power Plant

In a grid connected solar PV plant, the PV array is connected to the grid through an electronic "Power Conditioning Unit" (PCU), which converts DC to AC and enables the grid synchronization.

The grid-connected SPV can be connected to single phase MV, three phase distribution grid or MV transmission grid. The capacity of grid SPV can range from few W to several MW. It is envisaged that within next 5-10 years, solar power will become cost effective compared to conventional electricity.

The rooftop and ground mounted grid connected SPV, when connected to tail end of the grid, can effectively meet the requirements of local communities.

Solar Thermal Technology

Solar thermal systems absorb the incident solar energy raising temperature of absorbing surface, which transfers heat to a medium such as water aor air to perform desired application. Use of flat surfaces to absorb solar energy is employed for low temperature applications, while curved surfaces are used to concentrate solar energy from a large area on to a smaller area to produce high temperatures. For power generation, steam generated from a solar thermal systsem is used to turn a turbine.

Solar thermal energy systems can partially or fully replace the conventional fuels such as coal, oil and electricity, and therefore have tremendous potential in India. The variety of applications for which these can be used are usually categorised as:

  • Low temperature applications (operating temperatures upto 80 °C) such as heating of water and space, cooking, air heating and drying of food and agricultural products and water purification.
  • Industrial process heat applications requiring steam, solar cooling etc.
  • Solar thermal power generation

Financial Support: The financial support for various solar thermal applications is @ 30 % of the benchmark costs based on the area of solar colectors used. For different types of solar collectors, the available support is as follows:

So NoSolar collector typeCapital subsidy / collector area (₹ / m²)
1Evacuated Tube Collectors3000
2Flat Plate Collectors with liquid fluid3300
3Flat Plate Collectors with air as working fluid2400
4Solar Collector System for direct heating3600
5Concentrators for manual tracking2100
6Non-imaging concentrators3600
7Concentrators with single axis tracking5400
8Concentrators with double axis tracking6000

Uses of Solar Thermal Systems

Solar Water Heating

A solar water heating system is a device that used solar energy to heat water and is the most common application of solar energy in the world. A typical domestic solar water heater of 100 litres per day can save upto 1500 units of electricity in a year.

Design: A solar water heating system consists of solar collectors, a hot water storage tank, arrangement of cold water supply and connecting pipes etc. Solar collectors may be either flat-plate type or evacuated glass tube type. For solar water heatings of large capacity, an electric pump and some controllers are also used. Solar collectors are installed facing due south in inclined position at a slope decided by the latitude of the place and seasonal load profile of hot water requirements. These systsems may be installed on the open roof, on balcony/terrace, or on open ground. A continuous supply of cold water may be provided through an overhead tank. In solar water heating systems without pumps, hot water storage tank is placed higher as compared to the top of solar collectors.

Typically, a flat plate solar collector comprises a black absorber, covered on top with a toughened glass sheet with a small gap in between, and insulating material on the back of absorbers and sides. The entire assembly is placed in a well-sealed box. Absorber is prepared by bonding tubes through which cold water is circulated for heating by the heat transferred to it from the absorber.

Nowadays, another type of solar collector called evacuated tube collector (ETC) are being used instead of flat-collectors. These are of two types: (i) all-glass 'Dewar' type ,and (ii) single glass envelope metal fin-in vacuum type. All glass configuration have been extensively used in China. In this configuration, a special coating to absorb maximum solar radiation is applied on the outer surface of the glass tube which is inserted inside another tube of larger diameter. The open ends of the two tubes are sealed together and evacuation is done from the other end to form 'Dewar' like structure. The water is circulated for heating inside the double glass assembly. Initial cost of these is less than the cost of flat-plate collectors. The metal fin-in tube configuration may contain a heat pipe system connected to the absorber placed inside a glass tube. Air is removed from tube to create the vaccum. These provide thermal energy at higher temperatures (> 80°C) and are costlier than all-glass.

The water stored in the tank remains hot overnight as it is adequately insulated. Solar water heating systems are provided with electrical back-up system, which can be used on cloudy days. Electrical heating elements are usually placed in the storage tank. In some cases, solar-water heating system is led into an existing geyser, which needs to be switched on only on cloudy days. Most domestic systems have capacity of 100 - 500 litres of hot water per day.

Advantages:

  • Hot water is available 24 hours a day depending on use and capacity.
  • A solar heater pay backs its cost in 3 - 4 years.
  • Solar thermal lasts a long time (15-20 years) and requires only simple maintenance.

Cost:The smallest solar water heater available has a capacity of 100 liters per day and is suitable for a family of 4 to 5 members. It costs about ₹ 15 000 - ₹ 20 000 depending on solar collector type. Unit cost is lesser for higher capacities. The IS for flat plate solar collectors is IS 12933 : 2003.

Solar Cooking

There are two types of solar cookers available: Box type and Solar dish type.

Box Solar Cooker: 

Design: A box-type solar cooker consists of an outer box, typically 600 × 600 mm, made of either fibre glass or aluminium sheet, a blackened aluminium tray, a double glass lid, areflector, insulation and cooking pots. The blackened aluminium tray is fixed inside the box, with insulating material in between to prevent heat loss from all sides. A double glass lid with toughened glass acts as the cover of the cooking tray. A reflecting mirror, fitten on the inside of the outer box cover, reflects the solar radiation and helps in increasing the solar energy input. The cooking pots are made of steel or aluminium and painted black on the outer side. The food to be cooked ois placed in the cooking pots, which are then placed in the aluminium tray and covered by the doubleglass lid. The cooker is kept facing the sun appropriately to cook the food. Hybrid models are available which are provided with an elecrical heater for use on cloudy days. The IS standard for solar cooker is IS 13429.

Advantages: The box type solar cooker is ideal for domestic cooking during most of the year except on cloudy days. The cooker can be used for preparation of rice, dal, kadhi, vetetables, meat, fish, snacks, soups etc; but it cannot be used for frying or for making chapatis. If used regularly, it can save upto 3 LPG cylinders a year. Also, it saves time as no attention is needed while food is cooked in the solar cooker. There is no fear of scorching the food. Slow process of cooking in these cookers ensures better and more nutrious cooked food.

Costs: From ₹ 2 000 to ₹ 3 500 for normal size suitable for a family of 4 to 5 members. Life is 10-12 years and pay back period is 2-3 years.

Dish Solar Cooker: 

Design:A dish solar cooker uses a parabolic dish to concentrate solar radiation. This is also known as 'SK-14' type of cooker. This model is ideal for homes and small establishments. A typical dish solar cooker has an aperture diameter of about 1.4 m and focal length of 0.28 m. The reflecting material used is anodized aluminium sheet, which has reflectivity of 80 %. Manual adjustment of solar dish is required to follow the sun. The temperature achieved at the bottom of the vessel is 350-400 °C, which is sufficient for roasting, frying and boiling. This cooker can meet the needs of about 10-15 peaple and can be used 8-9 hours a day.

Cost:₹ 7 000 - ₹ 8 000. It can save upto 10 LPG cylinders per year. Payback is 2-3 years. Larger size cookers are also available.

Indoor Cooking:

Design:The unique feature of this model is that it makes possible to cook using solar energy within the kitchen. In this model, a larger dish having 7-12 m² of aperture area is used. This dish is placed outside the kitchen so that it reflects solar rays into the kitchen through an opening in its north wall. A secondary reflector further concentrates the rays to the bottom of the port / frying pan, which is painted black to absorb maximum heat. A mechanical clockwork arrangement is provided for automatic tracking of sun. Temperature attained is 400 °C on clear sunny days.

Cost:₹ 70 000 - ₹ 1 lakh for cooker suitable for 40-50 persons.

Solar Steam Cooking

It is possible to cook large quantities of food using the steam generated by solar heat. A solar steam generating system comprises automatically tracked parabolic concentrators, steam header assemblies with receivers, steam pipelines, feed water piping, steel structures and civil works, instrumentation like pressure gauges and temperature indicators, stream separators, steam taps etc. The system is generally hooked up with a conventional steam generating system already available with the user, to make it reliable under all weathjer conditions. A dish of 16 m² area is the most commonly employed. These systems can be used for large community kitchen and has potential to save fossil fuels.

Solar Drying

Many agricultural and industrail products need drying in order to reduce moisture for processing or preservation. While open sun drying may be most extensive and inexpensive option, the process is unhygenic and time-consuming. Conventional fuels such as biomass, oil and electricity is being used for drying. Solar dryers are viable option now in many industrial and agricultural application.

Depending on the mode of operation, the solar dryer could be designed in different ways:

Direct Type Dolar Dryers: A direct type solar dryer is one in which solar energy collection and drying takes place in a single unit. Cabinet dryers, rack dryers, tunnel dryers, greenhouse dryers and multi-rach dryers falls under this category. Normally these dryers are small in size and are stand-alone units. Usually these dryers are designed to use natural ventilation in updraft mode, however, some have DC fans powered by SPV for better air movement, and ensures faster moisture removal making it suitable for drying products with high moisture content.

Indirect Type Solar Dryers: In this solar energy collection and drying takes place in separate units. It has two parts; (1) a flat-plate air heater, and (2) a drying chamber. Air is heated in the flat-plate heater placed on the roof of the building or on ground. Hot air is circulated in the drying chamber with a blower. These dryers can be designed in different sizes. These are most useful in agricultural and food industry, where temperature requirement is in the range of 50-80 °C. Potential applications include tea leaves, cofee beans, fruits, spices, seafood etc. Solar concentrators are used to provide hot air at temperatures higher than 80 °C.

Mixed-mode Solar Dryers: These are the solar dryers in which solar energy collection takes place in the drying as well as the air heating unit. The drying, however, takes place only in drying unit. In large industrial drying systems, he solar heated is combined with air heated by conventional unit, thus improving reliability.

Advantages: Solar drying systems have low operation and maintenance costs and drying process is completed in hygenic and eco-friendly way. These are cost effective, but drying gets limited to daytime only.

Cost: Cost of solar drying is a strong function of climatic conditions and drying requirements in terms of operating temperatures, air flow rate and humidity levels. However, in most industrial products, payback period is 3 - 5 years.

Solar Water Purification

Water purification from brackish or seawater is one of the potential area for solar thermal. For this, conventional basin type solar stills are available, where solar energy is absorbed and pure water is obtained through evaporation and condensation process in an enclosure containing raw water. These units produces about 2-3 litres of water per sqm of collector area, and have utilities for small water requirements, such as batteries and laboratories. Speacial technoogies are also emerging involving multi-effect process of evaporation and condensation by employing advanced solar collectors for increased per unit area yield.

Industrial Process Heating

As per estimates, about 50 % of the energy, which is needed in commerce and industry for production processes is below 250 °C. A large potential of heat demand exists in paper, food, textile and chemical industries.

The thermal efficiency of flat plate collector based solar system becomes quite low at temperatures greater than 80 °C. Concentrating solar collectors are employed to obtain higher temperatures. Presently, solar dish concentrators, such as Scheffler dish and ARUN dish, are commercially available in the country.

Typically, a 100 m² concentrator saves about 8-10 tonnes of oil annualy, giving a pay-back of 5-6 years.

Solar Air-Conditioning and Refrigeration

In tropical countries, cooling is necessity both for storing perishable products and for thermal comforts inside the buildings. Vapour absorption refrigeration systems are driven by thermal energy and hence can be used to operate on solar energy. One of the important considerations in favour of these systems is that the need for refrigeration / cooling is maximum when the sun shines the most. The proven absorption systems are based on water-lithium bromide and ammonia-water as working fluid combinations. Water lithium bromide systems operate at temperatures compatible with selectively coated solar flat plate collectors (around 80 °C). Ammonia-water system needs generation temperature of more than 150 °C for economic operation.

Thermax has recently commissioned 100 kW solar AC in Gurgaon. In this system, a triple effect chiller has been integrated with indigenously developed solar parabolic concentrators.

Solar Thermal Power Generation

Solar thermal power generation is one of the key application of solar energy through thermal route. Solar thermal power technologies use solar energy to produce high temperature by focussing solar radiation from a larger area on a smaller area and then generate electricity by either driving prime mover with high pressure steam generated or using an engine (like Sterling engine) directly. These plants have thermal mass to store heat energy for short periods, and storage can further be increased as per requirement for continuing generation of electricity during periods of low shunshine as well as after sunset. Solar thermal power projects are seen as having the ability to provide reliable electricity that can be dispatched to the grid when needed. Also the flexibility of these plants to combine with conventional fuels enhance energy security. Solar thermal power can provide a reliable source of electricity production in regions with strong direct normal irradiance.

A number of technology options are available to produce electricity by heat from solar radiation:

Parabolic Trough Technology: It consists of large field of parabolic trough collectors through which a heat transfer fluid is circulated. The hot heat transfer fluid (at a temperature of about 400 °C) passes through a series of heat exchangers to produce steam, which runs a conventional turbine to produce electricity. The plant is provided with an optional thermal storage and / or fossil fuel backup systems. The solar field consists of single-axis-tracking parabolic trough collectors, which are normally aligned on the N-S horizontal axis. The collectors track the sun from east to west during the day to ensure that the sun is continuously focussed on the linear receiver. Given sufficient solar input, the plant can operate at full rating on solar energy alone. This represents the most successful tried technology the world over.

A new variant of the technology, known as Linera Fresnal Reflector is also available where shape of parabola is achieved by organizing various mirror strips at different angles. These strips are mounted on horizontal surface and therefore, the need of heavy structure is alleviated. This technology option is fast emerging and is expected to reduce the cost of solar power.

Solar Tower: A Solar Tower configuration consists of a field of heliostats or sun tracking mirrors, which reflect solar energy to tower mounted receivers. The concentrated heat absorbed by receivers is transferred to a circulating fluid that can be stored and later used to produce power. These systems can operate at temperatures greater than 500 °C. A couple of plants have been built using molten slat (a mixture of 60 % sodium nitrate and 40 % potassium nitrate) and steam as heat transfer fluid. The collector field can be sized to collect more power than demanded by the steam generated system and the excess heat can be accumulated in hot storage tank for later use.

Solar Dish Systems: In these, a large size paraboloidal dish is used as the solar concentrator, and a heat-to-electricity conversion system (Stirling engine) is mounted at focus of the concentrator on one single rack. The system is equiped with a tow axis tracking system. The heat absorber of the Stirling engine is placed in a focus of the dish reflector. Dish Stirling system is modular and stand-alone units of 10-25 kW capacity has been tried out. Recently some effort are also underway to generate power using solar-dish system through producing steam.

National Solar Mission

National Solar Mission was launched on January 11, 2010. The objective of the mission is to establish India as a global leader in solar energy, by creating policy conditions for its diffusion across the country quickly as possible. The mission targets include:

  1. To create an enabling policy framework for the deployment of 20 000 MW of solar power by 2022.
  2. To create favourable conditions for solar manufacturing capability, particularly solar thermal for indigenous production and market leadership.
  3. To promote programme for off-grid applications, reaching 1 000 MW by 2017 and 2 000 MW by 2022.
  4. To achieve 15 km² solar thermal collector area by 2017 and 20 km² by 2022.
  5. To deploy 20 million solar lighting system for rural area by 2022.

The mission is to be implemented in three phases and the proposed roadmap is as below:

Sl No.Application segmentPh 1 (2010-13) targetPh 2 (2013-17) targetPh 3 (2017-22) target
1Solar collectors7 km²15 km²20 km²
2Off-grid solar applications200 MW1 000 MW2 000 MW
3Utility grid power i/c roof top1000 - 2000 MW4 000 - 10 000 MW20 000 MW

The 1 000 MW of grid solar power will be developed through NVVN (NTPC Vidyut Vyapar Nigam Limited), which is designated as nodal agency by the Ministry of Power for entering into Power Purchase Agreements (PPA) with Solar Power Developers. Of the proposed 1 000 MW, 500 MW each will be through Solar PV and Solar Thermal. Apart from this, 100 MW of roof top power and 200 MW off-grid power has been targeted.

News: Govt to select solar plant bidders by Sept 2010

As many as 1000 entities are keen on setting up solar power generation plants with capacities of 5 MW each under National Solar Mission. The MNRE had invited bids for a total of 500 MW solar photovoltaic power generation capacity and it will be selecting only about 100 entities through a reverse bidding process.

According to Elctricity Regulatory Commission, cost of generation of a unit of solar power is ₹ 17.91. Under the reverse buidding process, to select the entities, the upper limit for cost of generation would be ₹ 17.80. Entities which quote the lowest cost for generation will be offered the mandate to put up these units.

Uniform tariff bad for Eastern Zone: A uniform national solar energy tariff is bad for Eastern states as radiation levels are well below the western region. Accordingly, solar power plants with identical capacities will generate less compared to solar units in the west. So revenue generation from a solar plant in West Bengal and Bihar will naturally be less than in Rajasthan and Gujarat.

News: Facts

News: Darkness at Noon

Over 3 years ago, the government launched one of the world's most ambitious solar power programs. But now the program is a mess. Inordinate delays in the execution of six big-ticket solar projects, have cast a long shadow on solar power infrastructure. Also crippling the sector are some dubious technology choices and foolhardy policy decisions.

Finances are another serious problem. Most developers find finances at exorbitantly high rates, forcing them to take funds from abroad. But loans from foreign banks have their downside: developers are forced to buy material recommended by bank's host country, and this has hit domestic manufacturers badly.

When the mission started, solar power was 17.8 MW, now we have over 1 000 MW; but these impressive numbers are due to program launched by Gujarat, which is independent of NSM. Solar plants in Gujarat account for over 800 MW - nearly two-third of the total capacity in the country.

But the mission has been applauded on many counts: it was carefully crafted, MNRE has gone for a transparent bidding process called reverse auctioning and the innovative manner in which it has bundled expensive solar power with cheaper conventional power to make it cost-effective.

During the first phase, as many as 500 bidders competed for 63 projects worth 12 000 crore. On the technology front, firms could either go for Solar PV or Solar Thermal. There was the concious decision to go for an equal spilt between PV and Thermal. Being a proven technology, PV project implementation has been easy. It has not the case with Thermal, but Solar Thermal is more suitable for India.

One of the major limitation of solar PV is that it can produce electricity only when the sun is out. Solar thermal, on the other hand, can generate electricity round the clock. Besides, over the years, thernal becomes more cost-effective.

Most of the delay, is on account of delay in thermal projects, most of them are in Rajasthan. There are several reasons for the delay: insufficient and inaccurate solar radiation data, expensive finance, unclear future of subsidies, difficulty in securing land and water, need for finding local manufacturing and tight deadlines. The solar thermal developers have already approached the ministry. The major reasons cited by developers are faulty data and non-availability of heat trasfer fluid.

Also, the prices of PV has come down due to glut in the market caused by recession. So developers may try to migrate from thermal to PV. Also Rajasthan is not the ideal place for setting up solar thermal project, as dust deposits obscures reflection and reduces yield. Studies has shown that even a month's accumulated deposit reduce efficiency by 35 %. Regions such as Jharkhand and Ladakh are more suitable for thermal projects.

PV projects, on the other hand, have different problem. For them financing has been a thorny issue. Most of them has taken loans at cheaper rate from US Exim bank, which charge 4 % interest against 12-15 % by Indian banks. But the US insists to procure materail from US firms, which specialize in thin-film technology. The global trend is towards silicon modules, which is time tested and more efficient. Worldwide, 86 % installations are silicon based.

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