הפקולטה להנדסה - המגמה להנדסה מכנית

מאי 2002

Design and Construction of a
Solar Radiation Measuring Instrument.

The Brief Management Summary

The Final Project

הכין: אולבינסקי אלכס

מנחה: דר. אברהם דיין

Brief management summary.

Project description.

The aim of this project is to design, to construct and to calibrate the instrument for solar radiation measuring.

Project assignment.

Measuring of the solar irradiance plays an important role in meteorology, agriculture, thermal design of buildings and especially in the field of the solar energy utilization.

Today's market offers very broad spectrum of such instruments, based on different principles and constructed by various manufacturers. However, the conducted survey shows, that the prices vary from 400 USD for the simplest device up to 1.500 USD for the most sophisticated (EPLAB pyrheliometer, which is taken as reference). Since these prices can be significantly reduced, this project is concerned with the building of an instrument for solar radiation measuring, with an adequate precision and response time, but within low budget.

The constructed instrument will be used by the Heat Transfer Laboratory in Tel-Aviv University. Its measurements will be used to evaluate the efficiency of heat-pipe type solar collectors. These solar collectors will be installed in the Heat Transfer Laboratory at the frame of the project conducted by the laboratory in cooperation with European Union Market.

The final design of the pyrheliometer consists of a small circular collecting surface, covered with a Nextel black coating (high emittance flat paint). The collecting surface is maintained suspended by the K-type thermo-couple, which is penetrated through the teflon holder. The collecting surface is located inside the cylinder tube, which can be air-evacuated, in order to prevent heat transfer from the surface, by any phenomenon, except radiation.

The final design description.

The thermo-couple is glued to the collecting surface in order to measure its temperature. The thermo-couple can be connected to the same temperature transmitter as the heat pipe solar collector. In this option the transmitter will convert automatically the measured temperature into the solar irradiation according to derived formula. The additional option is to connect the pyrheliometer thermo-couple to the mobile thermometer transmitter. In this case the obtained temperature must be converted to the radiation flux by the means of the calibration chart or by substitution to the characteristic formula. The cylinder is prepared from the thin copper plate, covered with Nextel black coating from inside and polished from the outside. This is done in order to make the surface diffusive, to absorb all the radiation emitted by the collecting surface from the inside and to emit all the absorbed heat by convection from the outside. On the top and the bottom of the cylinder, two flanges are tin-soldered. On the upper flange the glass lens is installed in such a way that its distance from the collecting surface is equal to the lens focal length – focusing the incident radiation at the center of a surface. From the bottom, the cylinder is closed by the teflon bushing. Likewise, as an option, the side-lock through which the air is evacuated, can be connected to the bushing. The pyrheliometer is mounted on two degrees of freedom holder, in order to enable free movement of the instrument, while directing it to the sun according to the azimuth and the altitude. The instrument precise direction to the sun is ensured by the means of adjustment of the upper flange shadow exactly as to cover the bottom one. After being directed, the pyrheliometer is screwed firmly by the means of two bolts.

Results

Pyrheliometer specifications.

Description		Initial Requirement	Specified
Sensitivity [W/m^2 oC]:		N.A	8.6112
Temperature dependence:		N.A	Independent
Linearity:		Linear	Linear
Response time [sec]:		180	130
Error:		< 5%	< 5.1%
Dimensions:	Length [cm]:	N.A	11
	Diameter [cm]:	N.A	4
Weight [gr]:		N.A	270
Total Price [USD]		150	133

1. The response time τ, obtained at the experiments and theoretically is 130 sec, which is less than the initial requirement, introduced at the job proposal and the intermediate report. Likewise, this time is less than the sun-loss time (5 min). Which is very important, otherwise, the temperature would not have been stabilized before the concentration dot have leaved the receiver.
2. Linear behavior in all the range. This feature is very important for two reasons:

a. The instrument possesses the stabile behavior,

b. The obtained temperature can easily be converted to the solar radiation flux with the aid of the characteristic formula or by Pyrheliometer Chart (Chart F in the Appendix).

3. Total Price: 133 USD. It is less than 150 USD as was required initially at the job proposal. Likewise, the above price comprises less than 10% of the EPLAB pyrheliometer cost).
4. The total error is not bigger than 5.1%. Although it is slightly bigger than the 5% error at the initial requirement, it can be reduced easily as will be shown in the discussion paragraph.
5. Applicability. The pyrheliometer has already been used successfully at the frame of the project, conducted by the Heat transfer laboratory in cooperation with European Union Market. Pyrheliometer measurements were used for evaluation of efficiency of the solar heat-pipe type collector prototype.
6. Compactness and mobility. Due to its small dimensions, the constructed pyrheliometer is much more compact and light, comparing to EPLAB pyrheliometer. Likewise, the pyrheliometer may be connected to the mobile thermometer. Thus, its geometry and transportability options make the pyrheliometer more mobile and allow measurements at the fields, where electricity may be unavailable.
7. The theoretically based formula, which transforms directly measured temperature into solar irradiation, was developed and it is in good agreement with experimentally obtained data and the calibration chart.
8. According to the experiments data, the usage of vacuumation pump had not improved significantly the measurement results. Consequently, it was decided to use the pyrheliometer without application of the pump. Which of course donate to the mobility and simplicity of the pyrheliometer.

Discussion.

Although the realization of the project was successful, there are several aspects, which may be improved.

1. The cost of the pyrheliometer can be significantly reduced. Most of its price is the manufacturing the tube from the flat paint coated plate. This was done, since the coating was performed, before the final design of the model was configured. Consequently, it was decided to coat the raw material and to convert it afterwards into the desired shape. If the coating is applied inside a ready tube, the cost will be reduced.
2. The heat loss through the thermo-couple can be reduced by usage of the thermocouple with thinner wires. Which will diminish the area of the conduction heat transfer and thus reduce the losses. Likewise, the thermo-couple with less error will diminish the relative error of the pyrheliometer. The T-Type thermo-couple l answers the both demands. With the error of 0.5% the relative error in the temperature measurement will be reduced to 0.6% and consequently the total error will not exceed 3%.
3. In the performed calculations and assumptions, the optical efficiency was deduced according to the properties of the involved geometry. However, they have not to be real. The calculations of the optical efficiency could be improved if the properties of the lens were properly defined by the manufacturer or by spectrophotographic test.
4. The additional experiments over the whole possible spectrum of solar fluxes must be conducted, in order to confirm the assumption of the linearity over the all range. Likewise, these experiments will reduce the error of the Pyrheliometer Chart.

The constructive parts of the pyrheliometer were those found on stock of the workshop and were not exclusively manufactured for this construction, since it was not part of the workshop schedule. Therefore, the work was completed at the free time of the workers. For this reason, the aesthetic appearance of the two degrees holder can be improved.