The sun is the world's largest source of energy. Each year the sun sends over 1.0 billion TWh of energy to the earth which is equal to 60,000 times the world's electricity needs. From a mathematical perspective, less than 3.0 per cent of the surface area of the Sahara would be sufficient to meet the world's energy demand with solar power plants. Solar energy in its various forms - solar heat, solar photovoltaics, solar thermal electricity, solar fuels - can make considerable contributions to solving some of the most urgent problems the world now faces, the climate change, energy security, and universal access to modern energy services. Solar energy offers a clean, climate-friendly, very abundant and inexhaustible energy resource to mankind, relatively well-spread over the globe.
Solar thermal electricity is a proven technology with close to 30 years of experience. Its strengths rest in its ability to make electric capacities firm and to time-shift electricity generation and storability. Concentrated Solar Thermal (CST), also known as Concentrated Solar Power (CSP), is one of the most viable options for large-scale solar thermal electricity.
The method is to convert solar energy into heat energy by absorbing it. Principal component is the solar collector to covert energy efficiently and it has the medium for energy transport and to use water/air/other fluid to heat to transfer the heat for generating electricity. The storage system is to overcome the mismatch between energy available and demand. It has the systems to transport and use energy.
A heat transfer fluid runs through the tubes which is heated up to 400°C by the sunlight through CST technology. The hot fluid passes through heat exchangers to generate steam in a central unit. As in a conventional power plant the steam powers a steam turbine with electricity generator. The integration of a heat store permits electricity generation almost around the clock. The main advantage of CSP technology against other renewable energy technology is the capability to provide dispatchable power by storing solar energy through thermal energy storage.
CST technology harnesses the sun's power to generate electricity. It uses lenses and reflectors to concentrate sunlight to generate heat energy. This energy is then generally used to heat a fluid, like water or oil, which in turn produces steam or hot air. The steam is then used to drive a turbine connected to a generator to produce electricity. Abundant sunshine and plenty of open space means Bangladesh is ideally placed to take advantage of solar thermal technologies for energy generation.
Photovoltaic solar or solar PV, traditionally known as Solver panel takes the energy collected from the sun by solar panels and converts it directly to electricity. On the other hand, solar thermal takes the energy from the sun and converts it first to heat, and then to electricity. One of the main benefits of solar thermal over solar panel is that the energy obtained from sun ray can be stored at the heat stage, meaning it can be a more reliable source of electricity. Energy can be stored during times of high output or low demand, and then converted to electricity later to provide a steady source of both peak and base-load electricity. CST does not need much space for installation of panels since it needs space for lenses to magnify the heat from sunlight.
Relatively low-cost thermal storage and fuel back-up could make this technology a useful complement to Solar panel and is likely to remain less expensive for electricity generation in the sunniest locations. Another option would be to design systems integrating production of heat, possibly cold, and electricity, either altogether or with different outputs at different times of the year.
CST energy is a proven technology that was first commercialised at large-scale in the Californian desert in 1984. CST power plants have delivered reliable renewable energy to communities in the United States, Spain and more recently, India, Northern Africa and the Middle East.
Most solar thermal plants also have a fuel-power backup, meaning CST can provide a firm and flexible source of renewable energy that can be dispatched when required. Almost all existing CSP plants use some back-up fuel to substitute or complement thermal storage. Back-up helps to regulate production and guarantee capacity, especially in peak and mid-peak periods.
CSP has another option of adding to existing power plants. It can also be hybridised by adding a solar field to fossil fuel plants, whether existing plant or greenfield plants. One option is to build a small solar field adjacent to a coal plant, pre-heating the feed water. In coal plants, successive bleeds on the turbine subtract steam during its expansion in order to preheat the feed water before it enters the boiler. The solar field can replace these bleeds, leaving more steam to be turned into electric power. One example of this is the 44-MW Cameo coal plant in Colorado, which added a 4-MW solar trough field in 2010.
CST is suitable for developing countries like Bangladesh as the optimal plant size is usually in the 200 to 300 MW range to benefit from scale effects. The most recent projects in the United States tend to target this range of capacities. CSP technologies are very much suitable for captive power plant as it can also generate electricity on a smaller scale of below 5 MW. The storability of generated electricity is very useful for end-user producers for reduction of cost of powers.
CST has another option of co-generation for heating and cooling system for industry and commercial and residential complex. This will further reduce the cost of utilities for both use of heat and cool simultaneously. The co-generation of heating and cooling system operated with other primary energy already exists in some industries in Bangladesh. The thermal power just replaces that primary energy.
Bangladesh has many remote islands and some remote places are without connectivity with national grid. Some small plants might be particularly relevant for isolated or mini-grids on islands, remote rural areas or requiring connection to weak grids with insecure supply.
At present some Indian city by-laws such as of Bangalore city has made the solar water heating and lighting mandatory. The by-law is under process to modify to provide solar water heating system for residential, commercial and institutional buildings to meet the byelaw requirement or a minimum of 50 per cent of water heating requirement on annual basis, whichever is higher. This will be mandatory for all offices, residential building blocks and hotels.
A domestic solar water heater in a family of five people with capacity of 100 litres can easily replace a 2kW electric geyser, which can save upto1500 units of electricity a year. The experts calculated that it will pay back of initial increment in cost is 3 -4 years, after which hot water is available almost free of cost.
International Energy Agency (IEA) in a report on Solar Energy Perspectives2011 projected that the global CST industry is quickly expanding, and is estimated to account for 28 per cent of total global electricity generation or 6000 gigawatts of capacity (with storage) by 2060.
Electricity costs would decrease even faster thanks to progressively greater capacity factors, making CSP technology competitive with conventional technologies for peak and intermediate loads in the sunniest countries by about 2020.
The writer is a Legal Economist.
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