Concentrated Solar Thermal Technology – Part III

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In the previous article, I talked about solar tower systems. In this article I will talk about dish/engine systems.

Dish/Engine Systems

Dish systems

Figure 17.1: Dish/engine system

The figure above shows a typical dish/engine system. The dish is a conventional parabolic dish and the engine is a Stirling engine. The Stirling engine converts the concentrated heat generated by the dish into electricity. Since the working principle of the Stirling engine is quite different from other types of engines, a brief description of it is in order.

The Stirling engine is a closed-cycle regenerative heat engine with a permanently gaseous working fluid. It is a heat engine that operates by cyclic compression and expansion of air or other gas (also called the “working fluid”) at different temperatures, such that there is a net conversion of heat energy to mechanical work. “Closed-cycle” means a thermodynamic system in which the working fluid is permanently contained within the system, while “regenerative” describes the use of a specific type of internal heat exchanger and thermal store, known as the regenerator. The inclusion of a regenerator differentiates the Stirling engine from other closed cycle hot air engines.

The working principle of the Stirling Engine is as follows:

fig2 1. Most of the working gas is in contact with the hot cylinder walls, it has been heated, and expansion has pushed the hot piston to the bottom of its travel in the cylinder. The expansion continues in the cold cylinder, which is 90° behind the hot piston in its cycle, extracting more work from the hot gas.fig3 2. The gas is now at its maximum volume. The hot cylinder piston begins to move most of the gas into the cold cylinder, where it cools and the pressure drops.
fig4 3. Almost all the gas is now in the cold cylinder and cooling continues. The cold piston, powered by flywheel momentum (or other piston pairs on the same shaft) compresses the remaining part of the gas.fig5 4. The gas reaches its minimum volume, and it now expands in the hot cylinder where it is heated once more, driving the hot piston in its power stroke.

Figure 17.2: The four phases of the Stirling Engine.

Stirling engine

Figure 17.3: The complete cycle of Stirling engine

The complete cycle can be seen in the animated GIF image above.

Stirling engines use gases such as hydrogen, helium and nitrogen as working fluids, the specific heats of which are 14.3 kJ/kg-K, 5.19 kJ/kg-K, and 1.04 kJ/kg-K respectively. The higher the specific heat, the higher the efficiency which means that hydrogen should be the most preferred working fluid. However, the use of hydrogen entails the risk of possible explosion. So although pressure losses are low due to the low viscosity of hydrogen, and it has higher thermal conductivity, it is not preferred. Helium and nitrogen are preferred since they are inert gases although their efficiencies are lower due to lower specific heats.

The merits of dish/engine systems are:
1.Can be used for different capacities due to its modularity.
2.No requirement of water.
3.Easy to repair and replace small engines.
4.Can be deployed in undulating terrain.

The demerits of dish/engine systems are:
1.Limitation on the maximum size of the dish that can be used due to structural and wind load considerations.Therefore, maximum capacity of a single system is limited to 25 kW.
2.Not perfected for commercial use although it has been for more than two centuries now.
3.Leakage of working fluid requires replacement.
4.Seals wear out frequently requiring change.

It is worth mentioning that dish systems without Stirling engines can also be used. Such systems can be used to generate hot water or steam (at the desired temperature and pressure). In the former case, water at any temperature can be generated; non-concentrating solar water heaters cannot do that and the water temperature is limited to around 60°C. In the latter case, the applications are in industries which require heat for various processes and are called IPH (Industrial Process Heat) applications. A good example is dairy industry where heat is required for pasteurizing milk. In the absence of a solar system, the heat is typically generated by burning furnace oil, which is expensive and also causes a lot of pollution. Solar systems, on the other hand, are non-polluting and environment-friendly in every way.

There is an Indian company called Clique Solar (http://www.cliquesolar.com) which has developed this technology 100% indigenously! Their product is rightly called ARUN and comes in three different sizes. I have visited the Mahanand Dairy in Latur where their system is installed and it was impressive to say the least.

I hope you liked this article. In the next article I will talk about installations of all the technologies in various parts of the world.

Sustainably yours,

Prashant Karhade.
Writer, Publisher, Entrepreneur

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