On Solar Thermal Energy

The Sun is one of the most promising sources of energy for our future. Energy from the sun may be captured in one of two ways. The first is through the direct conversion of sunlight into electricity, using a solar cell, technically known as Photo-Voltaic Module (Solar PV, in short). These are not very different from the solar cells that run many calculators. The second method of extracting the sun's energy is by collecting the heat we receive from the sun, and using the same to perform useful work.

In order to get the reader oriented, is useful to look at how much energy we get from the sun. Each square meter on the surface of the earth receives about 1 kW of solar energy. Of course, this number varies with location on the planet, time of day, and season. Taking this further, 1 sq. m of land on a sunny day in India can easily receive as much as 5-6 kWh of energy (8 hours of sunlight * 0.75 kW /m2 average flux). For more information on amount of energy received from the sun, see http://www.oksolar.com/abctech/solar-radiation.htm. How much of this we can actually extract between 8% and 25%, depending on the technology used. At 10%, 1 m^2 can generate at least 0.5Wh per day. Compare this with the typical household energy usage of about 3-5 kWh.

Solar cells or solar panels are definitely a very convenient way to put sunlight to use - they directly provide electricity, which can then be fed into the power grid. However, they are still 4-5 times more expensive to install than conventional coal-based plants. http://www.solarbuzz.com/index.asp keeps track of solar panel costs, and as of May 2008, it costs about $4,800 to install 1 kilowatt of solar PV capacity. Over the next 5-8 years, these costs are expected to come down with efficiency improvements, volume manufacturing and durability improvements. In the meantime, opportunities for solar panels exist in areas where conventional power is too expensive, or where the grid has not reached.

Over the last few months, though, I have started to get more excited about solar thermal energy. The central idea here would be to use a concentrator such as a parabolic dish to focus sunlight onto a small receiver, and thus heat up a thermal fluid such as water. There are several concentrator technologies, ranging from simple dishes, to long parabolic troughs, to foot-ball-field-sized arrays of reflectors concentrating sunlight onto the top of a tower (http://www.solucar.es/sites/solar/en/tec_termosolar.jsp). The process of transferring heat to the fluid can be very efficient. The next step would be to put this energy to use.

One option is to use this heat to boil water to steam, and drive a steam turbine to generate power. A 35 MW solar thermal steam power plant is being set up in Rajasthan to study the feasibility of this option. Given that the net efficiency of such plants is likely to be below 20%, it is possible that Solar PV will become a comparable and more convenient alternative in just a few years.

Another option is to focus sun's heat directly onto a "Stirling Engine", which is an engine that runs from heat supplied externally. Stirling Engines have been around for several years, but can be expensive to build to last. Stirling Energy Systems http://www.stirlingenergy.com/ is working on building such power plants. For a country like India, I suspect that this technology will be too expensive.

The option that excites me the most is to use concentrated solar power directly as a source of heat rather than as electricity. There are several process industries that could save on costs incurred in burning fuel for low-grade heat. For example, Pepsi Co. has installed a solar thermal plant in Modesto, CA to heat up the oil in which it fries its Frito-Lays chips (http://www.modbee.com/1618/story/259206.html). A group at IIT Bombay has developed an indigenous solar concentrator dish, which is being used in dairies in Anand, GJ to pasteurize milk.
There are several challenges with solar thermal energy. These include

(1) The source is limited to daytime. However, one needs to look into the economics of whether substituting fuel consumption during daytime alone is enough to justify the cost of solar thermal collectors.
(2) The land requirements could be significant. 1 MW heat generation capacity, assuming 40% land coverage and 50% heat transfer efficiency would require 5000 sq. m or 1.23 acres of land - which could be rather expensive.
(3) The cost of solar concentrators themselves is currently rather high. IIT Bombay's 160 sq. m dish is priced at Rs. 30,00,000, and has a heat generation capacity of about 80 kW.

In my next post, I plan to go into more detail on the costs and potential applications of solar thermal energy. For now, I will retire, and utilize this relatively light evening during my MBA to catch up on some sleep.