With India becoming a hub for mega solar projects, it is certain that in future solar power will play a significant role in India’s energy mix. More specifically, concentrated solar power (CSP) could have a unique role in India’s energy mix. Its potential to use hybrid technologies and easily add storage could unlock dispatchable and base-load power, setting the stage for larger renewable energy penetration.
History of Solar Thermal Energy
Simple techniques for concentrating sunlight to generate heat date back thousands of years. In ancient China and ancient Greece, people used the concentrated sun’s rays with mirrors or glass to light fires. There have been some recorded incidents of war where this technique was used to set enemy ships on fire. By the early 20th century several scientists had built simple machines that could run on concentrated heat from the sun.
However, the most significant milestone was reached in 1913 when an American inventor Frank Shuman created the first large solar-thermal pumping station in Meadi, Egypt. Shuman designed a system based on five large reflectors made of glass mirrors and arranged them to form a trough in the shape of a parabola. Each parabolic trough focused sunlight onto a tube and thus heating the water inside it. The steam produced by the process was used to power an engine connected to a pump capable of delivering 6,000 gallons of water a minute from the Nile to irrigate farms.
After the oil crises of the 1970s, many nations started research on alternatives forms of energy. Solar-thermal pilot plants were built for research purposes. The first company to implement the technology on a commercial scale was Luz International, an Israeli company founded in 1980. The first installation of solar thermal energy equipment, however, already occurred a more than a century ago in 1910 in the Sahara desert. It was a steam engine without a kettle and fire but with a mirror system for sun light collection to heat water for the needed steam pressure.
How it Works?
Solar Thermal systems use concentrated solar radiation as a high temperature energy source to produce electricity using its heat. The mechanism of conversion of solar to electricity is basically similar to the traditional thermal power plants except use of solar energy as source of heat. The basic process involves the conversion of solar into heat energy with the help of concentrating solar collectors or mirrors, and generated heat is used to heat the thermic fluids like heat transfer oils, air or water/steam. These thermic fluids act as both heat carrier and storage media. The hot thermic fluid is used to generated steam or hot gases, which are then used to operate a heat engine.
The efficiency of the collector reduces marginally as its operating temperature increases, whereas the efficiency of the heat engine increases with the increase in its operating temperature. The solar energy thus collected is carried from the circulating ﬂuid either directly to the hot water or space conditioning equipment, or to a thermal energy storage tank from which energy can be drawn for use at night and/or cloudy days.
Solar thermal systems can be either non-concentrating or concentrating types. They may also be either stationary or with sun-tracking mechanisms, depending on the application.
Four technologies, which are currently in use, are:
- Parabolic Trough
- Solar Tower or Central Receiver Plants
- Linear Fresnel’s Reflector (LFR)
- Parabolic Dish
The major component of any solar thermal system is the solar collector. This is a device, which absorbs the incoming solar radiation, converts into heat, and transfers this heat to a fluid (usually air, water, or oil) flowing through the collector. The solar energy thus collected is carried from the circulating fluid either directly to the hot water or space conditioning equipment or to a thermal energy storage tank from which energy can be drawn for use at night and/or cloudy days.
There are basically two types of solar collectors: Non-concentrating or stationary and concentrating. A non-concentrating collector has the same area for intercepting and for absorbing solar radiation, whereas a sun-tracking concentrating solar collector usually has concave reflecting surfaces to intercept and focus the sun’s beam radiation to a smaller receiving area, thereby increasing the radiation flux.
Storage and Hybrids
Both power-tower and parabolic-trough systems can store thermal energy in the form of hot, molten salt. It is then possible to generate steam, and thus electricity, even when the sun is not shining. Solar-thermal plants without storage can operate about 30% of the year; but with storage that number could climb to 70% or higher.
A cheaper alternative to storage is hybridization. All the original Luz plants also have natural-gas boilers that can generate steam when the sun is not shining. Because solar-thermal plants have a power block and turbine already in place, the extra cost is marginal. Hybridization could also be done the other way around, by using steam generated from solar-thermal collectors to help drive the turbines at existing coal or gas plants.
Advantages of the Solar Thermal Technology
CSP technology has its own advantages over the photovoltaic as it can readily incorporate thermal energy storage and hybridization to provide dispatchable power. Solar Thermal Plants are generally built on a much larger scale, and their costs have been much lower. Compared with other renewable sources of energy, they are better prepared to match a utility’s electrical load. They work best when it is hottest and demand is greatest. And the heat they generate can be stored, so the output of a solar-thermal plant does not fluctuate as wildly as that of a photovoltaic system. Moreover, since they use a turbine to generate electricity from heat, most solar-thermal plants can be easily and inexpensively supplemented with natural-gas boilers, enabling them to perform as reliably as a fossil-fuel power plant.
Another advance that makes solar-thermal power more economically and technologically viable than in the past is the ability to use a large number of smaller and less expensive mirrors, steered by computer systems, to ensure more accurate and automatic tracking and redirection of sunlight than was ever possible before.
Bill Gross, the chief executive of eSolar, a developer of “power tower” technology based in Pasadena, California, says his firm is using software to turn thousands of flat mirrors and shape them into a continuously evolving parabola around the tower.
Challenges before the CSP Sector
Despite these advantages, much more needs to be done to improve the sustainability of Solar Thermal Systems. Presently, the CSP systems are more expensive than photovoltaic (PV) technology. Setting up Solar Thermal projects take more time than the PV systems. Also these plants require more water per unit of electricity produced.
There are also many start-up challenges. Till now the solar thermal sector in India has significantly lagged behind the solar PV sector in the market. Attracting investment for solar projects, especially CSP, remains a bottleneck to the growth of the industry in India. The government should develop a strategy to optimize the potential role of different financial institutions and stakeholders.
There is no domestic manufacturing base for CSP equipment in India and there are only a handful of experienced technology providers abroad. The developers are finding it difficult to find reliable, low-cost options, a necessity to make their projects viable at the low tariffs. However a lot of these challenges can be addressed by innovation in technology.
We must address the following issues to ensure healthy growth of the solar thermal sector –
- Attracting Finance
- Storage Facilities
- Hybridization Opportunities
- Availability of heat transfer fluid and turbines
- Water use and availability.
While the challenges remain, with government’s increased attention and policy initiatives and also market interest, the revival of solar thermal industry is imminent. But competition from photovoltaic systems for large-scale power generation should not be underestimated. The falling prices of solar-cell modules can pose a tough competition to the thermal market. But no matter which approach comes out on top, competition between the two technologies is sure to foster continued innovation, and a growing supply of clean electricity, in the years to come.
Solar Thermal Power Generation Program of India
In India the first Solar Thermal Power Plant of 50kW capacity has been installed by MNES following the parabolic trough collector technology (line focusing) at Gwalpahari, Gurgaon, which was commissioned in 1989 and operated till 1990, after which the plant was shut down due to lack of spares. The plant is being revived with development of components such as mirrors, tracking system etc.
A Solar Thermal Power Plant of 140MW at Mathania in Rajasthan, has been proposed and sanctioned by the Government in Rajasthan. The project configuration of 140MW Integrated Solar Combined Cycle Power Plant involves a 35MW solar power generating system and a 105MW conventional power component and the GEF has approved a grant of US$ 40 million for the project. The Government of Germany has agreed to provide a soft loan of DM 116.8 million and a commercial loan of DM 133.2 million for the project.
In addition a commercial power plant based on Solar Chimney technology was also studied in Northwestern part of Rajasthan. The project was to be implemented in five stages. However, due to security and other reasons the project was dropped.
BHEL limited, an Indian company in power equipments manufacturing, had built a solar dish based power plant in 1990’s as a part of research and development program of then the Ministry of Non-conventional Energy Sources. The project was partly funded by the US Government. Six dishes were used in this plant.
Few states like Andhra Pardesh, Gujarat had prepared feasibility studies for solar
Prospects of Solar Thermal Energy In India
India is fortunate to be located in the region, which receives abundant radiant energy from the sun. In most parts of the country sunny weather is witnessed around 300 days a year. States like Rajasthan and Gujarat and parts of Ladakh region lead in receiving the solar radiation, followed by parts of Andhra Pradesh, Maharashtra, and Madhya Pradesh, which also get good radiation when compared to many parts of the world.
Currently, India has only 52.5 MW of CSP in operation (of which, 50 MW is contributed by the Godawari solar thermal plant, located in Northwest Rajasthan). However, there are seven projects of 470 MW aggregate capacities underway. Many private energy companies are also planning to venture the field. It is expected that in next decade, solar thermal power plants will contribute significantly in meeting India’s energy needs in a green and clean way.
Solar thermal power generation can play a significant important role in meeting the demand supply gap for electricity in India. India’s rural electrification mission can use solar dish collector technology for generating electricity at village and community levels. Normally these dishes have 10 to 25 kW capacities each and use striling engine for power generation. These can be developed for village level distributed generation by hybridizing them with biomass gasifier for hot air generation.
Solar thermal power plants can also be integrated with existing industries such as paper, dairy or sugar industry, which has cogeneration units. Many industries have steam turbine sets for cogeneration. These can be coupled with solar thermal power plants. Typically these units are of 5 to 250 MW capacities and can be coupled with solar thermal power plants. This approach will reduce the capital investment on steam turbines and associated powerhouse infrastructure thus reducing the cost of generation of solar electricity.
Integration of solar thermal power generation unit with existing coal thermal power plants can also save a lot of energy.
The recent budget announced a number of measures to boost the solar energy market in India including exemption for machinery and equipment required for setting up solar power plants. It is expected that solar thermal heaters will also become cheaper.
Policy Support to Boost Solar Thermal Market
In order to fully realize our potential in the realm of solar energy, solar thermal projects need encouragement. The government should ensure that second Phase of the Jawaharlal Nehu National Solar Mission continues to encourage CSP technologies along with other solar technologies. It should also look at fostering development of new technologies, supporting R&D and innovation to encourage domestic production and cost reductions in a growing CSP market.
The industry offers many avenues for efficiency improvement and cost reduction via research and development, technology transfer, and application engineering. Further research is needed in the fields of molten salts for storage and heat transfer fluids. Production subsidies to encourage CSP manufacturers to establish facilities in India can also help us in meeting the domestic content requirement.
It is also extremely important for the government to enforce commissioning timelines for first phase projects to bring discipline in the sector. Strong actions in this regard can help avoid any delays in the second phase. Ministry of New and renewable Energy should ensure that Phase 2 guidelines continue to strike a balance between encouraging new players to participate and attracting experienced developers by maintaining the requirement that selected developers work with experienced technology providers.
Attracting Investment in the sector is the key to maintain a healthy growth in the CSP sector. In order to encourage a domestic CSP industry, the government should maintain a domestic content requirement or consider other support mechanisms such as non-discriminatory equipment production subsidies for Phase 2.
Government should also give incentives for the adoption of storage technologies in CSP plants and evaluate mechanisms to spur storage-enabled generation.
In the first phase, the guidelines mandated a cost-based measure of “30 percent of local content in all plants/installations,” excluding land. Developers and local manufacturers have expressed confidence that the manufacture of CSP components would become possible in India as the market develops.
Other Applications of Solar Thermal
The technology to use the heat of sun can also be applied to many tasks beyond generating electricity. It can be utilized for a number of industrial and domestic tasks.
Crop Drying: Agriculture can benefit a lot from solar heating. Globally, the heat of sun is used for drying of agricultural products. Solar crop drying is commercially now increasingly being used.
Heating and hot water preparation: Solar energy can easily meet most heating demand with variations depending on climate and location.
Solar cooling: Solar thermal cooling can reduce conventional electric AC loads; the system uses parabolic concentrators integrated with thermally driven double effect absorption chillers. Solar assisted air-conditioning of commercial buildings is a promising concept.
The advantage of solar is that the demand for cooling coincides with the availability of high solar radiation. Continued development of high performance collectors and system components will improve the cost effectiveness of higher temperature applications.
Solar assisted cooling is an extremely promising technology as peak-cooling consumption coincides with peak solar radiation. Now it is necessary to support its commercialization and continued R&D. With increasing demand for higher comfort levels in offices and houses, the market for cooling has been increasing steadily over the past years.
Today, solar assisted cooling is most promising for large buildings with central air-conditioning systems. However, the growing demand for air-conditioned homes and small office buildings is opening new sectors for this technology.
Solar Cold Storage: The biomass gasifier and solar thermal based hybrid cold storage is another unique development in this direction. The system can be seen working in the National Institute of Solar Energy in Gurgaon and the institute is developing this system for a number of applications. This could be an alternative to the conventional cold storages in a large part of rural India, which still lacks access to grid electricity. This can prove to be a boon to millions of farmers in India’s villages who need an affordable storage facility for agriculture products, fishing, milk and dairy products etc.
In a country like India where energy access and affordability are critical priorities, giving preference to either one of PV and CSP technologies would be a policy mistake. Both these technologies are needed to make access to electricity more sustainable and affordable. Both CSP and PV can play important part in the India’s energy mix. Solar Thermal Energy could, thus, be the super-solution to the power crisis that our country faces. With the government backing large scale solar thermal plants, a complete overhaul of the energy production set-up in the country is possible which can further pave the way for a greener and brighter tomorrow.
This story is written by Rohan Singh. Rohan Singh is journalist with significant experience and has worked with different organizations including DD News. He writes extensively on environment, new and renewable energy, coal and petroleum industry.He has worked with organizations like FICCI, Greenpeace and WWF.