Jul-Sept 2002 The Second Solar Lime Experimental Campaign

Oct-Dec 2002 Improving the Reactor Numerical Model

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Starting from the beginning of July up to the end of August 2001, every sunny day will be good for solar experiments.

Tests are actually running in the Small Solar Furnace at PSI's High Temperature Solar Technology Laboratory (HTL).

This facility comprises a 51 m2 focussing heliostat and a stationary parabolic dish (concentrator) with a 2.7 m diameter.

A venetian blind-type flux attenuation device, matched with a 3-axis remote controlled reactor platform, allow solar chemical experiments to be ran under a peak concentration ratio of 4000 suns. (1 sun = 1 kW/m2).

During our tests, a maximum power of more than 19 kW was measured in the aperture area.

To collect the radiant power into the high temperature reactor the heliostat mirror, that tracks the sun position through the all day, reflects the direct sunlight to the parabolic dish. The heliostat mirror slightly concentrates the solar radiation (by a factor of 4 ca.) while the fixed parabolic dish highly concentrates (by a factor of 1000 ca.) and reflects it to his focus, where the solar reactor stands. To reach the maximum power, the circular sunlight aperture of the reactor has to meet exactly the focus diameter of the parabolic dish (8 cm ca. in our case).

The solar flux intensity, along with the power, can be controlled by adjusting the shutter.

When the shutter is completely closed (as shown in the picture here aside) the flux image reflected by the heliostat mirror can be observed on the white wall created by the closed venetian windows.

This configuration is set on purpose, from time to time, to optimize the heliostat sun tracking in respect of the fixed position of the parabolic dish; this operation can be easily done by manually moving the heliostat mirror so that the flux image is centered at best on the white wall.

Please note that, at solar noon, the flux image reflected by the heliostat mirror is much more circular than the one shown here since the reflection angle is much smaller.

In that condition the available power for the chemical reaction at the circular sunlight opening is also much bigger. It's indeed clear how easier it is to convey the radiant power into the solar reactor when the flux image reflected by the parabolic dish is circular itself.

As a matter of fact, during any experiment, the reactor stands on the moving platform so that the circular light opening sited on the reactor's front plate is continuosly centered at best on the focus of the parabolic dish.

A solar experiment usually starts at 10 am to last up to 4 pm.

High temperatures can easily be reached only when the focus is properly centered on the aperture; otherwise, big power losses as well as an undesirable temperature rise on the reactor's water-cooled front plate may occurr.

To keep the temperatures inside and outside the reactor under control, the reactor has been equipped with lots of thermocouples.

A data acquisition program records all the relevant parameters during each solar experiment, including the real power of the solar radiation, measured by a sun tracker placed on top of the small solar furnace building. On a very sunny day, the solar irradiation may reach, at noon, more than 950 W/m2.

A camera is used to continuously check and adjust the focus position of the concentrated sun rays on the reactor's front plate.

By measuring the concentrated solar flux when pointed on a clean white target it is possible to estimate, via software, the mean power entering the reactor through the aperture. The knoledge of the input power is crucial for making energy balances and estimating the reaction efficiency.

Unfortunately, this operation requires the test to be briefly suspended since the platform has to be moved down to allow the measurement of the solar flux on the white target positioned above the reactor front plate. This operation is done from time before and after each experiment, at least every hour, because the mean power, together with the shape of the focal spot, is continuously changing during daytime.

From an operational point of view the experimental procedure is quite simple: before any test can start, the silo has to be filled with limestone and all the instrumentation has to be checked in order to be sure that no inconveniences will show up during the working phase. The first exposition to sunlight will result in a sudden raise of the temperatures inside the reactor and any mechanical occurrence or structural modification will end up being quite dangerous to be done after the experiment has sterted.

When the sun is "turned on" it takes quite a long time (up to two hours of full power flux) to get to steady temperature conditions because of the big thermal inertia of the refractory lining. On the other side, once the reactor is hot, only a small power input is needed to maintain the reactor temperatures. A long run experiment may last up to four hours under steady temperature conditions.

While running experiments, samples of burned material are collected and right away analyzed by the means of a small electric furnace. A quick chemical analysis response is needed to rapidly check the degree of calcination and consequently vary the working parameters of the reactor (temperature, revolution speed of the drum and limestone feeding rate).

The first tests of the current solar experimental campaign gave hopeful results concerning the chemical feasibility of the calcination reaction by using solar energy and only minor mechanical problems occurred.

Since the very first test it has been astonishing how good were both the efficiency of the reaction and the degree of calcination.

As a consequence of the encouraging results, the whole team is motivated to get a complete overview and a thorough understanding of how our innovative apparatus works.